Small triple expansion engine

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Hello all
<p style=”text-align: center;”>Having not made anything for nearly a year I’m thinking of making a Triple expansion engine for a winter project my main problem is the crankshaft as it will be fabricated, I’ve made a double but always that slight wobble that takes ages to get true when the bearings are in place. Is there any information on how best to make it out of one piece of mild steel and made on a lathe, and can it be made for 7mm bearings</p>
Thanks Bob

[Nigel Graham 2Participant @nigelgraham2]

I don’t recall  ever seeing detail instructions for making a single-piece crankshaft, but I have made a two-throw one (cranks at right-angles to each other). The principle is the same.

To machine the pins you rotate the shaft between centres that coincide with the pins’ axes.

There are two approaches.

1) Turn a large, temporary boss on each end with the main centres and the pin centres, centre-drilled on those.

2) Make a pair of blocks both appropriately centre-drilled, to be aligned and secured on the shaft ends.

 

Mine was the second option because I could mill two faces on the square blocks at right angles to each other to act as data for setting out the centre-drillings, and for aligning them on the shaft, where they were secured by grub-screws, four per block (2 per face).

For a three-throw crankshaft, its cranks 120º apart, such blocks might be made from large-diameter hexagon-section bright mild-steel, giving automatic indexing against suitable angle-plates or Vee-blocks for drilling the pin-centres.

The key to success is aligning the centres true to radii and angles, end-for-end; and I could not ensure that on my crankshaft, about 13″ long, by Method 1.

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Turning the pins themselves requires something like a long, relatively thick parting-tool, set carefully radially to the lathe, and going very gently but steadily. For the finishing cuts the tool needs a tiny chamfer or rounding on both corners to leave little roots. Edge chamfers on the big-end bearings will accommodate those.

My crankshaft has 0.6″ dia pins on throws of 1″ (2-inch stroke), and I ran the lathe in back-gear at maybe 60-70rpm.

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Once you have turned each crank-pin and had a celebratory cup of tea (or …) you need prevent the webs springing inwards, for the next machining stages. I would make spacers beforehand, that will also be plug-gauges for the pin-lengths with a close sliding fit between the web faces- they must not be rattly or that defeats the object, though you could shim the gap. Nor tight enough to spring the webs part. Either fault will effectively bend the shaft.  These can be secured in place by wire or worm-drive hose-clips round the pin and spacer. Protect the pin surface with some soft packing, e.g. aluminium cut from a food can, or a strip cut from an expired bank-card.

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On diameters, the bearings between the crank-webs will of course all have to be of split form, and because you need make them anyway, of whatever the appropriate diameter for the engine. Full-size triple-expansion engines always had all-split journals anyway, not single-piece bearing bushes or ball-races, to allow construction and servicing.

Presumably too, at least one, or one pair if the engine has reversing gear, eccentric sheave will be between two cranks, so that / they would need be of split construction. The outer eccentrics are normally outside the cranks so can be of one-piece, as mine are.

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I recall seeing an exquisite two-throw (2-cylinder) crankshaft of about one-inch stroke, turned from solid, complete with the eccentrics. That would be a challenge, and would need the valve-gear designing very carefully, with no way to adjust the eccentric angles – as really, there should be no need to do anyway! Nor to replace worn eccentrics.

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I am sure I had photographs of this process but cannot find them. I seem to have lost a lot of photos in peculiar ways, suspiciously since Mickeysoft up-dated / down-graded my PC from Win10 to 11, causing me a lot of problems I have never previously encountered.  I even found the Seattle shower was randomly taking photos of mine into some strange Bing / MSN folder!

I will search elsewhere and I find suitable ones, will post them on this thread.

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Thanks for the information and time Nigel, very much appreciated.

Bob

[Speedy Builder5Participant @speedybuilder5]

I think this is what Nigel was explaining.  Note the big lump of lead to balance the shaft whilst turning it. The piece of EN8 bar weighed 46lbs before I started!

[JasonBModerator @jasonb]

At that sort ofg size I would probaly start with round bar, black EN1A would be my choice if it is just for display running, Black EN8 if it will be put to work. Face the ends and Ctr drill and then add a further set of 3 ctr drill holes at 120deg apart at each end making sure they are in line at each end.

Depending on the size of yor spidle bore and hole in teh 4-jaw it may be possibly to slowly increase the amount sticking out of the chuck to turn each section of shaft and the pins setting the appropriate ctr hole to run true and using that for tail stock support. Just do the final shaft diameter finishing cuts with it between ctrs. If it won’t fitt like that then between ctrs turning will have to be use dfor all of it.

To avoid the knocking you can mill out a lot of teh waste to leave say a 9mm square where each pin falls and as nigel says pack the spaces so you don’t compres sthem with the tailstock.

End of a 2 throw shaft with it’s ctr drilled positions being done.

Milling out the waste on a single throw crank

Same crankshaft having teh pin turned with an insert parting tool, middle of tip ground away to reduce chances of chatter

Packing wired &/or hot glued into place. This one has 8mm dia pins so 7mm should be possible

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