For dynamic sealing, O-rings still remain the simplest solution for sealing. While they may come with additional considerations, O-rings can functionally perform in properly designed reciprocating and rotary glands.
For O-rings in reciprocating glands, the recommended squeeze is reduced by about 7% for all cross sections. So, for a ring with a 0.070” CS, a static seal has a recommended squeeze from 22-32%, but for a dynamic seal, the same size ring has a recommended squeeze of 15-25%. Additionally, O-rings are preferably used in applications where the stroke is short and the diameter of the ring is small. Following this general guideline will help prevent spiral failure, the most common concern when it comes to O-ring in reciprocating glands. Spiral failure is caused by the O-ring rolling in the groove and getting twisted as opposed to translating along with the gland. This puts additional stresses on the ring and can lead to damaged rings.
As for rotary seals, many of the same considerations apply as reciprocating seals. Squeeze should be reduced, but by an even greater amount. A 0.070” CS ring should have 1 – 11% squeeze in a rotary application, compared to 22-32% in a static application. Due to surface area and frictional considerations, O-rings with cross sections larger than 0.139” are not recommended for rotary seals and all rotary seals designed with O-rings should be a female gland, with the ring being held in the bore rather than the piston. The largest consideration when designing a rotary seal is the feet per minute calculation (see chart). This simple equations helps determine what size ring will work best in a given application based on the diameter of the shaft and speed of rotation.
Finally, material selection is critical for O-ring dynamic sealing. Ideally, the material should be an 80 durometer or above, as a harder ring will give less frictional resistance than a softer ring. Additionally, Parker offers several compounds that are internally lubricated and will maintain lower frictional resistance continually as they perform in application. Below is a list of recommended compounds for dynamic sealing. The compounds shown below are recommended based on their internal lubrication and/or wear resistance, both being advantageous traits of dynamic seals.
EPDM: E1022, E1244, EJ274
NBR: N1090, NW163, N0818
FKM: VA163, V0848
N1090 is our Extreme Low Friction (ELF) compound, which offers the lowest frictional resistance of any compound Parker offers. We recommending using this compound for any applicable application.
|O-Ring Sections for Rotary Seals|
|Speed (fpm*)||Maximum Recommended "W" Dimension|
|0 to 200||Usually not critical (see ORD 5700)|
|200 to 400||.139|
|200 to 600||.103|
|200 to 1500||.070|
|*Feet per minute = 0.26 x shaft diameter (inches) x rpm|