A double square thread power screw is to support a load of $60kN.$ The screw has a

major diameter of $36mm$ and a pitch of $6mm.$ Coefficient of friction may be as low as

$0\mathrm{.}1$ and as high as $0\mathrm{.}15.$

a$)$ Will this screw always be self$-$locking?

b$)$ The same as $($a$)$ when a single thread is used?

c$)$ What torque is necessary to raise the load when $=0\mathrm{.}15.$