Balancing of parts - metalwork and mechanical assembly work. Static and dynamic balancing of parts What is static and dynamic balancing
Dynamic imbalance of the rotor is characterized by the presence of both static and moment imbalance, when both the main imbalance vector (D) and main point imbalances (M):
When the rotor is dynamically unbalanced, the axis of its rotation and one of the main axes of inertia either intersect outside the center of mass or intersect in space.
Elimination of dynamic imbalance of the rotor is carried out by dynamic balancing methods, in which the static and moment imbalance of the rotor are simultaneously reduced. In practice, dynamic balancing is the process of checking the mass distribution of a rotating rotor and, if imbalances exist, changing this distribution using corrective masses until the permissible unbalance value is achieved.
The choice of one or another dynamic balancing method is primarily determined by the type of rotor - rigid or flexible. If the rotor does not bend during rotation and behaves like an absolutely rigid body, making only movements caused only by vibrations of the bearing assembly, then such a rotor is called rigid. In fact, in any real existing rotor there are always dynamic bending deformations caused by the distribution of imbalances along the length of the rotor. But if these deformations are negligible compared to the displacements characteristic of rigid rotors, and are within tolerances at all rotor speeds, then such a rotor is considered rigid. It is important to note that with an increase in rotation speed and a decrease in the permissible imbalance value, the rotor, previously considered as rigid, begins to exhibit the properties of a flexible rotor and requires a change in the choice of balancing method.
Balancing of rigid rotors is carried out using methods regulated by GOST ISO 1940-1, and flexible rotors by GOST 31320. The choice of one method or another is determined by the configuration of the rotor and its rotation speed.
The rotors of most known machines at operating speeds can be considered as rigid and dynamic balancing methods regulated by GOST ISO 1940-1 can be applied to them. These methods involve eliminating the main vector of imbalances by installing a correction mass in one correction plane, and eliminating the main moment of imbalances by distributing masses in two correction planes.
As for GOST 31320, as can be seen from Table 1, it provides for several methods of dynamic balancing:
Modern methods dynamic balancing are based on the proportionality of the amplitude and phase of vibration to the existing imbalance. In other words, by measuring the vibration characteristics of a rotating rotor, it is possible to accurately determine the size and location of installation of correction masses in selected correction planes. Let us illustrate this using the example of a mobile balancing device BALTECH VP-3470 from the Baltech company, which allows dynamic balancing in the own supports of most rotor mechanisms: smoke exhausters, cooling towers, turbines, compressors, electric motors, etc. The balancing procedure with the BALTECH VP-3470 device takes just over half an hour and includes only three stages:
- Determination of initial vibration and vibration phase.
- Installation of a test weight of known mass and obtaining data on the mass of the correction weight and the angle of its installation.
- Installing a correction weight and conducting a control measurement of the vibration level.
The BALTECH VP-3470 balancer allows balancing in 1-4 planes at 16 control points and is equipped with software BALTECH Expert, which stores all trends, protocols and reports.
The BALTECH VP-3470 set is not the only portable balancing device from the Baltech company. Together with it, the company offers the PROTON-Balance-II device and a balancing system based on the CSI 2140 vibration analyzer, as well as the BALTECH-Balance balancing program (calculator).
In addition to the above-mentioned devices for balancing in their own supports, the Baltech company produces a wide range of horizontal, vertical and automatic balancing machines that allow you to balance rotors of a wide variety of configurations and weights.
Baltech balancing machines are distinguished by high structural strength, fully automated processing of measurement results, and high balancing accuracy - up to 0.5 g*mm/kg.
Modern balancing devices and Baltech machines require professional skills in handling them. Taking this into account, the Training Center of the Baltech company regularly conducts the Course TOR-102 “Dynamic Balancing” for the professional training of technicians to work on Baltech machines and devices.
The purpose of balancing is to eliminate imbalance of the part. assembly unit relative to its axis of rotation. Unbalance of a rotating part leads to the emergence of centrifugal forces, which can cause vibration of the unit and the entire machine, premature failure of bearings and other parts. The main reasons for the imbalance of parts and assemblies can be: errors in the shape of parts, such as ovality; heterogeneity and uneven distribution of the material of a part relative to its axis of rotation formed when...
Share your work on social networks
If this work does not suit you, at the bottom of the page there is a list of similar works. You can also use the search button
BALANCING PARTS AND ASSEMBLY
Types of imbalance
Balancing rotating machine parts is an important stage in the technological process of assembling machines and equipment. The purpose of balancing is to eliminate the imbalance of a part (assembly unit) relative to its axis of rotation. Unbalance of a rotating part leads to the emergence of centrifugal forces, which can cause vibration of the unit and the entire machine, premature failure of bearings and other parts. The main reasons for the imbalance of parts and assemblies can be: error in the shape of parts (for example, ovality); heterogeneity and uneven distribution of the material of the part relative to the axis of its rotation, formed when obtaining a workpiece by casting, welding or surfacing; uneven wear and deformation of the part during operation; displacement of a part relative to the axis of rotation due to assembly errors, etc.
Unbalance is characterized by an imbalance - a value equal to the product of the unbalanced mass of a part or assembly unit by the distance of the center of mass to the axis of rotation, as well as the angle of unbalance, which determines the angular location of the center of mass. There are three types of imbalance of rotating parts and assemblies: static, dynamic and mixed, as a combination of the first two.
Static imbalance occurs if the mass of the body can be considered as reduced to one point (center of mass), located at a certain distance from the axis of rotation (Fig. 6.52). This type of imbalance is typical for disk-type parts whose height is less than their diameter (pulleys, gears, flywheels, impellers, pump impellers, etc.).
The centrifugal force Q (N) generated during rotation of such a part is determined by the formula
Q = mω 2 ρ,
where m body weight, kg; ω angular velocity of rotation of the body, rad/s; ρ distance from the axis of rotation to the center of mass, m.
In practice, it is usually accepted that the specified centrifugal force should not exceed 4 x 5% of the weight of the part.
The type of imbalance under consideration can be detected without causing the object to rotate, which is why it is called static.
Rice. 6.52. Types of imbalance of a rotating body: a static; b dynamic; in general case of imbalance
Dynamic imbalance occurs when, during rotation of a part, two equal, oppositely directed centrifugal forces Q are formed, lying in a plane passing through the axis of rotation (Fig. 6.52, b). The moment of a pair of forces M (N) created by them is determined by the equation
М =mω 2 ρa,
where a the distance between the directions of action of forces, m.
Dynamic imbalance manifests itself during the rotation of relatively long bodies, for example, the rotors of electrical machines, shafts with several installed gears, etc. It can occur even in the absence of static imbalance.
The general case of imbalance, also inherent in long objects, is characterized by the fact that a reduced pair of centrifugal forces SS (Fig. 6.52, c) and a reduced centrifugal force T simultaneously act on a rotating object. These forces can be reduced to two forces P acting in different planes and Q, located, for example, for ease of measurement in its supports. The values of these forces are determined by the formulas:
Р =m 1 ρ 1 ω 2;
Q= m 2 ρ 2 ω 2
When a part rotates, in addition to reactions from external forces acting on it, reactions also occur from unbalanced forces P and Q, which increases the load on the bearings and shortens their service life.
To reduce imbalance to acceptable values, balancing of rotating parts and assemblies is used, which includes determining the magnitude and angle of imbalance and adjusting the mass of the balanced product by reducing or adding it in certain places. Depending on the type of imbalance, static or dynamic balancing is distinguished.
Static balancing
Static balancing achieves alignment of the center of mass (center of gravity of an object) with the axis of its rotation. The presence of imbalance (imbalance) and its location are determined using special devices of two types. On devices of the first type, it is determined without reporting the rotation of the part by balancing its imbalance, and on devices of the second type (balancing machines) by measuring the centrifugal force created by the unbalanced mass, so rotation of the part is mandatory.
In mechanical engineering, devices of the first type are usually used as simpler ones: with two horizontally installed parallel prisms (Fig. 6.53, a) or two pairs of disks mounted on rolling bearings (Fig. 6.53, 6), as well as balancing scales (Fig. 6.56 ). In the first two cases (see Fig. 6.53), the balanced part 1 is tightly placed on the mandrel 2 or secured concentrically with it, usually using sliding cones. The mandrel is installed on horizontally located prisms 3 or disks 4.
The method for detecting imbalance depends on the magnitude of the imbalance. If the torque created by the unbalanced mass relative to the axis of the mandrel exceeds the moment of resistance of the frictional forces to the rolling of the mandrel along the prisms (the case with a pronounced imbalance), then the part together with the mandrel will roll along the prisms until the center of gravity of the part takes the lower position. By attaching a load of mass m to the diametrically opposite side of the part, you can balance it. To do this, holes are also drilled into the part, which are filled with a denser material, for example, lead. Usually, balancing is achieved by removing part of the metal from the weighted side of the part (by drilling holes to a certain depth, milling, sawing, etc.).
Rice. 6.53. Schemes of devices for static balancing with prisms (a) and disks (b); 1 balanced object; 2 mandrel; 3 prism; 4 disk
In both cases, to perform balancing of a part, you need to know the mass of metal being removed or added to it. To do this, the part with the mandrel is mounted on prisms so that their center of gravity is located on the plane passing through the axis of the mandrel. At the diametrically opposite point of the part, a load Q is attached such that the unbalanced mass m can rotate the disk through a small (about 10°) angle. Then the mandrel with the part is rotated in the same direction by 180° so that the centers of application of the load Q and mass m are again in the same horizontal plane. If you release the disk in this position, it will turn in the opposite direction at an angle α. Near the load Q, an additional weight q (magnetic or sticky) is attached that would prevent the specified rotation of the mandrel 2 and could ensure its rotation by the same small angle in the opposite direction.
Knowing the masses Q and q, determine the required mass of the balancing weight Q 0 :
Q 0 = Q + q/2.
To ensure balancing, such a mass of metal should be added to the part at the point of application of the load Q or removed from the part at a diametrically opposite point. If it is necessary to change the calculated mass of the balancing load or the point of its application, then use the relation
Q 0 = Q 1 R,
where r radius of position of the calculated balancing load Q 0 ; Q 1 mass of constant balancing load; R distance from the axis of the mandrel to the point of its application.
A case of hidden static imbalance is also possible, when the moment created by the unbalanced mass of the part is insufficient to overcome the moment of rolling friction between the mandrel and the prisms, and the mandrel with the part remains motionless when installed on prisms or disks.
In this case, to determine imbalance, the part is marked around the circle into 8 x 12 equal parts, which are marked with the corresponding points, as shown in Fig. 6.54. If it is difficult or impossible to mark the part to be balanced, use a special disc with divisions, which is fixed motionless at the end of the mandrel.
Then roll the mandrel with the part along the prisms in the direction indicated by the arrow, and alternately align the marked points with a horizontal plane passing through the axis of rotation of the mandrel. For each of these positions of the part, a load q is selected, which is installed at a distance r from the axis of the mandrel. Under the influence of this load, the mandrel with the part should rotate at approximately the same angle (about 10°) in the direction of rolling along the prisms. The position for which the value of this load is minimal, for example 4, determines the plane of location of the center of the unbalanced mass G.
Rice. 6.54. Scheme for determining hidden imbalance at the initial (a) and final (b) stages
Then the weight q is removed and the mandrel is rotated 180° in the direction shown in Fig. 6.54 arrow. At point 8, at the same distance from the axis of rotation of the mandrel, a load Q is attached (Fig. 6.54, b), which ensures rotation in the same direction and at the same angle. Mass Q 0 the material removed at point 4 or added at point 8 to balance the part is determined from the condition of its equilibrium:
Q 0 =Gp/r=(Q-g)/2.
When choosing the type of device, it should be taken into account that its sensitivity is higher, the lower the friction force between the mandrel and supports, therefore devices with balancing disks are more accurate (see Fig. 6.53, b). The advantage of these devices is also less stringent requirements for the accuracy of their installation compared to prisms and more convenient and safe working conditions, since when the mandrel is located between two pairs of disks, the possibility of it falling with the part being balanced is eliminated. To reduce friction in supports with disks, vibrations are applied to them. The contact surfaces of the mandrel and the prisms or disks must be accurately manufactured and maintained in perfect condition. They are not allowed to have nicks, traces of corrosion, or other defects that reduce the sensitivity of the device.
To increase it, balancing devices with aerostatic supports are also used (Fig. 6.55). In this case, the mandrel with the product is in suspension due to the fact that compressed air is supplied to support 1 through channels 2 and 4 under a certain pressure.
High performance and the accuracy of determining the imbalance of some parts is ensured by balancing scales (Fig. 6.56). For a number of types of parts, they are more effective than prismatic and roller devices, since they allow direct determination of the unbalanced mass and its location in the part.
Rice. 6.55. Scheme of a stand for static balancing on an air cushion: 1 stand support; 2, 4 channels for compressed air supply; 3 mandrel
Rice. 6.56. Scheme of balancing scales for small (a) and large (6) parts: 1 balancing weights; 2 rocker arm; 3 balanced part
A mandrel with a balanced part 3 attached to it (Fig. 6.56, a) is installed at the right end of the rocker arm 2 of the scales. Balancing weights 1 are suspended at the left end of the rocker arm. If the center of gravity of the part being tested is shifted relative to its axis of rotation, then at different positions of the part the scale readings will be different. So, if the center of gravity of the part is located at points S1 or S3 (Fig. 6.56, a), the scales will show the actual mass of the part being tested. When the center of gravity is located at point S2, their readings are maximum, and when the center of gravity is located at point S4 they are minimal. To determine the position of the center of gravity of the part, the readings of the scales are recorded by periodically rotating it around its axis at a certain angle, for example, equal to 30°.
It is convenient to determine the imbalance of products such as large-diameter disks on special scales (Fig. 6.56, b). They have two arrows located in mutually perpendicular directions and are brought into a balanced (horizontal) state with the help of weights located diametrically opposite to the arrows.
The part to be balanced is installed using a special device on the scales so that its axis passes through the top of the scale support, made in the form of a conical point and a corresponding recess in the base. If a part has an imbalance, the scales with the part deviate from the horizontal position. By moving the balancing weight along the part, the scales are brought to the initial (horizontal) position, controlling it using the arrows. Based on the mass and position of the balancing weight, the magnitude and location of the imbalance are determined.
Devices of the second type for static balancing are based on the principle of recording the centrifugal force that occurs during rotation of an unbalanced part. They are special balancing machines, a diagram of one of which is shown in Fig. 6.57. The machine allows not only to determine the presence of imbalance, but also to eliminate it by drilling holes.
The part to be balanced 1 is installed concentrically and fixed on a table 9 equipped with an angular scale. Motor 7 imparts rotation to the table with the part at an angular frequency ω, therefore, if the part has an imbalance a, a centrifugal force arises, under the influence of which and the reaction of the springs 8, the system receives oscillatory movements relative to the support 6. The latter are recorded by a measuring transducer (MT) connected to the counter. logical device (SLU).
At the moment of maximum deviation of the system to the right, the SLU turns on the stroboscopic lamp 4, which illuminates the angular scale on the table 9, and transmits a signal proportional to the imbalance to the indicator device 5. Device 5, which can be of a pointer or digital type, shows the value of the required drilling depth.
The operator records the angular location of the imbalance displayed on the screen 3. After stopping, the table is turned manually to the required angle and with drill 2 a hole is drilled in part 1 at a distance r from the axis of rotation to the depth required to ensure balancing of the part. There are also balancing machines on which the disk is rotated to the required point (or several points) to perform drilling and the drilling process is performed automatically.
Rice. 6.57. Machine diagram for static balancing: 1 part to be balanced; 2 drill; 3 screen; 4 strobe lamp; 5 indicator device; 6 articulated support; 7 electric motor; 8 spring; 9 table; IP measuring transducer; SLU calculating and logical device
The accuracy of static balancing is characterized by the value e 0 ω р, where e 0 residual specific imbalance; ω R - maximum operating speed of the part during operation.
Balancing on prisms (see Fig. 6.53, a) ensures e 0 = 20 x 80 µm, on disk supports (see Fig. 6.53, b) e 0 = 15 25 µm, in aerostatic supports (see Fig. 6.55) e 0 = 3 x 8 µm, on the machine according to Fig. 6.57e 0 = 13 µm. International standard MS 1940 provides for 11 balancing accuracy classes.
Static balancing is not sufficient to eliminate imbalance in long objects when the unbalanced mass is distributed along the axis of rotation and cannot be brought to a single center. Such bodies undergo dynamic balancing.
For a dynamically balanced part, the sum of the moments of centrifugal forces of masses rotating relative to the axis of the part is equal to zero. Therefore, dynamic balancing is used to ensure that the axis of rotation of the part coincides with the main axis of inertia of the given system.
If a dynamically unbalanced body is placed on flexible supports, then during its rotation they perform oscillatory movements, the amplitude of which is proportional to the value of the unbalanced centrifugal forces P and Q acting on the supports (Fig. 6.58). Dynamic balancing methods are based on measuring the vibrations of supports.
Dynamic balancing of each end of the part is usually performed separately. First, for example, support I (see Fig. 6.58) is left movable, and the opposite support II is fixed. Therefore, the rotating object in this case makes oscillatory movements within the angle α relative to support II only under the influence of force P.
To increase the accuracy of determining the imbalance of a part, the vibration amplitude of the supports is measured at a frequency of its rotation that coincides with the natural frequency of the balancing system, i.e. under resonance conditions. During dynamic balancing, the mass and position of weights that should be added to or removed from the part are determined. For this purpose, special balancing machines of various models are used, depending on the mass of the parts being balanced. Balancing the free end of a part consists of determining the value and direction of the force P and eliminating its harmful effects by installing a balancing weight in a certain place or removing a certain amount of material. Then support I is secured, and support II is released and the part is similarly balanced from the second end. To simplify the design of the machine, usually one support is made movable, and the ability to balance the part at both ends is ensured by reinstalling it 180°.
Rice. 6.58. Vibration diagram of a part during dynamic balancing
The machine diagram (Fig. 6.59) for dynamic balancing, similar to that discussed above (see Fig. 6.57), is based on this principle.
Rice. 6.59. Machine diagram for dynamic balancing: 1 part to be balanced; 2 angular scale; 3 screen; 4 strobe lamp; 5 indicator device; 6 spring; 7 base; 8 support; 9 electric motor; 10 electromagnetic clutch; IP measuring transducer; SLU calculating and logical device
Devices IP, SLU, 5,4,3 and angular scale 2 have the same purpose as similar elements in the machine according to Fig. 6.57.
The part to be balanced 1 is installed on the supports of the base 7, which can perform under the action of a pair of inertia forces Q 1 Q 2 and the reaction of the spring 6 oscillations relative to the axis 8. The part is driven into rotation by the engine 9 through the electromagnetic coupling 10, with an angular velocity ω slightly greater than the resonant frequency of the natural oscillations of the system.
After balancing the part in the bb plane, it is rotated 180° to carry out balancing in the aa plane. The quality of dynamic balancing is judged by the vibration amplitude, the permissible value of which is indicated in technical documentation. It depends on the rotation speed of the balanced part and at a rotation speed of 1000 min-1 is 0.1 mm, and at 3000 min-1 0.05 mm.
Other similar works that may interest you.vshm> |
|||
| 7702. | BALANCING PARTS (UNITS) | 284.44 KB | |
| Acquiring technical skills to perform statistical balancing of the clutch driven disc and dynamic balancing of the crankshaft with flywheel and clutch assembly. Contents of the work: familiarization with balancing technology, study of equipment and accessories for statistical and dynamic balancing, elimination of static imbalance of the driven clutch disk of ZMZ and ZIL engines. Equipment and equipment of the workplace: balancing machine TsKB 2468 device for static balancing of driven clutch discs... | |||
| 9476. | REPAIR OF TYPICAL MACHINE PARTS AND ASSEMBLY. DESIGN OF TECHNOLOGICAL PROCESSES FOR PARTS RESTORATION | 8.91 MB | |
| The high economic importance of this when repairing cars is due to the fact that their most complex and expensive parts are subject to restoration. Types of technological restoration processes The technological process of restoring a part is a set of actions aimed at changing its condition as a repair workpiece in order to restore its operational properties. Unit technological process designed to restore a specific part, regardless of the type of production. A standard technological process is being developed... | |||
| 9451. | CLEANING MACHINES, COMPONENTS AND PARTS | 14.11 MB | |
| Operational pollution is formed on external and internal surfaces machine components and parts. Precipitates are formed from combustion products and physical-chemical transformation of fuel and oil, mechanical impurities, wear products of parts and water. Experience and research show that thanks to high-quality cleaning of parts during their restoration, the service life of repaired machines increases and labor productivity increases. | |||
| 18894. | Fitting and assembly of individual parts and assemblies of the ballast pump mechanism | 901.45 KB | |
| Main part: Fitting and assembling individual parts and assemblies of the ballast pump mechanism. Applications. Even the correct arrangement of cargo cannot always normalize and stabilize the draft of the vessel, as a result of which it is necessary to fill it with cargo that is useless from the point of view of sale. Water ballast is the most acceptable corrective weight on a watercraft. | |||
| 1951. | Imbalance of rotors and their balancing | 159.7 KB | |
| If the rotation of the rotor is accompanied by the appearance of dynamic reactions of its bearings, which manifests itself in the form of vibration of the frame, then such a rotor is called unbalanced. The source of these dynamic reactions is mainly the asymmetrical distribution of the rotor mass over its volume.1 b when the axes intersect at the rotor center of mass S; Dynamic fig. If the mass of the rotor is distributed uniformly relative to the axis of rotation, then the main central axis of inertia coincides with the axis of rotation and the rotor is balanced or ideal. | |||
| 4640. | MODELING DIGITAL NODES | 568.49 KB | |
| On the chips of modern LSIs it is possible to place many functional blocks of old computers along with interblock connection circuits. The development and testing of such crystals is only possible using mathematical modeling methods using powerful computers. | |||
| 15907. | PURPOSE AND CLASSIFICATION OF STATIONS AND UNITS | 667.65 KB | |
| Railway stations, their classification 2. Railway stations, their classification All railway lines are divided into sections or block sections. These include: sidings, passing points, stations, junctions. Stations ensure train movement on schedule; departure of all trains in strict accordance with the train formation plan; technically and commercially sound; ensure traffic safety when performing operations for receiving departures and passing trains, performing maneuvers, stowing and securing cargo... | |||
| 9483. | Assembling units with plain bearings | 10.89 MB | |
| Assembly of solid bearings. The main factors influencing the operation and durability of the bearing are the dimensional accuracy of the bushing and shaft journal, as well as the alignment of the bearings, which must be ensured during their assembly. Bearing alignment is checked using optical device or a control shaft that passes through all the holes in the housing. The control shaft journals should fit snugly against the bearing surfaces. | |||
| 11069. | Calculation of elements and components of communication equipment | 670.09 KB | |
| The work uses a bipolar transistor circuit with a passive RC circuit as a master oscillator. The generator sets oscillations with a frequency of 12.25 kHz and a certain voltage of 16 V. The nonlinear converter distorts the signal shape and multiple harmonics appear in its spectrum, the intensity of which depends on the degree of signal distortion. | |||
| 11774. | the process of disassembling the components of the hydraulic part of the turboprop engine | 1.24 MB | |
| Before disassembling the turboprop engine, the casing of the entire turbine is removed. Before opening the turbocharger, the turbine insulation must be removed, since during the repair process the metal of the cylinders is stripped under control. Air compressor and turbine rotor high pressure the assembly forms a compressor and turbomachine rotor assembly. | |||
One of the reasons for the reduction in engine life is vibrations resulting from an imbalance of its rotating parts, namely the crankshaft, flywheel, clutch basket, etc. It's no secret what these vibrations threaten. This includes increased wear of parts, extremely uncomfortable operation of the engine, worse dynamics, increased fuel consumption, and so on, and so on. All these passions have already been discussed more than once both in the press and on the Internet - we will not repeat ourselves. Let's talk better about balancing equipment, but first let's briefly look at what this imbalance is, and what types it comes in, and then consider how to deal with it.
To begin with, let's decide why introduce the concept of imbalance at all, because vibrations are caused by inertial forces that arise during rotation and uneven translational movement of parts. Maybe it would be better to operate with the magnitudes of these forces? I converted them to kilograms “for clarity” and it seems to be clear where, what and with what force it presses, how many kilos are on what support... But the fact is that the magnitude of the inertial force depends on the rotation speed, more precisely on the square of the frequency or acceleration during translational motion, and this, in contrast to mass and radius of rotation, is variable. Thus, it is simply inconvenient to use the force of inertia when balancing; you will have to recalculate these same kilograms each time depending on the square of the frequency. Judge for yourself, for rotational motion the inertial force is:
m– unbalanced mass;
r– radius of its rotation;
w– angular velocity of rotation in rad/s;
n– rotation speed in rpm.
It’s not rocket science, of course, but I don’t want to recalculate it again. That is why the concept of imbalance was introduced, as the product of an unbalanced mass and the distance to it from the axis of rotation:
D– imbalance in g mm;
m– unbalanced mass in grams;
r– distance from the axis of rotation to this mass in mm.
This value is measured in units of mass multiplied by a unit of length, namely in g mm (often in g cm). I specifically focus on units of measurement, because on the vastness of the world wide web, and in the press, in numerous articles devoted to balancing, you won’t find anything... Here you can find grams divided by centimeters, and the definition of imbalance in grams (not multiplied by anything, just grams and whatever you want, think about it), and analogies with units of measurement of torque (it seems like kg m, and here g mm..., but the physical meaning is completely different...). In general, let's be careful!
So, first type of imbalance– static or, they also say, static imbalance. Such an imbalance will occur if some load is placed on the shaft exactly opposite its center of mass, and this will be equivalent to a parallel displacement of the main central axis of inertia 1 relative to the axis of rotation of the shaft. It is not difficult to guess that such imbalance is characteristic of disk-shaped rotors2, flywheels, for example, or grinding wheels. This imbalance can be eliminated using special devices - knives or prisms. The heavy side3 will turn the rotor under the influence of gravity. Having noticed this place, you can simply select a load on the opposite side that will bring the system to equilibrium. However, this process is quite lengthy and painstaking, so it is still better to eliminate static imbalance using balancing machines - both faster and more accurately, but more on that below.
Second type of imbalance– momentary. This imbalance can be caused by attaching a pair of identical weights to the edges of the rotor at an angle of 180° to each other. Thus, although the center of mass will remain on the axis of rotation, the main central axis of inertia will deviate by a certain angle. What is remarkable about this type of imbalance? After all, at first glance, in “nature” it can only be found by “happy” chance... The insidiousness of such imbalance lies in the fact that it only appears when the shaft rotates. Place the rotor with a moment imbalance on the knives, and it will be completely at rest, no matter how many times it is shifted. However, as soon as you spin it, a strong vibration immediately appears. Such imbalance can only be eliminated using a balancing machine.
And finally, the most common case is dynamic imbalance. Such an imbalance is characterized by a displacement of the main central axis of inertia both in angle and location relative to the axis of rotation of the rotor. That is, the center of mass shifts relative to the axis of rotation of the shaft, and with it the main central axis of inertia. At the same time, it also deviates by a certain angle so that it does not intersect the axis of rotation4. It is this type of imbalance that occurs most often, and it is the one that we are so accustomed to eliminating in tire shops when changing tires. But if we all go to the tire shop as one in the spring and autumn, then why do we ignore the engine parts?
A simple question: after grinding the crankshaft to repair size or, even worse, after straightening it, can you be sure that the main central axis of inertia exactly coincides with the geometric axis of rotation of the crankshaft? Do you have the time and desire to disassemble and reassemble the engine a second time?
So, the point is to balance shafts, flywheels, etc. necessary, no doubt. The next question is how to balance?
As already mentioned, during static balancing you can get by with prism knives if you have enough time, patience, and the tolerance margin for residual imbalance is large. If you appreciate work time, care about the reputation of your company or are simply worried about the life of your engine parts, then the only balancing option is a specialized machine.
And there is such a machine - a machine for dynamic balancing of the Liberator model manufactured by Hines (USA), please love and favor!
This pre-resonance machine is designed to determine and eliminate imbalances in crankshafts, flywheels, clutch baskets, etc.
The entire process of eliminating imbalance can be divided into three parts: preparing the machine for operation, measuring the imbalance and eliminating the imbalance.
At the first stage, it is necessary to install the shaft on the stationary supports of the machine, attach a sensor to the end of the shaft that will monitor the position and speed of rotation of the shaft, put on a drive belt with which the shaft will unwind during the balancing process and enter the shaft dimensions, position coordinates and radii into the computer correction surfaces, select unbalance measurement units, etc. By the way, next time, you won’t have to enter all this again, since it is possible to save all entered data in the computer’s memory, just as it is possible to erase, change, overwrite, or change it temporarily without saving it at any time. In short, since the machine computer runs under operating system Windows XP, then all the techniques for working with it will be quite familiar to regular user. However, even for a mechanic inexperienced in computer matters, it will not be very difficult to master several on-screen menus of the balancing program, especially since the program itself is very clear and intuitive.
The process of measuring imbalance occurs without operator participation. All he has to do is press the desired button and wait for the shaft to start rotating, and then it will stop. After this, the screen will display everything necessary to eliminate the imbalance, namely: the magnitude and angles of the imbalances for both correction planes, as well as the depths and number of drillings that need to be done to eliminate this imbalance. The hole depths are derived, of course, based on the previously entered drill diameter and shaft material. By the way, this data is displayed for two correction planes if dynamic balancing was selected. With static balancing, naturally, the same thing will be displayed, only for one plane.
Now all that remains is to drill the proposed holes without removing the shaft from the supports. To do this, there is a drilling machine located behind it, which can move on an air cushion along the entire bed. The drilling depth, depending on the configuration, can be controlled either by a digital spindle movement indicator or by a graphic display displayed on a computer monitor. The same machine can be used when drilling or milling, for example, connecting rods when weighing. To do this, you simply need to rotate the support 180° so that it is above the special table. This table can move in two directions (the table is supplied as additional equipment).
Here it only remains to add that when calculating the drilling depth, the computer even takes into account the sharpening cone of the drill.
After eliminating the imbalance, the measurements must be repeated again to ensure that the residual imbalance is within acceptable values.
By the way, about residual imbalance or, as they sometimes say, balancing tolerance. Almost every motor manufacturer must provide residual imbalance values in the repair instructions for parts. However, if this data could not be found, then you can use general recommendations. Both domestic GOST and global ISO standard offers, in general, the same thing.
First you need to decide which class your rotor belongs to, and then use the table below to find out the balancing accuracy class for it. Let's assume we are balancing a crankshaft. It follows from the table that “the crankshaft assembly of an engine with six or more cylinders with special requirements” has accuracy class 5 according to GOST 22061-76. Let's assume that our shaft has absolutely special requirements– let’s complicate the task and classify it as the fourth accuracy class.
Next, taking the maximum rotation speed of our shaft equal to 6000 rpm, we determine from the graph that the value of est. (specific imbalance) is within the limits between two straight lines that determine the tolerance field for the fourth class, and is equal to from 4 to 10 microns.
Now according to the formula:
D st.add.– permissible residual imbalance;
e Art.– tabular value of specific imbalance;
m rotor– rotor mass;
trying not to get confused in units of measurement and taking the shaft mass equal to 10 kg, we find that the permissible residual imbalance of our crankshaft should not exceed 40 - 100 g mm. But this applies to the entire shaft, and the machine shows us an imbalance in two planes. This means that on each support, provided that the center of mass of the shaft is located exactly in the middle between the correction planes, the permissible residual imbalance on each support should not exceed 20 - 50 g mm.
Just for comparison: the permissible imbalance of the crankshaft of the D-240/243/245 engine with a shaft mass of 38 kg, according to the manufacturer’s requirements, should not exceed 30 g cm. Remember, I paid attention to the units of measurement? This imbalance is indicated in g cm, which means it is equal to 300 g mm, which is several times greater than what we calculated. However, nothing surprising - the shaft is heavier than the one we took for example, and it rotates at a lower frequency... Calculate in reverse side and you will see that the balancing accuracy class is the same as in our example.
It should be noted here that strictly speaking, the permissible imbalance is calculated using the formula:
D st.t.– the value of the main vector of technological imbalances of the product that arise as a result of rotor assembly, due to the installation of parts (pulleys, coupling halves, bearings, fans, etc.) that have their own imbalances due to deviations in the shape and location of surfaces and seats, radial gaps, etc.;
D st.e.– the value of the main vector of operational imbalances of the product arising due to uneven wear, relaxation, burning, cavitation of rotor parts, etc. for a given technical life or until repairs involving balancing.
It sounds scary, but as practice has shown in most cases, if you choose the value of the specific imbalance at the lower limit of the accuracy class (in this case, the specific imbalance is 2.5 times less than the specific imbalance defined for the upper limit of the class), then the main vector of the permissible imbalance can be calculated using the formula given higher, according to which we actually calculated. Thus, in our example, it is still better to take the permissible residual imbalance equal to 20 g mm for each correction plane.
Moreover, the proposed machine, unlike ancient domestic analog machines, which miraculously survived after the well-known sad events in our country, will easily provide such accuracy.
Well, okay, but what about the flywheel and clutch basket? Usually, after the crankshaft has been balanced, a flywheel is attached to it, the machine is switched to static balancing mode and only the flywheel imbalance is eliminated, considering the crankshaft to be perfectly balanced. This method has one big advantage: if the flywheel and clutch basket are not disconnected from the shaft after balancing and these parts are never changed, then the unit balanced in this way will have less imbalance than if each part was balanced separately. If you still want to balance the flywheel separately from the shaft, then for this purpose the machine includes special, almost perfectly balanced, shafts for balancing flywheels.
Both methods, of course, have their pros and cons. In the first case, when replacing any of the parts previously involved in balancing the assembly, an imbalance will inevitably appear. But on the other hand, if you balance all the parts separately, then the tolerance for residual imbalance of each part will have to be seriously tightened, which will lead to a lot of time spent on balancing.
Despite the fact that all the operations described above for measuring and eliminating imbalance on this machine are implemented very conveniently, they save a lot of time, and protect against possible errors, related to the notorious “human factor” and so on, in fairness it should be noted that at the very least, many other machines will be able to do the same. Moreover, the example considered was not particularly complicated.
What if you have to balance a shaft from, say, a V8? The task is also, in general, not the most difficult, but still it’s not balancing an inline four. You can’t just put such a shaft on a machine; you need to hang special balancing weights on the connecting rod journals. And their mass depends, firstly, on the mass of the piston group, that is, the mass of parts moving exclusively progressively, and secondly, on the weight distribution of the connecting rods, then depends on how much of the connecting rod mass relates to rotating parts, and how much to translationally moving parts, and finally, thirdly, on the mass of only rotating parts. You can, of course, sequentially weigh all the parts, write down the data on a piece of paper, calculate the difference between the masses, then confuse which entry refers to which piston or connecting rod, and do all this several more times.
Or you can use the “Compu-Match” automated weighing system offered as an option. The essence of the system is simple: electronic balance are connected to the machine computer, and when sequentially weighing parts, the data table is filled in automatically (by the way, it can also be printed). The lightest part in the group, for example the lightest piston, is also automatically found, and for each part the mass that needs to be removed to equalize the weights is automatically determined. There will be no confusion with determining the mass of the upper and lower connecting rod heads (by the way, everything necessary for weight distribution is supplied with the scales). The computer directs the actions of the operator, who simply needs to carefully follow the instructions step by step. After which the computer will calculate the mass of the balancing weights based on the mass of the specific piston and the weight distribution of the connecting rods. It only remains to add that when calculating the masses of these cargoes, even the mass motor oil, which will be in the shaft lines during engine operation. By the way, different sets of weights can be ordered separately. The weights, of course, are stacked, that is, washers of different weights are hung on the stud and secured with nuts.
And a few more words about weighing the piston and weight distribution of the connecting rods. At the very beginning of this article, we noted that “one of the reasons for engine vibration is the imbalance of its rotating parts...”, “one of...”, but far from the only one! Of course, we will not be able to “overcome” many of them. For example, uneven torque. But something can still be done. Let's take a conventional inline-four engine as an example. From the course on internal combustion engine dynamics, everyone knows that the first-order inertial forces of such a motor are completely balanced. Amazing! But in the calculations it is assumed that the masses of all parts in the cylinders are absolutely identical and the connecting rods are weighted impeccably. But in fact, during the cap. repair, does anyone weigh the pistons, rings, pins, equalize the masses of the lower and upper connecting rod heads? Hardly…
Of course, the difference in the masses of the parts is unlikely to cause large vibrations, but if it is possible to get at least a little closer to the design diagram, why not do it? Especially if it's so simple...
As an option, you can order a set of devices and equipment for balancing cardan shafts... But wait, that’s a completely different story...
* The OX axis is called the main central axis of inertia of a body if it passes through the center of mass of the body and the centrifugal moments of inertia J xy and J xz are simultaneously equal to zero. Unclear? There's really nothing complicated here. Simply put, the main central axis of inertia is the axis around which the entire mass of a body is distributed evenly. What does evenly mean? This means that if you mentally isolate some mass of the shaft and multiply it by the distance to the axis of rotation, then exactly opposite there will be, perhaps, another mass at a different distance, but having exactly the same product, that is, the mass we have identified will be balanced.
Well, what is the center of mass, I think it’s clear.
** In balancing, rotors are everything that rotates, regardless of shape and size.
*** The heavy side or heavy point of the rotor is usually called the place where the unbalanced mass is located.
**** If the main central axis of inertia nevertheless intersects the axis of rotation of the rotor, then such imbalance is called quasi-static. There is no point in considering it in the context of the article.
***** Among other classifications of balancing machines, there is a division into pre-resonance and post-resonance. That is, the frequencies at which the shaft is balanced can be either lower than the resonant frequency or higher than the resonant frequency of the rotor. The vibrations that occur during the rotation of an unbalanced part have one interesting feature: the amplitude of the vibrations increases very slowly as the rotation speed increases. And only near the resonant frequency of the rotor is a sharp increase observed (which, in fact, is what makes resonance dangerous). At frequencies above the resonant one, the amplitude decreases again and remains virtually unchanged over a very wide range. Therefore, for example, on pre-resonant machines there is little point in trying to increase the shaft rotation speed during balancing, since the amplitude of vibrations recorded by the sensors will increase very little, despite the increase in the centrifugal force that generates vibration.
****** Some machines have swinging supports.
******* The correction surface is the place on the shaft where holes are supposed to be drilled to correct the imbalance.
******** Please note that the specific imbalance is indicated in microns. It's not a mistake here we're talking about about specific imbalance, that is, per unit mass. In addition, the index “st.” indicates that this is a static imbalance, and it can be indicated in units of length, as the distance by which the main central axis of inertia of the shaft is displaced relative to the axis of its rotation, see above for the definition of static imbalance.
The rotor as a whole may have an uneven metal weight distribution relative to the axis of rotation and its center of gravity will not be located on this axis, i.e. the weight of the rotor will be unbalanced relative to the axis of rotation. Such imbalance of the rotor or its parts is called imbalance.
When the rotor rotates, the imbalance causes the appearance of a radially directed disturbing force. This force tends to tear the shaft together with the part attached to it from the bearings. The disturbing force changes its direction all the time, remaining radial, so its effect on the bearings varies in direction; such an action inevitably leads to vibration of the mechanism.
When vibration occurs, the parts of the mechanism experience impacts, shocks and overload, which causes accelerated general wear, disruption of centering and fastenings, and this in turn further increases the vibration.
To eliminate the disturbing force, the rotor is balanced, i.e. eliminate its imbalance. Operations to eliminate imbalance are called balancing. You can balance each part of the rotor individually or the entire rotor as a whole; the latter method is more economical and more accurate.
To balance the unbalance of the rotor, it is necessary to fuse (hang) a load of the mass necessary for balancing at the same distance from the axis (where the imbalance is detected), but in the diametrically opposite direction; after which the rotor will be balanced and no disturbing force will arise during its rotation.
The magnitude and location of the unbalance is found when performing various types balancing.
Distinguish static And dynamic rotor balancing:
1. Static it is called balancing because rotor rotation is not required to identify and eliminate imbalance; Equilibration is achieved when the rotor is at rest.
2. Dynamic imbalance is observed when the unbalanced masses of the rotor produce two disturbing forces, equal in magnitude, but oppositely directed and located at different ends. In this case, it may turn out that the general center of gravity of the rotor is located on the axis of rotation, i.e. The rotor is statically balanced. Such imbalance can be detected only when the rotor rotates, since the general center of gravity of the rotor is located on its axis, and only during rotation do both unbalanced masses form a pair of disturbing forces of alternating directions. Consequently, a statically balanced rotor may in some cases be dynamically unbalanced. The operation to identify and eliminate dynamic imbalance is called dynamic balancing.
Installation of smoke exhausters
Smoke exhausters (D) are designed to suck flue gases from the boiler furnace and eject them under pressure through chimney in atmosphere.
Smoke exhausters are of centrifugal (1) and axial (2) types.
1. For boilers with a steam capacity of 420-640 t/h, centrifugal double-suction smoke exhausters of the type D-25x2Sh and D 21.5x2 are used.
These smoke exhausters consist of the following main components:
Bearings
Guide vanes and their drives
Installation of the smoke exhauster begins with accepting the foundation and installing an electric motor on it.
The significant dimensions D of double-sided suction predetermine their delivery for installation in disassembled form. Therefore, the initial installation operation is the assembly of supporting structures D (frames) and volute bodies with suction pockets on the assembly site.
Installation D begins with the installation of a support frame, which is attached to the foundation with bolts. The frame is installed on metal pads, the total thickness of which can be up to 25-30 mm, with the number of pads in one package no more than three.
The pads are located on both sides of each foundation bolt and regulate the height marks, the deviation of which from the design ones is allowed no more than + - 6 mm.
Bearings D are installed on the support frame, the alignment of which is carried out along the string and plumb lines.
After installing the bearing housings, housing D is installed on the foundation, then its rotor is laid.
After installing housing D, control gates are installed on its suction side. The gate valves first undergo an inspection, during which the smoothness of their opening and closing is checked.
The assembled D is tested at idle; in this case, the radial and axial runout of the impeller is allowed to be no more than 3 and 6 mm, respectively.
2. In boiler plants with a steam capacity of 950 t/h or more, axial D type DO - 31.5 is used. The main advantages of these D (compared to centrifugal D) is their compactness. Two-stage axial D consists of:
Suction pocket
Housings
Guide vanes
Impellers
Diffuser
Chassis
Oil pump station with oil pipeline system
Ventilation for cooling
The suction pocket is made of two halves (upper and lower), connected by flanges. The total mass of the suction pocket is approximately 7.5 tons. The lower part of the suction pocket is mounted on two foundation supports.
Housing D is made of three parts designed to accommodate:
i. guide vane and stage 1 impeller;
ii. guide vane and second stage impeller;
iii. straightening apparatus.
All parts are connected to each other on flanges with bolts.
The chassis consists of a shaft, two bearings and a coupling connecting shaft D to the electric motor.
Bearings D - roller type, spherical, self-aligning, running on liquid lubricant, which is supplied by an oil station through an oil lubrication system (One oil station is installed on two D's. Thermal protection of the support bearing installed in the diffuser body is carried out using a special fan and a heat and sound insulating coating.
Installation D begins with the installation of supporting structures and acceptance of the foundation. The concrete surface is first cleaned of uneven surfaces and notched at the locations of the foundation bolts and shims for supporting structures D. The shims are made of sheet steel 100-200 mm wide and a length corresponding to the width of the lower plane of the supporting structure. The number of pads should not exceed three in one place.
Technological installation sequence ____ axial smoke exhauster DO - 31.5
| Sequence | Knot | Main works |
| I | Lower body | Installation on supporting structures. Installation of longitudinal stop keys. Alignment of thermal gaps in support attachment points. |
| Thrust bearing | Installation and fastening of the thrust bearing and rotor to the foundation support structures, maintaining axial clearances. | |
| Electric motor | Installation of half-coupling shafts. Installation of frame and electric motor. | |
| Nodes 1,2,3 | Alignment of the main axes and elevation marks of the lower part of the body, chassis and electric motor. | |
| Chassis | Aligning the lower part of the housing to the rotor while maintaining radial clearances. | |
| Supports for the exhaust fan body | Pouring concrete for the foundation bolts of the housing supports. | |
| Platforms and stairs | Installation of the guide vane drive on the foundation. Installation of platforms and ladders around the electric motor and the exhaust fan housing. | |
| Removing the smoke exhauster rotor. Installation under | ||
| foundation rates. Lubricate the supporting surfaces of the stands with a mixture of grease and graphite. Installing the lower part of the suction pocket. | ||
| Lower part of the fairing (shade) | Installing the lower part of the fairing and the lower cover of the support bearing guard. Rotor installation. | |
| Upper body | Installation of the upper part of the smoke exhauster housing on asbestos gaskets in a horizontal joint. Installing the upper part of the fairing. | |
| Bottom of suction pocket | Final installation and fastening to the body of the lower part of the suction pocket. | |
| Protective devices | Installation of the support bearing protective casing and stuffing box seal. | |
| Guide vanes | Installation of rotary rings, levers, rods and drive of guide vanes. | |
| Diffuser | Installation of the diffuser pipe on a temporary support. Sequential installation of three diffuser sections. Installation of spacer ribs between the pipe and the diffuser cone. | |
| Cooling Fan | Installation of cooling fan and air duct. | |
| Upper part of the suction pocket | Installation of the upper part of the suction pocket, installation of the shaft guard | |
| Smoke exhauster and electric motor shafts | Alignment and connection of the smoke exhauster and electric motor shafts. |
To balance any rotating part, it is necessary that its center of gravity lies on the axis of rotation, and the centrifugal moments of inertia are equal to zero. The discrepancy between the center of gravity of a part and the axis of rotation is usually called static imbalance, and inequality to zero centrifugal moments of inertia - dynamic imbalance.
4.1 Static balancing of parts
Static imbalance is easily detected when the part is mounted with support journals on parallels or rollers. Typically, static balancing is carried out on parts whose diametrical dimensions are much greater than the length along the axis of rotation (flywheels, disks, pulleys, impellers, etc.), since in this case the dynamic component can be neglected.
During static balancing, the location and magnitude of the imbalance are determined by installing test weights. Unbalance is corrected by removing an equivalent amount of material from the part or installing corrective weights. Excess material for massive parts (flywheels) is removed by drilling or milling, and for thin-walled parts (pulleys, disks, rotors) - by eccentric turning or grinding.
After eliminating the imbalance, repeat (control) balancing is performed. If the residual imbalance exceeds the permissible technical requirements values, balancing is repeated
4.2 Dynamic balancing of parts
Dynamic balancing is carried out when working at high speeds rotating parts or assemblies whose length along the axis of rotation exceeds the diametrical dimensions (for example, beater drums of combine harvesters or co engine ribbon shafts).
Even in a statically balanced part, there may be an uneven distribution of mass along the length relative to the axis, which, at a significant rotation speed, creates a moment of centrifugal forces on the arm L (see Figure 1) and, consequently, additional loads on the supports and vibration.
Unbalance is detected on special balancing machines when the part rotates at operating speeds and is eliminated, as with static balancing, only in two or more correction planes, selected depending on the design of the part.
Dynamic balancing eliminates the need for static balancing.
To perform dynamic balancing, installations are required that ensure rotation of the part, control of the centrifugal forces of unbalanced masses or moments of these forces acting on the supports, as well as identification of the plane of location of the unbalanced masses.
Figure 1 Reduction of forces acting on the rotor to two planes of force correction
This circumstance is precisely used in dynamic balancing of parts. For balancing, two planes are selected on the part, perpendicular to the axis of rotation and convenient for installing balancing weights or removing part of the material of the part - the so-called correction planes. The machine is set up so that it is possible to determine the location and size of the weights that should be added (or removed) in each of the planes to completely balance the part.
Dynamic imbalance is detected on balancing machines. In the repair industry, electric balancing machines with elastic supports(see Figure 2).
Unbalanced masses of the part cause mechanical vibrations of the moving supports (1). With the help of sensors (2), these mechanical vibrations are converted into electrical ones. Moreover, the voltage of the electric current in the sensor is directly proportional to the magnitude of the mechanical vibration of the support, i.e. imbalance. In the measuring device (3) the current is amplified and read on the milliammeter (4) in the form of imbalance readings.
Figure 2 Diagram of a machine for dynamic balancing of crankshafts:
1 - movable supports (cradles); 2 - vibration sensor; 3 amplification and measurement unit; 4 - milliammeter; 5 - strobe lamp; 6 - electric motor; 7 - strobe dial; 8 - dial for counting the angle of rotation of the shaft.
The angular location of the unbalanced masses is determined by a stroboscopic device. The stroboscopic lamp is controlled by the voltage of the oscillation sensor, and each time the vector of unbalanced masses passes the horizontal plane on the front side of the machine, the lamp (5) flashes and displays a certain number on the strobe light (8). Due to the stroboscopic effect, the numbers on the dial appear motionless.
