|
|
|
|
 |
Viscosity |
|
 |
Kinematic viscosity |
|
|
Dynamic viscosity |
|
|
Viscosity index (VI) |
|
|
Long-term low temperature |
|
|
Temperatures of use
|
|
|
Penetration |
|
|
Oil Separation |
|
|
Dropping point |
|
|
Flow pressure
|
|
|
Density |
|
|
Surface tension |
|
|
Color |
|
|
Evaporation rate
|
|
|
Wettability/drop strength
|
|
|
Aging resistance |
|
|
Corrosion resistance |
|
|
Plastic resistance
|
|
|
Tribological test
|
|
|
Friction behavior |
|
|
Wear behavior |
|
|
|
|
Viscosity
The viscosity is the index for the inner friction of the lubricant.
Highly viscous (high index) means thick (such as honey), and low viscous means thin (water = index of 1). The viscosity of a lubricant strongly depends on the temperature: Under cold, oils thicken, and they become thin under heat. The viscosity is therefore determined at specific temperatures such as 0°C, 20°C and 40°C.
|
|
|
|
|
|
|
|
Kinematic viscosity
The kinematic viscosity is measured with the Stabinger viscosimeter SVM 3000 according to ASTM D7042-04 in the unit of mm²/s. The density of the fluid influences the measurement. If the kinematic viscosity is multiplied by the density of the fluid, the dynamic viscosity is obtained.
|
|
|
|
|
|
|
Dynamic viscosity
The dynamic viscosity is calculated from a flow curve that is recorded using a rotation viscosimeter (a plate/cone measuring device) in accordance with DIN 53018. The dynamic viscosity is indicated in the unit mPa.s.
This type of measurement is preferred for high viscosity oils (DIN 53018) and greases (DIN 51810).
|
|
|
|
|
|
|
|
Viscosity index (VI)
The viscosity index (VI) is a measure of the viscosity/temperature relationship (VT behavior) of a lubricant, and is calculated according to DIN 51563 and ISO 2909 from the kinematic viscosity of the lubricant at different temperatures. A VI of 100 means a satisfactory VT behavior approximating that of a good mineral oil. The best oils such as plastic oils have a VI of approximately 400.
|
|
|
|
|
|
|
|
Long-term low temperature
The long-term low temperature indicates the low temperature at which a lubricant should not be used. Since hardening processes take a very long time, the low temperature is indicated at which the oil is still liquid after 72 hours. The test method follows that of FTMS Method No. 3458.1 at which the lubricant is cooled in steps of 5°C until it hardens.
|
|
|
|
|
|
|
Temperatures of use
The bottom limit for a lubricant is determined by the hardening of the oil, and the upper limit is determined by excessively low viscosity, fast evaporation, or the chemical decomposition of the oil. The indicated temperatures of use are guidelines.
|
|
|
|
|
|
|
Penetration
The penetration characterizes the hardness of a grease. The cone penetration is determined with reference to the former DIN51804. It indicates in mm/10 how deep a standard one-quarter cone can penetrate a grease sample within a specific period. The penetration is determined for fresh grease (mpr) and sheared grease (mpw). The mpw value is used to categorize the greases into consistency classes according to DIN 51818.
|
|
|
|
|
|
|
|
Oil separation
Greases consist of an oil that has be adjusted to a specific consistency with a thickener. In order for a bearing system to be properly supplied with lubricant, the oil must separate from the grease structure and actively penetrate the bearing gap. The oil separation is measured as a function of the temperature and time according to FTMS Method 321.3 with standardized, funnel-shaped nets.
|
|
| Return to beginning |
|
|
|
|
Dropping point
Lubricating greases that for example are thickened with metal soaps are characterized by their dropping point (DIN ISO 2176). A grease sample is heated in a standardized nipple until a liquid drop exits the hole in the bottom. The dropping point is the temperature at which the grease transitions from its plastic grease structure into a liquid state. The thickener "melts" at this point. The dripping point cannot be determined for lubricating greases with solid lubricants or gel thickeners.
|
|
| Return to beginning |
|
|
|
Flow pressure
The flow pressure is determined according to DIN 51805 and indicates the pressure in mbar that is required to press a certain amount of grease through a standard test nozzle. The flow pressure can be determined at any desired temperature such as 0°C and 25°C.
|
|
|
|
|
|
|
Densitiy
The density is a physical index of the lubricant. It indicates the weight of a sample with a volume of 1 cm³. It is measured according to DIN 51 757 at 20°C in the unit of g/cm³.
|
|
|
|
|
|
|
Surface tension
The surface tension is a physical index of the lubricant. It is measured with a platinum/iridium ring using the Baron de Nouy method in units of mN/m. Lubricants with a low surface tension generally wet the surfaces well (such as silicone oils at 20 mN/m). Liquids with a high surface tension (such as water at 72 mN/m) form individual drops.
|
|
|
|
|
|
|
Color
The color can range from water-clear over all the colors of visible light to black and is described verbally. UV light or aging and oxidation processes can change the color of the lubricant (to brown or green, for example). Changes must always be considered in reference to a new lubricant.
|
|
|
|
|
|
|
Evaporation rate
The evaporation rate indicates how fast a lubricant evaporates under certain conditions. The lubricant is tested according to ASTM method No. 3480.1 with 10 g lubricant on a specific surface for 24 hours at at temperature of 105°C. The loss of weight after this period is indicated in percent which provides information on the temperature at which the lubricant should be used, but it is not an absolute value for practical use.
|
|
|
|
|
|
|
Wettability / drop strength
The wettability or drop strength of a lubricant does not just depend on its surface tension. For example, lubricants spread much more on plastic than on metal surfaces. Small drops of the lubricant are therefore applied to the corresponding materials and the spreading is observed.
|
|
|
|
|
|
|
|
Aging resistance
The aging resistance or oxidation resistance of a lubricant is generally determined by its chemical composition and the amount of protective stabilizers. Bearing metals, high ambient temperatures, wear particles or aggressive surrounding media accelerate the aging of a lubricant. The aging is tested under the effect of catalytic metals and temperatures.
Publication:
Test method for evaluating aging and the stability of the quality of lubricants
Click here to obtain the publication in  PDF format
Article in the US journal, Practicing Oil Analysis:
The Method for Simulation the Aging and Oxidation Stability of Lubricants.
Click here
|
|
|
|
|
|
|
Corrosion resistance
Lubricants may not corrode the utilized bearing metals and they should also protect from humidity.
|
|
|
|
|
|
|
Plastic resistance
Lubricants may not cause any alteration of the utilized plastics, that is, the plastics may not swell or shrink, become brittle or form stress cracks. The lubricants may not age or form acids, and their viscosity may not increase or decrease.
|
|
|
|
|
|
|
|
Tribological test
The influence of the lubricant on friction and wear is determined by simulating the properties of a bearing in practice (tribological test). At Dr. Tillwich GmbH Werner Stehr, the ball/prism model is used as the tribological test system (ISO 7148 part 2). The ball/prism is used in contrast to the frequently employed pin/disk method since a dynamically changing gap between the ball and prism can simulate the behavior of lubricated radial friction bearings. All of the tribological measurements and parameters generated on the test benches are saved in the "TRIBODATA" database.
|
|
|
|
|
|
|
|
Friction behavior
Friction lowers the effectiveness of assemblies and dissipates energy. Friction causes the sliding elements to heat up. The overall frictional torque of a bearing depends on the bearing diameter, the component surface, the combination of materials and the inner friction of the lubricant arising from rubbing. Other parameters influencing friction are surface pressure, sliding speed and ambient temperature.
Since friction depends on many different parameters, a single friction value may not be indicated without noting the influential parameters, and may lead to a complete misestimation of the practical behavior of a bearing. The friction behavior may only be transferred from a model system to practice when the simulation parameters largely correspond with those of practice.
In our documents, the friction behavior of a lubricant is portrayed for a typical pair of materials.
The test parameters are: Load of 3 N and ambient temperature of 25°C.
The index for friction, the friction factor f (earlier: friction coefficient µ) is indicated for four characteristic sliding speeds: 0, 20, 50 und 200 mm/s that can serve as references for the three primary friction conditions of static friction, boundary friction and mixed friction, and hydrodynamics.
|
|
|
|
|
|
|
|
Wear behavior
Wear directly causes a loss of quality and failure of assemblies. A simulation of the bearing wear processes in practice is essential before the bearings are mass-produced. The wear depends on the combination of material, lubricant, tribological state of the sliding system such as the surface roughness, ambient temperature, sliding speed, specific surface pressure, and the sliding path.
Only conditional qualitative conclusions regarding practical behavior can be made using a laboratory test bench with tribological model systems. If an appropriate model test system is used and all the tests are carried out under the same test conditions, a comparative relative evaluation of the different materials and lubricants can be made.
In our documents, we indicate the wear reduction of a lubricant for two typical pairs of materials in comparison to an unlubricated pair. The test parameters are load 30 N, ambient temperature 25°C, sliding speed 28.2 mm/s, sliding path approx. 10 km.
The wear parameter is the wear depth in mm. This indicates how far the rotating ball penetrates the fixed prism. The measure of the wear depth is logarithmic.
|
|
|
|
|
|
|
 Viscosity
Kinematic viscosity
Dynamic viscosity
Viscosity index (VI)
Long-term low temperature
Temperature of use
Penetration
Oil separation
Melting point
Flow pressure
Density
Surface tension
Color
Evaporation rate
Wettability/drop strength
Age resistance
Corrosion resistance
Plastic resistance
Tribological test
Friction behavior
Wear behavior
Return to beginning
Kinematic viscosity
The kinematic viscosity is measured with the Stabinger viscosimeter SVM 3000 according to ASTM D7042-04 in the unit of mm²/s. The density of the fluid influences the measurement. If the kinematic viscosity is multiplied by the density of the fluid, the dynamic viscosity is obtained.
Return to beginning
Dynamic viscosity
The dynamic viscosity is calculated from a flow curve that is recorded using a rotation viscosimeter (a plate/cone measuring device) in accordance with DIN 53018. The dynamic viscosity is indicated in the unit mPa.s.
This type of measurement is preferred for high viscosity oils (DIN 53018) and greases (DIN 51810).
Return to beginning
Viscosity index (VI)
The viscosity index (VI) is a measure of the viscosity/temperature relationship (VT behavior) of a lubricant, and is calculated according to DIN 51563 and ISO 2909 from the kinematic viscosity of the lubricant at different temperatures. A VI of 100 means a satisfactory VT behavior approximating that of a good mineral oil. The best oils such as plastic oils have a VI of approximately 400.
Return to beginning
Long-term low temperature
The long-term low temperature indicates the low temperature at which a lubricant should not be used. Since hardening processes take a very long time, the low temperature is indicated at which the oil is still liquid after 72 hours. The test method follows that of FTMS Method No. 3458.1 at which the lubricant is cooled in steps of 5°C until it hardens.
Return to beginning
Temperatures of use
The bottom limit for a lubricant is determined by the hardening of the oil, and the upper limit is determined by excessively low viscosity, fast evaporation, or the chemical decomposition of the oil. The indicated temperatures of use are guidelines.
Return to beginning
Penetration
The penetration characterizes the hardness of a grease. The cone penetration is determined with reference to the former DIN51804. It indicates in mm/10 how deep a standard one-quarter cone can penetrate a grease sample within a specific period. The penetration is determined for fresh grease (mpr) and sheared grease (mpw). The mpw value is used to categorize the greases into consistency classes according to DIN 51818.
Return to beginning
Oil separation
Greases consist of an oil that has be adjusted to a specific consistency with a thickener. In order for a bearing system to be properly supplied with lubricant, the oil must separate from the grease structure and actively penetrate the bearing gap. The oil separation is measured as a function of the temperature and time according to FTMS Method 321.3 with standardized, funnel-shaped nets.
Return to beginning
Dropping point
Lubricating greases that for example are thickened with metal soaps are characterized by their dropping point (DIN ISO 2176). A grease sample is heated in a standardized nipple until a liquid drop exits the hole in the bottom. The dropping point is the temperature at which the grease transitions from its plastic grease structure into a liquid state. The thickener "melts" at this point. The dripping point cannot be determined for lubricating greases with solid lubricants or gel thickeners.
Return to beginning
Flow pressure
The flow pressure is determined according to DIN 51805 and indicates the pressure in mbar that is required to press a certain amount of grease through a standard test nozzle. The flow pressure can be determined at any desired temperature such as 0°C and 25°C.
Return to beginning
Density
The density is a physical index of the lubricant. It indicates the weight of a sample with a volume of 1 cm³. It is measured according to DIN 51 757 at 20°C in the unit of g/cm³.
Return to beginning
Surface tension
The surface tension is a physical index of the lubricant. It is measured with a platinum/iridium ring using the Baron de Nouy method in units of mN/m. Lubricants with a low surface tension generally wet the surfaces well (such as silicone oils at 20 mN/m). Liquids with a high surface tension (such as water at 72 mN/m) form individual drops.
Return to beginning
Color
The color can range from water-clear over all the colors of visible light to black and is described verbally. UV light or aging and oxidation processes can change the color of the lubricant (to brown or green, for example). Changes must always be considered in reference to a new lubricant.
Return to beginning
Evaporation rate
The evaporation rate indicates how fast a lubricant evaporates under certain conditions. The lubricant is tested according to ASTM method No. 3480.1 with 10 g lubricant on a specific surface for 24 hours at at temperature of 105°C. The loss of weight after this period is indicated in percent which provides information on the temperature at which the lubricant should be used, but it is not an absolute value for practical use.
Return to beginning
Wettability/drop strength
The wettability or drop strength of a lubricant does not just depend on its surface tension. For example, lubricants spread much more on plastic than on metal surfaces. Small drops of the lubricant are therefore applied to the corresponding materials and the spreading is observed.
Return to beginning
Aging resistance
The aging resistance or oxidation resistance of a lubricant is generally determined by its chemical composition and the amount of protective stabilizers. Bearing metals, high ambient temperatures, wear particles or aggressive surrounding media accelerate the aging of a lubricant. The aging is tested under the effect of catalytic metals and temperatures.
Publication:
Test method for evaluating aging and the stability of the quality of lubricants
Click here to obtain the publication in PDF format
Article in the US journal, Practicing Oil Analysis:
The Method for Simulation the Aging and Oxidation Stability of Lubricants.
Click here
Return to beginning
Corrosion resistance
Lubricants may not corrode the utilized bearing metals and they should also protect from humidity.
 Return to beginning
Plastic resistance
Lubricants may not cause any alteration of the utilized plastics, that is, the plastics may not swell or shrink, become brittle or form stress cracks. The lubricants may not age or form acids, and their viscosity may not increase or decrease.
Return to beginning
Tribological test
The influence of the lubricant on friction and wear is determined by simulating the properties of a bearing in practice (tribological test). At Dr. Tillwich GmbH Werner Stehr, the ball/prism model is used as the tribological test system (ISO 7148 part 2). The ball/prism is used in contrast to the frequently employed pin/disk method since a dynamically changing gap between the ball and prism can simulate the behavior of lubricated radial friction bearings. All of the tribological measurements and parameters generated on the test benches are saved in the "TRIBODATA" database.
Return to beginning
Friction behavior
Friction lowers the effectiveness of assemblies and dissipates energy. Friction causes the sliding elements to heat up. The overall frictional torque of a bearing depends on the bearing diameter, the component surface, the combination of materials and the inner friction of the lubricant arising from rubbing. Other parameters influencing friction are surface pressure, sliding speed and ambient temperature.
Since friction depends on many different parameters, a single friction value may not be indicated without noting the influential parameters, and may lead to a complete misestimation of the practical behavior of a bearing. The friction behavior may only be transferred from a model system to practice when the simulation parameters largely correspond with those of practice.
In our documents, the friction behavior of a lubricant is portrayed for a typical pair of materials.
The test parameters are: Load of 3 N and ambient temperature of 25°C.
The index for friction, the friction factor f (earlier: friction coefficient µ) is indicated for four characteristic sliding speeds: 0, 20, 50 und 200 mm/s that can serve as references for the three primary friction conditions of static friction, boundary friction and mixed friction, and hydrodynamics.
Return to beginning
Wear behavior
Wear directly causes a loss of quality and failure of assemblies. A simulation of the bearing wear processes in practice is essential before the bearings are mass-produced. The wear depends on the combination of material, lubricant, tribological state of the sliding system such as the surface roughness, ambient temperature, sliding speed, specific surface pressure, and the sliding path.
Only conditional qualitative conclusions regarding practical behavior can be made using a laboratory test bench with tribological model systems. If an appropriate model test system is used and all the tests are carried out under the same test conditions, a comparative relative evaluation of the different materials and lubricants can be made.
In our documents, we indicate the wear reduction of a lubricant for two typical pairs of materials in comparison to an unlubricated pair. The test parameters are load 30 N, ambient temperature 25°C, sliding speed 28.2 mm/s, sliding path approx. 10 km.
The wear parameter is the wear depth in mm. This indicates how far the rotating ball penetrates the fixed prism. The measure of the wear depth is logarithmic.
Return to beginning
|
