Gearmotors for motorized valves
A valve actuator is sized by seat breakaway, not running torque, and it usually ends its travel by stalling into a stop that the gearhead was never rated to survive.
- Wedge
- Industrial instrumentation and automation
- Catalog reviewed
- 2026-08-18
- Configurations screened
- 32 published
A quarter-turn actuator rotates a ball or butterfly valve 90 degrees between hard stops. A multi-turn actuator drives a globe or gate valve stem through a screw. A pinch valve compresses tubing. In all three the torque profile is the same shape: a high breakaway to unseat, a low running torque through the travel, and a rising torque as the closing member reaches its seat. The travel ends at a mechanical limit, and how the drive is told to stop there is the central design decision.
Torque comes from the valve manufacturer, not from a calculation: seat breakaway torque, running torque and seating torque are published per valve size, pressure and seat material, and they vary by a factor of several between a new soft seat and an aged one. What the motion engineer computes is the drive side. Output speed is the travel divided by the stroke time: for a quarter turn, n = 0.25 rev / t. Motor torque at any output torque is T_motor = T_out / (N·η). Then the two checks that matter: whether the breakaway torque fits inside the motor's short-term capability, and whether a stall at the end stop fits inside the gearhead's rating.
What normally controls the selection
Breakaway torque against the motor's short-term capability, and the end-stop condition against the gearhead rating. Running torque never governs, because it is a fraction of breakaway and the duty is low. The failure that ends valve actuator programs is the end stop: an actuator that drives to a hard stop and stalls delivers stall torque times ratio into the output stage, and on a 361:1 planetary that is 2.3 times the gearhead's rating on every single cycle. Limit switches, a torque limit, or a compliant stop is not an accessory. It is what makes the actuator survive its own travel.
Four conditions, and they are usually decided by four different numbers.
Sizing on one of them is how a mechanism gets a part that works on the bench and fails in the field.
Failure modes and misleading specifications
What follows is what actually fails, and which published number sends people the wrong way.
| Failure mode | Physical cause | What it looks like in the field |
|---|---|---|
| Output stage failure after a predictable number of cycles | Stalling into the end stop at full command on every close. The overload is not a fault condition, it is the control scheme. | Fatigue at the output stage tooth roots, failures clustered at a consistent cycle count, and no correlation with the process fluid or with running hours. |
| Valve fails to open after standing closed | Breakaway torque risen as the seat takes a set or the process fluid crystallizes. The actuator was sized on the new-seat breakaway figure. | Failures after a shutdown or after a long closed period, clearing when the valve is manually cracked. Current trace shows stall current with no rotation. |
| Overheating on a modulating duty | An actuator sized for open-close service used for throttling, where the drive repositions continuously and never gets a cooling interval. | Thermal cutout trips or winding failure on units in modulating service while identical units in on-off service run for years. |
| Position uncertainty after a power interruption | An actuator with no absolute position reference, and a geartrain that back-drives under the seat reaction. | The actuator has to re-home against a stop after every power event, which is another stall cycle it was not counted for. |
| Specification | Why it misleads in this application |
|---|---|
| Valve torque quoted as a single figure | Breakaway, running and seating are three different numbers, and all three change with pressure, temperature, seat material and age. Sizing on a single figure sizes on the wrong one. |
| Continuous output torque | The actuator needs its peak torque for a second or two at zero speed. A continuous rating describes a thermal steady state that a quarter-turn actuator never reaches. |
| Gearhead rated output torque as a stall rating | It is a continuous rating derived from tooth strength and life, not an impact or single-event rating. It does not tell you what one stall does, and it does not bound what the motor can push through it. |
| Stroke time | Stroke time sets the output speed, which for a quarter turn in a couple of seconds is a few rpm and is trivially satisfied by every configuration. It contributes nothing to the selection and it is often the only requirement written down. |
The tradeoffs that matter here
Spur or planetary reduction
Brushed coreless or brushless
How the travel is ended
Size a quarter-turn actuator for a small ball valve, and check both the breakaway and the end stop.
- Seat breakaway torque
- 350 mN·m
- Running torque
- 90 mN·m
- Travel
- 90°, in 2.0 s
- Supply
- 12 V
- Design factors
- 1.4 on running, 1.25 on breakaway
| Step | Expression | Result |
|---|---|---|
| Output speed | 0.25 rev ÷ 2.0 s × 60 | 7.5 rpm |
| Motor speed at that output speed | 7.5 rpm × 361 | 2 708 rpm, 44 percent of the 6 200 rpm no-load speed |
| Torque requirement after design factors | 90 × 1.4 and 350 × 1.25 | 126 mN·m continuous, 438 mN·m peak |
| Motor torque at breakaway | 350 mN·m ÷ (361 × 0.43) | 2.25 mN·m, 115 percent of the 1.96 mN·m rated torque and 19 percent of the 11.8 mN·m stall torque |
| Current at breakaway | 2.25 mN·m ÷ 12.421 mN·m/A + 150 mA | 332 mA against a 290 mA rated current, acceptable for a one to two second transient |
| Motor torque running | 90 mN·m ÷ (361 × 0.43) | 0.58 mN·m, 30 percent of rated |
| Current running | 0.58 mN·m ÷ 12.421 mN·m/A + 150 mA | 197 mA, 68 percent of rated |
| Output torque at a hard stall | 11.8 mN·m × 361 × 0.43 | 1 832 mN·m, 2.33 times the 785 mN·m gearhead rating |
Result
MM-B2419-P03610-120A resolves the running and breakaway cases. It is a 24.2 mm brushless configuration, 361:1 planetary, 12 V, rated 14.4 rpm at the output against the 7.5 rpm demand, 304.3 mN·m continuous against a 126 mN·m requirement, and a 785 mN·m gearhead rating against a 438 mN·m peak requirement. The motor sits at 19 percent of its stall torque at breakaway and draws about 332 mA for one to two seconds against a 290 mA rated current, which is a normal short-term overload. The end stop does not resolve. At 12 V with no limit this drive develops 1 832 mN·m into a stalled output, 2.33 times what the gearhead is rated to carry, and it would do that on every close. The actuator needs a current limit set to bound the output near the 438 mN·m seating requirement, which is 56 percent of the gearhead rating and leaves the seating torque intact. Being brushless, it already has the driver that makes that limit free.
What would change it
Aged-seat breakaway is the largest uncertainty and it moves in one direction only. If breakaway rises to 600 mN·m the motor torque requirement becomes 3.87 mN·m, still only 33 percent of stall, and the peak requirement becomes 750 mN·m, which is 96 percent of the gearhead rating and no longer acceptable. Modulating duty changes the problem entirely: continuous repositioning removes the cooling intervals and the continuous rating starts to matter. A slower stroke time buys nothing here because speed was never binding. The alternative published configuration with the same 785 mN·m gearhead rating, MM-C1736-P02560-060B, is coreless brushed at 6 V, rated 14.5 rpm, 319.2 mN·m continuous, and stalls at 1.67 times the gearhead rating, so it needs the same limit without a driver already present to provide it.
A short qualification plan
Each step names the measurement, not the intention.
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Get the real valve torques
Breakaway, running and seating torque measured on the actual valve at the process conditions, new and after an accelerated seat-aging exposure.
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Verify the torque limit at both temperature extremes
Measured output torque at the commanded limit, cold and hot. The torque constant falls as the magnets warm and the limit moves with it.
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Confirm the seating torque is still delivered
Leak test at the limit setting. A limit low enough to protect the gearhead has to still seat the valve.
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Cycle to the program life
Full open-close cycling with the limit active, then teardown of the output stage and, on a brushed candidate, the commutator.
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Prove the power-interruption case
Behaviour on loss of supply mid-travel, including whether the train back-drives under the seat reaction and how position is recovered.
Tools, products and articles for this problem
Send the valve torque data for an application review.
Send the breakaway, running and seating torques, the stroke time and the supply. You get back candidates checked against all three, and the torque limit that protects the gearhead without losing the seat.