Holding torque is the maximum force a stepper motor can resist while staying locked in position. When the motor’s coils are energized but the shaft isn’t spinning, holding torque is what keeps the shaft from being pushed or twisted out of place by an external load. If the applied force exceeds the motor’s holding torque rating, the rotor slips and loses its position.
How Holding Torque Works
A stepper motor moves in precise angular steps, and between those steps it needs to stay put. When current flows through the motor’s windings, it creates a magnetic field that locks the rotor into a specific position. Holding torque is the peak of that magnetic “grip.” As long as any outside force on the shaft stays below that peak, the motor holds steady. The rotor may deflect slightly under load, but it snaps back to its intended step position once the force is removed.
This is one of the defining advantages of stepper motors over other motor types. A stepper can deliver full torque at a complete standstill, holding a load in place without any rotation. Servo motors, by contrast, typically need a mechanical brake to hold position at zero speed. That makes steppers a natural fit for applications where a load must be locked in place reliably, like positioning a tool head or holding a camera platform steady.
Holding Torque vs. Detent Torque
Motors with permanent magnets inside have a weaker, passive resistance to rotation even when the power is off. This is called detent torque. It comes from the attraction between the permanent magnet on the rotor and the steel teeth of the stator, and it exists without any electrical input at all. You can feel it as the subtle “cogging” when you turn a stepper motor shaft by hand while it’s unpowered.
Holding torque is fundamentally different because it requires current flowing through the coils. That active electromagnetic field is far stronger than the passive magnetic pull of detent torque. A motor’s holding torque rating assumes rated current is applied. Detent torque is typically a small fraction of that value, nowhere near enough to securely hold a meaningful load.
Holding Torque vs. Dynamic Torque
Holding torque is a static measurement, taken when the motor is standing still. Once the motor starts spinning, different torque values come into play, and they’re almost always lower than holding torque.
- Pull-out torque is the maximum load the motor can handle at a given speed without skipping steps. As speed increases, pull-out torque drops.
- Pull-in torque is the maximum combination of torque and speed at which the motor can reliably start, stop, and reverse direction. It defines the envelope for responsive, start-stop motion.
Both of these dynamic torques are plotted on a torque-speed curve, which slopes downward as speed rises. Holding torque sits at the far left of that curve, at zero speed, representing the motor’s strongest possible grip. This is why a motor’s holding torque rating can be misleading if you only look at that single number. A motor rated at 2 N·m of holding torque will deliver considerably less usable torque once it’s spinning at working speed.
What Determines Holding Torque
The single biggest factor is current. Holding torque is specified at the motor’s rated current. Reduce the current and the magnetic field weakens, lowering the torque proportionally. This is why the driver electronics matter almost as much as the motor itself. A quality driver that delivers clean, stable current at the rated level lets the motor reach its published holding torque spec.
Temperature also plays a role. As the motor runs, its coils heat up. Hotter copper has higher electrical resistance, which means less current flows for the same applied voltage, which means less torque. Internal heat also comes from the constant realignment of magnetic particles in the stator material as the field changes direction. If a motor gets excessively hot, practical options include reducing the duty cycle, lowering the current when full torque isn’t needed, or adding a heat sink or fan to pull heat away from the coils.
Units and Typical Ratings
Holding torque is measured in the same units as any other torque. The international standard unit is the newton-meter (N·m). In the United States, many stepper motor datasheets list torque in ounce-inches (oz-in). To convert, 1 oz-in equals roughly 0.00706 N·m, or flipped around, 1 N·m equals about 141.6 oz-in.
Small NEMA 17 motors common in desktop 3D printers typically have holding torques in the range of 0.3 to 0.5 N·m (roughly 40 to 70 oz-in). Larger NEMA 23 motors used in CNC routers can reach 1.5 to 3 N·m or more. The right rating depends entirely on the load and the forces acting on the motor at rest.
Why It Matters in Practice
Holding torque is a critical spec for any machine where the motor must resist external forces while stationary. In a CNC milling machine, cutting forces push back against the axes. If the stepper motor’s holding torque isn’t high enough, the rotor slips by one or more steps, and the tool ends up in the wrong position. This is known as “losing steps,” and it ruins the workpiece with no warning, since most stepper systems have no position feedback to detect the error.
For lighter tasks like laser engraving, the forces on the motor are minimal, so holding torque requirements are low. Milling aluminum or hardwood, on the other hand, demands significantly more torque. Experienced CNC builders consistently emphasize that undersized motors are one of the most frustrating mistakes to make, because the machine works fine during testing but loses steps unpredictably under real cutting loads.
3D printers face a milder version of the same challenge. The print head needs to hold its position precisely between moves, and the motors must resist the slight drag of the belts and bearings. Holding torque matters less here than in milling, but it still determines how reliably the printer maintains layer alignment over hours-long prints.
What Happens During a Power Failure
When power is cut, holding torque drops to zero (or to the much weaker detent torque if the motor has permanent magnets). On a vertical axis, this means gravity can pull the load downward immediately. This is a serious safety concern in industrial equipment, robotic arms, and any system where an uncontrolled drop could damage hardware or injure someone.
The standard solution is a power-off brake, sometimes called a spring-engaged brake. These brakes work in reverse from what you might expect: they’re held open by an electromagnet while the system is running, and they clamp down automatically when power is removed. Springs push a friction disc against the shaft the moment the electromagnetic field disappears, locking everything in place. This provides reliable holding force during power outages, emergency stops, or intentional shutdowns, without depending on the motor’s own magnetic field at all.