Richard Renneboog, Sealing Machined Surfaces


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Sealing Machined Surfaces

Richard M. J. Renneboog
Information Technology Developer / Webmaster
Renaissance Aeronautics Associates Incorporated

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Don't blow a gasket! Who hasn't heard this old adage at some time? What does it actually mean, and for that matter, what is a gasket?

Engines contain gaskets for sealing machined surfaces

Image courtesy NOAA

Gaskets are simple structures used to fill in and seal the spaces where two surfaces meet, usually to prevent the leakage of a material under pressure. A good example is the interfacing of two machined flat surfaces, as occurs with various gasoline or diesel engine parts. The basic structure of any typical automobile engine is the engine block, containing the combustion cylinders, pistons, connecting rods, camshaft, and crankshaft components. Cylinder heads must be affixed to this basic unit, as must be the intake and exhaust manifolds, the oil pan, coolant circulating pump, and the transmission housing. These various add-on components all meet the engine block at machined flat surfaces, and each of these interfaces involves fluids under pressure: air and fuel intake gases, combustion exhaust gases, coolant, and lubricating oil. Unless the appropriate surfaces mate properly to form a tight seal, each of these fluids would leak out between the various parts.

The proper and continued functioning of an automobile engine, and most other machinery, requires that no fluids leak in or out in an uncontrolled manner. To prevent such leaks, the various machined surfaces must have all gaps and spaces between them perfectly filled and sealed. Machined surfaces, although quite smooth, contain numerous small imperfections and may not be true from one end to the other. When two such surfaces are brought together, it is generally true that they cannot form a tight seal against each other without being placed under undue or excessive stress when the bolts are tightened to join the two pieces. It is entirely possible to machine surfaces of parts so that a nearly perfect surface match between them is achieved, but this is a very expensive proposition, and does not work well in the context of any fast-paced high-production industry. To reduce machine operations and the associated costs, gaskets are commonly used to mate flat surfaces.

A gasket is a thin layer of material that readily conforms to the surface of the material around it, and ideally does not interact with the fluids that it must contain. The shape of the gasket matches the shape of the two surfaces that it joins. The gasket material deforms under the applied pressure to fill in the tiny imperfections and compensate for any lack of trueness in the machined surfaces. This ensures that fluids passing from one part to another do not leak out into the environment. Generally the more pressure that can be safely applied to a gasket, the better it serves.

"To blow a gasket" means that it has failed during operation and allowed pressurized fluids to blow out of the machine. Occasionally the proper pressure is not maintained on the gasket or the gasket material breaks down. Bolts can vibrate loose, or stretch under the effects of prolonged tension and operating conditions, resulting in the loss of proper pressure on the gasket. This can result in the breakdown of the gasket function at a particular location. A fluid leak can result, with fluid "blowing out" under pressure, and the effects can range from a relatively innocuous but noisy exhaust gas leak to a severely damaging internal coolant or oil leak.

Similar considerations and restrictions apply wherever flat, machined surfaces are joined face to face.

The Challenger disaster was caused by a failed O-ring

Image Courtesy NSSDC

So when is a gasket not a gasket? 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.

Materials used for gaskets are many and varied. Gaskets can be and have been made from practically any available material. Paper, hard fiber sheet, cork, rubber, neoprene and other plastics, aluminum, brass, and steel all see common use as gasket material, along with a very large assortment of specialized and specially manufactured materials. Under normal use, the restrictions on what materials can be used for gaskets are relatively few. Because gaskets cover a very broad, flat surface area in comparison to their thickness, and are maintained under a very high applied pressure between the two mated parts, gasket materials essentially only have one restriction. The materials must not interact chemically with the fluids they are meant to contain. For example, gaskets made from a combustible material such as hard fiber sheet would fail very quickly if they had to contain very hot fluids such as the exhaust gases from an automobile engine. This is why exhaust manifold gaskets and head gaskets are made of metal.

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.

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