Straigtening a bent crank

The previous posts discussed the manufacture of the webs and journals, and I felt very confident of the approach. Half the joints were to be shrunk together (very cold journals, warm webs, and a press (large vice)) and the others were to be loctited in place.

Here we see one part of the crank assembly, clamped in v-blocks while the loctite sets – guaranteed straightness (you would think!)

The theory being that the press/shrink fits are bound to be square (as the holes and journals already are, and there is no spare space for out-of-aligned-ness), and then the loctite joints are assembled with the journals fitted into V-blocks, thus ensuring everything was straight and true … how wrong can you be?

The press fits went together “OK”, but were too long to be pressed together in the bench vise so an Edwardian sash-cramp was “pressed” into service :-). Then loctite and v-blocks and Bingo!, it all comes out bent! 😦

Out with the engineer’s square and dial-gauge and we discover that the press fits are not square… Much head scratching, and a few tentative (and then very hard) clouts with rubber mallet prove that it’s all MUCH to solid to bounce back into shape.

(The current theory on how this happened, is that the sash-cramp was not really square, and the shoulders on the webs were too narrow and deformed under the stress of the shrink fit and pressing together. However it might have been that the webs were milled from oversized stock, and I have seen loco coupling rods bend like a banana from the retained internal stress which becomes unbalanced as material is removed – no matter what it was BENT!)

So I google “crank straightening” and discover “peening”. The idea being that by hammering (with ball-peen hammer (always wondered where the name came from)) the surface of the bent web you release surface stresses (as a result of stretching the surface with the hammering) and the web bends towards you.

A little peening to straighten the crank web – before it all went a bit Pete Tong

Well it almost worked, I cheated and used a pneumatic chisel with a domed tool in it, and as predicted the dial gauge confirmed that everything was coming back into shape. With a run-out of less than about 3 thou I was feeling very positive. I figured I would give it one more treatment and get it “bang-on”. But I seemed to over-cook it and it was bent the other way (quite a lot), so I set too on the other side, to bring it back, but it just seemed to get worse and worse (starting to feel stressed now!)… after a few more attempts I realised what had happened was that I had managed to break one of the loctite joints and the error I was trying to correct be peening web #1 was actually as a result of a joint on web #3 moving – by the time I realised this I was in a right two-and-eight!

Not looking good

It was now clear we were in serious trouble, so I heated the whole thing up until the Loctite let go, pull the pins from the webs, clean everything up and start again.

I really thought this time it would go better, but no, runnout on the journals when spun between centres was about 20-thou, and while you could get the crank into the bearings it was stiff to turn, and I figured it would just murder them over time, so it’s time to give up!

and the next post explains how we progressed… (let’s try that again)


One thought on “Straigtening a bent crank

  1. Malcolm Duckett

    Further thought and discussion suggests the real issue is that there is not enough meat on the webs (as drawn) to withstand the shrink fit stresses, and so they distort as they cool….. (should post somthing in the hints and tips section….


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