Bearings Lubrication

1. Purpose of Lubrication

The purpose of bearing lubrication is to prevent direct metallic contact between the rolling elements, raceway and cages. This is accomplished through the formation of a thin oil (or grease) film on the contact surfaces. However, for rolling bearings, lubrication has the following advantages:
  • To prevent or reduce friction and wear by the direct contact among rolling elements, raceways and cages.
  • To form an oil film on the friction surface and enlarge the contacted loading area of components. Therefore, it can reduce contact stress and prolonging rolling contact life.
  • To prevent against rust and corrosion. To dissipate heat and carry off the worn particles generated in operation or contamination ingress.
  • To improve tightness and prevent the contamination entering.
  • To reduce noise and vibration.

2. Lubricant Methods

The lubrication method for bearings can be divided into grease lubrication and oil lubrication. Each of these has its own features, so the lubrication method that best offers the required function must be selected. A comparison of grease and oil lubrication is shown as below.
Item Grease Lubrication Oil Lubrication
Handling Very Good Fair
Handling Good Very Good
Cooling Effect Poor Good
Seal Structure Good Fair
Power Loss Good Good
Environment Contamination Good Fair
High Speed Rotation Poor Good

2.1 Grease Lubrication

Considering the structure of the lubrication system, grease lubrication is used in many applications because of its simple sealing structure. Lubricant grease is a half-solid agent which is made of a base oil, thickening agents and additives. Base oil is about 70-95%, the thickening agent is about 5-30%, and the additive is minimum. The most widely used lubricating greases are calcium base grease, lithium based grease, calcium-sodium based grease and molybdenum disulfide grease, etc. The main properties of grease lubrication are shown as the following:

2.1.1 Base Oil

Lubricating properties of lubricants are based on the viscosity of base oil which is important to form lubricant film. Usually, the viscosity of base oil used by rolling bearing is in the range of 15 - 500mm2/s (40º). If the viscosity is over 500mm2 (40º), then the base oil will be very difficult to be dissolved out from the lubricant grease, which, could make the lubrication inadequate. However, if the viscosity is less than 15mm2/s (40º), it will be difficult to form a film.

2.1.2 Consistency

The ability of resisting deformation under outer force is named consistency. It can be measured with a needle penetrator. The designation of lubricant grease is indicated by the grade of needle penetration. The smaller the needle penetration, the thicker the consistency of lubricant grease is, and worse the liquidity. Needle penetration (or the designation) is divided into nine grades; grade 1, grade 2 and grade 3 are used by bearing. Grade 2 is used most frequently. Generally, greases with light consistency can be used at low temperatures. Greases with high consistency can be used at high temperatures. Its consistency changes little while the grease is used in the range of its normal working temperature. If the working temperature is higher than that allowed, lubricant grease will turn soft and easily leak. Generally, greases which soften at elevated temperatures may leak from the bearing or housing, those which stiffen at a low temperature may restrict the rotation of the bearing.

2.1.3 Thickening agents

The main function of thickening agent is to keep lubricant grease in a half-solid state. Therefore, some characteristics such as the working temperature, mechanical stability, heat resistance, and water resistance of lubricant grease are decided by thickening agents. The performance of lubricant grease changes with different thickening agents.

2.1.4 Additive

Currently base oil is hard to satisfy comprehensive lubrication requirements of friction couples. Therefore, in order to increase the performance of oils, some additives which will improve the quality of oil and must be added into the base oil in order to make greases suitable for various special working conditions. However, lubricant greases added with additives may bring up a negative influence on the bearing parts, such as corrosion.

Types of main additives:

  • Additives protecting the surface of metal, e.g. extreme pressure additive, etc.
  • Additives improve the capability of lubrication e.g. adhesive, etc.
  • Additives protect the lubricant grease itself, e.g. antioxidant, etc.

2.1.5 Miscibility

The miscibility of greases with different designations must be considered when changing the designation of greases. Combined lubricant grease with different designations are likely to make the consistency change considerably, which, would cause lubrication failure and leakage. Generally, greases with the same thickening agent and similar base oil can be mixed with each other.

3. Selection of Grease Lubrication

When selecting lubricant grease, the working temperature, load and rotating speed should be taken into consideration. The lubricity of grease is largely dependent on base oil, which, is chosen according to the usage and using condition of grease lubrication. Low viscosity base oil is suitable for bearings which work at low temperatures, light loads, and high speeds. Machine oil and engine oil are suitable as base oil for mid-speed bearings. Cylinder oil is suitable to be used as a base oil for bearings which are operating in high temperature, heavy load and slow-speed conditions. Synthetic base oil can be used in other special conditions. The structure and precision of the bearing can influence by the selection of lubricant grease. The available space inside the bearing or clearance is smaller, then the finishing precision on the working surface is higher and the base oil to be chosen would be of less viscosity. This allows for the unnecessary waste of energy. The viscosity value of base oil must be improved when the working surface is rough. Meanwhile, an oil film cannot be formed easily because of the comparably heavy pressure which would be caused by partial contacting. In watery working conditions, calcium base grease is the first choice because it will not dissolve in water. Sodium base grease will dissolve easily, it should not be used in a dry and hydroponic conditions
  • When selecting lubricant grease, attention must be paid to the following several points: Under normal speed, load, and temperature, a minimum movement viscosity of grease used by most kinds of bearings is not less than 15mm2/s.

  • The dropping point of the grease should be higher than its working temperature 20-30℃ to avoid leakage. Due to bad liquidity, heavy friction resistance, little thermal conductivity of lubricant grease, circle lubrication is inadvisable. If it must be used in the applications where concentrated lubrication is needed, lubricant grease with comparatively large needle penetration should be the first choice.
  • In specified range of temperature, grease lubricant is not sensitive to temperature. Grease lubricant has a high adaptability to great changes of load chrematistics and moving speeds, so it can be used in those machineries whose working ambient temperature and speeds change greatly.
  • Lubricant grease will not leak easily and need not be changed often. The needed sealing devices are comparatively simple, therefore, the lubricant grease itself has a certain sealing function, so it is suitable to be used in dusty situations where it is difficult to refill greases.
  • When lubricating in high temperature, grease with good resistance to oxidation, less evaporation, with a high dropping point should be the first choice.
  • For one-off lubricated sealed bearings, the life of lubricant grease should be longer than the fatigue life of bearing. If shorter, then the practical life of bearings is dependent on the life of greases.  Generally, the life of lubricant grease is corresponding with the time of medium repair and heavy repair of equipment.
The most widely used lubricating greases are calcium based grease, lithium based grease, aluminum based grease and molybdenum disulfide grease etc. Sealed bearings have been filled with grease before leaving factory and can be applied directly in service.

3.1 Grease Quantity

The quantity of grease to be packed in a housing depends on the housing design and free space, grease characteristics, and ambient temperature. For example, the bearings for main shafts of machine tools, where the accuracy may be impaired by a small temperature rise, require only a small amount of grease. The quantity of grease for ordinary bearings is determined as follows.
1/2 to 2/3 of the space --- When the speed is less than 50% of the limiting speed.
1/3 to 1/2 of the space --- When the speed is more than 50% of the limiting speed.

3.2 Replacement of Grease

As the lubricating performance of grease declines with the time, grease must be filled by proper intervals. The replenishment interval depends on the type of bearing, dimensions, bearing's rotating speed, bearing temperature, and type of grease.
When the temperature is getting higher, the base oil of lubricant grease would be vaporized, oxidative deteriorated, the grease net structure destroyed and indurate, which, would reduce the capability of lubricant grease. When base oil loses are 50-60%, especially under high temperature, the capability of lubrication descends more quickly. The higher the temperature is, the shorter the life of grease is. Accordingly, replace the old lubricant grease before the capability of the lubricating lost.  For the widely used lubricant grease, we judge the grease whether it loses its efficacy or not by examining the function parameter of the grease.

4. Oil Lubrication

Oil lubrication is recommended in the following situations
  • When bearing works at high speeds and high temperatures
  • When the frictional or applied heat must be removed from the bearing
  • When lubricant grease can't satisfy the needs when abutting machines adopt oil lubrication

4.1 Methods of Oil Lubrication

4.1.1 Oil Bath Lubrication

Oil bath lubrication is a method that allows of bearing into lubricant oil to be picked up by rotating elements, after circulating through the bearing, it drains back to the oil bath. Oil bath lubrication is one of the most widely and conveniently used lubrication methods. Oil bath lubrication can lubricate well, but the quantity of oil supplied is not accommodated easily; and there is no filtration equipment, which would bring impurities into the bearing easily and cause damages to the inside of the bearings. Oil bath lubrication is only suitable for low or medium speeds.

4.1.2 Drip Feed Lubrication

Drip feed lubrication is a method of dripping oil into bearings by a serving oiling orifice. The strong points of drip feed lubrication are that the device structure is simple, it can be used conveniently, more oil can be saved by this way. The amount of oil supplied can be measured and adjusted. A drop should be provided at intervals of 3-8 seconds. Too much oil may cause the temperature to rise. Drip oil lubrication is suitable for slow speed and light load.

4.1.3 Splash Lubrication

Splash lubrication is a lubricating system where the oil is induced into the bearing by a splashing motion created by the movement of the bearings rotating elements. The quantity of oil supplied by this splash motion is not easily controlled. Once again it is essential to maintain any pre-subscribed oil level, as over filling can also be detrimental, the results of over filling can lead to elevated temperatures, these elevated temperatures under heating and cooling can create a vacuum resulting in the possibility of the ingress of contaminants.

4.1.4 Circulating Oil Lubrication

Lubricant oil can be sucked out by an oil pump from an oil box, then it is transported to the parts in the bearing requiring lubrication, then it is returned to oil box from an oil jaw. The sucked oil can be used again after filtration. The circulation oil method is a superior system to all other lubrication methods as temperatures can be controlled by additional cooling injection; filtration can be applied to filter out contaminants. When applicable oil monitoring can be applied to that oil replacement is made automatically. Regulated flow is easily adjusted for optimum performance. These regulated flows can be matched to high speeds and high loads. These types of system are not suited for all applications and have the disadvantage of generally being expensive, however they do provide optimum performance.

4.1.5 Oil Mist Lubrication

Oil mist lubrication employs oil droplets which are transported to the bearing by an air current oil mist is produced in an atomizer. Dry compressed air and oil mixture are transported to every part of bearing where it needs lubrication. Oil mist lubrication allows little whisking wastage and temperature rise. Therefore, the air current in the housing will also serve to cool the bearing and produce a slightly higher temperature in the housing to assist in preventing impurities from entering. However, the cost of oil mist lubrication is high, there are some special requirement on the oil viscosity, generally, the viscosity of the oil is no more than 340mm2/s, otherwise, the effect cannot be made. Additionally, if the lubricating device dose not seal well and tightly, some oil mist may leak out and pollute the environment. If necessary, oil and gas separators can be used to collect oil mist. This method is often used for high speed applications.

4.1.6 Oil Jet Lubrication

Oil jet lubrication is a method that is adapted to jet oil into the bearings. When the bearing is operating at a high speed, rolling elements and cages are also rotating at high speed. By this way, turbulence air is formed around the bearing, which can make the lubricant oil reach the parts that needs to be lubricated. However, after the oil is sprayed out, the lubricating oil will flow back into the oil bath from the side because of centrifugal force. In order to provide adequate oil for fast rotating bearings, lubricant oil must be jetted out from the other side. When designing this lubricating system, the position of the high pressure standard nozzle should be placed between the inner ring and cage. By centrifugal effect, lubricant oil must be thrown out of the outer raceway. In order to keep the lubrication efficient, the jetting speed should not be less than 15m/s. No matter which way it is to be used, the cleanness of the lubricant oil or greases should be considered first, thus ensuring and lengthening bearing life.

4.2 Lubricant Selection

Lubricant selection is almost the same as the grease lubrication selection. It is mainly based on the viscosity of oil which can provide enough lubrication to the bearing at working temperatures. Lubrication viscosity would fall with the increase of temperature. Therefore, the lowest oil viscosity would be ensured at working temperatures. In normal working conditions, the lowest movement viscosity should not be over 15mm2/s to form a lubrication oil layer with enough thickness on the contact surface between raceway and rolling element. Mineral oil without any additive could be used in normal conditions. The lubrication with additive or compound oil is suitable for special working conditions. However, lubrication with additive would have a negative effect on the bearing, typically corrosion. When the bearing is rotating at high speed, the lubricant with a low viscosity could be used. For the bearing under heavy load, lubrication with a high viscosity should be chosen. The widely used lubricants are machine oil, highspeed machine oil, stream-turbine oil, compressor oil, transformer oil, and cylinder oil, etc.

4.3 Replacing Intervals

To ensure that the bearings lubrication in maintained good working condition, lubrications should be replaced regularly, even when oil lubrication systems are adopted. The replacement intervals for lubricants are dependent on working conditions, environmental conditions, oil mass, oil pollution and the type lubricant. Only careful monitoring of the lubricants condition can determine as to when to change the lubricant, after time a trend can be developed to establish a maintenance interval, premature oil changes can be wasteful in down time and expensive in purchasing, whilst there could be additional lubricating life remaining within the existing lubricant. When lubricating by Gas Oil Lubrication, the lubricant itself disposable or can be recycled to avoid environmental pollution.
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