V-belts transmit power through friction between wedge-shaped rubber and matching sheaves. That friction can slip — usually by 1-3% under normal load, more when belts age or get contaminated with oil. For most fans and pumps, slip doesn't matter. But when you need a driven shaft to stay in phase with the driver — a camshaft, a 3D printer axis, a robot joint — you need positive engagement. That's what a timing belt provides.

A timing belt is a reinforced rubber (or polyurethane) belt with molded teeth on its inner surface that mesh with matching grooves on the pulleys. Because the teeth physically engage, there's no slip and the two shafts maintain an exact, repeatable angular relationship. This is why every internal combustion engine either uses a timing belt or a timing chain to link the crankshaft to the camshaft — if that phase drifts, valves and pistons collide.

Tooth profiles matter more than you'd think:

• Trapezoidal (MXL, XL, L, H, XH) — the original 1950s profile. Cheap and widely available, but teeth deform under load, causing "tooth jumping" at high torque.
• HTD (High Torque Drive) — deep, curved (semicircular) teeth developed in the 1970s. Roughly 2x the torque capacity of trapezoidal at the same width because the curved root spreads stress and prevents shear failure at the tooth base. Common pitches: 3M, 5M, 8M, 14M (the number is tooth spacing in mm).
• GT / GT2 / GT3 — refined curvilinear profile with even better load distribution and less backlash. GT2 (2mm pitch) dominates desktop 3D printers.

Sizing rule of thumb: for HTD 5M at moderate speed, a 15mm-wide belt handles roughly 200-400 W per meter of belt speed. So a belt running at 2 m/s carries roughly 400-800 W. Always check the manufacturer's chart for the actual pitch, width, and small pulley tooth count — belts wrapped around tiny pulleys have fewer teeth in mesh and derate hard.

Design gotchas:

• Minimum 6 teeth in mesh on the smaller pulley, or torque capacity collapses. Larger pulleys and wider wrap angles help.
• Timing belts need less tension than V-belts — over-tensioning kills bearings and stretches the tensile cords. "Just tight enough not to skip" is the target.
• They tolerate almost no misalignment. A pulley cocked 1° will walk the belt off the edge in minutes. Use flanged pulleys on at least one end.
• Belts don't like reversing loads unless specifically rated — the tensile cords fatigue faster.