Friction phenomena in mechanical systems

 1. Which statement is untrue? Screw joints exposed to high temperatures require special purpose thread lubricants for easy disassembly. Stainless steel screws require lubricants with extreme pressure additives. A low value of the friction coefficient during tightening is essential to obtain a reliable screw joint. A constant value of the friction coefficient during tightening is essential to obtain a reliable screw joint.

 2. Which statement is untrue? In a V-belt drive with three pulleys the maximum transmissiom torque is limited by the smallest pulley. In a V-belt drive with two pulleys the maximum transmission torque is limited by the smallest pulley. The effective friction coefficient between a V-belt and the pulley decreases with increasing groove angle β. The moment that a V-belt start to ride on the inner cylindrical part of the pulley, which may be the case when the belt becomes worn, the maximum transmitted torque strongly decreases.

 3. Which statement is untrue? The maximum torque that can be transmitted by the press-fit of a shaft-gear assembly is limited by tensile strength of the gear. coefficient of friction between shaft and gear. modulus of elasticity of the gear. ratio of gear diameter and shaft diameter.

 4. Which statement is untrue? The friction in a rolling contact is dependent on hysteresis losses and slip. Reynolds slip also occurs in a technical pure rolling contact. Heathcote slip also occurs in the contact formed between a cylinder and a plane. The rolling friction in a line contact caused by hysteresis is proportional to (F/L)^1/2 and inversely proportional to (E’R’)^1/2.

 5. The rolling friction of wheels is usually expressed in a coefficient of friction for rolling μ=Ff/F, where F is the load carried by the wheels and Ff the resistance to motion. Consider the case of wheels constructed with sliding bearings rotating on a spindle. The ratio of the spindle diameter / wheel diameter d/D=1/100. The coefficient of friction in the bearing μ=0.2. The system is rolling on four wheels. Calculate the coefficient of friction for rolling. μ=0.2 μ=0.05 μ=0.008 μ=0.002

 6. Which statement is untrue? The friction coefficient in polymer - metal contacts decreases with the contact pressure. The friction coefficient in lubricated metal - metal contacts is constant over a large range of the load. In polymer - metal contacts the friction remains minimal when the polymer surface has a roughness of around Ra=0.2. In metal - metal contacts the friction is independent of the roughness. This can be explained by the real contact area, which is proportional to the load but independent of the roughness.

 7. Which statment is untrue? Stick-slip or oscillation may be prevented by: Roughening the hardest surface. Introduction of side slip. Decreasing the stiffness. Solid lubricants.