One for the thinkers?

This topic has 22 replies, 16 voices, and was last updated 30 December 2014 at 14:27 by nigel jones 5.

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27 December 2014 at 19:23 #173777
nigel jones 5
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@nigeljones5
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?
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27 December 2014 at 19:23 #7416
nigel jones 5
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@nigeljones5
27 December 2014 at 19:27 #173778
fastjohnrs
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@fastjohnrs
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
27 December 2014 at 19:38 #173779
S.D.L.
Participant
@s-d-l
Posted by fastjohnrs on 27/12/2014 19:27:31:

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

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
27 December 2014 at 19:49 #173781
KWIL
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@kwil
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
27 December 2014 at 20:27 #173786
Nobby
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@nobby
No
some body will come up with the reason
Nobby
27 December 2014 at 21:00 #173792
Michael Gilligan
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@michaelgilligan61133
Posted by fizzy on 27/12/2014 19:23:17:

Just musing through my bath time and asked myself this question –

.

So good you asked it twice ?

MichaelG.
27 December 2014 at 21:31 #173794
Gary Wooding
Participant
@garywooding25363
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".
27 December 2014 at 21:37 #173795
Bazyle
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@bazyle
Is "the same amount" 10 thou or the same as in % ?
27 December 2014 at 21:43 #173796
FMES
Participant
@fmes
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
27 December 2014 at 21:53 #173797
Ian P
Participant
@ianp
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
28 December 2014 at 09:43 #173802
nigel jones 5
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@nigeljones5
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.
28 December 2014 at 10:02 #173804
Michael Gilligan
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@michaelgilligan61133
Posted by fizzy on 28/12/2014 09:43:12:

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

.

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.
28 December 2014 at 10:33 #173806
Chris Trice
Participant
@christrice43267
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.
28 December 2014 at 10:39 #173808
Chris Trice
Participant
@christrice43267
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
28 December 2014 at 13:52 #173816
Speedy Builder5
Participant
@speedybuilder5
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
28 December 2014 at 14:01 #173819
Ian P
Participant
@ianp
Posted by Speedy Builder5 on 28/12/2014 13:52:02:

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
28 December 2014 at 19:38 #173835
V8Eng
Participant
@v8eng
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
28 December 2014 at 21:52 #173843
John Stevenson 1
Participant
@johnstevenson1
Posted by fizzy on 27/12/2014 19:23:17:

Just musing through my bath time and asked myself this question

.

Did you remove the coal first ?
29 December 2014 at 09:01 #173851
V8Eng
Participant
@v8eng
Erratum.

My last post should have read 'deep knocking noise' rather than 'rumbling' .
29 December 2014 at 09:34 #173858
Hopper
Participant
@hopper
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.
30 December 2014 at 14:21 #173965
Neil Wyatt
Moderator
@neilwyatt
So you've turned your crankpin 10 thou undersize and you want an excuse not to rebush the rod……?

Neil
30 December 2014 at 14:27 #173968
nigel jones 5
Participant
@nigeljones5
Hi Neil – actually no, I was honestly pondering this as described. The lead up to said muse was that I have just reduced the link pins on my 71/4 Invicta from 5/16 down to 3/16 and wondered if they would me more or less likely to knock when they become worn. Cynic !!
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