For Want Of An O-Ring
Who can forget the Challenger disaster of 1986, the culprit, a failed O-ring. But what exactly is an O-ring and how did it cause the destruction of this space shuttle? When surfaces are flat, gaskets are used to form a tight seal. How about when the machined surfaces are not flat but round? The sealing function in that case is served by an O-ring. O-rings are commonly used in hydraulic and pneumatic applications, often at very high pressures. But while an O-ring nominally serves the same purpose as a gasket, it functions in an entirely different manner. A gasket must be compressed strongly to make it fill in any inconsistent regions on flat surfaces. Compressing an O-ring in the same manner as a gasket completely defeats the functioning of the O-ring. The O-ring becomes flattened and is destroyed. Unfortunately, there are many technicians out there who never seem to learn that lesson.
The proper use of an O-ring as a pressure seal is very much a balancing act. The O-ring is designed to meet certain strength specifications and material applications, and when properly selected and applied will provide a sure seal against high fluid pressures. The trick is to apply just enough pressure to the joint to cause the O-ring material to seat against the surfaces and to stiffen against the pressure exerted by the fluid it must contain. As pressure is applied through tightening the joint, the O-ring material compresses somewhat to fill the space available to it in a specially machined groove. It becomes stiffer and unable to shift under the influence of fluid pressures, thus securing the seal. Over-tightening results in over-compression and deformation that destroys the O-ring and the seal and allows fluids to leak, possibly with dire consequences.
The restrictions on O-ring materials are more stringent. Because of the way in which O-rings function, the materials from which they are made must not be rigid materials. O-rings must be chemically inert to fluids such as hydraulic oils, organic solvents, and a variety of acidic and caustic water-based solutions. This leaves only special rubber and plastic formulations, usually silicon-based. Unlike gaskets, O-rings must be made to precision dimensions and with close attention paid to uniformity of shape. An O-ring that does not meet these requirements will certainly fail at the first opportunity. In the case of the Challenger, the cause of the failure was the temperature. On the morning of the Challenger launch, the temperature was below freezing, causing the O-rings to become hard and lose their flexibility. The result was a catastrophic leak of fuel which, when ignited, engulfed the entire shuttle in superheated flames. A devastating result due to the failure of an O-ring.
About the Author
Richard M J Renneboog, MS
Richard M. J. Renneboog is an independent private technical consultant and writer in both chemical and computer applications. Endeavors have included preparation of scripts for instructional and promotional video, corporate website design, curriculum development for training in advanced composites technology, and development.