Being a runway, once positioned, the load can only be lifted, travelled and set down along the line of the runway. With a few specialist exceptions, general purpose mobile gantries are only intended to be mobile when unladen. They cannot safely be moved under load.
There are many variants of mobile gantry, with the most obvious variables being the safe working load, span and height. However there are several other features which should be considered. First, it should go without saying that the joint between the runway beam and the leg must be sufficiently strong and stiff to prevent collapse.
The simplest and most effective method of bracing any structure is to triangulate, that is to include an angled brace between the beam and the leg. This can be done either on the inside of the leg (internally braced) or on the outside of the leg (externally braced). Both options take up a certain amount of space.
The advantage of the internal brace is that it keeps the overall length of the gantry to a minimum – the disadvantage is that it reduces the amount of travel available along the beam. Conversely, an external brace maximises the available travel but requires additional space outside of the legs to accommodate the brace.
Another method of bracing is known as a box brace. This is slightly more complex but essentially it does the same thing by integrating the brace into the shape of the leg. The advantage compared to the simple angled brace is that it takes up less space and is a good compromise if both the amount of travel and overall size are critical.
A simplified version of the box brace is known as a rigid or splice brace. This is particularly suitable for smaller, lower capacity mobile gantries and again is a good compromise if the combination of movement and size is critical.
Another feature to consider is adjustability. For obvious practical reasons, it is not possible to make the span adjustable. However, adjustable height gantries are feasible, and available as a standard product. It can be a very useful feature either to maximise the height available without the expense of a bespoke design, or if the gantry is to be used in a variety of locations with differing height requirements.
Most designs involve a telescopic leg and there are two basic variants. The simplest requires the beam and leg to be lifted by external means then locked in place with a load bearing pin. Except for the very smallest gantries, the weight of the beam and moving part of the leg is more than can safely be lifted by human effort. Such designs are therefore only practical for long term installations or where some other means of mechanical handling is available to set up the gantry. The other variant includes a geared mechanism to allow the operative to crank a handle to effect the adjustment.
This is an opportune point to mention two more useful variants, the foldaway or demountable gantry and the self erecting gantry. As the names imply, the foldaway or demountable can easily be dismantled for transportation and storage. However as each element has to be manually handled, there are practical limits to the size and capacity available.
Its big brother is the self erecting gantry. Generally these comprise five major elements, the runway beam and the two legs – with each leg consisting of two parts connected to the beam by hinges. They are assembled flat on the ground in the shape of the letter H, with the runway beam acting as the crossbar. Integral hand operated winches pull the two parts of the legs together and raise the beam. Once erected the legs are structurally locked.
It has the advantage of portability combined with ease of erection without additional mechanical handling equipment. However it does require sufficient flat space on a suitable surface to lay out the elements and erect it. Remember also that a similar space must remain available afterwards in order to dismantle it.
General purpose mobile gantries are usually mounted on castors. Older models tended to use metal wheels, whilst modern ones typically feature wheels with solid plastic tyres which are kinder to the floor. Both types require a smooth level surface free of debris.
As anyone who has pushed a supermarket trolley will testify, it takes only a very small piece of debris to jam a wheel, an important point to remember when positioning the gantry prior to use. Being a tall narrow structure, it can easily tip if a wheel is suddenly obstructed.
Also the surface must be reasonably level, partly to ensure that the runway is horizontal but also to ensure that the structure is loaded in the way designed and not at an angle.
Once in position it is generally desirable to prevent the gantry from moving and there are two basic methods of achieving this. Some manufacturers fit parking jacks.
These are usually situated inboard of the castors and are simply screwed down until they contact the floor, thus preventing any movement.
Unless specifically designed for the purpose, they are not intended to take the weight of the gantry and load nor to level the gantry. Gantries with that facility are available but they are not usually classed as general purpose.
Another method of preventing movement is by using castors with integral brakes. These are manually operated spring loaded levers which are flipped down and engage onto the tyre.
A final consideration is the classification or duty of the gantry. Unfortunately, there is no standard specifically for mobile gantries to which the purchaser can specify. The best guidance to manufacturers lies in a combination of the crane and runways standards.
Within British Standards, those referred to are generally BS2853 – the Design and Testing of Steel Overhead Runway Beams, and BS 2573 – Permissible Stresses in Cranes.
Manufacturers often describe their products as light duty, medium duty or heavy duty. Usually it is an indication of ultimate life expectancy but there is a practical aspect to this. Heavy duty equipment generally weighs much more than light duty equipment and this may be a disadvantage when considering ease of mobility.
However as with slewing jib cranes which featured in the first article of this series, light duty gantries are generally more flexible than heavy duty ones. If the application requires precision positioning of the load, a bouncy gantry is not as user friendly as a more rigid one. Selecting the most appropriate lifting machine is equally important and will be the subject of another article.
