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What is Leadscrew Efficiency?

February 05, 2018 - Rockford, IL  -  Posted by PBC News

What is Leadscrew Efficiency?

The efficiency of a leadscrew refers to how well a screw converts rotary energy (torque) into linear motion. Leadscrew efficiency is a pure calculated value, empirical testing is the best method to determine performance.

            Efficiency % = 

 

The efficiency of a leadscrew is primarily influenced by these factors:

  • Lead distance – i.e. 1mm ~ 25mm (available leads)
  • The efficiency is highly dependent upon the lead angle of the screw thread.
  • Lead angle – the angle between the helix of the thread and a line parallel to the axis of rotation of the screw
  • Friction – created by the contact area between the screw and the nut
  • Torque – Translating Rotary to Linear Motion
  • Driving the Screw to Translate the Nut 
  • Driving the Nut to Translate the Screw

PBC Linear Leadscrew Efficiencies

     

What is Back Driving? 

Back driving - Back drive is the result of the load (thrust force) pushing axially on the nut to create rotary motion.  Which means the screw can be driven backwards by the load.

This can be a disadvantage in applications and may require a form of brake be utilized to support the load typically in a vertical orientation.  In some cases, back drivability may be desired to allow an object to be moved if needed

As a Rule of thumb:

Leadscrews: Leadscrews typically provide efficiency between 20% - 80%

  • Leadscrews that have an efficiency of 50% or greater will back drive.
  • When the screw lead is less than 1/3 of the leadscrew diameter back driving will not occur

Example:

  • 10mm dimeter screw with 2mm lead = 41% Efficiency (< 50% will not back drive)
  • 10mm diameter screw with 25mm lead = 83% Efficiency (> 50% will back drive)

Ballscrews: Typical Efficiency = 90%

  • With a 90% efficiency will back drive
  • This creates the need to provide a form of braking to prevent back driving

 

How to improve Leadscrew Efficiency?

Efficiency increases as the screw lead increases

Leadscrew efficiency can be improved by increasing the helix angle of the thread

Example:

  • 10mm dimeter screw with 2mm lead = 41% Efficiency
  • 10mm diameter screw with 25mm lead = 83% Efficiency

Lubrication – but not conventional oil or grease

Conventional lubricants are typically not recommended as a form of lubricant with PTFE leadscrews & nuts

Leadscrews can be provided both with & without PTFE coating

Leadscrews with PTFE coating provides self-lubrication

 

  • Provides extend life of both leadscrew & nut when compared to un-coated screws
  • Provides dry lubrication that will not attract dust & debris
  • Using conventual lubrication can reduce screw coating & nut life
  • Reduces the need for preventative maintenance vs. un-coated or ball screws

PBC Linear Leadscrew Configurator: http://theconfig.com/external/default.aspx?c=1&n=B&r=CustomScrew  

PBC Motor Leadscrew Catalog: http://www.pbclinear.com/Download/Catalog/LeadScrew-Catalog.pdf

 

 
 

 

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