23.4.
Tyre Valves
A tyre contains the air under pressure, which supports the vehicle load. The tyre valve permits air under pressure into the tyre chamber (formed between the casing and the rim) when required, and releases this air for adjustment of pressure or when removal of the tyre is necessary. The valve stem easily accepts a high-pressure air-line adaptor or a pressure-testing gauge.
23.4.1.
Valve Operation
The valve assembly (Fig. 23.17A and B) has a core-pin, which is fixed to a valve seat. This pin passes through an internally parallel sleeve and the outside of the sleeve is attached to a tapered seal. When the core assembly is screwed into the core housing, the tapered seal prevents any air leakage between the core housing and the valve sleeve. The return-spring normally holds the valve in the closed position, so that compressed air pumped into the tyre is trapped.
For inflation of the tyre, a compressed-air supply pipeline attached to a valve adaptor is fitted over the valve stem so that the core pin is pushed down. Air from the higher-pressure supply is then forced through the valve annular core space into the tyre. When desired pressure is reached, the adaptor is removed, which releases the core-pin so that the valve is closed. When the tyre pressure is high, the core-pin is depressed slightly to release the excess air from the tyre. The outer cap keeps the valve out of grit and acts as a secondary seal for the valve assembly.
Car tubeless tyre incorporates a snap-in valve, which is basically a rubber moulding bonded to the metal stem casing. A recess or groove is formed at the large-diameter base of the rubber moulding, so that when the valve is pulled through the valve hole in the wheel rim, the pliable rubber base snaps into position (Fig. 23.17A and B).
Commercial-vehicle tubeless-tyre valve stem is all metal. The stem is attached to the wheel rim by an extended thread formed at the base and secured by a nut. For air-tight sealing either an O-ring or a flat and flanged rubber washer (Fig. 23.17C 1 and 2) is used. Commercial-vehicle tubed tyre uses a thread-adaptor tube moulded to a circular rubber patch which is turn is vulcanised to the inner tube. The valve-stem casing is then screwed on to the tube adaptor (Fig. 23.17C 3). These valve stems may have a single, double, or triple bend to suitably accommodate different rim profiles and single and twin wheel combinations (Fig. 23.17C).
Fig. 23.17. Tyre valve assemblies. A. Externally spring-loaded. B. Internally spring-loaded.
C. Commercial-vehicle valve shapes and fixings : 1 and 2-tubeless tyres, 3-tubed tyres.
