I'm not really sure what the power screw is. If it's the same thing as the S-cam then yes, it's sort of like the piston in a caliper. The S-cam twists and a lobe pushes against the rod which activates the brakes. It's kinda the same as a camshaft on an overhead valve engine. The cam turns and when it gets to the lobe it opens the valve. In the case of air brakes the cam turns and the lobe pushes the pads against the drum.
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Is a power screw comparable to a caliper piston that you would find in disc brakes on a car or motorcycle?