Scissor lifts tables

For example, the scissors of our scissor lift tables are joined to the upper and lower frames by fixed bearings supplied by the leading manufacturer in the sector.

Description

The basic structure is formed by a supporting mechanism, an upper frame and a power unit.

The upper frame

The upper frame rests on the supporting mechanism and serves as support for the load.

In the simplest version, it is a metal structure with the necessary design to withstand the tensions for which the table has been designed, depending on the load capacity, its distribution, etc. The load can rest directly on it or on some tooling required for handling purposes. In the most usual version the frame has a platform or metal sheet on it which, depending on the use for which the table is designed, can be smooth, tear plate, with non-slip finish; or can be galvanised, in stainless steel, etc.

Other versions, more specific, can be located on the frame:
In accordance with current national and international regulations, the platform will be fitted with the normal systems described as mandatory for safety. Also, if the lift table is to be used by personnel, as is the case of work platforms, or entered by personnel, as in the case of cargo lifts or lift tables for loading and unloading, the platform must be fitted with specific systems for personal protection.

The supporting mechanism

The purpose of the supporting mechanism is to convert the force supplied by the power unit into movement, thus lifting the upper frame and the load resting on it.

The most normal type is the scissor or pantograph. Its construction will depend on the size of the platform, the distribution of the load on it, the useful run or lift and the construction height. Thus, depending on these variables, the resulting mechanism will be simple scissor; double, triple or quadruple scissor, for greater lifting; tandem scissor, for larger areas; or other constructions for specific needs: if a low construction height is required with only a short run, as in the case of pallet transfer stations or TRT, to be able to load the table with a pallet truck, the scissors can be shifted to the sides of the platform; to obtain space for a loading ramp and/or passage two pantographs will be located at both ends of the platform, operating independently; to obtain large square platforms it may be necessary to place two pantograph tables in tandem, parallel and synchronised, etc.

The power unit

The purpose of the power unit is to generate and/or transmit power to the supporting mechanism. The power can be produced by a mechanism attached to the lift table (an electric motor, hydraulic group, etc.), produced by an outside unit (compressor, etc.) or even, for small loads, be manual (pedal or lever action, etc.). Depending on the specific needs, the generator mechanism can be located under the lift table thus achieving a more compact unit; integrated into the lift table but outside the platform, in order to reduce the construction height; or it can be completely separate from the lift table, as when having to work in explosive atmospheres.

The mechanism for power transmission to the upper frame or the platform is always integrated into the lift table. Depending on the transmission we find the following types:

• Hydraulic lift tables, where the power is transmitted by hydraulic oil to the cylinders which in turn drive the supporting mechanism.
• Pneumatic lift tables, where the transmission element is a mixture of water and glycol under pressure which operates the bellows cylinders located inside the scissors;
• and, finally, mechanically operated lift tables, where the transmission mechanism is a mechanical construction. There are several examples of mechanical transmission technology.
  • In chain lift tables, one or more push chains, which become rigid on taking up a vertical position, due to their design, form a lift column which transmits movement to the pantograph or the upper frame.
  • In spindle lift tables, the rotation of a screw against an anchored nut is what produces thrust on the pantographs, directly or through some other mechanism.
  • Finally, in belt-drive lift tables, the traction exercised by the belts on rollers inserted in the scissors is the mechanism by which elevation is produced.

Other structural elements

Optionally, lift platforms can have a lower frame with functions varying in accordance with their construction and use. In general, this element gives the whole greater stability. Some examples of these functions:

• The frame can have legs, eyebolt holes for the whole assembly to be moved by crane; guides for forks, for movement by forklift truck (chassis for fork entry); etc.
• Can serve to support the whole assembly on a turret to produce a lift turntable.
• If dealing with a rolling or mobile lift table, or where it needs installation in a pit or fixing to the ground, the frame can form an anchorage point, for wheels in the first case, for the anchorages in the second.
• When the support mechanism is a pantograph, it can house the roller guides for the lower scissor rollers or even carry out their function.

Construction materials and finishes

The construction of the lift tables is in steel, in accordance with structural needs and the required load capacity. The finish is usually oven painted, with several coats to give great durability in protection.

Specific requirements, such as flammable or very corrosive working atmospheres, extreme working temperatures, sensitive loads (foods, pharmaceutical products), etc., may need partial or total construction in galvanised steel, stainless or food quality stainless steel and antistatic or anti-explosive finishes (ATEX).

Safety elements

As happens with all industrial equipment, it is essential to guarantee safety of operation by means of protection and control elements. In the case of lift tables these safety requirements are described in standard DIN EN 1570/A1 (2004/10), which regulates their use depending on the operational environment of the equipment.

Perhaps the most important of them all is the device to prevent the feet from becoming trapped, the contact strip, which, located under the upper frame, goes round the whole perimeter and immediately stops the lift table from descending when it meets an obstruction in its path.

Equipment that is operated by a button panel must be fitted with a “dead man’s switch” which prevents it from working without a voluntary action by the operator. As a supplement, there must also be an emergency stop device.

Another fundamental element is the device to prevent uncontrolled descent, either due to breakage of hydraulic hoses in the case of hydraulic lift tables; failure of mechanical or traction elements in the case of mechanical tables; or by loss of power due to an electricity cut-off. A cylinder with a safety electro-valve locks the platform in position and makes the safe and controlled lowering of the table possible.

For repair and maintenance work, lift tables are fitted with locking bars, folding safety supports which prevent the table from descending during these operations.

For the risk of becoming trapped between the scissors (in pantograph constructions) extending elements are fixed to the upper frame, such as mesh and rolling blinds or rolls, or also fixed to the lower frame, as in the case of PVC skirts. These barriers prevent access below the platform while it is lifting and descending. All these elements of protection against access to the inner part of the table can be replaced by presence sensors which stop it from working in the case of being activated.

Lift tables designed to be installed in a pit have to be fitted with a hatchway on the platform giving access to the pit for operations of repair and maintenance, without needing to lift the table.

Other notable safety elements are handrails, required in the case of equipment which allows the transit of personnel on the platform; antiroll devices to prevent the load from falling off; non-slip finishes on the platform, etc.