Potential for slip in conventional belts prevents their use where input and output shafts must be synchronized. Synchronous belts, often referred to as timing belts, were developed to overcome this limitation. They have a toothed profile that mates with corresponding grooves in the pulleys, thereby providing the same positive engagement of chain or gears.

A stable belt length is critical for synchronous applications, so they were originally reinforced with steel. Today the most common reinforcement is glass fiber, but aramid is used if maximum capacity is required. Standard sections are MXL, XL, L, H, XH, and XXH.

The trapezoidal tooth profile first used on synchronous belts is recognized as standard. Belts with this configuration are commonly used in machine tools, textile machinery, home appliances, business equipment, and as camshaft drives in engines.

Synchronous belts have a number of attractive features that put them in applications where there is no real need for shaft synchronization. They are thin and flexible, so they operate well on miniature drives and in applications involving high speeds or small pulleys. Preliminary studies indicate they may be the most efficient form of power transmission short of direct drive. In addition, they are able to sustain high loads. For these reasons, synchronous belts have proved to be cost effective in such nonsynchronous applications as drives for power saws, motorcycles, and vacuum-cleaner brushes.

The primary drawbacks of synchronous belts are their higher cost relative to other types, and a demand for fairly accurate alignment of pulleys. In all, synchronous belts find use on highly engineered and mass-produced drives, and continue to be widely used in general industrial markets.

Recent modifications to tooth profiles have improved on the original trapezoidal shape. The full-rounded profile, often called HTD, distributes tooth loads more evenly to the belt tensile member. It also provides greater tooth shear strength and hence carries greater load than trapezoidal designs.

A modified curvilinear tooth design, called STPD, appears similar to the HTD design. However, its pressure angle, tooth depth, and materials are chosen for improved load/life capacity and other characteristics such as nonratcheting. A newer, modified curvilinear tooth design is called GT. MXL pitch belts were developed mainly to convey paper past reading heads at constant speed. A high-friction coefficient polyurethane drives the paper with zero slip.

Some nonstandard belt pitches for special industrial use are sized between XL (0.20 in.) and L (0.375 in.) such as 0.233, 0.246, and 0.250 in. In the auto industry, new belts are designated STA, AL, and ZA. The main difference between industrial and automotive sections is the size and tensile cord, which changes the pitch line differential. Automotive and industrial belts are not interchangeable.