Final answer:
The correct equation for torque in the context of lowering a load with a power screw is d) W tan (θ – ϕ), where W is the weight, θ is the angle of inclination, and ϕ is the friction angle.
Step-by-step explanation:
The correct equation for torque when a load is being lowered by a power screw during machine design is represented by the equation W tan (θ – ϕ). Here, W stands for the weight in newtons, θ is the angle of inclination of the screw, and ϕ is the friction angle. When lowering a load, the torque required will be reduced due to the assistance of gravity. Therefore, you must subtract the friction angle (ϕ) from the angle of inclination (θ) before applying the tangent function. This results in the torque equation being expressed as option d) W tan (θ – ϕ).