Labyrinth seals for INAball bearings

Dipl.-Ing. Günter Gerhart and Dipl.-Ing. Michael Kurz

INA reprintOctober 1995

2

Labyrinth seals for INA ball bearingsDipl.-Ing. Günter Gerhart and Dipl.-Ing. Michael Kurz

1 IntroductionAbout 4000 years ago, the AncientEgyptians were already usingmaterials to help in the transportationof loads by reducing losses due tofriction. These included simplelubricants such as water in order to reduce the sliding friction of therunners. Later, they used mixturesof olive oil and lime to grease thewooden axles of the carriages.In relation to energy costs, drivedesign and environmental consider-ations, etc., friction is becomingincreasingly important. In rollingbearing engineering, a large part ofthe friction is produced by contact

seals. However, the bearings mustbe sealed to prevent the egress ofgrease and the penetration of con-taminants and moisture. Variousseals are used depending on theapplication. In general, improvedsealing leads to an increase in fric-tional torque. In the field of agricul-tural and construction machines,the level of contamination is suchthat very good sealing against dustis necessary and higher frictionaltorques are of secondary import-ance. In many applications, a non-contact labyrinth seal fulfils therequirements, which leads to amuch lower frictional torque thanbearings with contact seals.

2 Friction in ball bearingsThe friction in ball bearings is responsiblefor the development of heat and thus forthe operating temperature and the tem-perature of the lubricant. The total frictionaltorque of sealed bearings consists of thefollowing parts:

M = M0 + M1 + M3 Frictional torqueof bearing

M0 Lubricant frictionM1 Rolling frictionM3 Seal friction

Due to the numerous influences, theindividual components of the friction canonly be determined approximately and for constant operating conditions. The important factors are the type ofbearing, operating speed, load and heatdissipation.The lubricant friction M0 of a ball bear-ing, which is independent of load, isdetermined by the viscosity and theamount of grease as well as by the designof the bearing and the rotational speed.

Figure 1 Composition of the total frictionaltorque

Lubricantfriction M0 Rolling friction M1

Seal friction M3

Table 1 Various types of grease lubrication

3

In particular, if there is an excess of greaseand at high rotational speeds, the churn-ing effect increases the fluid friction. Dueto the operating temperature, the kin-ematic viscosity of the grease changes,which can lead to a reduction in the lubri-cant friction with increasing temperature.

M0 = f0 · (ν · n)2/3 · dM3 · 10–7 [1]

if ν · n ≥ 2000

M0 = f0 · 160 · dM3 · 10–7

if ν · n < 2000

M0 [Nmm] Frictional torque resulting from the fluid friction

f0 [–] Bearing factor for frictional torque dependent on the rotational speed

ν [mm2/s] Kinematic viscosity of the lubricant at operating temperature

dM [mm] Average diameter of the bearing 1/2 · (d+D)

n [min–1] Rotation speed

Special greases can be used to reducethe total friction. However, an optimizationof the lubricant in relation to lubricantfriction is not always possible becauseother criteria such as temperature stabil-ity, loading capacity etc. have to beconsidered.Diagram 1 shows the lubricant friction ofdifferent types of grease with the sametype of bearing. In order to eliminate sealfriction and rolling friction, the tests with-out any load were carried out with non-contact seals. The percentage distributionof the frictional torque was recorded upto the limiting speed of the bearing. The size and type of load are factorsinfluencing the rolling and slidingfriction M1. In ball bearings, the rollingbehaviour leads to stretching of the race-way and upsetting of the rolling element.Due to the load, a contact ellipse isformed at the points of contact of the ball raceways.Due to the different distances betweenthis contact point and the axis of the

Grease type Kinematic viscosity DIN standardat 40 °C

➀ Barium complex-soap grease 220 mm2 · s–1 DIN 51 825-KP2N-20

(mineral oil base)

➁ Polyurea 160 mm2 · s–1 DIN 51 825-KPE2R-30

(ester oil base)

➂ Lithium soap grease 68 mm2 · s–1 DIN 51 825-K3N-30

(mineral oil base)

➃ Lithium soap grease 15 mm2 · s–1 DIN 51 825-KE2K-50

(diester oil base)

bearing, pure rolling movement is nolonger possible; additional sliding motionoccurs. Additional sliding motion mightoccur for example at the contact pointsbetween the rolling element and thecage. However the sliding friction is lowunder good lubrication conditions. The frictional torque dependent on load isdetermined as follows:

M1 = f1 · P1 · dM [1], [2]

M1 [Nmm] Frictional torque caused by the rolling friction

f1 [–] Bearing factor for frictional torque dependent on load

P1 [N] Relevant load dM [mm] Average diameter of the

bearing 1/2 · (d+D)In bearings with contact seals, the frictionaltorque and heating of the bearing unit isdetermined decisively by the type and thepreload of the seal. The seal friction M3can only be estimated for some sealtypes. Since an exact calculation is notpossible due to the numerous influences,this must be determined by tests.Depending on the application and thedesign specification, a variety of require-ments are placed on the seal. INA there-fore offers the designer a range of sealtypes.

Diagram 1 Distribution of frictional torques with different greases, see also table 1

➃

➂

➁

➀

100

80

60

40

20

0

Per

cent

age

dist

ribut

ion

of fr

ictio

nal t

orqu

e

Speed

Figure 2 INA R type seal Figure 3 INA P type seal Figure 4 INA RS type seal

4

3 INA seals for ball bearingsThe main distinction is drawn betweencontact and non-contact seals. Theappropriate seal type is dependent on theapplication and the associated environ-mental influences. Contact seals have good sealing ability.However, the preload of the seal has asignificant influence on the total frictionaltorque of the bearing.The 3 piece INA R type seal has a zincplated inner and outer cover (Figure 2).Between the covers there is an NBR seallip running on a ground shoulder. The contact pressure of the seal lip is de-termined by the radial preload.The construction of the INA P type sealis similar to that of the R type seal (Figure 3).Between the zinc plated inner and outercover there is a radially and axiallypreloaded seal lip in a seal groove. The INA RS type seal is based on thesame principle, but a single piece vulcan-ised seal is used (Figure 4). The advantage of the R type seal com-pared to the P and RS type seals is thesize of the grease reservoir; it has a largergrease capacity.

The sealing effect of non-contact seals isachieved by one or more narrow gaps.Due to this non-contact seal type and theresulting low-friction running, the heatdevelopment of the bearing is muchlower. This gives a higher permissiblerotating speed and longer relubricationintervals; alternatively, relubrication is nolonger necessary.However, the Z sealing shield has only a limited sealing effect and is used inconditions of low contamination levels orin combination with additional sealing(Figure 5).

The 3 piece labyrinth seal has a signi-ficantly better sealing effect (Figure 6). It comprises an inner and outer sheetsteel washer rigidly rolled into the outerring. An angled sheet steel ring is locatedin a non-contact arrangement betweenthe washers and is pressed on the innerring. All parts of the L type seal are zincplated on all sides in order to improvecorrosion protection. Since the seal is extended in an axialdirection, this gives an enlarged greasereservoir, allowing longer lubricationintervals (Figure 7).

Diagram 2 Load free frictional torque as a function of the rotational speed

R type seal

RS, P type seal

Z, L type seal

100

80

60

40

20

0

Per

cent

age

of d

istr

ibut

ion

of fr

ictio

nal t

orqu

e

Speed

Figure 5 INA Z type seal Figure 6 INA labyrinth seal Figure 7 Textile bearing with INA labyrinthseal

5

4 Comparison of the frictional torque of individual seal types

In bearings operating under low to mediumloads, the frictional losses due to the sealaccount for the largest share of the totalfriction. Based on series of empiricaltests, the differences in frictional torque ofthe different types of seals were deter-mined. The frictional torque behaviour overthe whole speed spectrum is shown forbearings of equal size and with the samegreasing at operating temperature. Theproportion of rolling friction could be dis-regarded, since the bearings were testedwithout load. In order to allow conclusionsto be drawn independent of bearing size,no absolute values are given. Diagram 2shows a percentage distribution of thefrictional torques (lubricant friction andseal friction) as a function of the speed.

The frictional torque curves of P and RStype seals and those of Z and L type sealsare combined as one range due to theiridentical action and results. Since the Zand L type seals do not have any sealfriction, the measured frictional torquewithout load can be interpreted as purelubricant friction. The difference betweenthis and the friction values of P, RS and Rtype seals is the friction level due to thecontact seals.

The reduction for the R type seal in thehigher speed range can be explained by anumber of factors, including the decreasein the kinematic viscosity of the greasedue to strong heating of the bearing, thepreload of the seal material and thus thetotal friction. The disadvantage of thiseffect, however, is that the life of standardgreases is considerably reduced atoperating temperatures higher than 70 °C.Diagram 3 shows the influence of the sealtype on the equilibrium temperature ofthe bearing.

Diagram 3 Equilibrium temperature as function of the rotational speed

R type seal

RS, P type seal

Z, L type seal

100

90

110

80

70

60

50

40

30

20

10

0

Tem

pera

ture

[°C

]

Speed

6

5 Consequences for theapplication

Depending on the application, a reductionin drive energy can play an important role.This can be taken into consideration es-pecially in those areas where machineswith a high number of bearing locationsare used, in the design dimensioning of thedriving unit. A brief calculation example isgiven to explain the conclusions ofDiagram 2.Example: INA radial insert ball bearing GE 30 KLLH(B) (Figure 11) with d = 30 mmunder radial load:

Bearing: C0 = 11 300 NC = 19 500 NdM = 46 mm

Load:n = 500 min–1

P = 1 950 NC/P = 10f1 = 0,0006 · (P/C0)

1/2 = 2,49 · 10–4

Frictional torque:

Lubricant friction: M0 = from test

Rolling friction:M1 = f1 · P · dM = 2,49 · 10–4 · 1950 · 46

= 22,4 Nmm

Seal friction: M3 = from test

Bearing with contact RS type sealM0+M3 = 53,4 NmmMges = 75,8 Nmm

Bearing with non-contact labyrinth sealM0+M3 = 2,5 NmmMges = 24,9 Nmm

Saving = 50,9 Nmm/bearing= 67,2 %

Figure 8 Special housing unit for the textile industry with integral INA labyrinth seal Figure 9 Textile bearing with low frictiontorque and special greasing

Figure 11 Paper guide roller bearing arrangement

Authors:Dipl.-Ing. Günter Gerhart, engineeringand sales managerDipl.-Ing. Michael Kurz, applicationengineering managerSchaeffler Wälzlager oHG Homburg/Saar

7

6 Applications for bearingswith labyrinth seals

There is a wide range of possible appli-cations for bearings with very low frictionespecially in the field of textile and paperprocessing machines; they are particularlysuitable for application in guide rollers,pleating rollers and heel rollers. The INA labyrinth seal has particular ad-vantages over other bearing types es-pecially in the textile industry. In ballbearings with contact seals or 2 Z sealingshields, a thin film of lubricant is formedbetween the seal lip and the seal groove.Fabric fly then adheres to this lubricant

Literature:[1] Eschmann, P., Hasbargen, L. undWeigand K.: Die Wälzlagerpraxis, R. Oldenbourg Verlag München-Wien,1978[2] Catalogue GB 511, INA Schaeffler Wälzlager Homburg/Saar

film. Due to “capillary action”, this fly thendraws the base oil out of the lubricant.However, the design of the labyrinth sealprevents contact between the fly and thelubricant. This seal type also offers thesame advantage in relation to paper dust.The optimum combination would be lightrunning grease (see Diagram 1) withlabyrinth seals (Figures 8, 9 and 10).In machines with a large number ofbearings, the drive power required can be reduced by minimizing friction, givingsavings on manufacturing and energycosts.

Figure 10 Special bearing in transport chains for dryers

Art

.Nr.

910

467

-4/L

DW

GB

-D 0

5961

·Prin

ted

in G

erm

any

Schaeffler Wälzlager oHG

Postfach 15 53D-66406 Homburg (Saar)Telephone (0 68 41) 7 01-0