ENGINEERING REPORT
CONSIDERATION OF VALVE EVENT IRREGULARITIES FOLLOWING FAILURE AT
EASTLEIGH ON 15TH AUGUST 2012
The locomotive was taken off a charter train originating at Poole. The locomotive was
removed at Eastleigh following reported irregularities in the valve events. The Duty Engineer on the day reported an earlier concern over ‘missing beats’ at Eastleigh depot before the locomotive set off. It was subsequently reported to have run normally tender first to Poole and then normally as far as Southampton after which it appeared to miss 2 beats and was ‘knocking’.
The locomotive was examined on 6/7th October 2012 at Arlington Fleet Services, Eastleigh
Works by Gary Shannon, CME and Simon Holroyd, Deputy CME. Further investigation of
root cause will be carried out during dismantling to address remediation work which has
already been identified.
The following is a summary of initial findings:
The companion flange on the centre cambox camshaft which connects to the cross drive shaft was found to have a degree of free movement on the taper of the cam shaft. The cross drive shaft and companion flange were removed. It is clear that the flange has been driving on the key rather than the taper. Wear in the taper and keyway needs to be remediated by regrinding the taper and recutting the keyway. A new companion flange will be manufactured. The degree of rotational movement (c. +/-5 degrees) would affect timing of the inside cylinder but is not considered likely to be have been sufficient to produce the events reported. This problem is wear and tear related.
The LH cambox was examined internally. A defective inside exhaust cam was found, having lost hard facing over a large part of the active profile. This would not in any way directly cause a problem but it was observed that the softer substrate material had rolled on the edge adjacent the outside exhaust cam causing some resistance to relative rotation. It is possible that such resistance could cause the mechanism to hold the cut off at a significantly higher setting in that box as it would allow the springs to extend in the outside scroll reversing levers whilst the scroll would fail to move on the camshaft to affect rotation of the cams and change the cut off. Such would correct itself once cut off was reduced to about 30% however. It was not possible to reproduce the effect on examination although a slight resistance to movement of the reversing wheel was noted in the relevant position. The finding is therefore inconclusive, but the cam and follower must be changed before further operational service. Investigation will also focus on prevention of occurrence and root cause analysis has begun.
The RH cambox was examined internally and found to be in perfect order.
Freedom of movement of valve tappets on outside camboxes was checked and no apparent defects found. Freedom of movement of valve spindles on outside cylinders was examined and no apparent defects found.
The inside cambox and valves have not been inspected at this stage due to accessibility but this will now be done when the box is removed and stripped to remove the camshaft. The drive to inside cambox has been removed for transit movement.
It is proposed that the locomotive be returned to its home base to enable the remediation of identified issues and further examination of valves, camboxes and drive train to try to identify any defect that could have caused the irregularities or eliminate them and give them a clear bill of health. Adequate running in testing will then be needed to give confidence for return to main line service.
This is not the first time that the locomotive has experienced problems of this nature. On a previous occasion a problem with an irregular exhaust opening and sharp blow up the
chimney was encountered. On this occasion a problem with an intermittent binding tappet
was found to be responsible. Working back down the root cause analysis diagram showed an irregularity in the original machining of the cambox to be responsible!
Intermittent problems can be time consuming to track down. Several members have
questioned if the present problem has any connection with the incident in the summer of
2011. At present although the symptoms were fairly similar it is considered unlikely. The
conclusion from this earlier event was that water carry over was likely to have been the root
cause. On this latest occasion we do not have evidence of this.
The following is extracted from the official report of the 2011 event for interest:
“On 12th June, the locomotive hauled an excursion train from Bristol to Paignton. On the
evening return run to Bristol, the locomotive developed an uneven beat with some ‘knocking’ felt on the footplate. The train was halted in Tiverton loop for examination soon after. No defects were evident externally and the locomotive ran satisfactorily with the regulator shut off. It was therefore decided to request diesel motive power to take the locomotive and train on to Bristol. This was effected without incident.
The following day the locomotive was run briefly in Barton Hill Depot. Unfortunately it was possible only to move a short distance in reverse followed by rolling back down the gradient. This did not provide a really satisfactory test. It did serve to show all valve spindles lifting correctly on the outside cylinders. (The inside cannot be readily seen). Movement with drain cocks open showed presence of 6 inlet events. No blows were encountered up the chimney. Examination of the outside motion showed no apparent defects.
Subsequent examination of the inside motion over a pit showed no apparent defects.
At this stage of the investigation there was very little information to work upon. Observations indicated some problem around cylinders and valve gear leading to irregular valve events and probably cylinder relief valves lifting. Possibilities were considered that something had failed in one of the camboxes or in the reversing or drive train to the camboxes. A further concern was raised that a valve may have broken and be detached from the valve spindle. This concern was exacerbated by a view that the sound of the locomotive at the time the incident occurred was consistent with water carry over.
Due to location of the locomotive it was not possible to carry out a proper running trial as
would have been carried out as the next step had facilities been available. It was therefore decided to carry out a physical examination of the camboxes reversing gear drive train and valves.
The inside cylinder cambox was pulled out onto its service platform and examined internallyfor integrity and functionality. The outside camboxes were visually internally examined in situ for integrity and functionality. All valve tappets were checked for freedom of movement. No defects were found.
The inside cylinder exhaust valves and cages were removed and found to be in good order. Operation of the inside cylinder inlet valves was checked by manually lifting on to seats, checking lift and drop, and found to be in order. Outside cylinder camboxes were pulled out sufficiently to check lift and drop of all valves. Cylinder covers were removed to enable further inspection of valves and ports. No broken valves were found.
All Hardy Spicer shafts, intermediate drive shaft and couplings were examined and found to be in order. The cross drive gearbox, which drives the center cylinder, was dismantled for examination. Ultrasonic examination was carried out on the crown wheel shaft to confirm integrity within the crown wheel hub and found to be in order. Valve event timing marks were seen to align correctly on all cylinders confirming proper synchronization.
With camboxes connected and lids removed, operation of reversing mechanism was checked. All boxes were seen to be synchronized and the mechanism operated correctly in each box.
A test steaming was then carried out with cover plates off to allow observation of the
operation of all valves. This time movement was effected with the steam brake lightly applied to provide some load. All valves were seen to operate correctly with all beats present. The locomotive was moved to West Somerset Railway for testing. Here it operated for 7 consecutive days accumulating 560 miles without incident, following which DBS accepted it back into traffic.”
The examinations and testing concluded ‘no fault found’.
A key difference between Caprotti valve gear and say Walschaerts is that the valve operation does not have the same degree of direct mechanical linkage, the Caprotti system relying on actuating steam to close valves.
By far the most common problem operational problem encountered with poppet valve gear is sticking valve guides resulting in valves failing to rise on to their seats or being sluggish to do so. Exhaust valves tend to suffer more from our experience due to oil or tannin deposits. Symptoms then include blows up the chimney and uneven running as proper compression events do not occur. If the problem is on an outside cylinder, once the offending valve is identified with a static test, it can be fixed quickly without causing any undue delay.
A second operational issue is water carry over into the actuating steam system. This can
happen easily as the actuating steam comes off the superheater header before the superheater elements, i.e. it is saturated and if some water is carried over from the boiler it does not have the chance to evaporate in the elements and goes directly from the pilot valve of the regulator into the actuating system. In the worst event, such carry over could cause valve breakage in addition to potential for motion and cylinder damage. In lesser circumstances water in the valve actuating steam system would affect timely valve return and cause some temporary irregularities until it was cleared by shutting the regulator to drop the actuating steam pressure to below 40 psi to allow the pressure operated drain valves to open.
To end on a positive note, we have the resources and parts to address the issues and no major cost items have been identified so far. Opportunity will be taken to address other motion items coming due concurrently.
DR GARY M SHANNON
CME
If anyone has any technical queries relating to the above article please feel free to email the engineering team on engineering@71000trust.com
