When selecting a DC gear motor for an actuator, lifting mechanism, valve, lock, positioning system, or other motion-control application, two terms are often misunderstood: self-locking and holding torque.
At first glance, they may appear to describe the same function. Both seem to answer the question: Can the output shaft remain stationary when the motor is not running?
However, self-locking and holding torque are not exactly the same thing.
Understanding the difference is especially important when select a micro DC gear motor, because the power-off behavior of the complete gear motor depends on the motor, gearbox, gear ratio, internal friction, gear geometry, lubrication, and operating conditions.
This article explains the difference between self-locking and holding torque, how power-off holding torque is generated, the role of cogging torque in a DC motor, and how to specify these requirements correctly when selecting or customizing a DC gear motor.
1. What Is Holding Torque?
The term holding torque generally describes the amount of external torque that a mechanism can resist without allowing the output shaft to rotate.
However, the exact meaning depends on the motor type and the test conditions.
For a complete DC gear motor, one particularly important definition is:
Power-off holding torque is the external torque applied to the output shaft that the gear motor can resist when the motor is unpowered, before the output shaft begins to rotate.
For example, imagine a DC gear motor connected to an actuator.
The motor drives the actuator into a certain position and then the power is switched off. An external load continues to apply torque to the output shaft.
If the output shaft remains stationary until the external torque reaches a specified value, that value can be described as the power-off holding torque.
This is different from stall torque.
Stall torque
Stall torque is normally measured while the motor is powered and the motor shaft is prevented from rotating.
Power-off holding torque
Power-off holding torque is measured while the motor is unpowered, with an external torque attempting to rotate the output shaft.
These are fundamentally different operating conditions.
2. What Does Self-Locking Mean?
Self-locking describes a mechanical behavior rather than simply a numerical torque value.
A mechanism is considered self-locking when an external load applied to the output cannot back-drive the input mechanism under specified conditions.
In simple terms:
The motor can drive the load, but the load cannot easily drive the motor backward.
This is particularly common in certain worm gearboxes.
When the motor is powered, the worm drives the worm wheel.
When the motor is switched off, the load attempts to drive the worm wheel backward. Depending on the worm geometry, lead angle, friction, materials, lubrication, and other factors, the worm wheel may be unable to rotate the worm.
This produces a self-locking effect.
However, it is important to understand that not every worm gearbox is automatically self-locking. Self-locking behavior depends on the actual mechanical design and operating conditions. The relationship between worm lead angle and friction is one of the key factors.
3. Self-Locking and Holding Torque Are Related, But Not Identical
This is probably the most important point for engineers selecting a DC gear motor.
Consider two gear motors:
Gear Motor A
- Output shaft can be back-driven.
- When power is removed, the shaft can still resist a certain amount of external torque because of internal friction.
- The output shaft starts moving when the external torque exceeds a certain value.
This motor has a measurable power-off holding torque, but it is not necessarily self-locking.
Gear Motor B
- The output shaft cannot easily back-drive the gearbox.
- When power is removed, the load remains mechanically locked within the specified operating conditions.
This motor may be described as self-locking.
Therefore:
A gear motor can have power-off holding torque without being truly self-locking.
Conversely, saying that a gearbox is “self-locking” does not automatically tell you the exact numerical holding torque under every possible condition.
For engineering applications, a numerical torque requirement is often much more useful than simply specifying “self-locking.”
| Characteristic | Power-Off Holding Torque | Self-Locking |
|---|---|---|
| Motor powered? | No | Usually no |
| Describes a numerical torque? | Yes | Not necessarily |
| Output can eventually back-drive? | Usually yes if torque exceeds threshold | Ideally no under specified conditions |
| Strongly affected by gearbox? | Yes | Yes |
| Affected by motor friction? | Yes | Can be |
| Affected by cogging torque? | Yes, potentially | Can contribute |
| Common in worm gearboxes? | Yes | Yes, depending on design |
| Equivalent to braking? | No | No |
| Unlimited torque capacity? | No | No |
| Requires actual testing? | Strongly recommended | Strongly recommended |
4. Does a Bare DC Motor Have Holding Torque?
This is another common source of confusion.
A conventional small brushed permanent-magnet DC motor does not normally have a large, defined mechanical holding torque when it is unpowered.
When the motor is disconnected from the power supply and an external torque is applied to the shaft, the shaft can generally be rotated.
However, it will not rotate completely freely.
Several sources of resistance exist inside the motor:
- Bearing friction
- Brush friction
- Commutator friction
- Rotor magnetic effects
- Cogging or detent torque
- Lubricant resistance
- Other mechanical losses
These create a certain amount of resisting torque.
But this should not automatically be called the motor’s “holding torque.”
For a normal DC motor, it is usually more accurate to discuss its mechanical drag torque, cogging torque, or backdrive resistance, depending on the test condition.
A complete DC gear motor is different because the gearbox can greatly increase the resistance seen at the output shaft.
5. Is Cogging Torque the Main Factor in DC Motor Holding Torque?
Not necessarily.
This is a particularly important distinction for micro DC motors.
Cogging torque, sometimes called detent torque, is caused by the magnetic interaction between the permanent magnets and the stator/rotor magnetic structure. In permanent-magnet motors, its magnitude is strongly affected by factors such as magnet strength, pole-slot combination, magnetic geometry, and slot design.
Cogging torque can therefore contribute to the resistance felt when an unpowered motor is manually rotated.
For example, a motor may feel like it has a slight “notched” or “stepped” resistance as the shaft rotates.
However:
Cogging torque is only one component of the total resistance torque of an unpowered motor.
Bearing friction, brush friction, commutator friction and other mechanical effects can also contribute significantly.
Therefore, it would be incorrect to say:
“The holding torque of a DC motor is mainly determined by cogging torque.”
A more accurate statement is:
Cogging torque contributes to the unpowered resistance of a permanent-magnet motor, but the power-off holding or backdrive torque of a complete DC gear motor is determined by the combined behavior of the motor and gearbox.
6. Why Does a Gearbox Increase Power-Off Holding Torque?
This is where the gearbox becomes extremely important.
Consider a simple example:
DC Motor → Gearbox → Output Shaft
Suppose the motor itself has only a small amount of mechanical resistance.
After passing through a high-ratio gearbox, that motor-side resistance can be reflected to the output side.
In a simplified idealized relationship, a motor-side resisting torque can be multiplied by the gear ratio when viewed from the output side.
However, a real gearbox is not an ideal transmission.
Actual output behavior is affected by:
- Gear ratio
- Gear type
- Gear efficiency
- Number of gear stages
- Gear mesh friction
- Bearing friction
- Lubrication
- Gear preload
- Shaft loading
- Temperature
- Manufacturing tolerances
- Wear
- Motor cogging torque
- Motor bearing and brush friction
Therefore, it is not correct to simply calculate holding torque as motor cogging torque × gear ratio.
The actual power-off holding torque should ideally be measured on the complete motor + gearbox assembly.
This is also why a high-ratio gearbox may be difficult to back-drive even when the gearbox does not use a worm gear. Practical testing has shown that gear ratio, internal friction, and gear configuration can have a major effect on backdrive behavior.
7. Spur Gear, Planetary Gear and Worm Gear: What Is the Difference?
Different gearbox architectures have very different backdrive characteristics.
Spur Gearbox
Spur gears are generally relatively easy to back-drive compared with a properly designed self-locking worm gearbox.
However, a high-ratio multi-stage spur gearbox can still have significant resistance because every stage introduces additional friction and mechanical losses.
Therefore:
A spur gear motor is not automatically backdrivable just because it uses spur gears.
The actual behavior must be tested.
Planetary Gearbox
Planetary gearboxes are widely used where high torque density, compact size, and good efficiency are required.
They are generally designed to transmit motion in both directions and may therefore be backdrivable depending on the specific design and gear ratio.
The complete motor + planetary gearbox assembly can nevertheless exhibit considerable power-off resistance.
Again, this resistance should not automatically be classified as “self-locking.”
Worm Gearbox
Worm gearboxes are commonly selected when power-off load holding or reduced backdrivability is required.
The worm-and-wheel geometry can prevent the output from driving the input under certain conditions.
The key parameters include:
- Worm lead angle
- Friction coefficient
- Gear materials
- Surface finish
- Lubrication
- Load direction
- Temperature
- Wear
The lower the worm lead angle, the greater the possibility of self-locking, although the exact behavior must be verified for the actual design.
However, engineers should avoid the assumption that:
“Worm gear = guaranteed self-locking.”
That is not universally true.
8. Worm Gear Self-Locking: Why Does It Happen?
Worm gear motors are widely used in applications where the output shaft needs to resist reverse rotation when the motor is unpowered. This characteristic is commonly referred to as self-locking.
However, it is important to understand that not every worm gear motor is automatically self-locking. The ability of a worm gearbox to resist backdriving depends on its geometry, friction characteristics, lubrication, load conditions, and the design of the complete gear motor.
One of the most important factors is the relationship between the worm lead angle and the friction angle.
How Does a Worm Gear Resist Backdriving?
A typical worm gear transmission consists of a worm and a worm wheel. The worm rotates around its own axis and drives the worm wheel through the engagement between the worm threads and the gear teeth.
In normal operation, the motor drives the worm, which then drives the worm wheel and output shaft.
The situation becomes different when an external load attempts to rotate the output shaft in the opposite direction.
For a conventional spur or planetary gearbox, the external torque can often be transmitted backward through the gears and eventually rotate the motor shaft. This is known as backdriving.
With a worm gearbox, the geometry of the worm and worm wheel can make reverse power transmission much more difficult.
When the worm lead angle is sufficiently small relative to the effective friction angle, the friction in the contact can prevent the worm wheel from driving the worm.
This is the basic principle behind worm gear self-locking.
9. The Most Important Factor in Worm Gear Self-Locking: Worm Lead Angle
For worm gear motors, one of the most important factors affecting self-locking behavior is the worm lead angle.
The lead angle determines the geometry of the contact between the worm and worm wheel and has a strong influence on whether the worm gear mechanism can be back-driven.
In general:
- Smaller lead angle → greater tendency toward self-locking
- Larger lead angle → greater tendency toward backdriving
- Higher efficiency → generally greater backdrivability
- Lower efficiency → generally greater resistance to backdriving
This leads to an important engineering trade-off:
A worm gearbox designed for strong self-locking generally sacrifices some transmission efficiency.
Single-Start vs. Multi-Start Worms
The number of worm starts is closely related to the lead angle.
In general, a single-start worm has a smaller lead angle than a multi-start worm when other design parameters are comparable.
Therefore, the following general tendency can be observed:
| Worm Design | General Self-Locking Tendency |
|---|---|
| Single-start worm, small lead angle | Higher |
| Double-start worm | Lower |
| Multi-start worm | Lower |
| Large lead angle, high-efficiency worm | More likely to be backdrivable |
However, the number of starts alone does not determine whether a worm gearbox is self-locking. The actual lead angle, friction conditions, materials, lubrication and gearbox geometry must all be considered.
Lead Angle and Friction Angle
From a theoretical point of view, engineers often compare the worm lead angle, usually represented by λ (lambda), with the friction angle, usually represented by φ (phi).
The friction angle can be related to the coefficient of friction through:
φ = arctan(μ)
where μ is the effective coefficient of friction.
As a simplified theoretical guideline:
λ < φ → greater tendency toward self-locking
while:
λ > φ → greater tendency toward backdriving
This comparison provides a useful first-level engineering assessment, but it should not be treated as a universal guarantee of self-locking.
Real worm gearboxes operate under much more complicated conditions than the simplified theoretical model.
Why the Theoretical Formula Is Not Enough
The effective coefficient of friction in a worm gearbox is not a fixed constant.
It can change significantly depending on operating conditions, including:
- Lubricant type and viscosity
- Worm and worm-wheel materials
- Surface roughness
- Contact pressure
- Operating temperature
- Running-in condition
- Gear wear
- Manufacturing tolerances
- Shaft alignment
- Vibration
- Load direction
- Operating speed
For example, a gearbox may exhibit stronger resistance to backdriving when it is cold because the lubricant is more viscous. After the gearbox reaches operating temperature, friction may decrease and backdrivability may increase.
Similarly, after long-term operation, wear and changes in surface conditions can alter the friction characteristics of the worm and worm wheel.
Therefore:
A theoretical self-locking calculation should be considered a design guideline, not a substitute for testing.
10.Self-Locking vs. High Efficiency: An Important Trade-Off
One of the most interesting characteristics of worm gearboxes is the relationship between self-locking and efficiency.
A gearbox with a very small lead angle and strong self-locking behavior may have relatively high friction and therefore lower transmission efficiency.
In contrast, a worm gearbox optimized for high efficiency may use a larger lead angle and/or a multi-start worm. Such a design can reduce frictional losses and improve efficiency, but it may also become easier to back-drive.
This creates a fundamental engineering trade-off:
Higher self-locking tendency ↔ Higher friction ↔ Lower efficiency
versus
Higher efficiency ↔ Lower friction ↔ Greater possibility of backdriving
The optimum design depends on the application.
For example:
- A lifting or positioning actuator may prioritize holding capability after power is removed.
- A continuous-duty transmission may prioritize efficiency and thermal performance.
- A battery-powered actuator may prioritize efficiency to reduce energy consumption.
- A safety-critical mechanism may require a dedicated brake rather than relying solely on worm gear self-locking.
Why “Absolutely Self-Locking” Should Be Used With Caution
For this reason, a professional gear motor specification should avoid making an unconditional statement such as:
“This worm gear motor is absolutely self-locking under all conditions.”
Such a statement can be misleading.
A better engineering specification is to define the actual operating requirement.
For example:
The gear motor shall resist an external backdriving torque of XX N·m with the motor unpowered under the specified operating conditions.
Even better, the specification can define:
- Maximum external torque
- Temperature range
- Load direction
- Allowable angular displacement
- Test duration
- Gearbox condition
- Required service life
This converts the vague term “self-locking” into a measurable engineering performance requirement.
11.Self-Locking Does Not Mean a Specific Holding Torque
A common misconception is that a self-locking gearbox automatically has a specific or unlimited holding torque.
This is not correct.
Self-locking describes the direction of power transmission and the resistance to backdriving, while holding torque is a measurable torque value.
For example, a worm gear motor may be self-locking under certain operating conditions, but this does not mean that its output shaft can withstand an unlimited external torque.
The gearbox still has mechanical strength limitations determined by:
- Gear tooth strength
- Output shaft strength
- Bearing capacity
- Housing strength
- Worm and worm-wheel materials
- Gearbox construction
- Shock loading
- Duty cycle
Therefore, the following two specifications should be distinguished:
Self-locking:
Whether the output can drive the input backward under specified conditions.
Power-off holding torque:
The maximum specified external torque that the output shaft can resist when the motor is unpowered without unacceptable backdriving or shaft movement.
For demanding applications, the second specification is usually much more useful because it provides a measurable design target.
12. Static Holding Torque vs. Dynamic Braking
Another important distinction is between holding and braking.
Self-locking is generally a static mechanical behavior.
It means that when the system has stopped, the output cannot easily be driven backward.
It does not necessarily mean that the gearbox can rapidly stop a moving load.
For example, imagine a vertical actuator supporting a load.
If the load is already stationary, a self-locking gearbox may prevent it from moving.
But if the load is moving downward at high speed and power is suddenly removed, the self-locking mechanism should not automatically be assumed to provide instantaneous braking.
As a result:
Static self-locking should not be confused with dynamic braking.
This distinction is particularly important for lifting, positioning and actuator applications.
13. How Should Power-Off Holding Torque Be Specified?
For a custom DC gear motor, simply asking the supplier:
“Is the motor self-locking?”
may not provide enough information.
A much better engineering specification is:
With the motor unpowered, the output shaft shall withstand an external torque of XX N·m without back-driving.
You should also define the test conditions.
For example:
- Motor voltage: 0 V / unpowered
- Gear motor temperature: specified temperature
- Output shaft direction: clockwise / counterclockwise
- External torque: specified value
- Allowable angular movement: specified value
- Test duration: specified time
- Gear motor mounting orientation: specified
- Number of cycles: if applicable
This creates a measurable engineering requirement.
14. Why the Complete Gear Motor Should Be Tested
Another important engineering principle is:
Self-locking and backdrivability should be evaluated on the complete drive system, not only on the gearbox or only on the motor.
A gearbox that appears relatively easy to back-drive when tested by itself may behave differently after a motor is attached.
The motor introduces additional resistance from:
- Bearings
- Brushes
- Commutator
- Magnetic effects
- Cogging torque
- Rotor inertia
As a result, the backdrive characteristics of the complete gear motor can differ from those of the bare gearbox. This distinction is also emphasized in gearhead selection guidance from maxon.
Therefore, for a custom micro DC gear motor, the most meaningful measurement is often:
Complete Gear Motor Power-Off Backdrive / Holding Torque
rather than the holding torque of the gearbox alone.
15. A Practical Example
Consider a micro DC gear motor used in an actuator.
The motor drives the actuator to a target position.The controller then switches the motor off.
The actuator continues to apply an external torque to the output shaft.
There are three possible situations.
Situation A: Easy Backdrive
The output shaft rotates easily when the motor is unpowered.
This means the gear motor has relatively low backdrive resistance.
A separate brake or mechanical locking mechanism may be necessary if the load must remain in position.
Situation B: High Power-Off Holding Torque
The output shaft remains stationary until the external torque reaches a certain value.
This gear motor has measurable power-off holding capability.
However, it may still be backdrivable if the external torque exceeds that threshold.
Situation C: Self-Locking
Within the specified operating conditions, the output shaft cannot drive the input mechanism backward.
This is characteristic of a self-locking mechanical transmission.
A worm gearbox may provide this behavior depending on its design.
These three conditions should not be treated as identical.
15. What Factors Should Engineers Consider When Selecting a Gear Motor?
When the application requires the output shaft to remain stationary after power is removed, engineers should evaluate at least the following:
1. Required holding torque
How much external torque must the output shaft withstand?
2. Backdrive torque
How much torque is required to start rotating the output shaft backward?
3. Self-locking requirement
Does the application actually require true self-locking, or is a certain minimum power-off holding torque sufficient?
4. Gear type
Is the gearbox spur, planetary, helical, worm, or another configuration?
5. Gear ratio
Higher reduction ratios generally increase the influence of internal friction and motor-side resistance on output backdrive behavior.
6. Motor characteristics
Motor bearing friction, brush friction and cogging torque can affect the backdrive behavior of the complete gear motor.
7. Temperature
Lubricant viscosity and friction characteristics can change significantly with temperature.
8. Wear and lifetime
A gearbox that behaves as self-locking when new may exhibit different behavior after long-term operation.
9. External load
Static loads, dynamic loads, vibration and shock should all be considered.
10. Safety requirements
If failure to hold the load could cause injury or equipment damage, self-locking alone should not necessarily be considered a safety device.
Conclusion
Self-locking and holding torque are closely related, but they are not the same concept.
Holding torque is generally a measurable torque value describing how much external torque a mechanism can resist under specified conditions.
Self-locking describes a mechanical behavior in which the output cannot easily back-drive the input.
For a micro DC gear motor, power-off holding torque is not determined by a single factor. It is the result of the combined characteristics of the DC motor, gearbox, gear ratio, gear geometry, friction, bearings, lubrication, and operating conditions.
Cogging torque can contribute to the unpowered resistance of a permanent-magnet motor, but it should not be regarded as the sole or necessarily dominant factor in the holding torque of a complete gear motor.
For ordinary spur and planetary gear motors, the gearbox and gear ratio can have a significant influence on backdrive resistance. For worm gear motors, self-locking behavior is primarily related to the worm transmission geometry and friction conditions.
Most importantly, “self-locking” should not be interpreted as “unlimited holding torque.” A self-locking gearbox still has defined mechanical strength limits, and its behavior can change with temperature, lubrication, wear and vibration.
For an OEM or custom actuator application, the best approach is therefore to define a specific power-off holding torque or backdrive torque requirement, together with the test conditions and allowable shaft movement.
At HOTEC Motor, custom DC gear motor solutions can be developed around application-specific requirements such as output torque, speed, gear ratio, shaft design, backdrive resistance, power-off holding torque, noise, EMC, size and mounting configuration.
If your application requires the output shaft to remain fixed after power is removed, do not simply ask whether a motor is “self-locking.” Define the required power-off holding torque and let the motor and gearbox design be evaluated against the actual load conditions.
The difference between these two terms may seem small, but understanding it correctly can prevent significant problems during actuator and gear motor selection.
