torque to turn bird

[larbar]

hi all,

I am currently looking to create a gearing system in order to turn a model bird around in circles from a 12v stepper motor.

My main problem is I am not sure how to calculate the torque neccessary, from my geared system, to turn the bird.

I have taken the centre of gravity to act through the axis of rotation and the friction to be negligable.
please advise.

laurence

 

[MiketheEngineer]

Its all about friction. Theoretically - with no friction and no gravity - it would take virtually no torque. Just try something inexpensive and go from there....

 

[IRstuff]

Your bird has a mass and a maximum diameter. Figure out the moment, using something along the lines of 50%-70% of the maximum radius. Then, angular acceleration/moment = torque

TTFN

FAQ731-376

 

[drawoh]

larbar,

How fast do you want this thing to accelerate?

How accurately do you want to position it?

How big is it?

JHG

 

[GregLocock]

The maximum torque you can exert on a decent sized shaft between your thumb and forefinger is about 1.4Nm. So, can you twiddle any of the shafts in your system?



Cheers

Greg Locock

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

 

[larbar]

Thanks all, for the responses so far.

drawoh- position and acceleration are not key with this design. I would like the bird to turn at approx. 0.5 rev/sec.

Taking the bird as a sphere, it's diameter is 350mm.

My main concern is the torque neccessary to turn the shaft coneected to the bird.

cheers

 

[SincoTC]

For thirty RPM you could drive it directly with a stepper motor if it can handle the starting torque, but it would require a ramped acceleration to do this, so it will need a suitable controller/driver board. As MiketheEngineer said above, friction is the thing plus a bit if experiment with something cheap and available, you'll need to support the weight as it could be too much for a small motor (grooved thrust races would be good for this).

But if you're not going to position it, do you really need a stepper motor with its need for control circuitry? There are plenty of geared DC motors about, use belts or you could even vandalise an old record player if 33 RPM is acceptable.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
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