The sphere roller - Schaeffler Group · 2019. 5. 24. · of the ball diameter, more rolling elements can be fitted by means of eccentric assembly in a position rotated by90 degrees

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The sphere roller –Less is more!

Heinrich HofmannRainer EidlothDr. Robert PlankGottfried Ruoff

It started with the point, then along came the circle and in 1883 the ball was born.

It has always the rolling elements – whether theywere balls or rollers – that decisively influencethe design, types and ultimately the perform-ance capability of rolling bearings. The basicdesigns of almost all rolling bearings were devel-oped to a level ready for volume productionaround 1900, when it became possible to hardenreliably and grind precisely (Figure 1).

It was the new vehicles – whether they werepedal cycles or motor vehicles – that were mostnotably dependent on low friction, namely onrolling friction. The level of torque that could beapplied to the pedal crank of a cycle, or could besupplied by the earliest engines, was still toosmall.

The low rolling friction between a wheel and asolid road had been known and exploited for mil-lennia; with the invention of the ball bearing andtapered roller bearing, this was transferred tothe centre of the wheel. The balls or rollers act asnumerous small low-friction wheels between thewheel axle and the hub.

From a very early stage, the shafts in the earlygearboxes were supported in order to give pre-cise, low-wear guidance of the gears (Figure 2).

Although now more than 100 years old, the ballbearing is still the world champion of ball bear-

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88 TThhee sspphheerree rroolllleerr

Introduction

Figure 1 History of rolling bearing types

Figure 2 Wheel and gearbox bearing arrangementsaround 1900

Wheel supported by balls

Wheel supported by tapered rollers

Manual gearbox

Rear axle differential

ing series in terms of quantity. Almost 7 billionpieces are now produced worldwide each year.

Tapered roller bearings – also developed aroundthe turn of the century – are now number 3, witha quantity of almost 1 billion produced each year.

It took the 50 years from 1900 to 1950 until theneedle bearing became the third major seriesready for volume production. The technologyrequired to produce the thin cross-sections forcages and formed bearing rings was not readyfor volume usage until about 1950. In terms ofquantities – with production of 4 billion piecesper year – the needle roller bearing family isnumber 2 worldwide.

The scale illustration with a constant bore size of35 mm – arranged in order of outside diameter –shows considerable differences in size, load rat-ing and mass (Figure 3).

All other rolling bearing series fitted with cylin-drical rollers and barrel rollers are produced inquantities that are smaller by several powers often than the three series mentioned.

Differences betweenseries with balls,tapered rollers andneedle rollersA comparison of the power-to-weight ratio, i.e. theload carrying capacity in kN divided by the bear-ing mass in kg, clearly shows the differences.

In order to highlight the differences between theseries when mounted, a virtual twin-shaft gear-box was designed.

Shaft 1 is supported as a locating/non-locatingbearing arrangement in a ball bearing and adrawn cup roller bearing.

Shaft 2 is designed as a semi-locating bearingarrangement in two identical tapered roller bear-ings. The tapered roller bearings are axially abuttedagainst each other. This is also described as anadjusted bearing arrangement, since the axialclearance is adjusted to ensure optimum func-tion (Figure 4).

In the semi-locating bearing design, it is clearthat the inner ring rib of the tapered roller bear-ing takes up axial space. This rib is necessary in


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Figure 3 Comparison of types

order to guide the endface of the taperedroller. In addition torolling friction on theoutside surface of theroller, there is there-fore sliding friction atthe rib. In terms of fric-tion, the tapered rollerbearing is therefore aso-called hybrid bear-ing. The load carryingcapacity is high in boththe radial and axialdirections. The power-to-weight ratio for theexample bearing 32007is at a respectable levelof 223 kN/Kg.

With an outside dia-meter of 62 mm, the tapered roller bearing doesnot require much radial space.

The weakpoints of the tapered roller bearing aretherefore the section width and the friction.

In the non-locating/locating bearing design ofshaft 1, it is noticeable that the drawn cup rollerbearing as a non-locating bearing requires littleradial space. This is possible due to the integra-tion in the shaft of the inner ring raceway and thethin-walled formed outer ring.

The power-to-weight ratio is very high at 478kN/kg. The drawn cup roller bearing, however,can only support radial loads.

The drawn cup roller bearing, a relative of the needleroller, is a technically sophisticated and cost-effec-tive solution for a purely radial bearing arrangement!

The ball bearing 6207 as a locating bearing hasthe largest section size with an outside diameterof 72 mm and has the lowest-power-to-weightratio of 93 kN/kg. However, it can support radialas well as axial loads.

The ball bearing, from today's perspective, is asuccessful misdesign from 1900.

Weakpoints of theball bearingBall filling capacityOn the one hand, the eccentric assembly allowssingle-piece rings without weakpoints such as fill-ing slots. The deep grooves increase the axial loadcarrying capacity. In the USA, the ball bearing istherefore described as a “deep groove ball bearing”.

For reasons of assembly, the ball filling capacityis limited by the sickle-shaped space betweenthe bearing rings or, in other words, there is“lots of air but not many balls” around the pitchcircle after ball distribution. For example, theball bearing 6207 can only be fitted with 9 balls.



Figure 5 Ball filling capacity/Width utilisation

Figure 4 Twin-shaft gearbox, current level of technology

Width utilisationOnly 70 % of the ball width is used, i.e. 30 % ofthe ball width wastes space and increases mass.

Contact pressureUnder small loads, the contact zone is only“punctiform”, i.e. due to the poor osculation, thepressure ellipse is small and the contact pres-sure high. The contact pressure is therefore larg-er than in roller bearings with line contact.

Under purely axial load – see Figure 6 – a circum-ferential wear track is found on the ball equatorand increases in width with increasing load.

SolutionsThe answer is the sphere roller bearing! Thethick ball is slimmed down and becomes a leansphere roller (KXR) in the sphere roller bearing!

Under load, the rotational axis of the ball doesnot change. If, however, a load-free zone occursin the bearing, for example under radial load, therotational axis of the ball changes chaoticallywith every revolution.

In other words, the “lazy” part of the ball each 15 %to the left and right of the ball diameter that doesnot support load is cut away. The bearing rings aretherefore narrower by 30 % of the ball diameter.

In the design of KXR bearings, a new guidancemechanism for the rolling elements using thecage and end of the sphere roller was developed.

Under zero load and zero speed, this guidancemechanism allows alignment of the rolling ele-ments in start-up operation.

The pocket bases of the cage are designed suchthat the sphere roller under load can align itselffreely as a function of the contact angle.



Figure 6 Contact angle and osculation

Figure 7 Sphere roller bearing

Advantagesof the “lean” sphere roller bearingFilling capacityThe ball filling capacity is increased, i.e. thebasic bearing 6207 can accommodate elevenrolling elements instead of nine.

Since the width of the sphere roller is only 70 %of the ball diameter, more rolling elements canbe fitted by means of eccentric assembly in aposition rotated by 90 degrees (Figure 8).

More rolling elements means higher load carry-ing capacity.

Width utilisationIf the full width of the sphere roller is utilised,the bearing width is reduced. In the case underconsideration, this is a decrease from 17 mm to13,5, i.e. a reduction of 20 %. Due to the reducedwidth, the bearing mass is reduced by 20 %.

In the case of the sphere roller: Less is more,since despite the smaller width, more rolling ele-ments can be fitted.

Contact pressureIt is known that the point contact in the case ofballs leads to higher contact pressure. Thechange in osculation to a logarithmic profile onthe sphere roller further increases the basic loadrating. The logarithmic profile of the sphereroller outside surface prevents edge pressure inthe axial boundary areas of the sphere roller

(small radius) with very narrow osculation in thecontact area (large radius). Even under smallerand moderate loads, there is a wide pressureellipse with low contact pressure.

The sphere roller can be designed with a logarith-mic profile since the rotational axis is always per-pendicular to the variable contact angle. The oscu-lation conditions therefore do not change if the loadratio changes from axial to radial and the contactangle changes as a result. The osculation “creeps”in an optimum manner with the change in load.

All computer simulations and running testsdemonstrate: “Yes it does rotate” about theintended rotational axis!

The sphere roller rotates in an optimum mannerabout its axis due to the special mass distribution,this means that it follows the gyroscopic laws.

Note: The KXR bearing retains the deep groovesof the ball bearing and thus the high axial loadcarrying capacity. The sphere rollers are widerthan the distance between the ring ribs. Witheccentric assembly, however, numerous sphererollers can be fitted.

Areas of application ofsphere roller bearingsA distinction is drawn between single row locat-ing bearing applications and double row semi-



Figure 9 Contact angle and osculation

Figure 8 Filling capacity/Width utilisation

locating bearing arrangements (adjusted bear-ing arrangements).

The single row KXR bearing, e.g. 6207KXR supple-ments the ball bearing to DIN dimensions and is:

• either higher performance but narrower withthe same outside diameter and the same bore

• or higher performance with the same widthbut with an increased grease reservoir orimproved sealing

• or equal performance but smaller radially andeven narrower than the standard ball bearing.

The double row KXR bearing = KXRT (tandemarrangement) can replace the tapered rollerbearing and multi-row ball bearings, since 2sphere rollers can replace one tapered roller.

The KXRT bearing has the same performance butlower friction and is narrower than the taperedroller bearing.

The KXRT bearing is the logical development ofthe tandem ball bearing (used, for example, inpinion bearing arrangements) with 2 rows ofballs.

The four-row KXR4 bearing replaces double rowtapered roller bearings and four-row ball bear-



Figure 10 Applications of KXR bearings

Figure 11 KXRT bearing (double row)

ings of the same bore and outside diameter. TheKXR4 bearing can also replace double row ballbearings using smaller sphere rollers with areduced outside diameter.

For the virtual shaft arrangement in a gearbox asdescribed in Figure 4, there are advantages inusing the new KXR designs (Figure 11).

The locating bearing on shaft 1 is replaced by asingle row KXR bear-ing of equal perform-ance but of smallerdimensions and nar-rower section size.

The two semi-locat-ing bearings on shaft2 are substituted bydouble row KXRTbearings.

Design envelopes –centre distance andbearing spacing –can be reduced!

It is well known thatvehicles with frontwheel drive andtransverse engines

require manual gearboxes with short mountingdimensions.

It is also a fact that the friction of the KXRT bear-ings is smaller than that of the tapered rollerbearing since there is no sliding friction on ribs.

Bearing adjustment and lubrication is simplerand more robust for the KXRT semi-locating bear-ings.



Figure 13 Twin-shaft gearbox with KXR/KXRT bearings

Figure 12 Applications of KXRT/KXR4 bearings

VerificationThe following investigations were carried out inorder to demonstrate the performance of the KXRdesigns.

Computer simulation of the kinematicsDue to its moment of inertia, the KXR is morequickly stabilised than the “fully round ball”. It isknown, for example, that a pedal cycle runs withgreater stability as the speed increases. This canbe explained by the gyroscopic forces.

Tests under load and speed• Testing under moment load

• In this challenging test, the contact anglechanges from plus to minus for each sphereroller on each revolution. It is found that thesphere rollers “creep” with the contact anglein accordance with the theory.

• Dynamic tests under axial and radial load

• The KXR bearings fulfil the theoretically antic-ipated life.

• Drive tests

Correct function in the manual gearbox isassured in jump starting and “drive and coast”.

Summaryand outlookThe sphere roller is a new rolling element lead-ing to a new series of rolling bearings with awide range of possible applications in the auto-motive and industrial sectors.

The KXR and KXRT bearings are highly suitablefor applications in gearboxes.

Perhaps this is a law similar to that in the IT sec-tor according to which product performance isincreased according to a rhythm which may dif-fer for electronics or mechanical engineering(source: G. E. Moore, co-founder of Intel)

Advantages of the sphere roller familyFor an unchanged load case

• Less space required

• Lower mass

• Lower friction, reduced energy consumption

For the same design envelope

• Higher performance (load carrying capacity)

• Larger grease reservoir

• More space for improved sealing

The KXR family is a solution with high perform-ance density!

100 years after the ball bearing was invented,and 50 years after the needle roller bearingcould be put into production, a new series ofrolling bearings has been born!



Figure 14 Summary/advantages

Documents

The sphere roller - Schaeffler Group · 2019. 5. 24. · of the ball diameter, more rolling elements can be fitted by means of eccentric assembly in a position rotated by90 degrees