When the pressure of the liquid sealed by an O-ring is too high or the clearance is too large, the O-ring may extrude from the gap and break. Backup rings prevent such extrusion. Even at low pressures where extrusion is not a problem, backup rings prevent accidents due to scratching or spiraling, the main cause of O-ring breakage, and extends the life of the O-ring.
PTFE (polytetrafluoroethylene) is a type of super-engineered plastic, a fluororesins also known as Teflon. It has excellent heat resistance and the lowest coefficient of friction among plastics. Products with various characteristics can be made by filling PTFE with different kinds of fillers.
Filler types and properties
|Glass fiber||Improves abrasion resistance|
|Excellent chemical resistance, especially resistance to acid and oxidizing agents|
|Minuscule impairment to electric properties|
|Affected by alkali|
|Can be used in combination with graphite and MoS2|
|Carbon||Improves abrasion resistance in air and underwater|
|Improves creep resistance and can withstand high temperatures and heavy loads|
|Can withstand a wide range of corrosive atmospheres|
|Often used in combination with graphite|
|Graphite||Improves friction and abrasion characteristics|
|Reduces wear of soft mating metals|
|Resists corrosive atmospheres|
|Also used with glass fiber and carbon|
|Bronze||Improves creep resistance, compressive strength, dimensional stability, and hardness|
|Improves thermal conductivity resulting in improved heat dissipation of the product and thus, wear resistance is also improved|
|Decreases chemical resistance and electrical insulation due to chemical reactivity and electrical conductivity|
|Molybdenum disulfide||Greater improvement in creep resistance, bending and compression strength, hardness and abrasion resistance compared to glass fiber alone|
|Excellent low abrasion properties as molybdenum disulfide itself is a lubricant|
|Reduces initial abrasion time|
|Must not be used by itself. Use it together with another filler such as glass fiber.|
|Carbon fiber||Like carbon and graphite, carbon fiber improves physical properties but has greater effects even at small quantities|
|Superior tensile strength and elongation compared to using carbon|
|Improves both air and underwater abrasion resistance|
|Superb creep resistance at both normal and high temperatures|
|Resists corrosive atmospheres|
Shape of the Backup Ring
There are three types of backup ring shapes: endless, spiral, and bias cut. Endless backup rings have the greatest effectiveness in use followed by spiral, then bias cut, but bias cut and spiral types are easier to install.
Using Backup Rings
- If pressure is applied from both sides, install the backup ring on both sides of the O-ring. If pressure is applied only from one direction, install a backup ring on the opposite side of the pressure. If extra space is available, we recommend installing two backup rings even when pressure is applied only from one direction.
- Various synthetic resins are used as materials, but PTFE (polytetrafluoroethylene) is generally used in most cases.
- Spiral backup rings should be installed so that the spiral wraps around the deeper side of the groove when installing.
- It can be used in most operating fluids, including general mineral oil-based hydraulic fluids, water-based hydraulic fluids, and synthetic hydraulic fluids.
O-rings such as those made from nitrile rubber and fluororubber are generally used as seals for gear oil, turbine oil, gasoline, machine oil, and other organic solvents. However, when reciprocating motion or rotational motion is applied to the seal, there is significant danger of damaging the O-ring. In such cases, using a PTFE slipper seal made in the O-ring’s sliding area has many advantages over using the O-ring alone.
- Significantly reduces sliding resistance during operation
- Minuscule change in starting resistance even with a long period of inactivity
- No sticking or slipping during operation
- Can be used without lubrication
- Lifespan is greatly extended
- No backup ring is necessary even under high pressure (up to 20 MPa)
- Can be used without changing the dimensions of the O-ring groove
Taking advantage of PTFE’s low friction characteristics and rubber’s elasticity and sealing pressure, friction and abrasion can be significantly reduced and JIS standard O-ring grooves can be used as-is.
(Cap seal type, etc.)
ShapeVarious slipper seal shapes are available.
Applicable Pressure RangeUp to 20 MPa (1 ring is required for every 7 MPa of pressure)
- PTFE with bronze
For reciprocating motion and swirling motion
- PTFE with glass fiber, molybdenum disulfide
For rotating motion
General use (for low pressure and low sliding)
V-packing is a typical kind of automatic seal packing that seals by using the internal pressure of the V-shaped lip to act on the shaft and inner wall of the stuffing box to seal. It works effectively with relatively low tightening pressure and low friction. Depending on the application, other types such as H, M, L, and D are also available.
V-packing with Fillers
PTFE (polytetrafluoroethylene) has excellent low friction properties but poor wear resistance. In high sliding speeds, we recommend the highly water-resistant V-packing with PTFE filler.
Number of mounting rings
If too many rings are installed unnecessarily, friction will increase, causing problems for some applications. The recommended standard number of rings is as follows.
|Operating pressure range(kg/cm2)||30 - 300||20 - 70||0 - 30||0 - 20|
|Number of packings||3 - 6||3 - 6||3 - 4||1 - 3|
The adapter ring may be metallic, but the same material should be used as much as possible.
Face the V type lip in the direction of the internal pressure and tighten lightly to set the initial seal.
Adjusting with springs
Since V-packing tends to shrink when the temperature changes significantly, it is effective to install a spring for adjustment.
Use in moving parts
Considering the PV value (pressure P × velocity V), V-packing must be used within the PV value limit. When using in parts with rotary motion, remember to cool and use together with a metal spacer ring in order to dissipate the frictional heat.
Lip Seal Packing
Lip Seal Packing Shapes