Proper lubrication techniques extend life of gearing equipment
Written by Jules DeBaecke
For
owners and operators of gearing equipment, the ultimate goal is to
achieve a return on investment by maximizing the output, reliability
and efficiency while minimizing down time and operating costs. Keys to
doing this effectively are understanding your gearing equipment’s
lubrication system and ensuring that the system has the correct type
and supply of lubrication oil.
VISCOSITY
A variety of criteria must be considered before choosing a gear
lubricant: the unit’s operating speed and load, temperature range and
lubricant availability, to name a few. However, the most important
factor is viscosity, which changes appreciably with temperature and is
generally stated at two temperatures, 40°C and 100°C. Lubricating oil
viscosity is usually expressed in terms of the time required for a
standard quantity of a fluid (at a given temperature) to flow through a
standard opening.
High-speed units produce an acceptable oil film at the contact area
even with a low-viscosity oil. Gearing units that operate at lower
speeds produce a thinner oil film, which requires more viscous oils to
separate meshing tooth surfaces. Still, often a gearbox will contain
both high and low-speed gear meshes. In general, the lowest viscosity
oil capable of forming an adequate oil film at all operating conditions
should be chosen. However, in practice, the lubricant chosen is often a
compromise between the requirements of the various lubricated
components - such as gears and bearings - and the particular
application requirements - such as large ambient temperature
differentials.
PETROLEUM-BASED OR SYNTHETIC?
Petroleum-based mineral oils are complex mixtures derived from the
refining of crude oil and have been found to excel as lubricants in
most applications. These oils are usually compounded with different
chemical additives to improve specific properties, such as increased
lubricant life, resistance to rust and oxidation and increased
load-carrying capacity.
More specialized high-load oils, also called extreme pressure gear
lubricants, contain selected additives that increase the load-carrying
capacity of gearing by forming a film on the metal that provides
component separation under higher load conditions. Often times, these
lubricants will contain more than one chemical additive for load
capacity enhancement over a wide temperature range.
Plant managers may also consider synthetic lubricants. Operating
parameters may include high-temperature thermal and oxidation
stability; low-viscosity variation over a broad temperature range;
low-temperature capability; and long service life. Since synthetic
lubricants can be up to four times more costly than petroleum-based
oils, they are generally reserved for problem applications, such as
extremely high or low temperatures, equipment subjected to frequent
overloads, and equipment with a marginal lubrication system.
LUBRICATION SYSTEMS
There are two types of gearbox lubrication systems that are in current
use: splash and force-feed lubrication systems. Both types are used to
distribute sufficient oil to each component of the gearbox while
minimizing the generation of heat by oil churning and oil foaming.
• Splash lubrication: It requires that the gearbox be filled to a specific
lubrication oil level, then rotating gear elements within the gearbox
dip into the oil and “sling” it into troughs, pockets or directly to
bearings and gear meshes requiring lubrication and cooling oil. Feed
troughs are employed to capture oil that is “slung” onto the upper
gearbox-housing wall by the dipping gear element. This oil drips into
the trough, which, in turn, distributes the oil to the bearings.
Splash lubrication systems are far simpler and less expensive than
force-feed but are applicable only to low-speed gear units. As shaft
operational speeds increase, the heat generated in the gearbox becomes
excessive, requiring an external, force-feed lubrication system to
supply larger volumes of lubricant to lubricate and cool gearbox
components.
• Force-feed lubrication: A shaft-driven or motor-driven oil pump draws oil from the
gearbox sump through a suction pipe. The oil is directed from the
pressure side of the oil pump through a filter to cleanse the oil, and
through a cooler. A pressure relief valve is typically located before
this filter to protect the system from too high an operating pressure.
If the filter becomes clogged, the relief valve will permit the
unfiltered oil to bypass the filter so the gearbox will continue to
receive lubrication, albeit unfiltered.
Relatively little oil is required for lubrication using a force-feed
system, provided it is properly directed. The bulk of the oil flow is
required for cooling the gear tooth flanks and bearings, not just
lubricating.
HEAT REMOVAL
The lubricant’s ability to remove heat and cool the gearing unit is
just as important as lubricating gearing and bearings. For every gear
drive there is a thermal rating. If the thermal rating is less than the
mechanical rating, additional cooling must be supplied by a force-feed
lubrication system.
If the lubrication oil alone is unable to remove the necessary amount
of heat, auxiliary cooling can be used in combination. For example, to
increase the thermal rating of a gearbox with a splash lubrication
system, air can be forced past the radiating surfaces of the gear
casing by strategically placed fans external to the gearbox. A fan
attached to the high-speed pinion shaft external to the gear housing is
one such application. In addition, the unit can be cooled by a water
jacket, which consists of water passages that are built into the gear
housing, usually at the high-speed end.
Understanding these basics can help maintenance technicians and plant
managers stay on the forefront of preventive maintenance measures and
help extend the lifecycle of their rotating equipment.
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For
owners and operators of gearing equipment, the ultimate goal is to
achieve a return on investment by maximizing the output, reliability
and efficiency while minimizing down time and operating costs. Keys to
doing this effectively are understanding your gearing equipment’s
lubrication system and ensuring that the system has the correct type
and supply of lubrication oil.
