Synchronous belts (also known as timing belts) are widely used in conveying, indexing, and positioning applications requiring high torque, high force transmission, and high acceleration.

Unlike V belts, which transmit power via friction between the belt and pulleys, synchronous belts use positive meshing between belt teeth and pulley grooves, eliminating slip. This delivers extremely high power transmission efficiency and excellent positioning accuracy.

In addition, synchronous belts perform exceptionally well in high speed applications, especially under high torque or thrust loads.

One major factor enabling high speed operation is the belt tooth profile, which affects tooth strength, meshing characteristics, and the tendency for ratcheting (tooth jumping).

Synchronous belt tooth profiles fall into three main categories: trapezoidal, curvilinear, and modified curvilinear. Manufacturers also offer proprietary variations (especially modified curvilinear) for low noise, high speed, or higher precision.

Trapezoidal Tooth Profile

The original synchronous belt profile, still widely used today. It offers higher efficiency than V belts but creates high stress at the tooth root, leading to rapid wear in high speed applications.

Imperial trapezoidal belts:MXL (0.08″), XL (0.2″), L (0.375″), H (0.5″), XH (0.875″), XXH (1.125″), LT (0.0816″)

Metric T series:T2.5 (2.5 mm), T5 (5 mm), T10 (10 mm)Standard trapezoidal profile for general driving and conveying.

Metric AT series:AT5 (5 mm), AT3 (3 mm), AT10 (10 mm)Improved T profile with larger tooth capacity and stronger cord. Higher load capacity, better for high torque, high precision applications.

· L, H, XH, XXH: Neoprene with fiberglass reinforcement

· T2.5, T5, T10, AT3, AT5, AT10: Polyurethane with steel cord reinforcement

AT profiles provide better load distribution and less backlash than T profiles, ideal for linear actuators and packaging equipment.

Curvilinear Tooth Profile

Rounder shape with deeper grooves, reducing stress at the meshing zone. Enables higher force transmission and higher speeds.Deeper grooves also lower the risk of ratcheting. However, curvilinear profiles require larger clearances, resulting in greater backlash than trapezoidal profiles.

Originally developed by Gates as HTD (High Torque Drive).

Metric HTD pitches: 3 mm, 5 mm, 8 mm, 14 mm

Material: Neoprene with fiberglass reinforcement

Superior to trapezoidal profiles for high torque and high speed, with better tracking

Modified Curvilinear Tooth Profile

Proprietary designs with shallower tooth depth and larger flank angles than standard curvilinear profiles.

Key advantages:

· Less backlash

· Higher torque/load capacity

· Better flank contact

· Lower tooth stress

· Minimal tooth deformation

· Lowest ratcheting tendency even under heavy loads

The best choice for high speed, high acceleration, heavy load drive systems.

Common pitches: 2 mm, 3 mm, 5 mm, 8 mm, 14 mm

Materials: Neoprene + fiberglass; polyurethane + Kevlar/carbon fiber (high torque)

Noise Considerations

Noise is common in high speed synchronous drives, caused by:

1. Meshing impact

2. Air trapping and expulsion between teeth

Modified curvilinear belts generate the lowest noise due to smoother meshing and larger contact area. Noise is also affected by pre tension and material selection.