O-ring seals due to high pressure and temperature fluctuations may be damaged. Extrusion is one of the most common reasons that leads to damage to orings. The best solution to prevent this is to install Back-up Rings. They are made of hard plastic (PTFE, FKM) resistant to extrusion.
What is Back-up Rings?
Back-up Rings are also called spare rings, reserve rings, or retaining rings. These rigid rings are made of harder plastic than a gasket, but still flexible enough to deform under pressure. Back-up Ring is used in combination with orings in high-pressure applications. They are designed to fit tightly between the seal and the gap, keeping the orings in the right place, minimizing the risk of shape change.
What is a retaining ring for?
The back-up ring is used in static and dynamic applications. Protects o-ring rubber seals with a circular section from being pressed into a gap between two surfaces. The back-up ring is used to minimize the extrusion of O-rings due to high pressure and temperature. If the pressure is two-way, then the reserve rings are used on both sides of the seal. For single-acting applications, one Back-up Ring is installed in the slot.
What are spare rings made of?
The spare ring can be made of various types of plastics (NBR, PTFE, Viton (FKM)). The advantage of thermoplastic PTFE is very low friction in dynamic applications. The Back-up Ring acts as a protective and support element, which is made of a harder and more durable material than a gasket.
How do spare rings prevent orings from moving?
Back-up Rings is used for both dynamic and static applications. They are used to prevent sliding and extrusion in the gap of O-ring seals due to cyclical pressure changes, i.e. pressure changes. increasing and lowering. The properly selected Back-up Ring is mounted in the housing as an additional support for sealing to prevent the seal from squeezing under pressure. The spare ring protects the oring from unwanted shape and place changes. Any type of gasket may be exposed to extrusion due to high pressure. Back-up Rings act as a support element that is mounted behind a gasket on one or both sides. If the pressure is only on one side, simply mount the Back-up Ring on the bottom side of the seal. In the event that the oring is subjected to pressure on both sides, the spare ring shall be mounted both at the top and bottom. This is made possible by the fact that the retaining rings are made of a material harder than a gasket. The back-up ring is precisely and closely matched to the housing, which can also contribute to a slight increase in friction.
Back-up Rings – Application
The use of Back-up Rings has many advantages. Their use is recommended between sealing elements in case of pressure exceeding 5 MPa (50 bar). In case of one-sided operation of increased pressure, the Back-up Ring is installed only on one side, while in the case of piston pressure, which puts pressure on the gasket on both sides , the spare ring is installed on each side. Strong pulsations, elevated temperatures, temperature fluctuations, high speed and frequency – these are also indications for the installation of retaining rings.
Examples of indications for the use of back-up rings:
- hydraulic cylinders
- high-pressure valves
- valves for hydraulic circuits
- high-pressure hydraulicsystems
- in static applications up to 950 bar
- with dynamic applications above 100 bar
- o-ring made of low strength or hardness plastic used
- in applications where piston and rotational movements are possible
Spare rings are suitable for both high-pressure applications subjected to high temperature fluctuations and where low hardness orings are used. Back-up Rings protection is also used when the recommended hardness of the rubber from which the O-ring is made is less than the size of the slot. Excavators, agricultural machinery, machine tools, injection moulding machines are few and far between in which Back-up Rings can be used.
