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Hydraulic Systems and Components of Aerial Work Platforms

Aerial work platforms (AEPs) are flexible, temporary and mobile, elevated platforms primarily used for maintenance and construction activities in inaccessible areas and are generally limited to payloads of 1 ton or less. This limited payload capacity differentiates them from cranes, which have much higher capacities. Aerial devices, cherry pickers, vertical mast lifts, boom lifts, scissors lifts, hotel lifts, telehandlers, stock pickers and personal portable lifts are all specific types of aerial work platforms based upon applications and design configurations. Scissors lifts are a unique type of AEP because they can only move in a vertical direction (up or down) due to the mechanical design of its lifting mechanism, which can be powered hydraulically. Hotel lifts are a small version of scissors lifts and have a much lower maximum payload rating. All aerial work platforms utilize hydraulic systems to power the lifting or boom mechanism and the turret mechanism for 360 degree slewing.  The mobile function of AEPs is generally performed by battery-pack that powers an electric motor with traction drive mechanism.

Hydraulic System

The hydraulic system of aerial work platforms consists of a hydraulic pump, motor, directional control valves, safety check (brake) & bleed down (relief) valves, hydraulic cylinders (actuators), non-burst hoses, fittings and gauges. The maximum hydraulic system pressure is about 2000 psi. In some AEPs, remotely located hydraulic power packs with diaphragm-type accumulators provide the hydraulic fluid power. Hydraulic fluid is channeled to the lift or slewing mechanism and cylinder via a directional control valve block.

Hydraulic Components: Pumps, Valves and Cylinders

The hydraulic pump of most aerial work platforms is driven by an electric motor, although some types use a diesel engine as a prime mover. The hydraulic fluid reservoir is controlled by an electro-mechanical solenoid with onboard relief valve, with a tank capacity in the range of a few gallons. Variable volume, pressure compensated, axial piston pumps are common for AEPs because of their compact design, high volumetric efficiency and reliability. AEP pumps are equipped with an overload relief valve.  Both the vertical and horizontal (azimuth/slewing) rotation of the platform is hydraulically powered. The vertical motion of the platform (boom) is accomplished by steel booms that are operated by double acting, hydraulic cylinders which have an manually-operated, bleed valve for emergency purposes that prevents free fall as well as allowing for the lowering of the platform in the event of a loss of hydraulic power. The cylinders have safety interlocks that will prevent uncontrolled movement of the boom in the event of a hose failure. The speed of the lift platform is controlled by the speed of the pump motor which increases or decreases fluid flow to the cylinders via directional, solenoid control valves, which are electronically controlled via the operator control panel. The horizontal motion of the aerial work platform is controlled by a slewing motor, which is typically a hydraulic orbit motor with worm gear transmission drive. Check valves are essentially free fall protection components that hold the platform in place for safety purposes. Hydraulic pressure gauges monitor system pressure.