卡勾设计资料

卡勾设计指导

TICONA

SNAP-FITS FOR

ASSEMBLY AND DISASSEMBLY

Presented by:

Tim Spahr

Technical Service Engineer

November 1991

(Revised 1/00)TiconaA business of Celanese AG

卡勾设计指导

GENERAL DISCUSSION

An economical and quick method of joining plastic parts is by a snap-fit joint. A snap-fit joint can be designedso it is easily separated or so that it is inseparable, without breaking one of its components. The strength of thesnap-fit joint depends on the material used, its geometry and the forces acting on the joint.

Most all snap-fit joint designs share the common design features of a protruding ledge and a snap foot.Whether the snap joint is a cantilever or a cylindrical fit, they both function similarly.

When snap-fit joints are being designed, it is important to know the mechanical stresses to be applied to thesnap beams after assembly, the required mechanical stresses or strains on the snap beams during assembly,the number of times the snap joint will be engaged and disengaged, and the mechanical limits of the material(s)to be used in the design.· Reduces assembly costs.

· Are typically designed for ease of assembly and are often easily automated.

·Replaces screws, nuts, and washers.

·Are molded as an integral component of the plastic part.

· No welding or adhesives are required.

· They can be engaged and disengaged.· Some designs require higher tooling cost.

· They are susceptible to breakage due to mishandling and abuse prior to assembly.

· Snap-fits that are assembled under stress will creep.

· It is difficult to design snap-fits with hermetic seals. If the beam and/or ledge relaxes, it could decrease

the effectiveness of the seal.

TYPES OF SNAP-FIT JOINTS

There are a wide range of snap-fit joint designs. In their basic form, the most often used are the cantileverbeam (snap leg), Figure 1, and the cylindrical snap-fit joint, Figure 2. For this reason, these two designs anddesigns derived from these basics are covered in this text.

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卡勾设计指导

Using the standard beam equations, we can calculate the

stress and strain during assembly of the snap beam. If

we stay below the elastic limit of the material, we know

the flexing beam will return to its original position.

However, for such designs, there is usually not enough

holding power with the low forces or small deflections

involved.

Therefore, much higher deformations are generally used.

With most plastic materials, the bending stress calculated

by using simple linear bending methods (Equations 1-3)

can far exceed the recognized yield strength of the

material. This is particularly true when large deflections

are used and when the assembly occurs rapidly.Therefore, it often appears as if the beam momentarily

passes through the maximum deflection or strain, greatly

exceeding its yield strength while showing no ill effects

from the event.

What actually happens is described later.For the present, simply note that the snap beams are

usually designed to a stain rather than a stress.

The strain should not exceed the allowable dynamic

strain for the particular material being used. By

combining Equations 1-3, the design equation (Equation

4) can be produced. Note that the strain is written in

terms of the height, length, and deflection of the beam.

2Figure 3σ=FLZEquation 1=FL3Y3EIEquation 2σ=EεEquation 3σ = Maximum stress on beamF = Force on the beamL = Length of the beamZ = Ic, Section Modulusc = d2 = Half the beam heightI = bd312, Moment of inertiah = Beam heightb = Beam widthε = Maximum strain on beamY = Beam deflectionε=3YH2L2Equation 4

卡勾设计指导

Strain Guidelines

Generally speaking, an unfilled material can withstand a strain level of around 6% and a filled material ofaround 1.5%. As a reference, a 6% strain level could be a beam with a thickness that is equal to 20% of itslength (a 5: 1 L/ho) and a deflection that is also equal to 20% of its length (see Figure 5). A 1.5% strain levelcould be a beam with a thickness that is equal to 10% of its length (10: 1 L/ho) and a deflection that is equal to10% of its length (see Figure 4). If using a beam that is tapered so the thickness at the base of the snap foot is50% that of the base of the beam, the length of the beam will approximately 78% (0.7819346 calculated ) thelength of the 6% and 1.5% beams with uniform thickness.

A more accurate guideline for the allowable dynamic strain curve of the material may be obtained from thematerial's stress strain curve. The allowable dynamic strain, for most thermoplastics materials with a definiteyield point, may be as high as 70% of the yield point strain (see Figure 7). For other materials, that break atlow elongations without yielding, a strain limit as high as 50% of the strain at break may be used (see Figure

6). If the snap joint is required to be engaged and disengaged more than once, the beam should be designedto 60% of the above recommended strain levels. However, the best source for allowable dynamic strain is thematerial supplier.

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卡勾设计指导

Before going further, we need to examine the actual stresses and forces developed in a snap finger. Figure 8shows a stress strain curve for a brittle thermoplastic material. The straight line portion of the curve is theregion where stress is proportional to strain. Line A is drawn tangent to this region

The slope of Line A is generally reported as the modulus of elasticity (Young's modulus or initial modulus) ofthe material. Many plastics do not possess this straight-line region. For these materials, Line A is constructedtangent at the origin to obtain the modulus of elasticity. If we designed a snap beam at 1.5% strain for thismaterial in Figure 8 using Equations 1-4 and a modulus of elasticity of 1.6 x 106 psi (given by the material) asdetermined from Line A, the resulting stress would be 24,000 psi, Point A on Line A. However, from thestress strain curve it can be seen that the true stress at 1.5% strain is about 18,000 psi, Point B on the curve.In addition, the deflection force predicted by Equations 1-4 will be high by the same proportions.

Now, to make our math easier, we need some method to force Equations 1-4 to give us the proper stress andforce results. If we construct a secant line from the origin to Point B, Line B, the slope of Line B is thematerial modulus just as the slope of Line A is the modulus of elasticity. The slope of Line B is the secantmodulus for the material at Point B and is approximately 18,000 psi divided by 1.5% strain or 1.2 x 106 psi.Obviously, the secant modulus can be calculated for any point on the stress-strain curve. Plots of the secantmodulus vs. strain (or stress) can then be produced, if desired. Obviously, at the lower strains, the ScantModulus should approach the modulus of elasticity of the material.

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卡勾设计指导

Radii and Stress

In designing the snap beam, it is very important to

avoid any sharp corners or structural discontinuities,

as stress will concentrate in such areas. To avoid

such problems, inside corners should be designed

with a minimum radius of 0.020 in. and where

necessary to maintain a uniform wall, a radius equal

to the inside radius plus the wall thickness should be

placed on the outside of the corner. As indicated in

Figure 9, an inside radius of 50% of the wall

thickness is considered a good design standard.

Therefore, it is recommended that when possible a

fillet radius equal to ½ the beam thickness be added

to the base of a cantilever beam.Tapered Beams

(same length/stiffer snap)

An improved method of designing cantilever beam

snap-fits is to use a tapered beam. The beam is

tapered from the root to the base of the snap foot.

Stresses in a straight beam concentrate at its base, as

shown in Figure 10. Where as stresses in the tapered

beam are distributed more uniformly through its length,

therefore reducing stress, as shown in Figure 11.

The taper effectively decreases the beam's strain and

allows for a deflection greater than that of a straight

beam with the same base thickness. Another use for a

tapered snap beam is that when the straight beam is

not stiff enough, the base of the beam can sometimes

be increased to create a stiffer tapered beam.

Figure 11

Figure 10

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卡勾设计指导

Formulas for Cantilever Beams

Equations 5-8 can be used to calculate the following properties:

Strain level in a straight beamε=3Yho

2L2Equation 5

Deflection of a straight beam2L2εY=3ho

L=Yho

2εEquation 6Length of a straight snap beam

Thickness of a straight snap beamEquation 72L2εho=3YEquation 8

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卡勾设计指导

To calculate the geometry of a tapered beam, the ratio of the thickness of the beam at the snap foot vs. thebase (hL /ho) must be known. On Table 1, find the hL /ho (Column 1) value and the corresponding K factor(Column 2). K is a geometry factor and is required in all of the formulas related to the tapered beam.Example: A hL /ho of 0.50, equates to a thickness at the snap foot of 50% he base. The K factor for an hL/ho of 0.50 is 1.636.

Table (K Values)h/h0.33

0.34

0.35

0.36

0.37

0.38

0.39

0.400.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

0.49K2.1372.0982.0602.0241.9891.9561.9241.8931.8631.8341.8061.7801.7541.7291.7041.6811.658h/h0.500.510.520.530.540.550.560.570.580.590.600.610.620.630.640.650.66K1.6361.6141.5931.5731.5531.5341.5151.4971.4791.4621.4451.4291.4131.3991.3821.3671.352h/h0.670.680.690.700.710.720.730.740.750.760.770.780.790.800.810.820.83K1.3381.3241.3101.2971.2841.2721.2591.2471.2351.2231.2121.2011.1901.1791.1681.1581.148h/h0.840.850.860.870.880.890.900.910.920.930.940.950.960.970.980.991.00K1.1381.1281.1181.1091.1001.0911.0821.0731.0641.0561.0471.0391.0311.0231.0151.0081.000

Equations 9-12 can be used to calculate the following properties:

Strain level in a straight beamε=3Yho

2L2KEquation 9

Deflection of a straight beambho2ESεFd=×6L

L=3Yho

2KεEquation 10Length of a straight snap beamEquation 11

Thickness of a straight snap beam2L2Kεho=3YEquation 12

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卡勾设计指导

To calculate the deflection force of a straight or

tapered cantilever beam, use Equation 13.bho2ESεFd=×6LEquation 13

The assembly angle along with deflection force and coefficient of friction between the mating parts determinesthe assembly force. The greater the angle and/or coefficient of friction, the higher the assembly force. It maynot be possible to assemble parts with assembly angles 45º and a high coefficient of friction. It isrecommended that assembly angles between 15º and 30º be used.

To calculate the assembly force of a straight or

tapered cantilever beam, use Equation 14.Fa=Fdµ+tanα1 µtanαEquation 14

The retaining force is determined by the angle of the mating surfaces of the snap foot and ledge. To a point,the greater the angle, the greater the holding strength of the snap. This is true only up to the shear strength ofthe snap and the effects of bending moments applied to the beam. It is a common belief that a retaining angleof 90º will prevent the snap joint beam from failing. However, the forces on the snap foot can create abending moment that is high enough to rotate the snap foot back and disengage the snap joint without a shearfailure (beam retention is discussed later). For detachable joints, it is recommended that a retaining anglebetween 30º and 45º be used.

To calculate the retaining force, use Equations 13 and 14 and substitute the retaining angle for the assemblyangle.

When designing snap-fits that require the ability to be engaged and disengaged repeatedly, a safetyfactor is needed to predict the beam performance. Therefore, when designing such snap-fits, replace εin Equations 5 through 13 with 0.6ε.

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卡勾设计指导

The following examples can be used in cases where snap-fits are used on a circular part, such as a boss.

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卡勾设计指导

Snap-fit Retention

It is often thought that if the retaining angles (the interfacing surface between the snap foot and ledge) are 90ºfrom their base and parallel, the snap foot or ledge must shear to fail. However, the forces that are applied tothe snap foot after it is engaged will create a bending moment (Figure 17) on the beam. In some cases, thisbending moment will roll the foot back off of the ledge.

To reduce the risk of this occurring, the following are two recommendations.

The snap loop in Figure 18 is one alternative. Since the force is directly in-line with the snap loop, the failuremodes, after assembly, are shearing of the support ledge or a tensile failure of the snap loop. The roll offfailure, as with the standard cantilever beam, is eliminated.

The length, deflection, thickness, and beam taper, (if required) are all calculated in the same manner as thecantilever beam snap-fit. To reduce stress concentrations, the inside corners should have fillet radii. Toaccommodate the radii on the snap loop, the ledge may have mating radii or clearance for the radii.

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卡勾设计指导

卡勾设计指导

APPENDIX

Example of a Cantilever Snap-Fit CalculationThe material specified has an allowable fiber strain of 1.5% (0.015), and at a 1.5% strain the secant modulusis 1,200,000 psi. The length of the beam has to be 0.50 inches, the width of the beam is 0.200 inches, therequired deflection is 0.030 inches, and the assembly angles are both 30. (Figure 23)

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卡勾设计指导

To increase the strength of this beam and maintain the same deflection and strain level, the beam can betapered (Figure 24).

There are now two unknowns:

1. the thickness at the base

2. the thickness at the snap foot.

These two unknowns are related by the K factors in Table 1. A decision on the base thickness and thicknessratio (hL/ho) is needed. A good starting point is with a base twice as thick as the straight beam with athickness ratio of 50%. For an hL/ho ratio of 0.5, the K factor is 1.636.

The strength of the beam's deflection may be increased by decreasing the hL /ho value or decreased byincreasing the hL /ho value. An hL /ho equal to 1 would be the same as calculating the straight cantileverbeam.

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卡勾设计指导

NOTICE TO USERS: To the best of our knowledge, the information contained in this publication is accurate, however we do not assumeany liability whatsoever for the accuracy and completeness of such information. Further, the analysis techniques included in this

publication are often simplifications and, therefore, approximate in nature. More vigorous analysis techniques and/or prototype testingare strongly recommended to verify satisfactory part performance. Anyone intending to rely on such recommendation or to use anyequipment, processing technique or material mentioned in this publication should satisfy themselves that they can meet all applicablesafety and health standards.

It is the sole responsibility of the users to investigate whether any existing patents are infringed by the use of the materials mentioned inthis publication.

Any determination of the suitability of a particular material for any use contemplated by the user is the sole responsibility of the user.The user must verify that the material, as subsequently processed, meets the requirements of the particular product or use. The user isencouraged to test prototypes or samples of the product under the harshest conditions likely to be encountered to determine thesuitability of the materials.

Material data and values included in this publication are either based on testing of laboratory test specimens and represent data that fallwithin the normal range of properties for natural material or were extracted from various published sources. All are believed to berepresentative. Colorants or other additives may cause significant variations in data values. These values are not intended for use inestablishing maximum, minimum, or ranges of values for specification purposes.

We strongly recommend that users seek and adhere to the manufacturer’s or supplier’s current instructions for handling each materialthey use. Please call 1-800-833-4882 for additional technical information. Call Customer Services at the number listed below for theappropriate MaterialSafety Data Sheets (MSDS) before attempting to process these products. Moreover, there is a need to reducehuman exposure to many materials to the lowest practical limits in view of possible adverse effects. To the extent that any hazards mayhave been mentioned in this publication, we neither suggest nor guarantee that such hazards are the only ones that exist.

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