Explainer: Holding Torque vs Peak Torque in Stepper Motors

What is holding torque in a stepper motor?

Holding torque is the maximum torque a stepper motor can apply without moving. It represents the static force available when the motor is energized but not rotating. This rating matters most when the machine must keep a part in place, support a weight, or hold a position under gravity.

The value depends on the motor’s magnetic structure, winding design, and driver current. Most motor datasheets list holding torque in newton-meters or ounce-inches. When you buy a stepper, this is the first number you see. It tells you how much load the motor can resist while stationary.

Holding torque is often higher than peak torque at low speeds. As speed increases, the motor’s ability to produce torque drops. The difference between the two ratings defines the torque curve, which is the core of your sizing decision.

What is peak torque in a stepper motor?

Peak torque is the maximum torque available while the motor rotates. It occurs at a specific speed, usually near zero or low RPM, depending on the driver and motor design. Peak torque is what matters during acceleration, rapid jogging, or when the load resists motion.

If your application moves a part quickly, the motor must deliver peak torque to overcome inertia. If it only holds a position, holding torque is enough. Confusing the two is a common sizing error. Engineers pick the wrong motor because they read only one number on the label.

Peak torque varies with speed. A motor may deliver high torque at 100 RPM but much less at 1500 RPM. The datasheet torque-speed curve shows this drop. Always check the curve, not just a single peak number.

How do holding and peak torque affect sourcing?

When sourcing a stepper motor, match the torque rating to the actual job. A motor sized only on holding torque may stall during acceleration. A motor sized only on peak torque may be larger and cost more than needed for static loads.

Look at the load first. Is the machine lifting a weight? Holding torque is the primary number. Is it accelerating a spindle or moving a gantry quickly? Peak torque is the primary number. Is it doing both? You need a torque curve that covers both conditions.

Driver settings also change the numbers. A stepper driver with microstepping can smooth torque delivery, but it does not change the fundamental holding or peak ratings. However, the current limit matters. If you run the motor at half its rated current, both holding and peak torque drop. Check the datasheet for torque versus current.

How to read a torque-speed curve

A torque-speed curve plots torque on the vertical axis and speed on the horizontal axis. It shows how much torque is available at each RPM. The curve starts high at zero speed and falls as speed increases.

The value at zero speed is the holding torque. The highest point on the curve is the peak torque. In many stepper designs, these two values are close but not identical. The curve may rise slightly above the zero-speed value before falling.

When you read the curve, find the speed your machine will operate at. Then check the torque available at that speed. Add a margin for friction, acceleration, and variations in the load. If the available torque is less than the required torque, the motor will lose steps or stall.

Worked example: sizing a stepper for a lab instrument

Imagine a small lab instrument that moves a glass slide across a stage. The stage weighs 0.5 kg. The slide moves horizontally, so gravity is not the main load. The main loads are friction in the rails and the acceleration required to move the slide quickly.

First, estimate the friction force. A typical linear rail has a coefficient of friction around 0.05 to 0.1. Multiply the weight by gravity and the coefficient to get the friction force. Convert that to torque using the stage radius or lead of the drive screw.

Next, estimate the acceleration. If the slide must move 100 mm in 0.1 seconds, calculate the required acceleration. Convert that to the torque needed to spin the screw at that rate.

Now compare both numbers to the motor datasheet. If the friction torque is 0.2 N-m and the acceleration torque is 0.3 N-m, you need at least 0.5 N-m with margin. Check the torque curve at your operating speed. If the curve shows 0.4 N-m at that speed, the motor is undersized. Pick a larger motor or reduce the acceleration.

This example shows why you need both ratings. The holding torque may be 0.6 N-m, which handles the friction. But the peak torque at speed may be only 0.4 N-m, which is not enough for acceleration. The correct choice is a motor whose torque curve covers both demands.

Common sizing mistakes to avoid

The first mistake is assuming holding torque equals peak torque. They are different measurements for different conditions. A motor that holds a heavy load may not accelerate it quickly.

The second mistake is ignoring the speed. A motor may have high torque at low RPM but low torque at high RPM. If your machine runs fast, check the curve at that speed.

The third mistake is not adding a safety margin. Real systems have friction, vibration, and load variations. Size the motor with at least 20 percent extra torque over the calculated requirement. This prevents stalling and extends motor life.

The fourth mistake is choosing the wrong drive method. Open-loop steppers rely on the driver and motor to stay in sync. If torque drops too low, the motor loses steps. Closed-loop steppers use feedback to correct position. They can handle more torque variation, but they cost more. Choose based on your application’s tolerance for error.

When to choose a servo over a stepper

Stepper motors are simple, cost-effective, and easy to control. They work well for low-speed, high-torque applications where precision is not critical. But they have limits.

Servo motors offer higher torque at speed, better acceleration, and closed-loop control. If your application requires high speed, high acceleration, or precise position control, a servo may be the better choice. The trade-off is cost and complexity.

Compare the two options using the torque-speed curve. If the stepper curve drops below your requirement at operating speed, a servo is likely the answer. If the stepper meets the requirement with margin, it is a practical choice.

Final check before buying

Before ordering a stepper motor, verify three things. First, confirm the holding torque meets your static load. Second, confirm the peak torque at your operating speed meets your dynamic load. Third, check the driver compatibility. The driver must supply the correct current and voltage to the motor.

If all three checks pass, the motor will perform. If any check fails, adjust the design. Reduce speed, increase motor size, or switch to a servo. Getting this right saves time, money, and field failures.

Reference table for torque ratings

Torque Type	Definition	Best For	Typical Use Case
Holding torque	Max torque at zero speed	Static loads	Holding a gate, supporting a weight
Peak torque	Max torque during rotation	Acceleration	Rapid jogging, high-speed moves
Continuous torque	Average torque at steady speed	Long runs	Conveyor drives, constant-speed loads
Torque ripple	Variation in torque during rotation	Precision work	Minimizing vibration in fine positioning

Use this table as a quick reference. When you read a datasheet, match each column to your application. If your job involves both holding and moving, you need both ratings to be adequate.