Mechanical Engineering - Theory of machines - Discussion

It's always p = Wtan(alpha-phi) for lowering the load and plus sign for raising load.

@Kishan option C is correct.

It will be Wtan (Phi-alpha) only.

It is C.

P = Wtan(φ - α).

If α > φ then P=Wtan (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).

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 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.

In case of irreversible screw Φ> α.

Good explanation, thanks @Prince Kumar.

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

Nicely explanation @Prince Kumar. Thank you very much.