Gears: Self-locking or back drivability

Topic:

• What means "Self-locking" or "back drivable"?
• How can it be judged if a gear or drive unit is "self-locking" or "back drivable?
• For what types of applications get "Self-locking" or "back drivable" an important aspect?
   

General important remarks:

• Take care of the risk: 
  • Any torque applied to a gear's output shaft has to be lower than the maximum torque specified by the gear's data sheet.
  • High torques can damage the gear!
     
• Distinguish self-locking and back drivability on the naked gearhead and on the complete(!) drive unit with gearhead, motor and maybe additional mechanical elements.
  • The naked gearhead might be back drivable while the complete unit is not.
  • Back drivability is a characteristic of the motor/gear combination! 
    • If a motor that has a low frictional torque, for example motors with metal brushes, the back driving torque is lower. With motors with graphite brushes, especially with high preload, or motors with cogging torque, the back driving torque increases significantly, i.e. back drivability is only feasible with smaller gear ratios. 
    • In principle also higher ratios are back drivable, but the motor's friction multiplied with the ratio and divided by efficiency leads to such high torques, that the output shaft or whatever is the weakest part inside the gearbox will be destroyed before it moves.
       
• maxon planetary gearheads (GP, GPX) are generally not specifically designed for reverse rotation.
  • Standard planetary gearheads (non "GPX UP") are back drivable up to 2 stages with ratio less than 100:1. Four stages and ratio higher than 200:1 is usually non-back drivable. But it's not just green/red, the border is soft and it depends on the gearhead efficiency, teeth geometry, vibrations, ...
    
  • If back drivability is a requirement, select maxon's "GPX UP" gear type.
    The “GPX UP” gearhead types have a high efficiency which is an advantage for back drivability even at high reduction ratios. 
     

Solution:

1.) Self-locking gears resp. motor combination 

1.a.) Meaning:

Self-locking means that when applying a torque to the gear's output shaft, it will not rotate and thereby is locked. Hence, self-locking always refers to a static situation.

1.b.) Special definitions

• Intrinsically self-locking
  • This is a gear where the torque in the backward direction lies within the cone of friction whereas in the forward direction it is outside of it.
  • Intrinsically self-locking gearheads cannot be moved by a steady force from the load. However, they may drift back if the gear is subject to vibrations.
  • Typical intrinsically self-locking gears are worm gears or linear screw gears, when the forward efficiency is below 50%. However, planetary or spur gears, even with 5 stages are never intrinsically self-locking, although most of the time functionally self-locking.
  • Among maxon gears, only the metric or trapezoidal screw drives are intrinsically self-locking.
     
• Functionally self-locking:
  • In functionally self-locking drive units, the highest expected and permissible torque on the load side (breaking torque) is too low to overcome the friction and/or detent torque of the primary side.
  • Functional self-locking properties are often achieved with 4 or 5-stage planetary gearheads.
    However, it is difficult to really grant for this property in general.

1.c.) Applications

Self-locking is desirable in applications ...

• ... where the most common operation state is standstill (without power).
• ... that need to lift and/or hold loads against gravity or other constant external force
  (e.g. crane applications or holding position in case of an emergency even if no brake is present).
• ... where the costs of a braking system are to be saved.

1.d.) Additional notes

• Many drives with gears of higher reduction are functionally self-locking because the load is not able to overcome the primary friction and/or detent torque. However, it is difficult to really grant for this property in general.
  • Functional self-locking properties are often achieved with 4 or 5-stage planetary gearheads.
     
• Secure self-locking can however only be guaranteed for custom specific drive designs.
  
   

2.) Back drivable gears resp. motor combinations

2.a.) Meaning

A gearhead is called back drivable if it can be driven from the gearhead shaft side applying a torque that is smaller than the max. continuous torque (or max. intermittent torque). Hence, back-drivability is always meant for a dynamic situation.

2.b.) Back drivable or not?

Whether a gear resp. motor combination can be back-driven or not depends on ...

• ... the gear's design type and its reduction ratio.
• ... the teeth geometry, which influences the friction and friction type. Pulling friction is dominant in forward direction and differs from pushing friction in backward direction.
• ... the mechanical play between gear's components and the tilting of floating gear stages.
• ... the gear's efficiency, which reflects the friction in the gear and the viscosity of the lubrication and its temperature dependency.
• ... the friction and cogging at the gear input, i.e., the motor friction
  ⇒ graphite brushes exhibit more friction than metal brushes
  ⇒ motors with iron core winding show cogging which can lead to increased self-locking

2.c.) Applications

Back-drivability is desirable in applications ...

• ... where the mechanism should be moved by hand in case of power loss or for maintenance reasons.
• ... where the drive is used as generator. The unit is constantly driven from the gear output.
• ... where robots work with humans (cobot).
• ... in which external forces must be estimated, e.g. contact forces of walking robots in unstructured environments (uneven ground).

2.d.) Additional notes

• maxon planetary gearheads are optimized for forward operation and not specially made to be back-driven.

Some rule of thumbs

• Units with standard planetary gearheads with 1 or 2 stages are generally considered back-drivable. It is just possible that the mechanical misalignment between motor and 1-stage gearheads might lead to a notchy behavior in forward direction and reduced back-drivability in backward direction.
   
• maxon planetary gear's with the remark "UP - Ultra Performance" are usually back-drivable due to their high efficiency.
   
• Planetary gearheads with 3 stages may or may not be self-locking or back-drivable.
   
• Planetary gearheads with 4-5 stages are typically rated as "functionally self-locking" within the permissible torque level unless there is some special back-drivable design present.
   
• A spur gearhead is always back-drivable. Even with more than 3 stages.

Cross Reference:

• maxon's training video: "Gearhead selection"
• maxon's catalog -> http://epaper.maxongroup.com
  • "Technology short and to the point: maxon gear"
    -> Page 72/73 of catalog 2023/2024
  • "Standard specification No. 102"
    -> Page 89 of catalog 2023/2024
  • "Explanations of maxon terminology gear and screwdrive"
    -> Page 368 of catalog 2023/2024