Trapezoidal Screws: Four Critical Design Considerations

Christopher Nook by Christopher Nook

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Regardless of the details of any given application, there are some pretty important design considerations that should be addressed each and every time you are considering trapezoidal screws as a means of linear motion. In this post we will explore five critical questions that must be answered to achieve success.

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Regardless of the details of any given application, there are some pretty important design considerations that should be addressed each and every time you are considering trapezoidal screws as a means of linear motion.  In this post we will explore five critical questions that must be answered to achieve success.

Lubrication

Proper lubrication is a must to achieve satisfactory service life when it comes to trapezoidal screws and assemblies.  Although the actual means by which you deliver lubrication to the sytem 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 trapezoidal 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 trapezoidal screws without PTFE coating.

Torque

The required motor torque to drive a trapezoidal screw assembly is the sum of three components: inertial torque, drag torque and torque-to-move load.  It must be noted that this is the torque necessary to drive the trapezoidal screw assembly alone. Additional torque when driving frictional bearings, motor shafts, moving components and drag due to general assembly misalignment must also be considered.

Inertial Torque: 
Tj = Iα
Where: 
I = screw inertia
α = angular acceleration
Drag Torque: Helix anti-backlash assemblies are typically supplied with drag torque of 1 to 7 oz.-in.  The magnitude of the drag torque is dependent on the standard factory settings or settings specified by the customer. Generally, the higher the preset force, the better the anti-backlash characteristics.
 
Torque-to-move:
TL = (Load x Lead) / (2Π x Efficiency)
Back Driving

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 backdriving is likely, the torque required for holding a load is:

TL = (Load x Lead x Backdrive Efficiency) / (2Π)
Temperature

With proper lubrication, Helix trapezoidal screws with bronze nuts operate efficiently between 15°F and 350°F, and plastic nuts between 15°F and 175°F.

There you have it, four major things to consider when you are going to be using trapezoidal screws to drive your linear motion application.  Determining these critical design considerations will definitely help to jumpstart your project in the direction of success.

As always, if you have any questions or need additional assistance, give us a call at 855-435-4958 and speak with one of our linear motion experts.  You can also contact us online here.