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With tribological systems, it is almost always essential to place a lubricant between the friction partners. The functioning and service life strongly depend on
whether the lubricant remains in the right place.
The amount of lubricant that is used in technical systems varies widely. A few thousand liters of oil can be used in the gears of a ship turbine. In the gears of
a watch, a microscopic amount of oil is sufficient: less than 8 microliters. 1 ml = 1000 microliters.
An average size tear of joy contains around 40 microliters.
In high-precision ball bearings that for example are used in momentum wheels for satellites, one milliliter of oil is diluted with 1,000 ml of highly-pure n-hexane.
The runways are lubricated with this lubricant solution. The lubricant film that remains after the solvent is diluted is discernable by the shimmer of its Newtonian rings.
The viscous friction of a larger amount of lubricant would excessively affect the gyration of the wheels. The projected life of this gyroscope is about 10 years.
Such momentum wheels that control position cannot be relubricated in space. If all three electrically driven momentum wheels fail, the satellite starts to wobble uncontrollably.
In the next NASA mission, the Hubble space telescope will get new wheels. The "repair men" will fly up in the space shuttle, change the wheels, and do a few other odd jobs
on a space walk.
When friction bearings that have been lubricated with grease are checked, an experienced service mechanic will always check the grease collar in the bearing gap. If the
grease is visible and light in color, the bearing will work just fine. Adding fresh grease also expels all the wear particles and impurities out of the bearing.
The same holds true for oil-lubricated porous bearings. If oil is visible in the bearing gap, the bearing will run smoothly with minimal noise. However, the oil can resinify
due to age.
Silicone oils change over a few days to transparent silicone rubber at temperatures above 150°C. In addition to a visual check, a haptic test is useful.
Presence is required
The most important property of a lubricant is not its highly-complex composition. It is the simple fact of its presence. Obvious, but true.
Despite the apparent triviality of this claim, it is incontrovertible and highly significant. Even hand sweat can sufficiently lubricate a polymer friction bearing for many hours
of operation. However, increasing the amount of lubricant by a factor of ten does not automatically increase the product life.
"More is better" is nonsense from a tribological vantage point. Interestingly, an additional drop of oil or daub of grease is still used here and there in most production lines.
The thought is, well, this should help things a bit.
Not present
Once the lubricant is gone, the tribological system runs dry.
Lubricated metal friction bearings can have friction factors of 0.01 and less in normal hydrodynamic operation. If they run dry, the friction factor increases to nearly 1. This
in an increase by a factor of one hundred. Of course, the friction performance is also 100 times higher. The ratio of developing friction is therefore 1:100.
A comparison by way of illustration: A light bulb in a bicycle light puts out about 1 watt, and a thick soldering iron emits about 100 watts, which yields a ratio of 1:100.
A properly dimensioned lubricated friction bearing is only a few degrees centigrade above the ambient temperature, even when under a substantial load. With mixed friction,
the bearing temperature quickly rises to about 80°C to 150°C. When the bearing runs dry, it can take only a few seconds before it fails.
When there is wear, the differences can be be even more dramatic. A well-lubricated bronze bearing can work perfectly for fifty years. When it runs dry, abrasion particles dribble
out of the bearing gap after a few seconds. Service life ratios of 1:10 4 or even 1:10 6 are quite possible.
What influences the presence of a lubricant in the tribological system?
We will leave aside the discussion of leakage from damages seals for the moment. Let us discuss the forces that can keep the liquid lubricants within the bearing gap.
Can oil dry?
Service personnel in the fields of micromechanics and clock engineering have repeatedly found that bearing gaps which were sufficiently filled earlier have become empty.
The oil apparently drained, evaporated or dried.
Of course, all liquids and hence lubricants have a measurable vapor pressure. With the exception of ionic lubricants, someone always likes to point out. In normal atmospheres,
the loss of volume from evaporation is so slight that it should not be problematic.
The service technicians were absolutely right. However, the oil did not evaporate; it spread.
The term spreading (or migration) means that lubricants creep and wet the surfaces of solid bodies. There are a series of simple but important relationships that will be presented
and can be understood without a great deal of effort by performing your own experiments.
? How does a liquid behave on a surface?
The surface is wet or de-wet, depending on various parameters.
Influential parameters for the wetting behavior:
- Surface tension of the base oil
- Surface tension of the solid body
- Cleanness of the surface
- Separation in the case of grease
- Surface texture
- Roughness of the surface
The viscosity and temperature influence the behavior over time.
Fig. 9.01: Wetting behavior
In a room without any gravity, a volume of liquid forms a sphere. This is caused by the surface tension of the liquid. Astronauts have fun with this phenomenon and drink the
ball of liquid with a straw. This is one of the few experiments where it is absolutely clear that the scenes were not shot in a studio.
If a small spherical drop of liquid contacts another surface, it either is absorbed by it (water drops), or not (mercury). The property of the surface plays an important role Fig. 8.02.
Fig. 9.02 Water drops on an impregnated tablecloth
If the drops on the tablecloth are drops of red wine, do not rub them to get rid of them. Only daub at them or carefully absorb them. Rubbing presses the liquid into the fiber
capillaries of the tablecloth, and a red wine spot arises despite the impregnated water repellant.
If a drop of mineral oil lands on a dull surface, a grease spot arises that becomes increasingly larger depending on the amount oil.
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| Fig. 9.03 |
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Fig. 9.04 |
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Fig. 9.05 |
Normally, spreading is radial with no preferential direction
Fig. 9.06 Polyglycol oil and silicone oil.
Situation a few seconds after the drops are applied to a slide.
In an experiment with polyglycol (with a surface tension of 38 mN/m) on a very smooth surface such as a mirror, the drop remains localized and retains a domed shape.
A silicone oil (Osp 21 mN/m) spreads.
Silicones like to creep so much that they can contaminate an entire production facilities without it being noticed. The squeaking of rubber shoe soles on floors is frequently an indication
of such contamination.
Many companies, especially electronics companies, strictly forbid silicone.
The surface tension of the liquid and apparent surface tension of the solid body compete with each other. If the surface tension of the solid is much higher, the surface becomes
completely wet. Mercury does not wet, it rolls away under gravity.
? What is the effect of capillary force?
Due to the cohesion and adhesion of a liquid and the material, the liquid arises by itself in the narrow tubes.
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| Fig. 9.07 |
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Fig. 9.08 |
Different capillary diameters (left), and different surface tension (right).
The height that the liquid rises directly depends on the capillary diameter. The water can rise for meters in a very narrow capillary tube. All plants uses this effect. In a capillary with a
internal diameter of one millimeter, the water column does not visible rise. Water with its high surface tension has a high capillary rise, and mineral oil rises one-third as high. The higher
the surface tension, the higher the capillary rise.
Cold water has a higher surface tension than hot water. When you spray hot water on walls in a shower, all the water drops run off.
The narrower the capillaries, the greater their suction. If one end of a narrow capillary is placed against the end of a wide glass capillary, the narrow one will completely drain the wide one.
? What happens with oil-saturated porous bearings that are left over night on blotting paper?
They will be completely drained.
? Why does this not work on a glass plate without filter paper?
In contrast to blotting paper, the glass plate does not have any capillary force, but the porous bearing has strong capillary force.
Once you life the saturated porous bearing from the glass plate, you see a wet spot. This results from the narrow gap between the bearing face and glass plate.
Capillary force acts between parallel surfaces as well as within tubes.
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| Fig. 9.09 |
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Fig. 9.10 |
If two surfaces are placed together, a flat gap with active capillary force arises. The closer the surfaces are, the larger the force. If the surfaces are gradually separated, the opposite happens.
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Fig. 9.11
Experiment in a corner.
Experiment with two slides and colored water. This works well with coffee!
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Under the effect of capillary force, oil creeps 15 mm in a corner within a few seconds. The corner may not be strongly rounded.
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Fig. 9.12
The blue water always creeps wherever the gap of the two slides is narrowest. The surfaces opposite the gap are sucked dry.
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Fig. 9.13
Two straightened paper clips and a low-viscosity mineral oil can nicely illustrate the gap-width-dependent capillary effect. The liquid drop always moves to where the gap is narrower.
The liquid follows the gradient.
? Is there such a thing as an anti-capillary force?
Apparently so. The opposite of a narrow capillary tube is a pointed sewing needle.
Fig. 9.14 Oil drop at the tip of a needle
If the needle point is dipped in oil and is slowly removed (the operative word is slowly), there is no oil drop at the end. However, if you withdraw the needle quickly, the oil cannot flow
down fast enough due to its viscosity. Now a large magnifying glass is needed. If you look at the needle tip very closely, you will not find any oil there. The drop is about one millimeter up.
The tip appears to repel it. In addition: This experiment does not work with a razorblade.
Fig. 9.15 The edge repels water.
The repulsion is strong enough for the edge to project a few millimeters beyond the liquid.
An anti-capillary force arises at the edge that repels the liquid. The edge may not be strongly rounded, however.
If you tip over a wine glass, the wine only flows to the edge of the table.
Fig. 9.16 Oil flow in a bearing
In a friction bearing a gap always arises between the shaft and bearing hole. This gap has a strong capillary force and retains the liquid lubricant. The same holds true for the
separation of oil from a grease.
When the gaps are narrow, this force is large enough to lubricate the bearing once for life. Examples are clockworks, measuring devices, electricity meters, etc.
If you want to prevent the oil from creeping out of a bearing, or even the active flow of oil into the bearing, the described effects can be exploited and design the bearing gap so
that the oil is in a gap that narrows toward the bearing.
The spreading and migration of lubricants can cause substantial problems.
Typical damage processes
Lack of lubricant
From the spread of bearing lubricants: In this case, the oily component of the lubricant creeps toward neighboring housing surfaces of the bearing, and there is a significant loss
of lubricant over time until the bearing dries out and fails. There are known examples where the greases used to lubricate gears have coated all the parts in the housing and have
reduced the actual lubrication by more than 80%, and the components failed before they reached one-tenth of their useful life. A second, typical type of damage arises from the
uncontrolled and undesirable creep of large amounts of lubricant. The selection of a high-viscosity product does not prevent this effect. It only delays it. The viscosity scarcely influences
the surface tension.
Physical and chemical reactions
The creeping lubricants reach parts of the components where they can cause hazardous changes. Such changes can affect the materials or electrical system. In physical and chemical
terms, the lubricant interacts with materials that are incompatible with the lubricant. Amorphous thermoplastic materials can form stress cracks. In this case, even minute amounts of
creeping lubricant contributes to the the initial formation of stress cracks. Surface cracks form in the components where there are significant differences in wall thickness, or where stress
is frozen in the component due to injection molding processes. These cracks can propagate until the parts of the components such as snap-in connections or printed circuit board columns
break off. To date, there has been on complete explanation of this process. Amazingly, amounts of oil below the existing detection limit are enough to cause problems.
In addition to the worst-case scenario of influencing materials, there can be swelling, softening or brittling materials, seals, paints, adhesives, etc.
Electrical problems
There are several mechanisms that can damage electrical components. One of these is the direct influence of electrical contacts by the property of the lubricant. If contact fires occur, the
lubricant decomposes and reaction products form that can have catastrophic results. Brush fires can also burn residual lubricant and render it harmless. Electrically conductive adhesive
wear particles can fill commutator gaps. A fire hazard exists. Silicone oil reaction products are critical that form glass-like coatings in the microscopic area of contact. Silicone-containing
products are especially feared because small amounts can be transferred to surfaces in vapors.
Example: Potentiometer errors
A film of lubricant in conjunction with wear processes have caused previously unknown problems on potentiometer tracks that have highly complex layers. All potentiometer tracks and the
associated wipers are designed so that a certain amount of wear is desirable, and the components are constructed to erode. This wear cleans the sliding elements within defined parameters
and hence yields constant contact resistances. The arising wear particles are brushed off by the wiper out of the wear zone. At the surface wet by the lubricant, the abraded material from the
resistance track forms a paste with the lubricant. The particles are no longer loose and separate; they agglomerate to form larger masses that can lift the wiper finger. This effect can cause
an unforeseeable total failure of the system. In components that affect safety such as electronic control elements in automobiles, this effect can cause extremely annoying and hazardous
malfunctions.
? Can you increase the surface tension of a lubricant with additives to eliminate these problems?
No, not without ruining the lubricant properties.
? How can you prevent the spread of oils?
By coating the surface of the components.
Fig. 9.17 Oil drops on an untreated surface (bottom) and an epilamized surface (top)
Fig. 9.18 Leaf of the lady's mantle (Alchemilla vulgaris)
Epilamizing process
As described, the spreading of the lubricant can be very problematic because the lubricant can leave the friction site, and the friction and wear can substantially increase. In addition, physical
and chemical reactions can destroy components and functions.
? Can the spread or creep of oils on metals or plastics be effectively prevented?
Yes, by a highly special treatment of the surface.
Epilamizing is an effective method for solving the technical problem of creep. When a component is epilamized, the apparent surface tension of the components is lowered by applying a
PTFE-like film to the surface. The word epilamizing is derived from the Greek word epilam, which means skin.
Fig. 9.19 Spreading on an untreated surface
Fig. 9.20 Spreading prevented on a treated surface
Application and advantages:
- Dipping, spraying, stamping
- Easy and economical
- Not visible
- Does not influence the other properties of the base materials
- Layer thickness: ~0.01 to 0.04 µm (invisible)
- Effect on all materials (metals, polymers glass, sapphire, etc.)
- Restricted effect on fluorinated materials
Fig. 9.21
The epilame molecules form a fluorine brush with oil-resistant properties. Oil cannot creep on the "brush."
By epilamizing component surfaces that are to be lubricated, lubricant loss or insufficient lubrication can be prevented. Lubricants cannot creep on epilamized surfaces. Sensitive components
remain lubricant-free. Contamination or damage is prevented.
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