Figure 1 [3] – Cracked “O”- Ring

By Dan Herring

We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems.

Tip #4: Maintain Your O-Ring Seals

O-rings are an integral part of any successful vacuum system, however O-rings lose their elastic nature over time and eventually crack (Fig. 1) leading to air infiltration into the vacuum chamber. When replacing O-rings it is critical to use the correct type, normally specific by the original equipment manufacturer.

O-rings materials are chosen for specific locations depending on the anticipated temperature exposure during operation (Table 1). The most common general-purpose types are Buna N, Viton and Teflon.

Table 1 ^[2] – Operating Temperature Limits for O-ring Sealing Materials

Figure 2 [3] – Debris Ladden Door Seal

In order for the O-ring to function properly it must be smooth, free of debris (Fig. 2), rounded (as opposed to flat), crack or scratch free and properly lubricated. Whenever an O – ring seal is exposed, such as the opening and closing of the front door of a single chamber vacuum furnace, the O – ring should be wiped clean prior to each run. Carefully running ones finger in a tight fitting latex glove over the O – ring surface is a good way to detect minute defects or particles (e.g. dirt, metal fines) on the surface. The O – ring should be wiped clean with alcohol using a clean soft cloth then high-quality silicone vacuum grease applied in a very light coating to aid in sealing. Despite what one may think, excess vacuum grease does not aid in sealing and should be removed, as it is getter for shop debris, which will interfere with the O – ring’s ability to seal.

O-ring Failures ^[4]

O-ring seals often fail prematurely in applications because of either improper design or material selection. From the end-user’s point of view, a seal can fail in three (3) general ways:

• Leaking
• Contamination
• Change in Appearance

Contributing factors are pressure/vacuum induced stress and thermally induced stress. Elevated temperatures may cause seal degradation, swelling or outgassing. Pressure or vacuum environments (or altering between the two) can cause outgassing and weight loss.

O-ring failures can be classified into the following general categories:

1. Abrasion (Fig. 3a) – The seal or parts of the seal exhibit a flat surface parallel to the direction or motion. Loose particles and scrapes are often found on the seal surface. Contributing factors include: rough sealing surfaces; excessive temperature; process environment containing abrasive particles; dynamic motion and poor elastomer surface finish.
2. Flattening or Over Compression (Fig. 3b) – The seal exhibits a flat-sided cross-section, the flat sides corresponding to the mating seal surfaces. Contributing factors include: excessive compression; excessive temperature; incompletely cured elastomer; elastomers with high compression set and excessive volume swell.
3. Degradation – The seal exhibits blisters, cracks, pits, void or pockmarks on its surface. Absorption of gas at high pressure and the subsequent rapid decrease in pressure. The absorbed gas blisters and ruptures the elastomer surface as the pressure is rapidly removed. Contributing factors include: rapid pressure changes; low-modulus/hardness elastomers and incompatibility with the pressure/vacuum or thermal environment.
4. Extrusion (Fig. 3c) – The seal develops ragged edges (generally on the low-pressure side) that appear tattered. Contributing factors include: excessive clearances; excessive pressure; low-modulus/hardness elastomers, excessive gland fill; irregular clearance gaps, sharp gland edges and improper sizing.
5. Installation damage (Fig. 3d) – The seal or parts of the seal may exhibit small cuts, nicks or gashes. Contributing factors include: Poor techniques; improper tools, sharp edges on glands or components; improper sizing of elastomer; low-modulus/hardness elastomer and elastomer surface contamination.
6. Over compression (Fig. 3e) – The seal exhibits parallel flat surfaces (corresponding to the contact areas) and may develop circumferential splits within the flattened surfaces. Contributing factors include: improper design (failure to account for thermal volume changes or excessive compression).
7. Thermal Degradation – The seal may exhibit radial cracks located on the highest temperature surfaces. In addition, certain elastomers may exhibit signs of softening—a shiny surface as a result of excessive temperatures. Contributing factors include: elastomer thermal properties; excessive temperature excursions or cycling.
8. Plasma Degradation (Fig. 3f) – The seal often exhibits discoloration, as well as powdered residue on the surface and possible erosion of elastomer in the exposed areas. Contributing factors include: Chemical reactivity of the plasma. Ion bombardment (sputtering). Electron bombardment (heating). Improper gland design. Incompatible seal material.
9. Other – Spiral failure (Fig. 3g) in which the seal exhibits cuts or marks which spiral around the circumference. Contributing factors include: difficult or tight installation (static); slow reciprocating speed; low-modulus/hardness elastomer; irregular O-ring surface finish (including excessive parting line); excessive gland width; irregular or rough gland surface finish and inadequate lubrication.

Figure 3 ^[3] – Typical O-ring Failures

(a) Failure Due to Abrasion	(b) Failure Due to Compression (i.e. Flattening)
(c) Failure Due to Extrusion	(d) Failure Due to Installation Damage
(e) Failure Due to Over-Compression	(f) Failure Due to Arcing	(g) Failure Due to Over Pressure

Common Misconceptions

Here are some of the most common misconceptions when working with O-rings:

1. One of the most common beliefs is that the vacuum grease applied to the O-ring surface is responsible for actual sealing and that the more grease present, the better (and longer) the seal will last. In reality, the layer of vacuum grease is intended only to act as a lubricant to seat properly under the applied compression forces and should be very thin layer, so much so that when you move your finger along the O-ring surface it will slide or glide unhindered with no appreciable amount of grease build up.
2. O-rings do not need to be re-greased after every run. False. O-rings should be wiped clean followed by running a finger (protected by a latex glove) over the surface of the O-ring to feel for damage. This is an excellent way to detect minute particles of dirt or grit and reveal nicks or areas that should be cleaned and re-greased. Thoroughly cleaning the surface of the O-ring with alcohol, methyl-ethyl-keytone (MEK) or acetone before re-greasing (or applying vacuum grease initially) is critical. Be careful to use only a company-approved solvent.
3. The shelf life of a typical O-ring is forever. In reality, O-rings have a shelf life of only about six (6) months.
4. O-rings cannot be spliced. Yes, they can as long as the part line is in the plane of the cross section. They must be joined together by gluing the material with a proper adhesive.

Next Time: Even more tips for enhancing vacuum operation.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: dherring@heat-treat-doctor.com

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

Reference

1. Herring, Daniel H., Vacuum Heat Treatment, BNP Custom Media Group, 2012.
2. Moyer, Michael, Keeping it Bright, ASM International Vacuum Maintenance Seminar, Anaheim, CA, 2008.
3. Herring, Daniel H., The Ubiquitous O-Ring, Industrial Heating, November 2009.
4. Marco Rubber & Plastic Products, Inc. (www.marcorubber.com), private correspondence.
5. The Vacuum Technology Book, Volume 1, Pfeiffer Vacuum (www.pfeiffer-vacuum.net)