Walschaerts valve gear

The Walschaerts valve gear is a type of valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 used to regulate the flow of steam to the pistons in steam engines. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. It was extensively used in steam locomotives from the late 19th century until the end of the steam era.

History

The Walschaerts valve gear was slow to gain popularity. The Stephenson valve gear remained the most commonly used valve gear on 19th-century locomotives. However, the Walschaerts valve gear had the advantage that it could be mounted entirely on the outside of the locomotives, leaving the space between the frames clear; which resulted in it being adopted in some articulated locomotives.

In 1874, New Zealand Railways ordered two types of Double Fairlie locomotives from Avonside. Both the B class and E class Double Fairlies were fitted with Walschaerts valve gear. This was certainly the first use of this technology to be used in New Zealand, and is possibly the first time a British manufacturer had supplied it.