General Design Principles for DuPont Engineering Polymers - Module
General Design Principles for DuPont Engineering Polymers - Module
General Design Principles for DuPont Engineering Polymers - Module
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Figures 10.07 and 10.08 show calculated interference<br />
limits at room temperature <strong>for</strong> press-fitted shafts and<br />
hubs of Delrin ® acetal resin and Zytel ® nylon resin.<br />
These represent the maximum allowable interference<br />
based on yield point and elastic modulus data. The<br />
limits shown should be reduced by a safety factor<br />
appropriate to the particular application. Safety factors<br />
of 1.5 to 2 are used <strong>for</strong> most applications.<br />
Where press fits are used, the designer generally seeks<br />
the maximum pullout <strong>for</strong>ce which is obtained by using<br />
the greatest allowable interference between parts,<br />
consistent with the strength of the plastics used.<br />
Allowable interference varies with material properties,<br />
part geometry and environmental conditions.<br />
Interference Limits<br />
The general equation <strong>for</strong> thick-walled cylinders is<br />
used to determine allowable interference between a<br />
shaft and a hub:<br />
I = S dD s ( W + μ h + l – μs) (15)<br />
W E h E s<br />
and<br />
1+( D s) 2<br />
D h<br />
W = (16)<br />
1–( D s) 2<br />
D h<br />
where:<br />
I = Diametral interference, mm (in)<br />
S d = <strong>Design</strong> stress, MPa (psi)<br />
D h = Outside diameter of hub, mm (in)<br />
D s = Diameter of shaft, mm (in)<br />
E h = Modulus of elasticity of hub, MPa (psi)<br />
E s = Elasticity of shaft, MPa (psi)<br />
μ h = Poisson’s ratio of hub material<br />
μ s = Poisson’s ratio of shaft material<br />
W = Geometry factor<br />
Case 1. (SI Units) Shaft and Hub of Same Plastic.<br />
When hub and shaft are both plastic<br />
Eh = Es; μh = μs. Thus equation 15 simplifies to:<br />
I = SdDs W + 1<br />
×<br />
W Eh Case 2. (SI Units) Metal Shaft; Hub of Plastic.<br />
When a shaft is of a high modulus metal or any other<br />
high modulus material, E greater than 50 ×103 MPa,<br />
the last term in equation 15 becomes negligible and<br />
the equation simplifies to:<br />
I = S dD s × W + μ h<br />
W E h<br />
71<br />
Figure 10.07 Maximum interference limits<br />
% of insert θ<br />
6<br />
5<br />
4<br />
3<br />
1.5<br />
2<br />
d<br />
Figure 10.08 Theoretical interference limits <strong>for</strong> press<br />
fitting<br />
Interference limits<br />
cm/cm of shaft diameter<br />
0.10<br />
0.08<br />
0.06<br />
0.04<br />
0.02<br />
0<br />
3<br />
Ratio D/d<br />
2·d<br />
Hub of Delrin ® 500<br />
Max. interference limits<br />
Shaft in Delrin ®<br />
Shaft in Steel<br />
Shaft of Zytel ®<br />
Shaft of steel<br />
4<br />
Hub of<br />
Zytel ® 101<br />
d1<br />
100<br />
80<br />
60<br />
40<br />
0.2 0.4 0.6 0.8<br />
20<br />
1.0<br />
Ratio shaft diameter to<br />
outside diameter of hub, d/D<br />
Based on yield point and elastic modulus at room<br />
temperature and average moisture conditions<br />
D<br />
Interference limits<br />
mils/in of shaft diameter<br />
Theoretical Interference Limits <strong>for</strong> Delrin ® acetal<br />
resin and Zytel ® nylon resin are shown in Figures<br />
10.07 and 10.08.<br />
Press fitting can be facilitated by cooling the internal<br />
part or heating the external part to reduce interference<br />
just be<strong>for</strong>e assembly. The change in diameter due to<br />
temperature can be determined using the coefficient of<br />
thermal expansion of the materials.<br />
Thus: D – Do = a (t – to) Do where:<br />
D = Diameter at temperature t, mm (in)<br />
Do = Diameter at initial temperature to, mm (in)<br />
a = Coefficient of linear thermal expansion