Scale gear DP for a 1/12 scale beam engine

[stevensons rocketParticipant @stevensons-rocket]

I’m half way through making a Stuart and Turner beam engine. I’m thinking of adding gearing and pulley reduction to ‘drive’ a faux dynamo, in practice a small geared motor to drive the engine. 2 questions:-

1) What is a typical showman’s engine dynamo speed. As seen at a steam fair, not max! I’m thinking a typical beam engine speed would have been around 30RPM?

2) What DP (or module) would look right on the gears? I think the Stuart model scale is about 1″

[JasonBModerator @jasonb]

The Stuart one is generally regarded a sbeing based on quite a small beam engine which would have had a flywheel in the region of 5ft so that makes it around 1/8″ scale.

The slightly larger ME Beam used 32DP gears so I would say 40DP or 0.5MOD would look about right.

A showmans engine will be generating at a fair speed as the belt comes off the large flywheel and runs on quite a small pulley so you have a step up of at least 10times. So around 750rpm at the dynamo is quite likely.

If you are just wanting to turn the engine over for show then a small cheap gear head motor and electric speed control could be hidden in teh base and just use a friction drive against the flywheel. Either set it into the provided cast iron box bed or chop that down to about 1″ thick so the flywheel can be let into a base and that makes it easier to hide the gubbins.

Mine is cut down but no motor

[stevensons rocket]

On 16 February 2026 at 16:14 stevensons rocket Said:

I’m thinking of adding gearing and pulley reduction…

As you mention a pulley, is there a belt involved?

As noted above, a dynamo would normally have a small pulley, and the beam engine might present a relatively large resistance to its driver. The friction between drive pulley and belt might not be enough at the scale used and (slow) speed required.

In the picture above, it could be like trying to drive the whole engine by turning the governor.

[BazyleParticipant @bazyle]

Gears wouldn’t look right. You are not going to drive a dynamo through noisy gears in full size. I think a set-up like Jason’s with a hidden motor, rubber sleeve against the flywheel, would be best with a dummy but looking right dynamo run off the engine’s smaller pulley. For show you want it to run slow so people can try to work out how the valve is moving relative to the piston etc.

[Michael GilliganParticipant @michaelgilligan61133]

For general encouragement …

MichaelG.

.

Edit: __ Have a look at small Poly-V belts … they should suit your application well, and there’s no need to groove the flywheel [just groove the small dynamo motor pulley]

[Michael GilliganParticipant @michaelgilligan61133]

Just because it’s an interesting problem … I asked ChatGPT for some guidance:

Here’s the key snippet

.

.

MichaelG.

[ back to bed now ]

[Bazyle]

On Bazyle Said:

Gears wouldn’t look right.

I think they could look right.

The OP mentions both gears and pulleys. I also mentioned the gear size used on the popular ME Beam engine which has a shaft driven at a higher speed than the crankshaft via 4.5:1 gearing. Plenty have built the ME beam and or it’s earlier incarnations and I have never seen a comment that the gears look wrong.

This gearing could be used as the first stage then just like the ME Beam which has a pulley on the end of that geared up shaft. The second stage would be from the pulley to the motor/dynamo. The gearing would mean you do not need such a small dia pulley on the motor so will get better belt grip.

If you went with a gear head motor then it would even be possible to drive the 2nd shaft pulley 1:1 and have the engine turn over at 30rpm or less depending on the chosen ratio of the gear head motor.

Sure it would have been unlikely to see a beam engine generating but it is a reasonable way to hide a motor to drive the engine for display which is something that was not done in full size and the Stuart beam would hardly scale up well if you were to make it with the same proportions at “full size”

[SillyOldDufferModerator @sillyoldduffer]

Never say never, but in full-size beam engines weren’t used to drive dynamos because the RPM are too mismatched,  Beam engines turn slowly, perhaps 30rpm, whilst dynamos work best at high-speed, 3000rpm or more.  It’s an inefficient combination. better alternatives available.

If a model dynamo is required to generate significant power, then a 100x step-up is needed, eek.   Not a step-down. Belts and gears both struggle to deliver a 100x ratio, so the answer is a faster engine – higher rpm.

Traction engines manage about 200rpm at the flywheel, which is geared up by a belt 4 or 5x to spin the dynamo at about +1000rpm.  (Large diameter to small dynamo pulley.)

Traction engines are too slow to generate electricity efficiently, but they made sense in a fairground.  Actually all reciprocating steam engines fall short, and they vibrate badly, though the high-speed Willan’s Steeple Engine, was once widely used.  Turbines are best for making electricity.

For demo purposes a slow turning dynamo (5×30=150rpm) spun by a beam engine might produce enough volts and amps to light up a string of LEDs, but not paralleled filament bulbs off a xmas tree.   Try spinning the dynamo by hand to see what the output is at low rpm.

Nonetheless, beam engine with dynamo makes a good display model.  Beam engines are a joy to watch because they move so slowly it’s clear how they work.  And always good to see models doing something, like powering a line-shaft that spins a miniature lathe or illuminating the world.

If verisimilitude matters so many rules are broken by driving a dynamo with a beam engine that I feel it’s not worth worrying about gear DP.  If done at all, full-size would use belts, and there would be no Light Emitting Diodes!

Belts are easier than gears too…

Dave

[JasonBModerator @jasonb]

Dave you have missed the point again. It will be a DUMMY Dynamo with a motor inside use dto drive the beam engine for display.

to ‘drive’ a faux dynamo, in practice a small geared motor to drive the engine

Also full size showmans as I said would run the dynamo at around 750rpm not the 3000 little model ones need. I suspect the OP asked about spee dto at least get the ratios looking right just like getting the gear tooth size to look right. From another forum

 These dynamos were made by Mather & Platt of Manchester and the one now on the engine is dated 1918. It is a type ‘P8c’ compound wound, 4 pole machine having a continous output of 270 amps at 110 volts at 750 rpm. It weighs 17 cwt. (1904 lbs).

[JasonBModerator @jasonb]

I just had a play with the Stuart Beam drawings. It would be possible to make something similar to the ME Beam’s gear arrangement.

If 30T:120T MOD 0.5 gears were used which are readily available for not much money then it could fit either side of the crankshaft.

A slightly modified eccentric with a second spigot to mount the gear on would be easy enough to do. Skim a bit off the protruding bearing on that side and you only need move the eccentric & rod out 1/8″ which would just need a simple spacer on the weigh shaft at the other end to get the rod line straight

Make two pedestalls, a shaft and a pully in the region of 65mm diameter and then flat belt that to the gear head motor hidden in a dummy dynamo casing.

Or you could slap another large gear on the end of the shaft instead of a pulley as was found on beam engines used for the likes of cane crushing

[JasonB]

On JasonB Said:

Dave you have missed the point again. …

No Jason, please read my post more carefully.  Covers whether or not a beam engine with dynamo and gears is realistic and the practicalities of generating electricity with reciprocating steam. I’m taking a wider view, not “missing the point”.

• In terms of realism, animating the engine with a geared motor is fine, but as gears aren’t period, the pedantic answer to “What DP (or module) would look right on the gears?” is none of them!
• Admittedly the generating bit isn’t strictly on topic but surely you’ve noticed we tolerate thread drift?  And that many engine builders are interested in getting power out of model dynamos…

🙂

Dave

[stevensons rocketParticipant @stevensons-rocket]

Thank you for your responses. I was thinking many a working beam engine might have been retrofitted with an auxiliary drive+dynamo in the late 1800s, early 1900s, if only to provide some electric light. E.G. in my late 1960s apprenticeship days I worked on machine tools with electric motors but had clearly been built to work from overhead shafting!

I’m more interested in it looking right (but where does one stop without creating an entire model village) and wondering about two lots of 4:1, maybe 3:1 ratios, the first by gears, second by pulleys. Thinking of running it slower than scale speed (15RPM?) to better show its mechanisms, as per the models I used to pour over in the Kensington Science Museum in the early ’60s. I might just fit it with a small crank handle.

Thanks again, Pete

PS You may have noticed, I’m new to this forum. I tried to fill in the bio page to say a little but failed. Finger trouble? Web problems? Any advice on what I may be doing wrong welcome.

[SillyOldDuffer]

On SillyOldDuffer Said:

On JasonB Said:

Dave you have missed the point again. …

• but as gears aren’t period, the pedantic answer to “What DP (or module) would look right on the gears?” is none of them!

🙂

Dave

Are you blind to the images I have posted of both the ME Beam and another six column engine with even more gears. Certainly period. I also just looked through one of George Watkins books many of the beam engines in there had gearing of one form or another being used to speed up or slow down the drive. Both plain and herring bone gears.

Pete, as you say a 3 or 4:1 initial ratio with gears would be quite in keeping and then you could belt or gear down again to your dummy motor which keeps all the diameters looking sensible.

Also worth noting that these smaller beam engines tended to run faster so 50rpm would have been quite typical but if you fit a variable speed drive to your motor you will be able to run the engine over a range of speeds.

[JasonBModerator @jasonb]

AI if you want to believe it also suggests that is was done. Much as Pete suggests.

 

Also there are some full size beam engines in preservation that get turned over by electric motors as they no longer have a steam supply.

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Just as an aside [being disinterested but not uninterested] … Although incorporating gears would be prototypical, and fun, I can see no practical reason for so doing.

The Poly-V arrangement from Motor to Flywheel should be quite sufficient to turn-over the the standard  model.

MichaelG.

.

Ref.

.

.

[JasonBModerator @jasonb]

It seems to me that Pete wants something that looks plausible and in keeping with a model hence the question about gear sizes and speeds/ratios.

Yes a modern belt onto the flywheel would be practical however it would not really look the part.

Beam engines seldom drove directly off the flywheel so a crankshaft mounted pulley to give a reasonable ratio would need to be as big as the flywheel.

Having the 4:1 initial geared ratio is not only prototypical is allows diameters to be kept smaller.

If a similar ratio were used from a pulley on the end of the geared shaft you would only need something like 4:1 again so sensible sizes.

4:1 driving 4:1 gives a total of 16:1 while keeping a bigger wrap angle and sensible sized gears/pulleys. And if you were wanting to replicate real full size ratios then you would be representing an engine running at 48rpm to drive a typical dynamo at 750rpm as mentioned above.

[Michael GilliganParticipant @michaelgilligan61133]

I happily defer to your wisdom, Jason

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Beam engine driving a dynamo is enough of an anachronism. Lots of early mills used gear drive to the line shafts, then they invented multi rope drives. The rope race at Trencherfidld mill is impressive, unfortunately it’s closed to public at present dud to shortage of cash.

Even water wheels used gear drive.

[Michael GilliganParticipant @michaelgilligan61133]

But, I wonder, how relevant is Trencherfield to a little Stuart beam engine ?

https://commons.wikimedia.org/wiki/File:Trencherfield_Mill_-_steam_engine_-_geograph.org.uk_-_3379989.jpg

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Having nothing better to do whilst I wait for some antibiotics to settle … I thought to ask Chat GPT about the origins of the Stuart model [just to broaden my education]

Question:

Can you identify a likely prototype upon which this miniature was modelled … or was it a freelance design.

Its origin seems to be lost in the mists of time … but YOU search wider and faster than I ever could.

.

An answer was delivered within two seconds,  and makes interesting reading

MichaelG.

[JasonBModerator @jasonb]

Don’t know about the mists of time but it can be found in Andrew Smith’s book. Took me a similar amount of time to open the page.

[JasonBModerator @jasonb]

The description from an old Stuart cataloge also says the same.

Good photo to show what I was saying about size, it would not have been a big engine and certainly way to chunky even if just scaled up to the size shown.

http://stuartturnersteam.com/6Pack/Beam/Beam.html

[Michael GilliganParticipant @michaelgilligan61133]

I have already admitted to being disinterested, Jason … and therefore do not have access to the book.

 

Incidentally … The ChatGPT answer was much more wide-ranging.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

There is a diddy beam engine in the Anson Engine Museum. Google has photos

 

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the reminder, Duncan

… we used to live only a few miles from the Anson.

MichaelG.

.

Ref. https://www.geograph.org.uk/photo/1739732

Edit: … and the Geograph rabbit-hole led me to this:

[which for some reason the forum has inserted as a clickable image]

[Author]

Posts