Threads
Acme and metric trapezoidal (Tr) threads
Acme and metric trapezoidal threads are flat-topped power-transmission threads used for lead screws, jacks, clamps and valve stems. Acme is the 29° inch thread of ASME B1.5, written as 1/2-10 ACME-2G, while metric trapezoidal is the 30° millimetre thread of ISO 2901–2904 and DIN 103, written as Tr 20×4.
Names by standard
Standards in each region
| Region | System | Designation | Standard | Match |
|---|---|---|---|---|
| USA | Acme | 1/2-10 ACME-2GGeneral-purpose (G) and centralizing (C) classes. Stub Acme (shallower thread) is a separate standard. | ASME B1.5 | Same material |
| UK | BS ISO metric trapezoidal | Tr 20×4Replaced BS 5346:1976 (withdrawn). Acme threads are supplied to ASME B1.5. | BS ISO 2901, 2902, 2903, 2904 | Same material |
| EU | ISO / DIN metric trapezoidal | Tr 20×4DIN 103 is the long-established German standard and is still the common catalogue reference. | ISO 2901–2904; DIN 103 | Same material |
| India | IS metric trapezoidal | Tr 20×4Adopts ISO 2901 (profile), 2902 (general plan), 2904 (basic dimensions) and 2903 (tolerances). | IS 7008 (Parts 1–4) | Same material |
| Japan | JIS metric trapezoidal | Tr 20×4Identical adoptions of the ISO trapezoidal standards. | JIS B 0216-1 to -3; tolerances JIS B 0217 | Same material |
Thread form
Geometry
- Acme thread angle
- 29° included
- Metric trapezoidal angle
- 30° included
- Basic thread height
- 0.5 × pitch (both forms); Stub Acme 0.3 × pitch
- Crests and roots
- Flat
- Pitch expressed as
- Acme: threads per inch; trapezoidal: mm
- Multi-start designation
- Tr: lead then pitch, e.g. Tr 20×8 (P4) is two-start; LH for left-hand
Size chart
Common sizes
| Designation | System | Nominal diameter (mm) | Pitch (mm) | TPI |
|---|---|---|---|---|
| 1/4-16 Acme | Acme | 6.35 | 1.588 | 16 |
| 3/8-12 Acme | Acme | 9.525 | 2.117 | 12 |
| 1/2-10 Acme | Acme | 12.7 | 2.54 | 10 |
| 5/8-8 Acme | Acme | 15.875 | 3.175 | 8 |
| 3/4-6 Acme | Acme | 19.05 | 4.233 | 6 |
| 1-5 Acme | Acme | 25.4 | 5.08 | 5 |
| 1 1/2-4 Acme | Acme | 38.1 | 6.35 | 4 |
| 2-4 Acme | Acme | 50.8 | 6.35 | 4 |
| Tr 8×1.5 | Metric trapezoidal | 8 | 1.5 | — |
| Tr 10×2 | Metric trapezoidal | 10 | 2 | — |
| Tr 12×3 | Metric trapezoidal | 12 | 3 | — |
| Tr 16×4 | Metric trapezoidal | 16 | 4 | — |
| Tr 20×4 | Metric trapezoidal | 20 | 4 | — |
| Tr 24×5 | Metric trapezoidal | 24 | 5 | — |
| Tr 28×5 | Metric trapezoidal | 28 | 5 | — |
| Tr 32×6 | Metric trapezoidal | 32 | 6 | — |
| Tr 36×6 | Metric trapezoidal | 36 | 6 | — |
| Tr 40×7 | Metric trapezoidal | 40 | 7 | — |
Acme rows show the common general-purpose TPI for each diameter; Acme pitch in mm = 25.4 ÷ TPI. Trapezoidal rows show the preferred pitch; other pitches are standardised for most diameters.
In practice
Uses and compatibility
Where it is used
- Lead screws and feed screws on machine tools
- Screw jacks, lifting screws and presses
- Vices, clamps and adjustable supports
- Valve stems and sluice gate spindles
- Threaded rod for linear motion where a ball screw is not needed
What interchanges
- Acme and metric trapezoidal threads are not interchangeable: the angles differ (29° vs 30°) and pitches are inch-based vs metric, even where diameters are close (1/2-10 Acme is 12.7 mm × 2.54 mm; Tr 12×3 is 12 mm × 3 mm).
- General-purpose (G) and centralizing (C) Acme of the same size and TPI share the basic form but have different clearances; specify the class on both nut and screw.
- Stub Acme has a shallower thread and does not mate with standard Acme of the same TPI.
- Trapezoidal and Acme nuts do not fit V-threads (metric, UNC, BSW) of similar diameter.
- Single-start and multi-start threads of the same pitch do not interchange; the lead must match.
Questions answered
Common questions
What is the difference between Acme and trapezoidal threads?
Acme is an inch thread with a 29° angle and pitch in threads per inch (ASME B1.5). Metric trapezoidal is a millimetre thread with a 30° angle and pitch in mm (ISO 2901–2904, DIN 103). They look similar but do not interchange.
What does Tr 20×4 mean?
A metric trapezoidal thread with 20 mm nominal diameter and 4 mm pitch, single-start and right-hand. A two-start thread with 4 mm pitch is written Tr 20×8 (P4).
Why use an Acme or trapezoidal thread instead of a V-thread?
The flatter flank angle and flat crests carry heavy axial loads with less radial force and wear, which suits lead screws, jacks and clamps that turn under load.
What is the pitch of a 1/2 inch Acme thread?
The common general-purpose 1/2 in Acme has 10 threads per inch, a 2.54 mm pitch.
From the Corona catalog
Products listed with Acme / Trapezoidal
Threaded Rods
Diameter: 1/4-2 1/2 in / M6-M60 · Length: 3-20 ft / 1-6 m
Specifications
Screw Jack with Base Plate & Wing Nut
Diameter: 1-1/4, 1-1/2 in (N.B.) / 33.7, 38, 38.1 mm (O.D.); 26-48 mm to order · Length: 24-36 in / 600-900 mm
Specifications
Hi-Load Screw Jack with Wing Nut
Diameter: 1-1/4, 1-1/2 in (N.B.) / 48.3, 60.3 mm (O.D.) · Length: 30-120 in / 760 mm-3 m
Specifications
Threaded Rods / Bars
Diameter: 1/4" - 2-1/2" / M6 - M60 · Length: 2 ft - 19 ft (3, 6, 10, 12 ft common) / 1 m - 5.8 m (1, 2, 3 m common)
SpecificationsKeep reading
Related references
References
Sources
- Trapezoidal thread form (Wikipedia) — Acme 29° and TPI, trapezoidal 30° and preferred pitches, designation format.
- AMES: Trapezoidal thread chart (DIN 103)
- BSI: BS 5346 ISO metric trapezoidal screw threads (withdrawn) and BS ISO 2901–2904
- BIS: IS 7008 (Part 1):1999 ISO metric trapezoidal screw threads, basic profile — Later parts and revisions align with ISO 2902–2904.
- JIS screw thread standards list (JIS B 0216, B 0217)
- ASME B1.5 Acme screw threads — US governing standard for Acme threads.
Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.