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"Notes from the Unit Shop"

Kevin O'Connor's advice for the beginning small scale live steamer





Reciprocating steam engines, like the ones that power small scale live steam locomotives, require the use of steam oil to internally lubricate their moving parts. These parts include steam admission valves, pistons and cylinders, and valve and piston rods. Steam oil is introduced into the steam supply in one of three ways: mechanical injection via a mechanically driven pump (Aster Big Boy), Roscoe (dead leg) displacement lubricator (most Aster locomotives), and the pass-through displacement lubricator (AccuCraft, Roundhouse, etc.)  Steam oil must possess unique characteristics to allow it to mix with saturated steam and hot water (condensate) within the admission valve and cylinder assemblies, and to provide a sufficient lubricating oil film between the engine’s internal moving parts at all times.




There are four types of oil used for purposes of lubrication. They are: animal, vegetable, mineral, and synthetic. Animal and vegetable oils were the first used by man as lubricating mediums. Products such as tallow and castor oil come to mind.  Much later the discovery of petroleum, as it naturally welled up from the ground, led to the use of mineral oils.  In the modern era scientists have manipulated molecules to give us synthetic oils that surpass the desirable characteristics of the naturally occurring oils, although at an increased cost. Animal oils are produced by the process of rendering (heating/boiling) animal carcasses, and collecting the liquid residue.  Vegetable oils are produced by squeezing the oil rich parts of the plant selected, and collecting the fluid that is pressed out.  Mineral oils are produced by heating the raw petroleum collected from wells drilled in the ground, and drawing off, at different temperatures, various products such as gasoline, kerosene, diesel, lubricating oil stock, and asphalt. Many synthetic oils use a form of silicone as their base. Most oils, with the exception of animal based oil, displace water from any surface where they contact.  This phenomenon is a key physical property in the selection of a proper steam oil.




Water will displace most oils, with the exception of animal based oils, and consequently special compounded oils which will lubricate in the presence of water are needed for successful steam engine operation.   In the early days of steam machinery pure animal oils were used for internal lubrication, but there were problems.  One was that the quality of animal oil products varied greatly by district and renderer.   Another was that common animal oil products were not stable at ambient temperatures and they would oxidize in the presence of air and become rancid.  These problems were overcome by the petroleum industry by blending a base mineral oil that would meet the requirements of viscosity, lubricity, and film strength , and then adding a small percentage of animal oil in the form of tallow.  Modern steam oils contain 4% tallow oil by volume.  It is this tallow oil that makes steam oil work in the hostile internal environment of the steam engine. In practice the petroleum producers place several compounds in steam oil to help stabilize viscosity and lubricity; hence the name compounded steam oil.  The color of steam oil is determined by the origin of the base oil and so it is imprudent to assess the viscosity of oil by its color. Lighter color steam oils are not necessarily of a lower viscosity than darker color steam oils




Small scale live steam locomotives (32 &45mm) never require boiler pressures in excess of 100psig, and most operate at pressures below 65psig.  In the full scale steam trade, steam produced at these pressures is known as low pressure steam (below 150psig), and as wet and very wet steam.  The use of “superheaters” in these scales, while beneficial to model operation, does not elevate cylinder temperatures above that indicated in the boiler. Steam pressure and temperature at the cylinder is the primary consideration in selecting the correct steam cylinder oil.  As a general rule the higher the steam pressure and temperature, the greater the need for a higher viscosity of oil, and the lower the amount of compounding which is required.  If temperatures and pressures are relatively low, then lighter viscosity oils with high percentages of compounding are most effective.  Small scale live steamers fall into the latter category.  It is wise to note that there is a marked difference in the selection of viscosity of lubricating oils in steam engines as opposed to automobiles.  The proper selection of the viscosity of steam cylinder lubricating oil is dependent on cylinder temperature while automotive lubrication is routinely determined by ambient temperature.  If one operates at a set boiler pressure no, mater the outside temperature, the temperature of the pass-through steam oil lubricator vessel will always be the same. Locomotives that utilize the Roscoe (dead leg) lubricator located on the front buffer beam or on the running board will see a lower temperature than experienced in the pass-through lubricator, but not by much.



No producer of steam cylinder oil provides a product tailored directly to the requirements of small scale live steam locomotives.  Standard product recommendations start at steam pressures of 150 to 200psig (366 to 388F). The grade of recommended steam cylinder oil for these conditions is ISO 460 which contains 4% tallow oil.  This is the grade of oil that the “ride-on” locomotive community uses. It is only available in 55 gallon drums. Small scale live steamers who live near a preserved railroad or a ride-on club track may be able to purchase a supply of ISO 460 by the bring-your-own-gallon-can method.  Chevron USA has a relatively new steam cylinder oil on the market that is lighter in viscosity than ISO 460 by about half  (1103 SUS vs. 2335 SUS @ 100F).  I know of no source for it in smaller quantities than a 55 gallon drum, and since I have not used it I can’t recommend its use to others. 




Small scale live steam locomotive operators should never use anything other than compounded steam cylinder oil, containing 4% tallow, for the lubrication of the internal parts of a locomotive’s steam engine.  Other mineral based oils such as internal combustion engine oil and machine oil should be avoided because they will not be able to maintain their lubricating properties in the presence of hot water and steam.  The result will be metal-to-metal contact between the internal parts of the steam engine assembly, which will result in the scoring and galling of mating wear surfaces.  Likewise vegetable oils, such as castor oil, should not be used either for the same reasons.  Castor oil is used as a lubricant in both two and four stroke model airplane engines because it mixes easily with both gasoline and alcohol based fuels and there is no free water present during the combustion process.  In the early days of aviation some full scale airplane engines, like the French built Le Rone rotary engine that operated on the two cycle principle, used castor oil mixed in with gasoline for internal lubrication.  




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