CED: Specify holding brakes where gravity can move a servo axis

Nicholas Rodacz puts two servo motors on camera, neither wired to a drive. He spins the shaft of the first with no resistance. The shaft of the second will not turn.

The difference is a spring-set holding brake. In a MarketScale Studio segment for the CED Industrial Solutions Network, Rodacz makes the case that the second motor is the one to specify for any axis that gravity, or a wound spring, can move on its own. The person who has to get that call right is the controls engineer signing the motor spec, and the person who lives with a wrong one is the maintenance lead who answers the phone after a power loss.

The question is landing on more spec sheets than it used to. In a June 2026 report on Hannover Messe 2026, held in Hannover, Germany, from April 20 to 24, IoT Analytics said its team of 12 visited more than 400 booths and conducted more than 300 interviews, and it ranked physical AI as the second of 12 trends reshaping industrial technology, with humanoid robots entering early industrial pilots. More robots on the floor means more servo-driven joints holding a payload against gravity.

Control Design, in a December 2023 episode of its Control Intelligence podcast, described servo motors as the actuator of choice for robot joints and end effectors in pick-and-place, welding and assembly, and for the retrieval systems in automated warehouses. Every one of those axes can stop with a load in the air.

A spring holds the shaft when the voltage does not

Rodacz defines the part before he defends it. A holding brake, he says, is an electromechanical mechanism that comes as an optional accessory for many types of servomotor.

The industry term for that behavior is power to release. Apply voltage and the spring lets go; remove it and the spring clamps. A spring, not software, is what holds a servo shaft once the power is gone.

That distinction matters because the fault a holding brake covers is the one no controller can respond to, since the controller has lost power too. For a procurement director, the word optional is the operative one. Rodacz says the brake comes as an option on many servo motor types, which means a default order can arrive without it, and the buyer has to ask.

That test costs nothing, and anyone in the plant can run it on a motor sitting on the shelf. If the shaft turns by hand with the drive disconnected, the motor has no brake.

Stored energy is the test, not orientation

Rodacz calls the target case a vertical load application, then widens it. The load usually is vertical, he says, but it does not have to be; what it must have is some form of potential energy, and he names gravity and spring tension.

He lists the places a holding brake turns up most often: cranes, elevators, gantries and the joints of robotic arms. Each one stores energy the motor has to hold back at standstill. If losing torque would let the load move, the motor needs a brake.

The motor makers describe the same load class from their side. Douglas Brandão, a product manager for electrical automation at Festo Brazil, wrote on the company's own site in August 2025 that AC servo motors are built for high loads and continuous cycles and are widely used in CNC machines, robotic arms and automated assembly lines. Those are the axes where a stopped payload has somewhere to fall.

For a plant manager, the practical version is a walk down the line with one question per axis: if this motor lost torque right now, would anything move? A hoist, a lift stage or a robot joint holding a part in the air answers yes. That axis goes on the brake list.

Gravity already sits inside the torque math

The sizing calculation engineers already run points at the same axes. Advanced Motion Controls, in a servo motor selection guide on its own site dated May 2025, sets continuous torque as the sum of external torque, gravity torque and friction torque, with gravity torque equal to the gravitational force multiplied by the lever arm.

That formula sizes the motor for the work while it is energized. It says nothing about who carries the gravity term when the drive drops out. Rodacz's brake is the part that carries it.

He adds that holding brakes are rated for a certain amount of torque and tells viewers to double check that the brake on their motor is strong enough for the application. The motor overcomes the gravity load while running; the brake holds it when nothing is running. Rodacz's torque warning and the gravity term in the sizing guide describe the same load from two directions.

The CFO who signs the purchase order will see the brake as an accessory line on a quote. The engineer should be able to point to the gravity term on the sizing sheet as the reason the line is there.

A parking brake, not a service brake

Rodacz spends the back half of the segment on what the brake will not do. It is not meant to decelerate the motor, he says, only to hold the axis in place once it has reached standstill, and he compares it to the parking brake in a car. The drive brings the axis to a stop; the brake keeps it there.

That one line moves the brake from a maintenance-budget question to a specification question. A holding brake is not field replaceable; fixing one means taking the motor apart. For the operations leader, the moment to decide is before the motor is ordered, not after it is bolted to the machine.

Tighter capital, more robot joints

The case for the accessory lands in a cautious spending environment. Deloitte's 2026 Manufacturing Industry Outlook, published in November 2025 by the Deloitte Research Center for Energy & Industrials, said the Institute for Supply Management's manufacturing purchasing managers' index stayed below 50 for much of 2025, signaling contraction. It said costs rose, employment fell and manufacturing construction spending, an indicator of investment in new or expanded facilities, declined steadily through the year.

Deloitte attributed much of that to trade policy uncertainty and tariffs. It noted that more than three-quarters of manufacturers responding to the National Association of Manufacturers' 2025 quarterly outlook surveys consistently named trade uncertainty their top concern. Every optional line on a motor quote gets a harder look in that climate.

That is the honest complication in Rodacz's argument: a brake is a cost added before anything has gone wrong. The interview answers with its own third point. Because the brake is not field replaceable, skipping it does not defer the cost; it turns a spec-time accessory into a later motor disassembly.

Deloitte, for its part, wrote that manufacturers should prepare for continuation, contraction or renewed growth in 2026. It pointed to tax provisions in the One Big Beautiful Bill Act, revised trade deals with the United Kingdom and Vietnam, and interest rate cuts as possible relief.

Deployment is moving regardless of the spending mood. IoT Analytics reported that Hannover Messe 2026 drew approximately 110,000 visitors and about 3,000 exhibitors, short of 2025 and below pre-COVID levels, and still found the halls full of senior executives from industrial hardware, software and service providers. Fewer people walked Hannover Messe 2026, and robotics still ranked second on IoT Analytics' trend list.

The shaft test before the purchase order

Rodacz leaves viewers three reminders: the brake holds, it does not slow; it carries a torque rating that has to cover the load; and it cannot be replaced without opening the motor. His demonstration turns the first into a check any operator can run on a motor before signing off a spec: disconnect the drive and try to turn the shaft.

If it turns, and the axis it will drive stores energy the way a crane, an elevator, a gantry or a robot joint does, the order needs the motor that will not. The next check is the brake's torque rating against the gravity term on the sizing sheet, made before the purchase order goes out, because after delivery the only way to change the answer is to take the motor apart.