Rib designs

This line may be superimposed on the rib design data as shown in Fig. 2.28. Combinations of dimensions above this line are likely to provide ribs which are too slender and so are liable to buckling. Combinations below the line are likely to be acceptable but do remember the assumptions made in the determination of the buckling line - in particular, the ratio of Oy/E will increase with time due to creep and this will cause the buckling line to move downwards. 

The term bladed refers to raised, continuous flow restrictors with a standoff distance from the bit body of more than 1.0 in. In most cases cutters are affixed to the blades so that the cutter arrangement may also be described as bladed. The term ribbed refers to raised continuous flow restrictors with a standoff distance from the bit body of 1.0 in. or less. Cutters are usually affixed to most of the ribs so that the cutter arrangement may also be described as ribbed. The term open face refers to nonrestricted flow arrangements. Open face flow designs generally have a more even distribution of cutters over the bit face than with bladed or ribbed designs. 

Rib design An example of how ribbing will provide the necessary equivalent moment of inertia and section modulus will be given. A flat plastic bar of IV2 in. x 3/8 in. thick and 10 in. long, supported at both ends and loaded at the center, was calculated to provide a specified deflection and stress level under a given load. The favorable material thickness of this plastic is 0.150 in. Using... 

There are certain basic rib-design guidelines that should be followed (Fig. 3-9). The most general is to make the rib thickness at its base equal to one-half the adjacent wall s thickness. With ribs opposite appearance areas, the width should be kept as thin as possible. In areas where structure is more important than appearance, or with very low shrinkage materials, ribs are often 75 or even 100 percent of the outside wall s thickness. As can be seen in Fig. 3-10, a goal in rib design is to prevent the formation of a heavy mass of material that can result in a sink, void, distortion, long cycle time, or any combination of these problems. 

Plantation rubber in the form of sheets which have been dried in an atmosphere of wood smoke and finally sheeted in a mill having rolls engraved with a ribbed design. Ribbed smoked sheet comprises about 60% of the output of a plantation. See Plantation Rubber. 

A cross-ribbed design only provides stiffening in the two rib directions. There is a low bending stiffness along a direction at 45° to the ribs, and a low torsional stiffness. An isotropic stiffened plate needs ribs in at least... 

Rib designs with design elements principally in the circumferential direction are the most common type of tread pattern and show overall good service for all-wheel-position summer service. On heavy tmcks, they are used nearly exclusively on steer and trailer axles because of their lateral traction and uniform wear characteristics. Rib/lug combinations tend to find use on all-season tires, which require a balance of good tread wear, traction, and wet skid. On heavy-duty truck drive axles, where forward traction is a prime requirement and where fast tread wear occurs as a result of torque-induced slip, the highway lug design is required. For off-highway service conditions the tread pattern assumes a... 

Stone ejection rib Portion of the tread rib designed to throw off stones with the aid of normal tire fiexing. Located up to 75% down in the tread grooves to prevent small stones from locking down at the base of the grooves where they cannot be ejected. 

Figure 8,18 Comparing a rib design strength wise and weight wise with other materials...

Figure 4.38 Side and bottom views (left) of a rib designed to stiffen the product wall cross section (right) shows a stud added which is slightly larger than the ejector pin...

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Figure 8.14 Standing rib design (a) R = 0.25W and (b) R = 0.5W. (Source Jordan I. Rotheiser, Joining of Plastics Handbook for Designers and Engineers, Hanser Publishers, Munich-Hanser Gardner Publications, Inc., Cincinnati, 1999.)...

Ribs (B) and metal inserts (E, H) are not recommended for preforms, but they can be used with the other versions. The minimum inside radius (IR) is 0.125 in for preform-molded parts and 0.062 in for SMC-and BMC-molded parts. Long-fiber compression-molded parts can be trimmed in the mold. For Class A finishes in SMC parts, the inside radius at the base of a rib should be 0.020 in otherwise it should not be less than 0.060 in. The thickness at the base of the rib (B) can be equal to the nominal wall thickness (W), however, it must not exceed 0.75W under Class A surfaces. The height of the rib (C) should be 2 to 3 times the nominal wall thickness (W) and the draft should not be less than 1° per side except for ribs imder Class A surfaces (which can handle the cost premium) where it can go down to 0.5° per side. These same rib design rules apply to boss wall thicknesses for these parts. In-mold coating can be used for high-class finishes. 

Structural foam is also more generous in freedom of rib design. The thickness of the base of the rib (B) can follow the specification indicated in Table 8.18. However, the recommendations in Sec. 8.2.4 should be followed for co-injection-molded parts. The height of the rib (C) should not exceed 3 times the nominal wall (HO (C < 3H0 since the law of diminishing returns sets in at that point and taller ribs, while feasible up to 8 times the wall thickness, tend to become too thin to foam due to the reduction in thickness resulting from draft. Besides, the... 

If there is sufficient space, the use of ribs is a practical, economic means of increasing the structural integrity of plastic parts without creating thick walls (see Chapter 3). Ribs are provided for spacing purposes, to support components, and for other uses. Table 11-4 shows a summary of the results of using a rib design. 

According to standard engineering handbooks (under Stress and >eflection in Beams, Moments of Inertia, etc.), resistance to stress is expressed by the moment of inertia, and the resistance to deflection by the section modulus. By finding a cross-section with the two factors equivalent, we can assure equal or better performance in the ribbed design compared to the thick wall without ribs. 

If ribbing is used in domed or contoured parts, it should conform to the exterior surface contour of the part and the height should generally be no greater than 0.75 in. Fig. 6-4 shows poor and good contoured rib design ... 

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