Bearing Care & Maintenance


Bearing Temperature

Bearing temperature generally rises with start-up and stabilizes at a temperature slightly lower than at start-up (normally 10 to 40 °C higher than room temperature) in a certain time. The time before stabilizing depends on the size, type, speed, and lubrication system of the bearing and the heat dissipation condition around the bearing. It ranges from about 20 minutes to as long as several hours.
If bearing temperature does not stabilize but continues to rise, the causes shown in Table 1 are conceivable Operation should be stopped and an appropriate corrective action should be taken.
High bearing temperature is not desirable in view of maintaining an adequate service life and preventing lubricant deterioration. A desirable bearing temperature is generally below 100 °C.


Table 1: Major causes of high bearing temperature
(1) Extremely insufficient or excessive lubricant
(2) Poor installation of the bearings
(3) Extremely small bearing clearance or extremely heavy load
(4) Extremely high friction between lip and seal groove
(5) Improper lubricant type
(6) Creep between the fitting surfaces

Operating Sound of Bearing
The following Table 1 lists typical abnormal bearing sounds and their causes. Please note that the description of some of these sounds is rather subjective and thus could vary considerably from person to person.

Table 1: Typical Abnormal Bearing Sounds and Their Causes

Vibration of Bearing

Damage to the bearing can be detected early by measuring the vibration of the machine. The degree of damage is inferred from quantitative analysis of the amplitude and frequency of the vibration. However, values measured differ depending on the measuring point and the operating condition of the bearing. It is desirable to accumulate measurement data and establish evaluation criteria for each machine.

Lubricant Selection

The purpose of lubricating the bearing is to cover the rolling contact surfaces and sliding contact surfaces with a thin oil film to avoid direct metal-to-metal contact. Effective lubrication of the rolling-element bearing has the following effects.

  • Reduces friction and abrasion
  • Transports heat generated by friction
  • Prolongation of service life
  • Prevents rust (corrosion)
  • Keeps foreign objects (or contamination) away from rolling elements and raceways

Grease lubrication

Grease is generally used for lubricating rolling-element bearings because it is easy to handle and simplifies the sealing system.

Carefully examine the type and properties of the base oil, thickener and additives of the grease, and select a grease appropriate for the operating condition of the bearing. The general relation between consistency of grease and the application of the bearing is given in Table 1. The types and properties of greases are given in the bearing section of the NTN general catalog.

Table 1 Consistency Grease

Oil lubrication

Oil lubrication is generally suitable for high speed or high temperature operations. It is also suitable for carrying heat away from the bearing.

Oil viscosities required for the operating temperatures of bearings are given in Table 2.

Carefully study the viscosity, viscosity index, oxidation resistance, corrosion resistance, foaming resistance, etc. to select an oil. Table 3.5 gives a guide for selecting viscosity.

Fig. 1shows the variation of viscosity with temperature for several lubricating oils. Use Table 3 to select an oil with an adequate viscosity for the operating temperature of the bearing.

Table 2: Viscosities Require for Operating Temperatures of Bearings

Table 3: Selection Guide For Lubrication Oil Viscosity


1. The table above is applicable to oil bath lubrication and recirculation lubrication.
2. Consult NTN if your operating condition is not shown in the table.

Fig. 1:Lubrication oil viscosity-temperature line diagram


In grease lubrication, the lubricating characteristic of grease deteriorates with operating hours, thus requiring relubrication of the bearing at appropriate intervals. Relubrication intervals of grease depend on the type, dimensions, and speed of the bearing, and the type of grease.

A line diagram serving as a rough guide of grease relubrication intervals, is given in the bearing section of the NTN general catalog.

For oil lubrication, oil changing intervals depend on the operating condition of the machine and the type of lubrication system (a rough guide to oil changing intervals and for oil analysis intervals is given in Tables 1 and 2

Table 1:Lubrication Oil Properties and Serviceable Limits

Table 2: Frequency of Lubricating Oil Analysis

Severe operating conditions means :
(1)Severe water condensation or ingress
(2)Excessive ingress of dust, gas, etc.
(3)Operating temperature exceeding 120 °C