In this article from our series on the different types of lift tables and their advantages and disadvantages, we take a detailed look at the hydraulic lift table.
How a hydraulic lift table works
A hydraulic scissor lift table is powered by an electric motor. A hydraulic pump is directly mounted onto the motor, which transports the pressure medium into the lift cylinders. Since the cylinders are connected to the two scissor arms of the lift table, the platform and the load on it are raised by the extension of the cylinders.
The downward movement occurs by retracting the cylinders. In hydraulic lift tables, due to their often robust construction, the table's own weight is usually sufficient to lower the platform. However, in lighter constructions, it may be necessary to use double-acting hydraulic cylinders to ensure controlled downward movement.
Lower wear
The hydraulic drive of a lift table experiences less wear compared to other mechanical drive systems. This is due to the nature of the power transmission: Instead of mechanical friction, the force is transmitted through pressure buildup in the cylinders.
The hydraulic fluid used also contributes to the lubrication of mechanical components, which reduces friction between moving parts. At the same time, the fluid acts as a cooling medium within the hydraulic circuit.
Flexible placement of the drive unit
One advantage of hydraulic power transmission is the ability to spatially separate the drive unit from the lift table mechanism. The drive unit can easily be positioned outside the actual working area. The necessary hoses and lines can be flexibly routed, which facilitates adaptation to different environments.
Subsequent reduction of lifting times
In some cases, it may be necessary to shorten the lifting and lowering times of a lift table retrospectively, for example, to optimize cycle times in production processes.
Since many lift table manufacturers design scissor lift tables with mechanical overcapacity, there are often reserves in lifting and lowering speeds.
One way to reduce lifting times is to replace the hydraulic pump with a model that has a higher flow rate.
Simple and reliable overload protection
A hydraulic unit enables uncomplicated and reliable overload protection, as the maximum allowable load can be set using a pressure relief valve. This valve ensures compliance with the permitted maximum pressure within the hydraulic system through simple mechanics. This important safety function is therefore not dependent on potentially more error-prone electronics.
One unit for linear and circular movements
Lift tables with rotary platforms are quite common. Unlike the lift cylinders, which perform linear movements, the rotary drive requires circular motion.
For spindle lift tables, an additional drive unit is needed, as the lifting screw cannot easily be used for rotational movements. Hydraulic lift tables, on the other hand, offer the advantage that the same unit can perform both linear and circular movements.
Cost-effective
Hydraulic lift tables use components such as cylinders, pumps, valve blocks, and valves that are also utilized in many other industrial sectors. These parts are produced in large quantities, leading to cost advantages.
A general rule of thumb is that an electrohydraulic lift table costs about half the price of a comparable belt-driven or spindle lift table.
Energy consumption only during lifting motion
As mentioned in the functional description, the lift table's own weight is usually sufficient to lower the platform. As a result, energy is only required during the lifting process. Despite the relatively lower efficiency of the motor-pump combination, this leads to an overall positive energy balance for hydraulic lift tables.
Lower positioning accuracy
Achieving high positioning accuracy at multiple stop points is not easily achievable with hydraulic lift tables. Solutions often include locking pins or supports.
Locking pins require appropriate slots. If the lift table is installed in a shaft and the stop points are sufficiently far apart, this poses no issue. However, for freestanding lift tables or when the stop points are very close together, locking pins are not suitable.
For a limited number of stop points, supports are practical. Often, hydraulically foldable supports are used, on which the platform of the lift table can rest. However, if the lift table needs to serve a larger number of stops, an alternative solution is necessary.
Another option is to use electric sensors such as inductive sensors or rope length encoders.
Inductive sensors can be combined with a catch-up control system: If the sensor loses the signal, the control system activates the drive unit until the platform is back within the sensor's range.
Rope length encoders provide precise determination of the lift table's position and, with the appropriate electronic control, allow the lift table to reach any number of positions. The current position of the lift table is continuously monitored and deviations are corrected via the drive unit or the lowering valve.
Risk of leaks
When using a hydraulic lift table, there is a potential risk of leaks, which can be caused by various factors. The most common causes include:
- Routing hydraulic hoses over sharp edges
- Using hydraulic hoses outside the permissible temperature range
- Mechanical damage to hydraulic lines
While the risk of leaks can be significantly reduced through proper installation of the hydraulic lines, it cannot be completely eliminated. Material defects or other failures in the hydraulic circuit can also lead to leaks.
Unlike underfloor heating pipes, hydraulic lines should not be permanently embedded in concrete or screed. The supply line for a hydraulic lift table should be installed in such a way that it remains accessible for later maintenance. For example, empty conduits in the concrete can be a practical solution.
Heating of hydraulic oil
At very high cycle rates, there is a risk of excessive heating of the hydraulic oil. This can become problematic, as the higher temperatures accelerate the wear of the hydraulic pump and may lead to premature component failure. Additionally, there is a risk that the hydraulic oil could start to foam, allowing air to enter the system. Since air is more compressible than hydraulic oil, unexpected platform movements may occur.
There are two ways to address this issue:
- Increasing the available oil volume: By enlarging the hydraulic tank, more oil is available to absorb the heat. This prevents a critical rise in oil temperature.
- Installing specialized cooling systems: If enlarging the tank is not feasible due to space constraints, oil coolers can be used. These dissipate the heat to the surroundings and keep the hydraulic oil temperature within an acceptable range.
Viscosity of hydraulic oil at low temperatures
At extremely low ambient temperatures, the viscosity of hydraulic oil changes. The oil becomes thicker, which can result in the pump struggling or even being unable to circulate it. In such cases, the lift table may become inoperable. A possible solution is to install a heater in the hydraulic tank to bring the oil to an appropriate operating temperature, ensuring the functionality of the lift table even in cold conditions.
Noise emission
The operation of a hydraulic unit is always associated with a certain level of noise emission. In most cases, this is not an issue, as the lift table is often used in an industrial environment with a corresponding noise level, or the unit is housed in a separate room.
In other cases, where noise must be minimized, soundproofing the unit can be an effective solution. This method has already become standard in many technical fields, such as in the automotive industry.
Compression and rebound of the lift table
Without additional technical measures, a hydraulic lift table experiences compression and rebound when the load changes while in a raised position. This is due to the compression of the hydraulic oil inside the cylinders.
When an additional load is applied while the table is raised, the pressure in the cylinders increases, causing the piston rod of the hydraulic cylinder to retract slightly, resulting in the lift table slightly bouncing downward.
When the load is removed, the pressure in the cylinders decreases, and the piston rod extends minimally.
The extent of the compression and rebound depends on the pressure difference and thus on the amount of load added or removed. The smaller the load relative to the lift table’s own weight, the smaller the spring deflection.
Contamination of hydraulic oil
To protect the hydraulic system from contamination, wipers are used in hydraulic cylinders. However, these do not work perfectly, especially under harsh operating conditions, which, without additional measures, would increase the mechanical wear of the installed components.
In addition to external factors, mechanical abrasion within the hydraulic system also contributes to the generation of unwanted particles in the hydraulic oil. Excessive contamination of the oil accelerates the wear of the hydraulic pump and cylinders. For this reason, many lift table manufacturers standardly use filters to remove foreign particles and thus extend the lifespan of the components.
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