O-rings can fail in two primary ways: by leaking or by tearing. O-rings can leak if someone use the wrong size of o-ring for the application. Additionally, o-rings can tear if an individual forces an o-ring of the wrong size into the proper place on the equipment.
Because o-rings can fail in these ways, it is necessary to use a proper size chart to ensure that the person purchases the proper size of o-ring. A size chart would allow individuals to use measurements to find the proper size of o-ring for the application, instead of have to guess what the proper size would be. Every o-ring has three specific measurement associated with it.
How to Choose the Right O-Ring
The three measurements of an o-ring are the inside diameter, outside diameter, and cross-section. The inside diameter measurement of an o-ring determines in what way the o-ring will sit; either around a shaft or within a bore. The outside diameter measurement of an o-ring indicates the distance of the rubber material from the o-ring to the surrounding area.
The cross-section measurement of an o-ring is the thickness of the rubber material of the o-ring. By knowing these three measurements, an individual can properly replace an o-ring with another o-ring with the same dimension. These measurements can be displayed on a size chart for easy viewing.
Most o-rings in North America use the AS568 dash numbering system to denote the different sizes of o-rings. The AS568 dash numbering system groups o-rings according to their cross-section size first, and then groups those according to their inside diameter sizes. For instance, o-rings that have a 1/16” cross-section have a lower number in the dash system than o-rings with a 1/8” cross-section.
Similarly, o-rings with a 3/16” and 1/4” cross-section use higher numbers in the AS568 dash system. Engineers typically choose the cross-section first, as it is based off the required pressure and clearance of the o-ring, and then choose the inside diameter as the second selection. The size chart can help individuals skip the sizes of o-rings that will not work for there application.
The other standard for o-rings include the British Standard sizes of o-rings and the Japanese Industrial Standard sizes of o-rings. Both of these standard use metric measurements to indicate the sizes of o-rings. Size charts also include these measurements for those using metric measurements, especially for individuals whose equipment was manufactured outside of the United States.
While many use the AS568 standard, others will use the metric measurements for their required applications, especially if they are purchasing an o-ring that was manufactured with machinery made overseas. Another factor to consider when selecting the proper o-ring for an application is the material of the o-ring. Some fluid will react differently to different materials of o-rings.
For instance, people use nitrile materials for oils and fuels within a moderate temperature range. Viton materials are used for situations with higher heat and chemicals. For example, EPDM materials are used with water, steam, and brake fluids, but if the fluid contains petroleum, the EPDM material will fail.
Additionally, silicone materials can endure extreme cold and heat, but will fail in the presence of oils. These different materials come with different temperature ranges for which they work, and these ranges are included in the size chart. To measure the dimension of an existing o-ring, the individual can use a digital caliper and a flat surface.
The individual should lay the o-ring flat on the surface and not stretched. The cross-section and inside diameter of the o-ring will need to be measured using the caliper. The individual can also measure the outside diameter with the digital caliper, but it will be equal to the inside diameter plus twice the cross-section measurement.
These measurements can be compared to the quick reference grid that is often included in size charts. The type of application for an o-ring can impact the amount that the o-ring is squeezed into place. For instance, static seals between two flanges have a squeeze rate of 25% to 30%.
Dynamic seals, such as piston and rod seals, have a squeeze rate of 10% to 15% as too much squeeze will create friction between two moving elements. These percentages are displayed on a size chart so that individuals are aware of the range of percentages for different types of seals. Additionally, if too much squeeze is used for dynamic seals, the o-ring will wear rapid.
Too little squeeze will occur for static seals and lead to the leaking of fluids at low pressures. The fourth consideration for o-rings are the measurements of the hardness of the rubber materials. People measure the hardness of o-rings on the Shore A durometer scale.
For most equipment, the recommended hardness is 70A. 50A material are softer and are used for applications with rougher surfaces and for vacuum applications. 90A materials are harder and can withstand higher pressures of 1500 psi. A size chart will indicate these different hardness measurements as an option for those looking to purchase an o-ring for an application.
Failures of o-rings typically occur due to one of four main cause. These four causes include extrusion, chemical swell, heat cracking, and spiral failure. Extrusion occurs due to the high amount of pressure that is placed on the o-ring.
Certain fluids cause chemical swell by reacting with the rubber compound of the o-ring. Heat cracking occurs in instances in which the o-ring is exposed to temperatures beyond those for which the rubber compound is rated. Spiral failure is experienced in dynamic seals when the o-ring develops a spiral cut, and typically occurs due to installation issues for the o-ring.
These four causes are included in size charts to allow individuals to understand the cause of failure in there own equipment. In instances in which the current o-ring on an appliance is damaged or stretched, there are available o-ring kits that include assorted small sizes of o-rings. Prior to installing the o-ring kit, an individual should apply a thin film of grease to the o-ring to prevent the o-ring from twisting while in place.
Should the measurements for the o-ring be in between two size dash numbers, the individual should select the larger size for the o-ring to avoid the problems of forcing the smaller size into place. Finally, the individual must verify the material of the o-ring for the compatibility with the fluids, temperatures, and pressures within the equipment to which it will be installed. After these steps are followed, the individual will have avoided the problems of leaks in the equipment.
The value of a size chart is that it removes the guesswork of replacing an o-ring with another. By understanding the different measurements, the dash system for sizes, and the compatibility charts of the materials, an individual will know how to properly replace an o-ring. By following these steps, the individual will ensure that the equipment does not experience repeated leaks.
