Question: Explain overhauling and self locking of screw.
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Subject: Machine Design -I

Difficulty: Medium

md1(66) • 7.1k views
 modified 16 months ago by written 19 months ago by kiran.9870 • 10
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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 $\phi \ge \alpha$