General Design Principles for DuPont Engineering Polymers - Module

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