47 Motion & Control No.2—1997

1. Outline

After setting the preload for a tapered roller bearing,the preload sometimes disappears during initial operationdue to inadequate running-in. Especially for automotivetransmissions and differential gears, it is difficult toperform sufficient running-in, resulting in preload loss.When the preload is decreased, an adverse influence isexerted on bearing durability and gear noise is generateddue to the decreased support rigidity.

This paper considers the running-in procedure and howit can be utilized in preventing preload loss, while

introducing the NSK-developed QAT tapered rollerbearings which permit quick yet secure adjustments of thepreload.

2. Design

When combining the outer ring (cup) of the taperedroller bearing with the inner ring assembly (coneassembly), a clearance is formed between the roller’s large-end surface and inner-ring rib surface due to the roller’sown weight (Fig. 1).

QAT (Quick Adjustable Tapered)Roller Bearing

Clearance Clearance Clearance Clearance

Clearance(between roller-large-end surface and inner ring rib surface)

Fig. 1 Cause of generation of clearance

New Products

Load 49 N

150

100

50

0

Dis

plac

emen

t, µm

Number of rotations

Large clearance(General bearing)

Small clearance(QAT bearing)

Fig. 2 Axial displacement vs. running-in

Motion & Control No.2— 1997 48

It is necessary to eliminate this clearance in order toassure a proper preload. Between the inner ring racewayand rollers, however, since friction resistance (staticfriction) occurs, it is not possible to eliminate the clearanceby means of adding a force equivalent to the preload on thebearing. It is necessary to minimize the frictionalresistance by performing bearing rotation (a procedureknown as “running-in”) in order to apply a proper preloadand remove the clearance.

As shown in Fig. 2, for bearings with large clearances, it

is necessary to conduct running-in for twenty rotations orso while achieving axial displacement of up to 100 andseveral dozen µm to eliminate the clearance. Thisphenomenon is governed by the magnitude of theclearance. Also, bearing specifications are effective atcontrolling and minimizing the clearance for applicationswhere it is difficult to apply running-in, such as inautomotive transmissions and differential gears.

QAT tapered roller bearings are optimally designed tocontrol variation and minimize the clearance that is

15

10

5

0 300 300100 200

Axial load, N

QAT bearing

General bearing

No.

of r

otat

ions

unt

il st

abili

zed

150

100

50

0 100 200

Axial load, N

QAT bearing

General bearing

Dis

plac

emen

t unt

il st

abili

zed,

µm

Fig. 3 Comparison of running-in procedure

No. of rotations

0

0.5

5 10 15

QAT bearing

General bearing

Rot

atio

nal t

orqu

e, N

m

Clearance

Torque

Fig. 4 Comparison of rotational torque

200

100

0

Dis

plac

emen

t, µm

General bearing QAT bearing

200

100

0

Fig. 5 Comparison of bearing displacement

49 Motion & Control No.2—1997

generated from assembly of the inner and outer rings. Thisbearing may be mass produced thanks to improvementsmade in processing methods.

3. Features

QAT bearings carefully control and minimize clearanceand have a small displacement in the axial direction todecrease such clearance, requiring only a short running-inrotation.

QAT bearings provide the following advantages andbenefits.(1) Reduced time necessary for the running-in

procedure (Figs. 2 and 3).(2) A stabilized bearing torque is achieved more quickly

(Fig. 4).(3) Variation in bearing displacement is reduced (Fig. 5).(4) Since preload variation is reduced and controlled with

the preload set at a lower value, a reduced bearingtorque and prolonged fatigue life are obtained.

4. Conclusion

QAT bearings perform better than general bearingsthanks to their ability to adjust preload. QAT bearings areeffective in preventing preload loss, prolonging the rollingcontact fatigue life, reducing the torque by reduction of thepreload adjustment range, stabilizing the preload setting,and reducing assembly time.