Choosing the Perfect Scissor Lifting platform Solution
Choosing the Perfect Scissor Lifting platform Solution
Scissor lift tables and scissor lifting platforms are an excellent choice when objects, materials or equipment need to be moved from one level to another, whether that is between floors or to a sensible working height in a situation where overhead cranes forklifts or other machinery that lifts the item from its side is not practicable. The great advantage of the scissor lifting table is that the complete lifting mechanism is contained beneath the lift platform which results in a mechanism where the lifting equipment is contained with a minimal footprint.
The get the most suitable scissor lift design for your application it is necessary to consider all aspects of the required operation. Discussions with a Tecmach design engineer at an early stage of the project will be of assistance in getting the specification correct and thus installing the best scissor lift.
Consider the design of scissor lifting platforms. In the vast majority of cases the scissor lifts power is derived from hydraulic cylinders. Hydraulic power allows the cylinder to be at shallow angle and still easily raise the load which in turn allows for minimal closed height. Hydraulic power provides abundant force to raise the load on the platform where the force on the end of a cylinder may be six time that of the load.
The scissor lift construction comprises of the following components:-
The top platform is made from thick sheet steel with folded or welded sides and ends. To increase the rigidity and strength larger and heavy-duty platforms have a robust frame of rolled hollow steel section welded to the underside. This frame reinforcement will also constitute runners and guides for the wheels and minimise deflection.
The scissor mechanism consists of scissor arms constructed from heavy duty thick walled rectangular hollow section steel. The outer scissor arms are connected to each other by a heavy-duty lower crossbar and the inner scissor arms connected with a central box section. The upper mounting of the hydraulic cylinder is attached to the crossbar of the inner scissor arms and the lower mounting of the hydraulic cylinder (pivot pin) is attached to the crossbar of the outer scissor arms.
This construction, with the cylinder mountings fixed between inner and outer scissor arms, means that the cylinder’s power is not directly applied the top platform or base plate of the table.
The inner and outer scissors are joined together at the pivot point with long bearings which can be lubricated. A distance between the inner and outer scissors maintains the safety distance of 30 cm between the scissor arms (Bone Room!).
The final part is the base frame, which is on standard lift tables is fabricated from rolled steel
angle. This provides a protective plate and is fitted between the wheel runners and the foundation.
To keep the closed height to a minimum, this frame is not self-supporting as the lift is normally
fixed to a fully supporting foundation such as the factory floor. If required the base frame can be fully self-supporting which will increase the closed height slightly.
Depending on the lifting height compared to the load foot print, a single scissors assembly may be used (Single scissor lift mechanism) or multiple pairs that are stacked one above the other (Double or triple scissor lift) or alongside in a tandem construction. This is because there is a relationship between the frame length of a lifting table and its stroke or lifting height.
This relationship varies between the different types of lifting tables, but can be generalised as limiting the leg angle to 45 degrees this minimising the unsupported portion of the platform, the platform cantilever. This angle is controlled by the travel within the cylinders. The hydraulic system consisted of the following component. A hydraulic power pack consisting of a motor; a tank and filter; a pump with by-pass valve and non-return valve as well as a lowering valve.
If the table is subjected to overloading, a by-pass valve is opened and the oil is returned to
the tank to avoid mechanical overload. Hydraulic oil is transported via a non-return valve and
through pipes and hoses to the cylinders. Cylinders are fitted with break-pipe valves for increased safety.
When lowering, the oil is returned to the tank through the weight of the load and the
weight of the top platform, which press the scissor arms and cylinder together. Lowering speed can be adjusted with the aid of the return throttle valve.
Finally, the electrical control system of a scissor lifting platform should consists of a motor contractor box with safety control circuit. In its basic form this box contains the following components, transformer for low voltage control circuit, motor contractor and thermal overload protection, connection terminal block. The following components are connected to the contractor box: The push button control box, the upper limit switch, the anti-crush safety frame ,the lowering solenoid valve, and the electric motor. Depending on the application there may also be switch monitoring, control safety relay and alarm circuit.
Selecting the most appropriate Scissor Lift Table More than anything, scissor lifts should be selected, based on their intended use. Each application will have differing requirements whether the use is for an assembly bench, pallet transfer within a conveyor system, or a goods only lift, this use will determine the capacity and style of the lift, along with any optional features which might be needed.
Before selecting a scissor lift, it is important to fully examine and analyse the intended use for which it is being purchased. In this process, a list of minimum specifications should be developed, which will be used in the process of qualifying or eliminating contenders from the selection process. Some of the characteristics which must be considered will include:
Location
Is the scissor lift is going to be used in a fixed location, be integrated into a machine or conveyor system or shall it be possible to move the lifting platform.
Load
Decide how much weight the lift will need to handle. What is the load and how is it to be applied to the platform and how it is to be removed. For instance, loading with say, a forklift or powered pedestrian truck, will produce different forces to that of a stacking operation. A reel of paper manually rolled on to the platform and located in a “Vee” will apply significant loads along the side of the platform and a dynamic force as it stops.
Scissors lifts are designed to have the load's centre of gravity in the centre of the lift platform. For a given safe working load manufactures such as Tecmach allow loading of ½ the SWL on ½ the length and 1/3 of the SWL on ½ the width. Applying the load outside this can result in premature wear. Describing the full proves allows for appropriate measures to be considered in the design process.
Lift Height
Consider the maximum height to which a load will need to be lifted or required travel this should be considered in relation to the product size. Also consider the closed height and if this is important within the application.
Duty Cycle
Typically scissor lifts are design to operate over a full cycle up to 20 times an hour with a lifting and lowering speed of 50-80 mm a second.
Standard or Custom Scissor Lifts.
Although the general design of a scissor lift is consistent, the applications in which the lift is used will often require special features or options. If a standard unit is not suitable then custom
designed and custom built scissor lift tables and platforms. Custom scissor lift tables should be
designed form the ground up for the specific application, not just adding parts to a standard table.
Determining the Quality of a Scissor Lift
When selecting and working with a scissor lift, as well as low cost of ownership and long service life, safety is a major concern. This is an important consideration when choosing a scissor lift. The quality of the material, workmanship and the design philosophy of the lift manufacture will contribute to the inherent its safety. Exceeding the minimum safety standards on design and control are also important.
A few key components to review include:
All scissor lifts should have quality precision fit bearings with suitable lubrication points. The bearings and axles only rotate through 45 -50 degrees and are continually loaded in this quadrant. The higher the load capacities the more important bearing design becomes. Over engineered bearings are a good feature. look also at total bearing length, longer bearing length increases stability.
Look at the dimensions and wall thickness of Rolled Hollow Sections, This is one of the less obvious places where manufacturers can improve the design significantly. Thinner material can reduce service life. Hydraulic cylinder can be the first point of failure on any scissor lift is usually oil leaks at the hydraulic cylinder seals. Lower quality cylinders are likely to leak sooner than high quality ones. Each cylinder shall have a hose rupture valve fitted within the cylinder port to prevent the lift dropping uncontrolled in the event of hydraulic pressure loss. Having two or more cylinders increases stability. Wide platforms with a single central cylinder will not be as stable as twin cylinders well-spaced.
The platform must be fitted with a safety (anti crush) edge below the platform. If activated the lift stops, has to be reset to allow lift to move
The scissor lifts should be manufactured according the requirements of EN1570, look for the following features. All bearing pins are fixed in position to eliminate the risk of the pin working out, and at shear points the minimum gap should be at least 30mm.
Prior to leaving the factory there should be a full inspection and test procedure which should include proof testing over the full cycle prior at 10% overload and 25% overload at the point of maximum stress. The hydraulic pressure relief valve is set a pressure equal to an overload of 10% of rated capacity. As always, manufacturers of high-quality scissors lifts will have a good reputation and will be able to discuss and qualify the detail with in the design. A conversation with a Tecmach design engineer will allow you to be confident that you are buying the most appropriate scissor lift table and getting the best value possible for a long service lift.