Should You Increase RPM or the Lead of Your Lead Screw?

Christopher Nook by Christopher Nook

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Applications

When it comes to lead screw performance, it's all about axial load and rotational speed (RPM). As you likely know, the relationship between these two factors is known as "pressure velocity", or the PV curve. Simply put, this PV curve defines the safe limits of load and speed.

When it comes to lead screw performance, it's all about axial load and rotational speed (RPM). As you likely know, the relationship between these two factors is known as "pressure velocity", or the PV curve. Simply put, this PV curve defines the safe limits of load and speed. For instance, as load increases, speed must be reduced in order to prevent excess heating and premature screw wear.

In a situation where you want to increase linear travel speed, you essentially have two options; you can increase the rotational speed (RPM) or you can increase the screw's lead. In the tables below, we will look at the pros and cons of each method so that you can decide which option would be best for your particular application.

Increase RPM
PROS CONS
  • Can use a standard screw lead (lower cost;  shorter lead time)
  •        
  • Shorter screw leads are more likely to hold position without maintained torque
  •        
  • Shorter screw leads increase positioning resolution (less change in linear travel per revolution)
  • Requires a programmable motor (usually a stepper or servo) or mechanical components  (gears or pulleys) to increase speed
  •        
  • Stepper motors have a fairly low RPM limit and lose torque very quickly as RPM increases
  •        
  • May reach critical speed (cannot rotate a screw at or near its critical speed)

Helix Axial Anti-Backlash Lead Screw Assembly

                                                         

Increase Screw's Lead
PROS CONS
           
  • May be able to use a fixed speed motor (no tachometer/feedback controls = lower cost/complexity)
  •        
  • Lower motor RPM means more motor torque available; possibly use a smaller motor
  •        
  • Can run motor at closer to optimum speed (most efficient)
  •      
           
  • May need a custom lead (higher cost; longer lead time)
  •        
  • Larger screw leads are more likely to back-drive (may need a brake; more components = greater cost/complexity)
  •        
  • Longer screw leads reduce positioning resolution (more change in linear travel per revolution)

 

So when it comes to increasing the linear speed, which option is better - increasing the rpm or the lead of your screw? As you can see from this post, both have their share of pros and cons. It's up to you and the details of your particular application to determine which route makes the most sense.


  


BTW - If you missed part 1 of our Guide To Lead Screw Terminologyclick here to get it now!