David Joy

David Joy (1825–1903) was a British mechanical engineer and prolific inventor whose most lasting contribution to railway engineering was Joy's radial valve gear of 1879, a simpler and lighter alternative to the Stephenson link motion and the Walschaerts gear that was widely adopted by British locomotive designers in the 1880s and 1890s, particularly on LNWR locomotives under Francis Webb.

Born at Leeds on 3 March 1825, Joy was apprenticed at Fenton, Murray and Jackson — successors to Matthew Murray's Round Foundry in Leeds — and subsequently worked at E.B. Wilson & Co. of Leeds, the Railway Foundry, one of the most important early British locomotive builders. His Jenny Lind 2-2-2 single of 1847, designed for the London, Brighton and South Coast Railway and built by E.B. Wilson, was one of the most successful and widely-imitated British locomotive designs of the mid-Victorian era, establishing the inside-frame single with a deep firebox as a practical and capable passenger engine type.

Joy's radial valve gear, patented in 1879, derived its motion from a fixed point on the connecting rod rather than from a separate eccentric, eliminating the eccentrics and their associated shafting while giving equivalent control over valve travel and cut-off. The gear was adopted widely on LNWR locomotives under Webb and appeared on various other classes, though it fell from favour in the twentieth century as Walschaerts gear's superior geometry at long valve travels became better understood. Joy spent much of his later career in private engineering practice, designing marine and stationary engines as well as railway equipment. His detailed diaries and autobiographical sketches are an important historical source for early Victorian engineering practice. He died at Hampstead on 14 March 1903.