Repair of Parts by Coating

Abstract

The worn surface is usually coated with a layer, which thickness exceeds magnitude of wear. In order to provide required clearances (or tightness) in the assembling of parts, uniform clearances (this suggests the presence of the perfect mating should be provided. In the paper, the effects on the layer were discussed and forecasting the clearance sizes or (when the limit values are specified) determining the assembly life. In the process of repairing, the worn surface is usually coated with the layer, which thickness exceeds magnitude of wear. After the surface is coated it is machined by one of the available methods. During recent years, new so-called “cold” methods of coating have appeared, some of them (particularly galvanomechanical machining) enables to obtain required dimensions and quality metrics of surface layer without complete machining. The final goal of repair is to provide required clearances (or tightness) in the assembling of parts. It is required to provide uniform clearances (this suggests the presence of the perfect mating surfaces before assembly). Depending on accuracy grade, variations in the clearance dimensions may be within the limits from some microns to decile of millimeter. This depends on mating surface accuracies and is independent of the fact whether these surfaces were repaired or not. If the parts were machined separately, thickness of the layer shall be determined subject to the actual size of mating part areas, size and dimensional tolerances of clearance. When developing the process of parts repair, the following shall be considered: - wear degree of contact mating locations; - requirements for quality of surface layer in part assemblies; - permissible errors of mating surfaces; - allowance (or tightness) between mating parts and its variation limits in the assembly. The wear in mating locations is determined by supervision of part defects before repair. The limiting wear of products shall not exceed capability of galvanomechanical method. It is required for qualitative surface coating to limit the thickness to 0,5-0,6 mm. Depending on wear degree, two methods of repair can be used: -smoothing of macrosurface with a galvanomechanical coating, for example with chromium and thereafter application of qualitative surface layer. Such process shall be applied in the case when degree of surface unevenness does not exceed 50%. If wear degree is out of the stated limits, preliminary machining is required. This machining is necessary to reduce surface unevenness up to 50%. After this procedure, effective thickness of coating shall be applied. The acceptable surface unevennesses with different degrees of wear are shown on Fig.1 At the first stage of coating the stock shall be evened. After this, the surface roughness does not exceed the roughness coefficient specified for a new part, but other quality indexes of the surface (residual stress character, wear-resistance) are not in conformance with the requirements for the repaired parts. Therefore, after evening, another “slower” method of galvanomechanical repair with specified contact pressure and mechanical effect conditions is applied. As a result of this method, the following indexes are available. See Table 1.