Two-Axis Hydraulic Rotary Actuator Solutions for OEM Applications

1. What is a two-axis hydraulic rotary actuator?

A two-axis arrangement provides rotary motion about two distinct axes. Whether the two axes can be controlled independently depends on the hydraulic circuit and the control system. Typically one axis carries the primary motion — swing, boom rotation or arm indexing — and the second carries a secondary motion such as tilt, wrist rotation or jib movement.

The important point for specification is that the two axes are not one larger actuator. Each has its own required torque, rotation angle, speed and load path, and each is subject to a different moment. A unit that is adequate on the primary axis can still be wrong on the secondary one, and the reverse. That is why two-axis selection starts with two separate requirement sets rather than a single torque figure.

Two-axis rotary motion can be achieved through a purpose-designed integrated assembly or a combination of separate single-axis actuators. The mechanical architecture varies by application and manufacturer, so the useful question for a machine project is not which family is better in general, but which one the mounting envelope, load paths and service access will actually allow.

2. Can two single-axis rotary actuators be combined?

Combining two single-axis actuators is technically possible in some applications, but feasibility depends on the actuator’s allowable external loads, mounting interfaces, structural support and available space.

Where two actuators are used, they are arranged around two distinct rotational axes and connected through an engineered mounting structure. The axes may be orthogonal, or set at whatever angle the machine kinematics require; the layout is defined by the application rather than by a standard assembly, and it has to be drawn for the specific machine.

Such a layout changes the load path in ways that are easy to underestimate. The second actuator and its attached components introduce additional forces and moments into the assembly. How these loads are distributed between the first actuator, its bearings, the connecting structure and any external supports depends on the mechanical design. Whether an actuator may carry them directly depends on its published allowable external loads: radial and axial load limits, tilting moment and mounting stiffness. Some rotary actuators are not intended to absorb a cantilevered second unit. Where necessary, external bearings or structural supports may be required to carry loads that should not be transmitted directly through the actuator.

A combined arrangement also adds interfaces: the connecting structure, an additional bolt pattern, and a second set of ports and hoses. Each interface adds tolerance stack-up and a place where alignment can drift. That is the cost of flexibility, and it is why combined solutions are assessed against the specific machine rather than selected from a table.

3. Can different actuator sizes be used for each axis?

In principle yes, and it is often the sensible answer, because the two axes rarely carry equal torque. A large swing axis with a small tilt axis is a normal requirement. Sizing each axis independently also keeps the envelope and mass down compared with over-sizing both to the heavier duty.

The trade-off is that different sizes may require an engineered mounting interface and careful coordination of hydraulic flow, pressure, control functions and hose routing. The two axes do not have to share a control concept — standard valves and conventional control elements can often be used — but flow and pressure have to be checked against each actuator’s own requirement. So mixed sizing is an engineering review against per-axis torque, angle and space requirements, not a catalogue selection, and the interface has to be drawn before the concept can be costed.

4. How are the actuators mounted, and how is bolt access maintained?

Mounting and bolt access can be decisive factors in whether a two-axis concept is practical. Even when torque and rotation requirements can be met, the arrangement must still allow proper assembly, fastening and maintenance. The items that repeatedly cause problems:

• Fastener entry direction and tool swing radius — there must be room for the socket and the wrench handle to rotate, not just for the bolt head to sit flush.
• Assembly-sequence access — a bolt that is reachable on a bare housing can become unreachable once the arm, hoses and guard are on. Access has to be checked in the order the machine is actually assembled.
• Access for the specified tightening method — the assembly process must provide sufficient access for the specified tightening method at the stage when each fastener is installed, which is not always the finished state.
• Blind holes and thread engagement — shallow bosses and blind threads limit usable engagement and increase the risk of pull-out under reversing load.
• Clearance through the full range of motion — hose runs, brackets and the second axis itself move. Clearance should be checked throughout the full range of motion, including intermediate positions and any simultaneous movement of both axes.
• Service access — replacing a seal or a hose should not require dismantling the whole arm.

A typical example, and one that is easy to miss when each actuator is reviewed separately: an actuator may have accessible shaft mounting bolts when installed on its own, but those bolts can become inaccessible once a second actuator and its connecting structure are assembled. This is not inevitable in every two-axis layout — it depends on the geometry and the assembly order — but it is the kind of constraint that decides the concept.

The working rule is to design for the wrench, not just for the bolt. Mounting and bolt accessibility must be verified by engineering against the real envelope — they cannot be confirmed from a general arrangement drawing alone, and no assembly sequence should be treated as proven until it has been checked that way.

5. What information is needed to evaluate a two-axis solution?

An assessment can only be as good as the requirement set behind it. For each axis, the minimum input is:

• required torque — continuous and peak, with the direction of the load moment;
• rotation angle, including any special or limited-swing requirement;
• speed and duty cycle, since these drive both sizing and thermal behaviour;
• space envelope and mass budget at the mounting point.

For the external loads acting on the assembly:

• radial and axial load at each output, where the load acts, and its distance from the rotation axis — this is what sets the overhung moment;
• dynamic and shock components, and whether the load reverses in service;
• external supports or bearings already present in the machine, and what they are intended to carry.

For the assembly as a whole:

• mounting interface — bolt pattern, pilot register, shaft or spline type;
• hydraulic supply pressure and flow, port type and position;
• control arrangement, sequencing between the two axes, and position feedback;
• environment — dust, water, temperature and duty severity for mining or outdoor work;
• if replacing an existing unit, a nameplate photo plus mounting-face, shaft and port photos.

How HM can support a two-axis evaluation

HM Hydraulics supplies and sources rotary actuators and supports the early stage of a two-axis requirement. Within that scope we can:

• review the initial requirements with you, axis by axis — torque, angle, envelope and mounting interface;
• identify potential actuator configurations, whether a purpose-designed integrated assembly or a combination of single-axis units, that suit the machine layout;
• flag mounting, external-load and bolt-access risks early, before a concept is costed;
• coordinate further feasibility assessment with our engineering and manufacturing partners where a project moves beyond the initial stage.

What we do not do is treat a two-axis requirement as a catalogue item. There is no pre-verified off-the-shelf answer to be quoted from a code: mounting, bolt accessibility and the interface between the two axes have to be verified for the machine in question.

The three stages are distinct. An initial technical discussion and requirement review can start now. A feasibility assessment beyond that is carried out with our engineering and manufacturing partners and is confirmed per project. A complete design, validation and delivery programme for a new two-axis assembly is not a standard offering; where a project needs it, the scope is defined and agreed separately.