The depicted scissor lift table is used by a renowned gearbox manufacturer for testing assemblies. Uniquely, the tests are conducted underwater.
Adjustments for underwater use
Normally, the pit of a lift table is equipped with a slight incline and a drain to protect the table from constant moisture. However, this special project is different. Contact with water is unavoidable, especially since the entire lift table (not just parts of it) is periodically completely submerged.
The ideal solution would have been a lift table completely made of stainless steel. However, this would have involved significant additional costs - the price would have at least doubled. Therefore, we decided against a full stainless steel construction and instead developed a cost-effective alternative. In the following section, we will present our concept that ensures the longevity of the lift table even under extreme conditions:
The surface protection
Whenever a component is in contact with water over the long term, we have the corresponding assembly hot-dip galvanized. This type of surface protection is characterized by high resistance to adverse environmental influences and mechanical stress. For lift tables used outdoors, this usually only affects the platform and the lower frame, not the entire lift table as in this special project.
The fact that the scissor of the lift table also had to be hot-dip galvanized presents some manufacturing challenges, as a comparatively large volume of welding is involved in the scissor. This is accompanied by high temperatures and internal stresses. These stresses are relieved during the hot-dip galvanizing process, which can lead to distortion of the component. However, distortion of the scissor is not tolerable in a lift table.
To address these challenges, several measures were taken:
- Avoidance of hollow bodies: These could potentially lead to bursting of the corresponding component during heating.
- Reduction of welding stresses: By designing with as few weld seams as possible, avoiding "weld seam accumulations," and extending the cooling phase after welding, stresses can be reduced.
As the successful commissioning of the shows, these measures prevented the scissors from warping.
With these measures, distortion of the scissor in a completely galvanised lift table can be successfully avoided.
The drive chain
One of the great advantages of hydraulic lift tables is the flexibility in placing the drive unit. In this special case, the drive unit was installed outside the water's influence for safety reasons. This protects the electric motor, pump, and valves from contact with water.
Despite these precautions, some components, such as the hydraulic cylinders, cannot be easily isolated. To increase their resistance to water, specially manufactured cylinders were used. These differ from standard cylinders in that they have a nickel-plated, triple-chrome-plated piston rod and special seals designed specifically for use with HFC fluid.
The hydraulic hoses, which are sometimes completely underwater, were also specially adapted. They are made of fabric material and are equipped with stainless steel fittings to ensure long durability under water.
The use of HFC fluid, which has a high water content and is flame retardant, precludes the use of submersible power units, as the high water content in the pressure medium could lead to short circuits in the winding. Therefore, a compact unit was chosen for this project.
The mechanical components
Looking at the images, it is noticeable that the platform is not completely closed but has an open platform frame. The reason for this is functional: A closed platform would float when the lift table is lowered. To prevent this, large openings were made in the platform to allow water flow. To ensure sufficient stiffness of the open platform, the substructure was reinforced and the sheet thickness was increased.
Moreover, the platform features several elongated slots. These were not milled to combat the buoyancy force of the water but fulfill a special customer request. The elongated slots allow various tool carriers to be mounted on the platform, which enables high flexibility in designing the experimental setups.
