Bent Crank

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Model T Ford Forum: Forum 2013: Bent Crank
Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman Miller on Tuesday, January 01, 2013 - 02:17 pm:

My crank is bent.005 off center (.010 when rotating with dial indicator) I tryed pressing it straight and after flexing it almost .300 it just springs back to where I start. How far must I press it?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Thode Chehalis Washington on Tuesday, January 01, 2013 - 03:00 pm:

Norman,
Here is an old thread. Looks like it may take up to 1/2". Really, just work up to whatever it takes.
http://www.mtfca.com/discus/messages/118802/157680.html?1283285701


Be sure to have it checked for cracks after it is straight and make sure the flywheel flange is still true.

Jim


Top of pagePrevious messageNext messageBottom of page Link to this message  By joe bell on Tuesday, January 01, 2013 - 03:01 pm:

Norman, How do you know the crank is bent? Are you indicating off the rear flange or the journal? If your indicating off the rear journal it probably is wore on one side, something that is real common. Hope this helps. Joe


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman Miller on Tuesday, January 01, 2013 - 04:24 pm:

I have crank on a precision balance stand sitting on the first and third mains. Digital dial indicator set on middle main


Top of pagePrevious messageNext messageBottom of page Link to this message  By joe bell on Tuesday, January 01, 2013 - 04:37 pm:

Norman, set it in a lathe and use the centers and then measure it, the mains on an older crank may measure close but can be out of round. To true up some older cranks sometimes .020 has to bee ground off to get them back on centers. Check the rear flange when you have the crank on centers to see if the flange is true. I have found many that when grinding the crank it needs to be set up on outside of flange instead of the center hole. Hope this helps, Joe


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Cascisa - Poulsbo, Washington on Tuesday, January 01, 2013 - 04:47 pm:

Norman,

I have found that this works :

Bend it until you cringe;
Then until it scares you;
Then a half a turn more.

You will be amazed at how far past center you will have to go before it starts to move.

Be_Zero_Be in Leesburg, VA


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Bamford, Edmonton AB on Tuesday, January 01, 2013 - 07:33 pm:

Norman my '26 Touring crank was bent a similar amount.

Working with info gleaned off the forum, I rigged up a straightening jig using a junk block and three main bearing caps. First photo show the block upside down in my 20-ton press with the crank supported at each end by upside-down front and rear caps. Second photo shows details of my jury-rigged setup to transfer ram pressure to the centre main.

As you can see in the first photo, the crank is deflected 0.320" off straight! Sounds like a lot but this much was necessary to effect any permanent change.

I worked up to this amount of deflection, checking the crank each time in the lathe, with the flange in the chuck and the front in a centre. Final result was total runout of 0.0005", perfect in my books.




Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Shirley on Tuesday, January 01, 2013 - 08:05 pm:

Chris, I feel that Henry would be proud of you. Nice work.


Top of pagePrevious messageNext messageBottom of page Link to this message  By J and M Machine Co Inc on Wednesday, January 02, 2013 - 10:25 am:

Norman:
It's not so much that you must bend it as bending really doesn't work as efficiently in conjunction with peening the crank.
We do the work on a straightening press and "peening part is using a radiused chisel to shock the crank while under load back into concentricity.I don't really put much force on the crank to get it straight no more than 500lbs.

It's not just the center main you have to straighten as once you do that journal you have to check the front crank snout and the rear flange. Check those and then go back to the middle and make sure it's straight. It's not an easy process as I liken straightening a model T crank to a wet piece of Spaghetti.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Joseph Magedanz on Wednesday, January 02, 2013 - 11:01 am:

J and M Machine,
Do you happen to have a video of that process?
I would be interesting to see.
Thanks.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les Schubert on Wednesday, January 02, 2013 - 11:32 am:

I have often pondered the bent T crankshaft thing. I have observed it only takes a few pounds of push to get the bend reading out of the crank (get the dial indicator to read zero). Of course as soon as you let go it springs back. Still it seemed to me that the extra bearing load caused by this didn't amount to much. In the end I have assembled a couple of engines with cranks like Norman has described with out doing any straightening. One engine is still running fine after many thousands of miles with no bearing troubles. The other engine had to be disassembled for other reasons (ultimately deciding to replace the valves and not owning the equipment I took it to a shop). Here is the interesting part. I decided to recheck the crank. It was now perfectly straight after being driven for about 1000 miles.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Anthonie Boer on Wednesday, January 02, 2013 - 12:16 pm:

Can someone explain me, why most of the cranks are bend in the middle ? In the engine the middle bearing is holding the crankshaft on his place .And can only move the play in the bearing .
Toon


Top of pagePrevious messageNext messageBottom of page Link to this message  By lorenzo leon on Wednesday, January 02, 2013 - 03:05 pm:

I have crank that had a mild bent awhile back ,i tied a wire at the pulley hole and hung it to a beam and [awhile back year or so] i was cleaning house took it doun cheked it and it was straight.
and i used it ,i think they bend a little if stood on the floor on its flange too long
or just come back demselves ?dont know


Top of pagePrevious messageNext messageBottom of page Link to this message  By keith g barrier on Wednesday, January 02, 2013 - 04:06 pm:

Les, I'm with you on this. The shaft in my 24 was bent and after a few thousand miles I pulled it back down to install aluminum pistons and it was straight. Reckon it healed up. KB


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Thode Chehalis Washington on Wednesday, January 02, 2013 - 07:40 pm:

Here is a thought. I roughly calculated that a temperature difference from one side of the crank to the other of 0.01 Deg F will cause a bow in the crank of 0.005". This temperature difference could easily be caused by a overhead light or by touching with a finger. Even with no external temperature influences it may take a very long time for all the steel in the crank to be at the very same temperature.

The other thing that was mentioned is that it only takes very little force to flex a "bent" crank so it is straight. And this force is likely very small compared to the normal loads during engine operation. I'm thinking that at least for the very flimsy T crank that an allowable bend in the crank would be much larger then the allowable bend in a larger modern crank. Does anyone have anything that shows the allowable bend in a T crank?

Jim


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