The driving controls of a BR Standard 5 Caprotti

Cylinders (2):  19" diameter x 28" stroke
Driving wheel diameter:     6ft-2ins
Boiler pressure:    225 lbs per sq. inch
Tractive effort: 26,120 lbs (at 85% boiler pressure)
Length over buffers:  62'-7" (loco & tender)
Weight of loco in working order:   76tons 0 cwts.
Weight of tender (BR1B): 51 tons 5 cwt (7 tons coal)
Weight of tender (BR1C): 53 tons 5 cwt (9 tons coal)
Water capacity (both types):  4725 gallons

The vast majority of British steam railway locomotives were fitted with either WALSCHAERTS or STEPHENSON valve gear, both of which utilise a series of cranks and levers to actuate PISTON valves to control the admission and exhaust of steam to and from the cylinders.

CAPROTTI valve gear is considerably different in that uses cam actuated POPPET valves (similar to those in an internal combustion engine) to control the flow of steam into and out of the cylinders.

Separate inlet and exhaust valves are fitted at each end of the cylinders and these are actuated by cams enclosed in an oil bath mounted above the cylinders. The rotating cams are are driven through a triple jointed shaft from a worm gearbox mounted on a return crank on the centre driving wheel. The cams push the poppet valves open into cages as required.

A separate gearbox, mounted behind each cylinder and controlled by the driver, allows angular adjustment of the cams to provide an infinitely variable cut-off. The gearbox also advances or retards the cams to allow the locomotive to reverse.

A pilot valve in the regulator admits a supply of saturated steam to the underside of the poppet valves to close them before the main superheated steam reaches the cylinders. When the regulator is closed, the poppet valves automatically open to allow the loco to coast freely.

* * *

Invented in the early 1920's by Italian architect and engineer, Arturo Caprotti, the use of Caprotti valve gear was never widespread in the UK. In the late 1920's, the LMS experimentally rebuilt 10 ex-LNWR "Claughton" class locomotives with Caprotti valve gear, but in practice, the improved performance (over 20% reduction in coal consumption) did not justify the high initial cost of the gear.

In 1948, in the early days of British Railways, it was decided to build 20 new LMS "Black 5" locomotives (44738 to 44757) with Caprotti valve gear. The totally enclosed mechanism had advantages from the maintenance viewpoint, and although the locomotives were fast and free running, their acceleration was inferior and they were generally regarded as being weaker than their piston valved counterparts.

In 1951, the final two "Black 5" locomotives (44686 and 44687) were built with an improved type of Caprotti valve gear which had been developed in the UK by a private company, Associated Locomotive Engineers, under the leadership of Mr. L.A. Daniels. The British Caprotti valve gear, as it was known, overcame many of the earlier problems and as a result of the experience gained, it was decided to fit it to the new BR Standard Class 8P 4-6-2 no. 71000 Duke of Gloucester, and the final batch of 5MT 4-6-0 locos.

The Standard Class 5 Caprotti locos were particularly fast and free running and speeds of 90mph were not uncommon. One is even reputed to have reached 100mph whilst working a test train. There seems little doubt that had steam locomotive construction continued in the UK, the British Caprotti valve gear would have been extensively used.

NEXT