Sizing Toothed/Synchronous Belts for prop drive

[Mike Rometer]

Which one makes more torque at 1000 rpm?

You mean an HD can get to 1000 rpm?

[Lopez Mike]

The answer, of course, is a steam powered motorcycle!

It's not that the sports bike uses a roller chain for power handling capability so much as that the belt needs to be much wider for a given power and width along side the rear wheel is at a premium.

As the worst case I know of, top fuel dragster engines drive their superchargers with a 2.5-3" wide belt and are cramming over a thousand h.p. through that belt. I think they are getting remarkable life out of the belt too. I believe they are running them for as much as 15-20 seconds per belt!

[malcolmd]

Just FYI and following on from all your advice I thought you might be interested in the assembled components for the prop-shaft belt drive (see pics below), comments welcome.
Dimensions: propshaft 1.25" dia, three deep grove ball races in plumber's blocks (two to take belt loads, one as a thrust bearing. CV joint from Ford Sierra XR4. Nylon shock absorber, 8mm pitch HTC 30mm wide belt with ~10% gear up from engine.

[fredrosse]

Looks like a very nice setup.

One note of caution, marine engine crankshafts have bearings that may have been only designed for the radial load coming from force of connecting rods. Adding the force of the belt tension could possibly exceed allowable loads. In other words, the marine engine may have been designed only with direct connection to a propeller shaft on the same axis as the crank being considered. Using belt, or chain drive adds another radial load.

I think this is probably a non-issue, but then my job has always been to find potential flaws in machinery before the machine is put into service.

[barts]

Looks like a very nice setup.

One note of caution, marine engine crankshafts have bearings that may have been only designed for the radial load coming from force of connecting rods. Adding the force of the belt tension could possibly exceed allowable loads. In other words, the marine engine may have been designed only with direct connection to a propeller shaft on the same axis as the crank being considered. Using belt, or chain drive adds another radial load.

I think this is probably a non-issue, but then my job has always been to find potential flaws in machinery before the machine is put into service.

Well, the main bearings are taking the piston loads, so a 2" diameter piston w/ 150 psi is producing 450 lbs+ at least some of the time...

- Bart

[fredrosse]

OK, the connecting rod load for Bart's example is 150 PSI x Piston area = 3.14 sq inches = 471 pounds force (lbf)
Belt tension will be higher on the drive side, Power = Net Torque x Rotational Speed

Say 3 horsepower x 550 ft-lbf / HP-sec = 1650 ft lbf / sec
Say 300 RPM x 2Pi Radians / revolution x 1min / 60 sec = 31.4 Radians per second
Solving for applied Torque = Power / Rotational Speed = 1650 / 31.4 = 52.52 lbf-ft torque x 12 inches / ft = 630 lbf-in

Torque = Tangential Force x Moment Arm, where tangential force is the net belt tension in this example
Say a 4 inch diameter pulley, Pulley Moment Arm = Diameter / 2 = 2 inches
Solving for Tangential Force = Torque / Moment Arm = 630 / 2 = 315 lbf , This is the differential tension on the belt.

If the tension on the slack side of the belt is, Say 30 lbf, then the drive side of the belt would have a tension 315 lbf higher, or 30 +315 = 345 lbf. If the driver and driven pulleys are about the same diameter, then the total force in one direction would be 30 + 345 = 375 pounds force. This would be a significant adder to the bearing load, depending on the orientation of the belt tension in relation to the orientation of the connecting rod load.

This is just an assumed evaluation, anyone can input the real numbers it they wish.

[malcolmd]

Thank you gents for your input. This morning I licked my calculating pencil (aka Google sheets) and looked at the loadings involved (please make allowances for my amateur engineer status).

The interweb tells me that the loading on a plain bearing is a combination of the bearing area (1/3rd of the total bearing area being considered in respect of a radial load) and the rotational speed... aka "PV" and that the loading (in American units) that SAE 660 PhBronze (what I am using) can tolerate is 75,000 psi sFt/min.

The Leak Compound (my version) has three 1.25" dia main bearings each about 2" long, so I think we can say there is about 2.66sqin of bearing surface in play per bearing - The engine is meant to develop about 10hp at ~600rpm and has a 2"+5"x 3" configuration running at a maximum boiler pressure of 200psi. The two toothed-belt pulleys are 6.33" dia driving and 5.55" dia driven.

I think this means that each of the three main bearings can withstand a load of about 993lbs radial loading (I guess in reality most of the belt load will be born by the bearing adjacent to the pulley.) - However, perhaps Fred or Bart can work my figures with a little more accuracy!! (please).

So, it seems to me, that we stand a fighting chance of the bearings being OK with the loading, but I might see if I can squeeze in one more pillow-block as an outrigger (although a total of four 1.25" ID deep groove RHP pillow-blocks and two 4" long cutlass bearings does feel like we are straying into overkill territory and the job of getting them all sufficiently well aligned to ensure they do not increase the loads might be a challenge )

[fredrosse]

10 HP 600 RPM 6.33 INCH PULLEY DIA = 400 POUNDSFORCE (34 pounds slack side, 366 pounds drive belt tension on drive side).

With the pulleys mounted one directly above the other, this translates to a downward force on the crankshaft, 400 pounds.

The connecting rod force, taking the lp cylinder at 50 psid = 1000 pounds force downward, and that load would be taken half on the middle crank bearing, half on the outboard bearing, so the load on the outboard bearing = 500 + 400 = 900 pounds.

That comes out to a nominal bearing pressure of 350 psi, which should be OK. Also meets your bearing calculation of 993 pounds allowable load. Since the downward load on the drive end bearing is about double the value it would have had if driving a prop shaft in axial alignment, you should assure good lubrication on this bearing, and watch that it does not get hot.

Mount the pulley as close as possible to the bearing.

[malcolmd]

Thanks for the advice and information. ..I will take care with the lubrication

[cyberbadger]

Malcolm,

Do you have part numbers for any of what you got and are getting? I'm doing something very similar. Any part numbers would be very helpful.

I start off chain, I have no engine shaft take off with the Toledo. I have to use a short chain at least anyways so I might as well make it straight-ish. The Nyitra is a pontoon boat.

I plan to take very short chain to a jack shaft with a sprocket and a toothed belt pulley down to the prop shaft. I need thrust bearings and I don't understand what I should be getting.

Mcmaster has unit bearings and I can't find a good definition of that.

I also don't understand the two plates and a bearing thing. How do I package that together to work properly?

*confused*

-CB