Low operating temperatures, combined with adequate spindle rigidity, are important and highly desirable for precision machine tools. This is particularly true for high-speed grinding spindles where the preload of the bearings is the principle load imposed upon them. Some of the benefits derived from low operating temperatures and better dimensional stability of the processed work, less need for bearing lubrication, preven- tion of objectionable heat at the external surfaces of the spindle housing, and elimination of troubles due to thermal effects on mounting fits and preloads.
The heat developed at the ball bearings under load is a function of the operating speed and the bearing preload. Preloading is necessary for maximum axial and radial rigidity. Unfortunately, if speeds are increased, the bearing preload may have to be lessened to maintain proper operating tem- peratures at the bearing.
For high-speed operation, the bearing preload should be sufficient to maintain proper rolling friction for the balls but not so high as to generate excessive heat. In cases where lower operating speeds are desired, bearing preloads may be increased to obtain additional bearing rigidity, provided the proper operating temperatures are maintained. Thus, a balance between heat generation and spindle rigidity dictates the amount of bearing preload that is used, commensurate with the operational speed and the bearing life required.
How bearing preload affects the operating temperature is illustrated in Figure 15. This graph applies to 207 size, angu- lar-contact, duplexed superprecision ball bearings, mounted back-to-back. Curve A is a plot of operating temperature at the bearing outside diameter for the speeds indicated, using bearings with a 150 pound built-in preload. Curve B is for bearings having a 30 pound preload. The slope of Curve A is much steeper than that of Curve B. Using bearings with a 150 pound preload, the temperature rise at the bearing outside diameter is 60 F when operating at 3600 rpm. For the same temperature rise, using bearings with 30 pounds preload, an operating speed of 15,300 rpm is indicated. Therefore it is evident that for higher-speed operation the bearing preload should be kept to the minimum necessary to assure sufficient bearing rigidity.
For workhead spindles, the operating speeds are generally low and the loading conditions heavy. Maximum radial and axial spindle rigidity is required under these loads, making increased bearing preload mandatory.
EFFECT OF PRELOAD ON TEMPERATURE RISE
Temperature Rise Above Room F
Speed in RPM 1 = 1000
A High Preload B Low Preload Figure 15 — Temperature vs Speed
Permissible Operating Speed
When determining the permissible operating speeds corre- sponding to the bearing preloads used in machine tool spindles, many influencing factors are involved. Among those considered are spindle mass and construction; type of mount- ing; spindle rigidity and accuracy requirements; spindle loads’ service life; type of service, (intermittent or continuous); and method of lubrication.
Bearing temperatures, generally, vary directly with both speed and load. However, high speed applications must have sufficient thrust loading on the bearings to prevent heat generation from ball skidding. The amount of bearing preload is determined primarily from these operating conditions. At lower speeds, the operating loads are heavier and the bearing deflections are greater. Therefore, the bearing preload must be high enough to provide adequate bearing rigidity under the heaviest loads and still maintain reasonable temperatures when the spindle is operated at high speeds.
The following relationship may be used to estimate the effect of preload and lubrication method on the Permissible Operating Speed. (SP)
S P = F L x F P x F Where B x N
FL is Lubrication Factor FP is Preload Factor FB is Ball Material Factor
NG is Permissible Speed for single grease lubricated bearing with inner ring rotation. This value is found in the Physical Characteristics sections.
LUBRICATION FACTOR (FL)
L F = 1.00
L F = 1.50
L F = 1.70
L F = 2.00
Factors are as follows:
BEARING PRELOAD FACTORS = (FP) Bearing Mounting Arrangement
B F = 1.00
B F = 1.20
BALL MATERIAL FACTOR = (FB)
If a cage other than one shown in this catalog is used, contact the Engineering Department for recommendations.
Ceramic balls allow 20% increase to speed factor.