One for the thinkers?

Just musing through my bath time and asked myself this question - if all things are equal would a 1/4" diameter big end (crank pin) which was 10 thou under size knock more, less or the same as a 1 inch dia crank pin, worn to the same amount?

If they have the same amount of play I'd imagine they'd knock the same, unless the difference in weight of components has a part to play, be interesting to see what other people think

Edited By JasonB on 28/12/2014 07:25:54

For any class of fit the amount of clearance or interference increases with diameter so what may be sloppy on a small shaft would be proportionally tighter on a bigger shaft.

Steve.

Edited By JasonB on 28/12/2014 07:25:30

How are you defining "knock"? Loudness or frequency of the knock? Size of the component parts will change everything.

Edited By JasonB on 28/12/2014 07:25:07

No
some body will come up with the reason
Nobby

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So good you asked it twice ?

MichaelG.

Well, how about a 1/10" crank pin vs a 10" crank pin?

I reckon the 1/10" would knock a lot more than the 10".

Is "the same amount" 10 thou or the same as in % ?

Could it be considered that the smaller crank pin would be fitted to an engine that ran at a much higher speed, the knock rate (frequency) would possibly be above the hearing range of the human ear, and therefore inaudible or masked by the equally annoying exhaust note

Which of the 'All Things' you had in mind, are equal?

Maybe, radial clearance, 10 thou on both sizes or maybe the same percentage diameter reduction, (or increase if the bore is worn)

I am sure you have touched on a very interesting subject but the problem needs to be better defined so that we don't wait until 1/4/2014 before replying.

I think showers are better than baths, thinking time is not as long so there is less time for the ideas to be generated

Ian P

Like I said, all things equal, same speed, 10 thou under size on each pin.

I used to shower but 18 month olds don't like them as the duck doesn't float.

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Simple geometry; the smaller version will 'rattle' more.

For demonstration [rather than proof] extrapolate to even larger/smaller versions.

At very small sizes the fit will look "like a sausage up the Mersey Tunnel".

MichaelG.

The smaller would knock more because the angle the crankshaft will have travelled through at the top and bottom of each stroke would be greater and there would be a higher contrast of speed in the up and down direction betweent the the point the two components stopped touching each other and the moment the clearance was taken up again.

That's assuming that the stroke and proportions of the engine were proportionally matched to the crankpin diameter  but the clearance was the same in both cases.

Edited By Chris Trice on 28/12/2014 11:09:23

Depends upon the temperature and thickness of oil and probably Reynolds number. The larger would have more anular area to squeeze out the lubricant. - Probably

BobH

But Fizzy said all things were equal.

To get equal conditions maybe the 1" diameter journal would be correspondingly narrower than the 1/4" one so that lubricant trapped in the annular area would be the same.

Simplest would be to assume no lubricant at all.

Ian P

I must say that I'm not sure exactly how relevant this is, but here goes anyway:

I have always understood that a worn big end on an engine makes a deep rumbling noise, whilst a worn little end makes a lighter rattling type noise.

Edited By V8Eng on 28/12/2014 19:39:28

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Did you remove the coal first ?

Erratum.

My last post should have read 'deep knocking noise' rather than 'rumbling' .

LOL. I can tell you from experience that a Harley Davidson with its 1" crankpin worn just three thou undersize will make a knock to wake the devil. NO little model engine with a 1/4" crankpin could even come close to making the same noise. A ten thou undersize crankpin on a Harley engine would be deafening.

Why? Because each Harley con rod is hammering about 35 horsepower to the crankpin, compared with 3/5 of 5/8 of a gnat's whiskerpower being delivered to said 1/4" model crankpin.

Apart from that, try this: Bang together two large lumps of steel weighing several pounds each and listen to the clang. Now bang together two 5/16" nuts and compare the volume of the collision.