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:

• Any torque applied to a gear's output shaft has to be lower than the maximum torque specified by the gear's data sheet.
  • Caution: Higher 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.
    
    

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