The self-propelled scissor lift uses a hydraulic lifting system to raise and lower the working platform safely and smoothly. Compared with mechanical lifting methods, hydraulic systems provide high lifting force, stable motion, and precise control, which makes them ideal for aerial work platforms.
Understanding the working sequence of the hydraulic system is important for equipment operation, maintenance, and troubleshooting. The hydraulic system of a typical electric scissor lift platform mainly consists of a hydraulic power unit, hydraulic cylinder, directional control valves, hydraulic pipelines, and safety valves.
Below is the standard operating sequence of the hydraulic lifting system.
1. System Standby State
Before lifting begins, the hydraulic system is in a standby condition.
Key characteristics of this stage include:
The platform is in the lowered position.
The hydraulic pump motor is stopped.
Hydraulic oil remains in the oil tank (reservoir).
The solenoid directional valve is closed.
The hydraulic cylinder is fully retracted.
At this stage, the system pressure is minimal and the machine is ready for operation.
2. Start of Lifting Operation
When the operator presses the lift button on the control panel, the lifting cycle begins.
Sequence of actions:
The electrical control system activates the motor of the hydraulic power unit.
The hydraulic pump begins drawing oil from the oil tank.
Pressurized hydraulic oil is sent into the main pressure line.
The solenoid directional valve switches to the lifting position.
This allows hydraulic oil to flow into the bottom chamber of the lifting cylinder.
3. Cylinder Extension and Platform Rising
Once hydraulic oil enters the cylinder, the hydraulic cylinder begins to extend.
Mechanical sequence:
Oil pressure pushes the piston rod outward.
The piston rod applies force to the scissor arm assembly.
The scissor arms expand in a cross-linked motion.
The working platform rises vertically.
During this process:
The hydraulic pump continuously supplies pressurized oil.
A flow control valve regulates lifting speed.
A relief valve limits maximum system pressure to prevent overload.
This stage continues until the platform reaches the required working height.
4. Holding Position (Height Maintenance)
When the operator releases the lift control button, the platform stops rising and maintains its height.
The hydraulic system maintains position through the following mechanisms:
The solenoid directional valve returns to the neutral position.
A hydraulic check valve or load-holding valve closes.
Hydraulic oil is locked inside the cylinder chamber.
Because hydraulic oil is nearly incompressible, the platform remains stable at the selected height without continuous pump operation.
This feature significantly reduces energy consumption.
5. Platform Lowering Operation
When the operator presses the lower button, the lowering sequence begins.
Operational steps:
The solenoid lowering valve opens.
Hydraulic oil from the cylinder flows back to the oil tank.
The weight of the platform and scissor mechanism forces the cylinder to retract.
The scissor arms gradually fold inward.
The lowering speed is controlled by a throttle valve or flow control valve to ensure smooth and safe descent.
6. Platform Fully Lowered
When the platform reaches its lowest position:
The hydraulic cylinder is fully retracted.
Oil has returned to the hydraulic reservoir.
The lowering valve closes automatically.
The system returns to the standby state.
The scissor lift is then ready for the next operating cycle.
Important Hydraulic Safety Components
To ensure safe operation, several hydraulic protection devices are integrated into the system:
Relief Valve
Prevents excessive hydraulic pressure and protects components from damage.
Hydraulic Lock / Check Valve
Prevents sudden platform descent if a hydraulic hose fails.
Emergency Lowering Valve
Allows the platform to be lowered manually in case of power failure.
Flow Control Valve
Maintains stable lifting and lowering speeds.
These safety features are critical for maintaining stable and secure aerial operations.
Summary
The hydraulic system of a self-propelled scissor lift operates through a precise sequence of pressurizing oil, extending the cylinder, holding the load, and safely returning oil to the reservoir. This hydraulic principle allows the lift platform to achieve smooth vertical movement, strong lifting capacity, and reliable safety performance.
Because of these advantages, hydraulic scissor lift platforms have become one of the most widely used aerial work platforms in warehouses, factories, construction sites, and facility maintenance applications.
