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Mechanical seals for pumps are sealing devices belonging to the contact type of seals with a pair of friction surfaces of two parts. One part is mounted on the shaft and is movable, the other fixed part is located in the pump housing. The rubbing pair of the device operates under pressure drops with minimum consumption lubricant. The lubricant in these devices is often the sealed medium.

According to statistics, mechanical seals for any pump are considered the most vulnerable component among all structural elements of the pump.

Mechanical seal design for pumps

The standard design of the device consists of 9 basic elements:

• installation bolt for securing the seal to the impeller shaft;
• elastomer seal;
• a pin that transmits shaft rotation to the movable ring;
• ring is movable;
• the ring is motionless;
• rear wall of the pump housing;
• a pin that prevents rotation of the fixed ring;
• impeller water pump shaft;
• springs or bellows that ensure a tight fit between the movable and fixed rings.

Mechanical seals for pumps (video)

Operating principle of pump mechanical seal

In general cases, the pump shaft mechanical seal has two rings:

• a fixed ring located in the housing;
• a movable ring located on the shaft of the unit.

One of the rings can move axially due to the presence of an elastic pressing element (spring, bellows, membrane). This element, together with the pressure sleeve and the movable ring, form an axially movable block or pressure unit. They ensure contact between the end surfaces in the mating of the movable and stationary rings of a pair without the pressing force of environmental pressure.

Mandatory parts of the mechanical seal device are secondary (auxiliary) seals between the rotor and the rotating block, between the housing and the stator block. The design includes elements for fixing the sealing rings (drive pins, set screws), which drive the moving ring and prevent rotation (angular displacement) of the fixed ring relative to the body.

Types of end seals for pumps

Division of sealing end devices into different types occurs according to the following criteria.

1. By design there are:

• single;
• double mechanical seal of pump shafts;
• combined.

1. By location in the equipment:

• with internal location;
• with outdoor location.

1. By design:

• conventional, according to European standard EN 12756 (DIN 24960);
• special, can comply with European standards;
• cartridge (cartridge) can comply with European standards.

1. According to the hydraulic load coefficient:

• hydraulically loaded;
• hydraulically unloaded.

1. According to materials used:

• with standard (regular) materials;
• with special materials (for work in special conditions).

Groups of mechanical seals by load

The degree of load on mechanical seals varies and depends on operating conditions: pressure and shaft speed. In order to assess the loading conditions of the device at the sealing joint during operation, there are separate recommendations.

For general characteristics the degree of severity of the operating conditions of the seals, use the product of two indicators: the sliding speed V in the friction pair and the pressure drop P in the device.

Values indicators P,V and P xV for various seals are divided into 4 groups according to the degree of their loading:

• lowest, where P up to 0.1 MPa, V up to 10 m/s, P xV up to 1.0 MPa x m/s;
• average, where P up to 1.0 MPa, V up to 10 m/s, P xV up to 5.0 MPa x m/s;
• high, where P up to 5.0 MPa, V up to 20 m/s, P xV up to 50.0 MPa x m/s;
• the highest, where P is more than 5.0 MPa, V is more than 20 m/s, P xV is more than 50.0 MPa x m/s.

Methods for correcting pump shaft distortions

During pump operation, shaft curvature may occur under the influence of increased loads. The bent shaft of the unit can be restored using various straightening methods. The following shaft editing methods are used:

• hardening;
• thermomechanical;
• thermal;
• stress relaxation.

All of the above methods for straightening a shaft, with the exception of cold hardening, involve heating it. Indicators such as deflection value, length, diameter and shaft material are decisive in choosing the method of editing it.

Types of seals in centrifugal pumps

The main condition for the stable operation of a centrifugal unit is the design of its seal. The units differ in size, characteristics, purpose, and pumped media.

Based on these parameters, the optimal type of pump shaft seal is selected. The types of shaft seals are as follows:

• single and double stuffing box;
• end single and double;
• cuff;
• slotted (labyrinthine).

Single pump mechanical seal

It is used in pumps pumping solutions that leak and get into external environment unacceptable in large quantities. Such liquids include: hot, low-boiling, aggressive, inorganic and organic.

This type of seal requires increased accuracy of installation of the installation unit and high quality of the shaft surface. When processing rubbing surfaces, the tolerance for axial runout is minimal. Subsequent fine grinding is also carried out. Liquid leakage with such a single device is negligible.

Pumps with double mechanical seal

Such a device differs from a single device in the number of sealing lapped surfaces. The device is supplemented with a barrier fluid supply system, which prevents the working fluid from entering the external environment. The barrier fluid is water, glycerin and other liquids that do not interact with the pumped medium.

There are two options for placing double seals:

• back to back;
• tandem.

The first option is used more often. In this case, the pressure of the barrier fluid exceeds the pressure of the pumped liquid by 1-2 bar. This is achieved through the use of a dosing pump, a special vessel or a hydraulic booster. The advantage of this option is that the gap between the movable and stationary rings is filled with a barrier fluid, which prevents the penetration of solid particles and dirt from the pumped medium. This significantly increases the service life of the device in comparison with the Tandem option.

In the Tandem version, the barrier liquid has lower pressure than the pumped liquid. When the device is depressurized, it is the pumped liquid that enters the barrier. This is important where penetration of foreign liquid into the pressure line is unacceptable. In this embodiment, there is no need to exercise serious control over the pressure of the barrier fluid, which is essential in certain situations.

Material for mechanical seals for pumps

When choosing an end device, the determining factor is the selection of material for the friction pair and secondary seals. These elements are made from various materials.

The following materials are used for the friction pair:

• metal (stainless steel) – SUS;
• graphite – CAR;
• ceramics – CER;
• silicon carbide – SIC;
• tungsten carbide – TC.

Secondary seals are made from materials with different temperature resistance:

• nitrile butadiene rubber (NBR), from -20 to +120 degrees;
• ethylene-propylene rubber (EPDM), from -30 to +170 degrees;
• fluorine rubber (Viton), from -30 to +185 degrees;
• fluoroplastic (PTFE), from -260 to +260 degrees.

Mechanical seals for pumps from different manufacturers

Pumps produced by different manufacturers have their own markings. All sealing devices are made from modern materials with a long service life.

Here is a list of popular pumps and mechanical seals for them:

• to APV pumps – ends: SNAPV(W+), SNAPV1(W), SNAPV2(W), SNAPV(DW), SNAPV 3, SNAPV 4, SNAPV 5, SNAPV 6, SNAPV 7, SNAPV 8, SNAPV 9, SNAPV 10, SNAPV 11, SNAPV 12, SNAPV 13;
• for Allweiler pumps – ends: SNAR, SNM 3, SNAL 1;
• for Lowara pumps – ends: SNAR, SNM 3, SNLW;
• for Inoxpa pumps – ends: SNIXP 1, SNIXP 2, SNIXP 3, SNIXP 4, SNM 3, SNFN, SNMG, SN 2100, SNAR;
• to EMU pumps – ends: SNMG, SNEMU 1, SNEMU 2;
• for Hilge pumps – ends: SNFN, SNAR, SNM 3, SNHG, SNBT;
• for Johnson pumps – ends: SNJH 1, SNJH 2, SNJH 3, SNJH 4;
• for Calpeda pumps - SNMG, SNFN, SNAR, SNM 3, SN 2100.

Some pump manufacturers use mechanical sealing devices own production, others use devices produced by companies specializing in their production.

TO category:

Practice metalwork and assembly works

Dressing shafts and polishing their surfaces

It is convenient to edit curved shafts on prisms located on the table using two levers mounted on the axes of clamps secured with nuts in the plate on the table. Taking into account the ratio of arms A and B of the levers, it is possible to create a bending force of up to 1000-1500 N. Using a screw press, a force of up to 3000 N can be created. The shaft is placed on two prisms installed on the table. The press stands have T-shaped grips at the bottom with transverse bars and two pairs of rollers, which allow the press to move freely across the table and create an extension force with a screw anywhere on the shaft.

To straighten large shafts and cylindrical parts, it is often necessary to create significant forces (up to 15 kN or more). In such cases, it is advisable to straighten the shafts using a tabletop pneumatic press head mounted between two racks on the rotor. Before starting editing, it is necessary to determine the nature of the curvature of the shaft and place it on two prisms. With your right hand, turn the handle down. At this time, compressed air enters the cylinder chamber of the press head through a hose and creates pressure on the rod and clamp. While straightening, the shaft is rotated with the left hand and periodically moved along the prisms. Prism supports must be installed at different distances from the axis of the press rod.

When straightening the shaft, there is a danger of bending it in the opposite direction. To avoid this, pads of such a height should be installed under the shaft opposite the rod and the clamp that would limit the large bending of the shaft.

Rice. 1. Editing the shaft manually (a) and using a press (b, c)

In Fig. Figure 2 shows the control of shaft straightness using an indicator device and a prism.

Before assembling, the parts must be cleaned and washed, since metal filings, tiny pieces of shavings, remnants of cleaning materials, abrasive powder that get into the holes and channels of the part can subsequently get into the bearings along with the lubricant during operation of the machine and lead to heating and premature failure. bearing wear, and often to machine failure.

Polishing is a finishing process that is carried out after finishing surfaces by filing, cleaning with an abrasive wheel, etc. After polishing, the surface of the part becomes clean and shiny. Polishing is carried out sequentially: first polish the right half of the surface of the part, then rearrange it and polish the left half from the ends to the middle. The polishing direction must be alternated: to the right, to the left (at an angle of 30 or 60°), then longitudinal. Parts that have the shape of bodies of revolution and a complex profile are polished mainly obliquely to the right and left.

Rice. 2. Shaft straightness control

In mass and serial production, the surfaces of parts are polished on special polishing machines with a spindle speed of 2500-5000 rpm. In Fig. Figure 3 shows a method for polishing the outer surface of a cylindrical part with a felt wheel installed in the spindle of a polishing machine. During the work process, felt or fabric circles are put on the spindle (left-hand spiral thread), as well as mandrels with circles of small diameters for polishing internal surfaces parts and cover them with casings. Bring the part to the polishing wheel and, lightly pressing the part and rotating it in different directions, polish it. In order to avoid marks and scratches on the treated surface of the part, it is necessary to place the finished parts in the nests of wooden containers or on rags.

Hard-to-reach places are polished with special shaped felt and leather wheels fixed in metal mandrels, as well as with sanding tape. When polishing with an emery belt, first use coarse belts (240-280 grit), and then finer ones (320-400 grit). Kerosene is usually used as a lubricant and coolant.

The easiest way to polish the surfaces of shafts and other cylindrical parts on lathes. The shaft is inserted into the headstock spindle lathe and to the center of the tailstock. When polishing, apply sanding tape to the shaft and clamp it between two wooden clamps. Then, with your right hand, grab the presses, hinged to each other with a loop, and squeeze the sanding tape. Turn on the machine. Leaning the palm of your left hand (to support the body) on the headstock, lightly squeeze the presses with sandpaper with your right hand, move them along the surface of the shaft and polish it, periodically changing the direction of movement of the presses and replacing the sanding tape. Finally, the surfaces of the parts are polished with strips of leather, cloth, suede or rubber, lubricated with paraffin mastic and fine polishing powder. The direction of rotation of the shaft during polishing must coincide with the direction of its rotation under operating conditions.

Rice. 3. Polishing cylindrical surfaces of parts with felt wheels on a polishing machine

Rice. 4. polishing cylindrical surfaces of parts on a lathe

In Fig. Figure 5 shows a method for manually cleaning the surface of the teeth of a worm wheel with a wooden plate and sanding tape. Take a wooden (beech or birch) plate in your right hand and wrap it with sandpaper or coke tape, then with the thumb and forefinger of your right hand, press them to the surface of the wheel tooth cavities and clean or polish the surface in the longitudinal direction, making sure that blockages do not form in the tooth profile. At the same time, with your left hand, holding the gear wheel, slightly turn it, creating the correct direction for the plate with the tape in the process of processing the tooth profile.

Having cleaned all the working surfaces of the part and made sure that there are no greasy stains on the parts, they begin to polish them. In this case, for steel parts, abrasive powders with a grain size from M40 to M14, GOI paste or chromium oxide are used. For polishing parts made of non-ferrous metals, crocus (iron oxide Fe203), chromium oxide, tripoli (silicon dioxide-silica) are used, for polishing parts made of wood and leather - pumice (a product of volcanic origin).

To mechanize the cleaning and polishing work, special machines are used that perform processing with endless sanding belts on a fabric or paper basis. The quality of the surface after processing depends on the grain size of the abrasive powders, wear of the belt and the lubricating and polishing materials used.

For preliminary polishing, circles made of felt or canvas are used, on the surface of which a layer of abrasive powder of various grain sizes is applied. Abrasive powders are held on the surface of the wheels using glue (hidewood, casein, etc.). As they work, a new layer of abrasive is applied to the wheels, and thus their constant performance is maintained.

For final polishing, circles made of felt, felt, or fabric are used, onto the surface of which fine-grained abrasives, various polishing materials or pastes are applied. When polishing the decorative surfaces of parts for chrome plating, thin powders or special pastes are used.

Repair of shafts and axles

Shafts and axles are made of carbon and alloy steels. Most shafts and axles undergo improvement, i.e. hardening with high tempering, surface hardening of working surfaces.

Shafts and axles have smooth cylindrical or conical surfaces (necks), splines, keyways, collars, flats and threaded surfaces.

During the operation of machines and mechanisms, various defects may appear on these surfaces: bending and twisting, wear and crushing of support and landing journals and collars; wear of keyways and splines; wear and damage to threads and center holes; cracks and breaks in various places.

When repairing shafts and axles, welding and metalworking work is first performed, since during their implementation, deformation of the part is possible and cleanly machined surfaces may be damaged. After welding and surfacing work, shafts and axles are subjected to straightening and pre-machining. Finishing of the working surfaces of the shaft should be done last.

Repair of bent shafts and axles. Minor deflections of the shafts (less than 0.5 mm) are eliminated by grooving or grinding. Shafts with a diameter of up to 50 mm, the deflection of which does not exceed 0.01 of the shaft length, are straightened in a cold state using a press or screw clamps. In a shaft straightened without heating, the deflection is partially restored over time.

To ensure the unchangeable shape of the shaft and relieve internal stresses, after straightening, a heat treatment is performed, which consists of holding the shaft at a temperature of 400-500 °C for 0.5-1 hour.

Significant deflections of the shafts are eliminated by hot straightening under a press, for which the place where the shaft is bent is heated to 600 °C in a forge or with the flame of a gas burner. After straightening, it is necessary to recheck the shaft for runout and, if the bend is not completely eliminated, repeat the straightening operation.

Repair of seats for bearings and other parts is carried out different ways. Minor damage to the rubbing surfaces in the form of wear is eliminated by finishing with special pastes or grinding.

In case of large wear, as well as in the presence of taper and ovality, restoration of the seats is carried out by processing to the repair size, and if this is not possible, by surfacing, metallization or galvanic method.

Most in a simple way restoration is the processing of seats to a repair size. However, repair dimensions are established for a limited number of machine parts. Therefore, processing is often carried out to the largest possible size, and the sliding bearings associated with the shaft are manufactured anew.

If wear exceeds 2 mm, the shafts and axles of road vehicles are restored by surfacing.

Extension of shaft journals by chrome plating, plating and metallization during the repair of road vehicles is used at specialized repair enterprises, since this is associated with the use of special equipment. After building up using one of the indicated methods, the seats are ground, ground, and polished to obtain especially clean and smooth surfaces.

Repair of keyways and splines. Malfunctions of keyways and splines can manifest themselves in the form of wear and crumpling of their surfaces, and chipping of metal on the working surfaces.

When repairing worn key connections, the damaged key is replaced with a new one of normal or increased size. In this regard, it is recommended to repair the keyways on the shaft: by expanding the worn keyway (by 10-15%) under an oversized key; by milling a keyway for a key of normal size in another place, offset by 90 or 120° to the damaged keyway; surfacing the walls of worn grooves and then milling them to a normal size.

Regardless of the repair method, the final size of the splines is obtained by mechanical processing to a nominal or increased size, which makes it possible to compensate for the wear of the splines in the mating hole.

When repairing splines by spreading, they are annealed, after which they are distributed using a roller made of U6 or U7 steel. As a result, the width of the splines increases by 0.5-1 mm. Depending on the amount of wear, splines are distributed along the edge of the worn surface or along both edges of the protrusion. The grooves formed on the splines are melted by electric welding and cleaned, and the splines themselves are adjusted to the mating part by mechanical processing.
It is advisable to perform surfacing with electrodes TsN-250 or TsN-300, which provide high wear resistance of the splines without subsequent heat treatment. After surfacing, the shaft is machined to a given size, the beads are trimmed from the end and chamfered. Splines are processed on gear hobbing machines with hob cutters or on horizontal milling machines with disk or shaped cutters.

In small repair shops, splines are processed on lathes using a special gear-cutting spline-cutting device.
Repair of threads on the surface of shafts. Slightly damaged threads are corrected on a lathe or by metalworking. Threads that have lost their profile due to wear or failure are restored by surfacing. In this case, the old thread is removed by turning on a lathe, after which the resulting surface is surfaced, ground, and the thread is cut again to the required size.

Metal straightening- an operation by which unevenness, curvature or other imperfections in the shape of workpieces are eliminated. Metal straightening is the straightening of metal by applying pressure to any part of it, regardless of whether this pressure is applied by a press or by hammer blows (straightening). Editing is used when the shape of parts is distorted, for example during bending and twisting of shafts, axles, connecting rods, frames; for dents and distortions of thin-walled parts. Depending on the degree of deformation and size, the parts are straightened with or without heating. Steel sheets, sheets of non-ferrous metals and their alloys, steel strips, rod material, pipes, wire, steel square, steel circle, as well as metal welded structures are used. The metal is edited both in cold and heated states. Straightening plays a big role in restoring unusable equipment parts. Correctly applied editing can completely restore a part, returning it to its original qualities. Editing can be done cold, heated, or by thermal action. The processing of metals by pressure at a temperature below the recrystallization temperature is called cold working, and at a higher temperature - hot working.

Cold straightening is based on mechanical action that causes plastic deformation of the metal. Straightening of sheet metal parts is carried out using the cold method, either manually or using machines. When manually straightening, a metal sheet is pierced on a flat plate or anvils using a hand tool or a pneumatic hammer with a special chisel. Machine straightening of sheet parts is carried out by rolling and stretching. Rolling straightening is performed on roller sheet straightening machines (Fig. 1). Stretch straightening is performed on stretch leveling machines consisting of a roller table and a double-acting hydraulic cylinder with movable clamps in which the sheet part is clamped. With increasing pressure in the hydraulic cylinder, the clamps move apart and create tensile stresses in the shortened fibers of the fixed sheet, reaching the yield strength of the material. As a result of plastic stretching of the shortened fibers of the material, the sheet part is straightened. In some cases, straightening of sheet parts is performed by transverse bending on a hydraulic press by sequentially pressing the punch. Welded panels that have received deformations due to shrinkage of welds are straightened similarly to parts made from sheet metal.

Rice. 1.

Straightening of rolled profile parts is carried out using the cold method - rolling on roller machines, stretching on stretching machines, as well as transverse bending on horizontal bending and hydraulic presses. Straightening of welded T-beams and frames with unacceptable welding deformations is carried out using a cold method similar to straightening of rolled profile parts, as well as using a thermal method.

Cold straightening of a number of parts is a labor-intensive operation, during which it is necessary to monitor the effectiveness of its application. Therefore, in addition to conventional equipment and control tools (hydraulic presses, indicators), special stands and devices are increasingly used that allow straightening and comprehensive inspection of a part during its use.

Cold straightening does not affect the structure of the metal, since it actually helps to reduce the internal stress of the material. This significantly distinguishes it from hot straightening methods, when the material is heated to the temperature of the structural transformation of the metal and thus damages it. However, when straightening without heating, steel parts retain significant internal stresses. As a result, after editing they gradually take on their original shape. To relieve internal stresses after cold straightening, the part must be stabilized, that is, kept at a temperature of 400...450 °C for about 1 hour or at a temperature of 250...300 °C for several hours.

Disadvantages of mechanical cold straightening: danger of reverse action, reduction in fatigue strength and load-bearing capacity of the part. The danger of reverse action is caused by the occurrence of unbalanced internal stresses, which over time, when balanced, lead to volumetric deformation of the part. The deterioration of the fatigue strength of parts occurs due to the formation of places with tensile stresses in its surface layers, and the decrease in fatigue strength reaches 15...40%.

To improve the quality of cold straightening, the following methods are used: keeping the part under pressure for a long time; double straightening of a part, consisting in the initial bending of the part with subsequent straightening in reverse side; stabilization of part straightening by subsequent heat treatment. The latter method gives the best results, but when heated there may be a risk of disturbing the heat treatment of the part, in addition, it is more expensive than the first two.

Cold straightening of shafts

When operating machines, the shafts develop defects: bending; wear of working surfaces; Damage to threads, keyways and splines. The bending of the shafts is determined in the centers of a lathe, special devices or on prisms using stands with indicators (Fig. 2).

Rice. 2.

Shaft bending is eliminated by straightening: cold or hot. Cold straightening is performed under pressure. It should be borne in mind that during cold straightening, as a result of the appearance of cold hardening in the metal, internal stresses arise, the magnitude of which is higher, the greater the amount of deformation during straightening. In addition, during cold straightening, the required shape of the shaft is not always maintained (the shafts can return to their distorted shape). Therefore, it is recommended that after cold straightening, heat the shafts to 400...450 °C, hold for 1 hour and cool slowly.

Editing using the Buravtsev method . It was called “elemental cold editing.” In the process of straightening according to the Buravtsev method, a press is also used (Fig. 3). The know-how lies in a special device with the help of which the surface layer of the shaft journal is plastically deformed so that instead of the usual tensile stresses, compressive stresses are created in it. In this case, the fillet is not affected, which means that the fatigue strength of the crankshaft after straightening not only does not decrease, but even increases. Moreover, having gotten rid of the disadvantages of previously known methods, element-by-element cold straightening allows you to restore any crankshafts (both cast iron and steel) of any engines (from motorcycles to excavators) with almost any deflection. At the same time, the editing accuracy is very high. For example, it is possible to ensure a mutual runout of the main journals of 0.01 mm with an initial runout of over 1 mm.

Rice. 3.

Over the years of using the method of element-by-element straightening in practice, factual material has been accumulated on the further “fate” of straightened crankshafts of both domestic cars and foreign cars, including trucks and buses. Statistics have shown that these crankshafts do not return to a bent state over time. There were no complaints related to shaft breakage, which indirectly indicates their high fatigue strength.

Straightening of shafts by work hardening . The method is suitable for straightening crankshafts whose runout does not exceed 0.03...0.05% of the shaft length. It is produced by peening the cheeks with a pneumatic hammer with a special head. The crankshaft is placed on prisms with the upper main journals or installed in the centers. The duration of straightening and the depth of hardening (deformation of the cheek) depend on the force and number of blows per unit time. It is not recommended to make more than three or four blows on the same place; The effectiveness of straightening is monitored by measuring the shaft runout. The inner and outer sides of the cheek (from the crankpin side) are subject to work hardening, depending on the direction of the shaft runout. Straightening the crankshaft cheeks does not reduce its fatigue strength.

Hot straightening of metal

This editing method is universal. It is carried out using conventional means heating and is used for straightening parts of various configurations with a high degree of accuracy. One of the advantages of the method is that it allows you to straighten cast iron parts that would otherwise be almost impossible to straighten. If necessary, the process can be carried out in such a way that the correction of the part axis occurs slowly and is measured in tenths and hundredths of a millimeter. Thermal effects can be used to straighten large cross-section parts, which is especially valuable if the enterprise does not have sufficiently powerful pressing equipment.

During hot straightening, leveling results from the creation of shrinkage stresses. This phenomenon is explained by the fact that the heated part, due to an increase in temperature, tries to expand, and the surrounding area counteracts this. In this case, the heated part of the metal is plastically deformed. After the unevenness is settled, the heated part cools and the created tensile stresses contribute to the leveling of the metal. Straightening is more effective the faster the heating and cooling process occurs and the narrower the heated strip. At the same time, too narrow a heating band causes cracks in the material.

A part such as a shaft or axis of circular cross-section or a beam of rectangular cross-section, subject to straightening, is placed on two supports or placed in the centers with the convex upward. An indicator is placed under the point of greatest concavity, according to the readings of which the progress of the process is monitored. Heating is usually carried out with a welding torch (its power is selected depending on the cross-section of the part), the place of the highest bend is limited by pads. If one-time heating is not enough to obtain the desired straightness, the operation is repeated, heating the zone located next to the original one. It is not recommended to heat the same place twice. For example, you need to straighten the spindle milling machine, which is bent to a deflection of 0.2 mm. Editing is carried out on a lathe. The spindle to be corrected is fixed in the chuck and rest. To straighten the part, it is heated at the point of greatest convexity, followed by cooling with running water. The heating area is limited by a special shield made of sheet asbestos moistened with water. By heating and subsequent cooling, the spindle axis can be straightened to a straightness of 0.01...0.02 mm.

Sheet steel parts are straightened using the same method, placing them on a plate for convenience (Fig. 2.4). The progress of the straightening process is determined by the fit of the part to the plate. Heating is carried out to a temperature of 800...900 °C, but not higher than 1000 °C. The heating temperature can be determined by the cherry-red color of the part. Cooling can be intensified by blowing compressed air onto the heated area or wetting it with water. The moment at which cooling begins should be chosen so as not to harden the part.

Rice. 4. Thermal straightening of sheet steel

Good results are obtained by straightening the curved tables of milling, longitudinal planing, grinding and other machines by thermal action. To edit, the table is placed on the slab with the guides facing down. On the working surface of the table, draw a line with chalk across the table opposite the place of greatest convexity and heat the strip along the drawn line. If this operation is performed on a plate, then the results of editing are controlled by the gap between the table guides and the plate, as well as using an indicator.

Thermo-mechanical straightening method . It differs from the thermal one in that before the section of the shaft installed with the convex side up begins to heat up, elastic stresses are created in it in advance using mechanical pressure, for example with a clamp. The pressing device is installed close to the heating point, near the point of greatest deflection. Before heating begins with this device, the shaft is bent in the direction opposite to the initial deflection. Monitoring the amount of deformation of the shaft when bending it with a pressure device is carried out using indicators. When heated, the shaft tends to bend upward; encountering additional resistance as a result of this, the material at the heating site passes the yield point earlier than with purely thermal straightening.

Stress relaxation method lies in the fact that the shaft in the area of ​​its maximum curvature is subjected to heating along the entire circumference and to the depth of the entire section to a temperature of 600...650 °C. Heating occurs when the shaft rotates at low speeds. After holding at the specified temperature for several hours, the shaft is installed with the deflection upward, and immediately pressure is applied to the heated section of the shaft using a special device in the direction opposite to the deflection. The pressure is applied to create a slight stress in the heated shaft material (elastic deformation). The time during which the heated shaft is maintained in a stressed state must be sufficient so that, under the influence of load and high temperature the necessary part of the elastic deformation has turned into plastic. The main advantage of the straightening method, based on the phenomenon of stress relaxation, is the straightening of the shaft, ensuring shape stability during further operation. At the same time, during the straightening process, carried out at stresses significantly below the yield strength, no dangerous internal stresses arise.

Shafts

The main defects of centrifugal pump shafts are deflection, wear of journals, keyways and threads. Accidents with pump shafts when pumping oil and petroleum products lead to serious consequences, therefore, the choice of materials, manufacturing technology and shaft repair must be approached very seriously.

Shaft bending usually occurs as a result of bearing failure or rotor parts touching stationary pump parts.

The rotor may get caught when the plain bearings are worn out or the rotor is not properly aligned radially and axially into the housing, resulting in the gap between the rotating parts of the rotor and the non-rotating parts of the housing being distributed incorrectly. If these malfunctions are identified, it is necessary to re-center the rotor to bring the gaps to normal sizes.

The shaft journals wear out mainly as a result of mechanical impurities entering the bearing assembly, as well as due to poor quality or insufficient lubrication. The shaft neck is produced unevenly, and the cleanliness of the surface is lost.

Threads and keyways wear out as a result of repeated disassembly and reassembly from mechanical stress.

The method and technology of shaft repair in each specific case depend on the nature and size of the defect, as well as the technical equipment of the repair base. Bent shafts are straightened mechanically when cold or when heated. The first method is simple and allows one to achieve sufficient accuracy, however, in this case, overstresses occur in certain sections of the shaft, as a result of which its fatigue strength is noticeably reduced. Straightening is done using a press or jack.

For thermal straightening, the shaft is installed in the centers of the lathe with its convex upwards. The section of the shaft that has the greatest bend is covered with an asbestos sheet, which has a window for heating the defective area. Heating using burners is carried out intensively to a temperature of 500-5500C (the heated area should take on a barely noticeable dark red tint). The heated area of ​​the shaft is covered with asbestos to avoid hardening. If after this the shaft does not straighten, it is reheated.

After straightening the shaft, it must be annealed to eliminate residual stresses. Annealing is carried out with burners, uniformly heating the shaft along its entire length. At the same time, it should rotate at a frequency of 15-20 rpm. After the heating stops, the shaft must be rotated until it cools completely.

The shaft can be used if its runout is no more than 0.015mm. Worn shaft journals are ground on a lathe, followed by grinding with a portable grinding head mounted on a lathe support, or I simply grind when the damage to the shaft journal is not significant and the metal layer to be removed does not exceed 0.4 mm. This repair method can be used until the repair size of the shaft journal diameter decreases by more than 5% of the nominal journal diameter.

Severe wear of the shaft journals or the need to restore them to nominal sizes requires the use of methods for applying metals to the worn surface, which can be done by surfacing or metallization.

The surface of the shaft is pre-processed on a lathe, removing chips to such a depth that the entire surface to be deposited is processed. This allows us to ensure good conditions for surfacing and maintain the same thickness of the deposited layer. Surfacing can be done manually, but using a machine achieves greater uniformity and high quality deposited layer.

The beads of deposited metal can be directed along the axis of the shaft or in a spiral. With spiral surfacing, shaft warping is reduced to a minimum. In spiral surfacing, the welded shaft is slowly rotated in the centers of a lathe, on the support of which an automatic welding head is installed. Surfacing is carried out under a layer of flux.

The metallization process consists of melting the sprayed material, spraying it with a jet of compressed air or gas, and depositing it on the surface of products by impact and deformation of particles. Depending on the heat source used, gas, high-frequency electric arc, crucible and plasma metallization are distinguished. The sprayed material can be used in the form of wire, tape or powder. The most widely used are electric arc and gas metallizers of the wire type.

Metallization does not cause deformation of the part being restored. To obtain good adhesion of the applied metal layer, it is important to carry out the preparation correctly. It consists of cleaning the shaft surface from dirt, oil, oxides and creating a rough surface.

After applying metal to the worn surfaces of the shaft journal by any method, they are machined and ground, restoring the diameter to the nominal one, taking into account the tolerances in accordance with the technical requirements.

In case of nicks on the thread, the shaft is installed in the centers of the lathe and the thread is restored with a cutter. In case of significant damage to the thread, the section of the shaft with the thread is ground to its base and welded to the appropriate dimensions. Then produce machining welded area and thread cutting.

Worn keyways on shafts are restored in several ways. If the keyway connection should not fix the position of the part relative to the shaft, leave the worn keyway, having first cleaned its sharp edges, and mark it at a certain angle to the old groove, then mill a new groove to the original dimensions.

If the keyed connection is strictly fixed, it is necessary to restore the worn groove. This is usually done by electric arc surfacing of the crumpled edges or by welding the keyway completely. At the site of surfacing, a new groove is marked and milled.

Carrying out surfacing requires preliminary preparation of the surfaces to be deposited. They must be free of corrosion and degreased. The electrode material is selected in accordance with the quality of the base metal.

The deposited material has increased hardness, which significantly complicates processing. Therefore, sometimes they resort to expanding the worn groove, increasing its dimensions on both sides of the longitudinal axis. The greatest expansion of the groove should not exceed 15% of the original width. A key is made according to the size of the new groove, and on the mating part the groove for the new key is widened or the key is made stepped.

In case of serious defects of the shaft - cracks in the shaft body, the inability to correct the deflection using the above method, repeated restoration of the journals and threads, as well as keyways - it is replaced with a new one.