Hydraulic scissor lifts are a typical example of the efficient use of hydraulic power. These lifting platforms are used extensively throughout industry as well as vehicle scissor lifts for car repairs. Fixed installed lifts are often used to lift goods between floor levels from vehicles into a building at the goods inwards areas.
Mobile scissor lifts are are normally small lightweight lifts that are used to push goods or heavy objects around a factory or workshop. Using 12V or 24VDC Hydraulic power packs these lifts must be re-charged just like a forklift.
Principle of operation
The top platform of the scissor lift remains horizontal as it raises from the lower to closed position. In this way the goods on the lift can safely be moved between levels.
Its called a scissor lift because between the base and upper platform is a cross braced mechanism similar to the arms of a pair of scissors. One end of the scissor arm is fixed to the base and one is fixed the platform, the others are free to roll.
A hydraulic cylinder is placed between the base and the platform or mechanism. As the hydraulic ram extends the scissors open and the lift raises.
Hydraulic Scissor lift components
Hydraulic Power Pack
- Generally scissor lifts have a simple single acting hydraulic circuit, except where they are very lightweight or they require fine position control.
- Starting the AC or DC motor runs the hydraulic pump and the lift raises by pumping hydraulic oil into the hydraulci ram.
- A hydraulic control valve is used to lower the lift, this is usually electrically operated but can be manual in simple lifts, where they can control the lowering speed.
- Due to the mechanical disadvantage of the scissor mechanism a much higher hydraulic pressure is required to lift the scissor lift from the lowered or flat position. This hydraulic pressure decreases as the lift raises.
- Smaller lifts use a single ram in the centre of the lift, larger rams use a pair of hydraulic cylinders. As scissor lifts often are very shallow in height when closed the rams are barely above horizontal when lowered, so they must be sized very carefully.
- Safety valves are essential as the use of flexible hydraulic hoses is unavoidable. Should these hydraulic hoses burst the lift would fall. To prevent this hydraulic hose burst valves are used.