Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads or high accuracy[citation needed] are required, as in a vise or the leadscrew of a lathe.[1] Standardized variations include multiple-start threads, left-hand threads, and self-centering threads (which are less likely to bind under lateral forces).

The trapezoidal metric thread form is similar to the Acme thread form, except the thread angle is 30°.[5][6][7] It is codified by DIN 103.[8] Although metric screw threads are generally more prevalent worldwide than imperial threads, the Acme thread is very common worldwide, and may be more widely used than the trapezoidal metric thread. This is not surprising, as manufacturers today are usually capable of making whichever threads (metric or imperial) are best for any given application (based on customer expectations or tooling availability). It may be that the tooling for Acme threads has been so dominant (compared to trapezoidal metric) that customers tend to want Acme threads for power screws regardless of metric standards used elsewhere in the product.

The Acme thread form has a 29° thread angle with a thread height half of the pitch; the apex (or crest) and valley (or root) are flat. This shape is easier to machine (faster cutting, longer tool life) than is a square thread. The tooth shape also has a wider base which means it is stronger (thus, the screw can carry a greater load) than a similarly sized square thread. This thread form also allows for the use of a split nut, which can compensate for nut wear.[9]

Compared to squared threads used for the same purpose of sustaining axial loads, disadvantages of the Acme thread form are lower efficiency due to higher friction and some radial load on the nut (angular offset from square).[5]

When created prior to 1895, Acme screw threads were intended to replace square threads and a variety of threads of other forms used chiefly for the purpose of traversing motion on machines, tools, etc. Acme screw threads are now extensively used for a variety of purposes. Long-length Acme threads are used for controlled movements on machine tools, testing machines, jacks, aircraft flaps, and conveyors. Short-length threads are used on valve stems, hose connectors, bonnets on pressure cylinders, steering mechanisms, and camera lens movement.[10]

Finally, there also exists a Stub Acme Thread standard, identical in all respects to the one just described except for the height of the basic thread being 0.3P.

Standard Acme thread pitches for customary diameters[11]
Nominal diameter (in) Pitch (in) Thread density (1/in)
14 116 16
516 114 14
38 112 12
12 110 10
58 18 8
34, 78 16 6
1, 1 14 15 5
1 12, 1 34, 2 14 4
2 12 13 3
3 12 2

In case of the trapezoidal thread form the angle is 30° instead of 29°.[6][7] All dimensions are in millimeters.[6][7]

Trapezoidal threads are defined as follows by ISO standards:

$Tr\,60 \times 9$

where Tr designates a trapezoidal thread, 60 is the nominal diameter in millimeters, and 9 is the pitch in millimeters. When there is no suffix it is a single start thread. If there is a suffix then the value after the multiplication sign is the lead and the value in the parentheses is the pitch. For example:

$Tr\,60 \times 18 (P9) LH$

would denoted two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread.[12]

Standard trapezoidal thread pitches for metric diameters
Nominal diameter (mm) Pitch (mm) Reference
10 2
12 3
24, 28 5 [12]
32, 36 6 [12]
40, 44 7 [12]
48, 52 8 [12]
60 9 [12]
70, 80 10 [12]
90, 100 12 [12]

## References

1. ^ Bhandari, pp. 202–204.
2. ^
3. ^
4. ^
5. ^ a b Bhandari, p. 204.
6. ^ a b c Trapezoidal coarse thread
7. ^ a b c Trapezoidal fine thread
8. ^ Green 1996, p. 1703.
9. ^ Bhandari, pp. 203–204.
10. ^ American Society of Mechanical Engineers (1997), ASME B1.5 - 1997 Acme Screw Threads, ASME Press, ISBN 0-7918-2482-9.
11. ^ Shigley, p. 400.
12. Bhandari, p. 205.