Chain driven cams ?

I'd like to 'break my duck' with regard to model engineering and finally build a model. I've only been threatening to do that for 15 years now

I'm thinking about designing a single cylinder 4 stroke engine that captures the flavour of a 1930-1950 sports or racing motorcycle engine without actually depicting any specific make or model.
Gears worry me though - I'm not set up for cutting gears, have never cut a gear in my life and quite frankly all those DP's and addendums might as well be written in Greek. I understand them not.

So how about chain drive to an overhead cam ? AJS used it after all, so it's authentic. Will I be able to get suitable chain that small though ?

Ideas anyone ?

Hi Siddley,

I found these by googling miniature chains, may be you can use them? Seems they have with pitch from 0.1475in. to 0.375.

If you don't want to make gears for your model engine, it is possible to buy them.

Regards Thor

Thanks ! my search turned up little other than parts for mini-moto's

Just off the top of my head the 0.1475in chain would make the complete engine about 6 inches tall ( given the design ) - that could be doable but it's a little bit bigger than I hoped for. At least I've got a dimension to work from now, to see if it's feasible.

Hello Siddley,

Don't forget you can run the chain over an idler sprocket and there needs to be a tensioner somewhere, all of which may influence the overall size by opening out the chain loop. Your aversion to gears will lead you onto making or buying sprockets instead, so the swing could become a roundabout!!

Brian

Hi Siddley, It is certainly possible to drive your camshaft by roller chain. It has to be stretched and lubricated very well. In the Netherlands the smallest chain they sell has a pitch of 4 mm. The smallest gear wheel they sell for this chain has 12 teeth. You can take the combination of 12-24, 15-30 or 19-39 teeth. 6 mm pitch chain is more common and better available. It is used fore instance in copy machines. There is also more choice of gear wheels. You can take 8-16, 9-18, 10-20, 11-22 teeth and so on. The width of this gears is 2,8 mm and the chain is 7,4 mm wide. I advise to use endless chain. Success with the design of your engine. Kind regards, Johan.

> 19-39 teeth.

That should make for interesting running

Methinks you meant to type 19-38.

Neil

Hi Siddley,
Sounds like a great project, are you building the complete bike or just the engine ?
In my younger days was a very keen bike rider and at one time had a Beeza Victor 441cc single, tossed me over the handle bars just once too often so got traded in. But it like the Vellocett and AJS were a joy to ride (real bikes) and as we used to say 'back in the day' they 'fired once every telegraph pole'.

If building a whole bike how about toothed belt drive to the back wheel, I think a BMW uses this drive train so not too way out. As for machineing the toothed cogs, check out the article on this site covering re-belting SL and 3/4 Emco's, cogs are machined with a standard milling cutter out of AL.

Let us know how you go
Brian

Siddley,

Gears are a doodle. Most of the maths is just there 'for completeness'.

If you let me know what centres you would like the gears at and the approximate number of teeth (e.g. 1 1/2" centres and 20 and 40 teeth) I'll do the calculations for the cutter and the gear itself as a worked example.

Even if you go with a chain, it might surprise you how few calculations are actually really needed.

Neil

Brian W - the design I'm looking at does use a chain tensioner but it doesn't make much difference to the length of the run due to its shape. Oh no, I'm not making sprockets

Johan - that's helpful, I might try and find one of these copy machines

Neil - the 19\39 brings up an interesting point about chain cam drives and a theoretical weakness of the system, can you figure it out ?

Brian in Oz - I wouldn't try to build a complete bike ( although maybe I could teach one of the cats to ride it ) I've owned quite a few British bikes myself. In fact when my mates were riding Yamaha RD250's I had a Triumph... I just love any bike that's interesting or has character though, it doesn't have to be British. Those Victors were a handful according to what I have read. BSA used the same basic engine for a works racer I believe.
Nowadays my main bike is a Suzuki DR350 offroader - it's competent but a bit boring...

Neil - missed your last post. I'm going to design for a chain this time, but I WILL take you up on your offer regarding gears. It's appreciated.

Hi Siddley.

I have little experience of model ic engines but would it not be possible to use a toothed belt to drive your camshaft as in full size practice?

Although of a different design to the one you are proposing, Jan Ridders uses toothed belt cam drive on some of his i.c. engines. His collection of engines on his website are worth studying as he is a prolific designer and builder of steam, Flame eater, Stirling and i.c engines. He also offers his plans for free!

Regards

Terry

I've got some small toothed belts somewhere Terry - the only thing is that they aren't historically accurate for the time period. Overhead cams in those days were usually driven by gear trains or bevel shafts. Even the chain is a little bit unusual.
Great website, thanks for the link.

Siddley,

What about basing a design on the NSU eccentric drive principle ?

True you would be limited to an OHC design but the whole of the drive train consists of 3 links with holes at the end which is something reasonable simple to make on a mill with a boring head.

Another advantage is that depending on the length of the links it can be any scale.

**LINK**

And yes i know this was designed by W.O. Bentley but there is more info on the NSU design than the Bentley one.

John S.

Forewarned!

For the chain should have a long life, it should not be whole gear ratio in the chain. Both drives have gear exchange at 2:1, Let us say the gear has 12/24 gear tooth and the chain will have other ratio without whole ratio number, to example the chain has 145 links will give 1:6,16 in gear ratio. Not calculate such as here 144 links : 24 gear tooth = 6 which is whole gear ratio 1:6 cause reduced lifetime of the chain. To calculate the gear ratio: 145 links of chain:12 gear tooth= 1:12,083 and 145 links of chain:24 gear tooth=1:6,16 who will be better for chain life time.

Jens Eirik

Jens - I understand, it's like the 'hunting tooth principle' with gear drives

JS - I'd never considered the NSU ( forgotten about it to be honest ) but it ticks all the boxes - successful sport and race bike, technically interesting, the right era etc. The only thing is that it's unit construction and I don't want to make a gearbox. But maybe in an alternate universe NSU made a non unit version, or maybe Bentley got into manufacturing motorbikes...
This could be a real proposition, I'll do some research. Cheers

Siddley, yes correct.

When i was in education as carmechanic, there was a old NSU Prinz engine with eccentric links. It was smooth transfer of the rotation transfered from the eccenter to other eccenter when i rotated the engine. Not difficult to make the eccentric transfered links to camshaft will i say.

From internet source: The street-bikes (125cc Fox and 250cc Max) were SOHC singles with a unique cam-drive system consisting of two reciprocating connecting rods driven by eccentrics on a reduction gear and driving eccentrics on the end of the camshaft. This system was silent, efficient, trouble-free, and so compact that the top end of the engine can easily be mistaken for a two-stroke and the engine could rev-up to astronomical revolutions that would have destroyed chains.

Edited By Jens Eirik Skogstad on 16/12/2012 14:44:04