Sentinel 7109 - A 28-ton Mistress?

I'm told (honest) that a mistress can take a lot of time, money and attention to keep happy. "Joyce" (Sentinel 7109) is certainly expertly fulfilling this role!

This week, 'blogging the restoration of Sentinel 7109 led me to a whole new aspect of the restoration that I had not really thought much about before.

No comment!

Nigel, co-owner of 7109, kindly volunteered me to give a talk to the 82D Model Engineering Society group in the Wiltshire town of Warminster. About 20 knowledgeable members attended the evening and, judging from conversations afterwards, seemed to have enjoyed the event.

Simon Bowditch, chairman, thanked me and commented that it was the most detailed description of the workings of a Sentinel locomotive that he had ever come across.

My content included 7109's history and reasons for being at Midsomer Norton; details of the Sentinel boiler, superheater, chimneys, engines, feedpumps and chains plus additional material on other Sentinels and models. I interspersed a few video clips to provide variety from the pictures only. I also passed around some supplementary pictures and texts and brought a few of Joyce's renovated components for interest sake. The talk lasted about 1 hour 20 minutes including discussions on the way arising from audience questions.

So, having done it once, I have the basis to tailor the talk for other societies should they be interested. Please leave a comment with a phone contact number below if you would like to invite me to give a repeat performance.

Many thanks to the 82D society for inviting me.

Chains - Free at last!

After about seven weeks, Sentinel 7109's pair of big-time bicycle chains have finally given in to the lengthy diesel dunking and manipulation with hammers and Stillson wrenches; now all rollers and links are free to move.

Flexible chain links ...

... wound round semi-sprockets

So the work is now complete to ensure there are at least two workable drive chain sets. A deep oiling will be required before use.

How to make an ashpan (1)

Sentinel 7109, when originally brought to Midsomer Norton Station in December 2004, came without a few items, one of which was its ashpan for mounting below the boiler.

I looked at the design in a previous article but have now decided that I want to have a go at doing the fabrication myself. Another article had a look at the ashpan from Sentinel 9622 at Teifi Valley Railway so there is a fair amount of information to go on for the basis of a design.

This drawing shows the basic idea:

Ashpan Plan View

A side view also helps:

Side elevation

I also scrutinised many photos of 7109 and other Sentinels for what their ashpan looked like. For example: (Click the photo to enlarge it).

7109's Ashpan Highlighted in the centre
(Photo cropped from original provided by John Hutchings, ILS)

The photo shows that, unlike the drawings, there are no cut-outs around the sides of the ashpan - which makes it easier to make!

Another ashpan from a Sentinel tractor emphasises the point:

100HP Boiler ashpan on Sentinel 6426 (Two-speed tractor)

So, to begin, some design decisions:

• How big? The circular inside of the firebox is 34.5" in diameter so the ashpan needs to be slightly larger so that ash will fall into it rather than down the outside if not large enough. It must not be too large to prevent mounting brackets to be fitted.

Mounting at the base of the firebox (looking upwards)

The picture shows that about 1 to 2 inches in additional diameter will be fine. So it will be made 35.5" in diameter.

• How deep?
  The drawing indicates 5.5" depth of side wall; however, 6" seems more like the picture of 7109's ashpan. So 6" will be the height of the sides.
• What thickness of what material?
  Mild steel, 3mm. 9622's ashpan may have been this thick at one time but it certainly isn't now! Using non-stainless steel, there is a trade off between material thickness and weight. The difficulty of manoeuvring it below the firebox emphasises minimising the weight. 3mm mild steel is a good compromise for the base and side wall.
  
• Any cut-outs to allow air to the fire?
  No cut-outs. By butting the top of the sides on to the bottom of the mountings, a gap is created for air anyway so no additional gap is required. Also, by using long studs from the mountings, some height and hence air-gap adjustment can be made for fine tuning the performance of the fire. The cut-outs are additional work but also makes the side wall more difficult to roll to the right curvature before fixing to the base.
  
• How to fix the side wall to the base?
  Welded inside and outside butted corners of the side wall and base. Metal Inert Gas (MIG) is preferable because there are long continuous welds. Manual Metal Arc (MMA or 'stick') could be used but would not be so tidy.
• How to get the ash out of the pan?
  The plan drawing shows a flap and this is present on the 9622 ashpan. The flap on 9622's is made from a single sheet of mild steel and bent at the edges to produce the triangular sides. 7109's flap will be made from the piece cut out of the main base with 2mm mild steel sides welded in place. Sentinel would have been making loads of ashpans and hence the flaps could have been made in quantity. 7109's is a one-off so this different approach wastes less material.
• How long a 6" wide sheet is needed for the side wall?
  This is where we have to go back to school! The length is about the circumference of a 35.5" diameter sheet minus the bit cut out for the flap. So that's Pi x diameter = 3.14159262(etc) x 35.5 = 9' 3.5".
  
  So how big is the bit cut out for the flap? Now we have to go to big school for this one!
  At first, I thought this was about chord-lengths of circles but it's easier than that. We know that the flap is 19.5" wide from the drawing and we know the radius of the base (35.5/2 = 17.75"). So this isosceles triangle can be bisected to give two right-angled triangles for which we can find the angle subtended by the arcsine of the opposite over hypotenuse!
  This gives us an angle of 33.3 degrees which needs to be doubled to 66.6 degrees because there are two right-angled triangles in the isosceles.
  
  So this gives us 66.6/360 x circumference = 66.6/360 x 111.5 = 20.64".
  And thus the side wall length is the circumference minus the flap's bit which is 111.5 - 20.64 = 91".
  
  On a practical note, a 91" thing won't fit in my car so two 45.5" pieces it is!

I'd often wondered what trigonometry was actually for but now I know!

• And how big a piece of 2mm sheet is needed to make the sides of the flap? It depends on the offcuts in stock! No trig needed here!

When the sheet steel arrives, it looks like this:

For scale, the part-feet showing are about 8" long!

To be continued!

100 HP Sentinel Locos (Fry's Type)

I've been asked for information about the Fry's Sentinel loco which was rescued and returned to its original home area in 2010. Avon Valley Railway are doing the restoration work.

As a late addendum to '100HP Pre-war Sentinel Locos' posted in 2011, the following pictures are taken from the 1931 Sentinel catalogue '"Sentinel" Patent Locomotives'. It includes the following details of the Fry's loco type (Click the pictures to enlarge):

The Fry's Loco

100HP Loco as used at Fry's in Keynsham

See page 53!

Old catalogues do come in handy sometimes!

The Power of the Double-Engined Sentinels

What can you do with a Sentinel locomotive rated at 200 HP?

John Hutchings, of the Industrial Locomotive Society, kindly sent me a copy of a very interesting letter recently (September 2011).

Recall that Sentinel 7109 was the first of a series of eight pre-war double engined Sentinel locos with the unusual curvaceous profile. Being a first-off, its performance needed to be marketed to potential customers and so it was delivered to the LMS at Newton Heath, Manchester for evaluation.

The letter below describes the tests and shows how impressed the observers were with 7109 at the start of 1928. (Click the picture to enlarge it for reading).

Sentinel 7109 performance
(Letter courtesy of John Hutchings, Industrial Locomotive Society).
(Note: this was retyped by JH as the original is in poor condition).

So these are the results of the actual tests done on 7109 which inspired the S&DJR Co Ltd to purchase their two similar locos for use at Radstock.

Clearly, you can do quite a lot with a 200 HP loco so long as you don't want to do it too quickly!

To Inject or not to Inject?

Part of the fun of doing a loco restoration is trying to resolve mysterious features that aren't obvious as to their purpose or function.

Lately, whilst cleaning up and renovating the pair of water tank filter valves, I've been pondering the purpose of a small tube on the left hand side of Sentinel 7109's water tank that is there in some photos but not in others.

Have a look at this:

Sentinel 7109 at Shrewsbury prior to delivery, probably 1927
(Photo courtesy John Hutchings, Industrial Locomotive Society)

Just in front of the cab on the rear filter valve, there is a small tube:

7109 Tube and valve highlighted

But in this later photo, the tube has disappeared:

Missing 7109 Tube
(Photo cropped from C Verrall 1964 original)

Compare these to the similar Radstock Sentinel (7588) which has two tubes to 7109's disappearing one.

Radstock Sentinel 7191 (wks no. 7588) showing its twin tubes
(Photo cropped from R. G. Jarvis original, Richard Dagger collection)

So what was going on, I hear you ask?

Sentinel 7109 was originally supplied with an injector; however, it apparently did not work very well and was soon replaced by a Worthington Simpson type boiler feed pump mounted on the right hand driver's 'window ledge'. Hence the tube was there to feed the injector in the early photo but missing in the later photo.

So if an injector did not work for Sentinel 7109, why was one or more then fitted to the later Radstock Sentinels and left there?

That is the question!

Comments welcome!