Hydraulic drive for trailer rear axle
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• A transmission system that provides hydraulic drive to the rearmost axle of an articulated vehicle has been developed by Commercial Transmissions Ltd, 277 Glossop Road, Sheffield SIO 2HB.
Hydraulic wheel-hub motors replace the normal stub axle assembly fitted to the rear axle of a conventional semi-trailer. The traction provided at the rear wheels by this means, claims Commercial Transmissions, not only improves manoeuvrability of articulated vehicles over difficult ground but also, by using the motors to pull the vehicle backwards, eases the reversing of long vehicles.
The motors, which have been developed specifically for this type Of application, are made by Sisu of Finland, and each can have a normal load capacity of up to 12 tons ie 24 tons for the axle. Standard S cam brakes which are incorporated can have optional wire-operated parking brakes.
Adaption of a vehicle for hydraulic drive adds, says the manufacturer, at the most 3.5cwt to the unladen weight of the outfit.
Hydraulic power is obtained from a two-stage hydraulic pump driven from a standard pto — the first stage is for the hydraulic drive system, the second the tipping gear system. A hydraulic storage tank is required which can be fitted, if convenient, in place of the long-range diesel fuel tank.
Hydraulic and mechanical drive systems are synchronized in first and reverse gear; a slight lead of the hydraulic drive system over the mechanical allows the initial starting torque to be taken up by the trailer.
Engine horsepower is divided between each drive system in the same ratio as the weight distribution on the tractive unit and semi-trailer. When the normal mechanical system is sufficient the pto is disengaged and the hydraulic motors freewheel without oil flow or mechanical disengagement.
Control of the hydraulic drive is from the cab and comprises two levers, the normal pto lever and a three-position hydraulicdrive lever having forward, neutral and reverse locations.
The system is not recommended for tractive units with engines rated below 180 bhp unless the overall transmission geat ratio is higher than 90 to I.