Often times in your design you will encounter parts made out of rubber. These will probably be, stands, anti-vibrations devices, rollers, shaft couplings , O-rings…. For proper selection or calculations of components one of the most important characteristics in rubber parts is hardness of rubber. Material hardness is important when designing a machine part. It can help you to prevent or achieve a deflection in the material as needed by your design.
Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a compressive force is applied 
This means that hardness is a measure of how much a material will deform when a force is applied to it.
Measurements of hardness are similar for all materials. They consist of applying a known force to an object (called a foot) with known hardness and geometry, and pushing that foot into the material of the part. Then measuring the deflection in the material that this force causes provides the value that is the hardness. Depending on the conditions such as the force, the characteristics of the object that is impregnating the material and the values, there have been developed different methods and scales for measuring hardness.
The process of determining the hardness of rubber is the same. For polymers, elastoplast and rubbers this method is named Durometer, precisely Shore Durometer. Albert Ferdinand Shore developed the process and the method bares his name. He defined the test and developed Shore hardness scale. The process and the instrument that is used in this process are both named Durometer.
As you can see in the GIF above the process of measurement is very simple with these modern devices. Inside of the device there is a spring that provides a force to the foot. The foot can be different depending on the scale of measurements. The spring pushes the foot into the elastic material, that deforms under this force. This deformation is in direct correlation with the hardness of the material. The scale on the Durometer instrument measures the distance that the foot has deformed the material in reference to the surface of the material.
This process of measurement is also precisely defined by standards: DIN 53505 (replaced) , ASTM D2240, ISO 868, DIN ISO 7619-1 and ISO 761 and others. They differ slightly. Let’s take for example DIN ISO 7619-1. Some conditions that have to be met with this standard are:
- For measuring Shore A the foot indents the material while for the Shore D foot penetrates the surface of the material
- Material for testing needs to be in laboratory climate storage at least one hour before testing
- Measuring time is 15s
- Force is 1kg +0.1kg for Shore A, and 5kg +0.5kg for Shore D
- Five measurements needs to be taken
- Calibration of the Durometer is one per week with elastomer blocks of different hardness
There are different types of Shore hardness test scales. Each scale has values between 0 and 100 where bigger number represents harder material. Types of Shore scales are as follows:
- Shore A
- Shore B
- Shore C
- Shore D
- Shore E
- Shore M
- Shore O
- Shore OO
- Shore DO
- Shore OOO
- Shore OOO-s
Most common type in machine design applications is Shore A. The second most common is probably Shore D. You can see this scale if you look in most rubber machine components like O – rings, Shaft seals, Couplings, … Common values for an O-rings 60, 70, 80, 90 Shore A.
The visual representation of the different types of hardness and scales is the best way to get the feeling for the Shore scales. Some hardness scales like Shore A and Shore D in part overlap. So do not get confused if you find a, for example 80 Shore A and 40 Shore D describing the hardness of the same part.
Here is a great chart that visually represents the Shore scales and the hardness of some common materials. So the next time that you have to specify the hardness of a rubber component, look at this chart and compare it to something in your surroundings.
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