Hello,

I have been reviewing a belt and pulley assembly to better understand the loads at work. From my understanding, there is a tight side and a slack side created when the pulley drives the belt. This difference in tension creates the effective pull (Pe = T1 - T2). What I don't understand is how this works at the other pulleys. If you look at each one individually, it seems that the tensions conflict. That is to say that the tight side for one pulley is the slack side for another, because the pulleys are always pulling. Does that mean that tension decreases along the belt?

Thanks in advance!

i think your drawing is almost right ! ...

belt tension changes at the pulleys where work is done, where torque is added and where it is removed from the belt.

i think your top pulley tensions are the wrong way around, belt tension cannot change away from the pulley, so the top pulley is driving the system (i think, T1 > t2, yes?, it is adding to the belt tension) and the bottom pulley is being driven (it is reducing the belt tension).

Think about it:

The pulley does NOT "drive" the work.

Think of your system again but remember that the tight pulley leather "pulls" only one part of one side of the "pulled" pulley. That part of the pulley wheel then turns the work. The slack side of the pulley leather/rubber does nothing.

Therefore, the tension adjustment/tension maintenance device needs to be on the tight (pulling) side of the pulley leather.

Think of it as water in a closed loop pipe system. When it leaves the pump it is at its highest differential pressure. Every device or bend along the way consumes a little bit of that pressure. So that as the water leaves the last bend, the only differential pressure left in it is the amount required to get back to the pump.