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Bolting and Flange Information The gaskets function is to seal two different surfaces in the surface is required to develop enough friction to held together by one of several means, the most common prevent the gasket from being blown out or from extruding being screw-threaded devices such as bolts. Sometimes or creeping excessively. the fastener itself must be sealed, as in the case of a steel The lay of the finish should follow the midline of the drum bung. gasket if possible. Take, for example, concentric circles The bolt is a spring. It is an elastic member that has on a round flange, or a phonographic spiral. Every effort been stretched to develop a load. The more spring pro- should be made to avoid lines across the face, such as vided by the bolt, the better the retention of stress on the linear surface grinding, which at 180� points will cross the gasket to maintain a leakproof joint. It must not be over- seal area at right angles to the gasket, allowing a direct elongated (over-strained), or the elastic limit of the steel leak path. will be exceeded. The bolt then deforms and, with contin- Waviness is seldom a problem under normal condi- ued loading (stressing), may rupture. tions. There are two areas that must be watched, however, To avoid such problems with bolt tightening, the use since excessive waviness is very difficult to handle. of a torque wrench is recommended. The torque tables The first area is glass-lined equipment where the on page C-44 show the recommended torque values for � � natural flow of the fused glass creates extreme waviness. Garlock compressed sheet, GYLON and GRAPH-LOCK Often the answer here is to use thick and highly compress- gasketing materials in 150 lb. and 300 lb. raised face ible gasketing. flanges. The equipment designer may specify the recom- mended torque to prevent damage to the equipment from The second area of concern is warped flanges. If overtorquing. Garlocks recommended assembly stresses, warpage is caused by heat or internal stresses, re-ma- page C-43, may help the equipment designer determine chining is generally sufficient. However, warpage due to the maximum allowable torque per bolt. The load will be excessive bolt loads or insufficient flange thickness results retained better by using a bolt with a longer grip, thereby in what is generally called bowing. ensuring a leakproof joint. The solution is to redesign for greater flange rigidity. There are limits on the degree of flange surface Sometimes backer plates can be added to strengthen the imperfection that can be sealed successfully with a gasket. design without having to replace the parts. Another step Large nicks, dents, or gouges must be avoided, since a would be to add more bolts. When this is done, usually gasket cannot properly seal against them. The surface fin- smaller bolt diameters are possible, thus adding more bolt ish of a flange is described as follows: stretch and better joint performance. 1. Roughness: Roughness is read in millionths of an inch (or meter) as the average of the peaks and valleys Questions?CallGasketApplicationsEngineering measured from a midline of the flange surface. This is ex- at1-800-448-6688 pressed 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 aver- age roughness height). 2. Lay: Lay is the direction of the predominant surface- roughness pattern. Example: multidirectional, phonograph- ic spiral serrations, etc. 3. Waviness: Waviness is measured in thousandths or fractions of an inch. Basically, it is the departure from � overall flatness. Typical roughness readings can be from 125 to 500 GarlocGarlockk micro-inches for serrated flanges and 125-250 micro- Gaskets inches for non-serrated flanges. Fine finishes, such as Gaskets polished surfaces, should be avoided. Adequate "bite" C-42
Goodyear Rubber Products
