Lotus Elan

Fit of Woodruff Key in Crankshaft

PostPost by: CBUEB1771 » Fri Feb 20, 2009 3:17 pm

Does anyone know the correct fit for the woodruff key in the nose of the crankshaft? I am starting the build of a 1700cc Twin Cam with a new SCAT steel crank. The key seat in the crank seems to be a few tenths under 0.1875", checked with gauge pins. The key, a new Ford item with the correct part number, is about 0.1884" wide. I have checked two of these keys and they are the same width. This gives an interference of just over 0.001" which is the maximum allowed by ANSI B17.1 1967 for a Class 2 fit. This standard allows up to a 0.002" clearance fit. I am having bad visions of damaging the crank by pressing the key in with the interference fit.
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PostPost by: neilsjuke » Sat Feb 21, 2009 3:05 pm

Put the key in the freezer over night.
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PostPost by: CBUEB1771 » Sun Feb 22, 2009 4:03 am

neilsjuke wrote:Put the key in the freezer over night.


Given a coefficient of thermal expansion in the range of 7X10-6 in/in/F I'll have to cool the key about 760 degrees F less than the crank temperature to get it to a slip fit. My freezer doesn't quite make that. I'll keep looking. :shock:
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PostPost by: GrUmPyBoDgEr » Sun Feb 22, 2009 8:57 am

Woodruff keys are generally made from quite soft steel.
A trained fitter would "stone" the key to achieve the required fit.
Why not give it a try?
Cheers
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PostPost by: CBUEB1771 » Sun Feb 22, 2009 2:10 pm

D.J.Pelly wrote:Woodruff keys are generally made from quite soft steel.
A trained fitter would "stone" the key to achieve the required fit.
Why not give it a try?
Cheers
John


John,
Quite right, I am trying to determine what the required fit is. This information is not provided in the workshop manual. Perhaps I'll find it in some of the Ford service information.
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PostPost by: rgh0 » Mon Feb 23, 2009 6:57 am

I have never measured the fit and generally the keys show some fretting wear when removed so hard to determine the original fit by reverse engineering measurements. I generally measure all the assembly fits but this one has always escaped me - up till now - and now you have me worried what it should be

All the keys I have removed have been a press fit in the keyway and required a drift to remove them so a .001 inch interference fit does not surprise me. Similarly I have always have had to press new keys in with a few light taps with a drift, nothing that ever gave me any concern about damaging the shaft when fitting.

I am sure Ford did not individually fettle each key in production so they must have chosen an appropriate fit and tolerances and lived with it, what it was is the question and are current keys and new cranks within that fit tolerance or not ??

Looking forward to see what you decide

cheers
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PostPost by: elansprint71 » Mon Feb 23, 2009 8:17 am

With my Mech Eng head on I'd say that a thou interference fit is right; I used to lightly file a very slight bevel around the curved edges just to ease fitting, really just removing the sharp edge with a Swiss file rather than anything more.

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Pete.
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PostPost by: denicholls2 » Mon Feb 23, 2009 9:26 pm

My personal experience is that tight on a key is good. When I dismantled my Fiat Twincam for rebuild, I found a heavily fretted key and widened hole. A few thousand more miles and the key would not have existed. That engine is interference as well. :shock:
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PostPost by: paddy » Mon Feb 23, 2009 9:30 pm

Out of interest, have you compared these dimensions with the slot in the crank sprocket/pulley ? That has to be a clearance fit, doesn't it?

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PostPost by: CBUEB1771 » Tue Feb 24, 2009 5:01 pm

paddy wrote:Out of interest, have you compared these dimensions with the slot in the crank sprocket/pulley ? That has to be a clearance fit, doesn't it?

Paddy


The key is a sliding fit in the keyway of the timing chain sprocket which is the expected condition. If there is anything wrong here it is the width of the seat in the SCAT crankshaft. I have to retrieve my original vee belt pulley to check that fit.
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PostPost by: elansprint71 » Tue Feb 24, 2009 9:59 pm

I did not think to mention this before, the function of the Woodruff key is not to provide drive, it is there for location and to allow the tightening of the components on the locking taper, i.e. a very shallow angled taper which locks together to provide the drive, as opposed to a steeper-angled locating taper, an extreme example of which is found on the knock-on Elan wheel.
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