Actuator system eases Volvo Truck steering systems
A
Parker Hannifin - Parker Sales UK product story
Edited by the Engineeringtalk editorial team
Sep 11, 2003
A novel hydraulic-pneumatic master/slave actuator system is at the heart of the a new hydraulic steering system developed by Danish company, OA Opbyg.
A novel Parker Hannifin hydraulic-pneumatic master/slave actuator system is at the heart of the a new hydraulic steering system developed by Danish company, OA Opbyg, to provide Volvo Truck fleet operators and owners with reduced operating costs and faster journey times.
The new Parker hydraulic-pneumatic master/slave actuator system has been developed against a background of ever more sophisticated braking, steering and suspension systems, coupled with stringent safety legislation, higher loads and faster road speeds.
Parker Hannifin designers worked closely with engineers from OA Opbyg to develop the new Volvo steering system.
Control of the steerable axle involves three linked systems: an electronic control system; pneumatic intermediate link and safety lock; and the hydraulic system.
The electronic control determines whether the steerable axle is to be activated or locked based on the specific signals received.
The pneumatic system functions both as an intermediate link between the electronic system and the hydraulic system, and as a safety lock for the steerable axle.
When the steerable axle is not activated, the axle is locked in its middle position by a pneumatic cylinder located on the axle.
The closed circuit hydraulic system moves the steerable axle and is not connected to the truck's other hydraulic systems.
When the conditions for axle steering are met, a relay governs the flow of control air to the inlet on the hydraulic valve.
The control air functions as a switch, turning the valve on and off.
When axle steering is deactivated, the pneumatic system ensures that the axle is locked in the middle position.
A twin piston, 100mm bore, 2 x 60mm stroke pneumatic cylinder designed and assembled by Parker Denmark engineers only for this Volvo application is connected to the hydraulic slave cylinder located on the axle.
One piston is directly connected to the slave cylinder's piston and the other is floating.
To lock the axle, compressed air is fed to both ends of the pneumatic cylinder.
At the same time, the space between the two pistons is vented through an air filter.
This causes both pistons in the pneumatic cylinder to centre and to lock in the middle position, thereby locking the axle.
The force to move the axle is supplied by the closed circuit hydraulic system which includes hydraulic cylinders designed and constructed at Parker's Geringswalde manufacturing facility in Germany.
The 50mm bore, 240mm stroke hydraulic master cylinder is connected to the truck's cab via a pitman arm.
Movement of the steering wheel is therefore converted into flow of hydraulic oil.
When the steering is activated, oil flows from the master cylinder through the hydraulic valves, hoses and pipes to the 63mm bore, 120mm stroke slave cylinder located on the back axle.
The consequent movement in the cylinder piston moves the wheels.
When the steering is deactivated, the oil flow is "short-circuited" in the hydraulic valve so that oil flow from the master cylinder is not directed to the slave cylinder.
Normal working pressure in the closed system is 25bar.
If the pressure falls below around 8bar, axle steering is deactivated and is locked in the middle position.
The Parker cylinders feature clean external profiles, tough painted or plated surfaces, and have no unprotected threads.
The result is a corrosion free system, even though components may be mounted in exposed positions.
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