Fitted for failure

Author: Adam Gosling, TyreSafe Australia

Fitting an OTR tyre is not as easy as a good fitter makes it look.  To fit a tyre might be easy but to correctly mount an OTR tyre takes experience, the correct equipment and an understanding of the mechanical interaction between tyre and wheel. 

OTR tyres have a tapered bead face that matches the tapered bead seat of the wheel or rim. Specialised lubricants are used to help the tyre slide up the surface of the taper, but these lubricants must dry out once the tyre is seated so that the torque from the engine doesn’t cause the wheel to rotate in the tyre.

For the tyre bead to slide up the taper and seat correctly a large volume of air must be delivered in a quick blast.  This surge of air pushes the tyre bead outward and up the taper, forming a seal.  If the compressor being used does not have sufficient capacity or volume, small sections of the tyre bead can ‘hang up’ and will not be positioned correctly on the bead seat – although visually, or at least without close inspection, the tyre may appear to be correctly fitted.

When load is applied, the rubber around any section of the bead that is not correctly seated rolls and cracks, permitting high-pressure air to penetrate the casing of the tyre.  The resulting separation within the casing drastically weakens the tyre and leads directly to failure – a significant waste of resources and operating time.  The external evidence of separation within the casing is a bubble in the outer wall of the tyre, which is a definite indication of poor fitting practices.

Competent OTR tyre fitters are a special breed of people.  The work is physical and carries significant risks if safety procedures are not constantly followed or equipment is not fully maintained – indeed many mining operations have deemed tyre bay operations to be the most dangerous on site.  A careless error can lead to significant destruction and severe or even fatal injuries.

To supply the necessary volume of air in the shortest possible time requires high flow air supply. Using a single lung petrol engine compressor might seem to be a low-cost option, but it can destroy tyres due to incorrect bead seating – and consumes an inordinate amount of time inflating the tyres.

Using super large bore inflation equipment reduces the inflation time on a 40.00R57 tyre from 28 minutes to under 5.  Consider the saving in downtime for a machine fitted with four of these tyres – 20 minutes for inflation instead of nearly 2 hours!

But untrained personnel should never attempt to use high flow air equipment.  High pressure air is extremely dangerous and the forces at work are huge – at 100 psi a 40.00R57 tyre exerts more than 500 tonnes of force on each flange.  When you hear a tyre “pop” during inflation this is these forces at work.  Amputation of fingertips was once an indication of unsafe methods and in years past all too many tyre fitters were minus a fingertip or 2.

Using a trained OTR tyre fitter with the right tools not only minimises the potential for safety issues but also maximises the service life & ROI of heavy equipment tyres and ensures minimal downtime.

Then it’s up to the production team to look after the tyres in service, but that’s another story – or three – for next time!

More: Meet TyreDoctor – OTR tyre and rim specialists

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