What is Back Driving?
Back drive is the result of the load (thrust force) pushing axially on the nut to create rotary motion. The resulting torque is known as “back-driving torque” and is the torque required to hold a load in position. In a vertical orientation, back drive can occur by the load (or carriage) alone, causing the carriage to drift down. Vibration and other external factors can also cause this movement. Generally, a lead screw with efficiency greater than 50% will have a tendency to back drive.
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.
Below is the formula to calculate back drive torque.
Tb: Back drive Torque (inch pounds)
Pt: Thrust load applied to nut (lbs)
SL: Lead of screw (inches)
Eƒƒ: Ball screw efficiency (≈90%)
Rule of Thumb
Lead screws: Lead screws typically provide efficiency between 20% - 80%
- Lead screws that have an efficiency of 50% or greater will back drive.
- When the screw lead is less than 1/3 of the lead screw diameter back driving will not occur.
- 10mm diameter screw with 2mm lead = 41% Efficiency (<50% will not back drive)
- 10mm diameter screw with 25mm lead = 83% Efficiency (>50% will back drive)
Ball screws: 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 Lead Screw Efficiency?
- Efficiency increases as the screw lead increases.
- Lead screw efficiency can be improved by increasing the helix angle of the thread.
- 10mm diameter screw with 2mm lead = 41% Efficiency.
- 10mm diameter screw with 25mm lead = 83% Efficiency.
Conventional lubricants are typically not recommended as a form of lubricant with PTFE lead screws & nuts. Lead screws can be provided both with & without PTFE coating.
Lead screws with PTFE coating provides self-lubrication.
- Provides extended life of both lead screw & nut when compared to un-coated screws.
- Provides dry lubrication that will not attract dust & debris.
- Using conventional lubrication can reduce screw coating & nut life.
- Reduces the need for preventative maintenance vs. non-coated or ball screws.
Find more information please download the Lead screw Technology Catalog, or go to the Configurator and click on the Integrated Motor and Lead Screws icon.