It is obvious that a lift table for 500 kg is not designed to carry the same load as a lift table for 1,000 kg. For this reason, there is a pressure relief valve in the pressure line of hydraulic lift tables. If the pressure in the hydraulic system is too high, part of the oil is pumped directly into the power unit tank to protect the lift table components against damage.
Furthermore, the motor protection switch of the drive motor may have tripped. This is actuated, for example, when the motor draws an excessive current.

If the lift table is overloaded, the weight must simply be reduced or removed. Check the lift table settings if overloading of the lift table is not the cause. The settings of the pressure relief valve or the motor protection switch may usually only be changed by the lift table manufacturer.

It is possible that all phases or even just a single phase of the main power connection has failed. Depending on the extent of the fault, it may cause a complete failure of the lift table controller and drive, a failure of the controller only or a failure of the hydraulic power unit.

To rectify this fault, you must check the entire main power connection (including the main switch) and eliminate the cause of the power failure.

If the electric motor of the hydraulic unit turns in the wrong direction, no oil is pumped from the tank into the cylinders. This means that although you can hear the hydraulic motor being actuated and started, the lift table does not lift.

Check the phases at the electrical connection of the electric motor and replace them if necessary.

At least one check valve is installed in the hydraulic system of the lift table. It keeps the lift table in the current position when the hydraulic pump of the drive is turned off. The check valve will not close properly if the valve seat is dirty. Depending on where the valve is installed in the hydraulic circuit, the oil can then flow from the pump directly back into the tank.

You can first try to flush the check valve through alternating backward and forward movement. If this does not succeed, the check valve must be removed and cleaned. Replace the valve if it is found to be irreparably damaged.

Of course, a component can also fail. If the electric motor is running and its direction of rotation is correct, but the lift table does not lift, this could indicate a defect in the hydraulic pump.

A defect in the coupling is more likely than mechanical damage to the hydraulic pump, though. To check this, it is usually necessary to dismantle part of the power unit. For more detailed information, refer to the instruction manual of the lift table or the technical data of the power unit. If the coupling is not damaged, the hydraulic pump may need to be replaced.

The power supply to the drive is interrupted when the emergency stop switch is pressed. This means that the motor is no longer supplied with power and that is the reason why there is no power to move the lift table. Please note that this usually does not affect the control voltage of the lift table. This means that although, for example, a built-in programmable logic controller or existing relays continue to be supplied with power, it is possible that the electric motor is not supplied with power.

The emergency stop switch is often used to turn off the machine. This should be prevented through appropriate instructions, as this function is not intended by lift table manufacturers. Especially when several people are working with one machine, this fault is easily overlooked. To put the lift table back into operation, it is sufficient to unlock all emergency stop switches. Depending on the control system, a reset button may also have to be pressed to release the lift table movement again.

Hydraulic lift tables need oil to extend the cylinders. If there is little or no oil left in the hydraulic unit, the cylinders do not extend and the platform of the lift table does not lift.

Top up oil through the filler neck of the tank. Make sure that all cylinders are fully retracted. Otherwise you’re in for a nasty surprise the next time you retract the hydraulic cylinders (the hydraulic tank would overflow). It is also important not to exceed the maximum filling level mark. However, more important than rectifying the fault is the question of where the old hydraulic oil is. If the lift table initially had enough hydraulic oil to lift the platform, the oil must have leaked somewhere. If the hydraulic lines have been laid correctly, a leak directly on the hydraulic unit or on the cylinders (damaged seals) is most likely. Eliminate the cause of the leak before using the lift table again.

To prevent uncontrolled downward movement of the lift table platform, the cylinders are usually equipped with a pipe rupture safety valve. In the event of damage, the flow is closed by means of a spring retainer so that the cylinders cannot retract any further. To test the functionality of this safety device, some lift table manufacturers equip their hydraulic system with a "bypass" line. A ball valve is installed in this line, which connects the pressure line directly with the tank in the open position and so simulates a hose or pipe rupture.

The solution to this problem is relatively simple. The ball valve must be closed. To eliminate this source of fault in the future, it should also be secured against being re-opened. The ball valves are usually equipped with a locking plate to at least prevent them from being opened without tools.

There are several fuses in the electrical control system of the lift table that protect the components of the lift table against excessive current. There should be at least one fuse each on the primary and secondary side of a transformer. If one of these fuses is blown, the entire control system can fail, so that lifting or lowering the lift table is no longer possible.

Please note that you should not simply replace the blown fuse with a larger one. The lift table manufacturer most probably have something in mind when designing the fuse and you would risk at least one defect in the components, or in the worst case even cause a fire. As with all faults, it is important to question why it has occurred. In other words, what was the reason for the fuse to blow or to have blown? Voltage fluctuations on the mains side may be the cause. But it can also be an insulation fault and an associated short circuit.

If the lowering valve is defective, it does not open and the oil cannot flow back into the tank. Often, however, it is not the hydraulic valve itself that is defective, but the solenoid coil or the supply line of the valve. For this reason, it is recommended to first measure the control voltage on the valve. This means it should be checked on the valve connector whether the control voltage is applied to the contacts when the lowering button is pressed. If the supply line can be excluded as the cause of the fault, the next step is to test the solenoid coil of the valve. There are often more than one valve installed in a hydraulic power unit, so that you can simply replace the suspected defective solenoid coil with one that functions. If the solenoid coil can also be ruled out as the cause of the fault, the only option left is to replace the lowering valve.

Lift tables usually have a surrounding safety edge to prevent objects or limbs from being trapped between the platform and the base frame of the lift table.
If the safety edge is activated, the lift table can no longer be lowered any further and the platform can only be raised. The first step is to find and eliminate the cause that triggered the safety device. For example, the cause can also be that safety edge got caught. If the safety edge is not blocked by some or other object, and that it got caught can also be ruled out as the cause, the electrical connection of the safety edge must be checked. In these cases, a broken cable or similar fault has probably prevented the lift table from lowering.

There are several fuses in the electrical control system of the lift table that protect the components of the lift table against excessive current. There should be at least one fuse each on the primary and secondary side of a transformer. If one of these fuses is blown, the entire control system can fail, so that lifting or lowering the lift table is no longer possible.

Please note that you should not simply replace the blown fuse with a larger one. The lift table manufacturer most probably have something in mind when designing the fuse and you would risk at least one defect in the components, or in the worst case even cause a fire.

As with all faults, it is important to question why it has occurred. In other words, what was the reason for the fuse to blow or to have blown? Voltage fluctuations on the mains side may be a possible cause, but it can also be an insulation fault and an associated short circuit.

This fault can occur if one of the following activities has been carried out recently:

• Replacing the lowering throttle
• Changing the lowering throttle setting
• Replacing the pipe rupture safety valves

Pipe rupture safety valves react to a pressure difference between the supply line and the lifting cylinders. If a pipe ruptures, the pressure in the supply line would drop abruptly to 0 bar and the spring installed in the pipe rupture safety valve would close the flow-through opening and so prevent the lift table from lowering any further.
If the characteristic curve of the pipe rupture safety valve does not match the setting of the lowering throttle, a malfunction can occur. A very high flow rate acts like a defect in the supply line for the pipe rupture safety valve. For this reason the corresponding valve is closed and the platform cannot lower any further. To rectify this fault, you must raise the lift platform again to open the pipe rupture safety valves. The platform can only be lowered at a reduced lowering speed; the setting of the lowering throttle must be adjusted. Make absolutely sure that the pipes to be opened are depressurised.

The power supply to the lowering valve is interrupted when the emergency stop switch is pressed. As a result, the valve cannot be opened via the built-in solenoid coil and the lift table remains in its current position.
Unlocking all emergency stop switches is sufficient to rectify this fault. If necessary, a reset button must also be pressed to enable the lift table to move again.

A pressure relief valve can only protect a lift table from overload during lifting. An electronic pressure switch is often installed when the lift table can be loaded and unloaded on several landings. This is connected to the control system in such a way that the lowering and lifting movement of the platform is blocked when a programmed pressure is exceeded. Operation can usually be resumed immediately after the overload has been removed and the fault has been acknowledged.

How does this fault manifest itself?
The compressibility of air is significantly higher than that of hydraulic oil. Air in hydraulic systems regularly causes problems, as it allows the load to move in an uncontrolled movement manner. Let’s assume a load is only lifted by the hydraulic oil in the cylinders. The applied load exerts a force on the cylinders and so also on the hydraulic oil in the hydraulic lines. The hydraulic oil is compressed, the volume of the oil is reduced and therefore the cylinders do not extend as much as they would without a load (with a constant quantity of oil). This effect is still relatively small with hydraulic oil. However, if air gets into the hydraulic system, it is also compressed by the pressure and the volume is reduced. Due to the greater compressibility, the change in volume at the same pressure is much greater and the load drops noticeably. If air enters the hydraulic system several times during the lifting process, this process is repeated accordingly and is perceived as jerking.

Solution:
It must be ensured that the hydraulic system is properly bled to prevent this undesirable effect. 
If the lift table manufacturer has fitted the cylinder with a bleed screw, bleeding can usually be done effortlessly. In this case, you must bypass or disconnect the limit switches, open the bleed screw slightly and move the lift table with pressure against its top end position. The escaping hydraulic oil must be collected with a cloth. Move the lift table against the pressure until only hydraulic oil without air pockets escapes from the bleed screw. It goes without saying that the maximum duty cycle and the manufacturer's specifications must be observed.
The hydraulic connection on cylinders without bleed screws must be loosened slightly. If pressure is now applied to the cylinder, some oil will escape. As soon as no more air pockets can be detected in the hydraulic oil, the hydraulic screw connection can be tightened again. Make absolutely sure that the line cannot rip off the cylinder when the hydraulic line is loosened. Maintenance support struts must always be used and safety glasses etc. worn.

A dirty running surface prevents the non-locating bearing roller from running smoothly. The dirt must either be skipped over or it is pushed back and forth in front of the non-locating bearing roller. Both manifest themselves in an uneven lifting or lowering process. Depending on the material, there is still a risk that foreign particles will penetrate the running surface or the non-locating bearing roller and so permanently impair the lift table operation.
To prevent this, manufacturers usually prescribe regular cleaning of the lift table and especially of the running surfaces. Unfortunately, it is not possible to give a general recommendation of how often cleaning must be carried out, as this depends primarily on the on-site operating conditions.

Depending on the service life, the individual bearings of the lift table may have to be regreased or, in the case of non-greasable bearings, completely replaced.
Lift tables subjected to a high cycle rate usually have bearings that can be lubricated. These can be regreased either via a central lubrication system or on the individual grease nipples. Commercially available lift tables, on the other hand, are usually equipped with maintenance-free plain bearing shells that are adapted to the planned service life of the lift table. If bearings are nevertheless damaged, the manufacturer can replace the bearings if necessary. Depending on the extent of the damage, bearing bolts and other adjacent components may also have to be replaced.

Just like jerky lifting, air in the hydraulic circuit can also cause the lift table to lower jerkily. The solution is the same as the procedure described above. The hydraulic system must be bled.
If your hydraulic cylinders have a bleed screw, you can easily open it and move the lift table to its top end stop (disconnect limit switches). The escaping hydraulic oil must be collected with a cloth. Move the lift table against the stop until only hydraulic oil without air pockets escapes from the bleed screw. Of course, the maximum duty cycle specified by the manufacturer must not be exceeded.
The hydraulic connection on cylinders without a bleed screw must be loosened slightly. Then the procedure described above must be carried out, whereby it is essential to ensure adequate safety. Dangers arise in particular from the possibility of the hose rupturing.

Dirt on the running surface of the roller bearing can prevent the non-locating bearing roller from running smoothly. Simple cleaning of the running surfaces with a hand brush and cleaning cloth is usually sufficient to fix this fault. Please remember that the maintenance support struts must be inserted for all work underneath the platform.

If the cylinders remain extended for a long time, there is a risk that the piston rods will dry out. This means the lubricating film that ensures the smooth retraction of the cylinders is missing. This could cause the platform to lower jerkily.
Soak a clean rag in hydraulic oil and wipe it on the piston rod of the hydraulic cylinder to fix this problem.

Lift tables are usually tested extensively by the manufacturer before delivery, so that the fault described here rarely occurs. Nevertheless, it is possible that the cylinder seals on some lift tables are still very tight and consequently offer increased resistance during retraction. To solve this problem, the continued use of the ...

Apart from the safety edge getting caught, a cable break in the supply line is another possible source of the fault causing the jerking lowering of the lift table platform. A resulting loose contact acts like a short-term actuation of the safety edge for the control system. For this reason, the lowering movement of the lift table is stopped.

The loss of hydraulic oil or an oil level that is too low prevents the top lifting position from being reached. In this case, the hydraulic pumps simply lack the necessary oil to be able to extend the cylinders further.
Check whether hydraulic oil has leaked somewhere and fix the leak. Then refill the tank of the hydraulic power unit. Make sure that all cylinders are fully retracted and that the maximum fill level is not exceeded.

Most lift tables use electromechanical switches to detect when the top and bottom lifting position is reached. They are used to turn off the hydraulic power unit in the top position and to close the lowering valve in the bottom position.
In most cases, the limit switches are installed in such a way that the end positions can be set flexibly. Due to the transport of the lift tables, it is possible that the switches are no longer in the planned positions. As a result, the lift table stops in the wrong position.
The limit switches must be reset to correct this fault. To do this, move the lift table to the desired end position and move the limit switch until you hear the switching operation.

A leak in the hydraulic line or incorrectly tightened connections can cause hydraulic oil to escape from the pressure line. This oil is not available for use in the cylinders. They retract and the platform lowers.
If you can see oil leaking around the connections, it is sufficient to retighten the corresponding screw connection. Make sure not to overtighten screw connections on hoses. This could damage the hose. Therefore, please only hand-tighten the hoses.

Cylinder seals are another possible source of leaks in the hydraulic circuit. Unfortunately, a defective cylinder seal must be replaced, which can usually only be done by the manufacturer of the hydraulic cylinder.

Check valves are used to prevent the oil flow in one direction. For example, a check valve is often found directly behind the pump. This prevents the hydraulic oil under pressure to flow from the cylinders via the pump back into the tank. The flow-through opening is usually closed by a combination of ball, spring and ball seat. If this seat is dirty, the ball no longer seals properly. The result is hydraulic oil flowing back into the tank and the platform lowering.
Minor dirt particles can be removed by simply flushing the valve. This means that repeated lifting and lowering of the lift table platform already eliminates the problem. In more serious cases, the check valve must be removed and cleaned with compressed air, for example. In the worst case, the valve must be replaced.

Some lift tables are equipped with an emergency release valve. This allows the lift table to be lowered even when the controller or power supply fails. An open emergency release valve thus leads to a lowering platform. Normally you should have no problem with this, as most emergency release valves are self-resetting and therefore close automatically.

The lowering valve is another valve that could cause the described problem. It connects the pressure line directly to the hydraulic tank. A soiled lowering valve would have the same effect as a soiled check valve. Hydraulic oil would enter the tank from the cylinders and the lift platform would lower.
Troubleshooting this fault is also similar to the procedure for problems with the check valve. This means that you should first try to flush the valve by repeatedly lifting and lowering. The next step would be to remove the valve and clean it with compressed air. The last option is to replace the valve.

Most lift tables are equipped with maintenance-free plain bearings. Unfortunately, they are not protected against wear or a defect. So if you hear a noise coming from somewhere around the scissor pin, the non-locating bearing or the fixed bearing of the lift table, this could indicate a defective bearing.

If you can hear a slight 'bubbling’ sound, this indicates air in the hydraulic system. First check the oil level of the lift table. All hydraulic cylinders must be fully retracted. If there is no or too little hydraulic oil in the tank, first find the cause and, if necessary, eliminate it. Then top up the hydraulic oil.
Bleeding the hydraulic system has already been described several times. Move the lift table against pressure, hold down the button for a short time and repeat the procedure until the air has escaped from the hydraulic circuit. In lifting tables with a bleed screw, slightly opening the screw is sufficient to allow oil to escape from the pressurised cylinders. Close the bleed screw as soon as there are no more air bubbles in the hydraulic oil. A more detailed explanation of how to bleed the system can be found under the point: Lift table lifts jerkily -> air in the hydraulic system.

If the noise originates in the area of the hydraulic unit, this indicates a defective pump or electric motor. In most cases, this is bearing damage. This fault is usually not easy to fix. Replacing the complete defective hydraulic unit is usually quicker and cheaper.

If the lift table is brand new or if work has been carried out on the electrical connection, you may be lucky. The power unit may not be running on three phases, but only on two. The connection can be fixed much faster and cheaper than replacing components.

If the lift table is operated close to or above the load limit, it can be that hydraulic oil is fed into the tank via the pressure relief valve. The pressure relief valve is a kind of overload protection in the hydraulic circuit and protects the installed components against unacceptably high pressure. If the operating pressure exceeds the set pressure of the pressure relief valve, oil flows back into the tank.
In a very limited load range, the effect can be that the lift table still lifts, but only at a very slow speed. The acceleration forces that occur cause the set pressure of the pressure relief valve to be exceeded, resulting in some of the oil flowing back into the tank. The consequence is that the entire volume flow of the pump is not available for the lifting movement and the lifting speed is greatly reduced.
This operating state must be avoided at all costs. The slower lifting speed inevitably leads to a longer running time of the power unit. The longer the running time, the higher the thermal energy introduced. The heating of the hydraulic oil can in turn lead to a defect in the installed hydraulic components. The only correct decision in this case is to reduce the load.

If an overload can be ruled out, the only remaining problem is unfortunately a defective hydraulic pump. A replacement is inevitable. As a rule, you should leave this work to the lift table manufacturer.

You could only be lucky if the lowering valve was permanently activated, because of work done on the electrical or control system of the lift table, for example. If the lowering valve is open and the built-in throttle lets less oil through than the pump can deliver, this will result in a low lifting speed. You can quickly eliminate or rule out this source of the fault by disconnecting the valve connector of the lowering valve. The same applies, of course, to a bypass that may be installed.