HTD timing pulleys in 3 mm, 5 mm, and 8 mm pitches are governed by the ISO 13050 curvilinear tooth profile, and their PLD (pitch line difference, the offset between the pitch diameter and the belt's neutral axis running radius) varies systematically with pitch and tooth count [S1][S2].
SIT's TOP DRIVE HTD product line confirms the three common metric HTD pitches (3M, 5M, 8M) plus 14M and dual-pitch variants D5M, D8M, D14M, all with polychloroprene compounds, fiberglass tensile members, and a -20/+100 °C operating window [S1].
What PLD Actually Means on an HTD Pulley
PLD on an HTD pulley is the radial offset between the theoretical pitch circle (where the belt's tensile cord layer sits in theory) and the pulley's actual running diameter (where the cord layer physically rides once the belt meshes into the curved tooth flanks) [S2]. The value is small, typically a fraction of a millimetre, but it directly affects centre-distance calculation, belt length selection, and the engagement angle between belt teeth and pulley grooves.
For HTD profiles per ISO 13050, the cord lies below the pitch line because the curvilinear tooth has more material above the pitch circle than below it; the PLD correction is added (or subtracted, depending on convention) when converting pitch diameter to the diameter used for length and centre-distance math [S1][S2]. The standard publishes PLD values tooth-count by tooth-count, which is why manufacturer datasheets tabulate them rather than treat them as a single constant per pitch.
Typical PLD Values for 3M, 5M, and 8M
Mid-range PLD values for HTD pulleys follow the pitch: 3M PLD commonly sits around 0.280 mm, 5M PLD around 0.480 mm, and 8M PLD around 0.686 mm for representative tooth counts in the 40-60 tooth range [S2]. The MIS Belt datasheet, for example, lists 8M PLD at 0.686 mm across its 14-tooth-through-264-tooth stock range, and shows 5M PLD at 0.480 mm in the same tabular layout [S2].
PLD does drift slightly with tooth count because the curvature of the tooth root and flank geometry changes as pitch diameter grows, but for engineering-grade centre-distance work the value is taken as a constant per pitch from the manufacturer table. A 60-tooth 5M pulley running against a 20-tooth 5M pulley, for instance, uses the same 0.480 mm PLD correction on both wheels when calculating effective running diameter [S2].
Part Number Coding for HTD 3M, 5M, and 8M Pulleys
B&B Manufacturing's 7-segment coding scheme illustrates the field convention: a part number like 15-5M15-6FA3 breaks down as tooth count (15), pitch code (5M = 5 mm HTD), belt width (15 = 15 mm), inch/metric flag, flange/hub style (6F = with hub and flanges), material (A = aluminium, S = steel, I = cast iron, SS = stainless, P = plastic), and bore size in millimetres or inch code [S3]. The pitch code 3M, 5M, and 8M is the only segment that maps directly to ISO 13050, and it is the segment that fixes which PLD table to pull from [S1][S3].
For users comparing 3M versus 5M versus 8M, the width codes (06, 09, 15, 20, 25, 30 mm) stay consistent across pitches, so a 15 mm wide belt on 5M uses the same width code as a 15 mm wide belt on 8M; only the pitch segment changes [S3]. Hub style, material, and bore segments are decoupled from pitch and from PLD, which is why datasheets can publish PLD tables independently of the rest of the part number.
How PLD Affects Centre Distance and Belt Length Selection
Effective running diameter D_eff = pitch diameter - PLD for HTD (cord below pitch line); centre distance between two pulleys is then computed from these effective diameters plus the belt pitch length [S2]. A 60-tooth 5M pulley has a pitch diameter of 95.49 mm but an effective running diameter of 95.01 mm once the 0.480 mm PLD correction is applied [S2].
Skipping the PLD correction on a 5M drive introduces roughly 1 mm of error on a single pulley diameter, which propagates into the centre-distance term and the calculated belt length; for a 1 m centre distance this is on the order of 0.1%, usually inside stock belt length tolerances, but for precision timing applications (CNC axes, printing registration, indexing conveyors) the correction is non-negotiable. The same logic applies to 3M (0.280 mm PLD) drives, where the absolute correction is smaller because the pitch is smaller, and to 8M (0.686 mm PLD) where it is the largest of the three common pitches [S2].
Material and Hub Options Relevant to PLD Tables
HTD 3M, 5M, and 8M pulleys are stocked in aluminium (lightest, most common for low-inertia drives), steel (highest torque capacity), cast iron (heavy-duty industrial), stainless steel (food/pharma washdown), and occasionally plastic or acetal for very small 3M sizes [S1][S3]. PLD values are geometry-driven and do not change with pulley material, but pitch-line concentricity and bore tolerance do, and those are the dimensions a material choice actually moves.
SIT's HTD stock offering ships pulleys in aluminium, cast iron, or steel with solid hub or SER-SIT taper-bushing mount, with 14M and D5M/D8M/D14M dual-pitch options available in addition to the 3M/5M/8M single-pitch family [S1]. For drive designers comparing pulley material against CBAM-driven aluminium and steel cost deltas, PLD stays flat across material choices, so the trade is between inertia, torque, corrosion resistance, and unit cost rather than geometry.
Limits, Edge Cases, and Common Sourcing Errors
HTD 3M pulleys below 10 teeth and 8M pulleys above 200 teeth are non-standard and often have to be custom-machined; published PLD tables may not cover these extremes, and extrapolation is unsafe because tooth root geometry shifts at very small and very large tooth counts [S2]. The MIS Belt HTD catalogue, for instance, extends 8M stock up to 264 teeth but starts at 14 teeth; going below 14 teeth on 8M means the designer should request a custom PLD value from the belt maker rather than assume the 0.686 mm mid-range figure still holds [S2].
Another common error is mixing HTD and PowerGrip GT pulleys: GT2/GT3 use a different modified curvilinear profile with a different (typically smaller) PLD, and a GT belt on an HTD pulley (or vice versa) gives wrong mesh and accelerated tooth wear even though the pitch codes 3M/5M/8M appear in both systems [S3]. When in doubt, confirm the tooth profile designation (HTD vs GT) on both the belt and the pulley datasheet before trusting the PLD number, because the same pitch code on a PowerGrip GT pulley will reference a different PLD table than an HTD pulley of the same code [S3].
Standards, Sources, and Where to Verify PLD
ISO 13050 is the governing standard for the HTD curvilinear tooth profile across all three pitches; manufacturer PLD tables from MIS Belt, SIT, B&B Manufacturing, and Gates are derived from this standard and are the practical reference for engineering work [S1][S2][S3]. The MIS Belt HTD PDF in particular is one of the few publicly available sources that prints PLD alongside pitch diameter, outside diameter, and tooth count for the full 3M, 5M, and 8M stock range in a single table [S2].
For a drive that crosses into electric linear actuator sizing or other precision motion contexts, the PLD correction on the timing pulleys is the same physics but the tolerance stack changes: 3M and 5M drives tend to dominate small-package linear stages, while 8M drives are more common on packaging, conveyor, and hydraulic motor pump drives where torque capacity, not positioning resolution, sets the pitch. Watch for the next ISO 13050 maintenance review and for any HTD vs GT profile consolidation in manufacturer catalogues, since both would refresh the PLD tables that designers currently pull from MIS Belt and SIT datasheets [S1][S2].
For component-level specifications, see construction machinery and equipment, lamps and light fittings, and lighting equipment and electric lamps.