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Hydraulic Valve Applications of Aerial Work Platforms

Aerial Work Platforms (AEPs) are hydraulically controlled machines used for raising or lowering a platform, supported by a scissors, jib or boom, that holds work personnel while they perform construction, repair or maintenance activities. The high power density, force multiplication, characteristic of hydraulic control systems is leveraged in aerial work platforms and gives them the capability of transferring a large amount of hydraulic power with relatively compact-sized hydraulic components.

AEPs have a variety of designs that include vehicle-mounted elevating work platforms, manually propelled elevating work platforms, boom-supported elevating work platforms, self-propelled elevating work platforms, among others. Insulated aerial work platforms are a special type of AEP used for high voltage electrical maintenance work and constructed with an insulated boom that meets electrical safety standards (CSA Standard CAN/CSA-C225-M88 Vehicle Mounted Aerial Devices).

While aerial work platforms utilize mechanical, pneumatic and electrical systems to control the machine and manipulate the movement of the boom, mast, jib or scissors, hydraulic systems are the primary means of control. The hydraulic systems of AEPs use a variety of hydraulic valves for safety and control purposes.

Safety Valves

The hydraulic safety valves of aerial work platforms are primarily used to operate the machine in a safe condition in the event a hydraulic system failure occurs. Emergency lowering or “brake” valves are manually operated valves used in conjunction with hand pumps in the event of a hydraulic pump or power failure when the platform is raised or boom is extended.  The lowering valve allows an operator to manipulate it in order to return the platform to the ground safely. Holding check valves are one-way, failsafe valves that prevent the loss of hydraulic pressure when the platform is raised.  Counterbalance valves are used for safety purposes in instances where a hydraulic hose or fitting leaks or ruptures causing the loss of hydraulic fluid. Some types of counterbalance valves are a specific type of check valve that can be pilot controlled and functions as a type of flow control valve. 

Relief safety valves monitor hydraulic pressure in lines and system components. They are preset to a set-point pressure and when this level is exceeded, the valve opens and fluid is channeled to an auxiliary system to lower the system pressure. AEPs usually have several relief safety valves. A main relief valve monitors hydraulic pressure from the pump and reservoir. Other relief valves are distributed between the major components and ties into the platform control station.

Control Valves

Proportional or directional hydraulic control valves are used to control the speed and direction of the boom, mast or scissors platform. They perform this task by controlling the amount of hydraulic fluid that flows to and from the hydraulic cylinders. Control valves are usually constructed with a spool type, load-sensing design. The spool slides to divert fluid flow either to the hydraulic actuator or back to the supply reservoir. A derivation of proportional control valves is the flow diverter, which is a compact, multi-circuit, valve-module. Flow diverters can control several hydraulic circuits simultaneously, which simplifies the hydraulic system design.