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

272

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