Scissor lifts generally work safely and efficiently, ensuring order and productivity in loading bays and other materials handling facilities. As such they are often under-appreciated, so here Alan Ryder of sara LBS explains their design and operation.
Scissor lifts tend to be used with large loads, so safe operating practises need to be observed. These will include ensuring that the lift is designed for the duties it is undertaking, that it is maintained to a suitable level, that safety systems are in place, and that the operators are trained in safe working practices.
In fact the Lifting Operations and Lifting Equipment Regulations (LOLER) require scissor lifts to be strong and stable enough for their intended duties. They also have to be marked to indicate safe working load as well as being correctly installed, positioned and maintained to minimise any risk. Operator training must be provided with records of use and maintenance kept.
The lift may be installed as an individual item or as part of a total loading bay package. Although there are off-the-shelf scissor lifts available, for many applications a bespoke design is a more appropriate solution. So how do you go about designing a suitable lift?
The most obvious consideration is the load (or weight) to be lifted. Both the frame and the drive mechanism need to be able to cope not just with typical loads, but with the largest load that is likely to be experienced. A safety factor also needs to be added to this. The lift needs to have its safe working load clearly displayed in at least one prominent place. Ideally, the drive mechanism would include a safety cut-out that prevents the lift from operating if it is overloaded.
The size and shape of the load also needs to be considered. The lift table should be able to accommodate the load without any overhang, and preferably a good clear margin around all sides. At a recent installation carried out by sara LBS at the new Leeds Trinity shopping centre, engineers consulted with the stores logistical manager to calculated the expected load size and weight prior to installing 2.5 tonne capacity lift which integrates with a swing lipped dock leveller.
The next consideration is the rise height required. This affects the geometry of the scissor mechanism, so is a fundamental consideration of the lift designers. Scissor lifts typically have a lifting range of less than two metres; it is theoretically possible to go much higher than this, but in such cases an alternative mechanism may be more suitable.
With load and travel defined, design attention turns to the drive mechanism. Typically, this is a self-contained electro-hydraulic solution, comprising of a power pack and the extending hydraulic rams or cylinders. Usually there are two cylinders working in unison. The design of the hydraulic circuitry is such that each cylinder’s movements are perfectly synchronised and the load shared equally between them.
Most scissor lifts use a form of hydraulics where power is used to raise loads, but lowering is achieved simply by opening a relief valve so that the load sinks slowly under its own weight. Obviously, this is an energy-efficient solution, and it is notable that all safety systems will work during lowering operations.
Integrated within the drive system is a number of cut-outs and other safety features including overloads, temperature and pressure monitors for the hydraulic fluid, and sensors to detect the presence of unauthorised objects around the moving parts of the lift.
The majority of scissor lifts will use their own hydraulic systems, but as an alternative, hydraulic pressure may be supplied from a central power pack that drives several machines around the facility. All-electric drives are available for light duty lifts.
Scissor lifts are usually made of heavy-duty steel section, often coated in a strong protective layer of paint, and include additional features. Many of the features will relate to safety, such as safety edges under the load platform to enact an emergency stop in the event of an obstruction being detected. A ‘deadman’ emergency stop configuration is usually included, meaning that an operator has to consciously control the lift’s movements; should their concentration waver the deadman will stop the lift. Safety valves incorporated into the hydraulic circuitry are provided to stop movement (up or down) in the event of hydraulic failure.
Safe operation often requires integration of the control of various pieces of loading bay equipment, ensuring the safe working load of the scissor lift is respected and that safety criteria are met. For instance, the sara LBS range of lifts are all designed to exceed the BS EN 1570:1998 + A2 2009 safety requirements for lifting tables.
Many scissor lifts will also include weighing platforms integrated into their load table. These will provide a safety overload function and can also log loads and feed the readings to production management systems.
It is common for the table to include a ball table, rollers or even a powered conveyor, so that loads can be fed onto the table without excessive physical effort being required of the operators.
The final consideration for lift designers is ongoing maintenance and the availability of spares. Scissor lifts are essentially simple, so a regime of regular inspection and maintenance will be suitable in most situations. sara LBS offers a nationwide 24/7 repair and maintenance service which is operated by trained employees rather than contractors. This includes an emergency call out service. A well-maintained quality scissor lift will have a working life measured in decades, and will pay for itself through efficiency gains many times over.