Motion System Design

Brushing Up


Friction plays a dual role in machinery. On one hand, it helps conveyor belts turn on pulleys and threaded fasteners hold tight. But it also causes wear on surfaces that move relative to each other.

According to estimates, 33 to 50% of energy consumption is used to overcome friction.


Lubricants separate surfaces to prevent friction and wear.

They operate in full film, boundary, or mixed film regimes. The film thickness between surfaces is proportional to viscosity (Z) and speed (N), and inversely proportional to load (P). Fullfilm lubrication prevents metal-tometal contact, minimizing wear. Boundary lubrication surfaces rub at high points with a thin film between them, and they wear rapidly. Mixed-film lubrication comprises both boundary and full-film.

Sliding and rolling

When two objects slide across each other, friction resists the motion. The sliding force is proportional to the normal force or weight, as expressed by the coefficient of friction μ. On a flat surface, μ = P/N where P = force needed to slide and N = weight. On an incline, μ = tan Ø where Ø is the angle at which sliding starts.

Smooth surfaces generally reduce μ, especially machined and ground. But very smooth surfaces adhere to each other, causing μ to increase. Objects that roll, rather than slide, have lower resistance to motion.

Putting friction to work

Many devices depend on friction for their operation. Clutches and brakes use friction between two surfaces to transfer torque or stop a load. A flat belt wraps around pulleys, and friction between them transmits torque. V-belts go further, using a wedging action to increase force and reduce slip. A locking device uses friction with tapered rings to mount drive components on shafts.

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