A timing pulley with 25.40 mm pitch diameter has a listed adjacent-pitch tolerance of 0.03 mm and accumulated-pitch-error tolerance of 0.05 mm under the cited pulley table [S2].
Accumulated pitch error is the cumulative spacing deviation across successive teeth, so a drive can lose positional accuracy even when every local pitch looks acceptable. Pitch, profile, tooth count, and pulley diameter determine the practical acceptance criteria for a timing pulley.
What accumulated pitch error controls
Accumulated pitch error describes the departure of the actual tooth positions from ideal spacing over a defined measurement length. A 0.03 mm adjacent-pitch tolerance limits the error between neighboring teeth, while a 0.05 mm accumulated tolerance limits the running total over the specified number of teeth or circumferential measurement length [S2]. These are different controls and cannot be substituted for one another.
The reference table applies the 0.03 mm adjacent-pitch tolerance and 0.05 mm accumulated-pitch-error tolerance to the 25.40 mm pulley-pitch-diameter range shown in the extract. The upper boundary visible in that range is also 25.40 mm, so the value must not be generalized to every diameter or tooth count [S2]. Larger or smaller pulleys require the corresponding dimension-specific table rather than a copied limit.
For a 200 mm/s positioning axis, a 1% friction-drive slip produces 2 mm of cumulative position error in one second, illustrating why tooth-engagement accuracy matters in positioning systems [S3]. A timing belt is not friction-driven at the tooth interface, but accumulated pulley pitch error still affects where each tooth engagement occurs.
Pitch diameter and tooth count set the error budget
Pulley pitch is the centerline distance between adjacent teeth, while pitch diameter is the diameter at the theoretical pitch line. The pulley tooth count and pitch determine the relationship between one revolution, belt travel, and the number of accumulated tooth-spacing deviations encountered [S4]. Pitch and pitch diameter are therefore not interchangeable dimensions.
The cited MISUMI overview separates general torque drives from high-accuracy positioning pulleys. General torque examples include MXL at 2.032 mm, XL at 5.08 mm, L at 9.525 mm, and H at 12.7 mm pitch. High-accuracy positioning examples include 1.5GT at 1.5 mm, 2GT at 2.0 mm, 3GT at 3.0 mm, 5GT at 5.0 mm, and 8YU at 8.0 mm pitch [S2].
Those families serve different error requirements. The high-accuracy range is identified by reduced backlash, while the general-purpose range is intended for torque transmission and light-load conveyance [S2]. Selecting only by pitch diameter is therefore insufficient: the profile family, number of teeth, and required positioning resolution must be specified together.
The distinction becomes more demanding in small-pitch systems because one accumulated tolerance represents a larger fraction of a short positioning stroke. At 1.5 mm pitch, 20 teeth cover only 30 mm of belt travel, compared with 300 mm for 20 teeth at 15 mm pitch. A fixed linear error allowance therefore behaves differently across tooth counts and pitches.
Compare profiles, diameters, and error acceptance
Trapezoidal profiles such as MXL, XL, L, and H remain appropriate where legacy interchangeability governs, while curvilinear HTD and GT2 or GT3 profiles distribute tooth contact more evenly and provide 20–40% higher torque capacity for the same pitch and pulley width [S3]. The profile choice affects load capacity and contact stress, but it does not authorize a larger accumulated-pitch tolerance.
Use the right inspection method
Belt pitch can be checked by measuring across 10 teeth with calipers in the mold grooves and dividing the result by 10 [S5]. Pulley pitch, by contrast, is measured at the centerline, and the pulley specification must define the relevant inspection surface and datum [S4]. A belt measurement cannot substitute for verifying the manufactured pulley.
Record at least 3 separate observations: adjacent pitch, accumulated pitch error over the specified tooth count, and the pitch-line or outside-diameter relationship. A direct comparison of XL pitch-line geometry is covered in XL Timing Pulley Pitch Line Differential: Why the 0.200 in Pitch Misleads You.
Do not assign a universal measurement length from the abbreviated table extract. The adjacent and accumulated tolerances are meaningful only when tied to their stated diameter range, tooth count, and inspection definition [S2]. For procurement, require the manufacturer to state those conditions in the drawing, including units and whether the measurement is radial, tangential, or pitch-line referenced.
Where accumulated error becomes a system failure
Long belts with small pulleys on a short center distance create low-tension conditions, while short belts with the corresponding geometry can create high tension; these geometry combinations affect how tolerance accumulation propagates through a drive [S6]. Pulley accuracy must therefore be reviewed together with belt length, center distance, pulley size, and tensioning, not as an isolated machining requirement.
Small-pitch systems are commonly used where high speed and high positioning resolution are priorities, while larger pitches suit higher-load and higher-torque systems [S5]. The same accumulated displacement can create a larger fraction of a short indexing step than of a long conveyor increment. A conveyor using a conveyor chain is a different positive-drive architecture, but its comparison helps isolate the error: chain pitch, sprocket pitch, and roller-chain backlash must be assessed as a matched system rather than transferred from timing-pulley data.
A pulley that passes an adjacent-pitch check can still produce poor angular repeatability when several local deviations accumulate in the same direction. Inspection records should therefore preserve the sequence of deviations, not only the maximum adjacent result. That sequence reveals bias that a pass or fail reading at one tooth would conceal.
Specify the complete acceptance package
A purchase specification should identify pitch, profile family, tooth count, pitch diameter, adjacent-pitch tolerance, accumulated-pitch-error tolerance, measurement length or tooth count, bore, keyway, and face-width requirements. The visible MISUMI data support exact limits of 0.03 mm for adjacent pitch and 0.05 mm for accumulated pitch error in the cited 25.40 mm range, but they do not establish those values for other diameters [S2].
Material and manufacturing route also belong in the final decision. Cold-formed timing pulley stock is offered in aluminum and steel, and one supplier processes pulley stock in bar lengths with outside diameters up to six inches [S4]. These manufacturing facts do not alter the published pitch tolerance, but bore machining, keyway fit, runout, and surface treatment can add error outside the tooth-spacing requirement.
For a replacement pulley, do not assume that matching the visible outside diameter is sufficient. Confirm the pitch family and tooth count first, then verify pitch diameter and the manufacturer's dimensional limits. The XL Timing Pulley PLD 0.254 mm: Pitch vs Outside Diameter Math provides the relevant distinction between pitch, outside diameter, and pitch-line differential.
The next trackable signal is the inspection record: confirm whether it reports 0.03 mm adjacent pitch and 0.05 mm accumulated error for the applicable 25.40 mm range, and whether the measurement covers the required tooth count rather than only one local interval [S2].