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Product/technological information
9. Lubrication
Purpose of Lubrication
The life and performance of bearings change significantly depending on the selected lubrication methods and lubricants, so they must be chosen appropriately considering the conditions of use. Lubrication has the following effects:
(1) Decreased friction and wear
Lubrication decreases the rolling friction between the raceway surface and the balls, the sliding friction between the balls and the cage, and the sliding friction between the cage and the raceway guide surface.
(2) Removal of generated heat
Lubrication removes the frictional heat caused internally by rotation as well as other heat transmitted from outside, and prevents the heating of bearings and the deterioration of lubricants.
(3) Relaxation of stress and extension of fatigue life
Forming an appropriate lubricant film on the rolling contact surface during rotation relaxes the stress concentrated due to impact load, and extends the bearing’s fatigue life.
(4) Rust-resistant and dust-resistant effect
Lubrication prevents the generation of rust on the balls, rings, and cage, and the intrusion of dust, foreign matter, and moisture into the bearing.
Requirements of a Lubricant
(1) Low friction and wear.
(2) High thermal stability and good thermal conductivity.
(3) High-strength lubricant film.
(4) Free of corrosive properties.
(5) Free of dust and moisture.
(6) Maintains viscosity suited to the conditions of use.
Comparison between oil lubrication and grease lubrication
| Item | Oil lubrication | Grease lubrication |
|---|---|---|
| Rotational speed | From low speed to high speed | Low speed and medium speed |
| Lubrication performance | Excellent | Good |
| Cooling effect | Good | None |
| Torque | Relatively small | Relatively large |
| Lubricant life | Long | Relatively short |
| Lubricant replacement | Easy | Difficult |
| Lubricant leakage | Not suited for places vulnerable to oil leaks | Small leakage contamination |
| Dust filtration | Easy | Difficult |
| Sealing device | Complicated | Simple |
Standard Lubricants
The standard lubricants below are used unless specified otherwise:
| Lubricant | Standard grease | Standard oil |
|---|---|---|
| Brand | Multemp SRL | AeroShell Fluid 12 |
| Abbreviation | SRL | AF2 |
| Manufacturer | Kyodo Yushi | Shell Lubricants Japan |
| Operating temperature range | -50 to 150℃ | -54 to 135℃ |
| Specific gravity | 0.93 | 0.92 |
* Force-feed lubrication (oil bath, drip feeding, splash, circulation, and jet, etc.) should be performed as necessary when oil is used.
Grease
Grease is a semisolid lubricant that consists of base oil, thickener, and additive. Based on the combinations of these materials, you must select a grease that suits your application.
(1) Base oil
In general, the base oil of grease is mainly made from mineral oil. Synthetic oil, including silicone oil, diester oil, and fluorinated oil, is also used to improve the grease’s heat resistance and low-temperature fluidity.
(2) Thickeners
Thickeners can be roughly classified into the soap type or the non-soap type, and have an impact on properties such as mechanical stability, water resistance, and operating temperature range.
(3) Additives
Various additives may be added depending on the purpose of use.
- An extreme pressure additive improves impact load and heavy load properties.
- An antioxidant prevents degradation due to oxidation caused when the grease is not resupplied for a long period of time.
- A corrosion inhibitor prevents the generation of rust on and around the bearings.
(4) Consistency
Consistency refers to the hardness of the grease, and it is measured by how deep a metal cone with a specified weight penetrates into the grease in five seconds, in 0.1-mm units. Larger numbers indicate that the grease is softer.
Consistency and consistency number of grease
| NLGI NO. |
00 | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|---|
| Worked penetration (25℃, 60 times) | 400-430 | 355-385 | 310-340 | 265-295 | 220-250 | 175-205 |
| Application | For intensive degreasing | For intensive degreasing | For intensive degreasing and low-temperature use | For general use | For general and high-temperature use | For high-temperature use |
| Condition | Semi-fluid | Very soft | Soft | Normal | Firm | Very firm |
(5) Dropping point
Dropping point refers to the temperature at which the grease starts to drip from a hole with a specified size after the grease is heated to a fluid state. A higher dropping point indicates that the grease can be used at a higher maximum temperature environment.
(6) Mixing different types of grease
When different types of grease that contain different base oil, thickeners, and additives are mixed, their properties will change. Therefore, in principle, grease products from different brands should not be mixed.
●Types of grease that may or may not be mixed (reference)
| Base oil | Ester oil | Mineral oil | Synthetic hydrocarbon oil | Silicone oil | Polyglycol oil | Fluorine oil |
|---|---|---|---|---|---|---|
| Ester oil | Yes | Yes | Yes | No | Yes | No |
| Mineral oil | Yes | Yes | Yes | No | No | No |
| Synthetic hydrocarbon oil | Yes | Yes | Yes | No | No | No |
| Silicone oil | No | No | No | Yes | No | No |
| Polyglycol oil | Yes | No | No | No | Yes | No |
| Fluorine oil | No | No | No | No | No | Yes |
| Thickener | Lithium soap | Calcium soap | Aluminium soap | Urea |
|---|---|---|---|---|
| Lithium soap | Yes | No | No | Yes |
| Calcium soap | No | Yes | No | Yes |
| Aluminium soap | No | No | Yes | No |
| Urea | Yes | Yes | No | Yes |
(7) Amount of filled grease
We categorize the amount of filled grease according to the usage conditions as shown in the table below. The standard products are filled with grease G, L, or Q below, as defined in advance for each product model.
| Grease amount symbol | Amount of filled grease (%) | Usage conditions | |
|---|---|---|---|
| Speed | Load | ||
| Ｆ | 90±10 | Low | Heavy |
| M | 70±10 | Low | Heavy |
| S | 50±10 | Low | Normal |
| G | 40±10 | Medium | Normal |
| L | 30±10 | Medium | Normal |
| Q | 25±5 | Medium | Normal |
| K | 20±5 | High | Light |
| Y | 15±5 | High | Light |
| X | 10±5 | High | Light |
(8) Grease life (reference)
The life of grease used in a sealed/shielded bearing can be obtained by the following equation:
logL=6.1-4.4×10-6×dm×n-3.125(Pr/Cr-0.04)-
(0.021-1.8×10-8×dm×n)×T
| (a) | Applies when T≦120℃ However, T=50℃ where T<50℃ |
|---|---|
| (b) | Applies when dm×n≦5×105 However, dm×n=1.25×105 where dm×n<1.25×105 |
| (c) | Applies when Pr／Cr≦0.16 However, Pr／Cr=0.04 where Pr／Cr＜0.04 |
- L＝Grease life (h)
- dm＝(D＋d)／2(mm)
- n＝Rotational speed(min-1)
- Pr＝Dynamic equivalent radial load(N)
- Cr＝Basic dynamic load rating(N)
- T＝Bearing operating temperature(℃)
(9) Grease relubrication intervals
Even when a high-quality grease is used, its properties will deteriorate and its lubrication performance will decrease depending on the usage conditions, such as the load, rotational speed, temperature, and ambient atmosphere, so you must supply or replace the grease in a timely manner. In general, it is desirable to replace the grease once a year at an operating temperature of 50℃, and two or three times a year at a temperature higher than 100℃, even when the grease is heat resistant. The figure below, titled Rotational speed and relubrication intervals (life), shows the relationship between the rotational speed and the relubrication intervals by bearing bore diameter. The figure below, titled Bearing temperature and correction factor for calculating relubrication intervals, indicates the relationship between the bearing temperature (above 70℃) and the correction factor, and this value should be multiplied by the relubrication interval.
Rotational speed and relubrication intervals (life)
Bearing temperature and correction factor for calculating relubrication intervals
Properties of different types of grease
| Properties | Appearance | Dropping point | Operating temperature range (℃) | Water resistance | Mechanical stability | Remarks | ||
|---|---|---|---|---|---|---|---|---|
| General name | Thickener | Base oil | ||||||
| General-purpose grease | Li soap | Mineral oil | Butter-like | 170-190 | -30 to +120 | Good | Good | Common general-purpose grease widely used for medium- and small-sized ball bearings. |
| Diester grease | Diester oil | Fiber-like or butter-like | -50 to +130 | Suited for low-temperature applications. | ||||
| Silicone grease | Silicone oil | 220-260 | -50 to +180 | Wide operating temperature range from low to high, mainly used in light-load applications(P／Cr≦3%). | ||||
| Cup grease | Ca soap | Mineral oil | Butter-like | 80-100 | -10 to +70 | Good | Relatively good | Includes a small amount of water as structure stabilizer. Not suited for high-temperature applications. |
| Fiber grease | Na soap | Mineral oil | Fiber-like or butter-like | 160-180 | 0 to +110 | Bad (emulsification) | Good | Emulsifies with water and becomes unusable. May be used in relatively-high-temperature applications. |
| Mobile grease | Al soap | Mineral oil | Thread-like or butter-like | 70-90 | -10 to +80 | Good | Relatively good | Highly adhesive and suited for use in locations subject to vibrations. |
| Mixture grease | Ca+Na soap, etc. | Mineral oil | Butter-like | 150-180 | -10 to +120 | Bad (when Na is used) | Good | Applied to large-sized bearings. |
| Composite grease | Li composite soap, etc. | Mineral oil | Fiber-like or butter-like | 180-300 | -20 to +130 | Good | Good | Suited for high-temperature and high-load applications. |
| Non-soap grease | Benton |
Mineral oil or synthetic oil | Butter-like | 230 and higher | -10 to +150 | Good |
Good |
Suited for high-temperature and high-load applications. |
| Silica gel |
Relatively good |
Relatively good to bad |
Has low water resistance. |
|||||
| Urea | Good | Good | Suited for high-temperature applications. | |||||
Lubricant Oil
Mineral oil and synthetic oil generally have high-strength lubricant film, high oxidation stability, and rust-resistant properties, so these oils are used to lubricate the bearings. A lubricant oil with an appropriate viscosity for the relevant usage conditions must be selected. The desirable kinematic viscosity at the bearing’s operating temperature is 13mm2/s or higher.
If the viscosity of the lubricant oil is too low, this will cause an oil film shortage, resulting in wear and/or seizure. On the other hand, if the viscosity too high, this will increase the torque and generate heat. In general, high-viscosity oil should be used for high-load and high-operating-temperature applications, and low-viscosity oil should be used for high-speed applications.
As with grease, you should carefully consider when to replace the lubricant oil and whether to mix it with different brands. * See the “Grease” section.
Lubricant oil selection criteria
| Bearing operating temperature(℃) | dn value | ISO viscosity grade (VG) of the lubricant oil | |
|---|---|---|---|
| Normal load | Heavy load or impact load | ||
| -30 to 0 | Up to the limiting speed | 15,22,32 | 32,46 |
| 0 to 60 | Up to 15000 | 32,46,68 | 100 |
| 15000 - 80000 | 32,46 | 68 | |
| 80000 - 150000 | 22,32 | 32 | |
| 150000 - 500000 | 10 | 22,32 | |
| 60 to 100 | Up to 15000 | 150 | 220 |
| 15000 - 80000 | 100 | 150 | |
| 80000 - 150000 | 68 | 100,150 | |
| 150000 - 500000 | 32 | 68 | |
| 100 to 150 | Up to the limiting speed | 320 | |
Remarks:
- In general, high-viscosity lubricant oil should be used for heavy-load and low-speed applications.
- The figures in this table apply to oil bath and circulation lubrication systems.
- The dn value is: bearing bore diameter d(mm) × rotational speed n(min-1).
- When using oil lubrication, an appropriate lubrication method for the application (such as oil bath, drip feeding, splash, circulation, oil mist, and jet, etc.) must be implemented.