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Con rods, stressman needed

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duncan webster14/07/2020 11:36:58
2651 forum posts
36 photos

Looking at photos of full size locos, some have connecting rods which taper from the little end to the big end, some are parallel and a small number appear to be tapered for a bit then parallel (GWR Kings)

I can see that they want to blend into the big end with a nice radius to avoid stress concentration, but parallel would seem to be a lot easier to machine. Anyone got any thoughts?

Dave Smith 1414/07/2020 14:06:47
108 forum posts
13 photos

My two pennies. It is not about stress but stiffness. This what a lot of engineers miss, it may be adequately strong but deflections is sometime the overriding consideration. The rod is strong enough we know that from the smaller little end, but the crank end needs to be a larger to ensure the crank or whatever it is attached to does not flex unduly. Particularly where the attachment points are cantilevered.


duncan webster14/07/2020 14:21:35
2651 forum posts
36 photos

I'm OK with the big end pin being bigger than the little end, if nothing else it has a higher surface speed, but that doesn't mean the rods have to be tapered

I've just looked at some automotive rods, they are parallel until they splay out to the big end. chevvy rods

Was it just aesthetics with steam locos? If so it must have cost a fair bit.

I think buckling is important, but that will be governed by the cross section at the half span? Perhaps this is why some coupling rods were fish bellied?

Dave Smith 1414/07/2020 14:33:19
108 forum posts
13 photos

The other thing is they lighter, you remove mass from the point where you do not not want it. Buckling comes down to stiffness again for a given load which is controlled by the cross section as you say. In my racing days we did as much as we could to take the mass out of the pistons and the little end of conrods.


SillyOldDuffer14/07/2020 15:01:07
5932 forum posts
1282 photos

I think weight reduction is the main reason. Reciprocating masses upset dynamic balance, thump thump, and accelerating excess weight is wasted work. Best to reduce both effects by removing metal that doesn't contribute to strength.

A double acting steam engine conrod in tension need only be thick enough to take pull forces because there's no tendency to buckle. In compression, pushing, the same conrod has to be strong enough to prevent buckling. Compressed rods are weakest at the centre point and least likely to buckle at the ends. So it makes sense to size the ends small for tension and the middle big to resist buckling forces.

Simple tapering seems most evident on big stationary steam engines probably because their conrods are relatively long thin affairs. Locomotive conrods are comparatively stubby but heavy, and their geometry may favour flatter conrods, in which case girder forms and weight reducing holes may be easier to make and stronger than a taper. IC engines are even stubbier. As smooth curves are good at reducing fatigue, I'd guess conrod design uses several ways of reducing weight whilst managing stress. Be interesting to FEM model a few different designs to see what the pretty colours say.


Edited By SillyOldDuffer on 14/07/2020 15:02:48

Russell Eberhardt14/07/2020 16:45:26
2595 forum posts
85 photos
Posted by Dave Smith 14 on 14/07/2020 14:33:19:

The other thing is they lighter, you remove mass from the point where you do not not want it. Buckling comes down to stiffness again for a given load which is controlled by the cross section as you say. In my racing days we did as much as we could to take the mass out of the pistons and the little end of conrods.


Yes. Many years ago I restored a vintage sports racing car that had the OHV push rods originally produced by a knitting needle manufacturer as they were experienced in making fish bellied rods!


DrDave14/07/2020 17:27:45
203 forum posts
43 photos

Two loads are of interest: compression due to steam pressure and accelerating the piston, etc. And transverse loads due to the mass of the con rod being accelerated up and down by the crankpin. So a classical pin ended strut with compression and transverse load.

The transverse load gives a bending moment in the rod, zero at both ends and maximum, not in the middle, but offset more towards the crank pin. Hence con rods normally being an I-beam (or H-beam on some car engines), often tapering towards the little end. For the loco rods, I dare say that asthetics and ease of machining dominate the design.


DrDave14/07/2020 17:33:18
203 forum posts
43 photos

To answer SoD, this is what happens when you let a Stress Engineer loose with a conrod...


JA14/07/2020 17:38:28
936 forum posts
51 photos

Considering railway locomotives, the connecting rods are long and thin compared with most IC engines (that is really stating the obvious). They are subjected to a number of loads, compression and tension along the rod and their own weight. At speed this weight (remember, equals mass x acceleration and acceleration increases with speed) may be very high. The rod would be a simply supported beam with a continuous load with the maximum deflection would half way along it. The deflection can be reduced by increasing its second moment of area (that is B x D^3 where D is the thickness of the rod in plane of motion and B is the thickness across the rod). Considering the mass of the rod is B x D (per unit length) thickening the rod where it is going to deflect the most has a major effect on the stiffness.

IC engine rods are rather different, because they are short, but I believe most fail in bending close to the little end.

I am sorry if the above is rambling.


Edited By JA on 14/07/2020 17:40:53

DrDave14/07/2020 18:53:15
203 forum posts
43 photos

I missed the obvious: tensile loads at the ends of a connecting rod will want to be in the outer fibres to align with the lugs. So you have two driving criteria. Buckling gives a I- beam that is fattest somewhere between the crankpin & centre of the rod, and tension requires the ends to be the same size as the lugs. Fair it all together & you get the average conrod.

As an aside, the title of the thread is interesting. Over thirty-something years, I have worked with scores of Stressmen, but I only recall two Stresswomen.

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