Weighing the Weighwrite way
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By Trevor Longcroft
• Because the single-axle weighbridge made by Weighwrite Ltd, White Waltham Airfield Estate, near Maidenhead, Berks, which is used by the rDepartment of the Environment for roadside vehicle checks, has been confirmed for accuracy (CM December 22), I decided to give it a practical test. I found it simple to use and for my money it offers the operator a transportable and accurate alternative to the permanently sighted, and more expensive, conventional weighbridge.
I visited Oakleigh Animal Products at Ascot where I was able to compare vehicle weights taken on a conventional weighbridge and a Weighwrite unit which has been in service for approximately two years (see table). The standard Weighwrite appm reads axle loads statically; incorporatit a further module in the control ca enables axle loads to be read whit( vehicle is moving over the bridge. latter method which indicates axle Wax a printed read-out sheet is recomme for vehicles fitted with corn pens suspension bogies; static tests by company showed individual bogie readings to vary by as much as 2 tons.
The vehicles used for the test two-axle tractive units coupled to tan axle trailers. The sum of the axle rem from the Weighwrite unit were comr to the gross vehicle weight obtE from a conventional weighbridge which axle readings were not ra :curacy could not be guaranteed the bridge approaches were not tal. The attic was driven over the with the engine at tick-over speed first /crawler gear engaged. The e deflection of the instrument was though a unit capable of handling ids of up to 20 tons is available.
tch test the sum of the axle readings wer than the gross vehicle weight he conventional weighbridge, the an error being 3.2pc (see table). company claims an accuracy of one it per axle at full scale deflection t 10 tons). Individual errors need not positive or negative; therefore the tive error resulting can be small. apparatus comprises three pieces: a ig beam, control cabinet and transrailer.
site requires only a concrete-faced /.Sin.-deep, constructed to the recom I dimensions, to accept the 2.5ft337 10ft-long weighing beam. A single-axle drawbar trailer is • lower /raise the beam from the and transport it between sites.
control head, normally fitted to the can operate from a 230V 50Hz al supply but 12V and 24V systems liable.
beam, which weighs about a ton, n four hydraulic load cells, one to :orner. These cells are connected dinally in pairs to two pressure leers. An applied load to the bridge a pressure change in the cell which vetted by the transducer into an al signal which is fed into the control via cable.
dready mentioned, the standard basic =sures static loads only; the reading n from a meter. This reading method is considered more accurate than the dynamic (for vehicles without compensating bogies) because of the two-stage reading technique employed.
The meter has both an upper and lower scale, the upper scale deflection being 2 tons, the lower 10 tons. For the first stage reading an approximation is taken on the lower scale. Suppose the indicated load was between 6 and 7 tons. A tonswitch indicator is set to 6 tons and the excess load can then be read from the upper scale with greater accuracy because full deflection is 2 tons, not 10 tons. With use, readings to the nearest lqr should be possible.
The addition of the dynamic read-out module to the control cabinet does, in effect, translate the meter reading on to a read-out sheet. The paper is divided into 1 ton units which are further sub-divided into 2cwt units, Again, with use, an operator should be able to read to the nearest lqr.
At a pre-set bridge load, the paper is automatically set in motion travelling at 2cm per sec; a pen travelling at right angles across the paper plots an axle-load diagram (see diagram).
The control cabinet and weighing beam are a matched pair set before delivery and should not be interchanged. When setting up the apparatus a pre-weigh check, which takes only a few minutes, should be completed after the machine has been allowed to warm up for five minutes. Periodic checks should be made throughout the weighing programme.
The basic apparatus for the static weighing of axle loads costs £1,450, the dynamic reading module a further £432. The purpose-built trailer, which completes the transportable kit, is £540.