# Motor Torque Calculation

## Main Question or Discussion Point

[SOLVED] Torque Calculation

Trent

#### Attachments

• 10.4 KB Views: 468

Related Mechanical Engineering News on Phys.org
FredGarvin
The dynamic loads are going to be a big factor in this calculation. Statically you can see that the worst case torque is only 60 in-Lbf. However, once this thing starts rotating, there will be accelerations that will increase that requirement. Those dynamic forces are going to be dependent on how fast this is going to turn and, I believe, the stroke. Then you do have to add in other losses like frictional and heating.

http://en.wikipedia.org/wiki/Piston_motion_equations

I am not to worried about major accuracy. I had looked at that page before and i got really confused. Im not too worried about friction and such, im looking for a rough estimate. More or less i just need the formula for moving the load taking in the account of the diameter of the circle. I guess an example would be if i had a 20lb piece of metal welded to the edge of the circle, how much torque would the motor need to be able to spin with the weight attached.

If you need a very rough estimate, take a look the equation which gives the piston velocity "v" vs. time in that wiki page:
http://en.wikipedia.org/wiki/Piston_motion_equations#Equations_wrt_time

The force (or load) acting on the piston is 20 lbs. Thus, the power required to move that load is

$$P = 20 \cdot g \cdot v$$

The power suplied to the piston by the crankshaft is:

$$P = M \cdot \omega$$

where $$\omega$$ is the angular velocity, which is related to RPM by the equation given in http://en.wikipedia.org/wiki/Piston_motion_equations#Angular_velocity
and "M" is the torque which you're looking for.

If you want to neglect the friction and inertial forces, then all the power from the crankshaft is used to move the piston, so you have that

$$M \cdot \omega = 20 \cdot g \cdot v$$

and from this one you can calculate the torque M.
Note that your motor will be running against higly oscillating forces, so I recommend you to install a flywheel to this contraption, if it doesn't allready have one.

Now... Like FredGarvin said, we don't know nothing about the dynamic forces. They depend on engine components' masses and their geometry.
So, if I were you I would multiply this value of M by a nice "coefficient of ignorance" (say, 1.5 or even 2) and let some VFD do the job of controlling the RPM (is PWM = VFD? don't know much of electronics ).

Last edited: