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As an assembly technique, threaded connections offer a number of advantages.
They're a simple, effective method when a connection requires a predetermined
tension or compression. Their
torque can be measured in most cases,
even in service. They offer unequalled convenience for component
disassembly, permitting repeated use of a fastener and helping
ensure accurate reassembly.
However, treaded connections can present problems. Selecting
the right lubricant can mean the difference between a
failure in service and a trouble-free connection. As
exotic alloys and extreme temperatures become more common,
choosing a proper lubricant for treaded connections becomes
more significant. Sufficient lubrication takes on increased
importance in light of the potential costs of an assembly that
comes loose in service, or a seized bolt that prevents disassembIy,
especially in the held.
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Bolt Tension
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Most manufacturers rely on torque wrenches to determine bolt tension.
Yet the torque wrench fails to compensate for the fact that much of
die energy used to install an unlubricated bolt is spent overcoming
friction, not tightening the connection. Without a lube, lost energy
should be considered in calculating the bolt tension, a practice that's
often impractical. The most accurate methods for determining bolt tension
depend on measuring how much the bolt has been stretched during installation,
but these approaches are too complicated and expensive to use under normal
circumstances. Unless the threads and mating surfaces are lubricated properly,
it's uncertain how much of the torque applied to the bolt head is convened
into bolt tension. Correct bolt tension is essential to the life of a threaded
connection, especially when it's subjected to shock loading or thermal
cycling, either of which can pull the connection apart. Much like a wire
that breaks from repeated bending, continuous expansion and contraction
can fatigue the bolt. For maximum service life, the recommended preload
force holding the threaded connection together should be 80 percent of
the bolt's yield strength. The 80 percent value is how the fastener or
machine manufacturer calculated torque values for the field use. The
millwright simply uses a torque value to tension critical fasteners.
The idea is to apply enough torque to deform the bolt elastically while
avoiding the plastic region of Its stress-strain curve. Too much torque
permanently deforms the bolt and destroys its strength.
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Conventional lubricants
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Conventional oils and greases are almost always derived from a petroleum
base, and lubricate most effectively by separating metal surfaces with a
fluid film. Film formation is a result of
lubricant viscosity, surface
tension, applied load and the relative speed between moving metal pans.
Based on the characteristics of conventional oil-based lubricants however,
low sliding velocities, minimum clearances and high loads may cause the
surface peaks of the threaded connections to penetrate the lubricating film.
This penetration results in metal-to-metal contact and can lead to galling
or seizing.
Conventional lubricants are not designed to operate under the conditions
encountered in threaded connections. For instance, when fasteners are at
rest, conventional lubricants tend to drain or squeeze out. When exposed
to low temperatures, petroleum-based lubricants thicken, and when exposed
to high temperatures, they thin out or volatize off. Petroleum-based
lubricants also may present a fire or explosion hazard when exposed to oxygen
or oxidizing chemicals. Further, these "wet" lubricants attract din, dust and
other contaminants, which produce abrasion that can
damage a threaded
connection. An alternative to petroleum-based lubes can be found in a group
of products known collectively as specialty lubricants. They prevent
metal-to-metal contact when it's not possible to establish a fluid film with
a conventional lubricant. In a threaded connection, a specialty lubricant
acts as a protective layer that allows metal surfaces to slide over each
other without breaking through the lubricant film. Of the many types of
specialty products available, solid lubricants provide the best results
when used on threaded fasteners.
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Solid lubricants
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Solid lubricants prevent metal-to-metal contact when speed and load
characteristics prevent the formation of a hydrodynamic fluid film.
Solid lubricants include graphite, zinc, copper, nickel and various
metal salts, which cling to metal surfaces. Another popular solid
lubricant, molybdenum disulfide, attaches itself to the stir-face
and can be "burnished' into the metal. Under load conditions, the
powder's molecular structure prevents contact between metal surfaces
as they slide upon one another.
Coefficients of friction and thermal stability make solids effective
thread lubricants. The lube's coefficient of friction can be controlled
by varying the type and amount of solids, useful for varying the amount
of torque required to tighten a bolt to the necessary tension. In addition,
some solid lubricants, such as nickel, are effective at temperatures above
2,000 P. Powdered lubricants don't attract abrasive dust and dirt. A careful
choice of lubricant will improve the accuracy of bolt
tension measurements.
Solid lubricants minimize frictional losses and permit dose adherence to
specified tightness. In some cases, it's impossible to achieve the desired
tightness without the proper solid lubricant. For example, a leading manufacturer
of power generating equipment produces water-level gauges for high-pressure
steam turbine boilers. Each gauge requires eight gaskets of various materials,
including copper and mica. Following a specified sequence for bolting up the
gauges is as critical as it is for assembling an engine wad or turbine casing.
Proper torque is so essential to leak-tight, safe operation that the company
furnishes a special torque wrench with each gauge it sells and specifies that
cap screws be coated with solid lubricants and tightened to 60 inch-pounds of torque.
Solid lubricants can be used in various forms for different applications.
High concentrations of lubricating solids (greater than 50 percent)
are usually found in paste form, which increases the effectiveness
for extreme pressure applications. Greases for antifriction bearings
generally contain small amounts (about two percent) of solid lubricant
particles (typically molybdenum sulfide or graphite), which allow the
grease to function more effectively at higher loads and boundary conditions.
In addition, dispersions of solid lubricants in various fluids can be
used alone or added to conventional oils. Adding various binders to
solid lubricants allows them to be applied like a paint to leave a dry,
corrosion-resistant film for surface lubrication, known as anti-friction,
or A-F coatings.
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Metal Deformation
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Even the most carefully machined metal surfaces appear as
jagged peaks and valleys when examined under a microscope.
Mound the circumference of a thread, the high points of contact
are distributed asymmetrically resulting in off-center loading
and increasing the severity of contact pressures and frictional
conditions. With conventional lubrication, galling and seizing
could take place because the surface peaks of the threads collide
with each other and may interact. Wear particles are formed when
the peaks break off. Increased wear or calling occurs, and a
seizure takes place when metal fragments prevent further movement.
A specialty lubricant on a threaded connection acts as a protective
film that prevents the contact points from colliding and breaking
through the lubricant layer. For example, airline mechanics apply
solid lubricants to the threaded connections on jet engine
mountings to permit tensioning bolts properly while eliminating
the danger of piling and seizing, thereby allowing
nondestructive disassembly.
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Reduced maintenance
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Though threaded connections are convenient for disassembly,
even routine maintenance and repair operations involving these
fasteners can pose problems. Threaded connections that are
susceptible to corrosion, oxidation or carbonization can
represent significant maintenance expenses if disassembly
is required. For instance, a leading vinyl acetate manufacturer
discovered a problem on the threads of head bolts and grate
support pins used in its chemical reactors. The first time
the grates had to be removed for cleaning, corrosion seized
the bolts and pins completely. Four maintenance men worked
18 hours to burn out the threaded connections, which presented
an explosion hazard and held up other operating crews during
downtime. The company spent thousands of dollars on labor,
new parts and lost production. After the firm switched to
a solid lubricant, subsequent grate removal required two
maintenance technicians and two hours work. Maintenance costs
were reduced significantly, as the annual cost of solids-based
thread lubricants was comparatively negligible.
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Specify "smart" babes
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Proper lubricants for threaded connections are typically designed
to meet three fundamental
needs. The first is true torque values
that standard torque control techniques can replicate.
Further,
correct lubrication and tightening of critical connections ensures
proper assembly seating. Second, because they resist oxidation
and many chemicals, the lubricants protect against
corrosion.
They also reduce the destructive contact between dissimilar
metals and withstand
greater temperature stresses than
conventional petroleum-based products. And third, they allow
for nondestructive disassembly, which saves labor and equipment
costs. For maximum benefit, the wise engineer will specify the
proper lubricant before evaluating a threaded fastener for its
torque/tension conversion accuracy and dismantling performance.
Given the proper lubricant the bolt controls the torque/tension relationship.
The diverse group of solid lubricant materials offers improved performance
in applications that exceed the environmental or operational limits
of conventional products, especially under extremes of temperature,
loads and environment. Solids-based lubricants offer performance
capabilities that exceed those of conventional products.
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David Como is a Senior Lubricant Specialist/Product Steward
with the US Lubricantss Expertise
Group at Dow Corning Corp.,
Midland Mich.
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