ENGINE/PROP SHAFT ANGLE

[sailorise]

I intend using a shortened lorry prop shaft and a standard thrust bearing to connect my 48hp 6cyl 1500rpm Lister with 2:1 reduction, to drive a shaft and 22"prop. The angle of the prop shaft is say, 5deg to the horizontal whilst the engine I'd prefer level. My question is, therefore; will there be a problem with the UJ on the g/box side running at a different angle to that on the thrust bearing side. Is it essential for them both to be running at the same angle ?

 

[Peterduck]

In theory. the input shaft and the output shaft should be parallel. This is because a universal joint does not transmit rotation evenly. While the input shaft [from the engine] is rotating evenly, the short tailshaft is actually accelerating and decelerating very minutely. The second uni. joint rectifies this, and the final drive is smooth. In the set-up that you are proposing, the second joint would accentuate the uneven rotation. It may, however be so small as to not be noticeable. You won't know until you've tried it.
Another "in theory" one is that the angle should be greater than 5 degrees. This is because the tiny roller bearings move back and forth over the same little bit of steel, and slowly wear a groove in it. This could be overcome by replacing the Hardy-Spicer joints with bronze bushes. I fitted a short shaft as you describe to my old ketch, because it was built by eye, and the centreline of the engine beds [and thus the engine] does not line up with the inboard end of the propshaft. It has worked well. the shaft must not, of course, have a telescopic end as most tailshafts do, or you'll get a nasty surprise when you engage reverse gear.
Peter.

 

[sailorise]

Thanks; in your own experience - your ketch - was the engine and prop shaft parallel?
And what opinion would you have over the possible vibration two UJs operating 'out of tune', when their speed is as low as say 500rpm (which is about where I shall be operating)

 

[cliffordpope]

When setting mine up, admitedly a much smaller instalation, I was told that anything up to 15 degrees is OK for a Hardy Spicer. Ideally the two joints should have the same angle, so if say the total angle between engine and prop were 10 degrees, you would move the engine backwards or forwards so that each joint contributed 5 degrees to the total.

Of course the engine does not even have to be straight in a sideways sense either, if it helps to clear hull floors etc. It seems there is quite a lot of latitude permissible, but one should just try to keep each angle as small as possible.

 

[sailorise]

Thankyou Clifford. Yes, I shall have to pull the engine forward until the angle difference is split. But this can only be approximate and I wonder if you experienced any vibration in your own set up and what speed you ran it at.

 

[burgundyben]

"While the input shaft [from the engine] is rotating evenly," - not quite correct, the speed the crank rotates changes depending on where the power cycles are, its not much and should be largely compensated by the mass of the flywheel, but it is there.

You are quite right about the universal joints and hteir angular position to each other is important, thats why we now use CV joints on cars.

My guess is that in this application it will hardly be noticeable (but that is a wild ass guess).

 

[Peterduck]

In the case of my ketch, the crankshaft and the propshaft are very close to parallel. I haven't checked it, so I can't be certain whether they are or not, but they are very close to parallel. At 500 rpm, I would guess [and it's only a guess] that you'd be OK. I think that it's one of those cases where you'll have to try it to know for certain. I agree with 'burgundyben' that it will probably not be noticeable [which is, after all, what you are wanting].
Peter.