Published in Crane and Hoist Canada- July/August 2019
Written by: Brady Haugo, Hydraulic Engineering Supervisor
Boom trucks commonly use aerial work platforms with booms that rotate 360 degrees. This allows the operators more freedom to move the work platform to the desired position. Most of the different styles of aerial work platforms have hydraulic and electrical requirements after the rotation joint. This means fluid power and/or electrical power and signal are needed to pass through the rotational joint.
The rotational joint is key to allowing the boom and operator 360-degree rotation without getting the hoses and wires tangled up. The rotational joint consists of the slewing bearing, which takes the brunt of the mechanical loads and hydraulic swivel and electrical slip ring that transfer the fluid power and electrical power and signal. The hydraulic swivel and electrical slip ring combined are referred to as a combination unit.
The operation of aerial lift platforms entails the operator controlling the position, direction, and speed of the aerial work platform from within the platform. Precise control is critical when working next to potentially dangerous situations. These controls use electric signals to control hydraulic valves that open and close—allowing movement of the work platform. The electrical slip ring and hydraulic swivel work in conjunction to not only get the operator’s input signal to the hydraulic control valve but also get the fluid power from the hydraulic pump and/or hydraulic control valve to the cylinders and motors that move the platform.
The hydraulic swivel and electrical slip ring (combination unit) sits at the centre of rotation inside the lift platform boom. The combination unit needs to sit at the centre of rotation to allow 360- degree rotation. Due to the relative size and weight of both units, the electrical slip ring attaches to the hydraulic swivel. The hydraulic swivel will have much higher torque values than the electrical slip ring. The hydraulic swivel is also much heavier. Because of this, the electrical slip ring attaches onto the hydraulic swivel and piggybacks the torque restraints of the swivel.
Aerial lift platforms are a light-duty application for both the swivel and slip ring. The environment lends itself to not needing additional protection from debris; the combination unit is usually well protected deep within the machine. The slip ring in this application typically has more than a few circuits. Within the slip ring, the circuits are stacked on top of each other, so the more circuits, the longer the unit is. The specific requirements of this application tend to favor one type of combination unit over the other.
UEA offers three different types of combination units:
- Fully integrated – The slip ring fits into the housing of the swivel. There’s no additional slip ring cover.
- Partially integrated – The slip ring fits directly on top of the swivel. There’s a separate slip ring cover and no mounting tube.
- Separated – The slip ring stands off the swivel with a mounting tube and flange. There’s a separate slip ring cover and base casting.
With the higher number of electrical circuits on an aerial lift platform, the application lends itself more commonly to the separated style of a combination unit. With the lack of debris issues and long slip ring, the separated style is the most economical choice.
The separated style uses a base casting that contains the bearings, cord grip threads, and cover mounting surface. The partially integrated version still requires the cover mounting surface and cord grip threads, but not the bearings. The core of the slip ring on the partially integrated version mounts to the spool of the swivel. The base casting and brush studs mount to the housing of the swivel.
The separated style combo unit is more economical, it’s sold in higher volumes, and its component prices are lower due to better economies of scale. We cannot say the same thing for other industries and applications.
The partially integrated combo would be the second most reasonable choice. The partially integrated style uses a spun aluminum cover. The brush harness that exits out of the slip ring needs a liquid tight cord grip. The spun aluminum cover doesn’t provide the necessary strength and rigidity for the cable grip. This requires us to use a partially modified base casting bolted to the hydraulic swivel housing.
UEA sells more of the separated styles of combination units than any other style. The fully integrated style wouldn’t be economically feasible for this application unless there were extreme debris or water intrusion requirements. Due to the higher number of circuits, covering the slip ring with the hydraulic swivel housing as in the integrated version would end up being more expensive than using a spun aluminum cover to cover the slip ring.
Using specific customer requirements to come up with a custom solution to the exact application at hand is our company’s approach to solving issues.
Learn more about our integrated slip ring and rotary union solutions here.