Acme thread form
Encyclopedia
Trapezoid
In Euclidean geometry, a convex quadrilateral with one pair of parallel sides is referred to as a trapezoid in American English and as a trapezium in English outside North America. A trapezoid with vertices ABCD is denoted...
al outlines. They are the most common forms used for leadscrew
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
s (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
Vise (tool)
A vise or vice is a mechanical screw apparatus used for holding or clamping a work piece to allow work to be performed on it with tools such as saws, planes, drills, mills, screwdrivers, sandpaper, etc. Vises usually have one fixed jaw and another, parallel, jaw which is moved towards or away from...
or the leadscrew of a lathe
Lathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...
. 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 (icon ). 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
Square thread form
The square thread form is a common thread form for leadscrews. It gets its name from the square cross-section of the thread.-Advantages & disadvantages:...
, 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
Tool bit
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...
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. The Acme thread form name appears as early as 1895.
The trapezoidal metric thread form is similar to the Acme thread form, except the thread angle is 30°. It is codified by DIN
Deutsches Institut für Normung
is the German national organization for standardization and is that country's ISO member body. DIN is a Registered German Association headquartered in Berlin...
103. 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
Thread angle
The thread angle of a screw is the angle between the threads. This is a defining factor for the shape of a screw thread. Standard values include:...
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
Split nut
A split nut is a nut that consists of two pieces which can be clamped together to engage the thread of the nut with the bolt, or released, to allow the nut to move along the bolt without turning the bolt. The two pieces have chamfered ends . The nut is split lengthwise so that it may be opened for...
, which can compensate for nut wear.
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.
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.
| Nominal diameter [in] | Pitch [in] |
|---|---|
| , | |
| 1, 1 | |
| 1 , 1 , 2 | |
| 2 | |
| 3 |
Trapezoidal metric thread characteristics
In case of the trapezoidal thread form the angle is 30° instead of 29°. All dimensions are in millimeters.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
Lead (engineering)
Lead is the axial advance of a helix or screw during one complete turn The lead for a screw thread is the axial travel for a single revolution....
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.
| 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 |