BACKGROUND
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.
TYPES OF OILS
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.
COMPOUNDED
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
TEMPERATURE AND PRESSURE CONSIDERATIONS
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.
STEAM CYLINDER OIL RECOMMENDATIONS
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.
WHAT NOT TO USE
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.
