Date:23 February, 2005
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Snap fit design
Cantilever beam snap-fits
Cantilever beam type snap-fits can be calculated using a simplification of the general beam theory. However the calculations are a simplification. In general, the stiffness of the part to which the snap-fit connects, is important. The formulae mentioned only roughly describe the behavior of both the part geometry and the material. On the other hand, the approach can be ud as a first indication if a snap-fit design and material choice are feasible.
Cantilever beam with constant rectangular cross ction
A simple type of snap-fit, the cantilever beam, is demonstrated in the figure below, which shows the major geometrical parameters of this type of snap-fit. The cross ction is rectangular and is constant over the
whole length L of the beam.
幼儿园保育员职责The maximum allowable deflection y and deflection force F b can be calculated with the following formulas if the maximum allowable strain level ε of the material is known.
2 L2
y = -- . -- . ε
3 t
w . t2 . E s
卤味
F b = ------------ . ε
6 . L
Date:23 February, 2005
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where
E s = cant modulus
电脑维修知识L = length of the beam
t = height of the beam
w = width of the beam
ε = maximum allowable strain level of the material
The four dimensions that can be changed by the designer are:
- h, the height of the snap-fit lip. Changing the height might reduce the ability of the snap-fit to ensure a proper connection.
- t, the thickness of the beam. A more effective method is to u a tapered beam. The stress are more evenly spread over the length of the beam.
- increasing the beam length, L, is the best way to reduce strain as it is reprented squared in the equation for the allowable deflection.
- the deflection force is proportional to the width, w, of the snap-fit lip.
Beams with other cross ctions
The following general formulae for the maximum allowable deflection y and deflection force F b can be ud for cantilever beams with a constant asymmetric cross ction.
L2
y = ------- . ε
3 . e
E s . I
F b = ------- . ε
e . L
大学才子派where
E s = cant modulus
I = moment of inertia of the cross ction
L = length of the beam
e = distance from the centroid to the extremities
ε = maximum allowable strain level of the material
Normally tensile stress are more critical than compressive stress. Therefore the distance from the centroid to the extremities, e, that belongs to the side under tension is ud in the above-mentioned formulae. The moment of inertia and the distance from the centroid to the extremities is given in table 1 for some cross ctions.
DSM Engineering Plastics– Technical Guide Date:23 February, 2005
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中药药材种植Table 1. Moment of inertia and distances from centroid to extremities
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Tapered beams with a variable height
The following formulae can be ud to calculate the maximum allowable deflection y and the deflection force F b for a tapered cantilever beam with a rectangular cross ction. The height of the cross ction decreas linearly from t1 to t2, e figure above.
2 . L2
y = c .-------- . ε
3 . t1
w . t12 . E s
F b = -------------- . ε
6 . L
where
E s = cant modulus
L = length of the beam
c = multiplier
w = width of the beam
t1 = height of the cross ction at the fixed end of the beam
ε = maximum allowable strain level of the material
The formula for the deflection y contains a multiplier c that depends on the ratio t2 / t1, e table 2, where t1 is the height of the beam at the fixed end and t2 is the height of the beam at the free end.
Date:23 February, 2005
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Table 2. Multiplier c as a function of the height
t2 / t1
0.40 0.50 0.60 0.70 0.80 0.90 1.00
c 1.893
1.636
1.445
1.297 1.179 1.082 1.000
Tapered beams with a variable width
The following formulae can be ud to calculate the maximum allowable deflection y and deflection force F b for a tapered cantilever beam with a rectangular cross ction. The width of the cross ction decreas linearly from w1 to w2, e figure above.
2 . L2
y = c .-------- . ε
3 . t
w1 . t2 . E s
F b = -------------- . ε
6 . L
where
E s = cant modulus
L = length of the beam
c = multiplier
w1 = width of the beam at the fixed end of the beam
t = height of the cross ction
ε = maximum allowable strain level of the material
Date:23 February, 2005
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The multiplier c depends on the ratio w2 / w1, e table 3, where w1 is the width of the beam at the fixed end and w2 is the width of the beam at the free end.
Table 3. Multiplier c as a function of the width
w2 / w10.125 0.25 0.50 1.00
c 1.368 1.284 1.158 1.000
Cylindrical snap-fits永矢
One must distinguish between a cylindrical snap-fit clo to the end of the pipe or remote from the end
