Tag Archives: crank failure

When a broken crank is good news!

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Well we got Befur’s engine back into the workshop, and stripped it down to see if we could locate the source of the knocking we have been suffering all season.

Befur’s Engine ready to be stripped.

On lifting the crank out it looked perfect, and did not have any obvious loose or moving joints My heart sank, as this was really my only theory on what was wrong. Continue reading →

Straigtening a bent crank

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DO NOT FOLLOW THIS APPROACH – Loctite and crankshafts are not a good mix. Either carve/cast from solid or shrink it all together with larger webs.

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)

Pressing,Peening and Glueing – a crankshaft for Befur

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DO NOT BUILD A CRANK THIS WAY – MINE FAILED IN 20 MINUTES!!!!

See this post on the 2nd crank, for a better approach!!!!

The theory

Reading the notes for the engine a number of approaches to building the crankshaft are suggested. Forged Blanks (would need to buy a steam hammer), turn from solid (I’ve done that for a model car engine, but the waste with a 1/2 inch stroke was too much, so this would be mad!) or fabrication from journals and webs with a combination of shrink fits and Loctite (with taper pins for security.)

I opted for the latter as I have had previously good experience with  heat shrinking the wheels on the loco, and metal adhesive technology has advanced a deal in the last 10 or so years.

The round bits

Turning the journals with lots of suds, between centres ensures everything is really square

So the game plan is to start by  preparing all the “round” bits (crank pins and journals) with shouldered ends – no great problems here, just centre drill the ends and then do all the work between centres. (Used EN24T for the journals, and had some stock provided in the casting box for the crank pins – this looks suspiciously like stainless, so I am a little concerned about the wear on this, but time will tell)…

To be really sure you also skim the headstock centre, (with it in place in the headstock) to make sure it runs straight, and check the alignment of head and tail stock centers (stick something of good dimensional quality (silver steel bar) between the centres, and then run a dial gauge along the side, to make sure there are no big errors…

This should ensure everything is nicely central and square. I checked the run-out on the shaft before starting to make sure this was OK,(I had centred the bar OK)  and had run-out of about a thou or so (and this it about as good as it gets on the Harrison, as the Timken taper-roller bearings in the head-stock produce a wobble of about 1/2 a thou when running – not slack but some sun & planet effect as the bearings precess?).

No real problems, spend some time miking the journals and making notes, so you have the data for making the holes in the webs.

Then I mounted two v-blocks on the milling table, locating them by clamping a large bar into them, clocking this true, then clamping the blocks to the table and removing the setting bar. Once this was done then I could mount the crank journals in place and machine the woodruff key ways –  here’s a couple of pics of this bit…

large bar ensures the blocks are square, then the dial gauge ensures they are square on the table – then you can clamp the blocks in place and remove the setting bar.

now the blocks are clamped in place the crank journal can be clamped in the blocks, and you know it’s square – a slot drill finishes the job..

The Webs

These were milled (shaped) from some over-size mild steel bar – as follows:

  •  First reduce stock to size (see vid),
  • face one end of each piece to provide a datum,
  • then clamp, drill and pin into pairs ( I learnt my lesson on the crank pins on the loco – any inaccuracy is magnified by two as the crank turns!) – the theory is if you then jig-bore them, the pairs MUST come out matched. If the two pairs are not identical then this is not a problem, the stroke on one cylinder will just be slightly longer than the other, HOWEVER if the two in a pair are not matched, then a bent crank is an absolute certainty!
  • Then you can bore the holes. 1.5-thou undersize of the shrink fit journals, and 1 thou over for the loctited ones… in case you haven’t figured it out, the reason for the Loctited joints is that these can be slid into place, ensuring that all the journals align.
  • Lastly round the crank-pin ends of the journals (the other end get balance weights later).

milling the rounded ends with a rotary table – having set everything up with DRO (god’s gift of a thing)

Assembling the thing

….actually this did not turn out to be the simple process I envisaged… so I think another post is in order for this bit, as I learn a bunch of new stuff in the process….