Rail
Wear
Abrasive wear occurs
when there is contact between the side of the flange of a wheel and the gauge
face of the rail. This contact usually takes place between the leading outer
wheel of a vehicle bogey and the outer rail of a curve.
On curves careful
periodic check must be carried out of the outer rail to ensure that side wear
is kept within prescribed limits. Failure to do this could result in a
derailment. Where curves are tighter than 200 metre radius, continuous check
rails should be provided inside the inner rail. This check rail is to be set
not more than about 50mm inside the running rail or at a distance that will
ensure that the inside face of the flange of the inner wheels will bear on the
check rail thus sharing the centrifugal force between the check rail and the
outer rail through flange bearing. Abrasive wear of rails can be reduced by the
use of rail lubricators placed at strategic positions. Great care needs to be
exercised in the use of lubricators to ensure that only flanges are lubricated.
Lubricant deposited on the top of rail heads can cause problems with braking,
acceleration and wheel-spin. This is particularly important where trains are
automatically driven or where stopping positions are critical such as when
rolling stock doors have to line up with platform doors.
When wheels run along
fairly straight track with flanges just clear of the rails, the contact area
between wheel and rail is extremely small. In theory the contact would only be
a point which would make contact pressures infinitely high. In practice both
surfaces deform slightly to give a contact?patch'. Evensonlyso,anarea oftypicallyabout such
ap 100 sq.mm under the heaviest wheel load. This gives pressures as high
as 1200N/sq.mm which is higher than the yield point of the steel. This has the
effect of causing the contact patch to become plastic and to flow causing
various wear patterns and irregularities over time.
Where rails become side worn near to
limit on curves, extra life can be obtained by either turning the high rail on
jointed track or transposing the two rails on continuously welded rail. Close
inspection of the existing inner rail outer edge must be carried out before
transposing to ensure that there are no other defects present such as
roll-over, ?lipping' would make the ride or plastic rough and precipitate
failure of the new running edge.
If speeds in excess of 120 kph (75 mph)
are expected, transposing should only be carried out if re-profiling of the
existing inner rail is carried out. Wear on point and crossings needs to be
carefully watched on a regular basis. Some repair of bad wear can be done by
welding but in most cases components need to be changed.
In jointed track excessive wear often
takes place at rail joints or fishbolt holes and is the main reason for
re-railing. Joints also increase wear on rolling stock. This is one of the main
reasons why main line railways are progressively changing to continuously
welded rails.
When a derailment occurs on any railway
at any location, rail wear must be fully investigated as this can often prove
to be the root cause. All rails should be closely inspected including any
tell-tale signs of where wheels ran at the time of and just prior to the
derailment.
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