The LMS, LNER and SR introduced upper quadrant signal arms soon after the Grouping of 1923. At first, the arms were of the corrugated steel pattern, but this changed around 1936 to the plain type. These signal arms were used for all new installations from the mid-1920s, and also gradually replaced the lower quadrant arms on signals inherited from the relevant pre-group companies (a process that is still not complete!). The design was adopted by BR and Railtrack, and new semaphore signals are still being installed today, where replacement by colour lights is inappropriate.

All three companies began by installing the arms on wooden (S06, S017, S028) and lattice steel (S7/02, S7/02/2, S7/22, S7/41, S7/42) posts and dolls. Concrete posts were also used, particularly on the LNER. In the 1930s, the LMS and LNER switched to tubular steel posts (T18, T532), whilst the SR began making posts from bolted lengths of scrap rail (S032). BR continued these designs.

Plain arms are provided on the fret. To convert them to the corrugated type, score lines on the front face, and add 0.45mm brass wire to the rear.

The actual size of the fret is 95½ x 70½mm; it is etched in 0.015" brass.

To see exactly what is involved in using these parts, download the instruction sheet as an Adobe Acrobat® file (131kb).

Identification of components on fret:
1. Plain home arm
2. Plain distant arm
3. Spectacle plate
4. Ladder mounting block (tubular posts)
5. Balance lever (5a for SR/BR(S))
6. Balance weights (6a for SR/BR(S))
7. Balance lever bracket
8. Crank bracket
9. Signal wire cranks (3)