Selecting a gearmotor for an infusion pump
A syringe pump drive is a lead screw pushing a plunger against fluid-path pressure. Screw torque, flow turndown and stall pressure decide the gearmotor, not rated watts.
- Wedge
- Medical and laboratory equipment
- Catalog reviewed
- 2026-08-18
- Configurations screened
- 32 published
In a syringe pump the gearmotor turns a lead screw. The screw nut drives a pusher block against the syringe plunger. The plunger displaces fluid through the administration set, and the set, the filter, the catheter and any partial occlusion all appear at the plunger as a back pressure acting on the full barrel area. Nothing else in the mechanism does useful work: the load is one axial force on one screw. Peristaltic and cassette pumps differ in that the load is a cam or roller train and the pressure appears as a cyclic torque ripple rather than a steady axial force, but the sizing logic below is the same once the mechanism relation is written down.
Write the axial force first, then convert it once. Axial force is the pressure force plus the plunger seal friction: F = p·A + F_seal, where A is the barrel bore area. Screw torque follows the standard power-screw relation T = F·L / (2π·η), with L the screw lead and η the screw efficiency including the nut. Output speed comes straight out of the flow rate: n = Q / (A·L). Because both relations are linear in L, the lead is the one geometry choice that trades torque against speed, and it is usually fixed before the motor is chosen.
What normally controls the selection
Two things decide it, and neither is the running torque. The first is flow turndown. A pump specified from 1 mL/h to 250 mL/h asks the same drive for a 250:1 speed range, and the reduction ratio has to keep the motor above the speed where commutation ripple and cogging become visible flow ripple at the low end while still reaching full flow at the high end. The second is the stall condition. A syringe drive runs into an occlusion and stalls, and the torque it can develop at stall is set by the motor stall torque times the ratio, which for most published configurations is well above what the gearhead is rated to carry and far above what the syringe can survive.
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 |
|---|---|---|
| Occlusion alarm fires late or not at all | Occlusion is inferred from motor current, but at a high reduction ratio the pressure-dependent part of the current is a small increment on top of a no-load current that itself drifts with temperature, grease state and brush condition. | Alarm threshold passes at 20 degrees C on a new unit and drifts out of tolerance after run-in or in a warm room. Field reports of delayed alarms with no reproducible fault. |
| Start-up bolus or delayed onset after a pause | Lost motion in the geartrain and screw, plus plunger seal compliance, must be taken up before the plunger moves. The compliance stores energy during the take-up and releases it as a short over-delivery. | Volumetric accuracy fails only in the first minutes after a restart, or only at low programmed rates where the take-up time is a large fraction of the measurement window. |
| Gearhead output stage failure after an occlusion event | The drive stalled at full voltage with no current limit. The output stage carried several times its rated torque for as long as the stall lasted. | Broken or plastically deformed final-stage teeth, or a spun output shaft, found at teardown on a unit with no other damage. Often a single event, not wear. |
| Flow ripple visible at low programmed rates | Reduction ratio too low, so at minimum flow the motor is turning at a few tens of rpm where cogging and commutation dominate the instantaneous speed. | Trumpet-curve short-term accuracy fails at the low end of the rate range while long-window accuracy passes. |
| Specification | Why it misleads in this application |
|---|---|
| Rated output in watts | A syringe drive at full programmed flow needs on the order of tens of milliwatts at the output. Every configuration in the catalog clears that. Watts tells you nothing about the two things that matter, which are the speed at minimum flow and the torque at stall. |
| Stall torque | Stall torque is not a capability here, it is a hazard. It is the number that tells you how much pressure the drive can develop into a closed line, and for most configurations it is above the gearhead's own rating. |
| Gearhead rated output torque | It is a limit, not an operating point, and it is a continuous rating. It does not tell you what a short stall does, and it does not bound the stall torque the motor can push through it. |
| No-load output speed | The pump never runs there. What matters is the motor speed at the lowest programmed flow rate, which is the no-load speed divided by the ratio and then divided again by the turndown. |
The tradeoffs that matter here
Spur or planetary reduction
Brushed coreless or brushless
Where the feedback goes
Size a syringe drive for a 20 mm bore barrel at 250 mL/h maximum flow, and then check what the drive does into a closed line.
- Syringe bore
- 20 mm
- Maximum programmed flow
- 250 mL/h
- Minimum programmed flow
- 1 mL/h
- Screw lead
- 1 mm
- Occlusion alarm pressure
- 100 kPa gauge
| Step | Expression | Result |
|---|---|---|
| Plunger area | A = π/4 × (0.020 m)² | 3.1416 × 10⁻⁴ m² |
| Pressure force at the alarm threshold | F_p = 100 000 Pa × 3.1416 × 10⁻⁴ m² | 31.42 N |
| Total axial force | F = 31.42 N + 8 N | 39.42 N |
| Screw torque at the alarm threshold | T = 39.42 N × 0.001 m / (2π × 0.30) | 20.9 mN·m |
| Output speed at maximum flow | n = 250 cm³/h ÷ (3.1416 cm² × 0.1 cm) | 13.3 rpm |
| Output speed at minimum flow | n = 13.3 rpm ÷ 250 | 0.053 rpm |
| Motor speed at maximum flow, 366.12:1 | 13.3 rpm × 366.12 | 4 869 rpm, 66 percent of the 7 350 rpm no-load speed |
| Motor torque at the alarm threshold | 20.9 mN·m ÷ (366.12 × 0.66) | 0.087 mN·m, 17 percent of the 0.5 mN·m rated torque |
| Current at the alarm threshold | 0.087 mN·m ÷ 3.763 mN·m/A + 19.5 mA | 42.5 mA |
| Current running free, seal friction only | 4.24 mN·m output ÷ (366.12 × 0.66) ÷ 3.763 mN·m/A + 19.5 mA | 24.2 mA |
| Output torque at a hard stall | 2.98 mN·m × 366.12 × 0.66 | 720 mN·m, 2.45 times the 294 mN·m gearhead rating |
| Plunger force at a hard stall | 0.720 N·m × 2π × 0.30 ÷ 0.001 m | 1 357 N |
| Line pressure at a hard stall | 1 357 N ÷ 3.1416 × 10⁻⁴ m² | 4.32 MPa, about 627 psi |
Result
MM-C1321-P03661-030A resolves the running case with margin. It is a 13 mm coreless brushed configuration, 366.12:1 planetary, rated 16.7 rpm at the output against the 13.3 rpm demand, and 120.8 mN·m continuous output torque against the 20.9 mN·m required. The drive runs at 42.5 mA into an occlusion and 24.2 mA free, so a current-based occlusion detector is working with an 18 mA difference sitting on a 19.5 mA no-load pedestal that drifts. The stall case does not resolve. Into a closed line at 3 V the drive develops 4.32 MPa, roughly 300 times the alarm threshold, and 2.45 times the gearhead's own rating. A current limit set near the alarm current, or a plunger force sensor, is a functional requirement of the mechanism, not an option.
What would change it
A lower reduction ratio makes the stall case less violent but the low-flow case worse. MM-C1024-P00640-030A also clears the running torque at 40.3 mN·m continuous, in a 10 mm body, but at 64:1 the motor turns 851 rpm at maximum flow, only 6.7 percent of its 12 650 rpm no-load speed. At 1 mL/h it would be turning 3.4 rpm, which is not a controllable operating point for a brushed motor. Changing the screw lead moves torque and speed together in opposite directions and is usually the cheapest lever. Changing the barrel bore changes force with the square of the diameter and is usually fixed by the syringe standard.
A short qualification plan
Each step names the measurement, not the intention.
-
Measure the mechanism, not the catalog
Plunger seal breakaway and running friction on the actual syringes at the temperature extremes, with the screw efficiency backed out from a torque measurement rather than assumed.
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Establish the current signature
Motor current against line pressure across the flow range, on at least five units, at the temperature extremes and after the run-in the geartrain will see in service.
-
Prove the stall protection
Deliberate occlusion at every programmed rate with the limit active. Record peak line pressure and peak output torque. Confirm the gearhead rating is never exceeded.
-
Measure lost motion as delivered volume
Start-up and restart bolus at the lowest programmed rate, measured gravimetrically, on the assembled drive rather than on the gearmotor alone.
-
Run the duty cycle
Full-life cycling at the real duty profile with periodic accuracy checks, and a teardown of the units that complete it.
Tools, products and articles for this problem
Send the pump mechanism for an application review.
Send the syringe geometry, screw lead, flow range and alarm threshold. You get back the load model, a candidate configuration with its margins, and the stall case worked through.