Timing Belt Terminology & Glossary

This glossary defines the most commonly used terms in timing belt selection, specification, and troubleshooting. Whether you are a maintenance technician replacing a belt, an engineer specifying a new drive, or a purchasing agent matching a part number, these definitions will help you communicate clearly with suppliers and understand product specifications.

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ANSI

American National Standards Institute. The organization that defines standards for trapezoidal timing belt profiles (MXL, XL, L, H, XH, XXH) used in North America.

AT-Profile

Advanced Trapezoidal. A modified version of the European T-profile with a wider tooth base for higher load capacity. Available in pitches AT3 through AT20. See AT-Profile Timing Belts.

Backlash

The angular play or lost motion between a timing belt and sprocket when the direction of rotation reverses. Caused by clearance between the belt teeth and sprocket grooves. Minimized by correct tension and tight-tolerance sprockets. Critical in positioning and indexing applications.

Belt back

The flat outer surface of a timing belt opposite the toothed side. On some drives, the belt back runs against a flat idler pulley.

Belt length (pitch length)

The total length of a timing belt measured along the pitch line (the line where the tensile cords sit). This is the standard measurement used in part numbers. Not the same as outside length or inside length.

Belt width

The measurement across the face of the belt, perpendicular to the direction of travel. Specified in inches or millimeters depending on the profile system.

Center distance

The distance between the centers of the driver and driven sprocket shafts. Center distance determines the required belt pitch length.

Closed-loop belt

A timing belt manufactured as a continuous, endless loop. The standard format for most rotary drives. Contrasted with open-end belts.

Cord (tensile cord)

The load-carrying member embedded in the belt body along its length. Common materials include fiberglass (neoprene belts), steel wire (urethane belts), and Kevlar/aramid (specialty applications). The cord determines the belt's tensile strength and elongation.

Curvilinear

A tooth profile with a rounded, semi-circular shape. Used by HTD and GT belt systems. Distributes tooth stress more evenly than trapezoidal profiles. See Tooth Profiles Explained.

Deflection method

A method for measuring timing belt tension by applying a known force at the belt midpoint and measuring the resulting deflection. See the Timing Belt Tensioning Guide.

DIN

Deutsches Institut für Normung (German Institute for Standardization). The DIN standard defines the T-profile and AT-profile metric timing belt systems used on European equipment.

Double-sided timing belt

A timing belt with teeth on both sides, allowing it to drive sprockets on both the inside and outside of the belt. Used on serpentine drives and counter-rotating applications. See Double-Sided Timing Belts.

Drive ratio

The ratio of the number of teeth on the driven sprocket to the number of teeth on the driver sprocket. Determines the speed change between driver and driven shafts.

Elongation

The amount a timing belt stretches under load, expressed as a percentage of the original length. Steel cord belts have approximately 0.02% elongation. Fiberglass cord belts have approximately 0.1 to 0.3%. Kevlar is between the two. Lower elongation is better for precision positioning.

Endless belt

See "Closed-loop belt."

FDA-compliant

A belt material that meets FDA 21 CFR 177.2600 requirements for food contact. Polyurethane timing belts are available in FDA-compliant grades for food processing applications.

Fiberglass cord

The standard tensile cord material in neoprene timing belts. Provides good tensile strength with moderate elongation (0.1 to 0.3%). Used in trapezoidal and HTD neoprene belts.

Flanged sprocket

A sprocket with raised edges (flanges) on one or both sides to keep the timing belt from tracking off the sprocket face. At least one sprocket in a two-sprocket drive should be flanged.

GT (Gates Tooth)

A modified curvilinear tooth profile developed by Gates Corporation. GT2 and GT3 are improved versions with optimized tooth geometry for higher load capacity and reduced backlash compared to standard HTD. See GT Timing Belts and HTD vs GT Comparison.

HNBR

Hydrogenated Nitrile Butadiene Rubber. A specialty compound used in timing belts for elevated temperature resistance, chemical resistance, and extended service life. Available for some HTD and trapezoidal profiles.

HTD (High Torque Drive)

A curvilinear timing belt system with rounded tooth profiles designed for higher load capacity than trapezoidal belts. Standard pitches: 3M, 5M, 8M, 14M. See HTD Timing Belts.

Idler

A pulley or sprocket that does not drive or get driven, used to route the belt path, increase wrap angle, or maintain tension. Flat idlers contact the belt back. Toothed idlers engage the belt teeth.

Indexing

The precise positioning of a driven component at exact intervals using a timing belt drive. Timing belts are ideal for indexing because of their positive tooth engagement and zero-slip operation.

Kevlar cord (aramid)

A high-strength synthetic tensile cord material offering lower elongation than fiberglass and better flex life than steel. Used in polyurethane timing belts where a balance of low stretch and flexibility is needed. Elongation typically 0.3 to 0.5%.

Linear motion

A drive configuration where a timing belt converts rotary motor motion into straight-line movement of a carriage or platform. Common in CNC machines, gantry systems, and automation. See Timing Belts for Linear Motion.

Load capacity

The maximum force or torque a timing belt can transmit without exceeding its rated tooth shear strength. Determined by the belt profile, pitch, width, and material.

Matched set

In V-belt drives, a group of belts selected to be the same length within tight tolerances. Not applicable to single timing belt drives, but relevant when multiple timing belts run in parallel.

MXL

Miniature Extra Light. The smallest ANSI trapezoidal timing belt pitch at 0.080" (2.032mm). Used for instruments, encoders, and miniature drives. See MXL Timing Belts.

Neoprene

Chloroprene rubber. The standard body material for trapezoidal and HTD timing belts. Provides good oil resistance, heat resistance, and service life. Reinforced with fiberglass tensile cords and nylon tooth facing. See Neoprene Timing Belts.

Nylon tooth facing

A thin nylon fabric bonded to the tooth surface of neoprene timing belts. Reduces friction between the belt teeth and sprocket grooves, improving wear life and reducing noise.

Open-end belt

A timing belt supplied in continuous roll stock rather than as a closed loop. Cut to any required length and joined with mechanical connectors or welding. Standard for linear motion and custom-length drives. See Open-End Timing Belts.

Outside length

The total length of a timing belt measured around the outside (back) of the belt. Not the same as pitch length. Part numbers typically reference pitch length, not outside length.

Pitch

The distance from the center of one belt tooth to the center of the next adjacent tooth, measured along the pitch line. The most fundamental specification of a timing belt. Expressed in millimeters (metric systems) or fractions of an inch (ANSI system). See Timing Belt Pitch Chart.

Pitch diameter

The effective diameter of a sprocket at the pitch line (where the belt's tensile cords sit when meshed). The pitch diameter, not the outside diameter, determines the belt speed and drive ratio.

Pitch length

See "Belt length."

Pitch line

The imaginary line within the belt body where the tensile cords are located. This is the neutral bending axis of the belt and the reference line for measuring pitch length.

Poly Chain GT Carbon

A high-capacity timing belt manufactured by Gates using carbon fiber tensile cords and a polyurethane body with a modified GT tooth profile. Designed to replace roller chain drives. See Poly Chain GT Carbon Belts.

Polyurethane (urethane)

A synthetic polymer used as the body material in timing belts. Offers superior chemical resistance, abrasion resistance, and low particulate generation compared to neoprene. Standard material for T-profile, AT-profile, and open-end belts. Available in FDA-compliant grades. See Urethane Timing Belts.

Profile

The shape of the timing belt tooth. The three main profile families are trapezoidal (MXL, XL, L, H, XH, XXH, T, AT), curvilinear (HTD: 3M, 5M, 8M, 14M), and modified curvilinear (GT, GT2, GT3). See Tooth Profiles Explained.

Pulley

A general term for a toothed wheel that meshes with a timing belt. In timing belt systems, the technically correct term is "sprocket." See Timing Belt Sprockets.

Ratcheting (tooth skip)

When a timing belt jumps over one or more sprocket teeth under load, losing synchronization. Usually caused by under-tension, overload, or worn belt teeth. See Timing Belt Troubleshooting.

Registration

The accuracy with which a timing belt drive positions the driven component. Also called positional accuracy. Affected by belt tension, backlash, cord elongation, and sprocket quality.

RMA

Rubber Manufacturers Association (now part of the U.S. Tire Manufacturers Association). Published the standards for ANSI timing belt profiles and HTD profiles used in North America.

Service factor

A multiplier applied to the nominal load to account for the type of driven equipment and operating conditions (shock loads, continuous vs. intermittent duty, etc.). Used in drive design calculations. See Timing Belt Selection Guide.

Span

The unsupported length of belt between two adjacent sprockets or contact points. The longest span is used for tension measurement.

Sprocket

A toothed wheel that meshes with a timing belt to transmit motion. Also called a timing pulley. Sprockets must match the belt's tooth profile exactly. See Timing Belt Sprockets and Sprocket Selection Guide.

Steel cord

Steel wire tensile cord used in polyurethane timing belts. Provides near-zero elongation (approximately 0.02%) for maximum positional accuracy. Standard cord material for AT-profile and open-end urethane belts.

Synchronous belt

A toothed belt that provides positive, no-slip power transmission through meshing teeth. "Synchronous belt" and "timing belt" are interchangeable terms for the same product.

Teeth in mesh

The number of belt teeth that are simultaneously engaged in the sprocket grooves at any given time. More teeth in mesh means higher load capacity. Minimum recommended is typically 6 teeth in mesh on the smaller sprocket.

Tensile strength

The maximum force a belt can withstand before the tensile cord breaks. Determined by the cord material, number of cord strands, and belt width.

Tension

The force applied to a timing belt to keep the teeth engaged in the sprocket grooves. Unlike V-belts, timing belt tension does not create grip; it prevents tooth skip. See Timing Belt Tensioning Guide.

Tooth count

The total number of teeth on a timing belt. Calculated by dividing the pitch length by the pitch. Example: 500mm pitch length / 5mm pitch = 100 teeth.

Tooth profile

See "Profile."

Tooth shear

A failure mode where timing belt teeth are torn or sheared off at the root. Caused by overload, under-tension (tooth skip impact), or worn sprocket grooves. See Timing Belt Troubleshooting.

Trapezoidal

A tooth profile with straight, angled sides forming a trapezoid shape. The original timing belt design. Used in ANSI profiles (MXL, XL, L, H, XH, XXH), European T-profiles (T2.5, T5, T10), and AT-profiles (AT3 through AT20). See Trapezoidal Timing Belts.

T-Profile

The European DIN standard metric trapezoidal timing belt system. Available in pitches T2.5, T5, and T10. See T-Profile Timing Belts.

Urethane

See "Polyurethane."

Wrap angle

The arc (in degrees) of the sprocket that is in contact with the belt. A larger wrap angle means more teeth in mesh and higher load capacity. Minimum recommended wrap angle is 60 degrees (approximately 6 teeth in mesh at most pitches).