SELECTION GUIDE
Rotary Actuator Types Compared: Helical, Rack & Pinion and Vane
Hydraulic rotary actuators turn oil pressure into a controlled swing through a fixed angle — not continuous rotation. Three designs dominate: the helical sliding-spline, the rack-and-pinion and the vane. They differ in how they convert motion, how much torque they fit into a given space, the angles they reach, and how they behave under heavy, dirty, shock-loaded duty.
Last reviewed: October 2026
The three main designs
Helical sliding-spline
How it works
Linear piston travel is turned into shaft rotation by mating multi-start helical splines on the piston and output shaft.
Strengths
Highest torque for its size; compact, single-piece housing; high stiffness; integral mounting faces.
Angle: Commonly 90° / 180° / up to about 360° per stroke; angle set by the spline design.
Typical use: Heavy, space-constrained swing and positioning on mining, construction and utility equipment — the Helac L/T/H-style form factor.
Rack & pinion
How it works
One or two pistons drive a linear rack that turns a pinion gear on the output shaft.
Strengths
Proven and serviceable; predictable torque; wide size range; easier to find large-angle builds.
Angle: Often 90° / 180° / 220° / 360°; rotation set by rack travel.
Typical use: General industrial rotation, valve and chute positioning, and larger envelopes where compactness is not the main constraint.
Vane
How it works
Pressure acts on a rotating vane inside a circular chamber, moving the shaft directly.
Strengths
Simple, compact at small sizes, smooth and fast cycling, low backlash at light loads.
Angle: Fixed rotation stops, commonly up to about 270°–280° depending on the vane count.
Typical use: Light-duty, high-cycle automation, fixtures and material handling at moderate pressure.
Why the helical sliding-spline fits heavy equipment
Inside a helical actuator, the piston and output shaft carry mating multi-start helical splines. When oil moves the piston axially, the angled splines force the shaft to rotate. Because the “gear” is wrapped around the piston rather than placed beside it, a short, stout cylinder produces high torque — there is no external pinion, no long rack housing and no gear mesh to take up backlash.
That torque density is why the form factor is common on booms, jibs, grabs, clamps and drill positioning where the mounting envelope is tight and the load is dynamic. The shaft is supported within the housing and the mounting flanges are integral, which keeps the load path short and rigid.
What this means on the machine
- • High torque from a compact, often flange- or foot-mounted body.
- • Fixed, repeatable angles (commonly 180°/360°) with positive stops.
- • Stiff under side and shock loads typical of mining and construction.
- • Counterbalance-valve builds can hold an over-running load in position.
- • Replacement must match mounting, shaft, angle and valve — not just torque.
Side-by-side comparison
| Criterion | Helical sliding-spline | Rack & pinion | Vane |
|---|---|---|---|
| Torque density (torque vs size) | Highest | High | Low–moderate |
| Typical rotation angles | 90°–360° | 90°–360° | Up to ~270° |
| Stiffness / shock-load tolerance | High | High | Moderate |
| Compactness on the machine | Best | Moderate (longer housing) | Good at small sizes |
| Serviceability | Seal/bearing overhaul possible | Gear & seal service possible | Vane/seal service, wear-sensitive |
| Typical duty | Heavy mining / construction swing | General industrial rotation | Light, high-cycle automation |
How to choose for the application
Required angle and motion
Fixed swing under roughly 360° points to a limited-angle actuator; unlimited rotation points to a hydraulic motor. See our actuator-vs-motor guide.
Available envelope
If the mounting space is tight and torque is high, the helical design usually wins. A longer rack housing is acceptable in many industrial frames.
Load and duty cycle
Heavy shock loads and contamination favour helical or rack-and-pinion; clean, fast, light cycles suit a vane.
Holding requirement
Over-running or suspended loads may need a counterbalance-valve build, not just a bigger actuator — confirm the valve option on the original tag.
Replacement vs new design
When replacing an existing unit, match the exact model code and interface. When designing fresh, size from the load curve, then check the mounting and angle.
Comparing a rotary actuator with continuous-rotation options? Read Rotary Actuator vs Hydraulic Motor vs Slew Drive.
ENGINEERING CHECK
Not sure which type is fitted?
Send the model code, a tag photo and the machine it is fitted to. We identify the type and confirm the replacement build — mounting, rotation, shaft and valve option — before you order.
Or email directly: hm@hmhydraulics.com
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Questions
Frequently asked questions
Which rotary actuator type gives the most torque in the smallest space?+
The helical sliding-spline design generally delivers the highest torque for a given envelope. Its piston and shaft carry multiple-start helical splines, so a short axial stroke produces high output torque without external gears.
What is the main difference between a helical and a rack-and-pinion actuator?+
A rack-and-pinion unit converts linear piston motion through a gear on the output shaft and often uses two opposing pistons. A helical unit uses mating helical splines on the piston and shaft, which is more compact for the same torque but more precise to manufacture.
Are vane actuators still used?+
Yes, for lighter, lower-pressure and high-cycle duties such as automation, fixtures and small positioning tasks. For heavy, shock-loaded mining and construction loads, helical or rack-and-pinion designs are usually preferred for their torque and robustness.
Can a rotary actuator rotate continuously?+
Most rack-and-pinion and vane units are built for fixed angles such as 90°, 180° or 360°, and helical units are typically limited to about 180°–360° per stroke. If you need unlimited continuous rotation, a hydraulic motor (or a slew drive for slow heavy swing) is usually the correct choice rather than a limited-angle actuator.
Does a higher torque rating alone make two actuators interchangeable?+
No. Mounting interface, shaft configuration, rotation angle, port position, valve option and the load path all have to match the original unit. A similar torque number by itself does not confirm a direct replacement; the tag and mounting drawing should always be checked.
This guide describes general rotary-actuator operating principles for selection purposes. Actual performance, rotation angles and interfaces vary by manufacturer and model and are confirmed against the unit nameplate, the mounting drawing and the application. 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.