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 Post subject: MP3 Under Repair
PostPosted: 11 Apr 2011, 12:04 
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Location: North Cambridgeshire, UK
The NVR's 40 ton crane MP3 (aka RS1004/40, ADRR95207 et al) is under repair, as of Sunday 10th April. Before I do a running commentary on cause and effect, I'll mention that our crane team consists of David O'Connell (DOC), Richard Busby and myself and we take it in turns to be driver, banksman and slinger.

Richard was the main driver of the crane at the NVR's Mixed Traffic Gala on 3rd April, and I then took over. After positioning the hook for stowing the jib at the end of the day, I had difficulty in getting the Hoist motion to disengage. This suggested that there was friction between the teeth of the dog clutch even though the regulator was closed. I turned off the main steam valve and let the residual steam pressure decay but that made no difference.

I eventually managed to release the Hoist gear with great effort, but as I started to lower the jib for stowing there were two or three seconds of knocking from, it seemed, somewhere in the gear train. I guessed that the dog clutch hadn't fully disengaged but an inspection of the gear, regulator and reverser mechanisms appeared to show that all was fine. The stowing was completed without further incident and minds were put at rest. If only we were blessed with foresight.......

Our next job with the crane was scheduled for 7th/8th April, to lay sidings, with double slip etc, at Ferry Meadows in readiness for a museum building to be built there. I was booked to drive the crane on the 7th but opted to miss my turn since RB and DOC had done all the hard work in organising and prepping. DOC also stepped aside as he had sprained his wrist, so Richard took to the footplate.

The crane worked well all day on the 7th, though the noise level from the gear trains seemed slightly higher than normal and Richard remarked that the Hoist gear was sometimes difficult to disengage. DOC took to the footplate on the 8th and soon reported that the Hoist gear was becoming very stiff indeed to disengage. As we worked through the morning, he also found it increasingly difficult to get the flywheels turning from rest. To start the engines turning, the regulator needed to be opened further than normal and then quickly eased back before the engines raced.

There was also a metallic knocking noise from the area of the main driving shaft assembly as it turned, even with nothing in gear - in fact the same noise I'd heard on the gala day! Richard and I climbed up and watched the shaft as it turned, to see if anything was catching. It was jumping about and rattling the teeth of the hoist gear (which is keyed to it) against the rope drum's gear teeth. We then thoroughly inspected it when it was at rest, but could see nothing amiss.

Continuing to operate the crane could cause even more damage than there already was, and the difficulty in controlling the crane's motions could become dangerous, so we decided to call a halt to its operation. I explained the position to the CCE and we then closed down and stowed the jib, simultaneously operating the hoist motion with either slew or derrick as required since the Hoist lever was still locked in. DOC managed to force the lever out of engagement when the jib was partway down and he skillfully controlled the engine(s) to seat the jib gently onto the runner.

Yesterday, Richard and DOC released the big ends and found that the mainshaft turned easily and appeared undamaged. They then removed the end covers from the two cylinders and found a sizeable and mangled chunk of metal loose in the LH cylinder, between piston and cover. This piece of metal had forced the piston to try to come to a stop before it had completed its travel, which is probably why the mainshaft was jumping about with one end being turned whilst the other end was being brought to a halt. Thankfully the piston hardly suffered from its hammering and is fine for re-use!

Richard and DOC suspected that the chunk of loose metal had broken away from either the fixed or moving part of the slide valve, this yet to be determined. They removed the valve chest covers and observed that the LH slide valve was rusty on its valve-to-cover sliding face whilst the RH one was shiny. This difference may be connected with the problem. DOC reports that the cylinders appear to be in exceptionally good condition apart from some slight scuffing as a result of the incident, and the next step will be to remove the cylinders and valve chests for full inspection and repair.

Hopefully the cost of repair won't be overly high, and we expect it will be less than the cost of hiring in a road-mobile crane for a couple of days to finish the sidings work. We hope to soon have MP3 back at Ferry Meadows to finish the work, but the CCE has said that his team have plenty to be getting on with so we don't have to panic.

I'll post again when there's more to report.


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 Post subject: Re: MP3 Under Repair
PostPosted: 13 Apr 2011, 10:01 
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Location: North Cambridgeshire, UK
An update:

My colleagues decided not to remove the cylinders and valve chests from the crane unless it becomes essential to so do. Instead they have stripped both valve chests in situ.

No metal was missing from anything in the LH valve chest and it was suggested that the rogue lump of metal may have started off in the regulator valve - which was left open for a time during its overhaul several months ago - and then been carried by steam to the cylinder. I find it hard to envisage a piece of metal of some 1/2" square x 1.1/2" long being carried from within the regulator valve up the steam pipe and then via the slide valve into the cylinder, but it's a possibility that will be looked at.

Since the loose metal was about 1/2" thick and the steam space between piston and cylinder end cover is also about 1/2" when the piston is at the end of its travel, this probably explains why there was no damage of any concern within the cylinder. It seems that the piston was forced upwards to become tight in the cylinder, but that is all.

I've been told that quite a lot of wear has been found between the LH valve sliding element and the cover, causing the valve to drop away from the seat too far when steam is shut off and thus needing greater pressure to seat it when the regulator is opened. This is clearly not a new problem as the outer face of the LH valve is grooved and rusty whereas the RH one is smooth and shiny. The wear will be attended to before re-assembly, probably by building up the worn with weld and remachining.

We are left with two questions:
Q1. Where did the rogue metal come from - and is there a risk of any having found its way to the other side of the piston on either engine?
Q2. What clearance should we aim for between the valve's sliding element and the valve chest cover?

The answer to Q1 is down to our crane team, but if anyone can help with the answer to Q2 that would be most useful.


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 Post subject: Re: MP3 Under Repair
PostPosted: 26 Apr 2011, 19:24 
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Location: North Cambridgeshire, UK
The LH piston and rod was removed from the cylinder by my two crane team colleagues on 10th April and I joined them for inspection and remedial work on 17th April.

The cylinder has some slight scoring, the sharpness of which was removed with fine abrasive paper. Nothing was broken from within the cylinder - I inspected the top end of the steam passages with a mirror to make sure there was no hidden nasty 'around the corner'.

The piston was damaged to a greater extent than I had envisaged from the description given to me. The impacts had compressed the ring groove (there is just one groove, carrying two rings) to the extent that the rings were seized at one point on their circumference.
Attachment:
Piston, Cover and Foreign Body.jpg
Piston, Cover and Foreign Body.jpg [ 99.09 KiB | Viewed 17566 times ]

Attachment:
Damage to Cover.jpg
Damage to Cover.jpg [ 115.29 KiB | Viewed 17568 times ]

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Damage to Piston Face.jpg
Damage to Piston Face.jpg [ 100.67 KiB | Viewed 17568 times ]

We removed the piston from the shaft, cleaned up the damage and corrected the fit of the rings. The rings now fit and slide as with a new assembly, with no perceptible front/rear play. We also took the peaks off some not-too-serious scoring of the piston and one of the rings.

The piston rod was bent, just behind the piston, giving 0.275" runout when revolved between centres. Using the hydraulic press, we straightened the shaft until it had just 0.006" max runout. We are happy that no undue stresses have been introduced, e.g. from fatigue.

The LH valve chest had been re-assembled by my colleagues earlier and it was left to me to clean off the cover gasket from the RH valve chest. That was a long and hard job, and it was my bad luck that the gasket was stuck to the valve chest, whereas on the LH side it was stuck to the cover and could be removed on the bench! I then measured the clearance between the valve and the cover, which I'll elaborate on in the next report.
Attachment:
RH Valve Chest.jpg
RH Valve Chest.jpg [ 103.44 KiB | Viewed 17568 times ]


Attachments:
Piston and Cover.jpg
Piston and Cover.jpg [ 99.09 KiB | Viewed 17568 times ]
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 Post subject: Re: MP3 Under Repair
PostPosted: 26 Apr 2011, 19:39 
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Joined: 15 Dec 2010, 22:30
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Location: North Cambridgeshire, UK
I took another good look on Sunday 24th but couldn't see where the rogue metal came from. I suggested that it could have started off curved and become flattened by impact, which led a colleague to say that it might be part of an earlier piston ring that had somehow got trapped in a steam passage in BR days. The piston and rings seem in too good a condition to be the original, so he could have a point. I'm awaiting laboratory analysis of the metal but doubt if that will help. There follows a photo of the 1/2" square x 1.1/2" long foreign body. Any ideas from where it might have originated?
Attachment:
Foreign Body.jpg
Foreign Body.jpg [ 112.48 KiB | Viewed 17568 times ]

I measured the RH valve cover running clearance, i.e. the clearance between the valve and a raised pad on the cover when the valve is on its seat. Advice was received from the East Somerset Railway's respected steam engineer Steve Masters, via Roger Cooke, that 1/16" clearance ought to be fine, but anything greater than 3/16" (0.187") might lead to damage when the valve is slammed against its seat by steam pressure when the regulator is opened.

Allowing for a 1/16" thick gasket (the original was much thicker than this but the very good face-to-face fit of the cover didn't warrant it), the clearance on the RH side was calculated to be an excessive 0.295". The raised pad on the cover against which the valve rests didn't appear to be worn to any great extent but still needs building up with weld and remachining. A 'chisel test' showed the covers to be cast iron rather than cast steel (the swarf came away in chips rather than a spiral) which will govern the welding process.

I'd been told that the pad on the LH cover (which had been refitted in my absence) had worn away completely and the valve was eating into the cover, so the clearance on the LH valve will be larger than on the RH, and repair is definitely required. But we needed to carry out a test steaming ASAP in order to give the Civils Dept an idea of when the crane would be available for further lifts, so we refitted the RH valve cover 'as was' but with the gasket fitted dry so as to ease removal.

The LH piston & rod assembly was inserted in the cylinder, with gland loosely in place but crosshead not connected. The assembly slid smoothly back and forth under hand pressure, whereas I understand its removal had needed a large hammer. The rod end was then mated with the crosshead and fastened, and the big-end connected.

With everything assembled, the crane was test steamed. The LH valve in particular was reluctant to seat as the regulator was opened and it made a quite worrying bang as it did so. The bang from both valves is no louder than before but I'd always accepted it as normal, whereas it now sounds painful as a result of Steve educating me as to the damage that could result from an over-large clearance.

The difficulty in getting the engines started from rest is most surely due to the excessive valve clearances. The good news is that the 'slow idle', once it was achieved, was smoother and quieter than I can ever remember. It ticked over with the gentle rhythm of a traction engine; a joy to listen to.

With the engines running, I could see that the crankshaft was slightly bent, centred at the fast hoist gear position. I didn't get close enough to measure the extent of run-out whilst the gears were turning but the teeth on the fast hoist pinion gear slid in and out against the teeth on its mating gear by up to 2 mm per revolution of the crankshaft. Correction will require removal and stripping of the crankshaft, which will probably be left until the crane undergoes its next heavy general overhaul.

The bending is of no surprise as Roger had advised that this might result from one engine trying to turn the shaft whilst the other engine was at a forced stop... and by studying where the shaft is supported and the forces that were imposed upon it, he correctly predicted exactly where the apex of the bend would be!


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 Post subject: Re: MP3 Under Repair
PostPosted: 16 May 2011, 21:03 
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Joined: 15 Dec 2010, 22:30
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Location: North Cambridgeshire, UK
Another update:

My colleagues built up and remachined the pads on the inside face of the valve covers so as to reduce the clearance between valve and seat when the steam is shut off and the valves fall away.

I had earlier 'chisel-tested' the material and decided it was cast iron as the swarf came off in crumbs rather than as a curl, however grinding-wheel spark tests suggested that it was cast steel, and the way it welded confirmed such. It was pre-heated, and hydrogen-free welding consumables used (MIG in this instance), in case the metal had a high 'carbon equivalent' as had the crane's axleboxes that we welded a couple of years ago.

Though one cover was worn much more than the other, each had a clearance 'as found' of about 0.300" which was reduced to 0.060". I was concerned that this might be too small a clearance but all seems fine in practice.
Attachment:
RH Valve Cover - Welded.jpg
RH Valve Cover - Welded.jpg [ 90.42 KiB | Viewed 17523 times ]

Attachment:
RH Valve Cover - Machined .jpg
RH Valve Cover - Machined .jpg [ 117.33 KiB | Viewed 17523 times ]

Starting the engines and controlling the speed is so easy now! A flywheel speed as low as 30 rev/min is achievable and this helps considerably with dog-clutch engagement. And no longer is there a 'bang' from the valves hitting their seats when steam is admitted to start the engines moving. It's quite eerie!

Turning to the debris, i.e. the lump of metal found in the LH cylinder: We have so far failed to identify where this came from. A laboratory analysis has indicated that it is a low carbon steel, which seems to exclude it coming from a casting or a piston ring, and the metallurgist has found possible indications of it having been splined. Investigation ongoing!


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 Post subject: Re: MP3 Under Repair
PostPosted: 26 Jun 2011, 12:34 
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Joined: 15 Dec 2010, 22:30
Posts: 298
Location: North Cambridgeshire, UK
It's about time I posted the outcome of this problem.

From study of the metallurgist's report and photos, we concluded that the debris started off as a hexagonal or round item with an internal thread and it became flattened by being squashed between the piston and the end cover. We can't decide what it actually was. If it was a nut, it was a very long one.

We are certain that it didn't belong to any part of the crane steam system between regulator valve and cylinder, so it must have found its way in from outside. It could have lain somewhere in the system for years before being carried into the cylinder.

The good side is that, as described earlier, in searching for the source of the debris we found that the valve clearances on both engines were too great and we were prompted to take corrective action.

I was away on holiday when the most recent series of lifts were carried out, on 14th June. I haven't received any panic telephone calls so I am assuming that the crane performed without further problem.


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