Sizing Toothed/Synchronous Belts for prop drive

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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malcolmd
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Sizing Toothed/Synchronous Belts for prop drive

Post by malcolmd » Sun Jan 17, 2016 3:41 pm

Further to my other post today, I am also considering using an indirect drive from engine to propeller shaft. Clearly chain would be one option, but I am also considering toothed belts (aka cam belts or synchronous belts). Looking at the specs for these, and the tables suggest that a belt only 8mm (1/4") wide would be easily capable of passing the ~10BHP @ 700rpm we might get from the Leak Compound. However this "feels" very light, as we have a fair weight of shaft and prop - and lots of start-up torque.

Does anyone have any practical experience I can use - or thoughts on the design process please.

ALSO I was intending to use one of those plastic shock absorber engine couplings when I was thinking of coupling the propeller shaft directly to the engine - if I went down the belt-drive route I am thinking this could be dispensed with - do you agree?

thanks
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by malcolmd » Sun Jan 17, 2016 3:50 pm

ooops - the 8mm was the pitch of the belt, not the width! - however the question stands - do you have any experience, suggestions on drive design and the need to retain a shock absorber... sorry...
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by johngriffiths » Sun Jan 17, 2016 4:38 pm

Because Tenacity's prop is only 14" diam, the vessel is fitted with a 2:1 speed increaser which utilises a 1 1/2"" wide toothed belt off the rear of the Leak compound. When the prop has hit rock or picks up a spring mattress (the canals are used a a rubbish tip by some people) I'm very glad to say there has never been a problem the the drive. Bits missing from the prop is another matter however. It would not be possible to change the belt without moving the engine thus the speed increaser was made big and strong.

Quite a number of steamboats use flex couplings but trad vessels like Shamrock use solid. No reason why you should not, still need to line everything up though.

PS I carry and have had to use bolt cutters and hack saws to clear the prop
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by malcolmd » Sun Jan 17, 2016 5:24 pm

Yes, we have family memories of touring the canal network and a memorable trip down the Old BCN Main Line in the late 80's that certainly required major use of boat hooks and weed hatches to make progress - removing bikes, shopping trolleys and dead sheep!

Having worked thru the Fenner design guides it seems to suggest that a 20mm wide 8mm pitch belt should do the job, but I think I might go for 40mm, which marries up with your arrangement, and I am only running the smaller Leak, but with a larger prop meaning more "flywheel" effect. However, I should not need to gear up - but the belts leave that option should it prove necessary.

I think it would be tough to get the engine lined up with the end of the propshaft, as I will need a UJ or CV to allow the engine to sit level, and the overall length of that approach looks hard to accommodate - so the belt drive will give me a little more room, and I think the belt will provide some of the "give" that the flex coupling would have provided.

I think I am decided - belts it is!

On one other note; I was thinking of trying to incorporate a clutch in the drive train (a dog clutch, I guess) - to allow for battery charging, or sailing with less drag. Does anyone have a proven design.
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by steamboatjack » Sun Jan 17, 2016 5:40 pm

Malcolm,
I have used tooth belt drive for many years without problems, (8mm pitch HTD type). However be careful on selection as your engine develops far more torque at lower revs than an internal combustion engine. (we have had all this discussed some years ago on this forum including some non believers).
Using the belt suppliers chart calculate your 10hp at 350 RPM which is a more realistic figure. The way to keep the belt width down is to use as large a diameter pulley as you can fit in. Also use a jockey or some easy way to adjust the tension as a loose belt will rattle as you reduce speed (with the prop driving the engine).
p.s. A Leak compound at 200 PSI boiler pressure and 400 RPM produces 8.97 IHP (about 8 BHP) The casting suppliers info is incorrect!

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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by DetroiTug » Sun Jan 17, 2016 6:08 pm

I third or fourth the use of cogged "timing" belt. The HTD 15mm width should be ample. It has very high ratings for it's size - much higher than the similarly sized (width and pitch) English size offerings. HTD series is just heavier, and it uses a rounded deeper tooth design rather than trapezoidal. I've been building CNC machines and retrofitting Bridgeports and Jet Mills for close to 20 years and I'm 100% sold on the use of timing belts for the transfer of torque/rotation. I have one CNC Bridgeport with L series belts on it from 1978, still has the original belts on it. Z axis powers the knee (2hp servo, 2.5:1 ratio), which is whipping 6-700 pounds of cast iron up and down on each Z move and has probably done so a million times. Folks often unknowingly compare timing belts to V-belts - there is no comparison, the timing belt has about a 10 times better service cycle in comparable size. High end and high performance motorcycles use them to drive the rear wheel.

But as with any transmission part that may fail, make it easy to swap out and carry a spare. A lot cheaper and quieter than a chain.

Quote: "I think it would be tough to get the engine lined up with the end of the propshaft, as I will need a UJ or CV to allow the engine to sit level, and the overall length of that approach looks hard to accommodate - so the belt drive will give me a little more room, and I think the belt will provide some of the "give" that the flex coupling would have provided."

Not sure that would work. The only experience I've had is with drive and driven shafts parallel to one another. One caveat with timing belts is that they must be free running on the pulleys and nothing sharp or rubbing that can damage the belts. At least one of the pulleys has to have flanges on the side to keep the belt on the pulley. Trying to run the belt twisted may cause the belt to run hard against the flanges on entry and exit and wear it out prematurely. Than again it may not be an issue. As my old steam buddy says "Try it"..

-Ron
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by malcolmd » Sun Jan 17, 2016 6:53 pm

Thank you both for your feed back:

Jack, your point about low-speed torque is exactly what I was driving at when worrying if the computation came out right.. I also understand the need for "as big as possible" pulley size, not quite sure until I get on my hands and knees exactly how much room we have :-) . You and I have discussed the probable output of the Leak before, and I agree that Camden's figures are very optimistic, so I have fitted geared down pumps to allow us to "thrash it a bit" if needed in moments of abject panic (Befur has more of the profile of a bulk carrier than a svelte river launch) - I think a max RPM of 600 or maybe 700rpm is in order, although others have suggested that an alloy LP piston might be needed to achieve this! I will recheck the figures based on 10HP at 350rpm, as you suggest.

Ron, I agree on HTD, I have used it to drive a vertical head on an old A&S mill I had, with some success - it is strong stuff! My explanation on the UJ was poor - I agree everything HAS to be parallel, I just think by having the engine shaft next to the propshaft (with a UJ) I can save a bit of length and make the set-up simpler...

Do either of you have thoughts on dog-clutch arrangements.

thank you both...
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by malcolmd » Sun Jan 17, 2016 7:02 pm

...OK re-ran the numbers at 350rpm and (if my understanding of the process is right) then this pushes it just over the 20mm width. Bearing Boys seem to do 30mm and 50mm width taperloc pulleys in 8mm HTD so I think it's 30mm...

thanks again...
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by fredrosse » Sun Jan 17, 2016 8:08 pm

"I think the belt will provide some of the "give" that the flex coupling would have provided." Not really, these belts do not "give" much at all, and are not designed to allow any "give"or significant mis-alignment.

It is my understanding that you intend a CV joint between the engine and the pulley shaft? In that case you should have heavy duty bearings on the pulley shaft, as well as the propeller shaft, to take the tensile load of the belt. Of course the propeller shaft must also take all of the thrust developed for propulsion.

One other option that is also workable, a common V-type belt drive. With the larger profile belt, and pulley diameters around 8 inches you can adequately traqnsmit the engine power, with good reliability. Then you can use a conventional idler pulley for clutching, another simple and reliable arrangement. This arrangement will suffer a couple of percent efficiency loss compared to the timing belt, but not a significant loss.
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Re: Sizing Toothed/Synchronous Belts for prop drive

Post by barts » Mon Jan 18, 2016 5:03 am

Remember to employ service factors suitable for a multi-cylinder gas engine; this will help compensate for the variation of turning effort w/ crankshaft position.

- Bart
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Bart Smaalders http://smaalders.net/barts Lopez Island, WA
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