SELECTION GUIDE
Rotary Actuator vs Hydraulic Motor vs Slew Drive
All three turn hydraulic power into rotation, but they answer different questions. A compact rotary actuator swings a load through a fixed angle and holds it there. A hydraulic motor spins continuously. A slew drive slowly rotates a large platform while carrying heavy overturning moment loads. Choosing correctly starts with the motion the machine actually needs — not with a torque number alone.
Last reviewed: October 2026
The three options at a glance
Rotary actuator
- Motion
- Fixed angle per stroke (commonly 90° / 180° / 360°)
- Torque
- High torque in a very compact, integrated housing
- Holding
- Holds over a fixed angle when oil is trapped; counterbalance-valve builds resist an over-running load
- Mounting
- Flange or foot mount, short rigid load path, drives the load directly
- Typical uses
- Boom and jib swing, clamps and grabs, drill-feed positioning, hatch and chute rotation
Hydraulic motor
- Motion
- Continuous, unlimited rotation
- Torque
- Torque depends on displacement; higher torque usually needs a gearbox
- Holding
- Needs an external brake or counterbalance/load-holding arrangement to hold when stopped
- Mounting
- Shaft drive to a gearbox, wheel, winch or driven component
- Typical uses
- Wheels and tracks, winches, fans, pumps, conveyors and continuous augers
Slew drive (slewing drive)
- Motion
- Slow high-torque rotation, sector or continuous, through a worm-gear slewing bearing
- Torque
- Very high output torque plus built-in resistance to axial, radial and tipping moments
- Holding
- Self-locking tendency on many worm designs; large-diameter bearing carries the structure directly
- Mounting
- Bolted between two large flanges / platforms
- Typical uses
- Upper-carriage swing, crane and aerial-platform rotation, heavy tables, solar tracking
Decision matrix
| Factor | Rotary actuator | Hydraulic motor | Slew drive |
|---|---|---|---|
| Motion required | A fixed, repeatable swing | Unlimited rotation | Slow rotation of a whole platform or structure |
| Dominant load | Torque in a tight envelope | Continuous torque / speed | High tipping (overturning) moment |
| Holding at rest | Built-in with a valve option | Separate brake usually needed | Large slewing bearing, often self-locking worm |
| Mounting | Compact flange / foot | Shaft to gearbox or wheel | Large-diameter flange-to-flange |
Why an actuator is not simply a slow motor
A compact rotary actuator is, mechanically, a hydraulic cylinder arranged to produce rotation: oil moves an internal element and the shaft turns through a defined angle until it reaches its stop. Trapping the oil locks that position, and an integral counterbalance valve can hold an over-running load. The angle, the holding force and the mounting faces are all built into one compact body.
To do the same job with a continuous motor you would normally add a gearbox for torque, a brake or valve for holding, and a stop or position control to repeat the same angle — then engineer a mounting frame to tie it all together. That is the right system for continuous duties, but it is a roundabout way to swing a boom through 180°.
Quick rule of thumb
- • Does the part stop at a fixed angle and hold? → rotary actuator
- • Does it spin without stopping? → hydraulic motor
- • Does a whole platform rotate slowly under heavy tipping load? → slew drive
HM Hydraulics supplies compact rotary actuators today. Our worm-gear slew drive range is in preparation — see the slew drives page and tell us your requirement; we are taking sourcing enquiries ahead of the launch.
ENGINEERING CHECK
Tell us the motion and the load
Replacing an existing unit or designing a new function? Send the model code or describe the required angle, torque, mounting and holding duty. We help confirm the right component and match the replacement build before you order.
Or email directly: hm@hmhydraulics.com
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Questions
Frequently asked questions
Can I replace a rotary actuator with a hydraulic motor?+
Not directly for a fixed-angle swing. A motor turns continuously, so it needs external gearing, a brake or counterbalance and a stop or position control to repeat a fixed angle. A limited-angle actuator already provides the angle, holding torque and mounting interface in one compact unit. The motion the machine requires should drive the choice.
When is a slew drive the right choice?+
A slewing drive suits slow, high-torque rotation of a platform or structure that also carries large axial, radial and tipping (overturning) moment loads — for example an upper-carriage swing, a crane or aerial platform rotation, or a heavy positioning table. It combines a slewing bearing with a worm-gear drive in one large-diameter assembly.
Does a rotary actuator hold position without a brake?+
A limited-angle actuator is essentially a cylinder, so when oil is trapped and a load-holding valve is fitted it resists back-driving over a fixed angle. A hydraulic motor generally needs a separate brake or counterbalance arrangement to hold a load when stopped.
Which gives continuous 360° rotation?+
A hydraulic motor rotates continuously, and most slewing drives can also rotate continuously. A compact rotary actuator is built for a fixed angle per stroke (commonly 90°, 180° or 360°), not unlimited turns. If the function must spin without a stop, an actuator is usually the wrong component.
How do I pick for a new machine design?+
Start from the motion: a fixed swing favours an actuator; unlimited rotation favours a motor; a slow platform rotation under high tipping moment favours a slew drive. Then check required torque, envelope, mounting interface, holding need and duty cycle, and confirm the selection against the load calculation.
This is a general selection guide based on typical component characteristics. Final component choice and sizing depend on the application load calculation, required safety factors, mounting interface and the machine service manual. Helac® and all OEM names are trademarks of their respective owners; HM Hydraulics is an independent manufacturer and is not affiliated with or endorsed by them.