PSA: Self-locking gearmotors aren’t brakes

Fractional-horsepower gearmotors must satisfy unique application requirements. In some cases, OEM design engineers specify gearmotors containing worm gearing to leverage the self-locking nature of this gear type. However, the locking characteristic of worm gears is often misunderstood and misapplied.

Some engineers think, “If I get a self-locking gearbox, when I cut power, the axis will come to a quick stop and will hold its position.” The problem is that no gear assembly is absolutely self‑locking. Rather, worm gears’ locking behavior is only exhibited from a static (stopped) state … and static self-locking isn’t the same as dynamic braking.

Note: In this context, we use the phrase dynamic braking to refer to an electromechanical design’s ability to actively slow or stop an axis … not the braking method that renders an electric motor a generator for kinetic-energy dissipation via resistors or regenerative means.

Basically, a worm gearmotor-driven axis’ inertia will keep it turning even after electrical input is removed, and the assembly only becomes self-locking once it’s already at a dead stop.

Some designs benefit from the combination of a self-locking gearbox with an actual brake. For example, certain stair lifts prioritizing efficiency need a brake to quickly slow and stop the assembly and then a self-locking gearbox to hold it in place sans power.

Rotisserie ovens for roasting must pause the turning of meat when a person opens the oven door so as to prevent burns, but gravity can keep spits turning if there’s any imbalance of the roasters’ weight — say, with two chickens on one side and none on the other. Here, a brake needs to bring the rotisserie spits to a full stop … and then the self-locking gearbox can hold them in place.

Other designs actually need the opposite — for axes to be fully backdrivable. Here, design engineers don’t usually ask for backdrivable gearing outright, but the need for low resistance to backdriving is requested in other ways.

Consider some applications: Railroad-crossing gates have very strict requirements for how easily gravity brings the arms down. In the event of a power failure, the gates must fall down and block vehicle traffic — because once there’s a power failure, no other signal will alert drivers of an oncoming train. This function relies on a gearbox that’s so easily backdriven that gravity alone on the arm’s weight brings it down into its failsafe position. In fact, the design of this seemingly simple solution was greatly complicated by how the arms are fitted with counterbalances to let a relatively small low-power motor lift the arm.

It’s the same for barrier-arm gates on parking garages. Either the gates must have electronics with overrides, or the electromechanical assembly (including the motor’s gearbox) must be backdrivable. That way, if something fails or somebody loses their ticket, an attendant can manually lift the gate.

Because it causes so much confusion, some manufacturers deemphasize references to “self‑locking” gearmotor behavior. However, the behavior can provide real benefit to some designs … especially where application engineers help guide OEMs to its proper use.

These insights were gained in a recent conversation with Terry Auchstetter of Bodine Electric Co. For more information, visit bodine-electric.com.