Rotary actuators for positioning work tools

The rotary actuator or swing motor is a compact and complex work tool in hydraulics. It is typically used as an auxiliary device for attachment tools. With its help, attachments such as clamshell buckets, pneumatic hammers, or chisels can be positioned very precisely without having to move the boom.

Rotary actuators – compact yet complex

The greatest advantage of the rotary actuator, also called “swing drive” or “swing motor,” is its compact design.

Occasionally it is also called a “rotary motor.” However, this designation is actually misleading: the swing drive can only operate within a limited range, typically at most within an angle of 270°. A complete rotation is not possible with the rotary actuator.

As compact as its external dimensions and as simple as its operation are, its internal workings are complex. The rotary actuator consists of a housing with integrated gearing. The driven inner shaft also has gearing. The connection to the tool is made with a flanged intermediate piece that is driven from the center shaft via a conventional spline connection. On the top of the rotary actuator there are two connections for hydraulic pressure lines.

The heart of the rotary actuator is a bushing with internal and external threads. It seals the left and right halves of the swing drive from each other. When hydraulic pressure acts on one side of the rotary actuator, it pushes the bushing in the other direction. On its way to the opposite housing half, the bushing drives the inner shaft via the gearing. It rotates in one direction. This movement is usually additionally supported by a correspondingly large suction on the other side of the bushing. When the flow is reversed, the shaft rotates in the other direction. As soon as the bushing reaches one housing wall or the other, the swing movement ends.

Differences between rotary actuators and hydraulic rotary motors

The question naturally arises as to why a rotary actuator is useful when hydraulic radial or axial piston motors are also available. The answer lies in the very compact design of the rotary actuator and in its simpler and more cost-effective construction. Although the assembly of the swing motor is quite complex, it is still simpler than that of a powerful and precisely operating hydraulic motor. Work tools never need to be rotatable 360° transverse to the boom. Therefore, the swing motor is the ideal compromise between performance and technical complexity at this point on a work vehicle.

Alternative to the swing motor

One disadvantage of the swing motor is its limited force. When moving the inner bushing from one housing half to the other, much frictional energy is lost. The swing motor can therefore typically only move an empty or unloaded work tool.

However, if a swing movement is to be performed under load, then a deflection drive (ULM) is required. This is a separate, conventional linear actuator that can drive a work tool, e.g., a trenching bucket, via a deflection lever. Deflection drives are installed as standard on backhoes, for example.

Application of the swing motor

A swing motor is frequently required on demolition excavators. It can position the attached work tool very precisely at the desired point. This is very helpful when demolishing a building using pneumatic hammer attachments or steel shears. Demolition excavators in particular often lack the required precision due to the length of their booms. A swing motor can compensate for this disadvantage.