Grinding and polishing

Thursday, July 04, 2013

Tags :  crankshafts

Crankshafts are very precise pieces of equipment, and are machined accurately for a number of reasons. For racing, we need to get as close to the maximum allowed capacity without exceeding it, so the throw of the crankshaft is tightly controlled. However, possibly the most critical and most accurately controlled machined features of the crankshaft are the main bearing journals and crankpins. They are usually specified with very tight limits on size, and other geometric tolerances such as cylindricity and circularity. Furthermore, tolerances are placed on the form of the bearing surfaces along their length to ensure control over any concavity or convexity. The surface finish of the bearing surfaces is also very tightly specified.

Grinding and polishing are the processes by which all these bearing surface specifications are achieved.

The grinding process needs to be carefully undertaken and has to be considered when machining the crankshaft prior to grinding. Where crankshafts are surface hardened, the depth of the hard ‘case’ on the component is relatively shallow. In order to maintain a constant depth of hardened case, the pre-grinding machining stage needs to be a constant offset from the ground surface. If too much material is removed, the crankshaft will lose the benefit of both the hard-wearing surface and the residual compressive stresses that offer improved fatigue.

Unfortunately, the most common mistakes with grinding allowance happen in the fillets of crankpins and main bearings, where the stress concentration factors are high and from which fatigue cracks are likely to emanate. Equally, there is the danger of not removing enough material, as that can leave a hard, friable layer at the surface which then easily flakes in service.

The resulting debris is very abrasive and can cause further damage in the engine. I have seen an engine that this has happened to, and although the damage was limited to the crankshaft and some of the con rods, it caused a much more rigorous inspection regime to be enacted at the crankshaft manufacturer at the customer’s behest. This was caused by insufficient grinding allowance being left on the crankshaft before nitriding. It should be noted that both under- and over-removal of the hardened case can result from poor machining prior to grinding, or lack of accuracy in grinding.

Even when the dimensions from the drawing are closely adhered to, damage can be caused by grinding if the operation causes excess heat at the surface. Grinding cracks may be parallel in appearance, or a network similar to dried mud. The heat caused by ‘abusive’ grinding can be enough to cause a transformation in the structure of the steel, although cracks can often form without the surface reaching temperatures sufficient to cause a change in structure. Factors affecting the heat generated in grinding include material removal rate, coolant flow rate, the condition of the grinding wheel and the grinding wheel material. Even if the crankshaft is not cracked, abusive grinding can leave the surface of the component in a state of tension, which can be disastrous for fatigue life.

Written by Wayne Ward

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