Displacement Lubricators

Lubrication is required for every moving part of moving machinery, including steam engines. And the cylinders are one of the most important. A lubricator is a required item to inject lubricating oil into the cylinders. There are many types of lubricators, but one of the oldest, and simplest, is the displacement lubricator, also known as a hydrostatic lubricator.

But first, a little bit of history about this type of lubricator.

English engineer John Ramsbottom is usually credited as making the first commercial displacement lubricator, which he patented in 1860 [British Patent No. 2460]. It was offered commercially soon after.

Ramsbottom ad
Ad in The "English Mechanic" Sep 1865
Roscoe lubricator

James Roscoe, a district superintendent for England's Midland Railway patented a variant in 1862 [British Patent No.1337] with a regulator valve to set the feed rate of the lubricant. In 1870, the Society of Engineers in London reported that Roscoe's lubricator was already being used on "some thirty lines of railway in Great Britain and on a great number of railways on the [European] Continent, and in America and Australia". It was also employed on "many stationary and marine engines".

E. L. Ahrons, in his 1922 book, "Lubrication of Locomotives", said "The Roscoe lubricator, which was invented in 1862, was certainly one of the most successful and widely applied of any form of cylinder lubricator, and held its own to the exclusion of others for some twenty-five years, until it gradually began to be superseded from about 1887 by the 'sight-feed' type. Even in the early 'seventies' practically every Midland engine was fitted with Roscoe lubricators."

American inventor Elijah McCoy (1843-1929) received a patent [US patent No.129843 of 1872] for an automatic lubricating device. Previously, engines had to be stopped before necessary lubrication could be applied. McCoy's invention allowed engines to be lubricated while they ran. The McCoy lubricator was superior to others available at the time, and its reputation spread. Users of heavy equipment, wary of buying cheap substitutes, often asked for "the real McCoy", a phrase that is still in use today.

How do these devices work?

Displacement lubricator
Displacement lubricator

The principle is quite simple.

A displacement lubricator is basically just a closed oil tank with a small pipe connecting it to the main steam pipe which feeds the cylinders.

Steam from the main steam feed pipe enters the oil tank and condenses into water. Due to the difference in specific gravity between water and oil, the oil floats on top of the water. As the water level increases it displaces the oil which runs into the main steam pipe. The oil is then picked up by the steam flow where it is atomised and emulsified and carried to the cylinders.

The process is continuous, and repeats until all the oil is gone.

A filler plug is fitted to permit filling with oil, and a small drain plug is fitted to the base of the oil reservoir for draining the condensed water.

A needle shut off valve is sometimes provided in the oil feed pipe to regulate oil flow, and to prevent any steam under pressure in the piping escaping though an open filler.


The displacement lubricator has been successfully used in model steam engines for many years, and as it has virtually no moving parts, is extremely reliable. The alternative is some form of mechanical lubricator.

Type of oil

There are 4 types of oil used for lubrication each has is its own characteristics and properties :-

  • Animal oils come from rendering animal carcasses. Example :- tallow, lard, sperm oil
  • Vegetable oils come from squeezing the oil rich parts of some plants. Example :- castor, olive and coconut oil
  • Mineral oils are derived from raw petroleum from oil wells, and drawn off at different temperatures during processing. Example :- kerosene, diesel, machine lubricating oil
  • Synthetic oils are manufactured, and use silicone as their base replacing the natural carbon.

Most oils, with the exception of animal based oils, displace water and do not mix with water. This property is required for operation of a displacement lubricator.

Depending on the oil, no oil in its original form is totally successful in steam engine use. Some carbonise (burn) at a low temperature, others are washed off the moving surfaces by the wet steam and are of little use. And in general, oil and water don't mix and tend to stay separated. Consequently specially compounded oils, which lubricate in the presence of water, are used for steam engine lubrication. Compounded steam oils are usually mineral based and contain 4%-6% tallow, and oxidation inhibitors, giving an oil which is thermally stable, and provides lubrication in wet conditions.

Only steam cylinder oil should be used for cylinder lubrication.

The use of automotive or machine lubricating oils is not recommended. They tend not to stay on 'wet' surfaces and are washed off, leaving no effective lubrication, and often contain additives which may be corrosive at elevated temperatures to brass or bronze which is commonly used in model steam cylinders.

Similarly vegetable oils, such as castor oil, should not be used as they also wash off. Castor oil is used as a lubricant in model aeroplane engines because it mixes easily with both gasoline and alcohol based fuels, but there is no free water present during the combustion process.


It is best to obtain and use the specially formulated steam oil to use on steam engines.