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Constant double-drive conversion by Primrose
Page 87
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TPrimrose Third Axle Co. Ltd. is now oting conversions of four-wheeled rigid truck chassis into double-drive six-wheelers. These incorporate a lockable third differential, double drive being in constant use.
The new conversion is being called "Constant Drive" to avoid confusion with the Primrose Selectadrive double-drive conversion in which the drive to the trailing axle can be released.
Unlike other third-axle conversions, two fresh axles replace the original rear axle of the chassis being converted. Leyland running units are used, the drive being divided by a transfer gearbox embodying a lockable third differential. Two propeller shafts from the transfer box transmit the drive to the two rear axles, the drive line for the trailing axle passing through a steady bearing in the casing of the leading axle.
The axles are double-reduction units. Primary reduction is by small diameter spiral-bevel gearing which reduces the size of the differential housing and gives under-axle ground clearance more generous than is usual. The secondary reduction is by epicyclic gearing in the hubs.
Cam-expanded brakes, 15+ in. dia. and 6 in. wide, are used. The whole six-wheeler's braking system is usually arranged so that there is a separate hand control in the cab giving an independent secondary system of braking in addition to the normal footbrake and parking brake.
Not practicable
Ten-stud wheel-fixings make the bogie suitable for six-wheelers up to 22 tons gross weight, and Primrose is prepared to plate chassis at this weight provided the Ministry of Transport Code of Practice on goods vehicle design is conformed with, and provided the outer axle spread is sufficient. Ultra-short wheelbase six-wheelers will not, in fact, he practicable with the Constant Drive arrangement because of the chassis length occupied by the transfer box and twin-shaft drive layout. In practice, the minimum body length is 15 ft. 6 in.
Six-point spreading of load is provided by the four-spring balance-beam compensated bogie suspension, which is of the standard Primrose design except that the balance beam has a 1 to 1 ratio to equalize the load on both rear axles.
The frame of the chassis being converted is flitched at the same time as being adapted to take the double-drive bogie. A 12 in. deep pressedsteel channel is sleeved over almost the full length of the chassis side-Thembers and additional fabricated cross-members, which pick up the full depth of the frame, are fitted. The added weight depends on the wheelbase, but is typically 27 cwt.
With 9.00-20 tyres the list price of the Primrose Constant Drive conversion is £1,390. With 10.00-20 tyres the price is £1,420.
SEMI-TRAILER LINK-UP
A NEW RANGE of frameless heavy-duty stainless steel semi-trailers will go into production later this year following the signing of an agreement between Charrold Ltd. and the Ohio Body Manufacturing Co. of New London, Ohio. The agreement gives Charrold exclusive manufacturing rights in the UK and Europe.