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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Rim <strong>Design</strong><br />
Rim design requirements will vary depending upon<br />
whether or not a tire is used and whether the tire is<br />
solid or pneumatic.<br />
Tireless wheels are frequently used on material<br />
handling equipment where vibration and noise are not<br />
critical. Impact resistance is of prime importance in<br />
this type of service, and rims are frequently molded<br />
with wall thickness up to 3 ⁄ 8 in (9.5 mm). The lengthened<br />
molding cycle can increase processing costs to<br />
a point where it can be more economical to mold a<br />
wheel in a thinner wall and—using the wheels as an<br />
insert—mold an elastomeric tire around it.<br />
If a pneumatic tire is used, the rim will be under<br />
constant pressure and the effect of creep on rim<br />
geometry must be taken into account. It can be shown<br />
that the outward <strong>for</strong>ce exerted on the rim is a product<br />
of the pressure in the tire and the radius of the tire<br />
cross section, plus the direct pressure <strong>for</strong>ce on the rim<br />
itself. Referring to Figure 5.04:<br />
(F = 1 pr + pL)<br />
For example, where tire cross section diameter is<br />
1.40 in (3.56 mm) and inflated pressure 30 psi<br />
(207 kPa), <strong>for</strong>ce on the rim will be about 21 lb/in<br />
(3.68 kN/m) of circumference. In addition, with a rim<br />
height of 0.50 in (12.7 mm), the 30 psi (207 kPa)<br />
pressure against the interior sidewalls of the rim will<br />
add an additional 15 lb/in (2.63 kN/m) of load. The<br />
total <strong>for</strong>ce will create bending stresses in the rim<br />
section (Figure 5.04). These, however, can be held to<br />
a low level by specifying rim height (L) at the minimum<br />
point required to retain the tire bead. Radial<br />
ribbing can be added as shown to further stiffen the<br />
rim without substantially affecting the cross section<br />
wall thickness.<br />
Figure 5.04<br />
r<br />
p<br />
F Tire<br />
p<br />
Inner Tube<br />
r<br />
p<br />
L<br />
33<br />
Finite Element Analysis in Wheel<br />
<strong>Design</strong><br />
Results of a finite element wheel analysis conducted<br />
on a computer are within 5 percent of laboratory wheel<br />
test data. This suggests that computer analysis will<br />
provide more accurate determinations of wheel per<strong>for</strong>mance—including<br />
stress levels and deflections—while<br />
the wheel design is still in the drawing stage.<br />
The design which was analyzed, called “Dervish,” is<br />
shown in Figure 5.05. In the computer modeling, two<br />
dimensional elements were used to allow variation in<br />
wall thickness of the rim, spokes and hub. This<br />
flexibility provided <strong>for</strong> analysis at various wall<br />
thicknesses and wheel weights. In the two cases run,<br />
a vertical load of 91 kg (200 lb) was applied, both<br />
on a spoke and between two spokes, with the wheel<br />
supported at the hub. The results are shown in Figure<br />
5.06 and tabulated below.<br />
Figure 5.05 Dervish model isometric view<br />
Figure 5.06 Dervish finite element (analysis of rim and<br />
web stress)