[Electric Boats] Re: "L" drive with U-joints test

 

Hi, Craig,

 

U-joints are probably better suited for small misalignments, where the shaft assembly is basically in a straight line.

Because of the nature of the way they behave dynamically, the torque required will vary slightly during each revolution. Although this can be reduced by correct indexing of one UJ to the other, it is still a tendency, I believe. This is caused by speed variation during each rev.

You would be better off using a constant velocity joint.

Another option, if the power being transmitted is relatively low, and the angle not too steep, would be a fabric UJ, or fabric style UJ. In the simplest form these are basically a disc of rubberised fabric with a yoke connected from each face. They are used in steering columns of cars, I suspect (never looked at a steering one closely!) as a metal disc with rubber inserts where the yokes connect.

I know for sure that the more simple fabric UJ's were used on the driveshaft of older RWD Fiats, notably the pre-war Topolino.

Another source would be a 1960's Lotus Elan car style joint, where rubber doughnuts were used on the rear axles to allow the wheels to have independent suspension. But again, angles would be limited, as otherwise 'cogging' would probably occur as the joint turns.

Used in higher angle designs, I believe any material that works by deforming as it turns will no doubt consume power, and create heat.

For connecting to 2 parallel shafts, my guessed opinion is that you might not be able to beat a toothed 'timing belt' belt. It might even be possible to reduce power losses by slicing the belt width down, to increase the flexibility, but the better option would be to get the manufacturers specs, as this type of belt is becoming more popular in smaller applications such as vacuum cleaner brush drives.

Another setup to consider is that longer vee belts can be twisted through 90 degrees, maybe narrower (as opposed to wider) toothed belts also?

Good Luck,

John

 

2.2. Re: "L" drive with U-joints test
    Posted by: "Craig Carmichael" craig@saers.com
    Date: Thu Jun 21, 2012 12:20 am ((PDT))

The longtail type motor has little appeal for me. I might try doing a
submerged motor straight to the prop but the 'pancake' motor shape is
about the worst possible for that. (But any typical 5KW BLDC direct
prop drive motor would be too fat.) That leaves installing as an
inboard, and that's not my vision either.

For an outboard there seems to be no avoiding a 90º turn at the foot,
and I'm guessing that the double U-joint will prove to be the least
lossy way (and quietest) to make that turn.

I was refining the U-join assembly today (ditched one link; instead
using the end of the prop shaft itself as the final 45º turn element)
when the 90º bevel gears in my angle grinder broke up - teeth
everywhere inside. Not the first time either. This does little to
give me confidence that gears is a better way to turn 90º. (But
they're pretty cheap angle grinders. Maybe I'll splurge this time.)

Cheers,
Craig

 

 

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