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25th February 1984
Page 50
Page 50, 25th February 1984 — Th rycle g Yre --- 4cri (0) (0) r gi Th rycle g
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Steering, 7

FIGURE 1 shows a worm and roller type steering box in which the worm is of hour-glass shape, which allows the roller to have full engagement throughout its travel. ZF Steering Gears produces steering boxes of this type and Figure 2 shows a sectioned steering box suitable for private cars and light vans manufactured by this company. They are made in two basic versions: (a) those with double tooth rollers suitable for private cars and light vans and (b) those with treble tooth rollers for heavier vehicles. In either case the maximum usable drop arm movement is 90 degrees.

The worm, see Figure 3, which is connected to the steering wheel by a welded-on input shaft or couplings, is carried in the housing of the steering box on two ball bearings.

The roller, which engages with the worm, is fitted in a forked arm on the cross shaft and is also carried in ball bearings. Polished thrust washers cater for the axial forces on the roller. The cross shaft, or steering roller shaft, is carried on bearings in the housing and housing cover. It is supported in the housing by a shaft spigot which also acts as an adjustment device. The adjustment screw is easily accessible, so free play that may occur can be easily removed.

When the load on the steering axle or axles is high, the steering becomes unduly heavy, making the driver's job tiring and difficult. Some improvement can be made by lowering the steering box gear ratio but there is a limit to how low a gear ratio can be employed without the number of steering wheel turns from lock to lock becoming excessive.

Most modern heavy commercial vehicles, and luxury private motor cars, are fitted with power-assisted steering to help the driver turn the steering wheel. This is particularly beneficial when the vehicle is travelling at low speeds and being manoeuvred in confined Power steering must have a fail safe characteristic, that is if the power system fails, the driver must still be able to retain complete and effective control of the vehicle. This feature also allows a broken-down vehicle to be steered when it is being towed, even if the engine is not running. The degree of assistance must be proportional to the effort applied by the driver at the steering wheel and he must be able to retain the "feel" of the wheels.

Generally the power assistance system consists of an engine-driven oil pump which supplies oil under pressure to one side of a piston when the steering wheel is turned. The side of the piston to which the oil is directed depends on which way the steering wheel is turned. The piston is connected to the steering mechanism and gives the assistance which is required.

There are two different basic types: one where a power steering ram and valve assembly is fitted between the steering box drop arm and the drag link, and the other a fully integral system where the valve and power unit is incorporated in the steering box itself. The former system is now almost obsolete although it could be used as an "add on" package to turn a basic manual system into a powered assisted one. Both systems will be described in future articles.

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