Screw threads are an important part of many industrial applications, supplying motion and positioning control for everything from delta rockets to bone screws. If you’re just getting started, though, it can be difficult to navigate your choices. What is a screw thread? How do I know if I need an industrial screw thread? What should I keep in mind as I review my options?
First, the basics.
For most industrial applications, a screw thread typically serves one of two functions: converting rotating motion into linear motion or preventing linear motion (fastening) by anti-rotating the screw thread. There are seven types of threads:
- Standard V-Threads (Inch-ASME B1.1, Metric ASME 81.13M): V-threads are used to hold components together and control position. They are not well suit for linear motion or power transmission.
- Acme Screw Treads (ASM B1.5): Typically used for power transmission and linear motion applications, with slower rotational speeds and heavy load requirements.
- Trapezoidal Screw Threads (ISO 2901 thru 2904): There are many similarities between trapezoidal and acme screw threads. However, trapezoidal threads use metric measurements and have a flank angle difference of .5 degrees.
- Ball Screw Threads (ISO 3408): Like Acme threads, ball screw threads are used for linear motion applications. They are more efficient than Acme or trapezoidal screws, but can be more costly.
- Buttress Screw Threads (ASME B1.9): Buttress threads are used in projects requiring movement of heavy loads with unidirectional force. (A good example of this in action is a thumb screw on an adjustable wrench.)
- Worm Threads or Gears (AGMA A98): As the name suggests, these are used for gear applications, rather than linear motion or fastening.
With so many choices, it can be tough to determine which thread will work for a specific application. Keep the following considerations in mind to ensure your needs are met.
Choosing Your Screw Thread.
Consideration One: The load you are moving or holding together.
Are you moving or holding something together? If you are holding something together, choose a standard V-thread.
Occasionally, “V” threads can be used for linear motion in lighter loads, although it is not recommended. The very nature of a “V” thread design is inefficient, and is best suited for use on components that need to hold something together.
For bidirectional motion loads, there are three options: acme, trapezoidal and ball screw threads. Acme and trapezoidal can handle greater loads than ball screws, but have a lower efficiency when comparing screw types, size for size.
Very heavy, one-directional loads call for a buttress thread. The low angle on the load bearing side of a buttress thread make it possible for the tread to absorb more axial force without a low radial resultant force.
Worm threads and wheels (gears) are a totally different species. Instead of providing linear motion, their motion is rotational. Typically it is a means of turning or redirecting an output rotation at some angle, most often 90 degrees to the input rotation. Overall, worms and worm wheels are relatively inefficient by their nature. They are always made and used as a set.
Consideration Two: Desired Efficiency
Efficiency plays an important role in linear motion and power transmission applications.
If you’re concerned about efficiency, rule out acme and trapezoidal; they are the least efficient options.
Ball screw threads have an efficiency between 85 – 90%. Because of efficiency differentials between screw thread types, input torque can vary greatly. Be sure to consider the cost, weight, speed and size limitations of your application when selecting a screw thread type for application.
Consideration Three: Torque.
Ball screw threads have lower torque requirements, but are more expensive. While Acme or trapezoidal screw threads are less efficient and require more torque, they can usually get the job done.
V-threads are an option for light loads, while buttress threads work for loads moving in one direction. Worm threads and gears are not designed for applications calling for a change in direction of 90 degrees, so, if torque is a concern, rule them out.
Consideration Four: Travel Time.
Travel time, the time required to traverse a given linear displacement, is an especially important consideration for fast components. (By fast we mean a long lead that makes the screw or nut move a greater distance per revolution than a “slow” or short lead would yield.)
As a general rule, ball, trapezoidal, or Acme screw threads work best for multiple starts. A V-thread or buttress might meet your needs, but, ultimately, if travel time is a concern, reverse engineer the component while factoring in other considerations, like torque, before making a final decision. (If you’re stuck, our engineers can help.)
There are many industrial applications requiring industrial screw threads. Even though there are various options that may meet your needs, defining components, and how they’ll fit together, will make it easier to select a thread that works.
At PST Group, we can solve your linear motion problems with standard products, customized screw threads, or fully engineered, complex system solutions. Call us today to have our 200 years of combined engineering experience help your team to be successful with all of your screw thread projects.