Lead Screws: Four Critical Design Considerations
Regardless of the details of any given application, some pretty essential design considerations should be addressed every time you consider lead screws as a means of linear motion. This post will explore four critical design elements that must be handled appropriately to achieve success.
Proper lubrication is a must to achieve satisfactory service life when it comes to lead screws and assemblies. Although the actual means by which you deliver lubrication to the system and the specific lubrication intervals will vary from application to application, screw assemblies should be lubricated enough to maintain a film of lubricant on the screw. An exception to this general rule is when the assembly itself is self-lubricating.
Helix produces both "wet" and dry lubricants for our lead screw products. Helix PTFE dry coating is excellent for many applications such as food, medical, and semiconductor, where more conventional greases and oils are not desired. Our E-100 spray lubricant or our PAG-1 grease is recommended for applications using precision lead screws without PTFE coating.
The required motor torque to drive a lead screw assembly consists of inertial torque, drag torque, and torque-to-move load. It must be noted that this is the torque necessary to drive the lead screw assembly alone. Due to general assembly misalignment, additional torque must also be considered when driving frictional bearings, motor shafts, moving components, and drag.
Tj = Iα
- I = screw inertia
- α = angular acceleration
TL = (Load x Lead) / (2π × Efficiency)
Back driving is the ability of a screw to be turned by a thrust load applied to the nut. Generally, back driving will not occur when the screw lead is less than 1/3 of the diameter for uncoated screws, or 1/4 of the diameter for Helix PTFE coated screws. For higher leads where back driving is likely, the torque required for holding a load is:
TL = (Load x Lead x Backdrive Efficiency) / (2π)
With proper lubrication, Helix lead screws with bronze nuts operate efficiently between 15°F and 350°F, and plastic nuts between 15°F and 175°F. When assembly components are employed in extreme conditions, premature failure is always an immediate concern, and overall service life will be adversely affected. Please consult with one of our experts to help determine a suitable solution for more extreme environments.
There you have four main things to consider when utilizing lead screws to drive your linear motion application. There are undoubtedly other concerns, but determining these critical design considerations will help jump-start your project in the direction of success.