An automatic lubricator is a device fitted to a steam engine to supply lubricating oil to the cylinders and, sometimes, the bearings and axle box mountings as well. There are various types of automatic lubricator, which include various designs of displacement, hydrostatic and mechanical lubricators.
Displacement lubricator
Ramsbottom type
The displacement lubricator was introduced in the United Kingdom in 1860 by John Ramsbottom. It operates by allowing steam to enter a closed vessel containing oil. After condensing, the water sinks to the bottom of the vessel, causing the oil to rise and overflow into delivery pipes. The oil from the delivery pipes is introduced into the steam pipe, where it is atomised and carried to the valves and cylinders.
In early applications in steam locomotives, either two displacement lubricators (one for each cylinder) would be positioned at the front of the boiler near the valves, often on either side of the smokebox or one lubricator would be placed behind the smokebox. The behind-smokebox configuration has the advantage that a good connection can be made to the steam pipe and it was used by the Great Western Railway. It has the disadvantage that the lubricator's accessibility is reduced and additional drain pipes are required to be connected to the waste to avoid it dripping onto the boiler.
Elijah McCoy, a Canadian who moved to Michigan and became a U.S. citizen, received a patent for his automatic lubricator in 1872. Piping delivered oil by gravity from a central reservoir to where it was required. Later types of lubricator (from around 1887), referred to as the "sight-feed" type, allowed a sight glass to be positioned in the cab where the rate of oil feed could be observed. In 1898 McCoy's further patent added a glass tube mounted below the reservoir so that the rate of delivery could be monitored, with a bypass pipe available in case the main feed was seen to be blocked.
The displacement lubricator was a useful stop-gap but had the disadvantage that it was difficult to accurately control over the rate of oil feed and lubrication was only supplied when the engine was doing work (when a locomotive is coasting with the regulator closed, no steam is present to operate the lubricator).
Roscoe type
The Roscoe type lubricator improved the situation by providing a valve with which to regulate the flow of steam and hence lubricant. Setting the valve correctly required experience and depended on the speed of the train.
The Roscoe lubricator was invented by James Roscoe in 1862 and patented in British Patent 1337. It incorporated two improvements over the original Ramsbottom type, the ability to control the quantity of steam which entered the lubricator, via a control valve, and the addition of an air filled chamber within the oil reservoir. The chamber expanded when steam was cut off, the aim of which was to supply oil even when the locomotive was coasting. However, this proved ineffective in practice, particularly if the locomotive was coasting for a long time (e.g. down a hill).
Replacement
When more sophisticated lubricators, such as the Wakefield and Detroit types, were developed, displacement lubricators fell out of use but they are still used on model steam engines.
Hydrostatic lubricator
The one-piece lubricator body is a gunmetal casting and incorporates the condensing chamber, the oil reservoir and the sight-glasses fitted at the front and back of the body. Passages within the body interconnect these areas. The body is fitted with a steam valve, a water valve and oil control valves. In operation, steam enters the condensing chamber and the condensed water passes into the bottom of the oil reservoir and the sight-glass chambers. When the sight-glass chambers have filled with water the oil delivery for each feed, regulated by control valves, is released upwards into the water-filled sight-glass chambers. The rate of delivery, typically two or three drops of oil through each feed per minute, is monitored by watching the progress of the drops as they rise through the water. The oil gathers above the water and passes into the delivery tubes. The body is fitted with a drain for emptying the condensed water from the oil chamber before it is refilled with oil.