What is a "Screw Pattern Tablet Instrument"?

Any aspect related to the structures and equipment on the Caledonian Railway Company.
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dunalastairv
Posts: 262
Joined: Wed Dec 12, 2012 7:01 am

What is a "Screw Pattern Tablet Instrument"?

Post by dunalastairv »

I recently purchased a set of three C.R. Traffic Department Circulars, which keep appearing on E-bay. Fascinating reading but the one dated Week Ending 27th. November 1908 contains this paragraph:-

"Blairgowrie Branch - Tablet Working - Fouling Single Lines for Shunting Purposes. After substitution of shunting for Screw Pattern Tablet Instruments between Blairgowrie and Coupar Angus, Exception III of Clause (c) of Tablet Regulation 16 may be put into operation between Blairgowrie and Coupar Angus South, and Coupar Angus South and Blairgowrie, and these places are to be added to the list at page 2 of Supplement to Appendix No. 38 to the Working Time-Table, dated 1st. May 1908".

So what is a Screw Pattern Tablet Instrument and what is its relation to the normal Tyer's Tablet Machines?

Thanks in advance, Michael Dunn.
Graham R
Posts: 97
Joined: Sat Sep 14, 2013 8:44 am

Re: What is a "Screw Pattern Tablet Instrument"?

Post by Graham R »

Hi Michael,

The General Instructions part of the Appendix to the working timetable defines three types of electric train tablet instrument in the “Regulations for Train Signalling on Single Lines of Railway Worked on the Electric Train Tablet Block System”: the No. 1 or Cylinder type, No. 2 or Screw type, and No. 3 or shunting type. Only No. 3 permitted return of a tablet to the same instrument once withdrawn; for the other two, the tablet had to travel to the other instrument. In all three cases, after a train was offered and accepted by bell code, the signalmen had to co-operate by plunging their instruments while the accepting signalman unlocked the cylinder or slide of his sending colleague’s instrument, allowing the latter to withdraw a tablet. The machines then stayed locked until the accepting signalman placed the tablet into his instrument’s cylinder or slide, then gave Train Out of Section to the sender. Both men plunged again and the receiver unlocked the sender’s instrument so its slide could be replaced. In the No. 3 the slide was more complicated with half and full out positions.

I am not sure if these Caley type numbers correspond directly to Tyers instrument numbers. There is an image of a Tyers Type 1 here which shows the slide for withdrawing the tablet and the red-topped cylinder for replacing it. I can’t find an image of a No. 2 instrument however. the No. 3 seems to have been developed into the more familiar No. 6 type which was common into the 1970s.

Anyway the significance of the Blairgowrie reference in your traffic notice relates to the practice of shunting into an occupied single line section. This was permitted only when a train was travelling away from the tablet station in question, in this case Blairgowrie. As long as the train had not reached Coupar Angus, the Blair signalman could allow a Loco to shunt beyond his overlap or his home signal, but as soon as he received “train out of section” from Coupar Angus he had to send a “Blocking Back” 2-4 or 3-3 bell code to signify that he had a train shunting in the section, then send 2-1 (Obstruction Removed) once the shunt was complete and the section was unoccupied. Where a section was controlled by No. 3 instruments, the procedure described above had to be explicitly authorised, and your notice confirms that this was permitted at Blairgowrie (and thus neatly confirms the date on which shunting type instruments were introduced at Blairgowrie). This is all detailed in the WTT Appendix as well.

This is very interesting (well, ok, my wife would probably disagree) and I hope you will share more details from these notices when you have time.

Best regards
Graham Roberts
Graham R
Posts: 97
Joined: Sat Sep 14, 2013 8:44 am

Re: What is a "Screw Pattern Tablet Instrument"?

Post by Graham R »

Silly me … I should of course have also looked in “Signalling the Caledonian Railway” (probably my wife had hidden my copy) which has an extensive explanation of the various patterns of machine on p38 et seq. It makes it clear that the Caley No. 3 macihine was mainly a Tyer’s No.7 instrument, with a squatter form and a small round galvanometer rather than the office-safe-plus-tombstone form of the Tyer’s No. 6, though there are photos of that as well. However, it does not mention the cylinder/screw/shunting terminology for the different patterns which was the subject of your original post. Perhaps the reason the Caley persisted in calling a Tyer’s pattern 7 a “No. 3” was simply because that’s how it was termed in the WTT Appendix.

Graham
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