XL profile timing pulleys carry a 0.200 inch (5.08 mm) tooth pitch, and the published Pitch Line Differential (PLD) is 0.254 mm, the constant subtracted from pitch diameter to land on the outside diameter of a flangeless XL pulley [S1].
This 0.254 mm PLD holds for aluminium, steel, polycarbonate and acetal XL pulleys alike, because the PLD is a function of the trapezoidal XL tooth geometry, not the body material [S1][S3]. With the pitch fixed at 5.08 mm, the only variable in the pitch-to-OD conversion is tooth count.
PLD derivation and where the 0.254 mm number comes from
The pitch diameter of a timing pulley is a theoretical circle defined by P.D. = pitch x tooth count divided by pi, which is why the value cannot be measured with a caliper [S1]. Outside diameter is what you can measure, and PLD is the bridge between the two.
For XL, P.D. = (5.08 x N) / pi in mm. The published PLD is 0.254 mm, so O.D. = P.D. minus 2 x PLD when the PLD is given as a single-sided value, or O.D. = P.D. minus PLD when it is given as a full-diameter offset, depending on the catalogue convention [S1][S3]. A 30-tooth XL aluminium pulley from Maedler carries a stated outer diameter of 48.00 mm, which gives a back-calculated P.D. of about 48.51 mm and matches P.D. = 5.08 x 30 / pi = 48.51 mm exactly when PLD is treated as a 0.254 mm single-side deduction [S2].
Verification against stock XL pulley tables
The 0.254 mm PLD reconciles cleanly with SDP-SI stock data, where a 30-tooth XL aluminium pulley lists pitch diameter 1.917 inch (48.69 mm) and outside diameter 1.897 inch (48.18 mm), giving a 0.51 mm total offset or roughly 0.255 mm per side [S3]. The 0.005 inch round-trip is inside normal manufacturing tolerance for a 0.200 inch pitch profile.
McMaster-Carr's XL filter exposes a 0.2 inch pitch, tooth counts from 10 to 35 in stock, and pitch diameters such as 0.637 inch (10 teeth), 0.891 inch (14 teeth), 1.273 inch (20 teeth) and 1.908 inch (30 teeth), each within 0.001 inch of the formula P.D. = 0.2 x N / pi [S4]. The same offset convention applies to the McMaster OD values of 0.744 inch (10T) and 1.890 inch (30T), which are 0.107 inch and 0.018 inch below P.D. respectively, varying with the hub style and flange choice rather than with PLD itself [S4].
Material, bore and finish options at the same PLD
PLD is independent of the body material, but the catalogue offering at the same 0.254 mm offset is wide. SDP-SI stocks XL pulleys in aluminium with a clear anodized finish, in black-oxide steel, in polycarbonate, acetal and moulded nylon, all sharing the 5.08 mm pitch and the same PLD [S3]. McMaster-Carr confirms the same 0.200 inch pitch across aluminium, steel, acetal and rubber XL bodies, with bores from 3/16 inch up to 5/8 inch and metric equivalents 6 mm to 12 mm [S4].
AutomationDirect's SJT line further covers the same XL geometry in hard-anodised aluminium with set-screw or QD tapered-bushing mounting, and the 0.200 inch pitch is published as the defining feature of the profile, with tooth counts from 10 to 120 in the broader catalogue [S5]. Maedler's 30-tooth XL aluminium part 18013000 weighs 177 g at a 48.00 mm O.D. and a 54 mm flanged O.D., which is consistent with a thin-web aluminium body where the 0.254 mm PLD is machined into a relatively light hub [S2].
Comparison: XL PLD vs L, HTD and T profiles
The 0.254 mm PLD is XL-specific and does not transfer to other profiles, which is the practical reason engineers treat PLD as a profile constant rather than a material constant. [S2]
Using a 30-tooth example to make the divergence concrete: XL P.D. = 48.51 mm and O.D. about 48.00 mm at PLD 0.254 mm [S1][S2]; L 30T P.D. = 90.93 mm and O.D. roughly 89.4 mm at a 0.76 mm PLD; HTD 5M-30T P.D. = 47.75 mm and O.D. roughly 46.55 mm at a 0.6 mm PLD. The point is that PLD scales with tooth size, and you cannot borrow an XL PLD for an L or HTD design without breaking the OD math.
Practical consequences for drive design
When an engineer specifies a centre distance and a belt length, the caliper-checked outside diameters of the two pulleys are the actual physical constraints, so applying the wrong PLD pushes the belt either too tight or too loose by several millimetres on small XL pulleys [S1][S3]. The SDP-SI 30-tooth case shows the magnitude: a 0.254 mm PLD versus a 0.51 mm PLD would move the calculated OD by 0.25 mm per side, or 0.5 mm across the diameter, which is enough to throw a 6.35 mm wide XL belt off its centreline in a precision application [S3].
For flange selection, the same convention applies: SDP-SI publishes the belt-guide flange as a separate diameter that sits above the belt width, not the tooth, so the 0.254 mm PLD is unrelated to flange O.D. and is computed only against the toothed body [S3]. The practical takeaway is that the 0.254 mm PLD is one of the most stable numbers in a timing-pulley drawing, but only when the profile in question is XL.
When the 0.254 mm PLD rule breaks down
The PLD rule assumes standard XL tooth proportions, stock finish and no fillet radii changes, and three situations push the real OD off the formula. First, moulded nylon and acetal XL pulleys shrink slightly during cooling, so McMaster and SDP-SI publish OD values that are a few hundredths of a millimetre under the calculated number even when PLD is held at 0.254 mm [S3][S4]. Second, custom cold-formed steel pulleys from Grob Inc. are machined from bar stock and the OD is held to the same PLD only when the customer specifies the standard XL tooth form, not a modified one [S9]. Third, belt-guard flanges and flanged pulleys add a second diameter on top of the OD, so the published 54 mm flanged O.D. on the Maedler 30T part is independent of the 48.00 mm body OD and the 0.254 mm PLD [S2].
For belt-length and centre-distance calculators, the 0.254 mm PLD feeds only into the OD-to-PD conversion on the pulley side, not the belt pitch length itself, and ContiTech's design calculator in the Pfeifer technical reference applies the PLD as a fixed profile constant rather than re-deriving it from the belt [S1]. Martin's catalog of synchronous sprockets follows the same convention, listing OD as P.D. minus a profile-specific offset that matches the 0.254 mm figure for XL across its stock aluminium and steel ranges [S10].
For an engineer cross-checking a vendor's published OD against a hand calculation, take the vendor's pitch-diameter value, subtract 0.254 mm once for single-side PLD, and compare to the vendor's outside diameter. The Maedler 30T gives 48.51 mm minus 0.254 mm = 48.26 mm, versus the published 48.00 mm, which is a 0.26 mm gap, normal for a thin-web aluminium hub where the cutter toolpath rounds the tooth tip slightly below the theoretical value [S2].
To extend the analysis into broader profile behaviour, see the spec comparison of trapezoidal vs curvilinear tooth profiles, the geometry walk-through on identifying V-groove, timing and ribbed pulleys, and the cross-profile PLD compensation table. For readers new to the underlying components, the timing pulley and timing belt reference pages define the pitch-line terminology used above. Track on October 6, 2026 whether any major XL catalogue publisher revises the 0.254 mm PLD value upward for reinforced HTD-XL hybrid profiles, and whether automationdirect or McMaster-Carr extend their XL bore range above the current 5/8 inch / 12 mm maximum.
Spec-level background on the components involved: pressure transmitter.