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Con-rod design question

Why thicker in the middle?

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Tim Stevens17/12/2015 14:52:45
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Many stationary steam engines and some mobile ones have a connecting rod which bulges out in the middle. This makes no sense to me.

If the rod is treated as a column it would be lighter (and so better) if the rod had the same section throughout. But it is in reality a column which is flapping about, so extra mass in the centre is not just too heavy, it also promotes bending. Just what you don't want.

Were the designers of such machines quite ignorant of ordinary principles?

Cheers, Tim

JasonB17/12/2015 16:13:44
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They just had a good eye and liked the asthetics, I do believe the Greeks had a "Bulge" in their columns too.

Michael Gilligan17/12/2015 17:04:56
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Posted by JasonB on 17/12/2015 16:13:44:

... I do believe the Greeks had a "Bulge" in their columns too.

.

That's entasis, Jason, and typically very subtle

A very interesting question, Tim !!

MichaelG.

Tim Stevens17/12/2015 17:07:04
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The Greeks, who had little regard for science, were interested in the appearance of the columns, and they had them tapered, but with an increasing taper as they went upwards. Not a bulge, then, but nearly parallel at the base, more tapered at the top. And intended, so we are told, to look right (which in all credit to the Greeks, they do). Entasis is the name for this sort of tapering.

And anyway, their columns were stationary. I have no problem with bulges etc on stationary columns, such as those holding the plates of clocks apart, or supporting the headwork of Victorian mill engines etc.

Regards, Tim

Neil Wyatt17/12/2015 17:10:47
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If you try and bend a rod, where does it bend?

The greatest stress is at the middle. You can reduce the amount of metal at the ends without weakening it.

Neil

Geoff G17/12/2015 17:15:35
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I'm new to the forum, but happy to reveal how my memory has faded over the 50-odd years since I went to college.

In response to Tim's enquiry, I don't think that the old guys were short of knowledge of the basic principles, more lacking the better techniques and materials that we have today. Yes, the bulge in the middle of the long con-rods does add weight just where you don't want it, but it also provides increased resistance to the bending forces created by the acceleration / deceleration caused by swinging one way then the other. Now for the hazy memory bit: I seem to recall that the bending strength increases in proportion to the cube of the depth of the section, so that the improvement in the stiffness of the rod is much greater than the addditional loading imposed by the weight of the bulge.

For an extreme example, look at the Sydney Harbour Bridge. It works rather better with that wonderful arch than it would as a flat slab, despite the considerable weight of all that superstructure!

And yes, Jason, the bellying of the rods does look much better than a straight con-rod, but, unlike the entasis feature of Greek columns, I think that aesthetics was not the reason uppermost in the minds of the old engineers when arriving at their solution to a problem.

Regards to all,

Geoff G

Michael Gilligan17/12/2015 18:50:52
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Posted by Neil Wyatt on 17/12/2015 17:10:47:

If you try and bend a rod, where does it bend?

The greatest stress is at the middle. You can reduce the amount of metal at the ends without weakening it.

.

I don't 'have the maths' to offer any calculations, but: Consider a good palette knife [or a rapier, or a fishing rod, etc.] ... these are tapered from the 'fixed' end to the tip, so that the bending is nicely distributed along the blade. The fish-bellied connecting rod is rather like a pair of these joined at the hilt.

MichaelG.

dont know

Interestingly: a strand of dry spaghetti [i.e. a brittle, slender rod] will usually break into three pieces when bent.

Robert Dodds17/12/2015 19:29:09
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As with Geoff G, my college recollections from 50+ years ago are sometimes hazy but I do remember considering the con rod as a compound pendulum and the effects of transferring some of the mass away from the big end of the con rod to influence the rotational force arising from the radial acceleration of the whole rod.
It was thought to be a contributor to piston slap, particularly troublesome in petrol engines.
Our early engineering forebears may not have had the technological tools available today like finite element analysis but they did not lack in persistence and thorough testing procedures to try out their new ideas.

Bob D

JasonB17/12/2015 19:58:17
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Thanks for teh Link Michael, I did not know it had a name but at least I was correct in saying they had a bulge (convex) shape and not a taper

Tim Stevens17/12/2015 20:08:02
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I stand by what I said about the shape of Greek columns. A cylinder which gets smaller in diameter from one end to the other is tapered. Any 'bulge' exists at the larger end, and the central part of a Greek column (ie the part where the rods I described do indeed bulge) is in every place smaller than the large end. But let's not fall out about it, eh?

Tim

steve de2417/12/2015 21:25:02
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Here is my explanation. The ends of a con-rod are pin jointed by the big end and crosshead joints. When the con-rod is about to buckle under compressive load it will be bending into a half sine wave. Bending stress is proportional to rate of curvature which is zero at the pin joints (they are hinges and can't transmit bending moment) and max in the middle of the con-rod. So the rod section can be reduced at either end with no loss of strength - but it has to be large enough to carry the compressive load. Steve.

Andrew Johnston17/12/2015 21:58:47
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Posted by Tim Stevens on 17/12/2015 14:52:45:

Were the designers of such machines quite ignorant of ordinary principles?

Rather harsh methinks...............

I agree with 'steve de24', it's most likely all to do with buckling. A connecting rod is fairly slender and when in compression is more likely to fail by buckling rather than exceeding the compressive strength of the material. The connecting rod is also subject to some radial forces, which will promote buckling. I'd say that the centre of the rod is thickened to increase the resistance to bending, and hence buckling, although of course it amounts to the same thing as stated by 'steve de24'.

Andrew

Neil Wyatt18/12/2015 08:07:46
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Locos also often have fish-bellied rods - now you want them to be as light as possible, especially on an express. The fish-belly is no more decorative than the fluting. Indeed many stationery engine had bellied and fluted rods to maximise strength while reducing weight.

Neil

> Interestingly: a strand of dry spaghetti [i.e. a brittle, slender rod] will usually break into three pieces when bent.

It breaks in one place, but then the whiplash effect causes a second break.

Michael Gilligan18/12/2015 08:56:56
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Posted by Neil Wyatt on 18/12/2015 08:07:46:

> Interestingly: a strand of dry spaghetti [i.e. a brittle, slender rod] will usually break into three pieces when bent.

It breaks in one place, but then the whiplash effect causes a second break.

.

Ah, but ... The interestiing part is the location of that first break -- hence my mention of palette knives, etc.

MichaelG.

P.S. ... Just found this video -- Feynman would have been impressed. star

JA18/12/2015 09:23:54
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I went to sleep last night thinking about this and, as above, came to the conclusion that the compressive load, from steam/gas pressure and the acceleration/deceleration of the piston, on the conrod was the major factor. The rod, if long, will fail as a strut. If the engine speed is low bellying of the rod is adequate. However when the speed is increased the angular acceleration of the rod around the gudgeon pin will effectively increase the weight of the rod. The rod is also a simple beam with a built in weight (more correct to say that than mass) and will bend due to the angular acceleration. The last thing you want with a strut. To prevent the rod from collapsing as a strut at "high" speeds the second moment of area needs to be increased and the cross sectional area decreased. Therefore designs such as fluting the rod are used instead of bellying.

On real high speed engines, conventional IC engines, the strut problem disappears since the rod is very short but the bending loads increase due to the very high angular accelerations. These rods are very heavily fluted.

I hope this makes sense.

JA

Michael Gilligan18/12/2015 09:38:36
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Introducing an Astronomical note to this discussion devil

It's worth looking at the engineering of Transit Instruments, etc.

**LINK**

Although essentially static; they are built to minimise sag, and the "shaft" is made stiffer in the middle [admittedly, the cones are hollow, so the weight increase is not proportional].

Yes; the loadings are applied differently to those in the con-rod, but the bending effect is comparable.

MichaelG.

.

P.S. ... Is there a Finite Element Modeller in the House question

Neil Wyatt18/12/2015 10:06:12
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So 'whiplash', but in a more subtle sense the whipped rebound of the broken part creating a tighter bend rather than one going the other way.

We can now calculate the speed of sound in spaghetti.

Neil

Martin Kyte18/12/2015 11:00:08
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My take on this 'in words' is that connecting rods like columns fail by buckling under compression. In this type of failure bucking increases the bending stress on the shaft which in turn causes increased bucking. There is positive feedback in this mode of failure. The ends of the shaft or rod are constrained by the bearings and can be of smaller cross section. The portion in the middle is made stiffer by having a larger cross section. Therefore the weight is minimised and the stiffness is sufficient to resist bucking. I strongly suspect the bending moment is greatest at the centre but I have yet to find the maths for that. If you look at many beam engines the connecting or pump rods are of a X cros section as well as being fish bellied which reduces the weight whilst improving the stiffness.

regards Martin

Fowlers Fury18/12/2015 14:19:39
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Not perhaps a response to the original post, but there's an interesting section in The Engineers' Pocket Book of 1943 covering the design aspects of steam engines. (A fascinating little book despite its age & useful for diagrams of locomotive ancillaries such as lubricators, vacuum pumps etc).

In the part covering connecting rods with its formulae for determining rod diameter, it states "The inertia bending effect is covered by the rather greater factor of safety and the taper of the rod". From which, I suppose, it might be inferred that a fish-bellied rod having two tapers, is stronger ~ as Martin Kyle comments.

con rod_1.jpg

Gordon W18/12/2015 14:31:25
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Just to liven you up a bit more- If you watch a long con-rod, such as a stationary steam engine or the rod on a knife bar mower, you will see two nodes. These are at 1/3 and 2/3 rds along the length, due to combination of bending and compression loads. I have seen this using a strobe light. This does not happen with short I/C rods.

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