Axle weighing without guesswork
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by Gibb Grace • An inexpensive, reliable and remarkably accurate device for indicating axle load, which I have lately tested, is known as the Loadax system. British throughout, it is now available from the designermanufacturer, Wirral Automation Ltd, Wirral House, Clayhill Estate, Liverpool Road, Neston, Cheshire (051-336 5151).
The system, unlike some others, does not depend on axle displacement, but uses the bending of the axles themselves to indicate the load. This bending, which is minute even under full load, is sensed by a transducer — a thin metal strip which has bonded to it four very small strain gauges, two on each side. The strip is mounted in line with, and parallel to, the axle and, depending on whether it is located above or below the axle, it is stretched or compressed when load is applied.
The action of straining the strip, and thus the gauges, alters the tatters' resistance to an electric current passed through them. Although these changes are slight, they can be amplified and made to work a dashboardmounted indicating instrument which shows on its dial what load gives what movement — and the full range of axle loads can be catered for.
With the Loadax system reliable strain gauges are completely encapsulated, firit in silicon rubber and then in a casing of glassfibre-reinforced polyester resin. Even the connecting cable is bonded in, so there is no likelihood of performance deterioration due to rain, mud, oil and so on.
Only one transducer is required for nondriven axles but two are needed for driven axles. This is because the differential housing is never central and the two halves of the axle have different bending rates.
The signals from the transducers are added electronically so that the sum is always proportional to the load, no matter how the axle is loaded. In the case of a normal fourwheeler three cables run into the cab instrument. Here the load on either axle can be read, as desired, by pressing the appropriate button. The total weight is shown if no buttons are pressed. The cab instrument has been designed in such a way that two, three, four or five axles can be measured.
Simplicity itself From a driver's point ofview the instrument could not be easier to use. Under each button is shown the legal plated weight of the relevant axle and the driver simply has to press the button to see if the axle is overladen. The instrument is heavily damped and although the needle swings over briskly at first, it takes only 20 to 30 seconds to settle at the correct reading. A knob over the button may be turned to give either a net or a gross reading.
When the equipment is first fitted the unladen weight on each axle is determined and this amount is set electrically in the instrument. It is then subtracted automatically from the gross load whenever the knob is turned to "net". Thus, by setting each knob to "net", the instrument will read out payload. The scale varies in range, depending on the total load, and may be specified to read tons or metric tonnes.
The equipment with which I carried out a range of tests was fitted to a two-axle 16-tongvvv Seddon and, adjacent, are shown the results obtained. As a normal balance weighbridge was not available Wirral Automation's own prototype strain gauge single-axle weighbridge was used as a check throughout. To satisfy myself that the small weighbridge was indeed accurate, I watched it being check-calibrated with attested 500 Kg weights from Cheshire County Council's weights and measures department.
The Loadax, as fitted to the Seddon, was calibrated in metric tonnes and the dial divisions were at 0.5 tonne (500 Kg). The driver had to estimate lesser amounts. At the very worst it is possible to estimate a quarter of a division (125 Kg) and, with practice, estimates to a tenth of a division (50 Kg) are not unreasonable.
The WA weighbridge had an electronic display of the imposed load on a digital voltmeter, reading three decimal places, i.e. to the nearest 100 Kg. With these restrictions it would be possible to get 150 Kg variation between Loadax and weighbridge for the same applied weight. As the accompanying results show, considerably better correlation than this was obtained, the average error being, in general, about 60Kg.
Compared with the weighbridge, the Loadax gave axle weights about 1.5 per cent high, with the accuracy increasing as the payload rose. At six tonnes payload, for example, the errors were nil and less than one per cent; the total vehicle weight was accurate to better than 0.5 per cent.
A two-axle installation costs £88 and fitting and calibration adds £10.
Loadax is still very new and very little operating experience is as yet available, but Mr Stanley Crompton, whose Seddon we used for the test and who is an owner-driver working in and out of a local quarry, told me that the system fitted to his vehicle had stood up to six months of this work, including high-pressure washdowns, without causing any problems.
The Loadax would appear to be a very significant. breakthrough. At any price it would be a desirable acquisition; at under £100 for a four-wheeler it becomes an extra well worth serious consideration by operators concerned to keep strictly within the law. The only limitation which „comes to mind is the lack of "beam" length on some driving axles where springs virtually abut cliff. casings.