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Suspension 7
THE OAF FTG 3300 DKX twinsteer 6 x 2 tractor, designed for the new 38 tonnes maximum weight limit, uses air suspension or both the second steer and driving axles. A load transfer device is fitted to control the level of air suspension on the second steered axle allowing maximum traction to be maintained in icy conditions by putting a greater load on the driving axle.
To give ease of trailer coupling, the fifth wheel height may be altered by over 80mm (3in) by operating a height control valve positioned beside the driver's seat.
In a previous article it was pointed out that air springs do not provide or maintain precise location for the axle. Figure 1 shows air suspension on another Daf vehicle.
Air suspension is very versatile, as is illustrated by Rubery Owen-Rockwell's Neway system. With this system the driver can lift one axle, in a multi-axle configuration, off the ground when the vehicle is running empty, resulting in a saving on tyre wear and fuel consumption (see Figure 2).
The trailer deck can be raised, lowered or tilted to match dock or platform heights simply by the use of a switch, with the height of either side of the trailer being adjusted with a corresponding hand valve. This is a particularly useful option with a swop body system (see Figure 3).
In the recent controversy over
maximum vehicle weights and the need for some tolerance in their enforcement the Ministers concerned have advocated the use of weighing apparatus incorporated in the design of the vehicle. The Scale-o-matic axle load indicattor can be fitted in the RO-R Neway air
suspension System. This device automatically weighs trailer loads on RO-R air suspended axles; the unit measures air pressure in the springs that support the trailer load.
This results, it is claimed, in the operator being able to maximise his payloads without the danger of overloading — particularly useful when the trailer is being loaded where there is not a weighbridge facility. The instrument is illustrated in Figure 4.
During this review of air suspension systems the height control valve, or the levelling valves as some manufacturers call it, has been mentioned on several occasions and there is no doubt that this is a very important part of the system.
Rubery Owen-Rockwell in its suspension service manual has this to say about it: "Height control valves are the 'brains' of air suspension. They automatically respond to the relative position between vehicle frame and axle, and meter air to or from the air springs to maintain a constant body to axle relationship.
"Right and left hand height control valves are used on the air suspension system, be it single or multiple axle, and both valves must be attached to one axle only in all cases. They are so installed to isolate all righthand air springs from left-hand air springs, and operate each side independently thus maintaining a side to side tri levelness at all times.
"Off centred loads up to rated capacity will automatil be levelled by different air pressure in right and left hal springs. The valve has a 2-6 second time delay to prever unnecessary actuation whili vehicle is negotiating uneve terrain at operating speeds i so has a lOmm (0.375in) det zone at the lever arm end to eliminate 'hunt and seek' action."
Rubber springs will be de with in the next article.