HTD vs. GT vs. trapezoidal: which timing-pulley tooth profile?

Where the three profiles come from, and their geometry

Trapezoidal teeth are the original timing-belt profile: the imperial series MXL (2.032 mm pitch), XL (5.08 mm), L (9.525 mm), H (12.7 mm) and XH (22.225 mm), and the metric T series T2.5 / T5 / T10 with the reinforced AT5 / AT10. The straight-sided tooth concentrates stress at its root, so load per belt width is the lowest of the three — but the installed base is huge and belts are everywhere, so it remains the standard on many legacy machines and light drives.

HTD (High Torque Drive) changed the tooth to a rounded form in pitches 3M / 5M / 8M / 14M. The curved root removes the stress concentration, so a belt of the same width carries considerably more load than a trapezoidal one, meshes smoothly and runs quietly; it is the most common profile in automation, packaging and machine tools today. By design there is some clearance between belt tooth and pulley groove, so a small amount of backlash remains.

GT (2GT / 3GT / 5GT, also sold as BAR or PowerGrip-GT-type profiles) starts from the curvilinear tooth and makes it deeper, with the groove following the belt tooth more closely — less backlash and better positioning, which is why it is the first choice on 3D printers, linear modules, semiconductor and medical equipment where repeatable positioning matters. Wolfer's precision BAR / GT pulleys belong to this family.

Width, material (aluminium / steel / cast iron), flanges and bore are machined to drawing; fix the profile from the belt supplier's part number first, then choose the pulley.

How to choose

By torque and width: to transmit more torque in a limited belt width while keeping noise down, choose HTD, with 8M / 14M for high power. By positioning accuracy: reciprocating positioning axes that need low backlash beyond what the encoder loop corrects, choose GT (2GT / 3GT small, 5GT medium). By installed base and cost: replacements on older machines, light loads or cost-driven drives use trapezoidal MXL / XL / L / H; European machines often use T / AT, where AT's thicker tooth gives more stiffness.

The pulley itself matters as much as the profile. Aluminium is light and low in inertia; steel and cast iron carry more load and wear better. Flanges keep the belt from walking — at least one pulley in a drive should have them — and taper-bore pulleys suit drives where the bore changes or field service is expected. Wolfer machines every pulley from solid bar (no castings) with bore-to-pitch-circle concentricity held to ≤ 0.03 mm, outside diameters Ø10–400 mm and widths 9–150 mm; the size tables are on the selector pages, and a discontinued OEM pulley can be copied from a sample scanned on the CMM.