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, where:
Tb = back drive torque (inch pounds)
Pt = thrust load applied to nut (lbs)
SL = lead of screw (inches)
Eƒƒ = all screw efficiency (≈90%)
Rule of Thumb
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 and nuts from PBC Linear. Using conventional lubrication can reduce screw coating and nut life. This is because our proprietary manufacturing process delivers a superior running surface, which improves performance with reduced pitting and noise, and extends the life of the nut.
Our lead screws are made from 300 series stainless steel, and the nuts from polymer. Together, these materials offer outstanding wear and corrosion resistance. Our lead screws can run maintenance-free because the polymer nuts are infused with PTFE, offering a continuous form of lubrication. In addition, our lead screws are available with a PTFE coating, providing self-lubrication which results in:
- Extended life of both lead screw and nut when compared to un-coated screws.
- Dry lubrication that will not attract dust & debris.
- A reduction in preventative maintenance vs. non-coated or ball screws.
What happens when a lead screw and nut start making noise? Often, the first reaction is to apply lubrication to reduce friction and noise, regardless of the root cause. Lubrication should be the user’s last resort, with the evaluation of motor tuning or alignment of the system being the critical first-steps. In other words, lubrication may be only a “band aid” solution.
PBC Linear does NOT recommend use of dry film lubricant because:
- The ingredients of the dry film lubricant will degrade the coating on the screw.
- This is a short-term fix with damaging results.
Our lead screws and nuts are intended to be self-lubricating. Coated screws should only be lubricated as a last resort using a recommended PTFE-type lubricant such as Uniflor 8981. This lubricant is white in color, and since a very thin film is needed, the screw will have a semi-gloss black appearance when applied. If there is visible white on the screw, too much lubricant has been applied, allowing dust and debris to stick to the screw and nut. The customer will need to determine the frequency of reapplication, based on their preventative maintenance policy.
For more information please download the Lead Screw Technology catalog, or configure online with the PBC Linear Configurator.