David Redszus wrote: ↑Tue May 04, 2021 11:42 am
The physical properties of wrist pins has been well known for over 75 years. They can be modeled
very accurately. The bending and distortion effects can be calculated on your desktop. That's the easy part.
The more difficult part, is to know what forces will affect the wrist pin, what the distortions will be,
and what effect it will have on the piston.
Twenty five years ago, Mahle showed that a lightweight, flexible wrist pin will, under load, cause
a expansion and contraction of the piston skirt, in both the thrust and non-thrust axis.
Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller
diameter pin. Pin ID being fairly insignificant compared to OD and length.
they can be modelled accurately but it is not the same as using simple bending, shear and deflection formulas based on beam theory. The dimensions of the wrist pin and the type of support do not lend to that method being useful or accurate for more than a school assignment.
Case in point you state "Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller diameter pin. Pin ID being fairly insignificant compared to OD and length." that is not very useful for optimisation of a design. Like all real world things being stronger, stiffer, more durable is a relative thing but means nothing without absolutes as to whether it is actually strong enough, stiff enough or durable enough which requires specific criteria. Without accurate methods you can not optimise with confidence. the classical methods end up being far too conservative
the forces are straight forward to calculate with a 1D simulation, the load history is more time consuming to categorise, but proper analysis involves relatively complicated FEA to model the system rod, pin and piston assembly to capture the compliance of each part and local load transfer between each part.