Subject: Machine Design -I

Topic: Design against static Loads, Bolted and welded joints under eccentric loading. Power Screw

Difficulty: Medium

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: