Background to vertical track force testing
During these early days of research, the BRB was contemplating raising the general operating speeds of passenger trains and wished to know the effect of running at high speeds over specific track features.
Tests were conducted overnight at Wrinehill, near Madeley just south of Crewe on the WCML using a Deltic amongst the various test locomotives. These locos were usually to be found running on the ECML at 100 mile/h and were chosen as the representative locos which would exert the maximum vertical force on the track (known as the ‘P2 Force’) when traversing a ‘dipped’ rail joint at speed.
The tests were conducted overnight on Saturday 30th October 1971 (it wasn’t very warm!) and although Test Car 4 was used on one of the trains which consisted of Freightliner wagons with the LMW hauled by E3112, we were involved with the trackside monitoring. The equipment used consisted of strain-gauged baseplates under the rails, which could measure the vertical force exerted by the locomotive as it passed over the ‘dipped’ rail joint.
A dipped rail joint was the phrase used to describe a fish-plated track joint which had deteriorated and which therefore ‘dipped’ below the normal rail top when under load due to being traversed by a train. These are still a common phenomenon in jointed track on secondary lines even today due to the fact that rail joints in UK are not staggered and are unsupported. Another unpleasant side effect is the damage inflicted on the rail ends by this phenomenon.
I remember driving around the Cheshire countryside in the middle of the night trying to find the site, which like most sites we seemed to choose was in the middle of nowhere! No SatNav's in those days!
A summary of the subsequent report reads:
As part of the development process of AC locomotives, track forces at a dipped rail joint were measured in a series of tests at Cheddington in 1970. A comparison was also made with the peak forces induced by other locomotives passing over the same joint. Since the AL6 locomotive produced some of the largest forces, possible modifications to its design have been considered. A further set of track measurements were made at Wrinehill in October 1971 to check the effect of these modifications.
The SAB resilient wheel offers one of the most promising ways of reducing the track forces. This is due to the flexible rubber bushes between the wheel rim and the hub which significantly reduces the dynamic effect of the unsprung mass of the wheel. Accordingly, two AL6 locomotives with resilient wheels (one of which also had Flexicoil suspension) were run over a dipped joint to compare their force levels with those of a standard locomotive.
The report concludes that resilient SAB wheels, when fitted to a class 86 locomotive, reduced the maximum dynamic wheel loads recorded by measuring baseplates to between 30% and 50% of the load levels of the standard class 86 at a speed of 45m/s (100mph). The resilient wheel also produced a 20% reduction in stress range in the wheel web as measured by a shear gauge between the rail end and the first bolt hole. Calculations indicate that for resilient wheels of this design the loads should be virtually independent of the track stiffness at a joint, whereas for conventional wheels the loads are proportional to the square root of track stiffness.
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