Trapezoidal Thread Forms

A male Acme thread

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 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 original trapezoidal thread form, and still probably the one most commonly encountered worldwide, is the Acme thread form ( /ˈækmiː/ ack-mee). The Acme thread was developed sometime in the second half of the nineteenth century as a profile well suited to power screws that has various advantages over the square thread, which had been the form of choice until then. It is easier to cut via either single-point threading or die than the square thread is (because the latter’s shape requires tool bit or die tooth geometry that is poorly suited to cutting); it wears better than square (because the wear can be compensated for); it is stronger than a comparably sized square thread; and it makes for smoother engagement of the half nuts on a lathe leadscrew than square does.^[2][3] The Acme thread form name appears as early as 1895.^[4]

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 nonmetric 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 nonmetric) 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.

Acme thread characteristics

Dimensions of an Acme thread

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 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]

The disadvantages of the Acme thread form are the much lower efficiency and the greater radial load on the nut, due to the thread angle.^[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]

Trapezoidal metric thread characteristics

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:

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.^[12]

Mechanics

References

Bibliography

• Bhandari, V B (2007), Design of Machine Elements, Tata McGraw-Hill, ISBN 9780070611412, http://books.google.com/?id=f5Eit2FZe_cC .
• Flather, John Joseph (1895), Rope-driving: a treatise on the transmission of power by means of fibrous ropes (1st ed.), New York, NY, USA: J. Wiley & Sons, LCCN 06-034155, http://books.google.com/books?id=hQ9IAAAAIAAJ&pg=PR1#v=onepage&q&f=false. 
• Green, Robert E. et al. (eds) (1996), Machinery’s Handbook (25 ed.), New York, NY, USA: Industrial Press, ISBN 978-0-8311-2575-2, http://www.worldcat.org/title/machinerys-handbook/oclc/473691581 .
• Jones, Franklin D. (1964), Machine shop training course, 1 (5th ed.), New York, NY, USA: Industrial Press, ISBN 978-0831110390, http://www.worldcat.org/title/machine-shop-training-course/oclc/661244. 
• Oberg, Erik (1908), Handbook of small tools: comprising threading tools, taps, dies, cutters, drills, and reamers, together with a complete treatise on screw-thread systems, New York, NY, USA: John Wiley & Sons, http://books.google.com/books?id=nplKAAAAMAAJ&pg=PA29#v=onepage&f=false. Co-edition, 1908, Chapman & Hall, London, UK. 
• Shigley, Joseph E.; Mischke, Charles R.; Budynas, Richard Gordon (2003), Mechanical Engineering Design (7th ed.), McGraw Hill, ISBN 9780072520361, http://books.google.com/?id=j8xscqTxWUgC .

External links

• Standard Acme threads
• Trapezoidal coarse thread
• Trapezoidal fine thread
• Stub ACME Thread Sizes and Data Chart

This information originally retrieved from http://en.wikipedia.org/wiki/Trapezoidal_thread_forms
on Wednesday 19th October 2011 7:18 pm EDT
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