Which of the following is true for a self-locking screw?a)The coeffic...
For a self - locking screw, the coefficient of friction is equal to or greater than the tangent of the helix angle.
In the Power screw, the torque Mt required to lower the load is given by
Mt = Wdm/2 tan (ϕ−α) where, W = load on the screw in N, dm = the mean diameter of the screw-in m,
ϕ = friction angle and α = helix angle
Case 1. when ϕ ≥ α
tan (ϕ - α ) ≥ 0, thus Mt ≥ 0.
i.e., positive torque is required to lower the load. Thus the load will not turn the screw and will not descend on its own unless an effort is applied.
Such cases of screws are known as 'Self-locking screws.’ And the screw will hold the hold without any application of the brake.
Also, as ϕ ≥ α
tan ϕ ≥ tan α
Case 2. when ϕ < />
tan (ϕ - α ) < 0,="" thus="" />t < />
i.e., negative torque is required to lower the load. The load will turn down the screw and will descend on its own unless a restoring torque is required to hold the load.
Such cases of screws are known as 'overhauling of the screw' or back drawing of the screw.