O-Ring Durometer Hardness

What Is O-Ring Durometer?

O-Ring durometer is the hardness of the o-ring material. Durometer scales run numerically from 0-100, with lower numbers indicating softer materials and higher numbers for harder materials. Durometer is just shorthand for “Shore hardness”.

How is O-Ring Durometer Measured?

O-Ring durometer is measured by the depth of indentation in the material created by a given force using a standardized pressure foot. The indicating scale reads the hardness of the rubber. If there is no penetration, the scale will read 100. (For o-rings that are too thin or provide too small an area for accurate durometer readings, Micro Hardness Testing is recommended – less than 6 mm thick and less than 12 mm from the edge of the material)

The durometer gauge has a calibrated spring which forces an indent or point into the elastomer. The correct combination of the type of sample and type of hardness gauge used are required for an acceptable hardness reading.

* Please do not try to determine durometer without a durometer gauge.


The difference in gauge types can be significant and details on the tester contact points and allowable samples are found in ASTM D 2240.

  • The hardness scale will be on the gauge - example: It will say ShA (Shore A) as the units. DO NOT USE readings 1:1 for other durometer scales).
  • Read the specs for each gauge

    • Type A can only be used for 1"x1"- diameter buttons.
    • Microhardness -use a Type G hardness testing per ASTM D2240 and are allowed to measure parts instead of a button.

ESP Durometer Testing Based on ASTM D 2240 Recommendations

Type (Scale) Tested Material Examples Durometer Hardness
A Soft elastomers: nitriles, thermoplastics, natural rubber, and polyacrylics. 20 - 90 A
B Moderately hard elastomers: thermoplastics, paper, and fibeous materials. Above 90 A
Below 20 D
C Medium-hard elastomers: thermoplastics, medium-hard plastics. Above 90 A
Below 20 D
D Hard elastomers: theromoplastics, harder/rigid plastics like hard hats. Above 90 D
DO Moderately Hard elastomers: thermoplastics and very dense windings. Above 90 C
Below 20 D
M Thin, irregular shaped elastmoers: thermoplastics and plastics 20 - 85 A
O Soft elastomers: thermoplastics, soft plastics, and medium-density windings. Below 20 DO
OO Extremely soft elastomers: thermoplastics, extremely soft plastics, gummy candy, and gel shoe insoles. Below 20 O

What O-Ring Durometer Scale Should I Use?

The ASTM D 2240 standard recognizes 12 different durometer scales.

The most common and widely used scale is the Shore A hardness scale and is the standard instrument used to measure the hardness of most rubber compounds.

There are two Shore hardness scales typically used for elastomers, Shore A (ShA) and Shore D (ShD). Both scales are zero to 100 but shore D scale is for harder materials. Example: 95ShA is approximately 50ShD.

Typically, the only time you use these crossovers is in the range of 80-100ShA converting to 50-65ShD. See some of the material listings in the parker EPS handbook to see what I mean (Polyurethanes of 55ShD hardness listed next to 90ShA polyurethanes).

  • Shore A Scale
    • For most applications, compounds having a Shore A durometer hardness of 70 to 80 is the most suitable compromise. This is particularly true of dynamic applications where 90 durometer or harder compounds often allow a few drops of fluid to pass with each cycle, and 50 durometer compounds tend to abrade, wear, and extrude very quickly. Normally durometer hardness is referred to in increments of five or ten, as 60 durometer, 75 durometer, etc. — not as 62 durometer, 66 durometer or 73 durometer. This practice is based on: (1) The fact that durometer is generally called out in specifi cations with a tolerance of ±5 (i.e., 65±5, 70±5, 90±5); (2) The inherent minor variance from batch to batch of a given rubber compound due to slight differences in raw materials and processing techniques; and (3) The human variance encountered in reading durometer hardness. On a 70-durometer stock, for example, one person might read 69 and another 71. This small difference is to be expected and is considered to be within acceptable experimental error and the accuracy of the testing equipment.
  • Shore D
    • Hardness Scale measures the hardness of hard rubbers, semi-rigid plastics and hard plastics.
  • IRHD Scale (International Rubber Hardness Degrees
    • In the O-ring industry, another hardness scale, the IRHD, is used due to the curved surface of the O-ring cross-section causing problems with accurately reading Shore A. The size and shape of the indentor used in IRHD readings is much smaller, thus allowing for more accurate measurements on curved surfaces such as an O-ring cross-section. Unfortunately, there is not a direct correlation between the readings of Shore A and IRHD Scales.
  • Shore OO
    • Hardness Scale measures rubbers and gels that are very soft.
SHORE OO 0 10 20 30 40 50 60 70 80 90 100
SHORE A 0 10 20 30 40 50 60 70 80 90 100
SHORE D 0 10 20 30 40 50 60 70 80 90 100

Why Is It Important To Choose The Right O-Ring Durometer?

O-ring durometer impacts overall sealing function of the seal depending on application parameters.

Softer sealing materials, with lower hardness readings, will flow more easily into the microfine grooves and imperfections of the mating parts (the gland, bore, rod or seal flanges). This is particularly important in low-pressure seals because they are not activated by fluid pressure. Conversely, the harder materials offer greater resistance to extrusion.

In dynamic applications, the hardness of the O-ring is doubly important because it also affects both breakout and running friction. Although a harder compound will, in general, have a lower coefficient of friction than a softer material, the actual running and breakout friction values are actually higher because the compressive load required to achieve the proper squeeze and force the harder material into a given O-ring cavity is so much greater.

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