Skru Kuasa A power screw is a device used in machinery to change angular motion into linear motion Usually it is used to transmit power E.g.. Lead screw of lathes, vises, presses and jack.

Forces acting on the screw thread When using a power screw to exert a force either raising or lowering a load, we must know how much torque must be applied to the nut of the screw to move the load. Hence; the parameter involved in raising or lowering a load are: F the load) to be moved Size of the screw indicated by pitch diameter dm l the lead of the screw which is defined as the axial distance that the screw would move in one complete revolution. f the coefficient of friction Figure on the left show a square thread power screw with single thread having a mean diameter dm, a pitch p, a lead angle l and a helix angle y is loaded by axial compressive force F. Hence the forces acting on the screw thread are as follow:-

(a) For raising the load, the torque required to overcome thread friction and to raise the load is given by Tu = ( F dm/ 2 ) [ (p f dm + l ) / ( p dm – f l ) ] Where dm = d – p/2 and l = np and f = coefficient of friction of thread. (b) For lowering the load, the torque required to overcome a part of the friction . Td = ( F dm/ 2 ) [ p f dm – l ) / ( p dm – f l ) ] For well-lubricated steel screw f = 0.15 From the forces diagram, the lead angle is given as tan l = l / p dm Hence for both the square thread and acme thread (as well as others) the equations can be rewritten as:-

(a) For raising the load, the torque required to overcome thread friction and to raise the load is given by Tu = ( F dm/ 2 ) [ (cos a tan l+ f ) / (cos a – f tan l ) ] = ( F dm/ 2 ) [ ( l + p f dm sec a ) / (p dm – f l sec a ) ] (b) For lowering the load, the torque required to overcome a part of the friction . Td = ( F dm/ 2 ) [ (f - cos a tan l) / (cos a + f tan l ) ] Self-locking the lead angle should be small enough and the friction force is large enough to oppose the load and keep it from sliding down the plane. Hence condition for self-locking, p f dm > l or f > tan l For axially loaded power screw, a thrust or collar bearing must be employed between the rotating and stationary members in order to carry the axial component. Hence; if the coefficient of collar friction is fc , then the torque required to overcome this friction is Tc = F fc dc / 2 must be added to get the total torque.

Efficiency of Power Screw In general efficiency is the ratio of work output to work input. Hence, efficiency for the transmission of a force by a power screw can be defined as the ratio of the torque required to move the load without friction to that with friction. Letting f = 0, hence torque required without friction T’ is T0 = ( F dm/ 2 ) [ l / p dm] = F l / 2 p The efficiency is e = T0 / T where T0 = F l / 2 p The power to drive a power screw is given by the equation P = T n / 63000 ; P in horsepower hp , n in rpm P = 2 p n T ; P in kW , n in rpm

Stresses On Power Screw Stresses occurred in power screw can be analyze for stresses on body and on thread Stresses on body Nominal body stresses in power screws can related to thread parameters as follows:- Nominal shear stress t in torsion of the screw body is Axial stress s in the body of screw is Where dr = d - p

Nominal thread stresses in power screws can related to thread parameters as follows:- Bearing stress on thread sB is Bending stress on root of thread sb is Transverse shear stress t at the centre of the root of thread is Where nt is the number of engaged threads.