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Presenting examples of

MOTOR-OVERLOAD-

PROTECTION applications

from the APPLIANCE and

AUTOMOTIVE industries.

B Y J . W . S M I T H

E S IG N ER S A R E C O NS T AN T LY

FACING THE CHALLENGES of packing

m o r e an d m o r e f u nct i on al it y i n t o

ever-smaller spaces while improving power

and performance. Mechanical overload protection for

drive-train systems has long been desirable but, in the past,

has required bulky and expensive slip-clutch mechanisms.

Tolerance rings have traditionally been used to com-pensate for machining tolerances or differential thermal

expansion effects, but developments in tolerance ring

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COURTESYOFRENCOLTOLERENCE

RINGS

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technology outlined in this article are enabling them to bemore widelyused as simple, low-cost,nondestructive over-load-protection devices.

The principles of tolerance-ring design are outlinedhere, along with an explanation of thefactors affecting theresulting design parameters, such as assembly force, ra-dial load capacity, spring compression, and slip torque.We will also present examples of successful overload-pro-

tection applications from the appliance and automotiveindustries.

First Principles of Tolerance Ring DesignThe tolerance ring is a precision spring steel device, com-prising a thin strip into which corrugations, or waves, areformed, each ofwhichwillact asa spring(Fig. 1). Thisstripis then rolled into a ring [Fig. 2(a)].

Within the elastic limit ofthe waves that makeupa tol-erance ring, simple spring theory applies, i.e.,

( )Force N :F Kc=

where K is the spring constant (N/mm

2

) and c is the dis-placement (mm). The factors influencing KincludeI material specification represented by Youngs

modulusI material thicknessI wave pitchI wave widthI wave shoulder shapeI wave thinningI plannish widthI plannish thicknessI wave root radiiI wave crest radii.Of these, for a given wave shape, the two major factors

are thickness and wave pitch. So the spring constant(kN/mm) is given by

K E wt

p

48

3

.

where E is the elastic modulus for the material (kN/mm),w is the width, tis the thickness (mm), and p is the wavepitch (mm).

The cubic power relationship allows for the possibilityof engineering a very wide range of spring stiffness. Thecomplex wave geometry, with a formed wave and closedends, gives rise to a rigid structure, allowing very high

spring stiffness to be achieved with corresponding hightorque transmission possible.

Fig. 3 shows an output from an finite element analysis(FEA) model of one wavewithina tolerance ring. It can beseen that the wave shoulders make the main contributionto the stiffness. This is one of the reasons why rings withmultiple bands of waves around the circumference are of-ten used in high-torque applications, see Fig. 2(b) for ex-amples. In tests using rigid steel gauges, a duplex ringwith two sets of waves gives 1.7 times the torque of a sin-gle-banded ring of the same dimensions. The differingstress distribution within the wave is utilized to retain 75

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Each wave acts as a spring.

1

(a)

(b)

(a) Example tolerance rings.

(b) Example rings with modified wave design.

2

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some spring performance in applications where the wavehas been plastically deformed, such as torque overloadprotection.

The rings are specifically designed for each applicationby varying the complex ring geometry, material thickness,hardness,etc., to createan appropriate springconstantand,hence, a predetermined torque above which the ring willslip (Fig. 4).

Radial load (N) is given by

F n c K R

=

where n is the number of waves, c is the compression ofwaves (mm), andKis the spring constant (N/mm). The ax-ial assembly force (N) is given by:

F FA R

=

where is the coefficient of friction, see Table 1 for typicalfigures. The torque (Nm) is given by

T Fd

A= 2

where d is the diameter (mm).Table 2 shows a selection of ring configurations with

corresponding spring constants. It is important to note thewide range of spring constant and, hence, forces andtorques that can be designed for a given ring size.

Empirical research shows that the wave compressionrange for the elastic properties varies depending on thewave geometry. The typical range for elastic propertiesis from 3-7% of wave height compression, but for light

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FEA stress distribution on top wave surface.

3

FR

FA

FR FR

FR FR

FR FR

FR

FA

FA FA

FA

FA FAFA

FAFA

FA

Frictional forces generating torque performance and as-

sembly force.

4

TABLE 1. COEFFICIENTS OF FRICTION [3]

MaterialStatic(Dry)

Static(Lubricated)

Steel on Steel 0.8 0.16

Steel on Cast Iron 0.4 0.21

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duty rings, specifically designed for mounting bearings,the elastic limit can be as high as 50% with a usablerange of 20-40%.

Principles of Operationin Bearing Mount Applications

This normally involves rings designed to achieve themini-

mum percentage of spring compression at which bearingcompression can be guaranteed when at maximum clear-ance, leaving the remainder of the spring compressionrange availableto handleanyexternal radial loads,machin-ing tolerances, or dimensional changes caused by differen-tial thermal expansion of the mating components.

In a typical electricmotorbearing mountwith Aluminahousing,608bearing,H9 toleranceonthebore, anda tem-perature range of 20-100 C, an HVL22x7SS tolerancering with a 0.33 mm wave height would be used, giving aradialclearanceof 0.297-0.320 mmacrossthetemperatureand machining tolerance range (Fig. 5).

Principles of Operationin Slip-Clutch/Torque-Transfer Applications

In slip-clutch applications, the ring is designed such thatthe crests of the waves embed slightly into the matingcomponent, and the base of the waves acts as a bearing sur-face and slips against the mating surface. Rings can be de-signed to slip either on the shaft or the bore, depending onthe materials, surface finish, and hardness issues, etc.

By utilizing the plastic range of Fig. 6, dimensionalchanges due to the tolerance variations of mating compo-nents will lead to minimal force (and, hence, slip torque)variation. The ring geometry is, however, designed suchthat sections will continue to operate in an elastic fashion

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TABLE 2. EXAMPLE CALCULATED SPRING CONSTANTS

Diameter Width PitchNo.Waves

WaveHeight

Thick-ness

SpringConstantperWave

SpringConstantof Ring

CalculatedRadial Load@ 10% WaveCompression

CalculatedTorque @ 10% WaveCompression

mm mm mm mm mm kN/mm kN/mm N Nm

10 10 3.5 8 0.75 0.1 0.23 1.85 139 0.10

10 10 2.5 12 0.75 0.2 5.09 61.05 4579 3.43

10 10 2.5 12 0.75 0.3 17.17 206.03 15452 11.59

25 25 3.5 22 1.00 0.1 0.58 12.75 1275 2.39

25 25 2.5 31 1.00 0.4 101.74 3,154.08 315408 591.39

25 25 3.5 22 1.00 0.7 198.72 4,371.84 437184 819.72

50 25 3.5 44 1.00 0.1 0.58 25.49 2549 9.56

50 25 2.5 62 1.00 0.4 101.74 6,308.17 630817 2,365.56

50 25 3.5 44 1.00 0.7 198.72 8,743.68 874368 3,278.88

Depending on minimum/maximum design compression of the waves. At high loads/torques results are significantly

affected by the mechanical properties (such as the surface finish, strength and rigidity) of the mating components.

Elastic and Plastic Zones

RadialLoad (N)

Torque FitRange

Bearing FitRange

AssemblyForce (N)

Torque(Nm)

Wave Compression

Force

/Torque

Effect of thermal expansion on radial clearances.

5

Rc Max.

Rc Min.

Vr

Force and torque versus wave compression.

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even after the main body has undergone plastic deforma-tion. This allows the ring to still accommodate the minordimensional fluctuations during the overload slip and totransmit thedrivetorqueonce again after theoverload con-dition is removed.

Developments in Torque-Slip DesignsModifications to root radii andthewave base or plannisharea, as shown in Figs. 7 and 8, have improved the bearingsurface, removing the point contact ofthepreviousdesigns that ledto degrada-

tion of the mating components.The flatter plannish area between

the waves also improves distributionof loadsaround theshaft(or bore). Fig.9 shows this improved force distribu-tion; pressure sensitive film wasplaced between the base of the ringsandthe mating component. This is thesurface that is designedto slip, andthemore uniform load distribution isclearly visible.

The new design overcomes thestick-slipeffects that previouslyrequiredthe system to undergo several overload

slipsbeforebedding-in andachievingthenominal torque specification.

Investigation of the wave riser anglehas shown that optimum conditions allow the maximumnumber of waves around the ring circumference while

maintaining the spring properties of the waves and allow-ing progressive deformation during assembly.

Depending on mating components, materials, and re-quired torque transmission, slip torques up to hundreds ofnewton meters can be accommodated.Slip-torque accuracyof10% and the number of slips over 10,000 have beensuccessfully developed (Table 3).

Fig. 10 shows the performance of astainless-steel conventional ring with

mild steel-mating components usingas-machined surface finish and ShellAlbidagrease. Were this assembly to beused in a production application, aprebreak slip cycle could be used duringassembly to bed-in the tolerance ringand remove the higher first torque fromthe curve during operation in-situ, ifthe design required it.

The mating components, torquelevels, and heat generation during theslip condition limit maximum slip-cy-cle frequency and duration. Tolerancerings are suitable where the slip dura-

tion is ofthe order of a few seconds, andthe surface slip speed is less than 1.5ms. This is primarily due to the fric-

tional heat generated, which will tend to cause annealingof the rings if excessive.

The modified wave ring design reduces initial slip to asubsequent slip differentialby 50%and reduced rangeoverfirst five cycles by 60%. The specification for automotiveantitheft slip cycles is +90, 180, and +90 over five cy-cles (Figs. 11 and 12).

Lubrication

The choice of lubricant is important in overall system per-

formance, as with any bearing. Determining factors in-clude intended operating temperature, mating materials,cleanliness, and other environmental issues.

In typical high load/torque applicationswith steel mat-ing components operating between 40 C and 150 C,high-pressure, high-temperature lubricants, such as ShellAlbida grease, are used.

Proprietary solid lubricants, using resin-based molyb-denum or silicon, have been successfully used in applica-tions where grease application was deemed undesirable orwhere avoidanceof thecosts associated with greaseapplica-tion on automated assembly machinery was required.78

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Point Contact= High Range (R)= High T1

Point Contact

Standard wave profile.

7

Reduced Point Contact= Flat Bearing Surface=Low Range (R)

=Low T1

Reduced PointContact

Modified wave profile.

8

Pressure-sensitive film showing load distribution of conven-

tional ring (upper) and modified wave design (lower).

9

TOLERANCE

RINGS ARE USED

AS SIMPLE,

LOW-COST,

NONDESTRUCTIVE

OVERLOAD

PROTECTION

DEVICES.

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In fact, some of these solid lubricants, developed spe-cifically for this application, have produced slip-torqueresults on traditional design rings comparable with thenewmodified wave profile design. This allows for thepos-sibility of performance enhancement of an existing toler-ance ring design without retooling the manufacturingprocess (Figs. 13 and 14).

Black & Decker Hedge TrimmerOverload ProtectionBlack & Decker (B&D) wanted to develop a standard de-sign to cover sales in both Europe and the United States.Thedifferentmarkets had differentaspirations withregardto product life and robustness.

By using a tolerance ring to protect the sintered metalmotor drive gears, B&D was able to design a cost-efficientproduct that is able to withstand the shock loads of abnor-mal motor stalls, such as when the cutting blades hit ametal fence wire or a branch.

Thering (4010mm)was designedtoslipthe first timebetween 80-90 Nm and then between 50-70 Nm over 30

shock-stalls of the trimmer mechanism.For a ring 40 mm in diameter and 10 mm in width, we

select a pitch of 5 mm and an initial wave height of 1 mm.This gives us 25 complete waves around the circumferenceof the ring. We then select the material thickness to pro-duce the required torque performance.

From thecalculations, thespringconstant for thering isgiven by

K E wt

p

48

3

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TABLE 3. EXAMPLE SLIP TORQUE APPLICATIONS

Application Specification #Cycles Ring Size (mm)

Steering column anti-theft lock(standard ring)

150 Nm 50 Nm5reversing

SV20 30 CS typ.

Steering column anti-theft lock(modified wave ring)

150 Nm 20 Nm5reversing

SV20 30 CS typ.

Hedge Trimmer motor drive gear over-load protection

70 Nm 20 Nm 30 HV40 10 SS

EPAS 100 Nm 20 Nm 1,000 SV25 9

EPAS 70 Nm 10 Nm 10,000 SV24 10

EPAS 150 Nm 20 Nm 1,000 SV26 9

EPAS 3 Nm 2 Nm 1,000 SV36 12

Drive gear overload protection 12 Nm 2 Nm 5 SV14 3

Seat height adjustment mechanism 17 Nm 3 Nm 1,000 SV36 5

Seat height adjustment mechanism 0.55 Nm 0.15 Nm 60,000 SV10 5

EPAS 3 Nm 1 Nm 1,000 SV21 9.9

Windscreen wipermotor overload

1 Nm 10 Nm 200,000 HV10 5

Extended Slip Performance

0

1

2

3

4

5

6

0 200 400 600 800 1,000

Slip Cycle

TorqueNm

Torque performance over 1,000 cycles.

10

Slip Torques with Standard Ring

50

75

100

125

150

175

200

225

250

T1 T2 T3 T4 T5

Rotation Number

Torque(Nm)

Standard waveform results.

11

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Sowe haveKfor 0.3,0.4, and0.5mmthick stainlesssteel

48 1875 1003

51944

48 1875 100 4

5

3

. ..

.

. ..

=

=

=

3

3

4608

48 1875 1005

59000

.

. ..

. .

From work with similar rings,15%compression wouldbeexpectedtobe the start oftheplastic zoneof Fig.6, sowenext calculate the radial load using

F n c K R

= .

So, for our three material thicknesses, we have:

25 1 15 1944 729

25 1 15 4 608 17 28

25 1 15 9

=

=

% . .

% . .

% .000 3375= . .

This will give a predicted torque performance from

T Fd

R=

2.

Using a coefficient of friction of 0.16, and for our threematerial thicknesses of 0.3, 0.4, and 0.5 mm, we get

729 01640

2233

1728 016 402

553

3375 01640

. . .

. . .

. .

=

=

2

1080= . .

So we will select the 0.4 mm material.Because we are using the plastic region of the graph in

Fig. 6, as long as we maintain the compression at a mini-mum of15%,any increase incompression will only cause avery small rise in torque.

In the final design, component dimensions and toler-anceswere chosento give a 16%compressionof thewaves.

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Slip Torques with Modified Wave Ring

50

75100

125

150

175

200

225

250

T1 T2 T3 T4 T5

Rotation Number

Torque(Nm)

Modified waveform.

12

50

75

100

125

150

175

200

225

250

Rotation Number

Torque

(Nm)

Slip Torques with Shell Albida Lubricant

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10

Standard wave, standard lubrication.

13

50

75

100

125

150

175

200

225

250

Rotation Number

Torque(Nm)

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10

Slip Torques with RENC03 Lubricant

Standard wave, dry-grease lubrication.

14

Rotation Number

Torque(Nm)

Slip Torques Black & Decker

T1 T3 T5 T8 T10 T12 T14 T16 T18 T20

100

75

50

25

B&D actual results, 0.4 mm material.

15

Section of B&D drive gear assembly.

16

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We see, from Fig. 15, that the actual torque resultsshowthe first slip cycle at 90 Nm is higher than for subse-quent slips at 60 Nm. This is due to the surface polishingand lubricant distribution that occurs during the firstslips.

The correlation between the theoretical torque and theactual results were excellent forthis application.Due to thereasons outlined earlier, however, it is essential that design

verification work is undertaken with production-intentcomponents to establish the effects of actual materials andsurface finishes, etc.

Careful design of the two-piece gear incorporating thetolerance ring enabled a locating groove for the tolerancering to be accommodated with a simpleoperation. This al-lows high radial loads to be accommodated withoutovercompressing the ring by transmitting forces throughthegrooveshouldersonce thewave hasbeen compressed toa certain point (Figs. 16 and 17).

Automotive Steering ColumnAntitheft ProtectionMillions of tolerance rings have been fitted to vehicles

over the last 17 years to protect the steering column lockmechanismfromdamageduring attempted theft [1].TheU.K. Home Office [the U.K. equivalent to the U.S. Na-tional Highway Traffic Safety Administration (NHTSA)which tracks vehicle thefts] rated all those vehicles fittedwith tolerance rings as low risk in terms of thefts per 100vehicles [2].

Thetolerancering is fittedbetweenthesteeringcolumnshaft and a locking collar into which the lock mechanismengages (Figs. 18 and 19). During an attempted theft, thering allows the lock collar to rotate on the shaft and pro-tects the locking pin from being sheared off.

The particular design challenges in these applicationsrevolve around the use of lower grade steels and wide man-

ufacturing tolerances for the mating components, whilemaintaining torque performance in line with legislation.The legislation calls for a minimum torque of 100 Nm af-ter five slip cycles, and a typical maximum torque of 220Nm is chosen to protect the steering column itself, to-gether with its associated mounting components.

The modified wave ring design allows the relaxing ofmating-component tolerances further and can obviate therequirement for a prebreak, an initial torque slip cycleper-formed during the assembly process, thus reducing manu-facturing times and simplifying production equipment.

The requirements for higher torques are achieved inthese applications by using hardened steel rings, typicallyof thicker material (0.5-0.7 mm) and utilizing dual bands

of waves around the ring to increase the achievable torquewithin a given space.The hardening process does not affect the spring con-

stant itself, but produces a higher slip torque by shiftingthe elastic-plastic transition of Fig. 6 with higher hard-ness, allowing higher compression (and, hence, sliptorque) before plastic deformation occurs, and the torqueis stabilized.

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B&D gear assembly.

17

Steering column lock assembly.

18

EPAS gear assembly.

19

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Automotive Electric Power-AssistedSteering Gear ProtectionWith thecontinualdrive to improvevehicleefficiencyandre-duce weight, automotive designers are replacing traditionalhydraulic power-steering systems with electrically poweredunits. These systems only consume power when turning thewheels and do not represent a drain in the idle state.

Electric power-assisted steering (EPAS) drive trains

typically use sintered metal technology for gears. Thesecomponents, while offering significant benefits in terms ofcost, are prone to damage from shock loading when, for ex-ample, the wheel hits a curb during parking maneuvers.

To protect the gears, a tolerance ring is used betweenthe gear and shaft, designed to slip during a shock load.

SummaryTolerance rings have been used for many years for fixingcomponents andmounting bearings in theautomotive andappliance industries.

Thedevelopments outlined in thisarticle haveextendedtheir application to protecting mechanical systems, typi-cally electric-motor drive trains, from potential damageduring shock overload conditions.

References

[1] Theapplicationof tolerance ringsin anti-theft steeringcolumnas-

semblies, SAE International, Paper 940868, 1994.

[2]Car theft in England andWales: TheHomeOfficecar theft index,

Home Office Police Dept., Paper 33, 1990.

[3] D. Tabor and F.P. Bowden, The Friction and Lubrication of Solids,

vol. 1. London, UK: Clarendon Press, 1950.

J.W. Smith (jim.smith@rencol.co.uk) is with Rencol ToleranceRings in Bristol,England.This article first appeared in its origi-nal format at the 2001 51st Annual International ApplianceTechnical Conference.

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