In this article, we provide you with the hydraulic diagram for a simple hydraulic scissor lift table. Along with the electrical schematic and the manufacturing drawings of the lifting mechanism, this is one of the most important components for building a lift table yourself.
Note: Please read through this entire article before starting to replicate the hydraulic circuit. Throughout the article, we explain how the different operating states are implemented using the hydraulic circuit and what role each component plays in the system.
Structure of the lift table
The provided hydraulic diagram is designed for a simple hydraulic standard lift table. This lift table consists of the usual components: a base frame, a scissor and a platform.
Single-acting hydraulic cylinders are used. This means that the cylinders return to their original position by the weight of the platform and the lifted load.
For light loads or lightweight construction, the use of double-acting cylinders may be necessary to lower the platform. However, note that this configuration cannot be implemented with the 2/2-way valves described here.
Operating states of the lift table
Our goal is not only to provide you with the hydraulic diagram of a scissor lift table, but also to ensure that you fully understand what happens with the hydraulics in the various operating states and how a hydraulic lift table actually works.
To explain the functionality as simply as possible, we have included numbers next to the various components in the description text. These numbers correspond to the position numbers in the hydraulic diagram. Therefore, it makes sense to print out the hydraulic diagram and follow the process step by step alongside the description using the diagram.
Normal operation
In normal operation, our lift table is simply supposed to raise or lower a load. This task can be accomplished with relatively few components.
Lifting movement
For the platform of the lift table to rise, the hydraulic cylinders (Pos. 1) must extend by filling their chambers with oil. This is achieved by the hydraulic pump (Pos. 10), which is driven by an electric motor (Pos. 11).
During the lifting movement, all valves remain in their default position and do not need to be actuated. The electric motor (Pos. 11) is switched on, causing the hydraulic pump (Pos. 10) to generate the necessary flow rate. This flow is directed through the hose (Pos. 3) into the hydraulic cylinders (Pos. 1). Once the desired lifting height is reached, the electric motor is switched off, and the pump stops delivering oil. The check valve (Pos. 8) ensures that the hydraulic oil does not flow back into the tank, preventing the platform from unintentionally lowering.
Lowering movement
To lower the platform of the lift table, the hydraulic oil must flow back from the hydraulic cylinders (Pos. 1) into the tank. This is done by actuating the lowering valve (Pos. 6).
The lowering valve (Pos. 6) is closed when de-energized, so no oil can flow unless the valve is activated. This is indicated by the small check valve on the spring side of the corresponding symbol. Once the coil of the hydraulic valve is energized, the valve opens, and the oil can flow from the cylinders (Pos. 1) through the lowering valve (Pos. 6) into the tank, causing the platform of the lift table to lower.
To keep the control of the valves as simple as possible, no proportional valves are used. The installed valves only have two states: open or closed. Without additional measures, the lowering speed of the platform would vary greatly depending on ambient temperature and load. For this reason, a lowering brake valve (Pos. 5) is installed before the lowering valve (Pos. 6). This valve limits the flow rate during the lowering process and prevents the platform from lowering too quickly.
Faulty operating state
Although this hydraulic diagram is designed for a simple and robust lift table, malfunctions can still occur during operation. In practice, with such a simple lift table, two faults commonly occur: The hydraulic line of the lift table has ruptured or the control system of the lift table has failed (solutions for other possible malfunctions can be found in our article Common faults, their causes and possible solutions). Safety measures have been included in the hydraulic diagram for both scenarios, which are explained in more detail below.
Rupture or breakage of the hydraulic line
A rupture or breakage of the hydraulic line can occur for various reasons. For example, hydraulic hoses should be replaced after seven to eight years at the latest, as they become brittle over time and can no longer withstand operating pressure. Other common causes include incorrect installation or material defects in the components used.
If a hydraulic line ruptures or breaks, it must be ensured that the platform and load do not lower uncontrollably. This is ensured by the use of burst pipe protections (Pos. 2). These safety valves are installed directly on or inside the hydraulic cylinders (Pos. 1) and close the hydraulic line if there is an excessive pressure drop.
After such an incident, the damaged line must be repaired, and the lift table must be slightly raised to equalize the pressure difference on both sides of the burst pipe protection. Only then will the safety valve reopen.
However, it is important to note that a burst pipe protection does not detect all possible damages in the supply line. For example, a small leak does not create a sufficient pressure drop to activate the safety device. Therefore, additional safety valves may be required. You can find an overview in our article Safety devices on the lift table: Hydraulic valves.
Power failure or electrical fault
The lowering valve (No. 6) in the hydraulic diagram is actuated by an electric coil. A power failure or a fault in the control system could make it impossible to lower the load. An emergency release valve (No. 7) is installed parallel to the lowering valve (No. 6) to allow the lowering of the load in an emergency. This can be operated manually in case of damage and allows the hydraulic oil from the cylinders (No. 1) to flow back to the tank via the hose line (No. 3) and the lowering brake valve (No. 5).
Function of the installed components
In this section, we briefly explain the function of each component of the hydraulic circuit.
Pos. 01: Hydraulic cylinder
The hydraulic cylinders provide the lifting movement by pushing the scissor halves apart. When the cylinders retract, the platform lowers.
Pos. 02: Burst pipe protection
This safety valve closes the hydraulic line in the event of a sudden hose or line break, preventing the platform from lowering uncontrollably.
Pos. 03: Hydraulic hose
A flexible hose that connects the valve block to the hydraulic cylinders. Flexibility is necessary to compensate for the movement of the scissor.
Pos. 04: Test port
A test port should be included in the valve block for testing purposes. It is used to monitor pressure during the lifting process.
Pos. 05: Lowering brake valve
Ensures a smooth and load-independent lowering movement of the platform.
Pos. 06: Lowering valve
When activated, this valve allows the platform to lower.
Pos. 07: Emergency lowering
Enables manual lowering of the platform in the event of a power outage or control system failure.
Pos. 08: Check valve
Prevents hydraulic oil from flowing back into the tank and keeps the platform at the desired level.
Pos. 09: Pressure relief valve
Protects the system from overpressure by diverting the flow to the tank when the pressure in the circuit becomes too high.
Pos. 10: Hydraulic pump
Pumps hydraulic oil into the cylinders.
Pos. 11: Electric motor
Drives the hydraulic pump.
Pos. 12: Filter
Protects the hydraulic components from contamination and wear particles in the hydraulic oil.
Pos. 13: Filler neck
Used to fill the hydraulic tank.
Pos. 14: Oil drain
Allows for the draining of hydraulic oil, such as during an oil change.
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