How to Evaluate Acme Screw Assembly Loads

Christopher Nook by Christopher Nook

Configuring an acme screw assembly that will not only perform but endure the demands of a linear motion application, requires a thorough understanding of the load requirements. Evaluating your loading conditions extends beyond the load that you want to move and the distance that you want to move it - it includes the effects of your application on the screw assembly itself.

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Configuring an acme screw assembly that will not only perform but endure the demands of a linear motion application, requires a thorough understanding of the load requirements. Evaluating your loading conditions extends beyond the load that you want to move and the distance that you want to move it - it includes the effects of your application on the screw assembly itself. Understanding and assessing the forces that influence lead screw performance is paramount to configuring a screw assembly that is effective, reliable, and long lasting.

Acme lead screw assemblies can be subjected to several types of loads in different ways, which are known as load classes. Improper loading may cause decreased lead screw efficiency, shorter service life, or failure. This article will examine acme screw assembly load types and classes.

Understanding Load Types

There are five primary load types that can affect acme lead screw assembly performance:

  • Tension Load – A tension load describes a situation during which the load tends to “stretch” or pull the screw from end to end. Some degree of tension loading is usually acceptable for acme lead screw assembly operation. With lead screws that utilize a rotating acme nut design, tension loads are always present as the screw is continually being pulled.
  • Compression Load – A compression load describes a scenario during which the load tends to “squeeze” or compress the screw from the ends toward the middle. Like tension loading, some level of compression loading may be permissible. Some systems will have a small amount of tension and compression loading as part of normal operation. It’s imperative that the acme lead screw has enough column strength to accommodate compression loading because the acme lead screw may buckle when compression loading exceeds column strength.
  • Thrust Load – A thrust load describes a load that is parallel to and concentric with the acme lead screw. This is the preferred type of load for proper lead screw assembly performance because the load follows the direction of the screw. Achieving a thrust load situation is the optimal goal when designing an acme lead screw assembly.
  • Overturning Load – An overturning load describes a loading condition when the load tends to rotate the nut radially around the longitudinal access of the screw. A moment load is a type of overturning load that tends to overturn or bend the axis of rotation in an angular direction. It’s best to avoid this loading condition in an acme lead screw assembly if possible because it can unevenly load the nut, which can cause binding and uneven wear.
  • Side Load – A side load, also know as a radial loads, describes a load that influences the nut radially, perpendicular to the lead screw axis. Side loading will reduce the critical speed, may permanently bend the lead screw, and should be avoided. A common cause of side loads is a situation in which the end supports of a screw assembly are not properly aligned with the nut attachment point.
Assessing Load Classes

In addition to the types of loads that affect lead screw assemblies, there are load classes that describe how the loading condition is applied. These are:

  • Static Load – A static load is a loading condition involving actions having high acceleration. In the context of an acme lead screw assembly, this relates to the maximum thrust load, including shock, that can be applied to an acme nut assembly.
  • Dynamic Load – A dynamic load is one that varies very slowly. For acme lead screw assemblies, dynamic loads also relate to the maximum recommended thrust load that can be applied to a nut assembly while in motion.
  • PV Load – A PV load is a loading condition caused by a sliding load that causes heat buildup due to friction. The amount of heat generated relates to the pressure (P) on the acme nut and the surface velocity (V) at the major diameter. How severely a PV load will influence an application depends on the material used for the screw assembly, the pressure on the acme nut, and the surface velocity at the major diameter.

Helix Can Help You Choose the Best Acme Screw Assembly for Various Loads

Choosing the right acme lead screw assembly requires a thorough understanding of the different types of loading conditions and the specific loads for your linear motion control application. Helix Linear Technologies has the engineering expertise necessary for fully interrogating your unique load environment and the advanced manufacturing technologies that you need to be able to configure the right acme screw assembly for the linear motion control requirements of your application.

To learn more about how Helix can help you configure the best acme screw assembly for you particular loading conditions, download a copy of Helix Precision Lead Screw Sizes, which details... you guessed it -  all of our precision lead screw sizes (although we are contantly adding sizes so be sure to check out our website):