Snap Fit Design

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Snap-FitDesign Manual

Topic Part

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction

Snap-Fit Design Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

Types of Snap-Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II

Snap-Fit Beam Design Using Classical Beam Theory . . . . . . . . . . . . . III

Improved Cantilever Snap-Fit Design . . . . . . . . . . . . . . . . . . . . . . . . . . IV

U & L Shaped Snaps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

General Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI

English/Metric Conversion Chart . . . . . . . . . . . . . . . . . . . . . . Inside Back Cover

Table of Contents

Introduction

About AlliedSignalFounded in 1920 as Allied Chemical Dye, todayAlliedSignal Inc.,manufactures an enormous variety of products. Its sales exceed 14 billion dollars annually.This Fortune 30 company is a major force in threeindustry sectors: Aerospace, Automotive andEngineered Materials.

As one of the operating units of the Polymers Business,AlliedSignal Plastics produces more than 500 productsfor over 1,000 customers worldwide.

AlliedSignal Plastics is one of the world’s largestproducers of Nylon 6 resins. The company’s Capron®,Petra® and Nypel® resins are already part of your life–in the fibers of your carpeting, the tires and othercomponents of your car, the switches that light yourhouse, the plastic wrapping on your cheeses and meats,and a huge range of other products. In other words,AlliedSignal resins can be found in plastic productswhere mechanical strength and temperature, impact and chemical resistance are important.

These resins from AlliedSignal,coupled with thecompany’s concept-through-commercializationexpertise,can combine to help make possible the mostefficient,cost-effective snap-fit for your product. Ourtechnical support is ready to help you with all yourneeds. And for more information,you can always visitour web site at http://www.asplastics.com.

Snap-Fit Design

This manual will guide you through

the basics of snap-fit design,

including: types of snap-fit designs

and their applications; how to

calculate the strength of the unit and

amount of force needed for assembly;

and the three common causes of

failure in snap-fits and how to

overcome them.

Snap-Fit Design ApplicationsWhy use snap-fits? This chapter will give you athumbnail sketch of the benefits of snap-fits and thematerials used to make them.

Snap-fits are the simplest,quickest and most cost-effective method of assembling two parts. Whendesigned properly,parts with snap-fits can be assembledand disassembled numerous times without any adverseeffect on the assembly. Snap-fits are also the mostenvironmentally friendly form of assembly because of their ease of disassembly,making components ofdifferent materials easy to recycle.

Although snap-fits can be designed with many materials,the ideal material is thermoplastic because of its highflexibility and its ability to be easily and inexpensivelymolded into complex geometries. Other advantagesinclude its relatively high elongation, low coefficient offriction,and sufficient strength and rigidity to meet therequirements of most applications.

The designer should be aware that the assembly mayhave some “play”due to tolerance stack-up of the twomating parts. Some snap-fits can also increase the cost of an injection molding tool due to the need for slides inthe mold. An experienced designer can often eliminatethe need for slides by adding a slot in the wall directlybelow the undercut or by placing the snaps on the edgeof the part, so they face outward (see Figure I-1).

Part I

REQUIRES SLIDE IN MOLD

UNDERCUT

NO SLIDE REQUIRED

SLOT

NO SLIDE REQUIRED,MOLD LESS COMPLEX

Figure I-1

I-1

S n a p - F i t D e s i g n A p p l i c a t i o n s

I-2

Concluding points: Snap-fits solve the problem ofcreating an inexpensive component that can be quicklyand easily joined with another piece. Thermoplastics are the ideal material for snap-fits because they have theflexibility and resilience necessary to allow for numerousassembly and disassembly operations.

Part I: Snap-Fit Design Applications

Door handle bezel

Backside of bezel Detail of backside of bezel, cantilever design

II-1

Types of Snap-FitsThis chapter provides an overview of the different types ofcantilever snap-fits and gives an idea of when they are used.

Most engineering material applications with snap-fits usethe cantilever design (see Figure II-1) and thus this manualwill focus on this design. The cylindrical design can beemployed when an unfilled thermoplastic material withhigher elongation will be used (a typical application is anaspirin bottle/cap assembly).

Part II

����������Y��CANTILEVER

“U” SHAPED CANTILEVER

“L” SHAPED CANTILEVER���� ��Figure II-1

When designing a cantilever snap, it is not unusual forthe designer to go through several iterations (changinglength, thickness,deflection dimensions,etc.) to design asnap-fit with a lower allowable strain for a given material.

Other types of snap-fits which can be used are the “U”or “L”shaped cantilever snaps (see Part V for more detail).These are used when the strain of the straight cantileversnap can not be designed below the allowable strain forthe given material.

Concluding points: Most applications can employ acantilever type snap-fit in the design. In applicationswith tight packaging requirements, the “U”or “L”shapedsnap may be required.

Automotive oil filter snaps

Cordless screw driver housing, cantilever snap-fit

III-1

�������� OVERHANG DEPTH

ENTRANCE SIDE

RETRACTION SIDE

A design engineer’s job is to find a balance betweenintegrity of the assembly and strength of the cantileverbeam. While a cantilever beam with a deep overhang canmake the unit secure, it also puts more strain on thebeam during assembly and disassembly. This chapterexplains how this balance is achieved.

A typical snap-fit assembly consists of a cantilever beamwith an overhang at the end of the beam (see Figure III-1).The depth of the overhang defines the amount ofdeflection during assembly.

Part III

Friction Coefficient µ = tan b

Mating Force = W

W = P tan(a + b)

µ + tan aW = P ————————

1–µ tan a

Figure III-2

Figure III-1

The overhang typically has a gentle ramp on the entranceside and a sharper angle on the retraction side. The smallangle at the entrance side (a) (see Figure III-2) helps toreduce the assembly effort,while the sharp angle at theretraction side (a') makes disassembly very difficult orimpossible depending on the intended function.Both theassembly and disassembly force can be optimized bymodifying the angles mentioned above.

The main design consideration of a snap-fit is integrity ofthe assembly and strength of the beam. The integrity ofthe assembly is controlled by the stiffness (k) of thebeam and the amount of deflection required for assemblyor disassembly. Rigidity can be increased either by usinga higher modulus material (E) or by increasing the crosssectional moment of inertia (I) of the beam. The productof these two parameters (EI) will determine the totalrigidity of a given beam length.

����a' a

R

W

P

W

P

RFRICTION CONE

a

a + b

}

b

MATING FORCE

Snap-Fit Beam Design Using Classical Beam Theory

S n a p - F i t B e a m D e s i g n U s i n g C l a s s i c a l B e a m T h e o r y

III-2

The integrity of the assembly can also be improved byincreasing the overhang depth. As a result, the beam hasto deflect further and therefore requires a greater effortto clear the overhang from the interlocking hook.However,as the beam deflection increases, the beamstress also increases. This will result in a failure if thebeam stress is above the yield strength of the material.

Thus the deflection must be optimized with respect tothe yield strength or strain of the material. This isachieved by optimizing the beam section geometry toensure that the desired deflection can be reached withoutexceeding the strength or strain limit of the material.

The assembly and disassembly force will increase withboth stiffness (k) and maximum deflection of the beam(Y). The force (P) required to deflect the beam isproportional to the product of the two factors:

P= kY

The stiffness value (k) depends on beam geometry asshown in Figure III-3.

Stress or strain induced by the deflection (Y) is alsoshown in Figure III-3. The calculated stress or strainvalue should be less than the yield strength or the yieldstrain of the material in order to prevent failure.

When selecting the flexural modulus of elasticity (E) for hygroscopic materials, i.e.nylon,care should betaken. In the dry as molded state (DAM), the datasheetvalue may be used to calculate stiffness,deflection orretention force of snap design. Under normal 50%relative humidity conditions however, the physicalproperties decrease and therefore the stiffness andretention force reduce while the deflection increases.Both scenarios should be checked.

Where:E = Flexural ModulusP = ForceY= Deflectionb = Width of Beam

Figure III-3

Part III: Snap-Fit Beam Design Using Classical Beam Theory

b

b

tP

L

L

�"

t2

P

t�L

t

b4

P

��

I) Uniform Cross Section, Fixed End to Free End

Stiffness:

Strain:

II) Uniform Width, Height Tapers to t/2 at Free End

Stiffness:

Strain:

III) Uniform Height,Width Tapers to b/4 at Free End

Stiffness:

Strain:

k = PY

Eb4

tL

= ( ) 3

e = Y 1.50 tL2( )

k = PY

Eb6.528

tL

= ( ) 3

e =

b

0.92 Y tL2( )

k = PY

Eb5.136

tL

= ( ) 3

e = 1.17 Y tL2( )

Cantilever Beam: Deflection-Strain Formulas

S n a p - F i t B e a m D e s i g n U s i n g C l a s s i c a l B e a m T h e o r y

III-3

Concluding points: In a typical snap-fit, the strength of a beam is dependent on its geometry and maximumdeflection during assembly. The force to assemble anddisassemble snap-fit assemblies is highly dependent onthe overhang entrance and retraction angles.

Close-up of automotive fuse box snap

Close-up of automotive fuse box, full view

Close-up of automotive fuse box, snap on sides of box

IV-1

The cantilever beam formulas used in conventional snap-fit design underestimate the amount of strain at thebeam/wall interface because they do not include thedeformation in the wall itself. Instead, they assume thewall to be completely rigid with the deflection occurringonly in the beam. This assumption may be valid whenthe ratio of beam length to thickness is greater thanabout 10:1. However, to obtain a more accurateprediction of total allowable deflection and strain forshort beams,a magnification factor should be applied to the conventional formula. This will enable greaterflexibility in the design while taking full advantage of the strain-carrying capability of the material.

AlliedSignal Plastics has developed a method for esti-mating these deflection magnification factors for various snap-fit beam/wall configurations as shown in Figure IV-1. The results of this technique,which have been verified both by finite element analysis andactual part testing1,are shown graphically in Figure IV-1.Figure IV-2 shows similar results for beams of tapered cross-section (beam thickness decreasing by 1/2 at the tip).

Snap-Fit Design Examples 1 & 2 illustrate this procedurefor designing snap-fits, including calculating themaximum strain developed during assembly andpredicting the snap-in force required.

1 Chul S.Lee, Alan Dubin and Elmer D. Jones,“Short Cantilever BeamDeflection Analysis Applied to Thermoplastic Snap-Fit Design,”1987 SPEANTEC,held in Los Angeles,California,U.S.A.

Part IV

Improved Cantilever Snap-Fit Design

IV-2

DE

FL

EC

TIO

N M

AG

NIF

ICA

TIO

N F

AC

TO

R Q

ASPECT RATIO, L/t

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

ON A BLOCK(SOLID WALL)

1

ON A PLATE(OR THIN WALL)

2 4

5

3

I m p r o v e d C a n t i l e v e r S n a p - F i t D e s i g n

Part IV: Improved Cantilever Snap-Fit Design

Uniform Beam, Q FactorFigure IV-1

IV-3

I m p r o v e d C a n t i l e v e r S n a p - F i t D e s i g n

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

2T

5T

DE

FL

EC

TIO

N M

AG

NIF

ICA

TIO

N F

AC

TO

R Q

ASPECT RATIO, L/t

2T

5T

��t/2

t

Tapered Beam, Q FactorFigure IV-2

IV-4

I m p r o v e d C a n t i l e v e r S n a p - F i t D e s i g n

MATERIAL UNFILLED 30% GLASSPEI 9.8%(2)

PC 4%(1) - 9.2%(2)

Acetal 1.5%(1)

Nylon 6(4) 8%(5) 2.1%(1)

PBT 8.8%(2)

PC/PET 5.8%(2)

ABS 6% - 7%(3)

PET 1.5%(1)

MATERIAL µPEI 0.20 - 0.25PC 0.25 - 0.30Acetal 0.20 - 0.35Nylon 6 0.17 - 0.26PBT 0.35 - 0.40PC/PET 0.40 - 0.50ABS 0.50 - 0.60PET 0.18 - 0.25

Table IV-I

NOTES:(1) 70% of tensile yield strain value(2) G.G.Trantina. Plastics Engineering.

August 1989.(3) V.H.Trumbull. 1984 ASME Winter Annual Conference.(4) DAM - “Dry As Molded”condition(5) AlliedSignal test lab

Table IV-II

NOTES:(1) Material tested against itself

Coefficient of Friction(1)

Allowable Strain Value, eo

Figure IV-3

MAXIMUM STRAIN

tY e = 1.5 ———- L2 Q

MATING FORCE

µ + tan aW = P ———————1–µ tan a

bt2 EeoP = ———————6LQ

Where:W = Push-on Force

W’ = Pull-off ForceP = Perpendicular Forceµ = Coefficient of Frictiona = Lead Angle

a’ = Return Angleb = Beam Widtht = Beam ThicknessE = Flexural Moduluse = Strain at BaseL = Beam LengthQ = Deflection Magnification Factor

(refer to Figure IV-2 for proper Q values)

Y = Deflection

��t

Y

b

L

a

PW

Improved Formulas

Hub cap with cantilever snaps

I m p r o v e d C a n t i l e v e r S n a p - F i t D e s i g n

DETERMINE:

A) THE MAXIMUM DEFLECTION OF SNAPB) THE MATING FORCE

SOLUTION:

A) THE MAXIMUM ALLOWABLE DEFLECTION OF SNAP

tYmax eo L2 Qeo = 1.5 ———- Þ Ymax = ————L2 Q 1.5 t

L— = 5.0 Þ Q = 2.0 (from Q Factor Graph)t

(0.015)(0.5)2 (2.0)Ymax = ——————————= 0.050 in

(1.5)(0.1)

Therefore,in an actual design, a smaller value for deflection(Y) would be chosen for an added factor of safety.

B) THE MATING FORCE

bt2 EeoP = ——————6LQ

(0.25)(0.1)2 (1.3)(106) (0.015)P = —————————————————— = 8.1 lb

6(0.5)(2.0)

µ + tan aW = P ———————1–µ tan a

0.2 + tan30ºW = 8.1 ————————— = 7.1 lb

1 – 0.2 (tan30º)

Therefore, it will take 7.1 lb mating force to assemble parts.

(From Q Factor Graph,Figure IV-1)

IV-5

DETERMINE:

IS THIS TYPE OF SNAP-FIT ACCEPTABLE FOR USE INNYLON 6 (CAPRON® 8200 NYLON)

SOLUTION:

tY e = 1.5 ———- L2 Q

L— = 3.57 Þ Q = 2.7 t

(0.063)(0.090)eo = 1.5 ————————— = 6.2%(0.225)2(2.7)

Therefore, it is acceptable for unfilled Nylon 6 (See Allowable Strain Value,Table IV-1).

Concluding points: Unlike conventional formulas,AlliedSignal includes the deflection magnification factorin all calculations. The examples show how to calculatethe maximum strain during assembly and how to predictthe force needed for assembly.

Snap-Fit Design Example #1

GIVEN:

Material Þ Petra 130(PET)

t = 0.10 inL = 0.50 inb = 0.25 inE = 1.3 (106) psiµ = 0.2 (From Table

IV-II,Coefficientof Friction)

a = 30.0°eo = 1.5% (From Table

IV-I, AllowableStrain Value)

Figure IV-4

��t

Y

b

La

PW

Snap-Fit Design Example #2

GIVEN:

Material Þ UnfilledNylon 6

t = 0.063 inY = 0.090 inL = 0.225 inb = 0.242 in

Figure IV-5

tb

L

Y

��P

Close-up of automotive hub cap snaps

V-1

Part V

The cantilever beam snap-fit design isn’t appropriate for all applications. This chapter defines “L”and “U”shaped snaps and tells when they are used.

Occasionally a designer will not be able to design acantilever snap-fit configuration with a strain below theallowable limit of the intended material. This is usuallydue to limited packaging space which can restrict thelength of the snap. This is the ideal time to considerusing either an “L”shaped snap or “U”shaped snap.

The “L”shaped snap (see Figure V-1) is formed bydesigning in slots in the base wall which effectivelyincreases the beam length and flexibility compared to a standard cantilever beam. This allows the designer toreduce the strain during assembly below the allowablelimit of the selected material. It should be noted thatadding a slot to the base wall may not be acceptable insome designs for cosmetic or air flow concerns.

The “U”shaped snap (see Figure V-2) is another way toincrease the effective beam length within a limited spaceenvelope. With this design,even materials with lowallowable strain limits (such as highly glass-filledmaterials) can be designed to meet assembly require-ments. The “U”shaped design usually incorporates theundercut on the outer edge of the part to eliminate theneed for slide in the mold,unless a slot is acceptable inthe wall from which the snap projects.

Figure V-1

Figure V-2

U & L Shaped Snaps

“L” SHAPED CANTILEVER���� �“U” SHAPED CANTILEVER�

U & L S h a p e d S n a p s

“L” Shaped Snap-Fit ExampleA) Calculate the minimum length (L2) of the slot (seesketch, Figure V-3) in the main wall for Capron® 8233nylon in the configuration below. The requireddeflection is .38 inches.

B) Calculate the required force (P) to deflect the snap .38 inches.

GIVEN:

e8233 = .03 (with moisture)t = .1 inL1 = .5 inR = .12 inI = Moment of Inertia (rectangle)

I = 12 = 12 = 8.333(10-5)E = 1.3 (106)b = 1.0 inY = .38

6/e Yt(L1+ R) - 4L13 - 3R(2pL1

2 +pR2 + 8L1R)A) L2= —–––––——————————————————————

12(L1 +R)2

6/.03(.38)(.1)(.62) - 4(.5)3 - .36[.5p+.122p+ 4(.12)]= ——————————————————————————––

12(.62)2

L2 = .750 in.

B) Y= 12EI

[4L13+3R(2pL1

2 +pR2 + 8L1R) + 12L2(L1 + R)2]

.38 =(12)(1.3)(106)(8.333)(10-5)

[4(.5)3+(.36)[.5p+

.122p + 8(.5).12]+ 12(.75)(.62)2]

.38 = 1.3(103)

(4.714)

P = 104.8 lbs.

bt3 1(.1)3

P

P

P

Part V: U & L Shaped Snaps

V-2

“L” SHAPED SNAP–FIT

Figure V-3

6/eoYt(L1+ R) - 4L13 - 3R(2pL1

2 + pR2 + 8L1R)L2 = ———————————————— ----------–––——–—————

12(L1 +R)2

or,

Y= 12EI

[4L13+3R(2pL1

2 +pR2 + 8L1R) + 12L2(L1 + R)2]

Where:L2 = Length of slot as shown in sketcheo = Allowable strain of materialY = Maximum deflection required in

direction of forcet = Thickness

L1 = Length as shown in sketchR = Radius as shown in sketch

(at neutral axis)P = Forceb = Beam widthE = Flexural modulusI = Moment of inertia

P

� �P

L1

R

L2

A A b

SectionA-A

t �

V-3

“U” Shaped SnapExample #1

Case 1

A) Calculate the amount of deflection at the tip of the beam for a 1.0 pound load

GIVEN:

P= 1.0 lb.I = 0.833 x 10-4 in4 = bt3/12 (rectangular cross-section)E = 534,000 psiR= 0.15 inL1= 1.4 inL2= 0.973 int = 0.1 inb = 1.0 in

A) Y = 18EI

[ 6L13 + 9R{L1(2pL1 + 8R) + pR2} + 6L2(3L1

2 - 3L1L2 + L22)]

Y = 18(534,000)(0.833 x 10-4)

[6(1.4)3 +9(0.15){(1.4)

(2p•1.4 + 8 • 0.15) + p(0.15)2} + 6 (0.973){3(1.4)2 - 3(1.4)(0.973) + (0.973)2}]

= 0.064 in

U & L S h a p e d S n a p s

1

"#R

PL1

L3

L2

A A� b

SectionA-A

t��R L1

L2

P

P

“U” Shaped Snap–Fit

Case 1

Y = 9(L1 + R)t

[6L13 + 9R {L1(2pL1 + 8R) + pR2}+

6L2 (3L12 - 3L1L2 +L2

2)]

or,

Y = 18EI

[6L13 + 9R {L1(2pL1 + 8R) + pR2}+

6L2 (3L12 - 3L1L2 +L2

2)]

e

eo

P

Case 2

Y = 3(L1 + R)t

[4L13 + 2L3

3 +3R {L1(2pL1 + 8R) + pR2}]

or,

Y = 6EI

[4L13 + 2L3

3 +3R {L1(2pL1 + 8R) + pR2}]

Where:

Variables defined on previous page.

P

�� R

L1L2

P

Concluding points: Snap-fits can use either the “L”or “U”shaped design to overcome space limitations. Both the“L”and “U”shaped snaps effectively reduce strain duringassembly, thus making it ideal for materials with lowerallowable strain limits.

U & L S h a p e d S n a p s

V-4

“U” Shaped SnapExample #2

Case 2

A) Calculate the amount of deflection at the tip of the beam for a 1.0 pound load

GIVEN:

I = 0.833 x 10-4 in4

E = 534,000 psiR = 0.15 inL1 = 0.7 inL1 = L2

L3 = 0.273 int = 0.1 in

Y = 6EI

[4L13 + 2L3

3 + 3R {L1(2pL1 + 8R) + pR2}]

= 6(534,000)(0.833 x 10-4)

[4(0.7)3 + 2(0.273)3 +

3(0.15){0.7(2p • 0.7 + 8(0.15)) + p (0.15)2}]

= 0.012 in

P

1

�� R

PL1

L3

L2

Automotive wheel cover

Close-up of above cover backside featuring the “L” shaped snap-fit design(from a top angle)

Inset shot of a “U” shaped snap-fit design

VI-1

between the parts, relaxation at the joint can result inloss of seal pressure, resulting in leakage of the containedfluid. Another problem often seen is excessive playbetween the parts due to tolerance variations, sometimesresulting in noise and vibration. Several ways tominimize these phenomena include:designing a lowstress snap beam,designing the snap-fit to incorporate a90° return angle so that it relaxes in tension versusbending (see Figure VI-2). This will prevent the matingpart from slipping past or becoming loose. Another wayis to use a large return angle and increase the land lengthin the return angle area (see Figure VI-3). Increasing theoverhang depth and evaluating the worst case scenarioin a tolerance study will allow the design to retain givenpull-off force even after relaxation occurs.

Figure VI-2

Figure VI-3

Part VI

Three basic issues should be reviewed before finalizing a snap-fit design: stress concentration,creep/relaxation,and fatigue. Below are descriptions of these problemsand suggestions to prevent them. All should beconsidered as part of good design practice for anythermoplastic design.

The single most common cause of failure in snap-fits isstress concentration due to a sharp corner between thesnap-fit beam and the wall to which it is attached. Sincethis location normally coincides with the point ofmaximum stress,a sharp corner can increase the stressbeyond the strength of the material,causing pointyielding or breakage. This is more critical for rigidplastics like glass-reinforced nylon,which have relativelylow ultimate elongation. More ductile materials, likeunreinforced nylon, tend to yield and deform before theybreak, redistributing the peak stress over a broaderregion. One solution is to incorporate a fillet radius atthe juncture between the beam and the wall (see FigureVI-1), so that the ratio of radius to wall thickness (R/t) isat least 50%. Going beyond 50% results in a marginalincrease in strength and may cause other problems likeinternal voids and sink marks. If sink marks are an issue,a smaller radius can be used,but it may increase thestress in this area. Another option is to add the radiusonly on the tensile side of the beam.

Figure VI-1

Creep,or more accurately stress relaxation,can result in areduction of the holding force between the twocomponents connected by the snap-fit. Stress relaxationwill occur gradually over time. If there is a gasket or seal

General Design Guidelines

SHARPCORNER��R =

.5t MINIMUM

t��������POOR DESIGN GOOD DESIGN

��������RELAXED POSITION(EXAGGERATED)

P = MATING PART FORCE

UNDEFORMEDPOSITION

UNDEFORMEDPOSITION

PP

RELAXATION IN TENSION RELAXATION IN BENDING

RETURN ANGLE

LAND LENGTH

��� ����OVERHANG DEPTH

VI-2

G e n e r a l D e s i g n G u i d e l i n e s

Fatigue,or repetitive loading, is the third major cause of failure. Fatigue concerns primarily apply if hundredsor thousands of cycles are anticipated. While the designstress level might be well within the strength of thematerial, the repeated application of this stress can result in fatigue failure at some point in the future.Some polymers perform better than others in this regard,making them ideal candidates for snap-fits or livinghinges that must flex repeatedly. The first way to avoid afatigue failure is to choose a material known to performwell in fatigue. This can be done by comparing the so-called S-N curves of the materials,which show theexpected number of cycles to failure at various stresslevels and at different temperatures of exposure. Thesecond way,still using the S-N curves, is to choose adesign stress level,at the correct temperature, that resultsin the required number of load applications prior tofailure. This method will usually be conservative since S-N curves are typically generated at much higherfrequencies than would be anticipated for repeatedapplication of a snap-fit assembly.

For hygroscopic materials like nylon, the effects ofmoisture on final part dimensions and mechanicalproperties also must be considered. For furtherinformation,please consult the AlliedSignal PlasticsDesign Solutions Guide.

Concluding points: There are a number of ways toovercome the issues of stress concentration,stressrelaxation,and fatigue. A well thought out design andusing the right polymer for a given application willminimize these issues. This allows the application tobenefit from all the advantages of a snap-fit design.

Part VI: General Design Guidelines

Circular saw handle inset shot featuring snap-fit closure and mating part

Close-up of truck mirror patch cover

Close-up of automotive fuel rail cover, snap-fit design

Aerator

English/Metric Conversion ChartTo Convert To MultiplyEnglish System Metric System English Value by. . .

DISTANCEinches millimeters 25.38feet meters 0.30478

MASSounce (avdp) gram 28.3495pound gram 453.5925pound kilogram 0.4536U.S.ton metric ton 0.9072

VOLUMEinch3 centimeter3 16.3871inch3 liter 0.016387fluid ounce centimeter3 29.5735quart (liquid) decimeter3 (liter) 0.9464gallon (U.S.) decimeter3 (liter) 3.7854

TEMPERATUREdegree F degree C (°F–32) / 1.8 = °C

PRESSUREpsi bar 0.0689psi kPa 6.8948ksi MN/m2 6.8948psi MPa 0.00689

ENERGY AND POWERin lbf Joules 0.113ft lbf Joules 1.3558kW metric horsepower 1.3596U.S.horsepower Kw 0.7457Btu Joules 1055.1BTU • in / (hr • ft2•ºF) W/m • °K 0.1442

VISCOSITYpoise Pa • s 0.1

BENDING MOMENTOR TORQUEft lb N • m 1.356

DENSITYlb/in3 g/cm3 27.68lb/ft3 kg/m3 16.0185

NOTCHED IZODft lb/in J/m 53.4

© AlliedSignal Inc. 1998 Printed in USA 5350/012/0498/GEN070

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