The method of additional repair parts is based on the use of pre-manufactured additional repair parts, which are installed on worn-out pre-prepared surfaces of the part or with which
completely replace the worn part of the part. The need to use this method is due to the fact that many parts complex shape Individual surfaces wear out or become damaged: smooth and threaded holes, shaft journals, seating areas for rolling bearings in housing parts.
Processing of worn parts for additional parts is carried out different ways: boring, drilling, reaming, grinding, etc.
Selection of material for additional details should be carried out taking into account the material of the part being restored. The exception is the restoration of the seating surfaces of cast iron parts, where the material for the bushing is steel.
Working surface The additional part must meet all the requirements (hardness, cleanliness of processing, etc.) imposed on the surface of the part being restored. Fastening of additional parts (AD) is most often done through interference fits, in some cases by welding around the entire perimeter or at several points, locking screws, studs and other methods.
The amount of interference can be determined by the formulas:
where δ m is the tabulated fit interference; R Z1 and R Z2 - height of microroughnesses of mating surfaces; R a1 and R a2 arithmetic deviations of microroughness profiles on the surfaces to be joined, µm; K1 and K2 - coefficients that take into account the irregularities remaining near the shaft and hole after pressing, are taken equal to 0.6.
To facilitate the pressing of the bushing (without heating or cooling the mating surfaces), the mating surfaces must be lubricated with machine oil or molybdenum disulfide. In addition, for better centering of the bushing during pressing and to avoid scuffing, its edge along the outer diameter should be chamfered at an angle of 45.
When pressing and pressing in the bushings (Fig. 43), the outer diameters increase and the inner diameters decrease, respectively. This must be taken into account when assigning an allowance for processing the working surfaces of the bushings after they are pressed in or pressed.
When pressing the bushing, the reduction in its internal diameter is determined by the formula
where σ is the compressive stress on the contact surface of the part, MPa; d - external diameter of the part, m; d1- inner diameter details, m; E1 - modulus of elasticity of the covered part, MPa.
When pressing a bushing onto a shaft, the increase in its outer diameter is determined by the formula:
where E2 is the elastic modulus covering the part. The thickness of the sleeve is determined by the formula
where [σ] is the permissible stress of the bushing material, MPa; σ t - yield strength of the bushing material, MPa; d - outer diameter of the bushing, m; P - pressing force, N.
To the calculated thickness of the bushing, it is necessary to add another allowance for machining the bushing after its pressing or pressing, which can be determined by the formula
where μ is Poisson's ratio.
Pressing or pressing should be done with heating of the female part or cooling of the male part. In this case, the strength of the fit increases by 2 ... 3 times.
The heating temperature of the female part or the cooling temperature of the male part can be determined by the formula
where α is the coefficient of expansion during heating or compression during cooling of the material of the parts; d - diameter of the male or female part, m.
The resulting temperature value must be increased, and during cooling reduced by 20 ... 90%, taking into account its change during the process of transfer, installation and pressing.
The repair method by replacing a part element is used in cases where a complex part with a large number of working surfaces has excessive wear on some of them, while the rest are only slightly worn. In this case, the worn element of the part is removed and replaced with a newly manufactured one. The element being replaced is connected to the main part by threading or pressing, followed by welding, or both.
The disadvantages of the method of additional parts are that the use of additional parts leads to a weakening of the rigidity of the part, an increase in its thermal stress, and a change in the number of links in the dimensional chain that includes the part. Therefore, this method is not progressive.
A method for restoring the initial (nominal) dimensions of parts.
The most advanced, but at the same time expensive, method is to restore the initial (nominal) dimensions of parts. With this method, the gap or interference in the mating is brought to the nominal (initial) value by restoring the nominal dimensions of the parts, their geometric shape and surface cleanliness.
With this method, restoration of the original dimensions and fit is carried out by applying, as a rule, a layer of metal or polymer material of the required thickness to a specially prepared worn surface, taking into account the subsequent machining surfaces.
The application of a layer of material can be done in various ways: surfacing, electroplating, chemical coatings, electrical erosion methods, metallization of spraying, application of polymer materials, etc.
This method does not have the disadvantages that exist when using the method of restoring the landing by changing initial sizes and therefore is progressive.
In addition to wear of the seating surfaces, there may be other defects in the parts - cracks, holes, scuffs, chips, etc., which can be eliminated by other methods not included in this classification. To eliminate such defects, gas and electric arc welding, polymer materials, soldering and other methods are used.
Details Category: Press connections Views: 4376
The peculiarity of tension connections is that, even before the application of working loads, they are prestressed by forces from interference on the seating surface, and triaxial tensile stresses, which are unfavorable for strength, arise in the female part. When prestresses are added to the working ones, stresses may arise that exceed the yield strength of the material, as a result of which the connection fails.
At the same time, the formal calculation of tension connections, based on the assumption of constant cross-sections along the length of the parts and ignoring boundary conditions, does not reveal actual stresses. The actual load-bearing capacity and strength of the connection is highly dependent on the shape of the female and male parts. Uneven rigidity of parts (stepped shafts, hubs with disks, etc.) causes uneven distribution of contact pressures and stresses along the length of the connection. Sharp stress surges occur at the edges of the connection.
Formal calculation, even with a large safety factor, does not always ensure the operability of the connection, especially since the distribution of operating stresses across sections of the part, as well as the nature of their interaction with prestresses in most cases, especially in connections subject to cyclic loading, are unclear. Therefore, regardless of the calculation results, it is necessary to strengthen the interference connections in every possible way using constructive measures.
For increase bearing capacity and the strength of interference joints, the following is advisable:
- reduce the pressure on the seating surfaces by increasing the length or diameter of the connection (a more effective method);
- choose the tension within narrow limits, using high-quality landings;
- reduce stress by appropriately choosing the wall thickness of the female and male parts (increasing the wall thickness of one of the parts reduces the stress in it, but at the same time increases the stress in the other part);
- avoid sudden changes in the cross-sections of the parts to be connected at the connection site (and in areas close to it) to prevent stress surges;
- reduce stress surges at the edges of the connection by reducing the sections of the hub (and shaft) towards the ends;
- subject the seating surfaces to hardening heat treatment (for example, low-temper quenching, low-temper quenching, heating HDTV) and hardening treatment by plastic deformation (shot peening, rolling of shafts, rolling or mandrating holes);
- use assembly of connections with heating of the female part or with cooling of the male part;
- apply electroplating contact surfaces with soft metals (Cd, Cu, Zn).
The performance of interference fit connections largely depends on correct assembly. To facilitate pressing, the shaft and hole are equipped with lead-in chamfers at an angle α = 30–45° (Fig. 535, a), and for large interferences α = 10–15°. The height h of the chamfer is set so that the input diameter of the shaft d is 0.1-0.3 mm less than the diameter of the hole d0 (Fig. 535, b).
It is most advisable to round the end of the shaft with a fillet of variable radius (Fig. 535, c), although the manufacture of such fillets is more expensive.
Sometimes cylindrical collars are made on the shaft or in the hole with an H7/h6 fit (Fig. 535, d, e). The location of the centering collar in the hole requires the use of a shaft system.
The axial position of the parts is fixed by pressing them all the way into the collar (Fig. 535, f, g), into the step of the hole (Fig. 535, h), flush with the hole (Fig. 535, i). Smooth parts can be fixed in any position with measuring spacer rings 1 placed under the rolling pin of the press (Fig. 535, j).
It is important to prevent biting and distortion of the parts being connected, which complicates the pressing process and sometimes leads to irreparable damage to the connection.
When pressing, thin-walled parts such as bushings are guided using a centering mandrel (Fig. 536, a). When pressed into through holes the bushing is placed on a rolled mandrel with a guide shank 1 inserted into the hole at the landing H7/h6 (536, b). After pressing, the shank is unscrewed.
Parts connected by interference fits must not be subjected to heat treatment, since when heated, the interference is lost due to the loss of elasticity of the material. In precise connections, it is necessary to take into account the deformation of parts during pressing (reduction internal dimensions male part and increasing the external dimensions of the female part). The greater the tension and the smaller the thickness of the parts, the greater the deformation.
It is possible to reliably determine the change in dimensions by calculation and adjust the original shape of the part in advance only in relatively rare cases when the walls of the parts have a constant thickness. Parts with variable wall thickness are deformed unevenly. Thus, when a thin-walled bearing bushing is pressed into a housing with a central wall (Fig. 537, a), the bushing takes on a corset shape. If the wall is asymmetrically positioned, the corset shifts towards the stiffening unit (Fig. 537, b). Day of provision proper operation After pressing the bearing, it is necessary to finally process the inner surface of the bushing, providing appropriate allowances in the workpiece. Most often, the bushings are reamed, leaving an allowance of 0.02-0.1 mm per side for reaming.
When pressing parts into the shaft cavity outside surface the shaft bulges barrel-shaped, which requires finishing of the shaft after pressing (Fig. 537, c). When pressing thin-walled gears onto shafts (Fig. 537, d), it is necessary to finish the tooth after pressing. If this is not possible due to dimensions (long shafts), the thickness of the rim should be increased or a collapsible fastening should be used (on a key or splines).
Pressing does not affect the dimensions of elements located at a large distance from the seating surfaces (for example, teeth disc wheels). In such cases, parts can be pressed into the final processed form without fear of dimensional accuracy. Distortion and axial runout of disk parts large diameter prevented by increasing the length of the landing belt.
A common mistake when designing off-design (subject to small or uncertain forces) tension connections is the insufficient length of the press belt, as well as the small thickness of the walls of the female or male part (Fig. 538). Such connections quickly fail as a result of crushing of the seating surfaces and overstressing of the thin walls during pressing.
For approximate determination of the minimum length of landing belts in connections with interference general purpose you can use the formula l min = ad 2/3, where l min is the length of the belt (less chamfers), mm; d—connection diameter, mm; a is a coefficient equal for female parts made of steel, a = 4, for cast iron, a = 5, for light alloys, a = 6. Based on this formula, a graph was constructed (Fig. 539).
If the connection is subject to high bending moments or shear forces, especially alternating forces, as well as when it is necessary to accurately guide and firmly fit the pressed part (for example, a frame column), the length of the press is made significantly longer.
In designs with fitting into blind holes, it is necessary to ensure the release of air during the pressing process. Compression of air during pressing, accompanied by an increase in its specific volume due to heating, can cause rupture of the female part, especially if it has thin walls or is made of material of reduced strength (for example, light alloys). To release air, grooves are provided (Fig. 540, a) or holes (Fig. 540, b and c).
It is unacceptable to press parts onto two belts of the same diameter (Fig. 541, a). When passing a part through the first (during pressing) belt, a distortion occurs, making it difficult to insert the end of the part into the second belt. In addition, scoring may occur on the surface of the part and hole. In such connections, the landing belts should be made of different diameters (Fig. 541, b). The axial dimensions of the connection must be such that the part first enters the second chord by an amount of m = 2-3 mm (Fig. 541, c), obtaining a stable direction, and only then enters the first chord.
In the design (Fig. 541, d), to reduce precision machining, the hole is made with two short landing belts. The error lies in the same diameter of the landing belts. In addition, deformation of the bushing in the areas where the landing belts are located is inevitable.
If strict straightness of the hole walls is important, the bushing should be deployed after pressing or the bushing should be seated along the entire length or at least over most of the length (Fig. 541, e and f).
The female parts must be given sufficient rigidity to avoid deformation under the pressing force.
In the fork part (Fig. 541, g), the upper eye bends during pressing, as a result of which pressing into the lower eye becomes impossible. If, due to structural conditions, it is not possible to give the eyelet sufficient thickness, then for pressing it a device should be used that rigidly fixes the eyelet. Most in a simple way is the introduction of a horseshoe-shaped cracker 1 between the eyes. The possibility of using this method must be provided for in the design of the part: the distance between the eyes must be specified with an accuracy sufficient for the use of a cracker that is uniform for a series of given parts.
Other Possible Solution- assembly with heating of the female part (or cooling of the male part) to temperatures at which gaps form on the landing belts.
The female and male parts must have as uniform a rigidity as possible in the radial direction. Local weakening, cuts, etc. are undesirable. In the design in Fig. 541, h, pressing is difficult due to the inevitable displacement of the sleeve towards the cutout. In addition, in the area where the cutout is located, the sleeve is deformed under the action of one-sided radial tension. The situation improves somewhat if the bushing is pressed into two belts located on the uncut sections of the hub (Fig. 541, i). Most correct in in this case install the bushing according to the H7/h6 fit and fasten it with bolts (Fig. 541, j).
Press-fitting is used in cases where the male or female parts do not have through cutouts extending to the end (Fig. 541, l). If the cutouts cannot be eliminated, then the only solution is to use the H7/h6 fit.
In some cases, it is necessary to maintain a certain angular position of the parts to be connected (for example, pressing a key shaft into the hub). It is possible to ensure alignment of the key with the keyway if a belt is made on the lead-in side of the shaft (Fig. 542, a) with a fit H7/h6 or H7/g6, having a length l exceeding the distance k of the key from the end of the shaft. The key is first inserted into the groove, after which the shaft is pressed.
Another technique is also used: the key is released from the shaft at a distance k sufficient to fix the shaft along the keyway before pressing (Fig. 542, b). It is best to assemble such connections by preheating the hub or cooling the shaft until a gap is obtained in the connection. Angular fixation of the shaft in the hole in this case does not cause difficulties.
Cams with a given angle of arrangement of the edges (Fig. 542, c) must be pressed through a guide device with radial cutouts for the edges, based on the central hole of the disk. The design must provide for the possibility of using such a device.
Design in Fig. 542, c is incorrect: the base at the base of the cams does not allow them to be passed through the guide grooves of the device.
In the design in Fig. 542, g the width m of the cams is made larger than the landing diameter d, which ensures confident direction of the cams during pressing.