Clamps can be used to lift a wide range of loads, including sheet material such as steel, or more complex shapes such as various rolled steel sections, tubes, drums or prefabricated concrete construction elements. The clamping force can be produced in a variety of ways. The simplest is by means of a screw thread. Some designs incorporate cams or scissor mechanisms which utilise the weight of the load so that the gripping force produced is proportional to the weight being lifted. Clamps can also be power operated.
As well as gripping the load to lift it, many clamps incorporate the facility to tilt, rotate or orientate the load in some way. This is particularly useful for loads which need to be stored or transported in one plane but presented for use in another.
There are a wide range of standard clamps on the market and specialist manufacturers can adapt or develop their products to suit particular applications. Let us now look at some of the more common types.
Clamps for lifting sheet material are generally referred to as plate clamps and there are two basic types: horizontal plate clamps and vertical plate clamps. As the name implies, horizontal plate clamps only handle plate laid flat. They are therefore always used in pairs, one either side of the plate, in conjunction with a two leg sling. The clamps rely on either a lever or cam to grip the plate and the angle of the sling leg must be such as to give the correct line of pull. Therefore the length of the sling legs must be matched to the width of the plate.
Long plates often need more than one pair of clamps to be adequately supported in which case they are used in conjunction with a lifting beam. Variations of this type of clamp include designs for handling single plates within a range of thickness, to be adjustable to handle several ranges, or to handle bundles of several plates.
The original designs of vertical plate clamps were intended only to lift plate in the vertical plane or lay it down from the vertical in one direction only. Attempting to lower the plate in the opposite direction caused the mechanism to release the plate. They could not therefore be used to turn over plates in a single movement. Another limitation of early designs was that the weight of the lifting machine hook bearing down in the clamp lifting eye could also release the grip. Modern designs are rather more sophisticated and much safer. However they still have their limitations and it is essential to understand them and select the appropriate design for the particular application.
The simplest designs utilise a screw thread which is manually tightened to grip the load. They are particularly useful for applications where the clamp is to be left in place between a series of handling operations. The majority of vertical plate clamps use a cam giving a clamping force which is proportional to the load. Generally the initial grip of cam types comes from a spring operated by a lever which the user flips over into the locked position after positioning the clamp on the plate. Vertical plate clamps can only lift one plate at a time. Generally the line of pull should be square to the plate edge but most manufacturers also make a “universal” type which allows the line of pull to be at an angle to the edge.
Clamps intended for lifting steel plate generally have sharp teeth which bite into the plate, giving a degree of mechanical connection rather than relying just on friction. However there are variations designed to handle plates of harder material or which have delicate surfaces where indentations from the clamp teeth would be unacceptable.
Another variation of the vertical types are clamps designed to be released remotely, typically for applications such as pile pitching.
Other steel sections such as tubes, angles, channels, universal beams and railway lines can all be lifted by standard clamps designed for the particular section. Girder clamps which locate onto the flange of a beam are available either as a suspension point from a steel structure or as a means of attaching to a steel beam in order to lift it.
There are many applications in the construction industry for clamps to handle pre-cast concrete components. Whether it is manhole segments, kerbstones, safety barriers, or any other pre-cast concrete products, there are clamps designed to lift and position them. A frequent problem with such products is that they have no natural lifting points by which to attach slings and they have to be landed in such a way that would trap slings wrapped around them. Clamps offer a method of connecting to the load so that it may be presented tight up to a neighbouring component without trapping the lifting equipment. Moreover, because they have been designed for the particular application, it is relatively easy to locate them in exactly the correct position every time when lifting a series of identical components. The clamp can also incorporate a mechanism for orientating the load so, for example, the facing panel of a building can be transported and stored flat but lifted and turned to the vertical and presented to the building for fixing.
Some clamps are designed specifically as permanent attachments to an hydraulic arm on a road vehicle or site truck. However many similar clamps are designed to work on the hook of a crane. They can be power operated but are often of a simple mechanical design using a scissor mechanism which generates a clamping force in proportion to the load lifted. The self weight of the clamp tends to close it and therefore provides the initial grip. They have to be locked open in order to release the load and this can be by means of either a manual lever or by an automatic latch.
Until relatively recently there were no national or international standards for clamps so they were made to the manufacturer’s own specification. They are now included within the scope of EN 13155: 2003. Although this is the first attempt to standardise them, and may not therefore address every eventuality, its requirements and verification procedures nevertheless include a lot of valuable safety features and I strongly recommend that it be referred to when purchasing new clamps and re-appraising the features of older designs.
Whenever an application involves lifting a series of identical loads it is always worth considering whether purpose designed lifting accessories will be a better option than trying to use standard slings. The above will, I hope, illustrate the possibilities offered by both off the shelf and purpose made clamps. The investment in special lifting accessories can not only improve safety but also more than pay for itself through improved efficiency.
