Spare a thought for the humble level-crossing barrier power pack. Every day it makes the barrier gate booms go up and down, up and down, over and over again. On small half-barrier crossings, that’s no problem. However, when the boom is over 9.1 metres long, so the centre of gravity is something like 4.5 metres from the pivot, that requires a bit of effort – in fact, quite a lot of effort.
So, although they were designed for the job, it’s not totally surprising that the poor power packs start to struggle after a time.
Developing an upgrade
The problem having got worse over the last few years, it was time for a radical solution. Network Rail engineers analysed the problem over a 24-month period alongside a specialist team from Howells Railway Products.
A detailed analysis of both existing and potential failure modes involved the dismantling and analysis of over 70 power packs which had been removed from service at point of failure and were due to be sent for overhaul.
Along with information on service failures and the results of internal testing within the factory, a number of improvements to the existing design were developed and proven to ensure a trouble-free service life. All modifications were contained within the power pack itself, so no other modifications would be required to the infrastructure as the re-manufactured units were completely interchangeable.
To verify these improvements, a complete barrier assembly utilising a 9.1 metre boom and skirt was installed at the Howells factory. The revised power pack was subjected to continuous testing in which it was raised and lowered repeatedly every three minutes. Performance was monitored by a data-logger and daily condition monitoring, allowing comparisons between pre and post-modifications to be carried out with a typical test running to 50,000 cycles over a three- month period for evaluation purposes.
Two trial packs were fitted in the live environment in May 2014 and these were trialled for four months before being removed for inspection. Further ongoing trials are in place at several crossings including Navigation Road in Trafford – with an average of over 370 train movements every day, this is one of the busiest level crossings in the UK.
Casting, bushes and bearings
One of the largest areas of concern was the connection between the pump and the boom. Due to the angle of rotation when the barrier rises, the rubber metalastic bush employed on the existing units is stretched beyond its elastic limit and starts to shear. After relatively few operations, the rubber disintegrates and the bush separates. This bush has now been replaced by a spherical bearing which allows free movement of the barrier through the required 80°.
However, the metalastic bush utilised at the opposite end of the power pack is retained as the movement arc is within acceptable limits and it has a secondary function providing tank centralisation.
A new casting has been designed which maximises the distance between the barrier pivot point and the piston rod, increasing the effective leverage of the piston and reducing the working pressure required to lift the barrier. This is beneficial on all booms but particularly so when the pack is used on a 9.1 metre boom as the operational pressures required are greatly reduced.
Motor, damping and filtration
During testing, it was discovered that the current 0.25HP motor was inadequate when used with the long (9.1 metre) booms. Replacing it with a 0.35HP version solved that problem while also greatly reducing the running current and temperatures, increasing the longevity and reliability of the motor.
In addition, the previous fixed-rate damper design was compromised by the requirement to utilise the same oil and ram as the pump circuit. Howells has now separated the pumping and damping hardware and circuits by the use of an independent, sealed external damper, easily adjustable to reduce or increase the damping effort as required.
The damping can now be adjusted for individual boom lengths and weights, allowing a much softer touchdown of the boom. This increases boom and skirt life, minimises impact damage on the machine and reduces ambient operational noise levels.
Adding the external damper also allowed the existing paper filter, which was prone to blockage and collapse, to be removed and replaced by a metal edge filter which gives a much larger surface area and is more resistant to contamination and implosion. Free flow through the new filter is now 22 litres/min at 1-2 P.S.I. pressure drop at 16°C, far better than the 5.5 litres/min achieved with the old paper filter.
All of the above changes, and a few extra tweaks, have resulted in a much more reliable power pack that has now gained product approval. In time, they will be retrofitted to those crossings where the long barriers are causing problems.