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# truck tails shafts lengths2

## truck tails shafts lengths

(OP)
Hi, in Australia is there a max length a 5" drive shaft can be? We are stretching a Volvo FM12 , 8x4 rigid and currently have a need for 1 shaft to be 2300mm long witout cutting it and making 2 short shafts. Any advise?
Cheers Paul

### RE: truck tails shafts lengths

(OP)
Dont know steels tensile, but knowing Volvo it wont be low. RPMS max at max speed is about 2000rpm.
Thanks

### RE: truck tails shafts lengths

A drive shaft will turn into a skipping rope as it reaches a critial rpm. I've seen the formulas for calculating the rpm on the internet before. Try a Google search.

(OP)
many thanks

### RE: truck tails shafts lengths

2
It is not as simple as that. The skipping rope shape is longer than just the distance between the UJs and the inertia of the driveshaft is greater than just the tube because of the mass of the UJs.

So, yes, you will be able to find an equation for the critical speed of a shaft on the internet, but if it hasn't been adjusted to take those two effects into account then it will be misleading, and will overprdict the safe rpm.

I would suggest looking at a book that discusses driveshafts explicitly, a power transmission specialist may have one, the SAE certainly had a driveline manual, but I can't remember if it includes whirl speeds.

You also need to make sure that you run at significantly less than the critical speed, as the resonant peak has shoulders. I'd suggest 30% would be a good starting point.

Cheers

Greg Locock

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### RE: truck tails shafts lengths

(OP)
many thanks will look in to in more detail
Paul

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