So what exactly does ‘portable’ mean? Like most words, it becomes hard to pin down where you try to be rigid about it, but as a guide a portable crane must be able to be carried, or at least pushed, by one person, or at most two, from place to place. It should not have its own power source for movement.
And what do we mean by ‘from place to place’? Is it simply around a workshop, or does it imply that the crane can be put in the back of a van truck and driven from site to site? Not to be proscriptive, let us go for both.
So a gantry crane on castors, as used in a huge number small workshops, autorepair shops and the like, fits that brief perfectly. The design is usually simple and fairly standard: an I-beam is supported at each end by an A-frame, and the A-frame is on small wheels. The I-beam supports a trolley and hoist – the hoist can be a manual chain hoist, or an electric one pendant-controlled and supplied with power by some sort of trailing cable. The frames and beam can be of steel or of aluminium – light in weight being an obvious advantage in something that is going to be pushed by people. Capacities tend to range from 250kg to about 5.0t, after which the design becomes too heavy to move by manpower easily. The metalwork is not too difficult or complex to make, which is probably one reason why many different manufacturers make them, buying in the one complicated part, the hoist, from specialist manufacturers or selling the frame without the hoist as ‘lifting gear sold separately’. The gantries come in different heights and widths – some are extendable in one or both of these dimensions. Some are designed to disassemble easily for storage when not needed. Bespoke versions are also available – for example, for low headroom with maximum lift heights.
Abus, for instance, make portable cranes to order, to the width and height that you specify, up to a beam length of 7,900mm (depending on the load capacity) and a height of 5m; the maximum load capacity is 2.0t. Rather than A-frame legs their design is an inverted ‘T’, and it is possible for one person to push it to its station by hand.
They come equipped with standard Abus hoists, of which thousands of units are currently in use – the company’s electric chain hoist is available with either a manual or an electric trolley, and can have the power supply as either a festoon cable or a conductor system.
Crucially, some portable gantries are suitable for being moved under load on a smooth floor, though most are not.
One company that does make moveable-under-load portable gantries is Hoist UK. Its welded portable gantries can be made to that specification if required. It also makes modular steel mobile gantries, which are for ‘self-build’ assembly and erection. They are of lighter construction for easy handling, and arrive as a flat pack, which helps to reduce transportation costs, but they cannot be moved under load. James Jordan, business development manager of Hoist UK, explains that it isn’t a matter of the strength of the joints.
“It is all in the design,” he says. “There are criteria for moving under load that are part of our safety assessment. For example, the casters we use are different: they have to be rated for dynamic forces as well as static ones. It is also about how such hoists are used. We have a specific instruction manual for the movable gantries: there is a list of precautions and of things that you should check before you move a load. So the difference is in the casters, it is in the design, and it is in the end user instructions as well.”
The beam and the A-frame legs are basic components of a portable gantry, but, as we have said, there is another: the hoist itself. Sometimes the manufacturers supply it, sometimes customers buy their own from another source.
“We can supply any sort of chain hoist, manual or electric,” says Jordan. “We offer Verlinde ones as standard because we are the sole UK distributors for Verlinde. For electric winches we have three different power supply options: you can have a mains isolator switch mounted to the leg and the hoist comes wired to that and ready to go. The power can come by cable, or by reel-in drum.
“Spans are up to 5.0m, and capacities are generally up to 2.0t for the modular version and up to 5.0t for steel, though we can go higher and we are happy to customise.
“We have one crane that has just gone through the workshop and is ready for dispatch now that has a 10t safe working load with a 1.0t auxiliary hoist on it. What the client is going to be doing with it is positioning the gantry over a machine. Typical day-to-day operation is feeding the machine, with loads up to 500kg, so they’ve got the little 1.0t electric chain hoist to do that. But every now and then they might have need to lift the entire machine, so they have the bigger hoist for that, and the beam is rated safe for both. So they are getting two for the price of one there really.”
TWISTED
An interesting variant on the theme comes from UK-based Shearforce. This is a fairly new company, formed about five years ago, and both it and the variant came about from the owner’s own need.
“I also run a transport and haulage company,” says Ray Simmons, “and a lot of the warehouse was inaccessible to a standard portable gantry because it couldn’t get near to the walls. Many others have the same problem, especially if they have machinery or concrete piers that get in the way.”
His solution was to make a gantry with a base that can be twisted to an offset: the A-frame legs can be rotated, about a vertical axis, by up to 45°.This lets the crane get closer to walls and into tight corners and the like.
“There was nothing it on the market, so we made our own,” he says. The device is patented – UK patent No: GB2606690, if you want to know the details – and comes in versions with safe working loads of up to 3,000kg.
It has another clever feature as well. It is mounted on castors, so it can be pushed around the shop floor, but once in position the wheels can be folded into the frame so that it is the flat undersurface of the frame that is in contact with the floor.This of course, spreads the eventual load over a much greater area and avoids point loads on the floor, which might otherwise damage a concrete or similar surface.
‘Portable’, as we have said, for our purposes also include ‘transportable’. There is a widespread need for lifting apparatus that can be put into the back of a truck, taken to a job to be unloaded and erected in minutes, then at the end of the day dismantled again and reloaded into the truck. The shearforce gantry fits this need also. Its three parts – the beam and two legs – unbolt easily. And here again some imagination has been at work.
“I saw how emergency services – fire and rescue people – have the apparatus they need in individual bags in their vehicles, so they can be brought out immediately they are needed,” says Simmons. “So I copied that idea: each of the three parts has its own red PVC bag, and the heaviest of them weighs 60kg – that is the steel version; the aluminium one comes in at even less, 50kg – so can be easily handled by two people. And to make it even easier, the bags have wheels at one end.”
A common issue with portable gantries is that to erect them you have somehow to lift the steel girder to its final height. This usually needs some kind of lifting gear – a piece of circularity that is somewhat unhelpful, given that the gantry is supposed to be the lifting gear you want. Simmons has thought of that one also: “I explain to clients that they can mix aluminium and steel components – that they can have steel legs with an aluminium beam. That way, the beam is light enough to be erected without a forklift or hoist.”
Shearforce gantries come in sizes up to 3.0t capacity.
SMALL MOVEMENTS
Wallace Cranes, in the US, has been making portable gantries for more than 60 years from its base at Malvern, Pennsylvania, and claims to have more gantry cranes in use than any other manufacturer. It has over 400 models. The company, too, shows originality in design thinking: it developed its original thrifty and Tri-Adjutable gantry crane patents in 1954, and the system is still being used: aerospace, education and the military are just some of the applications. Key to their design are patented pin joints with a very elegant self-centring beam support. Instead of complete rigidity, which when unevenly loaded concentrates stresses at a few points, the Wallace cranes have a ‘pinned-atboth- ends’ four-bar linkage design that compensates for torsional or off-angle loads by allowing small movements that redistribute the stresses evenly between all the members. Pin joints are central here: a pin joint will transmit only straight-line forces, not torsional or twisting ones, and they allow the frame to flex so that all four wheels are always in contact with the floor. This allows the crane to handle uneven floors. The frame flex also ensures that the load is distributed among all the casters. Even if the load is entirely on one end of the crane, the load is evenly distributed between those two legs.
The pin joints also make assembly easy – a 16oz (0.5kg) hammer is the only tool needed, and parts rotate to make alignment simpler. The assembly instructions run to only two pages, which is something that Wallace Cranes is proud of. Reducing the stresses in the frame also has the effect of increasing its useful lifetime. All welded joints in the frame are in pure compression only, which has a similar effect. Capacities are up to 10 ton, or higher for made-to-order cranes, and the range includes the Tri-Adjustable (height, span and castor spread all adjustable), the Thrifty (fixed or adjustable height, with greatest clear span), Hippolift (economy alternative to the Tri-lift, fixed height, adjustable span) and Mighty-Mite (workstation crane for lighter materials, adjustable height and span).
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