The Rotary Actuator That Slammed at the Ends
We picked a pneumatic rotary actuator (vane type) for a flip station: 180° rotation, 50 N·m load. On the bench, it rotated fine. On the machine, it slammed hard at both ends — the vane hit the end stop at full speed. The cycle time was 0.5 seconds, and the actuator reached full speed in the middle. At the ends, it had no cushioning. The internal stops were taking the full impact. After a week, the end seals leaked. The mistake: we sized the actuator for torque but didn’t check the internal cushioning. We switched to a larger actuator with adjustable shock absorbers (or external hydraulic shock absorbers). The flip was soft. The mistake was ignoring the kinetic energy at the ends.
Pneumatic rotary actuator selection isn’t just the torque. The rotation angle, the energy at the ends (kinetic), and the cushioning determine whether it lasts. This article covers the choices.
Types of Rotary Actuators
Vane Type
A rotary vane inside a chamber. Air pushes the vane, which rotates the shaft. Simple, compact. Angles up to 100° (single vane) or 180° (double vane). Torque up to about 200 N·m. Fast. But the vane seals wear (especially at high speed).
Best for: Small angles (90–180°), light loads, compact spaces.
Rack-and-Pinion
A linear cylinder drives a rack, which turns a pinion (the output shaft). Any angle (up to 360° or more). Higher torque. But longer (the rack takes space). Standard for most applications.
Best for: Larger angles, higher torque, general automation.
Helical (Spline) Type
A piston with a helical groove rotates as it moves axially. High torque, compact. But expensive. For heavy-duty.
| Type | Angle | Torque | Best For |
|---|---|---|---|
| Vane (single) | 0–100° | 1–50 N·m | Compact, small angle |
| Vane (double) | 0–180° | 5–200 N·m | 180° flip stations |
| Rack-and-pinion | 0–360°+ | 10–500 N·m | General, adjustable angle |
| Helical | 90–180° | 50–1000 N·m | Heavy duty, compact |
Step 1: Required Torque
The actuator must deliver enough torque to rotate the load.
T = I × α + T_friction + T_external
- I × α: The torque to accelerate the inertia (rotational). I is the load’s moment of inertia (kg·m²), α is the angular acceleration (rad/s²).
- T_friction: Bearing friction (small, 1–5 N·m).
- T_external: Any process torque (clamping, resisting).
For a 1 kg plate at 0.2 m radius (I = m × r² = 1 × 0.04 = 0.04 kg·m²), rotating 90° (1.57 rad) in 0.2 s (α ≈ 78 rad/s²): T_accel = 0.04 × 78 = 3.1 N·m. Add friction (2 N·m) and safety factor (2×): T_required = (3.1 + 2) × 2 = 10.2 N·m. Pick an actuator rated for at least 10 N·m (at 6 bar).
Step 2: Check the Kinetic Energy at the Ends
The actuator rotates fast. At the end of travel, it hits the stop. The kinetic energy must be absorbed by cushioning (or the actuator breaks).
E = 0.5 × I × ω²
Where I is the total inertia (load + actuator) and ω is the angular velocity at the end (rad/s).
For our example: I = 0.04 kg·m², ω at end = about 10 rad/s (90° in 0.2 s). E = 0.5 × 0.04 × 10² = 2 J. The actuator’s internal cushioning can absorb up to about 1 J. We’re over. Add external shock absorbers (which absorb 5–20 J each).
This is why the actuator slammed — the kinetic energy (2 J) exceeded the internal cushioning capacity (1 J). The end stop took the impact.
The rotary actuator rule: Size the torque for acceleration (with 2× safety). Then check the kinetic energy at the ends: E = 0.5 I ω². If E exceeds the internal cushioning, add external shock absorbers. The actuator that slammed at the ends was sized for torque but the kinetic energy was too high for the internal stops.
Step 3: Angle and Adjustability
The actuator’s angle must match the application. Some actuators are fixed (90° or 180°). Others are adjustable (0–180°). For a flip station, 180° is standard. For a clamping arm, 90° is typical.
Adjustable-angle actuators let you set the exact angle with a screw. If the angle might change (different products), use adjustable. If fixed, use a fixed-angle actuator (cheaper).
Cushioning Options
Internal Cushioning
Most rotary actuators have adjustable pneumatic cushioning at both ends (needle valves that restrict the exhaust near the end of travel). The air is compressed as a pneumatic spring, slowing the rotation. For light loads and low speeds, this works.
External Shock Absorbers
Hydraulic shock absorbers (the same kind used on linear axes) are mounted at the end positions. The actuator hits the shock absorber, which decelerates it smoothly. For heavy loads or high speeds, use these. They absorb more energy than internal cushioning.
Flow Control (Speed Control)
Flow control valves (mufflers with adjustable needle) on the exhaust ports control the speed. Slower rotation = less kinetic energy = less impact. Turn down the speed if the actuator is slamming.
Rotation Speed
The actuator’s speed depends on the air flow (see article 67, valve Cv). A big actuator with a small valve rotates slowly. For fast cycles, size the valve Cv for the required speed.
Typical rotation times: 90° in 0.1–0.5 seconds (vane), 180° in 0.3–1.0 seconds (rack-and-pinion). Adjust with flow control valves.
Mounting and Accessory
The rotary actuator mounts to the machine frame. The output shaft has a flange or through-hole for the load.
- Flange mount: The actuator bolts face-down to the machine. Standard.
- Foot mount: The actuator sits on two feet. For side loading.
- Output: A keyed shaft, a hollow bore, or a flange. Match the load’s mounting.
A Rotary Actuator Checklist
- What angle? (90°, 180°, adjustable?)
- What is the load inertia? (kg·m²)
- What is the rotation time? (Seconds.)
- Required torque: T = I × α × safety factor.
- Does the actuator deliver this torque? (At 6 bar.)
- Kinetic energy at ends: E = 0.5 I ω².
- Is E within internal cushioning? (Add shock absorbers if not.)
- Is the valve Cv sized for the speed? (Article 67.)
- Flow control valves on the exhaust? (Speed adjustment.)
- Vane, rack-pinion, or helical? (Torque and space.)
- How does the load mount? (Flange, shaft, bore?)
- How does the actuator mount to the frame? (Flange, foot?)
The Bottom Line
Pneumatic rotary actuator selection is torque plus kinetic energy. The actuator that slammed at the ends had enough torque but the kinetic energy at the ends exceeded the internal cushioning. Calculate E = 0.5 I ω², add external shock absorbers if needed, and use flow control valves to slow the rotation. Pick a vane for compact small angles, rack-and-pinion for general use. The flip station that rotates softly and lasts wasn’t the biggest actuator — it was sized for both torque and end energy.
