The frictional torque for square thread at mean radius while raising load is given be (W=load, R0 = Mean radius, f = Angle of friction, a = Helix angle) (IES-93)
(a) WR0 tan ($\phi$– $\alpha$) (b) WR0tan($\phi$+ $\alpha$)
(c) WR0 tan $\alpha$ (d) WR0 tan $\phi$