17.12.2012 Views

General Design Principles for DuPont Engineering Polymers - Module

General Design Principles for DuPont Engineering Polymers - Module

General Design Principles for DuPont Engineering Polymers - Module

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

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)

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!