Jul

Cap that valve

2011-07-01 15:11
The right way - The extension to the valve on the inner tyre on this dual set of tyres made pressure checking easy for author of this article Dave Scott (right). However, having an extension is not the beginning and end. You actually have to check the pressure. This inner tyre was totally flat which resulted in the driver having to change the tyre after it was pointed out to him.

When inspecting a tyre, many will look at it in its entirety , if you will excuse the pun. However, there is one component of the tyre that needs closer attention for it plays a huge role in the lifetime of that tyre. It is the tyre valve and its cap. In this article, FleetWatch technical correspondent Dave Scott highlights this vital component.

A simple definition for a tyre is a container , a pressure vessel , that stores air under pressure. Its only feature that allows pressurised air in or out is a valve that is fairly complex in design, using springs, gaskets, stopper, shafts and containers. This means that contaminants and dirt ingress can easily result in a sticking valve and cause a fast or slow pressure drop.

The wrong way - This photograph shows a missing valve bracket.

The wrong way – This photograph shows a missing valve bracket.

Road detritus , debris, mud or grit , can get caught in the open valve head shaft entrance and this gets forced into the valve as soon as tyre pressure checks are made resulting in slow, or even fast, air leaks. Where winter roads are ‘˜salted’ to de-ice the surface , yes, it has been done in South Africa – rock salt and other chemical winter de-icers used are very damaging to valve brass components. Is the valve accessible? So many inner tyres on a drive axle or trailer dual set do not have extensions to access the valve.

The wrong way - This photograph shows the valve cap rubbing on the rim as a result of a missing valve bracket.

The wrong way – This photograph shows the valve cap rubbing on the rim as a result of a missing valve bracket.

More often, the inner tyre has been fitted in such a way that the valve stem is totally hidden so that it just cannot be checked. An obscured valve inevitably means there will be no valve cap as well. From a tyre audit viewpoint, if the valve cannot be accessed, the tyre must be marked as having no pressure because it just cannot be checked.

Written policy
Tyre valves don’t last forever. Bridgestone recommends that best practice is to replace the valve every time a tyre is replaced. This, of course, applies to tubeless tyres where a valve is a separate part attached to the rim and not an inner tube. Tracking valve replacement must be part of a tyre incident analysis report while valve replacement must become part of a written tyre policy and fitters job description. If there’s no protective valve cap, all one sees is the valve core shaft peeping out of the valve entrance , that’s a problem!

Not realising that a tyre valve is constructed of a number of components, the incorrect assumption is that it’s robust and dirt does not matter. Not so. The valve cap is there to protect the valve from corrosion and dirt with the ultimate aim of keeping tyre pressure at a consistent, recommended level. The outcome extends into ensuring road safety, longer tyre life, lower fuel consumption and less downtime. There’s a cascade of consequences when valve caps go missing.

Operating guideline: If a valve cap is missing, you can be sure there’s dirt in the valve and it’s advisable not to check air pressure that could result in a run-flat tyre on the road.

Not all caps are the same

This valve cap with a red seal insert can withstand high temperatures.

This valve cap with a red seal insert can withstand high temperatures.

There are valve caps and then there are ‘˜valve caps’. The vast majority of tyre valves are equipped with plain black plastic caps that only serve to keep contaminants out of the valve stem. A black plastic cap usually cannot stand the heat generated on the road and is easily subject to over-torque when screwed on, or being screwed against the thread. Metal valve caps usually have a small rubber insert to permit a good seal against the valve body while this type of cap also helps to prevent air escaping from a slightly leaking valve.

Not all metal caps are the same. Some metal caps have a short shank that makes them difficult to screw on which in turn means the cap is abandoned through frustrated neglect. Normally a steel valve cap has a black seal insert that is suitable for ‘˜normal’ ambient temperatures and one can get a steel valve cap with a red seal insert that can withstand very high operating temperatures. There’s a choice so standardise on the best suitable for the task!

valve-5When walking through a parked fleet, observe the number of missing tyre valve caps. Success in road transport is all about attention to detail and discipline and missing caps are a serious indicator that there are many more overlooked fleet disciplines. Fleet supervisors fail to see the connection between missing tyre valve caps, energy wastage, road safety and expense control.

And if management don’t care why should ground staff bother? The missing or leaking valve cap will affect a driver on the road and they must be trained into the reason why this too is reportable on daily log sheets and an item they must look for in daily checks. The devil and savings are in the detail.

References & acknowledgement Bridgestone South Africa (Pty) Ltd Wikipedia -http://en.wikipedia.org/ wiki/Schrader_valve

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