• 1. Why hydraulic lift tables compress and rebound
• 2. To what extent does a scissor lift table compress and rebound?
• 3. How can the deflection of a scissor lift table be limited?
• 4. What solutions are available, if a mechanical end stop is not feasible?
  • 4.1 Option 1: Mechanical solution for limiting deflection
  • 4.2 Option 2: Electronic solution for position correction

One phenomenon of hydraulic scissor lift tables is the compression and rebound during load transfer. But can the spring deflection be limited?
In this article, we explore the reasons behind compression and rebound and present various ways to reduce deflection.

The compression and rebound of hydraulic lift tables are caused by pressure changes within the hydraulic system. This process can be best explained with an example.

Imagine an empty scissor lift table being raised to its upper holding position, where the hydraulic system maintains a pressure of 100 bar. When the lift table is loaded, the system pressure increases to 140 bar. This pressure rise causes the hydraulic cylinder to retract slightly, making the entire lift table sink a bit — this is known as compression.

The opposite effect, known as rebound, occurs when the load is removed. As the hydraulic system pressure decreases, the cylinder extends slightly, causing the lift table platform to lift slightly.

The exact height of the compression and rebound travel of a hydraulic scissor lift table depends on a variety of factors, making it difficult to provide a general answer. However, some of the key influencing factors can be identified:

• Current position of the scissor mechanism
• Current operating pressure
• Height of the applied or removed load
• Hose lengths in the hydraulic system
• Diameter of the installed hoses
• Temperature and type of hydraulic oil used

For the lift tables we manufacture, we have developed formulas that allow us to approximately calculate the deflection. These calculations enable us to provide a rough estimate for your specific application.
Other lift table manufacturers typically have similar approximation formulas. Therefore, if needed, it is advisable to contact your manufacturer or contact person directly to obtain specific information.

Simply put, by adjusting the influencing factors mentioned in the previous section. In practice, the most common approach is to control the pressure difference within the hydraulic system to minimize compression and rebound.

A proven method for limiting deflection is the use of a mechanical end stop in the upper position. In this setup, the scissor lift table moves against the stop while the hydraulic power unit continues to run for a brief moment. This increases the pressure in the hydraulic system until the pre-set pressure of the pressure relief valve (PRV) is reached.
When a load is applied to the lift table in this position, the resulting pressure difference is minimal. As a result, the compression of the lift table is reduced. At the same time, the mechanical end stop prevents rebound.

In certain applications, such as vertical triple scissor lifts or simplified goods lifts with more than two stops, using a mechanical end stop is not possible. For these scenarios, alternative solutions must be employed to effectively limit the suspension travel.

With this solution, the pressure difference in the hydraulic system is completely eliminated. This is achieved by mechanically supporting the lift table system and releasing the operating pressure in the system.
The platform is either placed on folding supports or on hydraulic locking bolts. In this position, the hydraulic system no longer plays a role in the deflection process.

Another way to limit deflection is through an electronic position control system. With this solution, the platform is continuously monitored for deviations from its target position. If a deviation is detected, an automatic position correction is performed to maintain the desired lift position within a defined range.

The complexity of this control system largely depends on the required holding accuracy. 
If extreme precision at stopping points is not necessary, a simple feedback control system with basic sensors is sufficient.
For higher accuracy requirements, a combination of rope length sensors and proportional valves is needed.