How do you lift a large pane of glass? Very carefully; as the punch-line goes. Glass has a polished surface which makes it perfectly suited to be lifted by a vacuum, below-the-hook lifting device.
Below-the-hook lifting devices, as described in ASME B30.20, are designed to make rigging safer and more efficient. There are two devices that have the advantage of being able to lift loads without the need for eyebolts or welded eyes. Since these devices have no physical connection to the load, they can be dangerous. How does an advantage become a disadvantage; let's look at the two devices.
First is a vacuum lifting device and used when the load has a smooth enough surface allowing the device to create a suction sufficient enough for lifting. Devices are equipped with a vacuum pump and one or more suction cups or rings. If the pump stops, suction stops. If the ring seal goes bad, the load falls. To prevent the load from releasing, the pump and suction rings have to be serviced and tested daily. A common way of testing suction rings and pumps is to lift a test load. Test loads would be heavier than any intended load or equal to crane capacity.
The second device is a magnet or magnets that lift any ferrous metal. This makes it possible to lift steel, in all forms, before it is converted into a product. For light loads a permanent magnet can be used but for heavier steel, electromagnets are used. Electromagnets require high current electricity and can become dangerous. Magnets often hang from the crane hook and require an electrical cable to energize them. If the cable gets unplugged, or cut, the load will drop. If a rigger wants to disconnect the magnet from the hook and unplugs the magnet while it is energized, they could get burned from the electrical arc. There is a special crane operator signal used to prevent this hazard to the rigger. If the operator leaves the magnet on to long, it will heat up and loose magnetic energy, dropping the load. Magnets should be tested during the operator’s pre-operational inspection by picking up a test weight.
Rigging Equipment Inspector Training Program
to learn more about the inspection process of Below-the-Hook Lifting Devices and other rigging equipment.