Peak payloads without overload
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by Trevor Longcroft
INQUIRIES from readers in recent months suggest that operators are giving closer consideration to built-in vehicle weighing devices — perhaps because of increasing enforcement efforts and some substantial fines for overloading.
During the past two years CM has tested five axle-load indicators. Two companies (Salter and Telehoist) have since ceased marketing their devices. But we believe that operators may find it useful to have the characteristics of the remaining three that are still available. These we have summarized, together with reports on the performance we recorded at the time. Where quoted, prices have been brought up to date.
A point to consider when weighing up the economics of such devices is that they are not only designed to prevent inadvertent overloading, but to enable drivers and operators to check that vehicles are not being wastefully under-loaded for fear of breaking the law.
Smiths Industries Ltd, Commercial Vehicle Special Products Unit, 391 Edgware Road, London W2.
SMITHS uses a sensor or transducer unit to monitor the change of chassis-to-axle clearance with load. The transducer is mounted on the chassis above the axle and is linked to the axle by a lever and follower — the follower bears on a horizontal pad on the mid-part of the axle.
As the vehicle is loaded, movement of the lever rotates the transducer shaft, which is connected via a clock-spring and damper to a resistance-wound bobbin. As the bobbin rotates, a contact arm "moves up" a helix, altering the resistance in the circuit and consequently changing the reading.
The reading head in the cab is calibrated in percentage of plated axle load: graduations range from 80 per cent to 110 per cent: readings below 80 per cent are considered unimportant compared with accuracy at the maximum permitted axle weight zone. The 80 per cent to 100 per cent sector is coloured green, that above 100 per cent is red.
A needle at the reading head indicates axle load and ideally the head should be sited directly in front of the driver to avoid parallax errors.
Once the equipment is installed, initial calibration is completed with the vehicle unladen and stationary. An approximate method is used to position the follower at laden axle height —a simple gap gauge can be made to sit between axle pad and follower, its size determined from a simple spring-rate axle-load calculation. The
transducer shaft is turned relative to the lever to set the reading at 100 per cent (plated axle load).
The vehicle is then loaded so that each axle is laden to between 85 per cent and 110 per cent of its plated maximum — the weight having been checked on a weighbridge. Subsequent fine calibration is then made with the vehicle moving at a constant speed on a flat road, by trimming with a screwdriver the potentiometers in the reading head. The needle is set to the percentage load on each axle.
Smiths supply a separate junction box with trimming potentiometer for trailers. The box is fixed to the trailer and calibration is checked with the vehicle running, but adjustments made when it is stationary — unless it is safe to do so
when moving. No re-calibration is require when swa pping trailers.
Axle readings are taken when vehicle is moving at a constant speed on horizontal road.
A single transducer model suffices fi all applications and lists are suppliE giving the required lever-length/sens position for any vehicle application. 1 mount the transducer, Smiths have number of brackets which are suppliE in a kit with the transducer(s) for particular model or vehicle. The devil does not give gross vehicle weight real ings, but an adhesive label is supplied f each vehicle to give relative percentEq weights on each axle which add up to ti gvw. Thus, by consulting the label, a driv of a two-axle rigid, for instance, with
ear axle loaded to a known percentage )f its plated maximum, will be able to ead the maximum percentage weight le can have on the front axle to avoid ixceeding the gross weight.
Two transducers are sufficient for most ;hassis; a kit for a two-axle rigid ;osts E80, three-axle rigid £96, two-axle ractive unit and single-axle trailer £100 ir £121 with a tandem-axle trailer. A ingle-axle trailer only is £37, a tandemxle trailer £61, tractive unit rear-axle 63, two-axle rigid and two-axle drawbar ,utfit £175. Prices are exclusive of VAT, tting and wire; fitting takes an average of ight to nine hours per vehicle at a acommended charge of between £1822 and can be completed at any of miths 105 fitting stations.
'est results
he vehicle used to test the equipment 'as an Austin FJ plated to 12.5 tons -4.3 tons at the front axle and nine tons t the rear. After each loading, in a series [tests, the vehicle was driven for approxnately one mile and the axle readings iken at constant vehicle speed on a flat bad. One mile was covered because was convenient to turn at a round)out half-a-mile along the road, but miths claim that a distance of 40 yards sufficient to achieve an accurate reading. From the test results only a few errors • oved to be in excess of two per cent id for the gross vehicle load — using le percentages obtained at each axle — le majority of readings were accurate within one per cent.
Maxload: Specialised Instruments Ltd, Bank Chambers, 2 Bridge Street, Leatherhead, Surrey.
TVVO indications of axle load are given with the Maxload device; one a simple dial reading, the other a positive go/no-go signal using a red and green light. As the axle load approaches maximum plated or calibrated axle load the green light illuminates: at plated or calibrated axle weight both lights illuminate and when the axle is overloaded, only the red light remains on. Further overloading of the axle by a pre-determined amount a utomatically isolates the starter soldnoid, preventing a vehicle restart.
The sensor is fixed to a chassis-mounted bracket and a simple lever arrangement, or spring-loaded arm for front axles, converts the vertical movement of the axle-centre relative to chassis into a rotary movement of the sensor spindle.
Because its points are adjustable, the sensor can be bench-set so that the lights come on at axle loadings specified (within limits) by the vehicle owner.
To calibrate the equipment, the vehicle is loaded until its axles are at their maximum plated weight. The linkage between the sensor and axle is then adjusted so that the red light is just flickering — the indicator needle is calibrated simultaneously. Interchange of semi-trailers requires no re-calibration of the sensor.
The cab-mounted instrument head consists of two dials and two sets of lights. Each sensor actuates one needle and the green and red light mounted above it.
When the total permitted axle loads exceed the gvw there is no method of converting readings to figures. It is feasible that the axles would be below their plated weight although gvw may be exceeded. However, once the driver is familiar with the equipment he should be able to make a reasonable estimate with the dials to remain within the gvw.
The price of a two sensor and one reading head kit considered suitable for most vehicles is £84 (exc. VAT), though certain extra bracketry may be required for special vehicles. Fitting takes about six to eight hours.
Test results We used a two-axle rigid vehicle with calibrated axle loads of 3 tons 6 cwt 3 qr at the front and 7 tons 10 cwt 1 qr at the rear. To settle the road springs before each reading we drove the vehicle forwards and backwards for about a yard, braking sharply after each manoeuvre.
Loadax: Loadax Installations Ltd.
Midland Road, Chapel en le Frith Derbyshire.
PEAK INSTRUMENTS of Stockport, Cheshire, has now taken over the marketing and manufacturing of the Loadax axle-load device previously produced by Wirral and has formed a separate company to handle the equipment.
This is the only one of the devices tested that does not depend on axle displacement. Instead, it uses strain-gauge recording of the minute bending of the axles to indicate load. The changes of current are amplified and are represented as an axle-load reading on a dashmounted dial.
One transducer is sufficient for a dead axle, but two are needed for driven axles because the differential housing is rarely located mid-way along it and so each "half" of the axle has different bending characteristics.
The transducer signals are electronically summed. The load on either axle can be read by pressing the appropriate button, or the total weight is shown if no buttons are pushed.
The cab instrument has been designed so that two, three, four or five axles can be monitored.
The legal plated axle weight is shown beneath the appropriate corresponding "axle read" button on the instrument so a driver simply pushes the button and compares his reading to the given plated axle weight. The instrument is heavily damped and the needle takes 20 to 30 seconds to settle, the readings being taken with the vehicle stationary. A knob over the pushbutton may be set to give either a payload or gross weight axle reading. The scale varies in range depending on the total load and may be specified to read in tons or tonnes.
For two or three-axle vehicles a three. transducer kit costs £198 : the transducerE are fitted to the front axle and trailinc bogie axle of a six-wheeler. In a typica artic with tandem-axle semi-trailer ant two-axle tractive unit, the equipment cost! £187.10. All prices are exclusive of VAT Test results Compared with weighbridge reading: the Loadax gave axle weights about 1.l per cent high, with accuracy increasini as the payload rose. The maximum erro of 10.86 per cent occurred at half payloat and as such can be disregarded. Otherwist errors were within 3 per cent, with maim being less than 2 per cent, and close tc permitted axle weights the accuracy wa: better than 1 per cent.