Speed Gate Gearmotor: Ratio, Backlash, Fail-Safe

How a Speed Gate Gearmotor’s 100:1 Ratio Sets Backlash, Back-Drive and Power-Off Behaviour

Two Power Figures, One Gearmotor

Take a representative 60 mm integrated gate gearmotor: 24 V nominal, 100:1 planetary, rated 30 rpm at 9.3 N·m, nameplate power 40 W. Now multiply torque by angular velocity and see whether the numbers agree. They do not, and the gap is the first thing a buyer should understand before comparing quotes.

P = T · 2πn / 60 = 9.3 × 6.2832 × 30 / 60 = 29.22 W

So the rated point delivers 29.2 W of mechanical power, not 40 W. The published output curve, measured at 23.96 V and 2.38 A, gives 9.356 N·m at 30.80 rpm, which is 30.17 W. That is within measurement tolerance of the 29.2 W calculation and confirms the rated point. The 40 W nameplate value must therefore describe a different point on the same curve.

Two measured points are enough to find it by linear extrapolation. Between no load (0.755 A, 36.50 rpm, 0.085 N·m) and the rated point (2.38 A, 30.80 rpm, 9.356 N·m), the slopes are 5.71 N·m per amp and 3.51 rpm per amp. Run both forward to the 3 A current limit.

The extrapolation lands at 38.6 W, about 3.5 percent below the 40 W nameplate. That is the rounding you expect when the nameplate quotes a ceiling value at the maximum current rather than a continuous rating. The two figures are not in conflict; they simply describe two operating points on one curve.

Why this matters at the quotation stage

If 40 W is read as continuous output at 30 rpm, the implied torque is 12.7 N·m, 37 percent above the 9.3 N·m the unit is rated for. A barrier sized that way will run into current limit on every cycle, which shows up later as unexplained overtemperature trips rather than as an obvious overload.

The correct phrasing for a specification or a purchase order is short: 9.3 N·m continuous at 30 rpm, approximately 30 W; 40 W available at the 3 A current limit only. Anyone quoting the second number without the first is either reading a different curve or has not done the arithmetic.

Where the output point comes from is a separate question, and it depends on the mechanism. A swing arm swinging a leaf has its own load case, and the torque requirements for swing gate motors are the right starting point for that geometry rather than the figures above.

What 100:1 Does to Every Other Number

A reduction ratio is not one multiplication, it is five different operations applied to five different quantities. Getting any of them wrong produces a design that looks correct on paper and misbehaves on the bench. The table below applies the same 100:1 to the drive described above.

Speed divides by the ratio with no efficiency term. Torque multiplies by the ratio and by gearbox efficiency, because the gearbox consumes part of what it transmits. Inertia divides by the square of the ratio, so a 100:1 drive can accelerate a flap that would stall a direct-drive motor of the same size.

The asymmetry that catches designers

Backlash behaves like inertia, not like torque: it multiplies by the ratio when referred to the motor. Twenty arcmin at the output becomes 2,000 arcmin at the motor shaft, which is 33 degrees of free rotation. That sounds alarming and is in fact harmless, because the Hall sensors sit on the motor shaft and never see it.

It stops being harmless the moment a designer adds an encoder at the output to close the position loop, or uses the motor’s Hall transitions to infer flap angle. Then the 0.33 degree of output play becomes a dead band in the control loop, and the controller is asked to hold a position it cannot resolve. Either measure at the output or accept the dead band explicitly in the tuning.

One more ratio consequence is easy to overlook on a small drive: 100:1 makes the motor’s own detent and friction torque visible at the output. A 0.01 N·m of combined cogging and seal drag becomes roughly 0.9 N·m at the shaft. That is the number that decides whether a flap can be pushed open by hand, and it is examined in the self-locking section below.

Efficiency Has Three Boundaries

An end-to-end figure of 53.5 percent looks poor for a 24 V brushless drive until the boundaries are separated. Three efficiencies are stacked here: the motor converts electrical input to shaft power, the gearbox converts shaft power to output power, and the controller adds its own losses upstream. Mixing them produces arguments that cannot be settled.

Start with copper. For a three-phase machine with a line-to-line resistance of 1.79 Ω, total copper loss is 1.5 · I² · R_LL. At the rated point that is 1.5 × 2.38² × 1.79 = 15.2 W, or 27 percent of the 56.35 W input. Subtracting copper from input leaves 41.1 W, of which roughly 7.6 W goes to iron, friction and stray losses at 3,080 rpm. The motor shaft therefore carries about 33.5 W, and 33.5 / 56.35 gives a motor efficiency near 59.5 percent.

The gearbox figure is an industry-typical value for a grease-lubricated planetary of this size, not a measurement taken on this specific unit; treat it as ±2 points. It is included because the split matters: without it, a 53.5 percent end-to-end number invites the conclusion that the motor is badly designed, when in fact more than a quarter of the input is being spent as copper loss at a deliberately conservative rated point.

Where the gearbox number moves

Per-stage efficiency of 96–97 percent is normal for a spur or helical planetary stage under rated load, so three stages multiply to roughly 90 percent and two stages to about 93 percent. That headline figure degrades in three situations that all apply to a barrier.

• Light load. At 20 percent of rated torque the no-load spin losses dominate and overall efficiency can fall by 5–8 points. A flap that spends most of its life making short, unloaded moves runs far from the catalogue number.
• Cold start. Grease viscosity rises steeply below 0 degrees C. The -5 degrees C lower limit in the duty specification is a mechanical limit, not an efficiency-preserving one; expect a higher starting current for the first few cycles.
• Long holding periods. A drive that holds position against a spring for hours is dissipating motor current continuously, and that loss never appears in a gearbox efficiency figure at all.

The same split explains why planetary gearbox efficiency is worth reading as a boundary question rather than a single percentage, and why the case for a three-stage planetary rests on packaging and ratio granularity, not on efficiency alone.

Backlash: The Number Nobody Puts on the Drawing

Backlash is the angular play at the output shaft when the input is held fixed. In a planetary it comes from three sources: tooth flank clearance at each mesh, clearance between planet pins and carrier bores, and axial float in the output bearing. Catalogues usually quote one number for the assembled unit, and almost no barrier drawing specifies how it was measured.

Industry-typical nominal values for a standard-grade planetary in this size class sit at 15 arcmin or better for a single stage and 18–25 arcmin for a three-stage 100:1 unit. Twenty arcmin is a reasonable design figure. Precision grades reach 8–12 arcmin, at roughly 40–70 percent more gearbox cost, which is rarely justified for a flap that only needs to return to a mechanical stop.

Why three stages do not triple the play

The intuitive assumption is that three stages accumulate three times the backlash. They do not, and the reason is worth knowing because it explains why a 100:1 unit can still hold 20 arcmin. Play from an upstream stage is divided by the ratio of every stage downstream of it before it reaches the output.

So the output stage dominates, and the practical consequence is that backlash is bought or lost in the last reduction stage. It also means that quoting “15 arcmin per stage” as a total is wrong by a factor of nearly three, in the conservative direction, and that a supplier who does quote it that way is probably repeating a catalogue line rather than measuring.

Converting arcmin into something a mechanism designer can use

Twenty arcmin is 0.333 degrees, or 5.82 mrad. Multiply by the arm radius to get linear play at the flap tip, which is the number that matters for a gap or a pinch point.

The reversal delay is small: at 180 degrees per second of output speed, 0.333 degrees of play is consumed in 1.85 ms. Backlash therefore almost never shows up as a visible timing error in a working gate. It shows up as a click on direction reversal, as a flap that sits a fraction of a degree off its stop after a few thousand cycles, and as a slowly widening gap between two flaps that meet in the middle.

If any of those three symptoms is unacceptable, the specification has to say so explicitly: backlash measured at the output shaft, unloaded, in both directions, at 20 degrees C, after run-in, not exceeding 20 arcmin. Without the measurement conditions, two suppliers can both claim compliance while delivering units that feel completely different on the bench.

Self-Locking Is Not a Gearbox Property

The single most expensive misconception in barrier drive selection is that a high ratio implies self-locking. It does not. Whether a drive holds position without power depends on the reverse efficiency of the whole chain: gearbox, motor detent, seal friction and any brake.

The relation between forward and reverse efficiency for a gear train is η' ≈ 2 − 1/η. At 90 percent forward efficiency, reverse efficiency is about 89 percent. At 50 percent, reverse efficiency reaches zero and the train becomes self-locking. A planetary stage in this size class sits nowhere near that threshold, so a planetary gearmotor is back-drivable by design, at every ratio.

What actually resists a hand push

If the gearbox does not lock, the only resistance is the sum of motor detent torque and friction, reflected through the ratio. The formula is T_hold ≈ (T_detent + T_friction) × i × η'. For a 60 mm eight-pole motor the combined figure is typically 0.01–0.03 N·m, which at 100:1 and 89 percent reverse efficiency reflects to 0.9–2.7 N·m at the output.

Compare that with what a person can apply. Ten newtons of hand force on a 500 mm arm produces 5 N·m, roughly twice the upper end of the holding range. The flap opens. For an escape route that is exactly the behaviour required; for a paid-access lane where the flap must not be forced, it is exactly the behaviour that must be prevented, and no gearbox choice will prevent it.

That distinction drives the next decision, because a drive that cannot hold itself has to be held by something else: a spring, a brake, or a deliberate mechanical lock released by the controller.

Power-Off Behaviour: Three Architectures

Once the drive is known to be back-drivable, power-off behaviour becomes a system architecture question rather than a component question. Three arrangements cover almost every barrier on the market, and they fail in different directions.

The trap in the third row is that a self-locking drive looks like the safest option and is often the least verifiable one, because its emergency behaviour depends on a consumable. The trap in the second row is a brake that is spring-applied and power-released, which is the correct choice for a parking brake and exactly the wrong one for an escape route: it locks the flap precisely when power is lost.

Field experience points the same way. Most reported failures in this product class trace back to the release path and the controller rather than to gear teeth, which is the pattern set out in common turnstile motor failures.

Where the standard enters

Electrically controlled exit systems on escape routes are governed by EN 13637, and the machinery safety chain around the controller by ISO 13849-1 for the performance level of the safety-related parts. Both documents set requirements on the release function, not on the gearmotor, and neither can be satisfied by a data sheet claim. The relevant force and timing figures must be taken from the current edition of the standard and from the project’s own risk assessment.

What a gearmotor supplier can legitimately state is narrower and more useful: the drive is back-drivable, the holding torque reflected to the output is in a stated range, and the unit can be supplied with or without a power-off-released brake. Whether that satisfies an escape route is a question for the barrier integrator and the notified body. Brake-equipped door and gate drives are a separate design with their own trade-offs in holding current and release time.

Torque Reserve and the Thermal Ceiling

The 3 A figure on the nameplate is a current limit, not a working point, and the loss budget shows why. Copper loss scales with the square of current, and at 3 A the same 1.79 Ω line-to-line resistance dissipates 1.5 × 9 × 1.79 = 24.2 W, against 15.2 W at the rated point. Going from 2.38 A to 3 A raises copper loss by 59 percent for 26 percent more current.

Put those losses against a thermal resistance of roughly 3.5–4.5 K/W for a totally enclosed 60 mm unit and the picture is unambiguous. At the rated point the winding runs perhaps 85–110 K above ambient. At 3 A the rise approaches 130–165 K, which at a 40 degrees C ambient puts the winding well past the 130 degrees C limit that Class B insulation is rated for under IEC 60034-1.

What that means for anti-crush current limiting

Gate controllers usually limit current to protect a person caught by the flap. That limit is often set from the torque needed to detect an obstruction, with no reference to the thermal budget. The two constraints have to be satisfied together: a limit high enough to move a sticky flap on a cold morning, low enough that a jam held for several seconds does not damage the winding.

A workable rule is to set the electronic current limit just above the worst-case running current, and to add a separate short-time allowance with a timer, rather than relying on the drive’s thermal mass to absorb an unlimited stall. The motor’s own thermal time constant for this frame size is in the range of one to two minutes, so a five-second jam is survivable and a five-minute one is not.

What Ten Million Cycles Actually Means

Barrier datasheets in this market routinely claim ten million cycles MCBF. That is a system-level mean cycle between failures for the finished barrier, including flaps, hinges, sensors and controller, and it is measured under a defined load and travel angle. It is not a gearmotor bearing life figure, and it is not transferable between mechanisms.

A 90 degree move at 30 rpm takes 0.5 s of steady travel, plus acceleration and deceleration, so about 0.6 s per move. Each passage is an opening and a closing, which makes 1.2 s of mechanism time. Change the stroke angle or the output speed and the whole duty picture shifts, which is the trade documented in flap barrier motor speed and torque.

At realistic throughput the ten million figure is not the limiting factor; the grease service interval and the output stage tooth surface are. Above roughly 4,000 passages a day the constraint stops being mechanical life at all and becomes thermal duty, because the drive spends a larger fraction of the day energised and never returns to ambient.

This is also where a barrier differs from a turnstile drive: a turnstile rotates through a full passage while a flap only sweeps a quarter turn and reverses, so the flap drive accumulates far more direction reversals per million cycles, and every reversal loads the same tooth flanks in the opposite direction. The mechanical families themselves are compared in types of turnstile systems.

The 60 mm Envelope and What It Replaces

An integrated unit puts the motor, the planetary stages and the output bearing in one housing. That is a packaging decision with mechanical consequences that show up at installation, and the dimensions are the place to check them.

Compared with a separate motor, planetary gearbox and coupling, the integrated unit removes two coupling interfaces and their alignment tolerance, and shortens the stack by roughly 30–40 mm at this power level. On a barrier that translates directly into a narrower post or a deeper service cavity, which is usually the reason the integrated format gets designed in at all.

Two catalogue variants of the same layout are worth checking before the mechanism is frozen: the drive specified for a flap barrier and the one for a swing gate turnstile differ mainly in output torque and holding arrangement, not in the planetary stage count.

What the integration costs

The trade is serviceability. A failed bearing or a stripped planet set means replacing the whole unit rather than one component, and the ratio is fixed at order time, so a mechanism that later needs a different opening speed needs a different drive. The shared housing also means motor heat and gearbox heat see the same external surface, which is why the thermal budget in the previous section has to be evaluated for the assembly rather than for the motor alone.

The 49.5 mm hole spacing is close to, but not the same as, a NEMA 23 pattern. Treating the unit as a drop-in replacement for an existing 57 mm motor plus gearbox is the most common installation error in retrofit projects, and it usually surfaces as an elongated mounting plate and a re-machined arm hub.

Six Specification Mistakes

These are the six errors that appear repeatedly when a barrier drive is specified from a catalogue rather than from a calculation. Each one has a numerical signature, which makes them easy to catch on review.

1. Reading the nameplate watts as continuous output. 40 W at 30 rpm implies 12.7 N·m against a rated 9.3 N·m, a 37 percent overload that will not appear until the drive trips on temperature.
2. Treating the maximum current as a working current. At 3 A the copper loss is 24.2 W against 15.2 W at rated, and the winding rise exceeds the Class B limit within minutes.
3. Assuming a planetary is self-locking. At 90 percent forward efficiency the reverse efficiency is about 89 percent, so the flap can be pushed open with roughly 10 N on a 500 mm arm.
4. Quoting system MCBF as gearbox life. Ten million cycles is a barrier-level figure measured under a defined load; the drive’s real limits are grease life and output-stage tooth surface.
5. Never asking for backlash. Two suppliers can both ship a 100:1 unit and differ by 15 arcmin at the output, which is 0.35 mm at an 80 mm arm radius and visible as a gap.
6. Sizing on motor torque instead of output torque. A 0.10 N·m motor looks trivial to source until the 100:1 and the 90 percent efficiency are applied, and the output lands at 9.3 N·m rather than 10.4 N·m.

Mistake four is the one worth repeating to a purchasing team, because it is the only one that cannot be checked from a data sheet. Cycle ratings are quoted by the barrier maker, warranted by the barrier maker, and measured on a complete mechanism with a specified flap mass and travel angle. None of that transfers to a bare gearmotor.

Checklist Before You Ask for a Quote

A short list, ordered so that the first four items eliminate most wrong drives before price is even discussed. Items five to ten are the ones that turn a sample into a production order without a redesign.

Item seven deserves one sentence of explanation. A drive specified at 24 V nominal and then operated from a 21 V battery-backed supply loses speed in proportion to voltage, so a 90 degree move that takes 0.5 s at 24 V takes about 0.57 s at 21 V. If the opening time is a contractual figure, that margin has to be in the specification from the start rather than discovered during commissioning.

Everything above concerns the drive. The mechanism, the arm geometry and the controller still decide whether the installation works, which is why the ratio is best treated as a starting constraint rather than a specification on its own. If the mechanism rotates rather than swings, start from the turnstile motor selection process instead. For the matching mechanical side, the planetary stage layout and the planetary gearmotor range are the useful references.

One worked example of the whole chain, with the mounting envelope and the measured curve, is written up as a speed gate gearmotor application case.

References

1. International Electrotechnical Commission. (2022). IEC 60034-1:2022 — Rotating electrical machines, Part 1: Rating and performance. https://webstore.iec.ch/publication/69785
2. International Electrotechnical Commission. (2022). IEC 60034-30-1:2022 — Efficiency classes of line operated AC motors (IE code). https://webstore.iec.ch/publication/69790
3. International Organization for Standardization. (n.d.). ISO 13849-1 — Safety of machinery: Safety-related parts of control systems, Part 1. https://www.iso.org/standard/69883.html
4. European Parliament and Council. (2023). Regulation (EU) 2023/1230 on machinery. https://eur-lex.europa.eu/eli/reg/2023/1230/oj
5. American Gear Manufacturers Association. (n.d.). AGMA standards for gear rating and accuracy. https://www.agma.org/
6. SKF Group. (n.d.). Rolling bearings: friction, clearance and rating life. https://www.skf.com/us/products/rolling-bearings
7. maxon motor. (n.d.). Design-in help and white papers on motor and gearhead selection. https://www.maxon.com/en-us/design-in-help/white-papers.html
8. FAULHABER. (n.d.). Brushless DC drive technology and application notes. https://www.faulhaber.com/en/
9. Siemens AG. (n.d.). SIMOTICS low-voltage motors and drive engineering data. https://www.siemens.com/global/en/products/drives/motors.html
10. U.S. Department of Energy. (n.d.). Motor systems and energy efficiency resources. https://www.energy.gov/eere/amo/motor-systems

FAQ

Q: Is a 100:1 planetary gearmotor self-locking?
A: No. A planetary stage in this size class runs at roughly 88–92 percent efficiency, and any train above 50 percent efficiency can be back-driven. Self-locking appears when the combined motor detent, friction and gearing push reverse efficiency below about 50 percent, which in practice means a worm stage.

Q: What backlash should I specify for a speed gate gearbox?
A: For a standard-grade three-stage planetary, 18–25 arcmin measured at the output shaft is the usual band, and 20 arcmin is a reasonable design figure. A precision grade at 8–12 arcmin typically adds 40–70 percent to the gearbox price. Specify the measurement conditions as well as the number.

Q: Why does the nameplate say 40 W when 9.3 N·m at 30 rpm is only 29.2 W?
A: They are two operating points. 9.3 N·m at 30 rpm is the rated point and delivers 29–30 W. The 40 W figure corresponds to the 3 A current limit, where the curve extrapolates to about 12.9 N·m at 28.6 rpm, or roughly 39 W.

Q: Can the gate be opened by hand when power fails?
A: With a planetary at about 90 percent efficiency the drive is back-drivable, and the holding torque reflected to the output usually lands in the 1–3 N·m range, so 10 N on a 500 mm arm already exceeds it. Whether that satisfies the project depends on the escape route standard and the arm geometry, and it has to be verified on the assembled unit.

Q: What does a 10 million cycle rating actually cover?
A: Published MCBF figures are system-level for the finished barrier, not for the gearmotor, and they assume a defined load, travel angle and maintenance interval. At 2,000 passages per day, ten million cycles is about 13.7 years, which is beyond the grease service life of most small planetary units.

Q: Does the 60 mm frame fit an existing barrier mechanism?
A: The frame is 60 mm square with a 49.5 × 49.5 mm four-hole pattern, an 18 mm shaft with a 6 × 25 mm key, a 34 mm shaft height and 130.5 mm overall length. That is close to a NEMA 23 footprint but not identical, so the mounting plate and arm hub both need checking before treating it as a drop-in replacement.