On T-profile timing pulleys, outside diameter runs smaller than pitch diameter because the belt seats on the pitch line, not the tooth tip: the tooth projects above the pitch line by an addendum Hf, so De = Dp - Hf, per CHSSB's ISO 5294:1989-based timing pulley manual [S4]. For T5 (5 mm pitch) and T10 (10 mm pitch), this relationship is identical in form, with T10 simply double the linear scale.
Pitch diameter itself is mechanical: Dp = P × Z / pi, where P is the belt pitch (5.0 mm for T5, 10.0 mm for T10) and Z is the tooth count. A 16-tooth T5 pulley therefore has Dp ≈ 25.46 mm, with a measured O.D. of 24.62 mm on a stock polyacetal part, that is 0.84 mm below the pitch line [S2]. A flanged T5 idler in the same tooth range is dimensionally identical in the tooth body, with flanges adding 1.18 to 1.20 mm to the overall O.D. for belt tracking [S3].
Pitch and O.D. Relationship for T5 and T10
T5 pitch is 5.0 mm and T10 pitch is 10.0 mm, both with a trapezoidal tooth profile per the original ISO 5294 tooth outline [S4]. The outside diameter always sits below the pitch line: SDP-SI's Handbook of Timing Belts, Pulleys, Chains and Sprockets explicitly states that the outside diameter of a timing pulley is smaller than its pitch diameter, the inverse of a spur gear [S5].
For T5, a 16-tooth part shows P.D. 25.46 mm and O.D. 24.62 mm, a difference of 0.84 mm [S2]. Doubling tooth count to 32 keeps the same 0.84 mm offset on geometrically similar T5 parts. For T10, with double the pitch, the absolute offset is roughly double, around 1.65 to 1.70 mm, since the trapezoid scales linearly. CHSSB's catalogue gives De = Dp - Hf as the working formula across all T2.5 to T20 profiles [S4].
Stock-Size O.D. Reference: T5, 10 mm Belt Width
The 12-tooth T5 polyacetal pulley, PN 16251200, measures 18.26 mm O.D. on a 19.10 mm P.D., with 20.65 mm flanges and 15 mm hub, 5 mm pilot bore, 6.2 g [S2]. The 13-tooth version measures 19.85 mm O.D. on 20.69 mm P.D. (PN 16251300, 7.2 g). The 14-tooth version, 21.44 mm O.D. on 22.28 mm P.D. (PN 16251400, 8.1 g), 15-tooth: 23.03 mm O.D. on 23.87 mm P.D. (PN 16251500, 8.6 g), 16-tooth: 24.62 mm O.D. on 25.46 mm P.D. (PN 16251600, 9.6 g) [S2].
These polyacetal die-cast pulleys are rated to 80°C continuous service per the manufacturer data sheet, with a 10 mm belt width, 11.5 mm tooth width, 8 mm hub length, 1.25 mm flange thickness, and 22 mm total length [S2]. A misapplied steel or aluminium substitute should be checked against the same envelope, since belt-pulley geometry is the load path, not the material.
Flanged Idler Dimensions and the Pitch-vs-Effective Diameter Distinction
MISUMI's T5/T10 flanged timing idler programme offers teeth counts from 14 through 32, belt widths from 10 to 50 mm, and shaft bores from 6 to 25 mm, in aluminium, 2000-series aluminium, or 304 stainless, with optional clear anodize, hard clear anodize, or electroless nickel plating [S3]. Press-fitted bearings per JIS B1514 Class 0 are standard, with swaged flange options NFC, RFC, and LFC that do not appear in the CAD preview [S3].
A common spec error: Optibelt's ZRS documentation warns that the effective diameter of a timing belt drive is the diameter to the belt's centreline, not the pulley's outer diameter, so the effective line of the timing belt sits above the O.D. by roughly the belt's tooth height [S7]. Mulco reiterates the same point: their catalogue lists theoretical pitch circle diameters d0, while the Mulco belt-pilot software uses the effective diameter for centre-distance calculations [S8]. For an OEM doing a layout, use the pitch diameter (or effective diameter) for shaft-centre math, not O.D.
Manufacturing Tolerances on O.D.
Mectrol's timing belt pulley catalogue specifies O.D. tolerance as 0.003 in (≈ 0.076 mm) for diameters of 1 in. and under, with an additional allowance per inch of O.D. up through 10.0 in [S6]. Translating to T5 stock sizes, a 24.62 mm (≈ 0.969 in) T5 16-tooth part falls under the 1 in. threshold and is held to ±0.076 mm O.D., well within the 0.84 mm offset between P.D. and O.D. [S2][S6].
For larger T10 drive pulleys above 1 in. O.D., Mectrol's incremental allowance kicks in, so a 100 mm (≈ 3.94 in) T10 part typically runs ±0.10 to ±0.13 mm O.D. depending on the catalogue clause [S6]. Surface treatment adds risk: electroless nickel plating per the MISUMI idler programme builds 0.010 to 0.020 mm onto the O.D. and must be called out on the print, or the assembled drive will bind on the flanges [S3].
Material and Width Options Across T5 and T10
Polyacetal (POM) T5 stock pulleys are commonly stocked in 12 to 16 teeth at 10 mm belt width for lightweight instrument drives, with 80°C thermal ceiling [S2]. CHSSB's catalogue offers S45C carbon steel, GG25 cast iron, FFF40 ductile iron, aluminium (UN13571 TA16, anodised), nylon, and stainless steel as production materials, with black phosphate, black oxide, anodising, or zinc plating as finishes, and dynamic balancing grade 6.3 specified for GG25 cast iron drives [S4].
For T5 drives, aluminium and polyacetal dominate below 1 kW at sub-1000 rpm duty. For T10, steel and cast iron are the default for drives above 0.5 kW or with shock loading, since T10's larger tooth shares load across a 10 mm pitch. Misumi's T5/T10 idler range is stocked in 14, 16, 18, 20, 22, 24, 25, 26, 28, 30, and 32 teeth, belt widths 10/15/20/25/30/40/50 mm, and shaft bores 6/8/10/12/15/20/25 mm [S3].
Sizing Rule of Thumb and Cross-Series Comparison
For an 8 mm pitch HTD or T-class pulley, Pfeifer Industries' chart uses a quick rule: O.D. in inches ≈ number of teeth / 10 [S1]. For T5 (5 mm pitch), the same logic scaled: O.D. in mm ≈ 5 × Z / pi - Hf, so a 20-tooth T5 has P.D. ≈ 31.83 mm and O.D. roughly 30.99 mm. For T10 (10 mm pitch), O.D. in mm ≈ 10 × Z / pi - Hf, so a 20-tooth T10 has P.D. ≈ 63.66 mm and O.D. roughly 61.96 mm. The trapezoid's Hf term is what pulls O.D. below P.D. in both cases.
Direct comparison on the four decision axes that matter for a designer: linear pitch (T5 = 5.0 mm, T10 = 10.0 mm, ratio 1:2); typical minimum stocked tooth count in catalogues (T5: 12 teeth, T10: 14 teeth for flanged idlers, narrower for T5) [S2][S3]; torque capacity per tooth (T5 lower, T10 higher because of larger shear area); and minimum recommended pulley diameter for a given belt speed (T5 generally limited to small pulleys below 25 mm P.D., T10 used from 30 mm P.D. up). The decision is linear-pitch and torque, not surface speed: pick T5 for instrumentation, light indexing, and small printers; pick T10 for packaging lines, conveyor drives, and CNC feed drives where tooth shear is the constraint.
Standards Backing and Sourcing Constraints
ISO 5294:1989 is the governing standard for T-profile synchronous belt drives and explicitly defines the trapezoidal tooth geometry, the De = Dp - Hf relationship, and the materials list (aluminium, C45 steel, GG25 cast iron) cited by CHSSB [S4]. SDP-SI's handbook (published 2025-11-24) reproduces the same pitch-diameter-vs-O.D. relationship for all T-pitches, including a worked T5 dimension table [S5]. Mulco and Optibelt both push the distinction between pitch diameter (theoretical, used for tooth meshing) and effective diameter (the centreline of the running belt), which determines shaft centre distance [S7][S8].
For sourcing in 2026, polyacetal T5 stock pulleys in 12 to 16 teeth are off-the-shelf at Maedler North America with 4-piece inventory and 1-2 week backorder lead time [S2]. T5/T10 flanged idlers in aluminium or 304 stainless are configurable on MISUMI's US site with 6-7 day shippable options on common tooth counts [S3]. Larger T10 steel drive pulleys above 100 mm P.D. are typically custom-ordered through CHSSB or equivalent, with material choice (S45C, GG25, FFF40) and finish (black phosphate, zinc) called out on the RFQ [S4]. Before placing the order, verify the print shows pitch diameter, not O.D., on the centre-distance callout, and confirm the catalogue's Hf value matches your supplier's tooth profile.
For component-level specifications, see timing pulley, checking and testing apparatus, and construction machinery and equipment.
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