Mechanical Engineering - Theory of machines - Discussion

Does anyone know how to prove the relationship between P, W, and θ?

C is the right answer.

You are right @Prince.

Refer to self-locking back driving condition. The Coefficient of friction angle should be greater than the helix angle for self locking condition. Otherwise, overhauling or back driving condition will start.

Nicely explanation @Prince Kumar. Thank you very much.

In case of screw jack the angle of friction should be greater than angle of inclination for self locking.

Good explanation, thanks @Prince Kumar.

In case of irreversible screw Φ> α.

Option C is right answer. Because when angle of friction is less than angle of inclination or repose. Then, the body will slide automatically on inclined plane. In that case, screw Jack is behave like overhauling then it is in dangerous. So screw Jack is must be selflocking machine. If it is selflocking then it must be angle of friction is greater than angle of inclination or repose of a plane. So option C is correct answer in case of lowering the load.

Answer A) is correct.

To raise the load in screw jack effort required is p = W*tan(α + φ).
W-load, αis helix angle and φ is friction angle,
To lower the load p = W*tan(α -φ).

Option A is correct because,

We know that for raising the load,
P = w(α + φ).
&for lowering the load,
P = w(α - φ).

It is C.

P = Wtan(φ - α).

If α > φ then P=Wtan (α - φ).