Bridgestone South Africa

Agricultural Tyres Inflation Pressure

Inflation pressures should be checked at least on a weekly basis. Recommended inflation pressures based on total load on tyres should be used. For accurate inflation, use a special low-pressure gauge with 10 kPa graduations. Gauges should be checked occasionally for accuracy. Always use sealing valve caps to prevent loss of air.

A special inflation gauge is available for testing tyres filled with water or calcium chloride solutions. (If a stainless steel gauge has not been utilised, be sure to wash out the gauge with clear water after using on tyres filled with calcium chloride.)

To determine the true operating pressure for a liquid filled tyre. the valve should be at the top of the tyre. Tyres filled with water or air-and-water should be tested when the tyres are cold and before a tractor is put into operation since the pressure rises somewhat as the fluid increases in temperature. A tyre that has sufficient pressure when it is hot may be under-inflated when it cools down.

Tyre overload and inflation

Tyre overload or under-inflation have the same effect of over-deflecting the tyre. Under such conditions the tread on the tyre will wear rapidly and unevenly, particularly in the shoulder area. Radial cracking in the upper sidewall area will be a problem.

With under-inflated drive tyres in high torque applications, sidewall buckles will develop, leading to carcass breaks in the sidewall.

While an under-inflated drive tyre may pull better in some soil conditions, this is not generally true and not worth the high risk of tyre damage that such an operation invites.

Over inflation

Over-inflation results in an under-deflected tyre carcass. The tread is more rounded, concentrating tread wear in the centreline area. Traction is reduced in high torque service because ground contact of the tread shoulder area is reduced and the harder carcass - with reduced flexing characteristics - does not work as efficiently. In addition, the tightly stretched over-inflated carcass is more subject to weather checking and impact break damage.

More tyres go out of service prematurely from under-inflation than for any other reason. An agricultural tyre is irreparably damaged by under-inflation. Certain types of service, in which tractor tyres are used, subject the sidewall to severe folding action when the proper inflation pressure is not maintained. The most common condition causing this type of failure is in disc-ploughing where one tyre runs in the furrow and is distorted by the tilt of the tractor.

The tilt of the tractor causes a sideward thrust of the weight against the tyre which, combined with the heavy pull of the plough, causes the inner sidewall of the tyre to buckle.

Continuous buckling or folding causes cord separation and a series of breaks in the sidewall area. It also causes a series of cracks at the edges of the bars. Sometimes the cracks may extend into the sidewall. Inflation pressure checks are particularly important on hydro-inflated tyres because the air volume is relatively small, and any air loss results in a much greater decrease in pressure than when the tyre is inflated with air only. Inflation influences tractor tyre tread wear. The following are examples related to inflation pressure:

Correct inflation - On the road:
1. Beveling towards the shoulder.
2. Round front.

Over inflation - On the road:
Wear is produced at the centre end of the bars. The front edges of the bars become round.

Under inflation - On the road:
Excessive deflection gives heel and toe type wear, increasing towards the centreline.

Slow Speed Operation

Pressure adjustments require slow speed operation

Higher tyre loads are recommended for intermittent service operations at reduced speed. Under such conditions inflation pressure must be increased in order to reduce tyre deflection and ensure full tyre service life.

Furrow drive wheel tyres

In disc-ploughing operations, where tyres on one side of the tractor are run in a furrow, the inflation pressure in that drive tyre should be increased 30 kPa over the recommended value. This is to compensate for the additional load being carried by this tyre and to reduce sidewall-buckling tendencies under high torque.

Sidehill work

When working back and forth on the side of a hill, the rear tractor tyres will alternately be on the down side. It is recommended that the inflation pressure in both rear tyres be increased by 30 kPa under such conditions. When one tyre is continuously operated in the down slope position, it is only necessary to increase the inflation pressure in that tyre.

Bridgestone South Africa

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