I’m sure someone will argue a different point of view, but my opinion is that the piston is probably the single most demanding component in an engine in terms of detailed design. There is so much at stake in terms of performance if the piston is too heavy or creates too much friction that people push piston design more than any other component. The penalties of getting any single design aspect wrong can turn an otherwise good piston into a liability.
The pin bore is a case in point. If we look at it simply, it is little more than a hole through two bosses. Look more closely though and we perhaps notice some other features such as circlip bores and oil grooves; there is often more engineering involved than meets the eye here.
The first obvious aspect to get right with the pin bore is its size. It needs to provide a suitable clearance to the piston pin in all circumstances, at all operating temperatures and with just the right material conditions on the diameter of the pin.
The surface finish of the pin bore is also critical, and honing is the method most often used for finishing pin bores. Honing produces a low-roughness surface finish with a ‘structure’ that retains some oil in the bore. It also makes it easy to remove tiny amounts of material from the bore, so represents a good way to finish the bore to a tight tolerance.
Any axial oil grooves in the bore need to be placed in the correct positions. If they coincide with areas of significant loading then high contact stresses can result on the edges of the grooves, deforming the material and allowing the pin more movement off-centre. The circlip grooves machined into the pin bores are also critical, as they control the axial float of the piston pin.
We must be sure that the pin is not able to float in any circumstances. That means the circlip pin grooves need to have the correct tolerances applied on both size and position, and need to be designed in conjunction with the circlip and the piston pin. It is often the chamfers on the piston pin ends that interact with the circlip, so these seemingly innocuous features need to be tightly controlled.
If you look at the pin bore on a highly stressed engine, you will often see at its bore edges on the inside of the pin bosses a radius that blends as closely as possible with the pin bore – it probably won’t be perfectly tangent, but close. This is again in the interests of the piston pin but also of the piston durability.
The aim here is to provide a ‘soft’ contact. If we leave a sharp edge, when the pin and piston flex, there is a condition of high contact stress at this point, and this can cause fatigue cracks to be initiated. This is particularly important if you are using a piston material with low elongation. At a microscopic level, this design helps to prevent the material exceeding its yield strength and elongation limit, and cracking. People developing pistons from such materials can find that simply copying an existing design results in failure because cracks have started from locations of high local contact stresses.
Written by Wayne Ward
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