The Acme thread form (pronounced /ˈækmiː/, "ĂK-mē") is a common leadscrew thread profile that offers high strength with ease of manufacture. It is typically found where large loads or high accuracy are required, as in a vise or the lead screw 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 thread form is identical to the Acme thread form, except the thread angle is 30°.[2]
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Characteristics
The Acme thread form has a 29° thread angle with a thread height half of the pitch; the apex and valley are flat. This shape allows the use of a thread milling machine for manufacture, which is much cheaper than the single point cutter used in machining square threads. 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.[3]
The disadvantages of the Acme thread form are that much lower efficiency and the greater radial load on the nut, due to the thread angle.[2]
Classifications
Trapezoidal threads are defined as follows by ISO standards:
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:
would denoted two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread.[4]
| Nominal diameter [mm] | Pitch [mm] |
|---|---|
| 24, 28 | 5 |
| 32, 36 | 6 |
| 40, 44 | 7 |
| 48, 52 | 8 |
| 60 | 9 |
| 70, 80 | 10 |
| 90, 100 | 12 |
| Nominal diameter [in] | Pitch [in] |
|---|---|
| 1⁄4 | 1⁄16 |
| 5⁄16 | 1⁄14 |
| 3⁄8 | 1⁄12 |
| 1⁄2 | 1⁄10 |
| 5⁄8 | 1⁄8 |
| 3⁄4, 7⁄8 | 1⁄6 |
| 1, 1 1⁄4 | 1⁄5 |
| 1 1⁄2, 1 3⁄4, 2 | 1⁄4 |
| 2 1⁄2 | 1⁄3 |
| 3 | 1⁄2 |
Mechanics
References
Notes
Bibliography
- Bhandari, V B (2007), Design of Machine Elements, Tata McGraw-Hill, ISBN 9780070611412, http://books.google.com/books?id=f5Eit2FZe_cC.
- Shigley, Joseph E.; Mischke, Charles R.; Budynas, Richard Gordon (2003), Mechanical Engineering Design (7th ed.), McGraw Hill, ISBN 9780072520361, http://books.google.com/books?id=j8xscqTxWUgC.
External links
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