# Force needed to rotate a drum/cylinder

1. Nov 28, 2007

### sniffy

How do I calculate the HP (torque) needed to rotate a large cylinder mounted on bearings?

The cylinder in question is a 8 foot long 12" diameter piece of pipe weighing a couple of hundred pounds, mounted on two end stubs with bearings. Its the business end of a large flail mower and this assembly rotates at aprox 2000 rpm, and is powered by a hydraulic motor.

The problem here is I have to calculate what size of hydraulic motor to use (hp rating) and this will be determined by the rolling resisance and mass.

Could someone point me in the proper direction towards a formula so that I can plug in the known values?

Length 8'
Diameter 12"
Weight 250 lbs
RPM 1500 to 2000
(bearing resistance shall be minimal)

Thx,
Jim

2. Nov 28, 2007

### stewartcs

Find the torque first...

T = I*a

where,

T = Torque, N-m
I = Moment of Inertia
a = Angular Acceleration, rad/sec^2

Then the horse power...

HP = n*T/5252

where,

HP = horse power
n = rotational speed, RPM
T = Torque, ft-lbs

Moment of Inertia (I) for:

Solid cylinder = (1/2)*m*R^2
Hollow cylinder = m*R^2

Make sure you convert the units as necessary!

3. Nov 28, 2007

### sniffy

Cool,
Now before I drag out my lava lamp powered abacus, lets make sure I understand the variable thingies here...

For the MOI for a hollow cylinder (pipe tube),
m= mass of the cylinder structure (in the example I stated 250lbs)
r= radius of the cylinder (12" diameter, 6" radius)

Only part that I'm foggy on is the definition of rad, as in...
"a = Angular Acceleration, rad/sec^2"

Thx,
Jim

4. Nov 29, 2007

### FredGarvin

There are $$2 \pi$$ radians per revolution. You need to make sure that the acceleration you need is converted to those units.

Just as a note, this calculation will give you the required power to provide an acceleration. It will not be the continuous operating power. That will be lower if all the external applied loads are the same.

5. May 21, 2008

### anandt

torque required to rotate a conical drum

Bigger dia. of drum 500mm
smaller dia. of drum 200mm
wt. of drum 100 kg
rpm of drum 25

what will be Moment of Inertia, torque & HP required to ratate the drum

6. Feb 12, 2009

### mhkhattab

What about the power required for continuous operation of a rotary drum ?
i.e acceleration equals zero

7. Feb 12, 2009

### Cyrus

8. Feb 12, 2009

### mhkhattab

i need someone to help me with this
i got a hollow cylinder with known dimensions and known weight
i need an equation that i can use to calculate the power required to rotate such a cylinder during continuous operation at a certain rpm that i'll know later on i.e acceleration during operation = 0

9. Feb 12, 2009

### Cyrus

mhkhattab,

(2) Include a real problem statement
(3) Provide an engineering drawing
(4) Provide numbers
(5) Show what background information you found using google

We're not here to do work for you.

10. Feb 12, 2009

### Gokul43201

Staff Emeritus
...but unknown bearing friction?

11. Feb 15, 2009

### mhkhattab

a hollow cylinder of inner diameter 3.6 m , it's wall thickness is 3 cm , made of stainless steel of rel. density 7.7 , the cylinder's length is 7.2 m , find the power required (during continuous operation) to rotate the cylinder around it's axis at 20 rpm . (a motor is used to rotate the cylinder) . the figure shown is only for help , the dimensions mentioned previously aren't those of the shown figure (please assume missing data if needed)

Last edited: Feb 15, 2009
12. Feb 16, 2009

### Cyrus

You've got to be kidding me...........

13. Feb 17, 2009

### mhkhattab

just say u can't do it and stop fooling around instead of wasting my time , anyway the answer turned out to be so simple not the way ur overreacting about

14. Feb 17, 2009

### nvn

mhkhattab: For your stated problem in post 11, the power P required for continuous rotation of your given hollow cylinder at a constant angular velocity omega is P = Mf*omega, where Mf = resistive torque due to kinetic friction (N*m), and omega = 2.0944 rad/s. If the driving force applied to your cylinder is applied at radius r3, then excluding lost heat, I think Mf = mu*m1*g*r3*r4/(r3 - mu*r4), where mu = kinetic coefficient of friction of bearings, m1 = cylinder mass = rho*pi*(r2^2 - r1^2)*L = (7700 kg/m^3)(pi)[(1.83 m)^2 - (1.80 m)^2](7.2 m) = 18 967 kg, g = 9.8067 m/s^2, and r4 = radius of bearings. E.g., if mu = 0.10, r3 = 1.83 m, and r4 = 0.30 m, then Mf = 5673.1 J, and P = Mf*omega = (5673.1 J)(2.0944 rad/s) = 11.882 kW.

As FredGarvin implies above, I think the power required during startup acceleration might exceed this continuous operation power value, unless perhaps you accelerate the cylinder very gradually during startup.

15. Feb 18, 2009

### mhkhattab

first, thank u very much nvn

the power u calculated was considering the drum empty , what if the drum is rotating containing granules of mass 22922 Kg and relative density 1.57 and porosity of 0.4 ? how would this affect the power required ?

thanks for help

Last edited: Feb 18, 2009
16. Mar 30, 2011

### bios6666

i neeed to calculate the retardation effect for a torque produced by a axle of length 14 inches, dia .5905 inches with a 50 rpm, wats the electromagnetic field require to reduce the torque of this axle to reduce it to 0 rpm,
plz gime me details wit formulae... plz help