Con-rod design question

That follows the pattern Gordon constrained at both end so nodes. 2 nodes 1/4 along the bar and the middle showing maximum flexure. Failure must occur when the compressive stress from the bend plus the compressive stress from the normal operation exceeds the elastic limit of the bar and it collapses. Therefor put more meat in the middle to stop it flexing as much, keeping the material below the elastic limit. Loco con rods should be treated as long columns and I guess short con rod as short columns.

I read that short columns fail when the bending and direct stress exceeds the yield point of the material and long columns when the elastic limit is exceeded. Short columns are defined as l/r less than 100 (length and radius).

This essentially means that long columns will not tolerate non linear deformation in the cross section as the stress becomes self sustaining and increases rapidly to failure. Short columns do not suffer this.

regards Martin

Many years ago I rebuilt a 1930 Talbot sports car, identical to those that came first and second in class and on handicap at the 24 Heures Du Mans race in that year. Not quite the same but it had slender fish bellied push rods to reduce the weight while maintaining the required strength. They were originally made by a knitting needle manufacturer.

Russell.

Martin comes close to mentioning the important terms, I will mention them implicitly: "Slenderness ratio" and it references the radius of gyration to the length (the l/r mentioned above). If there are any crane designers or (probably) structural engineers they would be able to recognise the question and answer it eloquently but I will decline and leave you all to search the term I suggest!

Mark

I was going to say I can't recall coming across a con rod 50 times as long as its diameter - but then I realised the valve rod of a model I am making probably is, the pump rod isn't far off either. I can't think of a piston con rod that is that slender.

Neil

You need to start building bigger models Neil, the rod that activates the governor butterfly valve on my Tidman engine is 66 times longer than its diameter and there is no risk of buckling

Edited By JasonB on 18/12/2015 20:49:07