Justification of the method of surface plastic deformation to strengthen the surface of the shaft under the cuff during repair

. The shaft-seal connection is a critical element of any unit, and its reliability is important to ensure the service life of the entire assembly unit. Treatment by surface plastic deformation makes it possible to provide hardening to a depth of up to 2 mm, microhardness increases on average by 150% relative to the initial value, and reaches a value of 6500 MPa. Residual stresses after SPD always have a negative value, the depth of their distribution exceeds the depth of distribution of increased microhardness by 1.5 times, which makes it possible to increase the service life.


Introduction
The service life of joints can be increased in two ways -by increasing the wear resistance of friction pairs and by rationally choosing the accuracy of parts [1,2].This is especially true for critical connections of machine units, which are loaded and tend to wear out quickly [3][4][5].Otherwise, there is an increase in economic losses due to equipment breakdowns and downtime [6], which is not beneficial for the enterprise operating this equipment, and it will choose other brands in the future.
The shaft-seal connection is a critical element of any unit, and its reliability is important to ensure the service life of the entire assembly unit.Particular attention must be paid to the selection of materials, surface treatment and type of seal to ensure an effective, tight and wear-resistant connection [7,8].
When choosing materials, it is important to take into account operating conditions, such as temperature, pressure, chemical aggressiveness of the environment and shaft rotation speed.The materials must be compatible with each other and provide the necessary longterm strength, increased wear and corrosion resistance of the connection.
Rubber seals can be made from a variety of materials, each of which has its own advantages and disadvantages in terms of wear resistance.For example, nitrile rubbers have higher wear resistance than natural rubbers, but they can also be more expensive.
The surface of the shaft under the cuff must be free of sharp edges, burrs, cavities and other defects that could damage the cuff or lead to leaks.The shaft surface must be free of corrosion or oxidation.Allowed surface roughness is within the range of Ra = 0.16...0.32 µm for medium and high shaft speeds.The permissible roughness value is Ra = 0.64 µm for lowspeed shafts.Shaft surfaces must be machined with high precision and regular cleanliness to ensure a tight fit and minimize the differential pressure between the seal and the shaft resulting from radial runout of the shaft and misalignment of mating parts.
Installation of the seal must be carried out in compliance with all necessary requirements and manufacturer's recommendations.It is important to ensure correct positioning of the seal, without distortion, as well as uniform and stable compression by the rubber lip over the entire contact surface with the shaft.
During operation of the unit, it is necessary to regularly check the connection between the shaft and the cuff -inspect for wear, damage and leaks.Early detection and correction of seal problems can prevent serious problems and extend the life of the unit.
It should be remembered that the service life of the entire unit as a whole depends on the reliability of this connection -gearboxes, engines, gearboxes, which are widely used in modern technology.

Research materials and methods
Methods of surface-plastic deformation (SPD) are widely used as a finishing and strengthening treatment.This is due to their low cost, low labor intensity and lack of chips.All SPD methods are based on the use of the plastic properties of metals that can accept residual stresses without compromising the integrity and volume of the workpiece.In contrast to abrasive finishing methods, during plastic deformation, in addition to improving the geometric properties, there is also a significant hardening of the surface, which is important for increasing the durability of machines and mechanisms.After SPD, parts become more resistant to fatigue failure, their corrosion resistance and wear resistance of joints increase, and risks and microcracks remaining after cutting are removed.SPD methods can be applied to all metals capable of plastic deformation Static methods of surface plastic deformation are characterized by stability of the shape and size of the deformation zone due to the stationary contact of the tool and the workpiece.
Wear-resistant ball bearing steels and alloy steels are used as tool materials used for static surface plastic deformation; natural and synthetic diamonds.
Rolling allows you to reduce the friction forces between the tool and the workpiece and achieve high productivity of the process.The tool used is balls or rollers, the hardness of which must exceed the hardness of the material being processed.Rollers provide greater productivity.Rolling occurs under conditions of rolling friction with slipping.Thanks to the self-alignment of the ball, free rotation is ensured during rolling friction between it and the surface being processed.In production practice, rolling schemes with rigid and elastic contact between the tool and the workpiece are used.
Ball rolling is used when it is necessary to create a deep hardened layer, process hard parts, increase productivity when using large workpieces and process materials that cannot be smoothed with diamond due to strong adhesion (titanium alloys, etc.).In general, rolling has better performance and allows for greater depth but less hardening than diamond burnishing.Surface quality indicators are after rolling in Ra = 0.05...1 µm, σ'rest = -100...400 MPa; after rolling Ra = 0.05...0.32 µm, σ'rest = -150...400 MPa.
Diamond burnishing is characterized by low productivity and low durability of an expensive tool.Since diamond is an anisotropic material, to obtain the required strength and hardness when installed in a mandrel, it is necessary to achieve the correct crystallographic orientation of its position.Diamond smoothing is recommended for use when there are increased requirements for the roughness and hardness of the resulting surface, as well as when processing low-rigid parts.Burnishing Since the diamond is practically does not deform, and its radius is extremely small, the contact area of the tool with the part is insignificant.This causes the creation of high contact pressures even with small normal forces required to cause deformation.
The disadvantage of diamond burnishing technology is the impossibility of processing titanium alloys due to the increased adhesion component of the specified coefficient due to the adhesion of metal particles to the deforming tool.
In general, diamond smoothing makes it possible to achieve lower roughness and greater microhardness of processed surfaces than rolling, while reducing the depth of work hardening.It should be noted that in cases where it is necessary to obtain a clean surface with a large depth and degree of hardening, sequential processing should be used -first rolling and then smoothing.To reduce the number of repeated strokes, you should strive to select the optimal pressure that allows you to process the required surface in one working pass.It should be borne in mind that by repeated working strokes can only increase the degree of hardening, but not its depth.Surface quality indicators after smoothing are Ra = 0.05...1 µm, σ'rest = -100...1400 MPa.
Burnishing is a highly productive process that combines the capabilities of finishing, strengthening, calibrating and shaping treatments.It is used for machining holes, when it is necessary to obtain high geometric and physical-mechanical characteristics of the surface.The essence of burnishing comes down to moving the tool with tension inside an unsecured workpieces, while the dimensions of the tool are slightly larger than the diameter holes.Surface quality indicators after burnishing are Ra = 0.1...1.6µm, σ'rest = -100...500 MPa.
Dynamic SPD methods are characterized by intermittent pulsed penetration of an indenter into the workpiece.Numerous blows applied by a tool to the metal surface leave a large number of local plastic deformations (craters) on it, which gradually cover the entire treated area.The source of deformation is formed as a result of periodic penetration of the indenter into the surface of the material and depends on the impact energy and the degree of overlap of the indentations.In this case, the surface pattern, as a rule, consists of a grid of holes partially or completely overlapping each other.Various tool steels are used as a material for tools during dynamic SPD: carbon; high-speed; carbide (under dynamic loading with wear in order to increase resistance to shock loads), as well as under dynamic loading without wear in order to increase resistance to shock loads.
Vibratory rolling and vibration smoothing are used when it is necessary to squeeze out barely noticeable grooves on the surface of the workpiece to retain lubricant and wear products during operation.It is carried out by a ball or diamond, which additionally imparts vibrations.By changing the amplitude and frequency, you can obtain the desired pattern from the grid of holes.Surface quality indicators after vibration rolling are Ra = 0.06...1.6µm, σ'rest = -100...450 MPa.
Embossing the surface with a mechanical or pneumatic hammer, the shape of the working part of which corresponds to the profile of the workpiece being hardened, makes it possible to achieve significant compressive stresses and a maximum depth of the hardened layer, reaching several tens of millimeters.However, the roughness after processing deteriorates noticeably, so the next operation is usually grinding.The surface quality indicators after minting are Ra = 8...32 μm, σ'rest = -200...1000 MPa.
Shot blasting is carried out by using the kinetic energy of metal balls that fly in an air stream and hit the surface of the workpiece at high speed, strengthening it.In this case, the roughness deteriorates significantly.This method is usually used to strengthen the elastic elements of vehicle suspension -springs, springs and torsion bars, as well as for gears, connecting rods, propeller blades and other machine parts with complex profiles.
Ultrasonic treatment (UST) is a relatively new SPD method.The surface layer is deformed by an acoustic head, oscillating at an ultrasonic frequency, into which a carbide indenter is soldered.Due to the influence of ultrasonic vibrations, the resistance of the metal to plastic deformation is significantly reduced.Some scientists put forward the hypothesis of facilitating the movement of dislocations inside grains along slip planes under the influence of high-frequency vibrations, other researchers explain this phenomenon by a decrease in the yield strength, and others associate this fact with an increase in the instantaneous temperature in the deformation zone and an improvement in the plasticity of the metal.However, since the heating time of the deformation zone due to the high speed and discreteness of penetration is extremely short, relaxation of residual stresses and polymorphic transformations do not occur during ultrasonic treatment.
The process of processing by surface plastic deformation is carried out by the force contact action of a deforming tool, the working elements of which (rollers, balls or bodies of other configurations) interact on the surface of the workpiece itself under conditions of their relative movement (Figure 1).
Surface treatment by smoothing is carried out intensively, during which an increase in the strength of the surface layer occurs (improving microhardness and creating favorable compressive stresses); also treated surfaces of the part with abrasive particles eliminate saturation with abrasive.

Results
On the shaft surface for the seal, it is necessary to provide a small depth of solid material on the shaft surface -from 0.5 to 2 mm, within the wear layer.To make a reasonable choice of a surface hardening method, it is necessary to compare the data on the technological capabilities of known hardening methods, in particular on the microhardness and stress state of the hardened layer.The following main types of strengthening treatment are known [9]: -heat-strengthening treatment (HT); -chemical-thermal treatment (CHT); -application of strengthening coatings; -surface plastic deformation (SPD).
A graphical comparison of the basic characteristics of the above types of hardening treatment is shown in Fig. 2.After heat-hardening treatment, the microhardness of steel grades that are well hardened can reach 7500 MPa, and the depth of such treatment reaches 5 mm.Stresses in the hardened surface layer, depending on the selected hardening modes, can be either tensile or compressive.
As a result of chemical thermal treatment (CHT), the microhardness increases to 8000 and the depth of the hardened surface layer will be from 0.01 mm to 1.4 mm.The distribution of residual stresses after chemical treatment depends only on the type of subsequent heat treatment [9].
Coatings are mainly used to increase the corrosion resistance and wear resistance of friction surfaces of parts.Typically, the surface microhardness reaches 3000 MPa.The thickness will vary in the range of 0.003...2 mm.With this method of hardening, tensile residual stresses are formed.
In the conditions of small-scale and single repair production, the above types of hardening treatment are very difficult to implement due to the high costs associated either with the purchase of equipment or with ongoing costs associated with high energy consumption.
SPD processing makes it possible to ensure a depth of the hardened surface layer of up to 30 mm, microhardness increases on average by 150% relative to the initial value, and reaches a value of 6500 MPa.Residual stresses after SPD always have a negative value, the depth of their distribution exceeds the depth of distribution of increased microhardness by 1.5 times (Fig. 1).Most SPD methods make it possible to strengthen local areas of the surfaces of parts with the implementation of a uniform transition from the hardened to the unstrengthened layer.These facts make it possible to consider SPD as the most rational and effective way to improve the performance characteristics of heavily loaded machine parts in repair production conditions [10].
SPD can be provided by static and dynamic methods.Static methods, such as rolling, rolling out, removal of microrelief, etc., provide a large amount of hardening -microhardness can reach 6500 MPa, and residual stresses will be about 1200 MPa with a small depth of the hardened layer of 2 and 3 mm, respectively (Fig. 3).Dynamic methods, such as centrifugal, shot blasting, hydro shot blasting, embossing, etc., make it possible to increase microhardness up to 6500 MPa, and residual compressive stresses, respectively, up to 1000 MPa, with a reinforced layer depth of up to 35 mm and 45 mm.For the shaft-cuff connection under study, it is irrational to use dynamic methods, since such a depth is not required here.

Conclusion
Thus, to form a longer service life of the connection of a rubber reinforced cuff with a shaft during repair of an assembly unit, in the case when the shaft is not replaced with a new one, but there is a technological possibility to process the surface of the shaft to the repair size or in the case of restoring the shaft to the nominal size by surfacing, spraying, ironing, galvanizing and other methods, it is most advisable to use roller rolling after grinding -use traffic rules, which allows you to ensure a compacted layer depth of up to 2 mm, which significantly exceeds the amount of radial wear of the shaft during operation of the joint.

Fig. 1
Fig. 1 Interaction of the deforming tool on the surface being processed.I -rolling; II -sliding; IIIintroduction, a -deformation zone; b -hardened layer h -compaction depth

Fig. 2 .
Fig.2.Comparison of methods for hardening the surfaces of parts -depth of the hardened layer h  -microhardness H  -residual stresses σ -depth of the stressed layer h σ

Fig. 3 .
Fig. 3. Justification of the SPD method for the surface of the joint shaft "shaft seal" -depth of the hardened layer h  -microhardness H  -residual stresses σ -depth of the stressed layer h σ