Monitoring loads: Made to measure28 February 2023
In the age of big data, load monitoring can tell you all you need to know and more about what is hanging on the end of your hook. It can also tell you what has hung there before, for how long, how far it travelled, and what the total strain on your hoisting rope has been. Julian Champkin reports.
There are load pins, load links, load cells and tensometers; there is wired monitoring and wireless monitoring, and not merely of current load on the hook – they can be of peak loads and of cumulative loads as well, summed and averaged over any time period that you fancy; and you can be informed of times suspended, speeds of travel both horizontally and vertically, accelerations, decelerations, and time standing still.
All of this is part of the collection of data. You can read all of this data, in real time, on your smartphone, tablet or on your head office laptop on the other side of the continent. And you can analyse it with software that will tell you how long it will be until you have to replace your wire rope or the bearing on the trolley wheel of your hoist, plus it can add that “oh, by the way, it is time that you lubricated that main shaft because it seems to be running a little big hotter than it ought to”.
This is all part of Industry 4.0, which is all about data in huge quantities – obtaining it, analysing it, acting on it. Without Industry 4.0 no self-respecting operation can call itself efficient or can tell where those marginal – or possibly major and fundamental – changes need to be made that make the difference between profit and loss. And hoisting and lifting, and the monitoring and data gathering and analysis thereof, is an integral part of Industry 4.0.
Few have been in the business of load monitoring devices longer than Jim Murray, currently CEO – semi-retired, he says – of JCM, which from its base in Aberdeen, Scotland, makes, sells and rents such devices. He began in the business more than 30 years ago. He gives Hoist a guide to the past and current state of the technology and the market.
For our purposes we can divide load monitoring devices into two geometries – load links and load shackles – as well as two data transmission methods – wired and wireless – with self-indicating ones, which have the readout on a display panel on the device itself, as an extra.
“Wireless transmission of data has become so reliable and so cheap that there are very few wired devices nowadays,” he says. “Back when I started, the first wireless devices had antennae sticking out of big covers that had to house huge batteries, and if you were lucky they had a range of 10m downhill with the wind behind them as long as there was no interference nearby. But now, 600m reliable range is standard, so there is little reason why anyone should want a wired device.”
Dealing with geometries first, the main distinction is between load links and load shackles. Load links, says Murray, are used more at the lower end of the capacity range. “With a load link, strength-to-weight ratio is important. We use aircraft aluminium to give a good value there: it is not too heavy, but it’s strong.
“For larger-capacity loads you would use a shackle. One reason is that to give a 5:1 safety margin a load link starts to become crazily big and chunky, and since you need a large shackle at each end of it anyway to fix it to the hook and to the rope, the whole thing starts to get out of hand.”
A load shackle replaces three large items with just one. The company’s load links start at 2.5t capacity; its shackles start at 6.5t and go up to 500t.
“The disadvantage of shackles has been that in the past they were less accurate than load cells.” The reason, he says, comes down to geometry.
Both load links and load shackles have the same essential technology at their hearts: it is a strain gauge that consists of a fine grid of wires, the resistance of which changes under stress. “It is a very well-proven technology now; temperature variation does affect them, but accurate calibration now can account for that,” says Murray.
“In the load link the strain gauge is vertical, in the direction of the load, so measures it directly. In the shackle, the measuring gauge is in the pin of the shackle, which lies at right angles to the rope and the vertical load; it essentially measures a shear or bending stress, and the orientation of that pin is crucial.
“It is imperative to line that pin up correctly. We put in an anti-rotation pin at either end of it, to hold it in the same place, in the right orientation in the shackle, with absolute repeatability. The output reading a very sensitive to that: it varies with the cosine of the angle, so a small change in the angle gives a big change in the reading.
“A problem I have seen with others is that the pins are not substantial enough, and permit some rotation, which is why load shackles in the past have been less accurate than load links. And in load monitoring, repeatability and calibration are all-important.”
Calibration and repeatability are, of course, different things. “Both have improved greatly as electronics have developed so dramatically over the years,” says Murray. The output graph, of current from the measuring device against the load it carries, is what needs to be calibrated, so that you know what load a certain number of milliamps corresponds to. If the graph is a curved one, this involves taking very many calibration measurements to establish it. A straight line, on the other hand, needs only two points to define it.
“There is now a slight curve on the output from load pins,” says Murray, “so calibration has become very much simpler. “All load shackles are supplied with a calibration certificate and proof test certificate, but we can also supply instructions to the user on how to recalibrate them. The calibration is via the handset, which means that it can be done anywhere.”
Complex lifts, and the demands of big data, can require input from several different monitors, and they may need to be displayed and viewed simultaneously. Nowadays, this is generally on some kind of handheld device, and as important as the monitoring devices is the software that receives and analyses the data. JCM’s offering is the ALRS2, the handset mentioned above, which can take and display information from up to six separate monitors.
“It has two displays that show simultaneously: one shows the current load, the other the peak load. It is very important to have both on view,” says Murray. The cumulative effect of past overloads or near-overloads can weaken the wire rope, he adds, especially at its end where it enters the eye: “The wire can be half its normal thickness at that point because of overstressing.”
For more complex lifts still, JCM provide software that can handle up to 99 monitoring units at once.
“Big data and Industry 4.0, of course, have increased demand for monitoring of all kinds, and the progress of electronics has made the devices progressively cheaper,” says Murray. “The main problem now is the chips they use – not the price but the availability. Thanks to the pandemic and Russia’s invasion of Ukraine, they’re in short supply globally; they are the same type whose shortage is affecting carmakers and the like all round the world. We are fortunate we have a good supply of them in stock – occasionally, we have had to resort to workarounds for a short time, but we can produce to need now.”