DESIGNING WITH RUBBER

An elastomer is a type of material that exhibits rubber-like properties. Elastomers are viscoelastic—they are both sticky and very elastic. The term “elastomer” itself stands for “elastic polymer.”

Natural Rubber: The most well-known elastomer is natural rubber, which is made from the milky latex of various trees, typically the Hevea rubber tree. Natural rubber has been used for centuries and is still important in various industrial applications.

Synthetic Elastomers: In addition to natural rubber, there are synthetic elastomers derived from petroleum. These include:

Styrene-butadiene rubber (SBR): Used in tires, footwear, and other applications.

Butadiene rubber is also used in tires and various industrial products. Limitations on elastomer use are generally environmental, as rubber properties are affected by temperature, contact with solvents and oils, and exposure to ozone and radiation. However, the selection of the proper elastomer for a particular application can largely overcome these limitations.

The engineer designing components from rubber must have maximum information on the conditions to which the rubber parts will be subjected during their working life. Necessary data include maximum loads, normal working loads, permissible and desirable deflections, vibratory conditions, shock loads, and the desired life expectancy of the component.

To promote long life, stresses should be kept within recommended low limits. As with other engineering materials, highly stressed components become subject to fatigue failure, as fatigue life is predominantly a function of strain in the body. The design stresses for the rubber suspension system components of an LVT tank indicate static stresses of 75 psi (0.5 MPa), and 220 psi (1.5 MPa) at bump excursion conditions. The suspension system was designed for a 100-hour life, but the actual life experienced was 150 service hours. Newer military designs report using similar design stresses and obtaining longer life by designing for a smaller stress range. In passenger cars, suspension design stresses are 120 psi (0.8 MPa), and, in bus suspensions, designs are for 75 psi (0.5 MPa) empty and 105 psi (0.7 MPa) at a fully seated load. If strains are calculated based on a uniform stress distribution, a corrective factor should be applied to compensate for surface irregularities and section changes. Discontinuities, such as occur in a bonded assembly, must be so designed that flared edges at the interface provide a low strain gradient.

In practice, dynamic failure of rubber springs always occurs progressively, the interval between the start of failure and complete failure occupies a considerable portion of the fatigue life. This is an advantage of a rubber spring over a steel spring, in that sudden catastrophic failure rarely occurs.

Rubber volume is affected by temperature changes, and is subject to swelling by oils. The volume compressibility of rubber is so low that it can be considered negligible. Consequently, large compressive stresses may be induced in the rubber where the part is confined in rigid surroundings unless adequate provisions are made for such volume expansions. Such allowances must be made for rubber seals and other components that must function at a wide range of temperatures.

The question of "how the rubber will be loaded" depends on the application. In most cases, there is a choice between application in tension, shear, or compression. The choice should be made on the best utilization of the material. Where large production quantities are involved, the economic aspects of minimum rubber volume are major considerations. Load-deflection characteristics, space allotment, and other available design data determine other factors.

The following section presents design data for the use of rubber in shear, compression, and tension. It covers a discussion of rubber applications, stress-strain curves, and design equations, and includes design examples. The design equations are based on the theory of small strains typical for rigid materials, where Hooke's law of proportionality of stress to strain applies. The stress-strain curves presented are average curves and considerable deviation, as a result of variations in compounding, can be expected when components designed with the aid of these data are tested. One reason for requiring approximate solutions is the lack of a suitable theory for macroscopic behavior. Such theories exist but are not adaptable in practice, and therefore do not constitute practical design tools.