Advantages and disadvantages of friction; grip bolts

Advantages of friction

• It makes walking and the running of vehicles possible (grip of tyres and shoes on the ground).
• Brakes and clutches work by friction.
• Belt and rope drives, screws, nuts and bolts, nails and wedges hold by friction.
• Friction keeps objects at rest on slopes, helps in holding and gripping, and gives stability to structures.
• It is used for striking a match, sharpening tools, and in friction welding.

Disadvantages of friction

• It causes wear and tear of machine parts.
• Energy is lost as heat, so the efficiency of machines is reduced.
• It overheats parts, so lubrication and cooling are needed.
• It causes noise and vibration, and more power is needed to move parts.

High-strength friction grip (HSFG) bolts

HSFG bolts are high-tensile steel bolts used in steel structures to join plates. The bolt is tightened with a calibrated wrench to a high tension (proof load), so the plates are clamped very tightly together.

• The load is not carried by the bolt shank in bearing or shear. It is carried by the friction between the clamped plate surfaces: the clamping force (bolt tension) × coefficient of friction gives the slip resistance.
• Hence there is no slip between the plates under service loads, and no bearing stress on the bolt holes.
• The joint is rigid, resists vibration and load reversal, has good fatigue strength, and the nuts do not loosen.
• Used in bridges, steel frames and crane girders. The contact surfaces must be clean and free of paint and oil.