Tag Archives: crank

Let’s try that again!

Sometimes you just have to face the fact that things are not going well – and lift the lid off the scrap bin and cast out that the bit you have been carefully crafting and toiling over, in the knowledge that it’s for the greater good ūüė¶

And so it was with the crank and main bearings…

The fabricated main bearings were just not a good solution, and the approach I had taken to machining them had resulted in dimensional errors that needed “uncouth” solutions (shimming¬†etc.) to get them right. Moreover the crank would not press up straight (even after peening the webs to remove post-press distortions.

After much soul-searching I concluded that I needed to bite the bullet and¬†re-cut the main bearing journals after assembling the crank – this means that the bearings would be oversize, so would need re-making. Having reached that point I decided that I should re-make them from a block of¬†solid gunmetal¬†(a 62mm square block 7-inches long, only ¬£160!) as the cast blocks from Camden were too small to fit the over-size housings that the base castings arrived from France with….

Cutting the main-bearing material in half, to fit it back together!

Cutting the main-bearing material in half, to fit it back together!

As always, coming¬†to these “scrap and start over” decisions is accompanied by much wringing of hands, but delivers a calmness that lets you sleep better….

Pictures and progress in following posts…

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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! ūüė¶

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

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 thhis would be mad!) or¬†fabrication from journals¬†and webs with a combination of shrink fits and loktite¬†(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 centered 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 headstock produce a wobble of about 1/2 a thou when running Рnot 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 Рthen 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 Рas 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 slighty 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. 1.5thou 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 locktited¬†(how DO you spell locktite) joints is that these can be slid into place, ensuring that all the journals align.
  • Lastly round the crankpin 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….

Main Bearings; scraping thru!

Major decisions on Main Bearings

The bearing housings in the base casting were already machined a 1/16th  oversize when I purchased the castings, so the Camden-provided bronze bearing castings were not going to work.

So I set about trying to source some suitable “phozzy-bronze” from my normal supplier, and discovered there was none to be had of a suitable size. So I decided to build a fabricated version of the bearings with Cast Iron outers and bronze bearing “shells” – some friends recommended going the whole hog and white-metalling the bearings (as per loco or car practice), but I decided my approach was less error-prone, and more in line with my “skills”!

Given the “oversizedness” of the slots I also decided to up the main bearings from 1 inch to 1.25 inch diameter (I think bigger is always better in the bearing stakes.)….

So I purchased some (very) oversized square-section continuously cast iron bar, and some 1.5 inch phosphor-bronze bar for less than half the price of the Camden blanks – and enabled my “smug mode” – (However, I think pride may well come before a fall!) ūüėź

At this point¬†I looked at the size (oversize) ¬†of the square cast iron bar I had, and thought about the fact I was about to machine away about 90% of it and then do the same thing by putting a 1.25 inch hole in a 7 inch length of 1.5 inch bronze, and decided this was mad… so I found some 2-inch round cast bar, and decided this could be machined with “square” section to fit in the bearing slots and this was a more “economic” solution…. 2 months later this seemed like a very, very foolish decision! (Time will tell)

So we start cutting

My plan was to silver solder the “shells” to the inside of the bar. So the plan was bore the cast bar,rough bore the bronze bar solder one inside the other….

Well that didn’t work! 3 attempts proved that I could not persuade the easyflow to penetrate and adhere to the cast, and I just wound up with messy flux-covered non-stuck thing!

Plan B – locktite the shells in to the housings (well that is what is holding them at the moment, but I am not convinced it’s a real fix, so they will get small grub screws fitted in axially at the end to prevent any “spin” (as this would cut the lubrication and be a fast route to a bad place!.

The process in pictures – this is really an outline of how NOT to tackle this!

A bit disappointing Рbut a great workout!

So I assemble the machined bearings into the base casting, and try the bar stock that will form the crank main journals and “bingo”, it’s all locked solid!

The traditional approach is to line-bore this kind of assembly, but it’s too bit for my workshop, and I had hoped that my strategy for building the individual bearing assemblies would mean line-boring was not required. – but it was not to be!

Deep gloom ensues….

However, the model engineering community provides an answer – Neil and Darren say “simple Mal, ¬†just scrape them true”…

A close shave!

So after a quick review of the theory, and armed with a tube of engineer’s blue, 20-30 hours of hard graft (this is THE most effective¬†bicept¬†exercise¬†ever)¬†and very bloody knuckles, we have a shaft that sits in, turns and does not wobble! RESULT!

What I should have done….

Start with the square bar, make it¬†accurately¬†rectangular to provide some reference surfaces, machine to a good fit in the base casting bearing slots, then bored this (in the mill and square), then fitted the bronze bits, then cut them in half, then finally bore the assemblies using the inherent squareness of the main bar….

Well it seems like a good idea!

Now on to the crank – and hereby hangs a further take