Proper Lead Screw Selection Prevents Load Drift

There are many industry applications that require a critical need for precision, quiet operation, maintenance free performance, and life-long consistent performance from linear driven systems. Some common applications include medical and lab automation, bottling and food processing, automated retail and kiosks, and scanning and imaging. Within these applications, there are specific needs for vertical lead screw actuator applications.

In vertical lead screw actuator applications, lead screw selection is crucial to the safety and success of the system. Prevention of load drift, or screw back drive, during a power loss condition starts with proper selection of a screw’s lead. As a rule of thumb, the lead selected should be at least 3 times smaller than the screw diameter. Less aggressive, finer leads offer increased load drift resistance & safety, however it will result in a negative effect reducing linear carriage travel speed. Alternatively, a more aggressive, coarse lead will offer faster carriage travel speeds, but will not typically provide any resistance to load drift.

Load drift prevention through lead screw selection

Some other methods that can be used to prevent load drift are:

  • Integration of a gear reducer/gear motor with higher ratios. CAUTION: Integration of a reducer, motor with fail-safe brake, or counter-balance system will not prevent a load from falling in the event fail-safe brake internals fail or a belt/cable breaks or dislodges from the load being supported.
  • Using a motor with a fail-safe brake. This will ensure that the load will not suddenly drop in a crash situation.
  • Integration of a counter-balance system into machine design. This will prevent back drive and will reduce motor size requirements. 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. Generally, a lead screw with efficiency greater than 50% will have a tendency to back drive.