Timing belts

With matching 5 mm profiles, a 20-tooth driver at 1,200 rpm drives a 40-tooth sprocket at 600 rpm. Driver pitch diameter is 31.83 mm and belt speed is 2.00 m/s. A 100-tooth belt has 500 mm pitch length. These are nominal geometry results. Select width, center distance, wrap and tension for actual load and duty. Gates geometry and speed formulas.

A synchronous belt meshes with matching sprocket teeth. This guide covers industrial drives. Check shaft connections, supports, adjustment and guarding as part of the drive.

Geometry and compatibility

Pitch is the repeating tooth spacing along the belt's pitch line. Keep it distinct from tooth height, belt width, and outside circumference. For a belt with N teeth and pitch p, pitch length is Lp = Np; sprocket pitch diameter is dp = zp/π, where z is its groove count. These are pitch-line quantities, not measurements over the tooth tips. The speed ratio follows groove counts. Gates, Light Power and Precision Drive Design Manual, useful formulas.

Trapezoidal and curved tooth families are not interchangeable merely because their pitch matches. Mitsuboshi distinguishes its standardized Timing Belt G/U families, including MXL, XL, L, and H, from SUPER TORQUE and MEGA TORQUE round-tooth products, whose interchangeability is restricted. Record the complete manufacturer series and mating pulley profile. Mitsuboshi industrial-belt FAQ, compatibility.

Compatibility can also vary within a brand. Gates specifies GT-profile pulleys for PowerGrip GT3 2MGT, 3MGT, and 5MGT, but HTD-profile pulleys for 8MGT and 14MGT. This is a named product pairing, not permission to substitute any belt carrying a similar pitch marking. Gates PowerGrip GT3 product specifications.

Construction and positioning behavior

The tensile member resists longitudinal load; the body forms and supports the teeth; a facing can protect the meshing surface. Gates GT3 uses fiberglass cord, elastomeric teeth/backing, and nylon facing. Choose the whole construction rather than treating cord material as a capacity rating. Gates GT3 construction.

Polyurethane is another construction family. Continental's February 2023 Synchromotion announcement describes steel-cord reinforcement in a polyurethane body, offered in customer-specific endless lengths. It distinguishes this product from its established Synchroflex belt and open-ended Synchrodrive range. Endless rotary belts and open-ended linear-drive belts therefore need separate product identification; the material name alone does not establish their usable joint, length range, or rating. Continental Synchromotion announcement.

Synchronous does not mean perfectly rigid or backlash-free. Gates identifies cord elongation, clearance between teeth and grooves, and tooth deformation as contributors to registration error. Reversing positioning duty needs an error budget under changing load, not just a nominal speed ratio. Gates recommends prototype evaluation where tooth deflection matters. Gates PowerGrip GT3 Design Manual, printed pp. 171–172.

Power, speed, and selection

Start with driven torque/power and speed, acceleration, reversals, shock loads, operating hours, ratio, and available space. Gates' selection procedure applies a duty-dependent service factor, chooses pitch and sprockets, then corrects the width rating for belt length and teeth in mesh. A pitch designation supplies no universal kW rating. Check both belt capacity and sprocket speed limits; the cited 2014 manual limits its stock sprockets to 6,500 ft/min belt speed, rather than granting that speed to every synchronous drive. Use current data for the actual product. Gates selection procedure, printed pp. 3–4.

Wrap angle determines how many teeth share load. Tsubaki explains that insufficient engagement increases load per tooth; reduced engagement requires its catalog correction. Pulley diameter and repeated bending also affect cord fatigue. Tsubaki synchronous-belt Q&A, Q8 and Q11.

Geometry example, not a selected drive: assume matching 5 mm profiles, a 20-tooth driver at 1,200 rpm, and a 40-tooth driven sprocket. The output is 1,200 × 20/40 = 600 rpm; driver pitch diameter is 20 × 5/π = 31.83 mm; belt speed is 20 × 0.005 × 1,200/60 = 2.00 m/s. A hypothetical 100-tooth belt has 500 mm pitch length. Width, center distance, wrap, load rating, and allowable tension remain unselected. Gates geometry and speed formulas.

Installation and tension

Isolate power before handling the drive. Mitsuboshi's installation sequence checks pulley coplanarity, shortens center distance, seats the teeth, and moves the base to establish specified tension. Inspect pulley grooves and remove burrs; fit the required guide flanges. These flanges retain the belt but do not correct shaft misalignment. Mitsuboshi Timing Belt Design Manual V832-E, printed pp. 3-3 and 3-8–3-9.

Tsubaki warns that misalignment can force a belt hard against a flange, damaging the edge or detaching the flange. Its adjustable-idler guidance places the idler on the slack side and fixes it after adjustment. It also distinguishes reversible load elongation from permanent elongation: a belt can change position elastically without having permanently stretched. Tsubaki Q2 and Q6–Q7.

Measure tension against a calculated target. A sonic meter measures vibration of a stationary free span; Gates requires the appropriate mass constant, belt width, and span length. Frequency alone is not a transferable tension setting. Its string-based relation is T = 4μL²f² with μ in kg/m, L in metres, f in Hz, and T in newtons. Belt bending stiffness can bias the result; Gates recommends synchronous-belt measurement spans longer than 20 tooth pitches. Rotate the isolated drive by hand to seat the belt and equalize spans before measuring. Gates 550C manual, pp. 4–7.

Do not apply an arbitrary twist test or increase tension until noise disappears. Mitsuboshi allows deflection-load or sonic methods but excludes certain high-performance families from deflection measurement. Its FAQ specifies initial seating followed by prescribed tension and cautions against later routine retensioning. Follow the selected belt's instructions. Mitsuboshi tension and maintenance FAQ.

Failure clues and replacement records

Tsubaki recommends inspection before tooth loss or rupture because each symptom has several possible causes. Tsubaki Q8–Q9 and Q30–Q40.

For replacement, retain the full belt designation, pitch length/tooth count, width, reinforcement, pulley drawings, and tension procedure in the BOM. Also record the duty used for sizing, so a mechanically fitting substitute is not mistaken for a qualified replacement.

Related reference

Production planning references