13. NSK special · PDF file268 269 13. NSK special bearings 13.1 Ultra-precision ball...
Transcript of 13. NSK special · PDF file268 269 13. NSK special bearings 13.1 Ultra-precision ball...
268 269
13. NSK special bearings
13.1 Ultra-precision ball bearings for gyroscopes
(1) Gyroscope bearingsGyroscopes are used to detect and
determine traveling position and angular velocity in airplanes and ships. Gyros are structurally divided into two groups depending on the number of directions and speeds of movement to be detected: those with one degree of freedom and those with two degrees of freedom. (See Fig. 1)The performance of a gyro depends on the characteristics of the bearing. Thus, a gyro bearing is required to demonstrate top grade performance among the ultra-precision miniature bearings. Gyros have two sets of bearings. One set supports the rotor shaft running at high speed and the other set supports the frame (gimbal). Both must have stable, low frictional torque. Principal types and application environments of rolling bearings for gyros are shown in Table 1.
The inch series of ultra-precision bearings are almost exclusively used for rotors and gimbals. Boundary dimensions and typical NSK bearing numbers are shown in Table 2.
Special-shaped bearings dedicated to gyro applications are also used in large quantity.
Table 1 Type and running conditions of gyro bearings
Application Principal bearing type Typical running conditions
RotorAngular contact ball bearingEnd-cap ball bearing
12 000, 24 000 min1 or 36 000 min1, 60 to 80Chelium gas
GimbalDeep groove ball bearingOther special-shaped bearings
Oscillation within 2,Normal temperatures to 80C,Silicon oil or air
Fig. 1Gyro type
270
NSK special bearings
271
*Angular contact type bearing is also available for rotor.
Table 2 Boundary dimensions and bearing number of gyroscope bearings
Boundary dimensions (mm) Bearing numbers
d D B B1 r(min.)
Open Both-sideshielded
1.0161.1911.397
1.9842.380
3.175
3.967
4.762
6.350
7.9389.525
3.175 3.967 4.762
6.350 4.762 4.762 7.938
6.350 7.938 9.525
9.52512.700 7.938
7.938 9.52512.700
9.52512.70015.87519.050
12.70022.225
1.1911.5881.984
2.3801.588
2.779
2.3802.7792.779
3.9674.3662.779
2.7793.1753.967
3.1753.1754.9785.558
3.9675.558
2.3802.779
3.571
2.3803.571
2.7793.5713.571
3.9674.3663.175
3.1753.1754.978
3.1754.7624.9787.142
3.9677.142
0.10.10.1
0.10.10.10.15
0.10.10.15
0.30.30.1
0.10.10.3
0.10.150.30.4
0.150.4
R 09R 0
*R 1
*R 1-4*R 133
*R 1-5
*R 144R 2-5
*R 2-6
*R 2R 2 AR 155
R 156R 166
*R 3
R 168 BR 188
*R 4 B*R 4 AA
R 1810R 6
R 0 ZZR 1 ZZ
R 1-4 ZZ
R 133 ZZSR 1-5 ZZ
R 144 ZZR 2-5 ZZR 2-6 ZZS
R 2 ZZR 2 AZZR 155 ZZS
R 156 ZZSR 166 ZZR 3 ZZ
R 168 BZZR 188 ZZR 4 BZZR 4 AAZZ
R 1810 ZZR 6 ZZ
Boundary dimensions (mm) Bearing numbers
D1 C1 C2 Open,with flange
Both-side shielded,with flange
5.156 5.944
7.518 5.944 5.944 9.119
7.518 9.11910.719
11.176
9.119
9.11910.71914.351
10.71913.89417.526
13.89424.613
0.3300.580
0.5800.460
0.580
0.5800.5800.580
0.760
0.580
0.5800.5801.070
0.5800.5801.070
0.7901.570
0.7900.790
0.790
0.7900.790
0.7900.7900.790
0.760
0.910
0.9100.7901.070
0.9101.1401.070
0.7901.570
FR 0FR 1
FR 1-4FR 133
FR 1-5
FR 144FR 2-5FR 2-6
FR 2
FR 155
FR 156FR 166FR 3
FR 168 BFR 188FR 4 B
FR 1810FR 6
FR 0 ZZFR 1 ZZ
FR 1-4 ZZ
FR 133 ZZSFR 1-5 ZZ
FR 144 ZZFR 2-5 ZZFR 2-6 ZZS
FR 2 ZZ
FR 155 ZZS
FR 156 ZZSFR 166 ZZFR 3 ZZ
FR 168 BZZFR 188 ZZFR 4 BZZ
FR 1810 ZZFR 6 ZZ
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NSK special bearings
273
(2) Characteristics of gyroscope bearings(2.1) Rotor bearing
The rotor bearing is required to offer extremely-low, torque at high speed, and free from variation and long term stability. To meet these demands, the bearing uses an oil-immersed cage in most cases. A lubrication method of injecting the lubricating oil dissolved with solvent into a bearing is also available, but this method requires adequate concentration adjustment because the frictional torque is affected by the oil amount (Fig. 2). In such an event, the oil amount is adjusted through centrifugal separation to obtain variation-free running torque. A special bearing type, in which an end cap is integrated with the outer ring, may also be used (Fig. 3).
(2.2) Gimbal bearingRequirements on the gimbal bearing as a
gyro output axis include low frictional torque and vibration resistance. Table 3 shows the
maximum starting frictional torque for typical bearings. Much smaller starting torque can be obtained through precision machining of the raceway groove and special design of the cage.
Fig. 3Typical end-cap ball bearing
Fig. 2Oil amount and running torque
Table 4 Specifications of bearings for rotors and gimbals
Item Rotor bearing Gimbal bearing
Bearing type
Bearing accuracy
Lubricationmethod
Cage
Ball accuracy
Bearing contactangle ()
Angular contact ball
CLASS 7P or above
Oil-immersed cage and self-lubrication (dual use greaseavailable)
Laminated phenol
Around Grade 3
20 to 28
Deep groove ball or angularcontact ball
CLASS 5P or CLASS 7P
Oil lubrication, filled with anadequate quantity
Steel sheet (low torque design)
Around Grade 5 or above
Table 3 Maximum starting torque
BearingNo.
Measuring load
mN {gf}
Radial internal clearance (m)MC23 to 8
MC35 to 10
MC48 to 13
MC513 to 20
MC620 to 28
Maximum starting torque (Nm) {mgfmm}
R1
R1-5
R144
R2
R3
R4B
735{75}
735{75}
735{75}
735{75}
3 900{400}
3 900{400}
7.95{810}
13.2{1 350}
8.92{910}
14.7{1 500}
63.5{6 500}
68.5{7 000}
7.35{750}
12.3{1 250}
8.35{840}
13.7{1 400}
54.0{5 500}
59.0{6 000}
6.75{690}
11.8{1 200}
7.65{780}
12.7{1 300}
54.0{5 500}
59.0{6 000}
6.10{620}
10.7{1 090}
6.85{700}
11.8{1 200}
49.0{5 000}
54.0{5 500}
5.20{530}
9.70{990}
6.08{620}
11.4{1 160}
44.0{4 500}
49.0{5 000}
274
NSK special bearings
275
13.2 Bearings for vacuum use ball bearings for X-ray tube
A ball bearing for a rotary anode of an X-ray tube is used under severe conditions such as high vacuum, high temperature, and high speed. An X-ray tube is constructed as shown in Fig. 1, with the internal pressure set below 0.13 mPa (106 Torr). Thermoelectrons flow from a cathode (filament) toward an anode (target) to generate X-rays at the anode.
A rotor is a part of a motor and driven electromagnetically from the outside. Common speeds range from 3 000 to 10 000 min1. The anode rotation involves inner ring or outer ring rotation (Fig. 2). Generally, inner ring rotation enables high rigidity and low bearing temperature, but the construction becomes complicated.
Because of heat generation of the anode, the bearing reaches the maximum temperature of 400 to 500C on the anode side and the bearing on the opposite side reaches a temperature of 200 to 300C. The bearing is therefore made from high-speed tool steel which is superior in heat resistance.
Most X-ray tubes are used for medical purposes and thus silent rotation is essential. However, difficulty of enhancing the rigidity because of its construction and change in the bearing internal clearance under heavy temperature fluctuation are hindrances to vibration proof.
In this respect, minute care must be taken during the design of a bearing and its neighboring parts. The common range of bearing bores is 6 to 10 mm. Fig. 3 shows examples of typical constructions.
(a) is a type with pressed cage.(b) has the entire outer ring raceway shaped
as a cylindrical surface.(c) has one side of the outer ring raceway
shaped as a cylindrical surface to relieve deviation of inner and outer rings (such as caused by thermal expansion) in the axial direction.
Note that (b) and (c) normally apply to full complement type ball bearings.
Fig. 1Typical construction of X-ray tube
Fig. 2Anode bearing and rotating ring
Fig. 3Typical construction of X-ray tube bearings
276
NSK special bearings
277
One of the greatest challenges facing X-ray tube ball bearings is the lubrication method. Because of the vacuum and high temperature environment, a solid lubricant is used with one of the methods described below:(1) Provision of laminated solid lubrication (molybdenum disulfide) to the pocket surface of a cage(2) Provision of thin film of mild metal (silver or lead) over the surface of balls, inner ring/outer ring raceway
The method (2) above applies mostly to full-complement type ball bearings and the thin film is provided by plating, ion plating, etc. The results of a durability test performed on a ball bearing with a soft metal coating in a vacuum are shown in Fig. 4. By the way, Fig. 4 shows a comparison of the endurance time for different conditions of ball bearings (8 mm in bore and 22 mm in outside diameter) that are rotated at 9 000 min1 under an axial load of 20 N {2 kgf} at 0.13 mPa {106 Torr} while at room temperature. Fig. 5 shows a graph of the change in running torque as a function of time.
The