Caprotti valve gear

TheCaprotti valve gearis a type ofsteam enginevalve gearinvented in the early 1920s byItalianarchitect and engineerArturo Caprotti.It usescamshaftsandpoppet valvesrather than thepiston valvesused in other valve gear. While basing his design on automotive valves, Caprotti made several significant departures from this design to adapt the valves for steam. Having agreed a joint-venture withWorcester-based engineering companyHeenan & Froudefrom 1938, Heenan & Froude fully acquired Caprotti post-World War IIin 1947.

Duke of Gloucester

Caprotti valve gear on BR no. 73129

Usage in Italy

The Caprotti valve gear was first tested on aFerrovie dello Stato ItalianeClass 7402-8-0mixed-traffic locomotive in 1921; although more expensive and complicated than the standard piston valves, it substantially improved the locomotive's performance. Until the 1930s it was fitted on some 334 FS locomotives and on 77 narrow-gauge locomotives of other companies; of the former, some were new-builds, the others were rebuilds of non-superheated compound locomotives.

Given their need for more complex and expensive maintenance, most of these locomotives were generally withdrawn from service before those withWalschaerts valve gear,in the 1960s.

Usage in Great Britain

In August 1926, theLondon, Midland and Scottish Railwayequipped four-cylinder 4-6-0 locomotive no. 5908 of theClaughtonClasswith Caprotti valve gear and poppet valves. Following trials, nine more were rebuilt in 1928 with Caprotti valve gear, poppet valves and larger boilers, and also in 1928 ten others of the same class were given the larger boiler but retained theWalschaerts valve gearand piston valves with which this class was originally fitted, to enable comparisons to be made between the two types of valve gear. Later that year, no. 5908 was also given a larger boiler. It was found that the Caprotti-fitted locomotives were more economical on coal and water than those with Walschaerts valve gear, but it was later found that some of the losses of the Walschaerts locomotives was due to leakage of steam past the valve heads, where a single wide ring was used. New piston valves having several narrow rings were fitted to one locomotive, and it was then found that the Walschaerts valve gear could be just as economical as the Caprotti, but with considerably less cost of fitment. No moreClaughtonswere fitted with Caprotti valve gear, and the ten locomotives were withdrawn in 1935–36.

On theLondon and North Eastern Railway(LNER), two 4-6-0 locomotives ofLNER Class B3were rebuilt with Caprotti valve gear in 1929, followed by a further two in 1938–39. These locomotives also had four cylinders, and the poppet valves were mounted vertically, two at each end of each cylinder. One of the first pair of locomotives was rebuilt withWalschaerts valve gearin 1943, but the other three ran with Caprotti valve gear until withdrawal in 1946–47.

British Caprotti

In the 1950s, Caprotti valve gear was improved and thisBritish Caprotti valve gearwas fitted to the last twoBritish Railways-built'Black Fives'44686/7,the last 30BR standard class 5s,numbers 73125-54, and the uniqueBR standard class 8 71000Duke of Gloucester.Results were mixed, with the performance of theDuke of Gloucesterbeing particularly disappointing. That was later found to be due to errors elsewhere in the design and construction of the locomotive.

Although more expensive to manufacture than its rivals, the improved Caprotti valve gear is considerably more efficient than any other. A major improvement is that much of the mechanism is enclosed, leading to reduced wear and tear from the harsh steam locomotive environment, and completely independent control of admission and exhaust. The restoredDuke of Gloucester,with its flaws eliminated, has proved the concept.

External links

• British Caprotti Valve Gear on the Duke of Gloucester
• BR Standard Class 5, No 73129with Caprotti gear, Midland Railway - Butterley.