Specification of “Creeping” or Non-Holding Torque when Power Removed

If you’ve ever worked with a micro servo motor—those tiny, affordable workhorses found in everything from RC airplanes to 3D-printed robot arms—you’ve probably noticed something odd when you cut the power. The shaft doesn’t just freeze in place. Instead, it often gives a little resistance, maybe even rotates slightly under load, or “creeps” to a new position. This behavior, technically called non-holding torque or back-drivability, is one of the most misunderstood specifications in the micro servo world. And if you’re designing a mechanism that needs to stay put when the power is off—like a camera gimbal lock, a gripper, or a self-locking joint—ignoring this spec can ruin your entire project.

In this deep dive, we’ll unpack what creeping torque actually means, why it matters specifically for micro servo motors, how it’s measured, and how to design around it. We’ll also look at real-world examples, common misconceptions, and the physics that make these tiny motors behave the way they do.

What Exactly Is “Creeping” or Non-Holding Torque?

Let’s start with the basics. When you energize a micro servo motor, it has a “holding torque”—the maximum torque it can resist without losing position, typically specified at a certain voltage (e.g., 1.5 kg·cm at 4.8V). But when you remove power, that holding torque drops to near zero. However, it doesn’t drop to absolute zero. There’s always some residual resistance from:

• Internal gear friction (especially in plastic-geared micro servos)
• Motor cogging torque (magnetic resistance from the permanent magnets)
• Bearing drag (from the output shaft bearings)
• Grease viscosity (especially in cold environments)

This residual resistance is the non-holding torque, sometimes called “creeping torque” because under a constant load, the shaft may slowly rotate or “creep” over time. It’s not a locked position—it’s a frictional brake that can be overcome.

Why Micro Servos Are Especially Prone to Creeping

Full-size industrial servo motors often have electromagnetic brakes, mechanical locks, or high-ratio gearboxes that prevent back-driving. Micro servo motors, on the other hand, are designed for cost, weight, and size efficiency. They typically use:

• Plastic planetary gears (low friction, prone to wear)
• Brushed DC motors (low cogging torque)
• Simple potentiometer feedback (no brake circuit)

This makes them light and cheap, but also means that when power is removed, the output shaft can be rotated with surprisingly little force—sometimes as low as 0.1 kg·cm. That’s about the force needed to lift a single AA battery. If your mechanism has any external load (gravity, spring force, or even vibration), the servo will creep.

The Three Types of Non-Holding Torque You Need to Know

Not all creeping is the same. Depending on the motor design and load, you’ll encounter three distinct behaviors:

1. Static Friction (Stiction) Torque

This is the initial resistance you feel when you first try to rotate the unpowered shaft. It’s caused by static friction between the gears and the output shaft bearing. For most micro servos, this is the highest non-holding torque value—typically 0.2 to 0.5 kg·cm for a standard SG90 or MG996R.

Key point: Once you overcome stiction, the shaft moves more easily.

2. Dynamic Creeping Torque

Once the shaft starts rotating, the resistance drops to dynamic friction. This is what causes “creeping”—a slow, continuous rotation under a sustained load. For example, if you have a micro servo holding a 0.3 kg·cm load (like a small camera), and the dynamic friction is only 0.15 kg·cm, the shaft will rotate until the load is released or the servo hits a mechanical stop.

Real-world example: A pan-tilt camera mount using a micro servo. With power off, the camera slowly tilts downward due to gravity, even though the servo feels “tight” when you first touch it.

3. Cogging Torque (Magnetic Detent)

This is unique to brushless DC motors (common in high-end micro servos like the Dynamixel or T-Motor). The permanent magnets inside the rotor create “detent” positions—points where the rotor naturally wants to align. This gives a periodic resistance as you rotate the shaft. For micro servos with coreless motors, cogging is negligible. But for iron-core motors, it can be significant (0.1–0.3 kg·cm).

Why it matters: Cogging torque can make the shaft feel like it’s “clicking” into positions, which can be mistaken for a brake. But it’s not a lock—it’s just magnetic preference.

How to Read the Spec Sheet (and Why Most Are Misleading)

If you pull up a datasheet for a popular micro servo like the Tower Pro SG90, you’ll see something like this:

• Stall torque: 1.5 kg·cm @ 4.8V
• Operating speed: 0.12 sec/60°
• Weight: 9g

What you won’t see: “Non-holding torque: 0.25 kg·cm” or “Back-drive torque: 0.3 kg·cm.” Most manufacturers simply omit this spec because:

1. It’s not standardized (no ISO or NEMA standard for micro servo non-holding torque)
2. It varies with temperature, wear, and voltage history
3. It’s usually low enough to be embarrassing

How to Infer Non-Holding Torque

You can estimate it from the gear ratio and motor type:

• Plastic gear servos (SG90, MG90S): Non-holding torque is typically 10–20% of stall torque. So a 1.5 kg·cm servo might have 0.15–0.3 kg·cm.
• Metal gear servos (MG996R, DS3218): Higher gear friction gives 15–25% of stall torque. Expect 0.4–0.8 kg·cm for a 10 kg·cm servo.
• Coreless motor servos (HS-35HD, KST X08): Lower cogging and smoother gears give 5–10% of stall torque.
• Brushless servos (Dynamixel XL330, T-Motor AK80): Very low cogging, but gear friction still dominates. Typically 3–8% of stall torque.

Pro tip: If the datasheet lists “back-drive torque” or “free-running torque,” that’s your number. If not, assume 15% of stall torque as a conservative estimate.

Why Creeping Torque Matters in Real Applications

You might think, “I’ll just leave the power on.” But that’s not always possible. Here are five scenarios where non-holding torque becomes a critical spec:

1. Battery-Powered or Energy-Harvesting Systems

In solar-powered sensors or battery-operated robots, you can’t keep the servo powered indefinitely. When the system sleeps, the servo must hold position without power. If the non-holding torque is lower than the load torque, the mechanism will drift, potentially damaging itself or losing calibration.

Example: A solar tracker uses a micro servo to tilt a panel. At night, the system shuts down. If the servo creeps, the panel may tilt to an extreme angle and not return to the correct position at dawn.

2. Safety-Critical Mechanisms (Grippers, Clamps)

In a robotic gripper powered by a micro servo, the object must stay gripped even if the controller crashes or the battery dies. If the non-holding torque is too low, the object slips. This is why many micro servo grippers use additional mechanical locking (like over-center mechanisms) or worm gears.

Real failure: A 3D-printed prosthetic hand using micro servos dropped objects when the battery ran low because the servos couldn’t hold the grip without power.

3. Position Memory After Power Loss

In applications like valve actuators or camera sliders, the servo must remember its position after power-off. If the shaft creeps, the position is lost. Some high-end micro servos have “position feedback” that saves the last angle to EEPROM, but if the shaft moves before power is restored, the saved angle is useless.

4. Vibration and Shock Environments

In drones or RC vehicles, vibration can cause a micro servo to creep even if the static load is low. The vibration reduces static friction, making the shaft more susceptible to back-driving. This is why drone gimbal servos often have higher non-holding torque specs or use electromagnetic brakes.

5. Multi-Joint Mechanisms (Serial Arms)

In a small robotic arm with multiple micro servos, each joint must resist the weight of the downstream links. If the first joint (shoulder) has low non-holding torque, the entire arm will collapse when power is removed. This is a common failure in hobbyist robot arms.

How to Measure Non-Holding Torque Yourself

If the datasheet doesn’t provide it, you can measure it. Here’s a simple method:

Equipment Needed

• Micro servo (obviously)
• Digital torque gauge (0–2 kg·cm range, like a Mark-10 or cheap spring scale)
• 3D-printed or metal arm (to attach to servo horn)
• Power supply (to energize and de-energize the servo)
• Protractor or angle sensor

Procedure

1. Mount the servo securely with the output shaft horizontal.
2. Attach a lever arm to the servo horn (length L = 5 cm is standard).
3. Energize the servo to a known position (e.g., 90°).
4. Apply a load to the arm using the torque gauge, perpendicular to the arm.
5. Slowly increase the load until the shaft just starts to rotate (stiction torque). Record the force.
6. Continue applying load and measure the force required to keep the shaft rotating at a constant speed (dynamic torque).
7. Repeat with power removed. The difference between powered and unpowered torque is your non-holding torque.

Calculation: Torque (kg·cm) = Force (kg) × Lever Arm Length (cm)

Typical Results

| Servo Type | Stiction (kg·cm) | Dynamic (kg·cm) | Cogging (kg·cm) | |------------|------------------|-----------------|-----------------| | SG90 (plastic) | 0.25 | 0.18 | – | | MG996R (metal) | 0.55 | 0.40 | – | | KST X08 (coreless) | 0.12 | 0.08 | 0.02 | | Dynamixel XL330 (BLDC) | 0.08 | 0.05 | 0.01 |

Note: These values vary by unit and wear. Always test multiple samples.

Designing Around Creeping: Mitigation Strategies

If your application can’t tolerate creeping, you have several options—none of them perfect, but each useful in the right context.

1. Use a Worm Gear or Lead Screw

Worm gears are inherently non-back-drivable (within limits). A micro servo driving a worm gear can hold position without power because the gear angle prevents reverse rotation. However, worm gears add friction and reduce efficiency (typically 30–50% loss).

Best for: Grippers, lifts, and position-locking mechanisms.

Trade-off: Slower speed, more heat, and lower overall torque output.

2. Add an Electromagnetic Brake

Some micro servos (like the Dynamixel XM430) have optional brake modules. When power is removed, a spring-loaded brake engages. This gives true zero-creep holding, but adds weight, cost, and complexity.

DIY option: Use a small solenoid brake (like those from Mclennan) that engages when power is off.

3. Use a Higher Ratio Gearbox

A servo with a higher gear ratio (e.g., 1:500 vs. 1:200) will have higher non-holding torque because gear friction scales with ratio. But this also reduces output speed and increases backlash.

Rule of thumb: Doubling the gear ratio roughly doubles the non-holding torque (assuming same gear type).

4. Preload the System

You can use a spring or counterweight to offset the load, so the net torque on the servo is near zero when power is off. This doesn’t eliminate creeping, but it reduces the driving force.

Example: In a camera tilt mechanism, add a spring that counteracts the camera’s weight. With power off, the spring holds the camera in place, and the servo only needs to overcome friction.

5. Use a Servo with “Hold” Feature (PWM Trick)

Some micro servo controllers (like the PCA9685) can be programmed to output a low-power PWM signal even when the main system is asleep. This keeps the servo energized but at a lower voltage, reducing power consumption while maintaining some holding torque. Not true zero-power, but close.

Caution: This can overheat the servo if not designed carefully. Use a current-limiting resistor or a dedicated “sleep” mode.

Common Misconceptions About Creeping Torque

Let’s clear up a few things that often confuse engineers new to micro servos.

“Metal gears mean higher non-holding torque.”

Partly true. Metal gears have higher friction than plastic gears, so they resist back-driving better. But they also wear faster if back-driven repeatedly, and the friction can vary with temperature. A metal-gear servo from a cheap manufacturer may have rough gear meshing, leading to inconsistent creeping.

“Coreless motors have no cogging, so they don’t creep.”

False. Coreless motors have very low cogging (magnetic detent), but they still have gear friction and bearing drag. In fact, some coreless servos have lower non-holding torque because the motor itself offers less resistance. The creeping comes from the gearbox, not the motor.

“If I use a high-torque servo, it won’t creep.”

Not necessarily. A high-torque servo (e.g., 20 kg·cm) usually has a higher gear ratio, which increases non-holding torque. But the ratio is only one factor. A 20 kg·cm servo with smooth metal gears might have 1–2 kg·cm of non-holding torque, which is still low relative to its stall torque. You need to check the spec, not just the number.

“Creeping is the same as backlash.”

No. Backlash is the play between gears when direction changes. Creeping is continuous rotation under load. They often coexist, but they’re different phenomena. A servo can have zero backlash (e.g., harmonic drive) but still creep if the gear friction is low.

Real-World Case Study: A Creeping Failure in a 3D-Printed Camera Slider

Let me share a story that illustrates why this spec matters.

A maker built a motorized camera slider using a micro servo (MG996R) to drive a lead screw. The slider was supposed to hold position when not in use. But after a few days, the camera would slowly slide downhill. The maker assumed the servo was faulty and replaced it—same problem.

The diagnosis: The lead screw had a 2:1 reduction, but the servo’s non-holding torque (0.5 kg·cm) was less than the torque from the camera weight (0.8 kg·cm). Even with the lead screw’s friction, the system back-drove.

The fix: Replace the lead screw with a higher-pitch one (more friction) and add a spring-loaded brake. After the fix, the slider held position indefinitely with power off.

Lesson: Never assume a servo will hold position just because it has a high stall torque. Always calculate the net torque on the shaft with power removed.

How to Specify Creeping Torque in Your Design

When you write a spec for a micro servo in your project, include these parameters:

• Non-holding torque (minimum): The torque the servo must resist with power removed, measured at the output shaft.
• Back-drive torque (maximum): The torque required to rotate the shaft when unpowered. This is the inverse of non-holding torque.
• Temperature range: Non-holding torque drops in cold grease and rises in hot grease.
• Cycle life: After 10,000 cycles, gear wear can reduce non-holding torque by 20–50%.

Example Specification

Micro Servo Specification – Non-Holding Torque - Stall torque: 5.0 kg·cm @ 6V - Non-holding torque (static): ≥ 0.8 kg·cm @ 25°C - Non-holding torque (dynamic): ≥ 0.5 kg·cm @ 25°C - Back-drive torque: ≤ 1.2 kg·cm (to rotate shaft from rest) - Gear material: Steel (heat-treated) - Lubricant: Low-viscosity synthetic grease (operating range -10°C to 60°C)

The Future: Smart Servos with Active Braking

The micro servo industry is slowly moving toward integrated braking. For example:

• Dynamixel Pro+ series has an optional “holding brake” that engages when the communication bus goes silent.
• T-Motor AK series uses field-oriented control (FOC) to generate a holding torque even at zero speed (but this requires power).
• Some hobby servos now include a “brake mode” that shorts the motor terminals, creating a dynamic braking effect (like an eddy current brake).

But for now, most micro servos still rely on friction alone. If you need true zero-creep holding, you’ll need to add external hardware or choose a specialized servo.

Final Thoughts on Creeping Torque

The specification of “creeping” or non-holding torque in micro servo motors is one of those silent killers in electromechanical design. It’s rarely on the datasheet, but it can make or break a project. The key takeaways:

1. Always measure or estimate non-holding torque for your specific servo.
2. Design for the unpowered state—assume the servo will creep unless you prove otherwise.
3. Use mechanical advantages like worm gears, springs, or brakes to compensate.
4. Don’t confuse stall torque with holding torque—they are different numbers, and non-holding torque is an order of magnitude lower.

Next time you pick up a micro servo, give the shaft a twist with the power off. Feel that slight resistance? That’s your creeping torque. And now you know exactly what to do with it.