Over hauling:
- The torque required to lower the load can be given by the equation,
$T=\frac{Wd_m}{2}tan(\phi-\alpha)$
It can be seen when $\phi\lt\alpha$ the torque required to lower the load is negative.
It indicates a condition that no force is required to lower the load. The load itself will begin to turn the screw and descend down, unless a restraining torque is applied.
The condition is called overhauling of the screw. This condition is also called back driving of screw.
Self-locking:
- When $\phi$ is greater than or equal to $\alpha$, a positive torque is required to lower the load.
- Under this condition, the load will not turn the screw and will not descend on its own unless an effort P is applied.
- Screw will be self-locking if the co-efficient of friction is equal to or greater than the tangent of the helix angle, the screw is said to be self-locking.
Condition for:
Overhauling |
$\phi \lt \alpha$ |
Self-locking |
$\phi \ge \alpha $ |