|Bolt holes close to edge||Causes breakage in stripping and assembling.||Projection or "ear"|
Notch instead of hole.
|Very small bolt holes or non-circular openings.||Require handpicking; easy to miss.||Avoid hole sizes under 3/32" diameter. If small hole is for locating or indexing, change to notch.|
|Tear-away parts with open slots at attached edges.||Slots require handpicking and costly programming charges.||Simple perforation|
|Thin walls, delicate cross-section in relation to overall size.||High scrap loss; stretching or distortion in shipment or use. Restricts choice to high tensile strength materials.||Have the gasket in mind during early design stages.|
|Metalworking tolerances applied to gasket thickness, diameters, length, width, etc.||Results in perfectly usable parts being rejected at incoming inspection. Requires time and correspondence to reach agreement on practical limits. Increases cost of parts and tooling. Delays delivery.||Most gasket materials are compressible. Many are affected by humidity changes. Try standard or commercial tolerances before concluding that special accuracy is required.|
|Transference of fillets, radii, etc. from mating metal parts to gasket.||Unless part is molded, such features mean extra operations and higher costs.||Most gasket stocks will conform to mating parts without preshaping. Be sure radii, chamfers, etc. are functional, not merely copied from metal members.|
|Large gaskets made in sections with beveled joints||Extra operations to skive or glue. Difficult to obtain smooth, even joints without steps or transverse grooves.||Precision WaterJet cut dovetail joints.|
| The gasket's function is to seal two different surfaces held together by one of several means, the most common being screw-threaded devices such as bolts. Sometimes the fastener itself must be sealed, as in the case of a steel drum bung.|
The bolt is a spring. It is an elastic member that has been stretched to develop a load. The more spring provided by the bolt, the better the retention of stress on the gasket to maintain a leakproof joint. It must not be over-elongated (over-strained), or the elastic limit of the steel will be exceeded. The bolt then deforms and, with continued loading (stressing), may rupture.
To avoid such problems with bolt tightening, the use of a torque wrench is recommended. The equipment designer normally specifies the torque required for a product. This ensures that the maximum available load is applied consistently to the gasket. The load will be better retained by using a bolt with a longer grip, thereby ensuring a leakproof joint.
There are limits on the degree of flange surface imperfection that can be sealed successfully with a gasket. Large nicks, dents or gouges must be avoided, since a gasket cannot properly seal against them. The surface finish of a flange is described as follows:
1. Roughness: Roughness is read in millionths of an inch as the average of the peaks and valleys measured from a midline of the flange surface. This is expressed either as rms (root mean square) or AA (arithmetic average). The difference between these two methods of reading is so small that they may be used interchangeably. Roughness is also expressed as AARH (arithmetic average roughness height).
2. Lay: Lay is the direction of the predominant surface-roughness pattern. Example: multidirectional, phonographic spiral serrations, etc.
3. Waviness: Waviness is measured in thousandths of fractions of an inch. Basically, it is the departure from overall flatness.
Typical roughness readings can be from 125 to 500 micro-inches for serrated flanges and 125-250 micro-inches for non-serrated flanges. Fine finishes, such as polished surfaces, should be avoided. Adequate "bite" in the surface is required to develop enough friction to prevent the gasket from being blown out or from extruding or creeping excessively.
The lay of the finish should follow the midline of the gasket, if possible. Take, for example, concentric circles on a round flange, or a phonographic spiral. Every effort should be made to avoid lines across the face, such as linear surface grinding, which at 180° points will cross the seal area at right angles to the gasket, allowing a direct leak path.
Waviness is seldom a problem under normal conditions. There are two areas that must be watched, however, since excessive waviness is very difficult to handle.
The first area is glass-lined equipment where the natural flow of the fused glass creates extreme waviness. Often the answer here is to use thick and highly compressible gasketing.
The second area of concern is warped flanges. If warpage is caused by heat or internal stresses, re-machining is generally sufficient. However, warpage due to excessive bolt loads or insufficient flange thickness results in what is generally called bowing. (See example of bowing pictured below).
The solution is to redesign for greater flange rigidity. Sometimes backer plates can be added to strengthen the design without having to replace the parts. Another step would be to add more bolts. When this is done, usually smaller bolt diameters are possible, thus adding more bolt stretch and better joint performance.
|Bowing of flanges because of excessive bolt load or insufficient flange thickness:|
|Correct bolting patterns:|
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