Technical specifications for selection, installation and use of O-ring seals
1）Rubber sealing material raw material properties
| PROPERTIES OF SEAL ELEMENT MATERIALS | |||||
| ELEMENT MATERIALS COMMON NAME |
NITRILE | NEOPRENE | SILICONE | FLUORO- ELASTOMER |
ETHYLENE- PROPYLENE |
| ASTM DESIGNATION | NBR R |
CR C |
SI S |
FKM V |
EPDM E |
| TEMPERATURE PROPERTIES | |||||
| RANGE ℃ | -40～+120 | -30～+110 | -70～+190 | -40～+200 | -40～+150 |
| WEATHER | G | E | E | E | E |
| FLAME | P | Q | E | E | E |
| MECHANICAL PROPERTIES | |||||
| TENSILE STRENGTH | E | G | P | E | F |
| ELONGATION | G | G | P | G | G |
| COMPRESSION SET | E | F | E | G | F |
| RESILIENCE | G | G | P | F | G |
| ABRASION | E | E | F | G | F |
| IMPACT | F | G | F | G | G |
| TEAR | G | G | F | F | F |
| ELECTRICAL | F | F | G | F | F |
| ENVIRONMENTAL PROPERTIES | |||||
| LUBRICANTS | E | G | G | E | E |
| IND.FLUIDS | G | G | F | E | P |
| CHEMICAL | F | F | G | E | F |
| OZONE | P | G | E | E | E |
| ACID | F | G | G | E | G |
| ALKALI | F | G | F | G | G |
| ALCOHOLS | G | F | E | E | F |
| WATER | G | F | G | G | G |
| RADIATION | F | F | G | G | G |
| STEAM | G | F | G | G | G |
E=EXCELLENT； G=GOOD； F=FAIR； P=POOR
2）An O-ring is an automatic bidirectional sealing element. During installation, its pre-compression in both radial and axial directions endows the O-ring with initial sealing capability. This capability increases as system pressure rises.
| Classification | Static Sealing | Dynamic Sealing |
|---|---|---|
| Working Pressure | Without backup rings, it can reach up to 20MPa at most With backup rings, it can reach up to 40MPa at most With special backup rings, it can reach up to 200MPa at most |
Without backup rings, it can reach up to 5MPa at most With backup rings, for high pressures |
| Motion Speed | The maximum reciprocating speed can reach 0.5m/s, and the maximum rotational speed can reach 2.0m/s. | |
| Temperature | General occasions: -30℃～+110℃; Special rubber: -60℃～+250℃; Rotating occasions: -30℃～+80℃ | |
| Medium | See “Raw Material Characteristics Table of Rubber Seals”. |
3）Factors to consider when selecting O-ring seals
1. Working medium and working conditions
When selecting O-ring materials, the compatibility with the working medium should be considered first.
It is also necessary to consider the working conditions such as pressure, temperature, continuous working time, and operating cycle at the sealing point.
If used in rotating situations, the temperature rise caused by frictional heat must be considered.
Different sealing materials have different physical and chemical properties, as shown in the “Rubber Sealing Material Characteristics Table”.
2. Sealing form
According to the type of load, it can be divided into static seal and dynamic seal;
According to sealing purposes, it can be divided into hole seals, shaft seals, and rotary shaft seals;
According to its installation form, it can be divided into radial installation and axial installation.
When installing radially, for shaft seals, the deviation between the inner diameter of the O-ring and the sealed diameter d2 should be minimized as much as possible;
For hole sealing, its inner diameter should be equal to or slightly smaller than the diameter d1 of the groove.
When installing axially, the direction of pressure should be considered
When subjected to internal pressure, the outer diameter of the O-ring should be approximately 1-2% larger than the outer diameter d3 of the groove;
When subjected to external pressure, the inner diameter of the O-ring should be approximately 1-3% smaller than the inner diameter d4 of the groove.
4）Other factors affecting sealing performance
1. Hardness of O-ring
The hardness of O-ring material is the most important indicator for evaluating sealing performance.
The hardness determines the compression amount of the O-ring and the maximum allowable extrusion gap of the groove.
Due to the fact that the nitrile seal of Shore A70 can meet most of the usage conditions, no special instructions are given for the sealing material, and Shore A70 nitrile rubber is generally provided.
2. Squeezing gap
The maximum allowable extrusion gap gmax is related to system pressure, O-ring cross-sectional diameter, and material hardness.
Usually, the higher the work pressure, the smaller the maximum allowable extrusion gap gmax value.
If the gap g exceeds the allowable range, it will cause the O-ring to be squeezed out and damaged.
3. Permanent compression deformation
Another indicator for evaluating the sealing performance of O-rings is the compression permanent deformation of the material.
Under pressure, the O-ring, as an elastic element, undergoes elastic deformation. As the pressure increases, permanent plastic deformation occurs.
The compression permanent deformation d can be determined by the following equation:
In the formula: b0- original thickness (cross-sectional diameter W), b1- thickness under compression, b2- thickness after release.
Maximum Allowable Extrusion Gap
| Pressure (MPa) | O – ring Cross – sectional Diameter | 1.78 | 2.62 | 3.53 | 5.33 | 7.00 |
|---|---|---|---|---|---|---|
| Shore Hardness A70 | ||||||
| ≤3.50 | 0.08 | 0.09 | 0.10 | 0.13 | 0.15 | |
| ≤7.00 | 0.05 | 0.07 | 0.08 | 0.09 | 0.10 | |
| ≤10.50 | 0.03 | 0.04 | 0.05 | 0.07 | 0.08 | |
| Shore Hardness A80 | ||||||
| ≤3.50 | 0.10 | 0.13 | 0.15 | 0.18 | 0.20 | |
| ≤7.00 | 0.08 | 0.09 | 0.10 | 0.13 | 0.15 | |
| ≤10.50 | 0.05 | 0.07 | 0.08 | 0.09 | 0.10 | |
| ≤14.00 | 0.03 | 0.04 | 0.05 | 0.07 | 0.08 | |
| ≤17.50 | 0.02 | 0.02 | 0.03 | 0.03 | 0.04 | |
| Shore Hardness A90 | ||||||
| ≤3.50 | 0.13 | 0.15 | 0.20 | 0.23 | 0.25 | |
| ≤7.00 | 0.10 | 0.13 | 0.15 | 0.18 | 0.20 | |
| ≤10.50 | 0.07 | 0.09 | 0.10 | 0.13 | 0.15 | |
| ≤14.00 | 0.05 | 0.07 | 0.08 | 0.09 | 0.10 | |
| ≤17.50 | 0.04 | 0.05 | 0.07 | 0.08 | 0.09 | |
| ≤21.00 | 0.03 | 0.04 | 0.05 | 0.07 | 0.08 | |
| ≤35.00 | 0.02 | 0.03 | 0.03 | 0.04 | 0.04 |
4. Pre-compression amount
O-rings installed in grooves should have an initial pre-compression to ensure their sealing performance. The pre-compression relative to the cross-sectional diameter W varies depending on different applications. Typically, it ranges from 15% to 30% for static seals and from 9% to 25% for dynamic seals. Specific values can be referenced from the charts below.
5. Stretching and Compressing
When installing O-rings in grooves, they experience stretching or compressing. Excessive stretching or compressing can cause the O-ring’s cross-section to increase or decrease excessively. Stretching by 1% reduces the cross-sectional diameter W by approximately 0.5%. For hole seals, O-rings should be in a stretched state with a maximum allowable stretch of 6%; for shaft seals, O-rings should be compressed circumferentially with a maximum allowable circumferential compression of 3%.
6. O-rings as rotary shaft seals
O-rings can also be used for low-speed rotary movements and short-cycle rotating shaft seals. When the peripheral speed is below 0.5 m/s, the contraction of heated elongated rubber must be considered, so the inner diameter of the seal should be about 2% larger than the shaft diameter it seals. After installation in the groove, the seal ring experiences radial compression, forming slight undulations in the O-ring within the groove, thereby improving lubrication conditions.
7. Installation compression force
The installation compression force depends on the degree of initial compression and material hardness. The diagram below shows the relationship between unit compression force per centimeter of seal circumference and the cross-sectional diameter, used to estimate the total force required for installing O-rings.
5)Installation of O-rings
When installing O-rings, pay attention to the following:
– Ensure all edges or transitions are chamfered or deburred and burrs are removed.
– Check for defects on the sealing surfaces.
– Remove all machining residues.
– Protective sleeves should be added to threaded paths to prevent sharp edges from scratching the O-ring.
– To facilitate installation, lubricating grease can be applied to the O-ring installation surface.
– When installing manually, do not use sharp tools but try to effectively use tools to ensure the O-ring does not twist.
– Do not overstretch the O-ring.
– For O-rings made from sealing tapes, stretching at the connection points is prohibited.
– If automatic installation is performed, proper preparation must be done. For example: To facilitate installation, molybdenum, graphite, talc powder, or PTFE coating can be applied to the surface of the O-ring.