DMME - Stresses in a Gudgeon Pin

1. Introduction
The gudgeon pin (which connects the piston to the connecting rod in a conventional internal combustion engine, ICE) is subjected to a combination of shearing and bending loads. However the length to diameter ratio is short and simple beam theory only gives accurate results for beams with large length:depth ratios (greater than about 10:1) so conventional beam analysis will not be accurate. There will inevitably be some deformation of the bushes that hold the gudgeon pin in both the piston and the connecting rod. It is therefore likely that a simple analysis assuming pure shear will also not be accurate. It should also be noted that a simple calculation assuming a uniform shear stress is not accurate as even when a component is in pure shear due to transverse loading, the shear stress distribution is not uniform - consideration of complimentary shear stresses indicates that the shear stress must be zero at the top and bottom surfaces.
Although simple calculations of the types described above will give order of magnitude values for stresses, it is necessary to carry out FEA to obtain more accurate results. How this is done is described below.

2. Finite Element Analysis
To obtain reasonable results it is necessary not just to model the gudgeon pin, but also to use a simplistic model of the piston and connecting rod and include the effects of the contact between them. If appropiate contact zones are not specified, the FEA package will 'weld' the parts together giving incorrect results. This means the parts need to be individually modelled and then brought together into an assembly for analysis. This has been done using the student edition of PTC Pro/Engineer and Mechanica. The tutorials on the CDs which accompany the Student Editions of the software include instructions on assembling components and on setting up contact areas. (I have not done much 'assembly' or specifying contacts and I found both of these a little tricky).

Because of symmetry it was only necessary to model one quarter of the piston / connecting rod / gudgeon pin. The gudgeon pin dimensions and the width of the bushes in the piston and connecting rod are taken from a specific engine.

The appropriate constraints were placed on the symmetry planes and the top of the 'piston' was restrained from moving in the 'Y' direction.
Contact surfaces were specified between the gudgeon pin and the piston and between the gudgeon pin and the connecting rod.
From the Mechanica Structure menu use 'Contacts', 'Create', 'Part'. Then use 'Query Select' and pick on one of the surfaces (a small contact region symbol appears) then pick on the other surface and a second contact region symbol appears. The 'Review' command can be used to check the surfaces are correct.
Because the contact analysis is non linear, the load should be applied in steps - this is set up from the 'Analysis' form. I specified 3 steps. It should be noted that surface contact pressure values may not be accurate unless mesh refinement is specified, however for our purposes, shear, bending and von Mises stresses etc, this is not likely to be significant.

3. Results
The 3 stress plots that can be linked to below are each about 270 kB in size. Note the differences.
Although your geometry will be different, it will be similar and some extrapolation may be acceptable provided you explain and justify it.

Plot of Von Mises Stresses

Plot of Maximum Shear Stresses

Plot of Maximum Principal Stresses

Return to Module Introduction.

David J Grieve, 5th February 2003.