BUR, the O-ring's protector
BURs are special rings that provide extra support for O-rings with which they are fitted. Their main purpose is to provide increased extrusion resistance and prevent damage to the O-ring when it is subjected to high pressures and/or temperatures or larger extrusion gaps.
Extrusion is one of the most common reasons why an O-Ring can fail, compromising the seal. When the internal pressure of a system is too high, the O-ring tends to extrude into the tolerance gap. This will cause a sort of pinching that will quickly nibble away at the seal and cause damage. At this point, the seal will certainly fail.
Backup Rings are designed to fit between the O-Ring and the extrusion gap to prevent O-Ring extrusion. According to the direction of pressure in the seal, one extrusion ring or two can be used. When in doubt, it is always better to use two BURs for one O-ring. Double protection is the best option.
How to avoid extrusion?
- Reducing the gaps: By reducing the gaps where the seal will be installed, we will reduce the extrusion gap. This will surely be the most expensive option, as it implies changing the application.
- Increasing the hardness: The higher the shore hardness of the O-ring, the higher its resistance to extrusion. However, this solution has two minor drawbacks. On the one hand, buying O-rings with high shore hardness can increase your costs; on the other hand, O-rings with higher hardness have a limited seal for low pressures.
- Adding a backup ring: The last and best option would be to add a backup ring to the O-ring.
The standard BUR material we usually have in stock is the NBR90, although we can also manufacture it in FPM, PTFE, POM, PU, etc. on request. In addition to NBR90 BURs, we also have PTFE rings in a spiral cut.
Check our BUR chart for usual O-ring sizes according to the AS-568 standard.