Timing belt drives have one non-negotiable rule: the pulley pitch must match the belt pitch. A pulley built for a 3/8-inch pitch belt will not mesh correctly with a 1/2-inch pitch belt, no matter how closely the outside diameters appear. The good news is that stock timing pulleys are standardized around four classic pitch designations, and once you know how to read them, specifying the right pulley takes minutes instead of hours of cross-referencing.
The four stock pitch designations
- XL (Extra Light) — 1/5-inch (0.200") pitch. Used on light-duty, low-torque synchronous drives such as instrument and small conveyor applications.
- L (Light) — 3/8-inch (0.375") pitch. The workhorse for light-to-medium power transmission and general machinery.
- H (Heavy) — 1/2-inch (0.500") pitch. Handles higher torque loads and is one of the most common industrial timing pitches.
- XH (Extra Heavy) — 7/8-inch (0.875") pitch. Reserved for the highest-torque, largest synchronous drive applications.
Stock timing pulleys are manufactured to tight tolerances and stocked in minimum plain bore, taper-bushed, and QD-bushed styles, depending on size and pitch. Bushings are priced and selected separately, so the pulley and the bore adapter are two line items on the same order.
How to read a catalog number
A typical stock timing pulley catalog number is built from three pieces of information. Take TB 20L100 as an example: TB is the mounting style (taper bushed), 20 is the number of teeth, L is the pitch designation (Light, 3/8-inch), and 100 is the belt width code. Once you decode the style, teeth, pitch, and width, you have everything needed to order a pulley that will actually work on your shaft and with your belt.
When do you need flanges?
The 'F' designation in a pulley type means the pulley is flanged. Flanges keep the belt tracking on the pulley, and they matter more than most people expect. As a rule of thumb, when the drive center distance is eight times the diameter of the smaller pulley, or when the drive operates on vertical shafts, both pulleys should be flanged. Skipping flanges on a long-center or vertical drive is a common cause of belt walk-off and premature belt edge wear.
If you are replacing a timing pulley and only have a worn part in hand, bring us the pitch, tooth count, belt width, and bore requirements — or the part number — and we will match the correct stock pulley. Most stock timing pulleys are available for immediate shipment, which is exactly what you want when a synchronous drive is down.
Catalog item plug: RBC Industrial stocks Martin timing pulleys in XL, L, H, and XH pitches, in stock bore, taper-bushed, and QD styles — plus the matching bushings, flanges, and synchronous belts.