A labyrinth combination of slinger and end cover provides a highly effective seal against the intrusion of foreign matter. This seal is recommended for use over a wide range of speeds. For exceptionally slow-speed applications, a combi- nation of slinger and commercial contact-type seal is usually employed.
Slingers should be machined all over to assure true- running. Their diameters should be concentric with the bore. The outside diameter of the slinger is often tapered to throw off cutting compounds, coolants, etc., from the point at which such liquids may enter the spindle. A drip or run-off groove adjacent to the open lip of the end cover is highly desirable and practical.
The axial clearances of the internal faces between slinger and end cover should be about 1/16 inch (1.6mm). The first radial clearance opening on any design through which liquid may pass should be made very close, about .007 inch (.18mm) on the diameter .0035 inch (.089mm) on a side. The inner radial clearances should be between 0.15 inch (.38mm) and .0075 inch (.190mm). These are figures actually used in successful practice.
To position precision ball bearings on spindle shafts, precision manufactured self locking bearing locknuts are recommended rather than the conventional locknuts and lockwashers used for bearings made to ABEC-1 tolerances.
This precision bearing nut incorporates a locking feature in its design. The nut threads deform slightly as the locking setscrews are tightened. This slight deformation creates an interference with the shaft threads which prevents further rotation of the locknut. The precision threads of this locknut are cut square with the face to provide the necessary true- running clamping surface against the inner-ring face of the ball bearings.
Detailed assembly drawings on pages E45 to E48, are representative of successful applications of Fafnir precision bearings on such equipment as gear drive assemblies; automatic screw machines; high-cycle wheel heads; high- speed internal grinding spindles; superprecision work heads; and high-speed router spindles. It is hoped that these ar- rangements will stimulate questions regarding your particular application problems. They will gladly be examined by our Engineering Department.
High-speed grease-lubricated spindles and heavy precision work heads requiring unusual rigidity and running accuracy are a few of the many special problems involving precision bearings. These and many other applications generally require design features which will be recommended by the Engineering Department on request.
Even though ball bearings have the least amount of friction of any of the so-called anti-friction bearings, lubrication is re-
quired to minimize rolling resistance due to deformation or kneading action of the balls in the raceways under load, and to minimize any sliding friction that occurs between the balls, the raceway and the retainer. Lubrication also serves to protect the accurately ground and polished surfaces from corrosion. In addition, lubrication, in general, dissipates generated heat and helps protect the bearing moving parts from the entry of foreign matter.
Only enough lubrication to accomplish these purposes should be used since another source of heat may become present, namely friction between the lubricant and the moving parts, in the form of churning or internal shear of the lubricant itself.
Regardless of the method of lubrication or type of lubri- cant, it is important that quality lubricants be used to minimize oxidation, gumming or sludging and that the lubricant be clean and free of moisture to minimize wear.
In the lubrication of ball bearings, it is important to realize that a small quantity of oil or grease will, if constantly present in the bearing, suffice for its requirements. More trouble can result from excessive lubrication than from too little, although either condition should be avoided. Excessive oil or grease will result in high temperature and possibly failure. When grease is used, it is especially necessary to take into consid- eration the maximum operating temperature. Also particular attention must be given the housing design relating to the proximity of the grease to the bearing, in order to assure adequate purge room and grease retention.
Depending upon operating speeds, loads and tempera- tures, machine-tool ball bearings are lubricated with grease, oil or oil mist. In general, oils are required when bearings operate at high speeds and to provide greater cooling than is possible with grease.
When ball bearing spindles are grease lubricated, the heat generated is removed only by conduction through the sur- rounding parts. With jet or circulating oil lubrication, generated heat is dissipated by the oil passing through the bearings as well as by conduction through the shaft and housing. Both means of removing heat from the bearings are important but, generally, dissipation through conduction is less obvious.
As an example, in an oil mist lubricated grinding spindle the nose or wheel-end bearings are fixed and close to the grinding coolant. The pulley-end or rear bearings are secured axially on the shaft but permitted to float laterally in the housing to compensate for size variations due to thermal changes. Heat is conducted away from the front bearings at a faster rate because of the mass of the spindle nose and the intimate contact of the outer rings with the housing shoulder, the end cover, and the housing bore. This condition, coupled with oil mist lubrication and the proximity of the grinding coolant, takes away generated heat efficiently.
The rear or floating pair of bearings are not so favored. Usually, the mass of the shaft at the pulley-end is not so great. The pulley possesses some heat-conduction ability but also receives heat generated by belt friction. The absence of grinding coolant and the reduced area of conduction usually results in a slightly higher operating temperature.