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Hydraulic Cylinder Applications in Aerial Work Platforms

The raising or lowering of the booms, beams or jibs of aerial work platforms is performed by hydraulic cylinders that are controlled by directional control valves and powered by a hydraulic power unit consisting of a tank, a hydraulic pump and safety relief valves. Aerial work platforms use double acting, hydraulic cylinders in order to obtain the up and down movement of the platform’s lift components. It should be noted that not all designs of aerial work platforms use hydraulic cylinders as the primary means of platform component movement. For example, the scissors type of aerial work platform can use a hydraulic, pneumatic or mechanical power system to drive a lead-screw or a rack and pinion assembly to raise or lower the platform. But, hydraulically powered, aerial work platforms have three benefits over other types of systems since the hydraulic system can provide acceleration adjustments, control the speed of the platform or boom, as well as provide supplementary braking, dampening or cushioning. Some aerial work platforms use custom designed, telescoping, double acting hydraulic cylinders, which are multistage cylinders that extend the length of the cylinder’s stroke.

Double Acting Hydraulic Cylinders

All hydraulic cylinders consist of a cylindrical barrel, end caps, a piston, seals, and a rod or plunger. The two main types of hydraulic cylinders are: single acting and double acting. While single acting, hydraulic cylinders are simple in design and inexpensive to manufacture, they only provide actuation in one direction and therefore not the ideal choice for aerial work platforms, which require a bi-directional (retraction / extension) capability. Double acting, hydraulic cylinders provide this bi-directional, push and pull movement.  Double acting cylinders are constructed with hydraulic pressure ports on both ends of the cylinder’s barrel so fluid can be directed to either port in order to force the piston to move in either direction. Some double acting cylinders have built in safety holding check valves, flow control valves, internal bypass valves or counterbalance valves. These built in components are packaged into the cylinder via a cartridge assembly. All cylinders have seals to prevent hydraulic fluid leakage between the piston rod and the cylinder chamber.

Double acting hydraulic cylinders have two types of rod end configurations: single and double. Single rod end configurations are bi-directional but have greater force during extension than retraction due to the difference in piston area from end to end. Double rod end configurations have equal force in both directions. Double acting cylinders are welded or threaded to the aerial work platform’s lift components with a variety of mounting styles including, flanges, pin & trunnion, lugs and, the most common, a clevis with ball bearing arrangement, which provides free play against misalignment between the cylinder and the load. End caps are attached to the cylinder body with tie rods or welds; the welded type is the most common for aerial work platforms. To ensure a hard and protective surface, the pistons are commonly plated with chromium. For corrosion resistance, the cylinder barrels are coated with zinc, and the piston rods and rings are coated with a ceramic material. For mobile applications, double acting cylinders can be manufactured with bores up to 500 mm and strokes up to 20 meters.