It is generally recognized that only the grease which is immediately adjacent to and in contact with the bearing contributes to its lubrication. In most applications, unless the speed is exceptionally low, a large quantity of grease can be objectionable. An excessive amount increases churning action, which may lead to overheating and possible break- down of the grease.The tendency to churn depends upon the consistency of the grease, the bearing design, the housing construction and the quantity of grease that is used.
Ordinarily, bearings need to be packed only about 25% to 30% full of grease. As a guide, the chart shown in Figure 17 may be used to determine the approximate amount of grease which should be used in each bearing. The chart covers the extra-light series bearings for bore sizes of 10 to 55 mm’s inclusive.
Prior to mounting, each bearing is carefully packed with the required quantity of clean, suitable grease using a flat steel scale for inserting the grease into the bearing and around the balls. Grease voids within the bearing are elimi- nated and even distribution of the grease is accomplished by revolving the bearing slowly in the hands until the torque throughout the bearing becomes noticeably constant.
Although several grease products have been successful at DN values as high as one million, oils are generally required for bearings operating at high speeds or to provide more cooling and dissipation of heat than is possible with grease. High-grade spindle oil having a viscosity of 100 seconds Saybolt at 100F is recommended for use in drip-feed oilers, oil bath lubrication arrangements and oil mist systems. In heavily-loaded applications, oil in relatively large quantities must be supplied, and where temperatures run higher than normal, oil coolers will be required. Churning of a large pool of oil is to be avoided if speed is appreciable.
Oil Bath The conventional oil-bath system for lubricating the bearings is satisfactory for low and moderate speeds. The static oil level must never be higher than the center of the lowermost ball. When the shaft is rotating, the running level may drop considerably below the standstill level, depending on the speed of the revolving parts. A sight gauge or other suitable means should be provided to permit an easy check.
Where the speeds are considered high for oil bath and the bearings are moderately loaded, oil, introduced through a filter-type, sight-feed oiler, is recommended. This assures a constant supply of lubricant. The feed in drops per minute is determined by closely observing the operating temperatures.
In applications where the ball bearing is heavily loaded and operating at high speed and high temperatures or where the operating conditions are severe with high ambient tempera- tures encountered, oil jet lubrication may be required. In such cases it is necessary to lubricate each bearing location individually, and to provide adequately large drain openings to prevent excessive accumulation of oil after it has passed through the bearings.
Oil mist lubrication is recommended for spindles running continuously at high speeds. With this method of lubrication, oil of the proper viscosity is atomized into finely divided particles, mixed with clean, filtered, dry compressed air and directed to pass through the bearings in a constant stream. This oil is metered into the air under pressure. Thus, the system not only lubricates the bearings but it affords some cooling due to the air flow. This continuous passage of air and oil through the bearings and the labyrinth seals also serves to prevent the entrance of contaminants into the bearings.
To insure the “wetting” of the bearings and to prevent possible damage to the balls and raceways, it is imperative that the oil mist system be turned on for several minutes before the spindle is started. The importance of wetting the bearings before starting cannot be over stressed and has particular significance for spindles that have been idle for extended periods of time. To avoid such effects, most oil mist systems have interlocks which make it impossible to start the spindle until the lubricating system is working properly and the bearings are thoroughly wetted.
This method is similar to the oil mist; however, the oil is fed by periodic pulses to the lubrication line providing a higher air to oil ratio. Therefore, this method lowers the operating bearing temperature and lubricant shear effects, enabling higher operating speeds.
Typical * Speed (DN)
Grease Oil Bath Oil Drip Oil Mist Metered Oil Oil Jet
Low Low Low Medium High High
500,000 400,000 600,000 1,000,000 >1,000,000 >1,000,000
LUBE SYSTEM COMPARISON
Speed value is an approximation and assumes proper mounting
and preload techniques along with average loading conditions. For more specific guidance contact your local sales engineer.
The Speed, “DN”, value is obtained by multiplying the bearing bore size in millimeters by the shaft RPM.