While selecting the base rubber for any Rubber products, we first study the application and which are the basic properties are utmost important. Some of the basic properties are designed considering temperature range, fluid resistance, Weather, Ozone resistance etc

While selecting the most appropriate rubber for rubber products, bearing seals etc… the following data is needed:

• Highest and peak temperature
• Continuous temperature
• Lowest operational temperature?
• Contact fluids and with context to temperature?
• How long the fluid will come in contact and also if any accidental contact of hazardous chemicals etc…
• Environmental or weather (ozone) application

The accompanying chart shows the different rubbers and their properties as related to these questions. The cost factor is also shown, with the rubbers arranged in order of increasing cost from left to right.

Starting at the left, check the properties of the rubber against your answers to the questions above. Keep moving to the right until a rubber is found that meets your need. (Where no particular temperature or fluid resistance is required Natural Rubber is the most widely used material and offers the greatest scope of compounds and properties coupled with the most economic cost.It is also an environmentally friendly material)

If a rubber meets most of the requirements but is borderline on a particular property, Mech Spares should be consulted for further advice with a view to testing under the relevant conditions (if necessary).

Once a rubber type has been selected, the hardness range must be determined. Hardness is measured in degrees on the Shore “A” or IRHD scale (the values are similar although Shore “A” readings are usually one to three degrees higher than IRHD readings). Hardnesses are normally based on a nominal figure e.g. 50 ±5° or as a hardness range e.g. 50-60°.

Materials below 30° are extremely soft and comparable with foams. These are available but must be regarded as a special requirement.

Additional mechanical requirements should be specified only where necessary. They can be specified simply by description e.g. “good tear resistance required” or quantified by the use of a defined test method (such as ASTM or BS test methods) giving maximum or minimum values as appropriate. These values may be obtained by calculation based on the actual requirements of the application, by comparison with known values for similar applications or materials, or by testing a material which has been proved satisfactory by trial or experiment.

Where requirements are considerably more complex than the scope of this publication, the ASTM or BS framework for specifying rubber materials can be consulted. Materials may also be selected from the BS range of standard and special application specifications, some of which are listed below.

If the properties required of the rubber are difficult to determine, the production of a prototype mould should be considered. A variety of rubber compounds can then be moulded and parts tested for suitability in the actual application. Once a material has been proved suitable, it can be tested and its properties appropriately defined.

A method for specifying rubber materials can be found in ASTM D2000 and BS 5176. These provide a full framework covering all rubbers and rubber properties. The relevant requirements for an application are selected from within the framework and are expressed in the form of a “line call out”. (The same framework is used in both ASTM D2000 and BS 5176).

An example of a line call out is BS 5176 1MBC514 F2 7Z1. This can be broken down as follows indicated in this table:

The appropriate table for Grade 1 MBC materials shows the following basic requirements:

• Test: Elongation at Break
• Requirement: 400% minimum
• Test: Heat Resistance (70 hrs @ 100°C)
• Requirement: T.S. ± 30%, E.B. -50% max, Hardness ±15° max
• Test: Oil Resistance (Oil no.3 70 hrs @ 100°C)
• Requirement: Volume change + 120% max
• Test: Compression Set (22 hrs @ 100°C)
• Requirement: 80% max