# Building a rotating cylinder

AerospaceEng
So essentially I'm trying to build a rotating free floating rotating cylinder (like laying a coke can on its side and propping it up so that when it rotates it won't have contact with the ground) I hope this is clear
and I need to figure out what size of motor I need to turn my cylinder by calculating the amount of torque needed

Dimensions of cylinder

m=226.80Kg
Length of cylinder=2.44m

Moment of Inertia=42.14

But I'm not really sure where to go from here, how do I calculate the torque needed? and help is appreciated

Mentor
So essentially I'm trying to build a rotating free floating rotating cylinder (like laying a coke can on its side and propping it up so that when it rotates it won't have contact with the ground) I hope this is clear
and I need to figure out what size of motor I need to turn my cylinder by calculating the amount of torque needed

Dimensions of cylinder

m=226.80Kg
Length of cylinder=2.44m

Moment of Inertia=42.14

But I'm not really sure where to go from here, how do I calculate the torque needed? and help is appreciated

There was just recently a very similar thread. Does it help at all?

.

AerospaceEng
Thanks for the post Berkeman that kind of helped, although I know that Torque is

T=I(alpha)

but I dont understand how I can caculate alpha (the angular acceleration) because if you do some math you'll see that my cylinder turns very slowly, it's suppose to turn fairly slowly for long periods of time so I know to find alpha i do

alpha=w(angular velocity)/time

but thats fine whether it takes 10 seconds or a full min to reach its full speed doesnt matter to me its how much torque does it take to keep it rotating?

torquil
but thats fine whether it takes 10 seconds or a full min to reach its full speed doesnt matter to me its how much torque does it take to keep it rotating?

That would be determined by the friction that opposes the rotational movement. I.e. air resistance and e.g. resistance within ball-bearings. In other words, a complicated matter that also requires a lot more information.

Pies
Hi Guys,

I actually have a very similar problem I'm working on at the moment except my cylinder is alot heavier (2500kg). The dimensions are similar and it only needs to move slowly (30 degrees in 5 minutes).

I've worked out what the resistance due to Inertia is but I'm not sure how to go about accounting for the friction. The cylinder is located on a shaft with bearings at either end; what information do you need to calculate the resistance to rotation due to friction in the bearings.

In it's basic form, friction exerts a force on an object equal to mu.N, where mu = coefficient of friction and N equals the normal force (i.e. weight) of the object. Can the friction due to a bearing be calculated in the same way? Does the diameter of the shaft have any effect on friction force? Or shaft velocity?

Any help or suggestions would be appreciated.