Vacuum lifters15 October 2010
This month, Derrick Bailes, technical consultant with the Lifting Equipment Engineers Association, continues his lifting accessories buyer’s guide series, focusing on vacuum lifters. Continuing the new series of buyer’s guides, so far I have dealt with lifting accessories which make a direct mechanical connection to the load or rely upon friction to grip the load. Another possibility is to attach by adhesion and there are two methods, vacuum and magnetism. This month we look at the options presented by vacuum lifters.
We are all familiar with the simple ‘rubber sucker’ and, in principle, all vacuum lifters work in the same way. A difference in pressure between the atmosphere outside and the air inside the vacuum cup, applied across the area of the cup, produces an adhesion force. For a given pressure difference, the larger the area, the greater the adhesion force. Equally, for a given area, the larger the pressure difference, the greater the adhesion force.
The simplest form of vacuum lifter is the self priming type. In this type the vacuum is generated by a piston and cylinder which connect the lifting eye to the suction pad. When lifting begins, the piston moves within the cylinder reducing the pressure in the cylinder and hence in the suction pad. When the force arising from the pressure difference and the area of the piston equals the weight of the load, the piston will stop moving and the load will lift. By making the area of the suction pad bigger than the area of the piston, the adhesion force is proportionately greater than the weight of the load. This provides a ‘factor of safety’ (or ‘coefficient of utilisation’ as it is often called in current European legislation and standards) and should have a minimum value of two.
As well as the coefficient of utilisation, several other safety features can be designed into the self priming vacuum lifter. Because the vacuum is not created by a continuously running pump, they are only intended for applications where no leakage occurs. This means a non-porous load and a smooth surface finish so that there is no leakage around the suction pad arising from surface texture. Also the pad must be in good serviceable condition. However, from a safety point of view, the possibility of unintentional leakage and loss of vacuum must be addressed. By making the stroke of the piston longer, a small reserve of vacuum can be created. If a leak occurs, the piston will move to balance the loss. By giving a visual indication that the piston is moving and an indication that the end of the stroke is near, the operative can be warned before the loss becomes dangerous. Furthermore, initial movement of the piston beyond the position required to lift the working load limit indicates an overload.
Under normal circumstances, the vacuum is released by landing the load, thereby inherently preventing inadvertent release of the load.
Other designs of vacuum lifter depend upon a continuous source of vacuum. This is produced either
by pumping, by venturi, or by turbine. Pumped systems are the most suitable where a high level of security is required. Usually the pump is powered by mains electricity and, in the event of power failure, there are several backup options. A reserve vacuum can be provided by means of a vessel similar to the air receiver used with a compressor except it operates below atmospheric pressure instead of above. A further backup can be provided by means of a battery to power the pump.
Pumped systems are often used to handle large area sheet material which requires the use of several suction pads to support the load uniformly. Long flexible loads are particularly prone to sagging so a lifting beam may be used to suspend the pads and thereby the load. If incorrectly positioned, some suction pads could be overloaded and peel off the load. With a simple system
of linked suction pads, leakage at one suction pad for whatever reason could cause sufficient loss of vacuum that the others cannot sustain the load. Such problems can be addressed by built in redundancy of two or more independent vacuum systems each of which is capable
of sustaining the load.
There are certain applications for which a particularly high level of security is required. If sheet material is accidentally released from a height, as well as falling it can glide a considerable distance, thereby increasing considerably the area of the danger zone below. Typical situations when such a hazard can be present include construction sites or when ships are being unloaded. An alternative to built in redundancy is a secondary means
of holding the load. One advantage of handling sheet material using vacuum lifters is that the top sheet of a stack without separators can be lifted. The vacuum system can be used to raise the load just sufficient to engage mechanical arms or attach slings or fit a safety net or other device which will secure the load in the event of loss of vacuum.
Pumped vacuum systems are also the most suitable when the load has to be manipulated after lifting. Examples include those designed for lifting glass, rotating it to the vertical and fitting it to its frame.
However careful we try to be, human beings are prone to making errors and it is important to safeguard against inadvertent release of the load. Generally, except in no go areas where persons are excluded or the design prevents release until the load is landed, a two action control requiring the deliberate coordination of actions is specified.
Several other safety features should also be incorporated. Primarily these are indicating and warning devices to inform the user and warn of impending danger in sufficient time to take appropriate action. The indicator should measure the pressure and show the working range and danger range. The warnings should warn of loss of power, switching to battery backup and a reduction in vacuum to the point where vacuum losses cannot be compensated for.
In a venturi system, compressed air flows through a nozzle concentric with the open end of a smaller tube causing a reduction of pressure in the smaller tube which is connected to the suction pad. Effectively it continuously sucks the air out. It is a very simple system with no moving parts but it does rely on the source of compressed air. Compared to a pumped system, it usually works at a lower pressure difference and therefore requires a proportionately larger suction pad for a given load. However, the volume of air it can suck out is also higher and the system therefore lends itself to lifting loads with some porosity or surface texture such as stone or concrete products.
The reliability of the compressed air supply is critical. Although the static pressure may appear adequate, it can rapidly fall off if there is a long pipe run or other consumers take air from the supply. There are two ways of providing backup in the event of a reduction in pressure. One is to have a local pressure reserve tank with non-return valve. The other is a local vacuum reserve tank, again with a non-return valve. Indicator and warning devices similar to those for the pumped system will keep the operator informed.
The turbine vacuum lifter takes the balance between air volume and pressure reduction to the opposite end of the spectrum compared to the self priming type. Essentially it is a large fan integral with the suction pad which extracts air directly from the pad. Because of the high volume of air it can extract, it will handle relatively porous loads such as bales of material.
They are usually powered by mains electricity and backup can be provided either by a battery or an additional flywheel mass. However turbine systems are often used at low level and are controlled by the operator through steering handles. By controlling the maximum height and keeping the operator to the side of the load, this effectively keeps the operator away from the danger zone so a backup is not essential.
With all vacuum systems, the shape of the suction pads should be matched to the load being lifted. Although often illustrated as round, a variety of shapes are possible. If more than one suction pad is required, the layout and working load limit of the pads should be such that the share of the load on each is within its working load limit taking account of the rigidity of both the load and the vacuum lifter.
The European Standard for non-fixed load lifting attachments, EN 13155, covers all four types of vacuum lifter and includes the requirements for all the relevant safety features.
This brief overview will, I hope, illustrate that vacuum lifters can be a very useful way of lifting and manipulating a wide variety of loads. And, provided that the appropriate system is selected and matched to the intended load, they can do so with a high level of safety and reliability.