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4. BEARING LUBRICATION
The correct bearing lubrication has a direct infl uence on the bearing life. Lubricant creates between the rolling ele- ment and bearing ring a carrying lubricating fi lm which hinders their metal contact. It lubricates surfaces where fric- tion arises, it has cooling effect, it protects the bearing from corrosion and in many cases seals the bearing space.
In the most cases - approximately 90%, bearings are lubricated with grease or oil, in rare exceptions by other lubricating means. When deciding which lubricant and which lubrication type should be used, operating conditions, characteristic qualities of the lubricant, equipment design and operating economy should be taken into account.
4.1 GREASE LUBRICATION
In the design practice grease lubrication is preferred to oil lubrication from the point of view of arrangement simplic- ity, utilization of the sealing capabilities and simple maintenance. For reliable bearing operation 1/3 to 1/2 of its free space is fi lled with grease at the fi rst assembly. A greater grease amount has negative infl uence on the operation. Higher passive resistances cause the inner bearing space warming up undesirabely, which can lead to its break- down. Bearings making only a small number of revolutions during operation, from the point of view of corrosion protection should be completely fi lled.
4.1.1 Relubrication Interval
Relubrication interval is the period during which the grease has the necessary lubricating properties. After this period bearing must be relubricated, and old lubricant must be removed from the bearing space completely. Re- lubricating period depends on the bearing type and size, rotational speed, operating temperature and grease quality. The recommended relubrication period for individual bearing types at normal load (P ≤ 0,15 C) and normal operational conditions is shown in diagrams in Figure 21 and 22. The diagrams are valid for common grease and temperatures to +70°C. For temperatures over +70°C, the relubrication period is shortened for each 15°C on the half of original value. For temperatures under +40°C the relubrication period can be doubled.
For small sized, especially single row ball bearings, the relubrication periods are several times longer than the bearing life, that is why the bearings are, as a rule, not relubricated.
For this reason it is advantageous to use these bearings shielded or sealed on both sides and fi lled with grease. For some rotational speeds the relubrication period is out of the diagram curve i.e. the permissible limit for grease lubrication has been reached and oil lubrication should be used.
Necessary grease quantity for relubrication is calculated from the equation: Q = 0,005.D.B
For bearings with higher rotational speed requiring a more frequent relubrication, it is necessary to remove the used lubrication from the bearing space so that temperature increase should not occur. For this reason the grease 4.1.2 Bearing Grease
Bearing grease is produced most often of quality mineral or synthetic oils (sometimes with additives), thickened with fatty acid metallic soaps. Grease must have good lubricating properties and high chemical, temperature and mechanical stability. The grease list of bearing lubricants is in Table 36.
Rolling Bearing Grease Properties
Kind of Grease
Properties
Operating
Thickening Agent
Basic Oil
Temperature
Resistance
Extent [°C]
against Water
Application
lithium soap
lime soap
soda soap
aluminium soap
complex lithium soap
complex lime soap
multi-purpose lubricant suitable for higher complex soda soap
complex aluminium soap
complex barium soap
bentonite
polyurea
lithium soap
complex barium soap
suitable for higher temperatures and higher Valid for ball and cylindrical roller bearings
Valid for double row spherical roller bearings, single row tapered roller bearings and thrust ball bearings
4.2 OIL LUBRICATION
Oil lubrication is used, when operating rotational speed is so high that the grease relubrication period is too short. Another reason can also be the necessity of heat transfer from the bearing, or the high temperature of environment, which does not enable utilization of grease, or if surrounding parts are already lubricated by oil (e.g. geared wheels in the gear box). Except for some cases, spherical roller thrust bearings are always lubricated by oil.
When oil lubricating, lubricating must be secured both at starting and during operation. Excess oil increases tem- Oil feed into bearing is secured in various design ways, out of which oil bath lubrication with oil level reaching middle of the lowest rolling element, oil circulation lubrication, jet lubrication, oil mist lubrication etc., are the most 4.2.1 Bearing Oils
For bearing lubrication mostly refi ned oils with good chemical stability which can be improved by antioxidizing The decisive oil property is kinematic viscosity which decreases with increasing temperature. Suitable oil viscosity ν1 can be stated according to the diagram (see Figure 23) dependence on the bearing mean diameter ds = (d+D)/2 and rotational speed n. If the operating temperature is known or it can be found out, according to the diagram (see Figure 24) suitable oil and viscosity ν at internationally standardized temperature 40°C being necessary for By ratio κ < 1 it is recommended to use EP oil with additives which improve the oil fi lm load rating. By value κ decrease under 0,4 oils with EP additives are always used.
If the ratio κ is greater than 1, improved arrangement reliability is reached in operation.
- bearing d = 180 mm, D = 320 mm, ds = 250 mm For these conditions according to diagram (see Figure 23) the minimum kinematic viscosity is ν1= 17 mm2.s-1.
If the operating temperature is 60°C, the oil selected according to the diagram (see Figure 24) at standardized temperature 40°C must have kinematic viscosity ν min 35 mm2.s-1.
For thrust spherical roller bearings the lubricating oil kinematic viscosity is approximately stated in dependence on n x d, where n is the bearing rotational speed in revolutions per minute and d is the bore diameter in mm, according to table 37. Lower values are valid for bearings with lower load, for which is valid Pa ≤ 0,1 Ca. Higher values are Oil Viscosity for Spherical Roller Thrust Bearings
Kinematic Oil Viscosity
mm2s-1 at 40°C
4.3 LUBRICATION WITH SOLID LUBRICANTS
Solid lubricants are used for bearing lubrication when the grease or oil cannot fulfi l the requirements for reliable lubrication in conditions of limiting friction or from the viewpoint of high operating temperatures, chemical infl u- In this case it is necessary to consult the bearing use with mentioned special workstations.

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