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Hydraulic Pump Applications for Aerial Work Platforms

Aerial work platforms (AEPs) use gear and piston pumps for the hydraulic system drive to power the hydraulic cylinders or actuators that raise or lower the platform. Designed specifically for mobile, hydraulic applications, the current generation of gear and piston pumps used on aerial work platforms feature quiet operation, greater efficiency, reduced weight, high power density, and reversibility capabilities.

Gear Pumps

Gear pumps are used in aerial work platforms because they are simple in design and inexpensive to operate. There are two types of gears pumps: internal and external. The external rotary gear pump is most commonly used in aerial work platforms. Gears pumps are a type of positive displacement pump where the meshing and un-meshing of the pump’s driver and driven gear assemblies create a vacuum causing a pressure drop that pulls hydraulic fluid from the suction to the discharge side of the pump. External gear pumps can bear maximum operating pressures in the mid-pressure range of 3000 to 3500 psi. Gear pumps typically have medium flow rates and are self-priming. They are less efficient than piston pumps; however, they are more tolerant of contaminants. Gear pumps are inexpensive with long service lives. The current designs have more gear teeth than in previous pump designs in order to minimize the flow ripple for noise and vibration reductions. Generally, gear pumps do not catastrophically fail because, as the gears wear down, the pump’s efficiency decreases. As a result, gear pumps are replaced before they seize mechanically.

Piston Pumps

Piston pumps are specified for the hydraulic systems in aerial work platforms because of their compact design, low noise, high power density, low maintenance costs and greater fuel savings due to their reduced weight. They are most commonly applied in moderate flow, high-pressure applications up to 6000 psi. There are two categories of piston pumps: axial and radial. The axial piston pump is more commonly applied in mobile hydraulic applications such as aerial work platforms. In general, an axial piston pump is a fixed or variable, positive displacement pump that uses pistons to push hydraulic fluid from the inlet to the discharge section of the pump. The piston pump is constructed with a piston array, swashplate, shoe assembly, valve plate and cylinder block.  The cylinder block is driven by an electric motor, which in turn rotates the swashplate that the pistons via the shoe assembly ride upon. As the cylinder block rotates, the pistons reciprocate in an up and down movement. The upward movement of the piston causes a reduction in pressure thereby forcing the suction valve to open, which causes hydraulic fluid to flow into the pump. The downward movement of the piston causes the reverse to occur; pump pressure increases, the suction valve closes and the discharge valve opens. Designed with fewer parts, piston pumps for mobile, aerial work platforms are lighter in weight due to fewer parts, with the net effect of better fuel efficiency. For cold climate conditions, some mobile piston pumps have a built in cold start valve that reduces torque at start up by using a directional control valve to control outlet pressure. To adapt to changing load requirements, piston pumps have load sending and pressure compensating controls. The load-sensing feature ensures pump flow rates match load requirements, while pressure compensation modulates flow rates for variable load demands.