Gamet BearingsExclusive North American distributor for Gamet Bearing.

Koyo Technical Information

a. Grease lubrication

1) Grease lubrication

Grease is made by mixing and dispersing a solid of high oil-affinity (called a thickener) with lubricant oil (as a base), and transforming it into a semi-solid state.
As well, a variety of additives can be added to improve specific performance.

  • Base oil
    Mineral oil is usually used as the base oil for grease. When low temperature fluidity, high temperature stability, or other special performance is required, diester oil, silicon oil, polyglycolic oil, fluorinated oil, or other synthetic oil is often used.
    Generally, grease with low viscosity base oil is suitable for applications at low temperature or high speed; grease with high viscosity base oils is suitable for applications at high temperature or under heavy load.
  • Thickener
    Thickeners are an important factor to determine grease performance. Table 9-1 shows the properties of common thickeners.

Table 9-1 Characteristics of thickeners

ThickenerOperating temperature range °CRotational speed rangeMechanical stabilityWater resistancePressure resistance
Lithium soap-30 to 120Medium to highExcellentGoodGood
Calcium soap-10 to 70Low to mediumFair to goodGoodFair
Sodium soap 0 to 110Low to highGood to excellentBadGood to excellent
Urea compounds-30 to 150Low to highGood to excellentGood to excellentGood to excellent
Bentone-10 to 150Medium to highGoodGoodGood to excellent
Fluorine compounds-40 to 250Low to mediumGoodGoodGood
  • Additives
    Major additives and their uses are as follows:
    Extreme pressure agents -> When bearings must tolerate heavy or impact loads.
    Oxidation inhibitors -> When grease is not refilled for a long period.

2) Amount of grease

Generally, grease should be supplied into the housing such that one third or half of the internal space is filled, though the grease amount should be adjusted according to the housing construction and the size of internal space.

When the grease amount is excessive, it may be heated up by agitation and changing the properties of deteriorating or softening, grease.
If the bearing is used at a low speed, the space may be filled up to two thirds or entirely to prevent ingress of foreign matter.

3) Grease feeding interval, grease life

  • Grease feeding interval
    The service life of grease enclosed in the housing can be determined from Fig. 9-1.Add or replace grease before the end of the service life determined from Fig. 9-1.If a reduced amount of grease is initially enclosed to accommodate high speed, remove old grease to the extent that fresh grease is added.To determine regreasing interval, use the Menu entitled Grease Feeding Interval.

    Fig.9-1 Grease feeding interval

    Grease feeding interval

    [A] : radial ball bearings

    [B] : cylindrical roller bearings, needle roller bearings

    [C] : tapered roller bearings, spherical roller bearings, thrust ball bearings

    Temperature correction
    When the bearing operating temperature exceeds 70°C, obtained by multiplying tf by correction factor a, found on the scale below, should be applied as the feeding interval.
    tf‘=tf×a

    Temperature correction factor a

    Temperature correction

    Bearing operating temperature °C

    Example of calculating grease feeding interval:
    In case of a tapered roller bearings with a bore diameter of 100 mm and running speed of 300 min-1, grease has a service life of approximately 8,500 hours, as indicated in Fig. 9-1.
    If the amount of grease initially enclosed is 0.4 kg, it decreases 0.00113 kg a day (0.4 kg × 24 h / 8,500 h = 0.00113 kg/day).
    If grease is fed once a week, the total grease to be added is 0.008 kg (0.00113 × 7 days = 0.008 kg).

  • Grease life in shielded/sealed ball bearing
    The service life of grease enclosed in single-row deep-groove ball bearings, closed with a seal or a shield, can be estimated by the equation below.
    To determine accurate grease service life, use the Menu entitled Grease Life of Shielded/Sealed Ball Bearings.

logL= 6.10-4.40 × 10-6dmn-2.50 (Pr / Cr-0.05) – (0.021-1.80 × 10-8dmnT


where:
L :grease lifeh
dm :(D + d) / 2 (D : bearing outside diameter, d : bearing bore diameter)mm
n :rotational speedmin-1
Pr :dynamic equivalent radial loadN
Cr :basic dynamic radial load ratingN
T :operating temperature of bearing°C

The conditions for applying equation are as follows. When other conditions are applied, Koyo should be consulted.

  • Operating temperature of bearing : T°C

Applicable when T ≤ 120
50 ;

  • Value of dmn

Applicable when dmn ≤ 500,000
when dmn < 125,000 ; dmn=125,000

  • Load condition: Pr / Cr

Applicable when Pr/Cr ≤ 0.2
when Pr/Cr < 0.05 ; Pr/Cr=0.05

b. Oil lubrication
b.1. Lubricating oil

For lubrication, bearings usually employ highly refined mineral oils, which have superior oxidation stability, rust-preventive effect, and high film strength. With bearing diversification, however, various synthetic oils have been put into use.

Characteristics of lubricating oils

Type of lubricating oilHighly refined mineral oilMajor synthetic oils
Diester oilSilicon oilPolyglycolic oilPolyphenyl ether oilFluorinated oil
Operating temperature range (°C)-40 to +220-55 to +150-70 to +350-30 to +1500 to +330-20 to +300
LubricityExcellentExcellentFairGoodGoodExcellent
Oxidation stabilityGoodGoodFairFairExcellentExcellent
Radioactivity resistanceBadBadBad to fairBadExcellent

Selection of lubricating oil

When lubricating oil viscosity is too low, the oil film will be insufficient. On the other hand, when the viscosity is too high, heat will be generated due to viscous resistance.
In general, the heavier the load and the higher the operating temperature, the higher the lubricating oil viscosity should be; whereas, the higher the rotation speed, the lower the viscosity should be.
Proper kinematic viscosity can be obtained through selection by bearing type according to the following table.

Table 9-2 shows the relationship between lubricating oil viscosity and temperature.

Proper kinematic viscosity by bearing type

Bearing typeProper kinematic viscosity at operating temperature
Ball bearing
Cylindrical roller bearing
13mm2/s or higher
Tapered roller bearing
Spherical roller bearing
20mm2/s or higher
Spherical thrust roller bearing32mm2/s or higher
b.2. Kinematic viscosity

Table 9-2 Proper kinematic viscosities by bearing operating conditions

Operating temperaturedmvalueProper kinematic viscosity (expressed in the ISO viscosity grade or the SAE No.)
Light/normal loadHeavy/impact load
-30 to 0°CAll rotation speedsISO VG 15, 22, 46(Refrigerating machine oil)
0 to 60°Cup to 300 000ISO VG 46Bearing oil
Turbine oil
ISO VG 68
SAE 30
Bearing oil
Turbine oil
300 000
to
600 000
ISO VG 32Bearing oil
Turbine oil
ISO VG 68Bearing oil
Turbine oil
over 600 000ISO VG 7, 10, 22Bearing oil
60 to 100°Cup to 300 000ISO VG 68Bearing oilISO VG 68, 100
SAE 30
Bearing oil
300 000
to
600 000
ISO VG 32, 46Bearing oil
Turbine oil
ISO VG 68Bearing oil
Turbine oil
over 600 000ISO VG 22, 32, 46Bearing oil
Turbine oil
Machine oil
100 to 150°Cup to 300 000ISO VG 68, 100
SAE 30, 40
Bearing oilISO VG 100 ~ 460Bearing oil
Gear oil
300 000
to
600 000
ISO VG 68
SAE 30
Bearing oil
Turbine oil
ISO VG 68, 100
SAE 30, 40
Bearing oil

[Remarks]
1. dm= (D + d) / 2 × Small dotSmall dotSmall dot D : bearing outside diameter (mm) , d : bearing bore diameter (mm) , n : rotational speed (min-1) }

2. Refer to refrigerating machine oil (JIS K 2211), turbine oil (JIS K 2213), gear oil (JIS K 2219), machine oil (JIS K 2238) and bearing oil (JIS K 2239).

3. Contact Koyo if the bearing operating temperature is under -30°C or over 150°C.

b.3. Lubrication methods

1) Oil bath

  • Simplest method of bearing immersion in oil for operation.
  • Suitable for low/medium rotation speed.
  • Oil level gauge should be furnished to adjust the amount of oil.
    (In the case of horizontal shaft) About 50% of the lowest rolling element should be immersed.
    (In the case of vertical shaft) About 70 to 80% of the bearing should be immersed.
  • It is better to use a magnetic plug to prevent wear iron particles from dispersing in oil.

Oil bath

2) Forced oil circulation

  • This method employs a circulation-type oil supply system.
    Supplied oil lubricates inside of the bearing, is cooled and sent back to the tank through an oil escape pipe. The oil, after filtering and cooling, is pumped back.
  • Widely used at high rotation speeds and high temperature conditions.
  • It is better to use an oil escape pipe approximately twice as thick as the oil supply pipe in order to prevent too much lubricant from gathering in housing.

Forced oil circulation

Required oil supply in forced oil circulation and oil jet lubrication


G =
1.88 × 10-4 µ ⋅ d ⋅ n ⋅ P
60 c ⋅ r ⋅ ∆T

where
 G
: required oil supplyL / min
µ
: friction coefficient (see table at right)
 d
: bearing bore diametermm
 n
: rotation speedmin-1
 P
: dynamic equivalent load of bearingN
 c
: specific heat of oil, 1.88 to 2.09kJ/kgK
 r
: density of oilg/cm3
∆T
: temperature rise of oilK

Values of friction coefficient µ

Bearing typeµ
Deep groove ball bearings
Angular contact ball bearings
Cylindrical roller bearings
Tapered roller bearings
Spherical roller bearings
0.0010 to 0.0015
0.0012 to 0.0020
0.0008 to 0.0012
0.0017 to 0.0025
0.0020 to 0.0025

The values obtained by the above equation show amount of oil required to carry away all the generated heat, with heat release not taken into consideration. In reality, the oil supplied is generally half to two-thirds of the calculated value. Heat release varies widely according to the application and operating conditions.

To determine the optimum oil supply, it is advised to start operating with two-thirds of the calculated value, and then reduce the oil gradually while measuring the operating temperature of bearing, as well as the supplied and discharged oil.

3) Oil jet lubrication

  • This method uses a nozzle to jet oil at a constant pressure (10 to 50N/cm2), and is highly effective in cooling.
  • Suitable for high rotation speed and heavy load.
  • Generally, the nozzle (diameter 0.5 to 2 mm) is located 5 to 10 mm from the side of a bearing. When a large amount of heat is generated, 2 to 4 nozzles should be used.
  • Since a large amount of oil is supplied in the jet lubrication method, old oil should be discharged with an oil pump to prevent excessive residual oil.
  • Required amount of oil: see reference

Oil jet lubrication

4) Oil mist lubrication (spray lubrication)

  • This method employs an oil mist generator to produce dry mist (air containing oil in the form of mist). The dry mist is continuously sent to the oil supplier, where the mist is turned into a wet mist (sticky oil drops) by a nozzle set up on the housing or bearing, and is then sprayed onto bearing.
  • This method provides and sustains the smallest amount of oil film necessary for lubrication, and has the advantages of preventing oil contamination, simplifying bearing maintenance, prolonging bearing fatigue life, reducing oil consumption etc.

Oil mist lubrication

– Required amount of mist (mist pressure: 500mm in water column)

 


(In the case of a bearing)=0.11dR
(In the case of two oil seals combined)Q =0.0028d1

where:
Q :required amount of mistL/min
d :bearing bore diametermm
R :row numbers of bearing
d1 :inside diameter of oil sealmm

In the case of high speed rotation (dmn ≤ 400,000), it is necessary to increase the amount of oil and heighten the mist pressure.

– Piping diameter and design of lubrication hole/groove

When the flow rate of mist in piping exceeds 5m/s, oil mist suddenly condenses into an oil liquid.
Consequently, the piping diameter and dimensions of the lubrication hole/groove in the housing should be designed to keep the flow rate of mist, obtained by the following equation, from exceeding 5m/s.


V =0.167Q≤5
A

where :
V: flow speed of mistm/s
Q: amount of mistL/min
A: sectional area of piping or lubrication groovecm2

– Mist oil

Oil used in oil mist lubrication should meet the following requirements.

  • Ability to turn into mist
  • Has high extreme pressure resistance
  • Good heat/oxidation stability
  • Rust-resistant
  • Unlikely to generate sludge
  • Superior demulsifier

Oil mist lubrication has a number of advantages for high speed rotation bearings. Its performance, however, is largely affected by surrounding structures and bearing operating conditions. If contemplating the use of this method, please contact Koyo for advice based on our long experience with oil mist lubrication.

5) Oil/air lubrication

  • A proportioning pump sends forth a small quantity of oil, which is mixed with compressed air by a mixing valve.
    The admixture is supplied continuously and stably to the bearing.
  • This method enables quantitative control of oil in extremely small amounts, always supplying new lubricating oil.
    It is thus suitable for machine tools and other applications requiring high rotational speed.
  • Compressed air and lubricating oil are supplied to the spindle, increasing the internal pressure and helping prevent dirt, cutting-liquid, etc. from entering.
    As well, this method allows the lubricating oil to flow through a feeding pipe, minimizing atmospheric pollution.

(Example of spindle unit incorporating oil/air lubrication system)

Oil/air lubrication

  • Koyo produces an oil/air lubricator and, air cleaner, as well as a spindle unit incorporating the oil/air lubrication system.