Tuesday, 2 December 2014

Leaky Axle Box Gasket

Each of Sentinel 7109's axle boxes supports its corner of the loco on a plain bearing and has an oil bath in the bottom. A large sprung pad sits in the oil bath and is spring loaded against the lower surface of the axle bearing. The pad acts as a wick to lift the oil to the lower bearing surface. As the axle rotates, oil is carried round from the pad to the weight carrying upper surface.

Some time ago, I'd noticed that one of the axle boxes seemed to be retaining its oil level much better than the other three.

I checked whether there was any water under the oil in the offending box using an oversized horse-syringe (for want of a better name).
One of these
There was a lot of water!

On removing the covers from the two right hand side axle boxes, the oil retaining one showed signs that its gasket was split at the top. On further inspection, both the right hand side gaskets were in poor condition and weren't helped by being disturbed.

New gaskets were made for both of them. (As no problem has been apparent with the LHS boxes, they have been left alone).
Front RHS gasket of 1.5mm Rubberised Cork
With the cover on, the axle boxes look like this.
Assembled (complete with missing stud!)
The missing stud will be replaced, of course (and the wheels will be painted).

Note: These photos have been cropped. If there are any modellers wishing to copy the axle box detail, leave me a comment below. I have other photos with more detail that I can supply on request.

Sunday, 30 November 2014

No Longer a Screw (too) Tight!

Sentinel 7109's new chimney castings, having been machined to have flat bases, need a nice flat boiler top surface to sit on. Sounds simple enough except that lately I'd begun to wonder about the actual flatness of the boiler top plate on which the castings would come to reside. So one day I took along a straight edge and looked a bit more closely.
A Sinking Feeling
It was clear from the straight edge that all was not quite as I'd hoped and that there was probably 1/4" sag in the centre at the point where the superheater was supported.

Even more closely:
Another Sinking Feeling
And the other way:
And Yet Another
As the lowest point seemed to be where the superheater's support was located, I surmised that perhaps the superheater might have been tightened down so much that it was distorting the top plate.

Whether it would be possible for the top plate to be flattened simply by screwing both chimney bases down tightly, I did not want to entertain in case a casting cracked in the process.

I then checked the effect of loosening the superheater support (where the two nuts are). Indeed the sinking could be lessened to 1/8" instead of a 1/4" which I felt would be satisfactory.

It's clear that it will be better to slacken the superheater support and tighten down the chimney castings first to make the top plate flat. Then tighten the superheater with the chimneys in place.

Nice idea but see the picture below:
Space between the old chimney bases
The new castings leave even less space between them than the old castings shown above so it will not be easy to wield a spanner in there.

But where there's a will...

Tuesday, 25 November 2014

No Longer a Screw Loose!

I confess, this is not going to be one of the most engrossing articles on Sentinel 7109 but a very short piece to record which one of the chimney castings' fixing studs needed to be welded in place to the boiler top plate. It was the one in the photo below.
The regulator is at the top right.
So, when looking to the front from the rear of the cab, the stud is the rearmost outside left hand one...
...the hidden one at the bottom left near the spanners!
A few years ago, when the top plate was refurbished and the fixing holes rethreaded, this stud was so loose, it was more likely to pull itself out than hold down a chimney. Hence the reason for the welding.

Now, just to show off a little, even I felt I'd surpassed myself with the tidiness of the weld itself. No angle grinding was used at all to cover mistakes.

Since it was always going to be a very small weld, I judged that using a stick weld would add far too much metal and need grinding away afterwards to tidy it up leaving iron filings all over the place to rust. It would also be very difficult to wield the stick in such a confined space above the boiler. So I decided to use a TIG torch.

It was still not an easy task as, not only did I have to get the welder in the cab but an Argon gas bottle as well. There really is very little space in this cab!

It was a good decision, however, as the top photo shows.

Sunday, 5 October 2014

Exhausting Pipes (3)

In 'Exhausting Pipes (2)', I'd just received Sentinel 7109's machined chimney castings from Mendip Steam Restoration Ltd. with 'Interesting Times Ahead'!

I first had to fit the original pipes to the top of the new castings - the key point being that the outer shells' mounting holes also had to line up with the corresponding holes in the pipes. The pipes also had to end up being the same height as the originals.

Luck was on my side. Until this moment, I hadn't realised that the pipes were bored out at the lower end so that there was a lip to support the pipes at the correct height (assuming the new castings were the same height as the originals - which they were!).
Supporting lip at lower end of pipe
I could thus put the pipes in place over the castings (with a lot of assistance from a rubber mallet!).
Pipes on casting (ignore the bolt heads)
Outer shell mounting holes are highlighted
Then, using a couple of metal rods, I was able to support the outer shell in place and adjust the pipe orientation to suit.
Pipes line up with outer shell fixing holes
I marked out the fixing holes in the castings using the holes in the pipes, then drilled and tapped them (M10).
Tapping the castings
That sorted out the pipes but the outer shells were not in very good condition.
Corrosion around an outer shell upper fixing hole
Corrosion around outer shell lower fixing holes
I decided to attach sheets of metal over the corroded sections and weld, bolt or rivet them in place.
Outer shell upper fixing hole repaired
Outer shell lower fixing holes repaired
The sheet metal was 0.8mm thick and thus fairly pliable. To make it follow the curve of the original was a challenge but after a bit of thought I found a ratchet strap could pull it into shape so that I could drill the fixing holes.
So finally, putting it all together, it looks like this (both of them similar). The M10 fixings are all stainless steel for greater resilience in this environment.
Finished (nearly)
Next, there is some fine welding to tidy up a few rough edges and a decent coat of suitable paint. Oh, and hoisting them into place (not trivial!).

Thursday, 11 September 2014

Exhausting Pipes (2)

After some time in the making, the machined new chimney bases have arrived from Mendip Steam Restoration Ltd. It's worth a look back at an earlier article for where these fit atop the boiler.
New Chimney bases, Blast Nozzles & Exhaust Pipes
The photo above shows clearly how the exhaust steam flow divides into two to feed the pairs of blast nozzles.
Four Blast Nozzles showing blower rings
The short pipes showing against the orange pipes are the inlets for the blowers. More details are in this earlier article. Also try the label 'Chimneys' for all the relevant details on the subject.

The main difference between the old and new castings is the weight and hence strength. I used to be able to lift the old ones with difficulty; the new ones required help from a neighbour to get them out of my car! They are simply massive in construction and should last a very long time.

The trouble is that they will be very difficult to fit in place because of the weight! They also need the long chimney tubes to be fitted. Interesting times ahead!
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