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ENGINEERING

Bearing Selection

Fafnir angular-contact precision ball bearings are available with high (25) and low (15– 18) contact angles. Each type has inherent characteristics that are desirable for machine tool spindles. Low contact angle bearings are more rigid radially and less rigid axially than high contact angle bearings. A bearing having a low contact angle allows more axial yield and less radial deflection than one having a high contact angle.

In selecting the proper size and type of bearing, consider- ation is given to the size and construction of the spindle and the kind of mounting, since the latter relates directly to the spindle rigidity and the deflection characteristics. In general, where the operating load is principally radial, low contact angle type bearings are selected, but where the loading is chiefly in thrust and a high degree of axial rigidity is required, higher contact angle type bearings are recommended. Therefore, for the majority of applications, superprecision ball bearings with low contact angles are used. However, when maximum axial rigidity is required in combination with heavy thrust loads or when high ambient temperatures are involved, bearings having the higher contact angles are preferred. The three established preload values are necessarily higher for high contact angle bearings than for those having low contact angles. The axial and radial deflection characteristics of low angular-contact, light series, preloaded ball bearings of a standard size are shown in figure 12. Curves C and D, which are for a preloded tandem pair of such bearings, indicate the greatly reduced axial and radial deflections as compared to those for a preloded single

bearing of the same size (Curves A and B). For example, a tandem pair of bearings under a thrust load of 600 pounds would have an axial defection of 0.0010 inch, while that for a single bearing would be about 0.0017 inch. Similarly, the radial deflections for these bearings operated under 600 pounds radial loads would be 0.00024 inch and 0.00049 inch.

Axial deflection curves for the standard preload levels for both the 2MM (15contact angle) and the 3MM (25contact angle) are shown in Figure 13. Please note the force for the equivalent preload level for a 2MM bearing is about one-half that of the preload level for a 3MM bearing. Preload values for all Fafnir machine toll grade angular-contact bearings have been calculated to give optimum performance by machine designers.

A comparison of the curves in Figure 13 shows the 25contact angle bearing to be more rigid under axial loads than the 15contact angle bearing. Note that the axial deflection for the 2MM 15contact angle preloaded pair of bearings with a medium preload (60 pounds) is 0.0013" under a 300 pound thrust load. The 3MM 25contact angle preloaded pair with a light preload (60 pounds) deflects 0.0008" under the same 300 pound thrust load.

Light

30

60

Medium

60

120

Heavy

120

240

2MM 15Contact Angle Preload Force in Pounds

3MM 25Contact Angle Preload Force in Pounds

EFFECT OF SINGLE AND TANDEM MOUNTING ON AXIAL AND RADIAL RIGIDITY

.0030

.0008

Axial Deflection

.0025

.0020

.0015

.0010

A

C

Radial Deflection

.0007

.0006

.0005

.0004

.0003

.0002

B

D

.0005

.0001

100

200

300

400

500

600

700

800

100

200

300

400

500

600

700

800

Thrust Load – lbs. A Preloaded DB C Preloaded Tandem

Radial Load – lbs. B Preloaded DB D Preloaded Tandem Figure 12 – Axial vs Radial Deflections

EFFECTS OF CONTACT ANGLE ON AXIAL DEFLECTION Less Axial Rigidity

Low Contact Angle

High Contact Angle

Axial Deflection

.004

.003

.002

A

B C

D

Axial Deflection

.004

.003

.002

A

B C

.001

.001

D

100

200

300

400

500

600

700

100

200

300

400

500

600

700

Thrust Load – lbs.

A No Preload B Light Preload 40lbs.

C Medium Preload 125lbs. D Heavy Preload 250lbs.

A No Preload

Thrust Load – lbs. C Medium Preload 125lbs.

B Light Preload 40lbs. D Heavy Preload 250lbs.

Figure 13 – Axial Deflections

E32

102800

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