Protective devices on the lift table: Hydraulic valves

• 1. Safety valves: Essential protection for hydraulic lift tables
• 2. Burst pipe protections
  • 2.1 Functionality
  • 2.2 Failure in case of minor leaks
  • 2.3 Reopening the burst pipe protection after activation
• 3. Electrically unlockable valves
  • 3.1 Functionality
  • 3.2 Advantage over burst pipe protections
  • 3.3 Disadvantages of electrically unlockable valves
• 4. Hydraulically unlockable valves
  • 4.1 Functionality
  • 4.2 Advantage of hydraulically unlockable valves over electric lock valves
  • 4.3 Higher installation costs
• 5. Which safety mechanism should be chosen?

In this article, you will learn about the design of burst pipe protections, as well as electrically and hydraulically unlockable valves, their advantages and disadvantages and what to consider when selecting them.

A hydraulic lift table maintains its platform in position through constant pressure in the cylinders. However, in the event of an unexpected hose rupture, there is a risk of a sudden pressure drop, which could cause the platform to descend uncontrollably. To prevent this, nearly all hydraulic lift tables are now equipped with safety valves. The use of such valves is particularly crucial in areas where people are present near or on the lift platform, ensuring their safety.

The choice of the appropriate valve depends on the specific application: If the lift table is used solely for lifting loads, different requirements apply compared to industrial work platforms designed for personnel transport.

The main components of a burst pipe protection consist of the housing, the shut-off flap and a spring mechanism. Burst pipe protections are directly screwed into the hydraulic cylinder and respond to pressure differences between the supply line and the cylinders.

If the pressure in the supply line drops, the spring mechanism closes the flow opening. This prevents oil from escaping the hydraulic cylinders, ensuring that the lift table platform remains in its current position.

For a burst pipe protection to effectively prevent oil flow, a sufficient pressure difference must be present. This becomes problematic if the pressure loss is not caused by a ruptured or burst hydraulic hose but by a small leak in the hydraulic system. In such cases, the resulting pressure difference is too minor for the spring mechanism to close the flow opening. As a result, the platform cannot be securely held in its upper position despite the presence of burst pipe protections.

After a burst pipe protection is activated, it can only be reopened by raising the platform again. This increases the pressure in the supply line, which forces the shut-off cap to open against the spring mechanism.

Therefore, the use of burst pipe protections is not permitted on industrial work platforms, if there is no safe way to exit the platform. Lowering the platform would only be possible after repairing the drive system.

Electrically unlockable valves do not differ in their construction from other electrically actuated 2/2-way valves. They have two switching positions, which can be set using a coil and a spring.

In hydraulic lift tables, valves that are closed in a de-energized state are used. When no electrical voltage is applied to the actuation coil, the spring pushes the valve into its rest position. In this position, the flow opening of the hydraulic valve remains closed, preventing any exchange of oil between the supply line and the cylinders of the scissor lift table. This ensures that in the event of a hose rupture or pipe breakage, the oil is securely retained within the lift cylinders.

To move the lift table platform, the actuation coil is electrically activated. This causes the valve to open, allowing the oil to flow between the supply lines and the hydraulic cylinders. As a result, the platform of the lift table can be raised or lowered.

Electrically unlockable valves offer a higher level of safety compared to burst pipe protections. While burst pipe protections reach their limits with small leaks in the hydraulic circuit, electrically unlockable valves ensure that the lift table platform remains securely in position even in the case of minor leaks. This high level of safety is achieved because the valves only open when they are electrically actuated, allowing for a controlled movement of the platform.

Although electrically unlockable valves offer safety advantages, there are also some disadvantages to consider.

While installation is relatively simple and flexible, it is more complex than with conventional burst pipe protections. Each valve requires an additional cable to ensure direct actuation.

Moreover, the risk of malfunctions in the lift table increases, as each additional component represents a potential risk for defects. The coils of the valves, in particular, can be a weak point that may lead to failures of the scissor lift platform, even if the overall risk remains low.

Another issue is the load distribution in the event of a valve failure. In most cases, a lift table is equipped with at least two hydraulic cylinders, each with an electrically unlockable valve. If one of these valves fails, it remains closed while the other one opens, placing the entire load on a single cylinder, which leads to an uneven distribution of forces. To prevent damage, the lift table must be dimensioned in such a way that, in an emergency, a single cylinder can bear and transmit all the forces.

Furthermore, the higher costs associated with these valves should be mentioned. In addition to installation costs, there are extra expenses for protection against power failures. Electrically unlockable valves only open when electrical voltage is present, which can be problematic in the event of a power outage. In situations where lowering the platform is crucial, such as on industrial work platforms, an uninterruptible power supply is required.

Hydraulically unlockable valves offer a key advantage over the electrically unlockable variant: a lower probability of failure in the control unit. The failure of the control unit in a hydraulic valve is less likely compared to an electrical coil. As a result, the risk of a single cylinder having to bear the entire load of the lift table in the event of a defect is significantly reduced.

The main disadvantage of this type of safety mechanism is the higher installation costs. Compared to electrically unlockable valves, the installation of hydraulically unlockable valves involves significantly more effort, as not only must a hydraulic line be routed to the cylinders, but an additional control line must be laid to each unlockable valve.

Moreover, protection against power outages is necessary. Hydraulically unlockable valves only open when the control line is pressurized, which is typically achieved through the lift table’s hydraulic power unit. However, in the event of a power outage, this power unit cannot operate, meaning the lift table platform cannot be lowered. To solve this problem, a hand pump or an uninterruptible power supply (UPS) must be installed to allow the valves to be unlocked even during a power failure.

If, however, it is possible to leave the platform in the raised position until power is restored, installing a hand pump can be avoided.

The choice of the appropriate safety mechanism for a lift table cannot be made universally. It depends on the specific situation on site and the intended application of the lift table. In particular, if people are working on or beneath the platform, the hydraulic system must be designed for maximum safety.

We hope this article has provided you with a good overview of the various safety valves. With this basic knowledge, you will be able to better evaluate the recommendations of your lift table manufacturer and determine the right configuration for your requirements.