Affordability, quiet operation, precision, and reduction in regular maintenance—these are just a few of the reasons why Acme threads are the preferred choice for many applications.
- Long-length Acme threads: Used for aircraft flaps, controlled movements on machine tools, conveyors, jacks, and testing machines
- Short-length Acme threads: Used on bonnets on pressure cylinders, camera lens movement, hose connectors, steering mechanisms, and valve stems
Often associated with lead screws or CNC drive systems, the Acme thread form is also used in industrial applications requiring heavy torque and power transmissions, such as lathes, milling machines, and presses. Because of its availability, the Acme lead screw is widely used in the United States.
Compared to competing technologies, Acme threaded bars are very diverse. These threads come in a wide range of diameters and lead sizes. Diameters range from ⅛” to 6”, and leads are available from less than 1/32” to four times the diameter.
There’s more to Acme threads than just diameter and lead sizes, though. But before we get into thread types, materials, and more, let’s start with the basics.
What Is Acme Thread?
Acme threads were established over 100 years ago to replace square threads and a wide range of threads of other forms mainly used to produce traversing motions on machines, tools, and for heavy load work.
Acme threads have a 29-degree angle, which is easier to machine than square threads. Due to their trapezoidal thread profile, which provides greater load-bearing capabilities, Acme threads are generally broader, stronger, and squarer than unified threaded rods. Because Acme rods feature thicker and wider threads, they consistently operate smoother and for longer periods than others in environments with dirt and debris.
Acme Thread Types
There are three main types of Acme thread forms:
- General-Purpose (G) Acme
- Centralizing (C) Acme
- Stub Acme
General-Purpose Acme Thread
General-purpose Acme threads are “flank fit” threads meant for valve stems, piano stools, etc., wherever motion-producing threads are required but where sideloading is not a significant factor.
General-purpose Acme threads come in three classes (2G, 3G, 4G), with each class providing some clearance on all standard size diameters for varying degrees of free movement.
For general-purpose assemblies, designs should only use external and internal Acme threads of the same class and type. Class 2G is the preferred choice, however, classes 3G and 4G have progressively less variance in pitch diameter, which will reduce backlash and end play. There is also a 5G class, but it’s not recommended for new designs.
Centralizing Acme Thread
Compared to general-purpose thread forms, centralizing Acme threads, such as class 2C, are manufactured with tighter tolerances and reduced clearance at the major diameter of both the internal and external screw threads.
Centralizing Acme threads are used extensively for feed screws, and the limited clearance enables a bearing at the major diameter of the thread to preserve the alignment of the thread axis, preventing wedging on the flanks and part sagging.
Stub Acme Thread
Stub Acme threads follow the same basic design as general-purpose and centralizing threads but have a thread depth of less than one-half the pitch. This creates a shallower thread, which is ideal on small diameter screws.
Stub Acme threads are typically used for applications where, due to mechanical or metallurgical considerations, a coarse-pitch thread of shallow depth is necessary.
Material Considerations for Acme Threads
When it comes to lead screws, the materials with which they can be constructed are diverse. The most common types of materials used in the manufacturing of lead screws are:
- Carbon steels
- Alloy steels
- Alloy heat-treated steels
- Stainless steel
- Exotic metals
Choose the Best Acme Screw for Your Application
Keystone Threaded Products produces top-of-the-line, USA-made, unsurpassed parts that are recognized industry-wide for their quality, including Acme threaded products. Our Acme threaded bars are roll threaded, producing uniform, smooth, precise threads of superior physical qualities.
Our team is dedicated to meeting your project’s specific requirements. With over 750 years of combined experience on the production floor, we have the knowledge, skill, and proficiency to manufacture the highest-quality precision roll threaded and machined products that are integral to your design.