Final answer:
The torque to lift and lower the load is 34 kNcm. The efficiency of the power screw system depends on the number of starts and the mean collar diameter. The thread is self-locking if the coefficient of friction is greater than or equal to the tangent of the thread angle.
Step-by-step explanation:
The torque to lift the load:
To find the torque to lift the load, we can use the formula: Torque = Force * Distance. Since the load is lifted using a screw, we need to convert the force applied to the load to torque by multiplying it by the mean collar diameter. Therefore, Torque to lift the load = 4 kN * 8.5 cm = 34 kNcm.
The torque to lower the load:
The torque to lower the load is equal to the torque to lift the load. Therefore, the torque to lower the load is also 34 kNcm.
Efficiency of the power screw system:
The efficiency of a power screw system is given by the formula: Efficiency = (Ideal Mechanical Advantage / Actual Mechanical Advantage) * 100%. Since the thread is self-locking, the ideal mechanical advantage is the same as the number of starts, which is 4. The actual mechanical advantage can be calculated using the formula: AMA = π * Mean Collar Diameter / Pitch. Therefore, Efficiency = (4 / (π * 8.5 cm / 2 inch)) * 100%.
Thread self-locking or backdriveable:
A thread is considered self-locking if the coefficient of friction between the nut and the collar is equal to or larger than the tangent of the thread angle. In this case, the coefficient of friction is given as 0.150 (sliding). If the coefficient of friction is greater than or equal to the tangent of the thread angle, then the thread is self-locking.