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slow rpm, high torque model steam

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Thomas Gude25/09/2013 17:47:46
106 forum posts
26 photos

Just wondering, is there a model steam engine one could make that had slow rpm and high torque? Classic view at show is to see a blur of crankshafts and connecting rods but (I assume) very little torque. Thinking of something that could mostly be made on an S7.


David Jupp25/09/2013 18:35:01
836 forum posts
17 photos

How about a scale 'River Don Engine' ? Go and see the original running at Kelham Island Museum, Sheffield.

Scaling it down will tend to make it (or any other engine) end up running much faster, with less torque - laws of physics apply even to models.

Bazyle25/09/2013 18:35:38
6324 forum posts
222 photos

Probably a Beam Engine would be best for your slow speed requirement but not intended to do work in model form.

If you are after a workhorse that goes slowly to avoid gearing then torque depends on the size of your cylinders relative to the steam pressure. You would be advised ot use 2 or more cylinders to avoid needing a big flywheel and to produce more even power cycles.

Stub Mandrel25/09/2013 20:45:43
4315 forum posts
291 photos
1 articles

If it's slow running, rather than the torque that matters to you, then as long as the flywheel is heavy enough, the answer is don't put so much bloomin' air in. Mill engines typically ran at about 60 rpm.

Norden will run at less than 60 rpm. Just make sure that the o-rings/packing on seals and piston are not too tight, and that all bearings are free - you should be able to flick the flywheel and the engine will turn over a few times. This might not be ideal for a hard working engine, but for an exhibition model where slow running on a few PSI of air or steam is the aim, it's OK. With more steam, expect a few leaks (just like the real thing) or tighten up those glands!


Edited By Stub Mandrel on 25/09/2013 21:07:38

Ady126/09/2013 00:10:50
5092 forum posts
736 photos

Torque comes from long crankshafts

Steam efficiency is is only what? 15%

jason udall26/09/2013 00:19:14
2031 forum posts
41 photos

as has been said.. more flywheel and turn down the gas...

but torque derives from working presure ,bore, crank diameter.thus stroke .

small will always need to buzz to have any torque

".Its worse than that. Its physics Jim"

Thomas Gude26/09/2013 14:56:50
106 forum posts
26 photos

Ah, physics, that old chestnut. Fair enough. Would adding exess weight to anything other than the flywheel make a difference?

Stub Mandrel26/09/2013 17:18:41
4315 forum posts
291 photos
1 articles

A gram at the rim of the flywheel will have about as much effect as an ounce on the crankshaft, so it's not really worth adding mass anywhere else.


DerryUK26/09/2013 17:22:50
125 forum posts

I presume you've used a torque wrench Thomas? How would you increase torque with one of those ?

First apply more force (muscle) to the end of the wrench. To do that with a steam engine you would up the steam pressure and increase the area (diameter) of the piston.

How else can you increase torque with a wrench? Double the length of the wrench. In a steam engine that equates to a long stroke.

So in your high torque engine you want high pressure, large diameter bore and long stroke.

If you think about it, with a steam engine you can get maximum torque with the engine stationary, all you need is for the angle between the con-rod and crankshaft to be 90deg.

But like most things in life steam engines are a compromise.

Ian S C27/09/2013 11:49:38
7468 forum posts
230 photos

Thomas, you can gain a little extra flywheel effect with balance weights on the crank. Even without the balance weights on the crank shafts of a number of my hot air engines, these motors will run without a flywheel, but only at high speed, and low torque.

The Stuart S9 that I rebuilt earlier in the year would run steadily at about 40rpm, on less than 10psi, but not much torque. The engine has cast iron rings, and Teflon plumbers tape in the stuffing boxes.

The engine is now on display in the front window of the local museum, after being displayed for a month at the library. Ian S C

Crocadillopig27/09/2013 12:10:54
30 forum posts
4 photos

There is always the easy way if you just want to sit back and enjoy the beauty of the motion, I have attached a 220v 5 rev/min electric motor disguised as a generator/gearbox to the back end of my Stuart Triple that sits in the lounge in pride of place.


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