LOAD MONITORING | FEATURE


L


oad monitoring is of course essential. To start with the obvious: it is clearly a good idea to know what is hanging from the


end of your hook or hoist. One of the more basic things to know about it is how much it weighs. Lifting machinery has safe working load limits, and to exceed them is a very bad idea indeed. So, monitoring devices that tell you the


weight of your load are a good place to start. There are very many such devices. Generally, they fit somewhere between the lifting device and the load. Different geometries are available: they can be below-the-hook, in the form of load links, or actually part of the normal lifting


arrangements in the form of load pin or load shackles. Almost (but not quite – see box) without exception they all use the same technology. One or more strain gauges - delicate, thin networks of wire carrying electric current - are incorporated into them, glued on to the pin or shackle. When the support comes under load it stretches, so the strain gauge stretches with it; which changes its electrical resistance and therefore the value of the current passed through it. The change in current is amplified, measured, and sent to the readout device. The heavier the load on a pin or shackle


the greater the stretch or deformation, and the greater the change in current. Calibration of course is necessary, and devices come with appropriate certification from one of the recognised bodies. The degree of accuracy is also an issue: especially since the obvious reason for having load measurement in the first place is to prevent overloading your crane or hoist. You want, therefore, to be very sure exactly how heavy the load you are lifting is - and if you reach or exceed that limit your load monitor can set off alarms, or can stop the lift. Here, geometry plays a part. A lifting link


hangs vertically between hook and load. The force of the load is applied straight downwards; the link just stretches, and it does so in a more or less linear way: double the load and the extension more or less doubles. The strain gauge glued to the link also simply stretches with it.


Data from a load monitor can be sent


wirelessly to a laptop No load monitor means no data


Its resistance therefore changes in a relatively straightforward way (though calibration of course is required.) In a lifting pin, however, the situation


is different. The pin lies horizontally, with a shackle supporting the ends and the load pulling down on the middle. Without further precautions the pin therefore would not simply stretch: it would bend. The lower, outside, edge of it stretches


more than the upper, inner edge. Complicated things are happening, involving bending moments, arcs of circles and possibly compression of one part of the pin while another part is in tension. There are also shear forces, at each


end of the pin where it is supported by the shackle. All of this has consequences for the output of any strain gauges attached. It will be complex and probably not very linear, and probably not very repeatable. Load sensing shackles therefore come with bobbins, which partially resolve the issue. The bobbin is essentially a metal collar that tapers in the middle to give a shallow V-shape in which the load-bearing rope sits. The bobbin fits over the load- sensing shackle pin and can rotate freely upon it. Its inside dimensions align with shear grooves engraved on the load pin. It serves three functions. Here is how manufacturers Crosby


Straightpoint explain them: Firstly, the bobbin spreads the load


across the whole of the pin, to the shear grooves at either side of the load pin, rather than concentrating it in the middle. In doing so the bobbin absorbs the bending moment and ensures that the pin is under simple shear. Secondly, V-shape of the bobbin centres the rope and ensures that the load is always applied to the same


www.hoistmagazine.com | January 2025 | 33


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