To derive the force required to lift a load using a screw jack, we model the screw thread as an inclined plane. Let PL be the axial load (weight) to be lifted, and PR be the horizontal force app

To derive the force required to lift a load using a screw jack, we model the screw thread as an inclined plane. Let $P_L$ be the axial load (weight) to be lifted, and $P_R$ be the horizontal force applied at the mean radius of the screw to lift the load. Let $\alpha$ be the lead angle (helix angle) of the screw, and $\mu$ be the coefficient of friction between the screw and the nut. We define the friction angle $\lambda$ such that $\mu = \tan \lambda$.

We consider the forces acting on a block on an inclined plane, representing a segment of the screw thread.

• $P_R$: Horizontal force applied to lift the load.
• $P_L$: Vertical load acting