The Vertical Axis That Dropped on Power Loss
We used a servo motor (without a holding brake) to drive a vertical Z-axis. The axis moved up and down. On the bench, it held position when powered. When we killed the power (e-stop, or a power failure), the axis dropped — gravity pulled the load down. The motor’s rotor back-driven (it spun freely). The load crashed. The problem: the motor had no holding brake. A vertical axis needs a brake (spring-engaged, released by the motor’s power) that holds the load when power is off. We switched to a servo motor with an integrated holding brake. When power cuts, the brake engages (spring-applied). The axis holds. The mistake was using a motor without a brake on a vertical axis.
Servo motor holding brake selection is for vertical axes and load-holding. Without it, gravity wins on power loss. This article covers the rules.
What a Holding Brake Does
A holding brake (spring-engaged, power-released) is integrated into the servo motor. When the motor has power, the brake is released (the motor runs freely). When power cuts (or the e-stop fires), the spring applies the brake. The motor shaft locks. The load (on a vertical axis) doesn’t drop.
The brake holds the static load. It doesn’t stop a moving axis (use a controlled stop first, then the brake holds). It’s a safety/holding device, not a dynamic brake.
Step 1: When You Need a Brake
You need a holding brake when:
- Vertical axis (Z-axis): Gravity pulls the load down. On power loss, the load drops. The brake holds it.
- Cantilever (horizontal overhang): The load hangs off the axis (not perfectly horizontal). The load slowly drifts.
- Unbalanced load: The load isn’t balanced (no counterweight). The brake holds it.
You don’t need a brake when:
- Horizontal axis with no back-drive: The ball screw (or gearbox) self-locks (or the load doesn’t move when power is off).
- Balanced axis: A counterweight (or spring) balances the load.
- Conveyor (horizontal, light): The load doesn’t drift when stopped.
The brake rule: Any vertical (Z) axis needs a holding brake. The axis that dropped had no brake. A horizontal axis with a self-locking screw doesn’t need one. When in doubt (a cantilever, an unbalanced load), add the brake.
Step 2: Brake Torque
The brake’s holding torque must exceed the load torque (with safety factor). The load torque on a vertical axis (ball screw) is:
T_load = m × g × lead / (2π × η)
Where m is the load mass (kg), g is gravity (9.8), lead is the ball screw lead (m), and η is the ball screw efficiency (0.9).
For m = 20 kg, lead = 5 mm (0.005 m), η = 0.9: T_load = 20 × 9.8 × 0.005 / (2π × 0.9) = 0.98 / 5.65 = 0.17 N·m. Small. With gearbox (10:1), the reflected torque is T_load / ratio = 0.017 N·m on the motor. The motor’s brake (typically 0.5–1 N·m for a 400 W motor) holds it easily.
For a heavy load (100 kg, lead = 20 mm): T_load = 100 × 9.8 × 0.02 / (2π × 0.9) = 19.6 / 5.65 = 3.5 N·m. The brake must hold at least 3.5 N·m (with 1.5× safety = 5 N·m). Pick a motor with a 5 N·m brake.
Step 3: Brake Release (Control)
The brake is released when the motor has power. The servo drive controls the brake: it releases the brake just before the motor energizes (so the axis moves), and applies the brake when the motor stops (after the axis is at rest).
Don’t release the brake before the motor torque is on (the load drops for a split second). The sequence: motor torque on → brake release → move. Stop → brake apply → motor off.
Step 4: Brake Life
The brake is a holding device (static). It’s not designed for frequent dynamic stops (stopping a moving axis). If you use the brake to stop the axis (dynamic braking), it wears fast. Use the motor to stop the axis (controlled deceleration), then the brake holds.
For a vertical axis that cycles up/down, the brake engages/disengages every cycle. It’s designed for this (spring-applied, long life). But don’t use it as a dynamic brake.
| Application | Brake Needed? |
|---|---|
| Vertical Z-axis | Yes (gravity load) |
| Horizontal axis, self-locking screw | No (load doesn’t drift) |
| Horizontal axis, back-drivable | Maybe (if load drifts) |
| Cantilever / overhang | Yes (holds the overhang) |
| Balanced axis (counterweight) | Usually no (but add one for safety) |
A Holding Brake Checklist
- Is the axis vertical? (Gravity load?)
- Is the load unbalanced? (No counterweight?)
- What is the load torque? (m × g × lead / 2πη?)
- Is the brake torque rated for the load? (With 1.5× margin?)
- Does the drive control the brake sequence? (Motor torque before release?)
- Is the brake used for holding (not dynamic stopping?)
- Is the brake voltage correct? (24 V DC?)
- What happens on e-stop? (Brake applies?)
- Is the brake audible? (A click when it engages?)
- Is there a brake fault detection? (Brake didn’t release?)
The Bottom Line
Servo motor holding brake selection is for vertical and unbalanced loads. The axis that dropped on power loss had no brake. Use a spring-applied, power-released brake on any vertical Z-axis. Size the brake torque for the load (m × g × lead / 2πη, with margin). The drive controls the sequence (motor torque before brake release). The axis that holds position on power loss wasn’t the most powerful motor — it had a brake.
