Case Study:
Mechanical/Hydraulic Model Used to Increase Loading Capacity of Hydraulic Scissor Lift

Scissor lifts enable access to elevated work areas by raising a gantry with a scissor mechanism. The ascent and descent are controlled with hydraulic cylinders.

Scissor lifts are widely used in the construction industry and, accordingly, safety is critical in their design, manufacture, and operation.

The loading capacity of an existing scissor lift needs to be increased. To do this, elements of the lift, such as its hydraulic motor, are replaced with larger or more powerful components. In addition to increasing the capacity of the lift, the engineer must ensure that safe operating conditions are maintained for a given set of components. To do this, the engineer needs to determine the rates of ascent and descent and the forces and pressures generated in the hydraulic cylinders. Without computer simulation, this information cannot be obtained easily.



An engineer uses MapleSim to build a model of the scissor lift. The engineer uses

• Multibody components to model the mechanical scissor mechanism;
• Hydraulic components to represent the hydraulic circuit;
• Signal-flow and logic components to control the pumping and release of the hydraulic fluid from the hydraulic cylinders during the ascent and descent phases.

The plunger cylinder converts hydraulic flow into translational motion, and lifts a scissor mechanism and the associated gantry. At the top of the lift, the system holds for 10 seconds (with a check valve preventing backflow). A let-down valve then opens, allowing the scissor mechanism to retract back down (with the descent throttled by the restriction of the hydraulic circuit). When fully retracted to the bottom, the scissor mechanism rests on a back-stop.

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