Belt Drives

• Belt drives are friction drives; grip depends on area of contact, belt and pulley materials, and tension.
• A grooved pulley = a sheave. A crowned pulley is convex — it centres a flat belt.
• Installed tension = belt tension with the drive stopped; top and bottom strands are equal.
• Effective tension = tight-side minus slack-side tension (T1 − T2) while running: the net pull that drives the load.

• Older and lighter-duty use.
• Materials: leather, rubber, fabric (3–12 ply with alternating fibre direction), cord, steel-cable, nylon.
• Chemical splices are recommended over mechanical fasteners.
• Mechanical fasteners: wire or alligator lacing, steel hinges, plate fasteners.

Wedging action

more load → tighter grip. Efficiency 95–98 % new and correctly tensioned; it can drop to about 93 % if the belts aren't re-tensioned after run-in.

Four sections: extension (top — stretches), compression (bottom), neutral (the driving / tensile cords), cover.

Fitting V-belts

lock out, then move the motor in to shorten the centres so the belts drop into the grooves by hand. Never pry or roll a belt over the rim: that strains or breaks its tensile cords, and the belt then runs slack and turns over. Put all the slack on one side, tension, run briefly to seat the belts, then lock out and re-tension; recheck after the run-in.

• Positive-drive (timing / gear) belts have moulded teeth and need at least one flanged pulley.
• A timing belt's pitch line runs through its tensile cords, which ride above the pulley teeth, so a timing pulley's pitch diameter is larger than its OD (a V-belt sheave's is smaller).

• Idler pulleys cause a reverse bend that shortens belt life.
• Idlers do two jobs: increase the arc of contact and provide take-up.
• Outside idler: flat, ≥ ⅓ larger than the smallest drive pulley, best on the slack side near the drive pulley.
• Inside idler: reduces the arc of contact but gives unlimited take-up.
• Direction: every sheave inside the belt loop turns the same way as the driver; a sheave or idler driven by the back of the belt, outside the loop, turns the opposite way. Double-angle (hex) belts can grip with either face.
• Don't run mechanically-joined V-belts over flat idlers.
• Hubs: plain parallel bore, plain taper bushing, or flanged taper bushing (QD) — use the same bolts to install and to remove.

• Adjustable-centre drive: variable sheave on the driver, fixed sheave on the driven.
• Fixed-centre drive: both sheaves adjustable.
• Make speed adjustments while the drive is running. Spring flanges push the belt outward.
• Store rubber belts cool and dry; synthetics cool and humid but not touching water.
• Hang endless belts in loops — no kinks.
• Alignment goals: line up the pulley centrelines and keep all shafts parallel; re-check after run-in.
• Edge wear with belts tracking to one side points to sheaves out of line or shafts not parallel. Squeal at start-up that stops once the drive is running means the belts slip under the starting load: check the tension first.

A synchronous (timing / positive-drive) belt carries the load on the teeth actually in mesh, so the small pulley needs enough wrap for enough teeth. The minimum is six teeth engaged over a 60° arc on the small pulley; some references quote the same limit as a percentage of coverage. Below the minimum the belt ratchets — jumps teeth — under shock, or the drive must be derated to a lower horsepower.

• Check the small pulley first: count the teeth inside the arc of contact; fewer than six means a larger pulley, a longer centre distance, or an idler that increases wrap.
• Flange rule: one pulley must be flanged; flange both when the centre distance is 8 × the small-pulley diameter or more, or when the shafts are vertical.
• Never mix pitch codes (XL, L, H, XH, XXH) between belt and pulleys; the tooth form must match.