Shaft Design - Torsion, degrees of twist 1

[bpo]

I would like some advice on shaft diameter to use, considering only twist.
A 6m long shaft, carrying a blade (8mm plate - equally each side) supported by a bearing on each end and 3 bearings spread eaqually in the middle. Width of blade is 780mm.
Torque applied = 2000 N.m. Assembly - Blade/Shaft - is used in an inlet damper to a boiler.I Have considered only Torsion, as the shaft is well supported and bearing friction minimal.
The answers I get with calculations are confusing. Can someone please assist me, or can someone please tell me where i can get a program to download to calculate.
Thank you

 

[atad]

You must also consider speed of operation. If you plan actuate the damper blade very fast you will get significant momentum generated back through the shaft. Ideally the damper blade should have its own integral end position stops to help to minimise the amount of backdriven torque that will be transmitted to the driving mechanism.

However, this is only a major problem if the speed of operation is fast - I'm guessing but in your case if you were to actuate the blade faster than 5 seconds then you need to consider the torisional stress effect the damper blade will have on the drive mechanism.

 

[CoryPad]

The general equations for shaft torsion are:

elastic deflection &[ignore]theta[/ignore]; = (lT)/(KG)

where

&[ignore]theta[/ignore]; = deflected angle
l = shaft length
T = applied torque
K = polar area moment
G = shaft material shear modulus


stress &[ignore]tau[/ignore]; = Tr/K = (G&[ignore]theta[/ignore];r)/l

where

&[ignore]tau[/ignore]; = shear stress
r = shaft radius



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EnglishMuffin]

Since you have given CoryPad a star for providing the appropriate equations, can we assume that your confusion has now evaporated ?

 

[Cockroach]

Well done, CoryPad, I echo EnglishMuffin's remarks, except change the "?" for a "!"

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[bpo]

Thank you all for your responses,
The formula is the same as wghat I have been using.
I need to consider the problem from another angle.
The damper has a dia. 50mm B.M.S shaft in it at the moment, but is actuated from either side - i.e has a split shaft - with a Torque of 1000 N.m. We want to replace the two shafts with a single longer shaft, thereby eliminating an extra actuator and gearbox. The problem is to find a suitably larger dia solid shaft - or hollow shaft, without overdimensioning it. The blades have to seal against the frame of the actuator in closed position. This is why I thought by working with an angle of twist I could obtain an answer. I have not yet tried working with allowable stress, will attempt this and see where I get.
regards
bpo