Screw Jack

Overview

Engineering reference data for Screw Jack in mechanics.

Key Formulas

Newton's Second Law

$F = ma$

Force = mass × acceleration.

Work

$W = Fd\cos\theta$

Work = force × displacement × cos(angle).

Kinetic Energy

$E_k = \frac{1}{2}mv^2$

Energy of motion.

Potential Energy

$E_p = mgh$

Gravitational potential energy.

Variables

Friction Considerations

For screw jacks operating with friction, the effort force depends on the direction of motion relative to the load.

With friction, motion assisting the load (load assists motion): $F = Q \left( \frac{2 \pi \mu r - p}{2 \pi r + \mu p} \right) \left( \frac{r}{R} \right)$

With friction, motion opposing the load (load resists motion): $F = Q \left( \frac{2 \pi \mu r + p}{2 \pi r - \mu p} \right) \left( \frac{r}{R} \right)$

Where:

• r is the pitch radius of the screw (in, m).
• μ is the dimensionless friction coefficient.

Definitions and Notes

• Pitch (p) is the distance between adjacent screw threads. For inch-sized fasteners, it is commonly specified as threads per inch (TPI).
• Lead is the linear distance the nut (or screw) advances in one full revolution. For a single-start screw, the lead equals the pitch.
• Always distinguish between mass and weight (Q). In the given formulas, Q represents the gravitational force (weight), with units of lb or N.

References

• Original Source