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23rd July 2010
(*with apologies to any Beach Boys fans)
Regular readers will be aware that we have long been concerned with excessive vibration from the power unit of 14 901. For one thing, on a rigidly-mounted engine, such vibration my be injurious to mountings or crankcase, for another, it gives rise to unpleasant operating conditions, and on the comic side, things that you thought were tight start to unscrew themselves - Doctor Who's sonic screwdriver is almost matched by a DV8 with bad vibes!
In earlier stories we have explained how we have approached this problem, hoping that it was something simple, and certainly the vibration was markedly improved by the installation of a viscous damper on the front of the Rolls-Royce DV8TCE. (Viscous dampers are fitted to most R-R engines, but not gensets). But the vibration did not go away, and after running at Butterley and Peak Rail it was quite definitely deteriorating. Before letting the loco loose on general duties at Rowsley, Andrew and Pete decided that it was time to investigate the torsional coupling.
This coupling (above) takes the full engine horsepower from the engine flywheel and passes it to the propshaft and thence to the Voith, but elements within the housing reduce the torsional stress (the "ripples" in the torque caused by the reciprocating masses and the moments of ignition). Although not quite the same as that on a standard Class 14, it was thought to be the same make (Holset), and our initial assumption was that the rubber elements had deteriorated. That part of Holset was taken over long ago by Renold, and enquiries to Renold ellicited the reply that they "cannot identify the original supply" and instead offered a current production coupling at £2600 and long lead time.
So on Sunday July 18 the coupling was removed for examination.
Here's the inside of the torsional coupling just after it was lifted out. The outer casing transfers the drive through the rubber blocks, onto the inner "spider" which emerges through the end and on to the propshaft. Two aspects were immediately obvious - firstly that apart from a little water ingress, the rubber elements looked quite satisfactory, and secondly that only the stiffness of the rubber elements restrained the inner spider from rocking, bearing in mind the not inconsiderable weight of the propshaft. That the spider was most definitely rocking was evidenced by a glance at the engine flywheel....
Where each of the spider arms stuck out, there was considerable indentation on the relatively soft metal of the Rolls-Royce flywheel. This would have been difficult and expensive to have been machined, so was almost certainly damage.
Fortunately Pete has long been a satisfied customer of Centa Transmissions of Bradford, who have numerous applications in rail. Centa couplings are used on 14 901, "Pluto" and Drewry 72229, and he was already discussing the problem with a Project Engineer at Centa. Better still, Centa's Managing Director was a former Holset sales engineer, and took a personal interest. Consequently last Wednesday Pete took the coupling up to Bradford for examination. Picture if you will, 6 Centa employees, led by the MD, studying this rusty old coupling laying in the back of an estate car in the car park...
"Where's the rest of it?"
"That's all there is"
This, Pete was assured, was a standard "1.5 RB" Holset torsional coupling, but it was incomplete. When fitted to a Dorman 8QT (the engine which it had when at Grangemouth) there was a large adaptor plate that bolted to the outer ring of bolts on the SAE16 flywheel. When fitted to a Rolls-Royce DV8 (for this coupling was supplied to both engine manufacturers) a much smaller backplate, no bigger than the outer diameter of the casing was provided. But vitally important, both backplates carried a "support bearing" - a Tufnol ring that rests in the recess in the end of the spider and keeps it all in line.
The most likely scenario is that volunteers in Scotland removed the coupling from the Dorman and found the casing bolts lined up with holes on the flywheel of the Rolls-Royce DV8, so mounted it straight on without realising the importance of the missing support piece.
Centa are stripping and overhauling the coupling, and will manufacture a new back plate and support bearing ready for re-installation.
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