Stepper vs. servo for linear actuators
It is the question behind half the sizing requests we see, and the honest answer depends on three numbers the engine computes for both motors at once.
Three numbers that decide
Torque margin at speed. A stepper's available torque falls steeply with rpm (its pull-out curve); a servo's continuous torque is nearly flat. The engine reads both from catalog curves — stepper at 24 V, servo at 48 V — at the rpm your speed and pulley or lead demand, and requires at least 25% margin. Speed margin. How far the catalog speed cap sits above your request: 500 mm/s stepper versus 1,000 mm/s servo on an SBA45 is the difference between 42% and 620% headroom at a 300 mm/s spec. Inertia mismatch. Load inertia reflected to the motor over rotor inertia; above 10:1 the engine computes a gearbox remedy (3:1 or 5:1 PLE gearhead, its own inertia included).
| Stepper (NEMA 17/23/34) | Servo (NV17/23/34) | |
|---|---|---|
| Payload cap | 25 kg | 50 kg |
| Speed cap (XY) | to 1,200 mm/s | to 2,500 mm/s |
| Z speed cap | 500 mm/s | 750 mm/s |
| Behavior near limit | stalls (no feedback) — size with margin | follows with feedback; peak headroom above continuous |
| Controller | BLC-2T / 4T / 6T | BLC-2TC / 4TC |
| Price, SBA45 500 mm with 3:1 gearbox | $1,800 | $1,950 |
A worked example
2.5 kg over 500 × 500 mm at 8 cycles a minute. Stepper: SBA45, N17-1 with 3:1 gearbox, 166% torque margin, 43% speed margin, 8.2:1 inertia, 8 cycles/min at spec. Servo: SBA45, NV17-1 with 3:1, 171% torque margin, 621% speed margin, 5.2:1 inertia, 8/min at spec and 9.2/min at the catalog cap. The delta is $150 per X-axis. Both are correct; the servo buys future speed, the stepper buys price.
When the answer is obvious
Above 25 kg or 1,200 mm/s, servo is the only catalog option. Below 3 kg and 300 mm/s with no growth expected, stepper wins on cost and simplicity. In between — most real applications — the engine's two-column comparison is the fastest way to a defensible decision, and it prints the basis for either choice.
Size it yourself in about a minute
Type the task — payload, travel, speed — and the Engineering Decision Engine returns a validated, priced 3D system with the torque margins, inertia check, and cycle-time math shown. Free to design and quote.
Frequently asked
Does the engine size both motor types every time?
Ask for a comparison — "stepper or servo?" — and it sizes both columns from the same spec on real torque data and shows the price delta. Otherwise it sizes the type you specify.
What is inertia mismatch and why 10:1?
The ratio of load inertia reflected to the motor shaft over the motor's rotor inertia. Above roughly 10:1 a stepper loses positional accuracy and a servo becomes hard to tune; a gearbox reduces reflected inertia by the ratio squared.
Why 24 V for stepper and 48 V for servo?
Those are the standard supply voltages for the controller families (BLC-T and BLC-TC) the engine quotes; the torque curves are read at those voltages so the margins reflect the system as it will actually ship.