Innovation in lift axle control technology
04 December 2013
While lift axles have been available for commercial vehicles for many years, these systems have involved the mounting and plumbing of numerous components with many potential leak paths and failure points. John Morris describes a system design that integrates many of the key valve components into a single module.
Lift axles allow operators to change the number of axles in use on a commercial vehicle depending on the load being carried, ensuring compliance with axle loading legislation while also offering benefits in the areas of fuel economy and manoeuvrability when the axle is not needed and lifted away from the road surface.
When carrying a zero or very light load, for example, lowering the number of axles in service reduces the effects of ‘friction drag’ while also minimising tyre wear and making the vehicle easier to manoeuvre. With a full load, though, additional axles can be deployed as needed, stabilising the vehicle by ensuring even distribution of the weight of the load across the vehicle’s axles and chassis.
Traditional lift axle systems have been controlled by two pairs of rubber bellows working together to either lower or raise the axle. Of the two bellow pairs, one is pressurised and the other vented depending on whether or not the particular axle to which they are fitted is required to be in use.
Typically systems are controlled through a pair of three-way, pilot-operated brake relay valves each controlling one of the bellow pairs. Brake relay valves offer high flow and are proven in the demanding commercial vehicle environment. Industrial spool valves have also been used but have suffered from durability and freezing issues.
In most common configurations, the brake relay valve pairing includes an inversion valve to enable inversion of one of the relay signals, to allow one of the bellow pairs to fill while the other is being vented. The system is also fitted with a control of some kind, either a three-way air valve or an electrical switch operating a three-way solenoid valve
Brake relay valves will generally deliver air at the same pressure as the pilot pressure being applied. Often, however, this level of pressure may be inappropriate to the specific axle loading requirements, so an adjustable regulator is then fitted to the pilot ‘leg’ of the load bellows which allows the pressure in the load bellows to be adjusted depending on the load being carried. This is key to the performance of the system as an excessive load on the axle has the potential to signficantly compromise braking and vehicle control safety.
The disadvantage of this system set-up is the mounting and plumbing of numerous components with many potential leak paths and failure points. One obvious solution was to integrate the two brake relay valves and the inversion valve into a single module. These single module systems are now well-established in the marketplace.
How does it work?
Figure 1 shows a simplified representation of the basic design of the LACM. Two brake relay-like pistons are employed to control the load and lift bellows (or bags). Piston A controls the load bags while piston B controls the lift bags.
Piston A acts as a pilot-operated pressure regulator identical to common brake relay valves. Piston B acts as a pilot operated, normally open three-way valve. If pilot pressure is absent, both pistons are in their ‘up’ position as shown in Figure 1. The load bags are vented and the lift bags are pressurised – the axle is raised.
If pilot pressure is applied, both pistons move down as shown in figure 2. Piston A will move down, opening the supply poppet while simultaneously closing the exhaust path. The load bag pressure will rise until the pressure beneath the piston is balanced with the pilot pressure and will move up and down as needed to keep the load bag pressure the same as the pilot pressure.
Piston B will move all the way down and close the supply while simultaneously exhausting the lift bag pressure to zero – the axle is lowered and sharing the vehicle load. This layout eliminates any need for an inversion valve.
The LACM can be operated either pneumatically or electrically by the use of a three-way solenoid valve. The use of the solenoid valve allows the system to be tied into the vehicle’s control system with the ability to automatically deploy or lift the axle depending on vehicle speed and direction. Lift axles are often designed in ways that do not allow for operation in reverse.
By tying into the vehicle’s reversing light circuit, for example, the axle can be automatically raised when reverse gear is selected and then re-deployed when returning to a forward gear. Vehicle speed can also be a controlling factor. Often the auxiliary axle will need to be lifted at very low manoeuvring speeds to prevent tyre scrubbing. This can be done automatically by tying into the vehicle’s speed signal.
John Morris is with Norgren
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