O-ring Damage Is Simply Impossible
Failure can lead to the leakage of machinery and equipment, but when a unique new failure mode appears, the need for some additional research. In this case, a combination of various factors can create a new mechanism for the failure of O-ring.
In practical applications, aircraft fuel pump manufacturers contacted Parker Hannifin for help to solve a series of abnormal O-gasket failure leading to fuel leakage problems. June 2008 and July, three different aircraft fuel pumps are carried out daily maintenance. In all three examples, the O-type gaskets purchased from the accessories market and the fuel pump were assembled before the aircraft service, passing the initial leak test. Within two months, all repaired pumps have signs of fuel leaks. After disassembly, all three O-ring are broken at the groove, and each break produces a 90 ° bend
A critical step in this failure analysis is a thorough examination of the mechanical damage to the O-ring. Obviously, all three O-ring have been damaged, which will lead to leakage. Surprisingly, all three O-ring were ruptured in the direction of the pressure source, rather than deviating from it. However, ignoring other causes that may lead to failure may lead to the neglect of more critical factors that can lead to the best possible precautionary approach. In the example of this article, O-washers do not produce other mechanical damage. For example, this O-ring does not have evidence of pressure-related extrusion failure, when the fluid pressure exceeds the strength of the material, the rubber will be squeezed into the gap between the accessories, leaving "damage" Rubber area. This is clearly not present on this O-ring. And there is no indication that the O-washers have produced any way of compression between the assembled parts.
In addition, the rubber material does not have any signs of chemical decomposition or thermal degradation. Unlike other damage points, the O-ring has no significant expansion, hardening, softening, blistering, compression deformation, embrittlement or loss of tensile strength. Very simple, no damage compared to other obvious damage. For these reasons, the chemical and thermal incompatibility of these sealing materials can be proven to be a potential factor in the underlying cause of failure.
O-type gasket failure is most often seen in order to meet the application, O-washers are stretched during the assembly process. In this case, although the O-washers passed the initial leak test and were used for several weeks. For the damaged O-washers it is not possible. Therefore, O-type gaskets must be no damage during installation and initial pressure testing. This means that the O-ring is certainly damaged during use, that is, it is very rare in a static O-ring washer after it is installed in the tank. Destruction of this type of use can usually be traced back to one or two reasons.
The most common cause of the use of O-washers is torn, which is spread from the initial twist point. In this case, a part of the O-ring is crushed in the fitted metal part, usually when the groove is accidentally ejected during the installation. O-ring that are damaged in this manner will show signs of significant mechanical damage, including partial crushing, partial missing on O-ring, and semi-circular cuts in cross-section. In some cases, a tightened O-ring will pass through the initial leak test and is damaged during use, due to the initial shrinkage of the O-ring. However, there is no evidence of any shrinkage damage in these failed O-ring.
The less common use of the O-ring was damaged due to the molding defects in the O-ring. If the molten rubber material flows and sticks during the molding process, a mechanical defect is left in the O-ring, which is easily damaged under pressure. These can usually be diagnosed by studying the damaged samples in use. If the material does not achieve good bonding, the damaged parts will show "spherical - and - groove" damage form, with the match is convex and concave shape. O-shaped gasket inside the bubble so that the concave shape of different, can make a smooth, round and even wavy. And in this case the damaged O-ring has a very sharp and clean gap, as if by a sharp knife cut the same. O-ring manufacturing defects lead to this type of damage is simply impossible. This sharp, smooth fracture specimen is consistent with the elongation limit beyond the material.