Measuring a pulley comes down to a handful of key dimensions: the outside diameter, the bore (center hole), the groove profile, and the overall width. Whether you’re replacing a worn V-belt sheave on a motor or ordering a new timing pulley for a drive system, getting these numbers right ensures the replacement fits your shaft and matches your belt. Here’s how to take each measurement accurately.
Tools You’ll Need
A measuring tape or ruler handles most pulley measurements. For tighter tolerances, especially on bore diameter and groove dimensions, a set of calipers gives you much better precision. A pulley groove gauge is useful if you work with belt drives regularly, but it’s not essential for a one-time measurement. If you’re measuring a large pulley that’s still mounted, a flexible diameter tape (sometimes called a pi tape) wrapped around the outside edge reads diameter directly without needing to find center.
Outside Diameter
The outside diameter (OD) is the simplest measurement: the distance straight across the pulley from one outer edge to the other, passing through the center. For a flat or round-belt pulley, place your calipers across the widest point. For a V-belt sheave, measure across the top of the grooves, not down inside them.
If the pulley is still on the shaft and you can’t get calipers around it, wrap a flexible tape around the circumference and divide by 3.14159 (pi) to get the diameter. This method is surprisingly accurate on larger pulleys where caliper access is awkward.
One thing to know: the outside diameter is not the same as the pitch diameter, which is where the belt actually rides. For timing pulleys, the pitch diameter is the dimension that determines speed ratios. You can calculate it with this formula: (tooth pitch × number of teeth) ÷ 3.14159. For V-belt sheaves, the pitch diameter sits partway down inside the groove, roughly where the belt’s center of gravity contacts the sheave wall. Manufacturers list pitch diameters in their catalogs, but for ordering a physical replacement, the OD and groove count are what you’ll measure directly.
Bore Diameter and Keyway
The bore is the center hole that slides onto the shaft. Measure across the inside of this hole with calipers. If the pulley uses a tapered bushing (common on industrial sheaves), you’ll measure the bushing bore rather than the pulley hub itself. Common bore sizes in imperial units run in standard increments: 1/2″, 5/8″, 3/4″, 7/8″, 1″, and so on.
Most pulleys have a keyway, a small rectangular slot cut into the bore that accepts a key to lock the pulley to the shaft. You need two measurements here: the width of the slot and its depth from the bore surface. Keyway sizes follow standardized tables based on shaft diameter. For example, a shaft between 5/16″ and 7/16″ takes a keyway 3/32″ wide, while a 1″ to 1-1/4″ shaft uses a 1/4″ wide keyway. If you’re ordering an exact replacement, matching the keyway width to your existing key ensures a proper fit without slop or binding.
Groove Profile for V-Belt Pulleys
V-belt sheaves have grooves shaped like a “V,” and the groove dimensions tell you which belt profile the pulley accepts. The two critical measurements are the top width of the groove and its depth.
Standard belt profiles and their groove dimensions:
- A profile: 13 mm top width, 8 mm depth
- B profile: 17 mm top width, 11 mm depth
- C profile: 22 mm top width, 14 mm depth
- Z profile: 10 mm top width, 6 mm depth
The groove angle on all of these profiles is 40 degrees. Narrow-profile (wedge) belts like SPA, SPB, and SPC share the same top widths as their classical counterparts but sit deeper in the groove. An SPA groove, for instance, is 13 mm wide like an A but 10 mm deep instead of 8 mm. Measuring the depth tells you whether you’re looking at a classical or narrow-profile sheave.
To measure top width, lay a straightedge or ruler across the top of the groove and read the distance between the two upper edges. For depth, lower a narrow rule or the depth rod of your calipers into the groove until it touches the bottom. If the pulley has multiple grooves, count them and measure the center-to-center distance between adjacent grooves as well, since multi-groove pulleys need belts that match both the profile and the groove spacing.
Tooth Count for Timing Pulleys
Timing (synchronous) pulleys don’t use friction like V-belt sheaves. Instead, teeth on the pulley mesh with teeth on the belt. The two measurements that define a timing pulley are the number of teeth and the tooth pitch.
Counting teeth is straightforward: go around the pulley and count every tooth. Mark your starting point with a piece of tape or a marker so you don’t lose track on larger pulleys. The tooth pitch is the distance from the center of one tooth to the center of the next, measured along the pitch circle (not the outer surface). Common pitch standards include 2 mm, 3 mm, 5 mm (called HTD or GT profiles), and older imperial pitches like XL (1/5″), L (3/8″), and H (1/2″).
If you can’t find a marking on the pulley, measure the distance across several teeth and divide. For instance, if 10 teeth span 50 mm measured along the belt contact surface, the pitch is 5 mm. Once you have tooth count and pitch, you can calculate the pitch diameter: multiply pitch by tooth count, then divide by 3.14159. A 30-tooth pulley with a 5 mm pitch has a pitch diameter of about 47.7 mm.
Overall Width and Number of Grooves
Measure the total width of the pulley from one face to the other. This matters for fitment, especially in tight spaces or when aligning multiple pulleys on parallel shafts. On multi-groove V-belt sheaves, also measure from the center of the first groove to the center of the last groove. This “groove-to-groove” span ensures your replacement sheave positions the belts in the same plane as the mating pulley, which prevents belt tracking problems and premature wear.
Recording Your Measurements
When you’re ready to order a replacement, you’ll typically need to provide: the outside diameter, bore size, keyway width, number and profile of grooves (or tooth count and pitch for timing pulleys), and the overall hub width. If the pulley uses a quick-detachable (QD) or taper-lock bushing, note the bushing type as well, since these follow their own standardized dimensions. The bushing type is usually stamped or cast into the bushing itself.
Taking a photo of the pulley next to a ruler before disassembly gives you a backup reference. If any stamped part numbers or manufacturer markings are visible on the pulley face or hub, record those too. They can shortcut the entire identification process.