ISO Trapezoidal Thread Designations and Forms

Read the designation

A metric trapezoidal thread designation such as Tr20×4 identifies a nominal diameter of 20 mm and a pitch of 4 mm. For a single-start thread, lead equals pitch; for a multi-start thread, lead is pitch multiplied by the number of starts. The table gives the manufacturer’s listed nominal dimensions for selected designations. It includes the pitch diameter and specific internal and external diameters, each kept under its source symbol.

ISO 2902 defines the general plan of diameter and pitch combinations, while ISO 2904 specifies basic dimensions. The standards are related but do not make every field interchangeable: profile, basic dimensions, diameter/pitch selection and size tolerances are addressed in separate documents. ISO’s catalog lists ISO 2902:2016 as confirmed and ISO 2904:2020 as current when checked for this article. Confirm the live standard record before relying on it for a new design.

The Bornemann page labels its listed entries as nominal dimensions according to DIN 103. For example, its Tr20×4 row reports d2 = D2 of 18 mm, internal major diameter d4 of 20.5 mm, external core diameter d3 of 15.5 mm and internal core diameter d1 of 16 mm. Keep those source labels with the numbers; do not relabel a core diameter as a minor-diameter limit or a tolerance boundary.

What the dimensions mean

The pitch diameter is a theoretical cylinder where thread widths and spaces are nominally equal. It is central to fit and engagement, but a nominal value alone does not determine whether a real screw and nut will assemble. The major and minor diameters describe different thread boundaries. This article follows the symbols in the source table and focuses on nominal values. It does not provide tolerance classes, maximum material sizes, allowances, flank-angle tolerances or inspection gauge limits.

The diameter and pitch together identify a thread series entry. The pitch is the axial distance between adjacent corresponding points on one thread. Lead describes axial travel per revolution and only matches pitch for a single-start thread. Multi-start screw assemblies therefore need the start count as well as the designation. Do not infer lead or nut travel from pitch alone if the product is multi-start.

Trapezoidal profiles are commonly used for power transmission and traversing motion, but the name does not establish load capacity. A screw’s force and life depend on the screw and nut material, engagement length, lubrication, speed, buckling, wear, alignment, temperature and drive arrangement. A nominal dimensional lookup cannot replace a lead-screw or actuator design calculation.

Choose a source-matched size

Start with the drawing or equipment specification, then match the full designation including diameter, pitch, start count if applicable, hand and tolerance class. A dimension found in a vendor’s nominal table is useful for initial identification, but the manufacturer’s actual product drawing controls that product. If a mating nut comes from another supplier, confirm that both products use the same thread standard and class.

The listed examples span small sizes such as Tr8×1.5 and larger designations such as Tr40×10. Several nominal diameters have more than one listed pitch. They are separate combinations, not interchangeable choices based only on the major diameter. A change in pitch alters the thread geometry and, for a single-start thread, the lead and travel per revolution.

Never mix an ISO metric trapezoidal series with an ACME thread just because the forms appear similar. Their flank profiles and dimensional specifications differ. Unified inch threads also use a separate system. A replacement decision should verify the actual standard, thread form, pitch, lead, hand, start count and tolerance designation instead of relying on a visual match or an approximate nominal diameter.

Use nominal data within its limits

For a new component, consult the applicable controlled standard or a qualified manufacturer’s drawing for the complete geometry and tolerance requirements. ISO standards are copyrighted publications; this reference links to their official catalog pages and does not reproduce their full text. The open vendor table is the source for the values shown here. It should be treated as a dimensional reference to the specific entries, not as proof that a component satisfies a current acceptance specification.

For inspection, select gauges and measurement methods specified by the governing drawing or standard. A caliper check of outside diameter will not establish pitch diameter, profile or functional fit. Thread wires, gauges, optical measurement or a coordinate method may be appropriate depending on the requirement, but the acceptance method must be chosen for the specified class and geometry.

Where a project requires interchangeability or carries a load, resolve uncertainty before cutting material. Confirm thread series and tolerance class with the design authority, then verify the supplier’s material and manufacturing data. This table helps identify nominal forms and compare listed diameters; it does not establish strength, efficiency, back-driving behavior or suitability for a particular mechanism.

Source and scope

Dimension source: Bornemann DIN 103 nominal-dimension table. Standard scope: ISO 2902:2016 and ISO 2904:2020. These standards are not reproduced here.