Door Opener Gear Motor Selection: Torque, Speed & Holding Without a Worm Gear

Almost every article about door opener gear motors repeats the same claim: use a worm gearbox, because worm drives are self-locking. That advice is correct in principle and misleading in practice, for two reasons. First, self-locking in a worm drive depends entirely on the lead angle and the friction state — high-efficiency worm boxes with a small lead angle are না reliably self-locking under vibration, and many door operators use exactly those. Second, if you are sizing a compact right-angle AC gear motor with a spur or bevel final stage, the gearbox is না self-locking at all, and any selection process that assumes it will produce a door that creeps, drops or can be pushed open.

This guide works through door opener selection the way it should be done: three numbers for the drive, then a separate decision about how the door is held when the power goes off.

What a door opener actually asks from a gear motor

A bare motor spins too fast and produces too little torque to move a door panel directly. The gearbox does two jobs at once: it reduces output speed to roughly 10–120 r/min where door mechanisms work, and it multiplies torque by roughly the ratio. On top of that, door openers add four requirements that a conveyor drive does not have:

• High breakaway torque. The hardest moment in every cycle is the first movement — weather seals, gaskets, track friction and accumulated dirt all resist at once.
• Frequent starts and reversals. A door operator is an intermittent-duty machine that starts, stops and reverses dozens to hundreds of times a day.
• Holding position on power loss. The door must not creep, drop or be pushed open.
• Quiet, compact running. Operators live in occupied buildings and often mount inside a slim header or jamb.

Only the third item concerns the gear train, and there is more than one way to solve it.

Step 1: Calculate the torque the door needs

Torque selection starts with the resistance the drive must overcome, not with the motor nameplate. For a linear drive (rack-and-pinion or belt), the required output torque is:

T_{বাইরে} = F × r ÷ η_drive

where F is the total door resistance force in newtons (seal drag, track friction, weight distribution, wind), r is the pinion or pulley radius in metres, and η_drive is transmission efficiency between gearbox and door — about 0.9 for a clean rack or belt drive, 0.7–0.8 for a worn or fouled track.

For a swing door driven at the hinge, the torque is a moment rather than a force:

T_hinge = m · g · d · cosθ + T_closer

with m the leaf mass, d the horizontal distance from hinge to centre of mass, θ the opening angle and T_closer the spring torque added by the door closer. The closer spring is the term most often forgotten — on a standard overhead closer it can add 30–50% to the required drive torque through the middle of the travel.

Reference figures for indoor doors: running resistance of a sliding panel with new seals is roughly 50–100 N. Sizing on that running value is a mistake, because peak load occurs at breakaway. Apply a service factor of 1.5–2.0 to the breakaway figure.

Step 2: Set output speed, then pick the ratio

Door opening time is specified, not optional. A 1.2 m wide sliding door that must open in 1.5 s at a pulley radius of 40 mm needs a belt speed of 0.8 m/s, which gives 191 r/min at the pulley — then divide by the pulley-to-gearbox ratio. In practice, sliding operators land at 100–250 r/min at the gearbox output; swing and gate operators at 10–60 r/min because the hinge ratio is much larger.

Once you have output speed, read allowable load at that ratio rather than inferring torque from motor power. On the 25 W 4GN right-angle series (80 mm frame), the real numbers are:

For a light indoor swing door or a small gate, 25 W at 50:1 gives 6.18 N·m at 30 r/min, which covers a broad class of pedestrian doors. Heavier or wider leaves move to a 40–90 W unit; vehicle barriers and large industrial gates belong to the 120–300 W range.

Step 3: Decide how the door is held — the part everyone gets wrong

There are four legitimate ways to hold a door, and only one of them is the gearbox.

Option A: A self-locking worm gearbox

Worm drives become self-locking when the lead angle is below the friction angle — roughly below 5°, which in practice means ratios above about 40:1 with a single-start worm. Two caveats matter:

• Self-locking is a static property. Vibration from traffic, wind buffeting or a slammed neighbouring door can walk a nominally self-locking worm backwards over time.
• Efficiency drops sharply. A self-locking worm box may run at 40–60% efficiency, against 90%+ for a spur or bevel right-angle stage. That heat has to go somewhere.

If you use a worm box for holding, treat self-locking as a secondary safety and still provide a mechanical stop or brake for the primary hold.

Option B: A brake motor

A spring-applied, power-released brake holds the shaft when the supply drops. It is definitive and independent of gear geometry. Cost adder is modest, and it works with any gear type — spur, helical or bevel. This is the normal specification for any vertical or inclined axis, and for gates that must not be pushed open.

Option C: Mechanical stop or detent

For end-of-travel holding — a closed door resting against its jamb, a barrier arm sitting on its stop — the structure holds the load, not the drive. This is free and completely reliable, and it is what most sliding door operators actually rely on. The drive only has to reach the stop, not hold position mid-travel.

Option D: Counterbalance

On overhead and sectional doors, springs carry the leaf weight so the drive only overcomes friction and imbalance. A correctly balanced door with a non-self-locking gear train will sit still because there is no net load. Verify the balance, and provide a brake or stop as a backstop for spring failure.

Step 4: Duty cycle, power and environment

Door operators are intermittent-duty machines, which usually suits a reversible 30-minute-rated AC gear motor well — a busy shop entrance at 60 cycles per hour still leaves the motor unpowered most of the time. Two cautions:

• High-cycle installations (airport, retail mall, hospital entrance at 300+ cycles/hour) need the cycle count checked against the motor’s thermal rating, not just its torque. Reversing current is higher than running current and heats the winding faster than the duty table suggests.
• Outdoor gates need the enclosure and the shaft seal checked specifically. Water enters at the shaft, not through the housing.

Power selection follows the frame, not the other way round: a 25 W unit on an 80 mm frame is single-phase 110V/220V or three-phase 220V; 400 W and above on the medium frames are three-phase 220V/380V. If the site only has single-phase supply, that constrains you to the small frames and therefore to door sizes the small frames can move.

Selection checklist

• Compute breakaway torque, including the door closer spring on swing doors.
• Apply a 1.5–2.0 service factor to breakaway, not to running load.
• Derive output speed from the required opening time and mechanism ratio.
• Read allowable load at that ratio from the catalogue, not motor nameplate torque.
• Choose the holding method explicitly: brake, stop, counterbalance, or a genuinely self-locking worm.
• Check cycle rate against the thermal rating, not the torque rating.
• Confirm supply phase before choosing the frame size.

প্রায়শই জিজ্ঞাসিত প্রশ্নাবলী

Is a right-angle gear motor self-locking?

Not if the final stage is a bevel or spur gear. Self-locking requires a worm gear with a lead angle below the friction angle. A compact right-angle bevel unit will back-drive under load.

What ratio do I need for a sliding door?

Most sliding operators want 100–250 r/min at the gearbox output. With a 4GN 25 W unit, that is a 3:1 to 10:1 ratio — but at 3:1 the allowable load is only 0.45 N·m, so verify the door resistance before choosing the shallowest ratio.

Can a 25 W gear motor move a swing door?

Yes for light indoor pedestrian leaves where the hinge ratio is favourable, typically with 30:1 to 50:1 giving 4–6 N·m at 30–50 r/min. Heavier leaves, wind-exposed locations and doors with strong closers need 40–90 W.

How do I stop the door being forced open?

A friction brake is not enough. Use a mechanical latch, a brake with sufficient holding torque, or a self-locking worm stage, and specify the holding torque against the forcing load rather than the door weight.