Rectangular Nitrile Gaskets
Rectangular nitrile gaskets larger than sheet stock on hand can be fabricated by cutting strips and corners with a table saw or a utility knife with razor blade. Applying a little transformer oil or WD-40 oil makes cutting easier. Nitrile is also available in spools in standard ribbon sizes. The ends may be joined using a cyanoacrylate adhesive (super glue). Although there are many types of this glue, only a few of them work well with nitrile. Since all these glues have a very limited shelf life, remember to always keep them refrigerated to extend their shelf life. Lawson Rubber Bonder No. 92081 best withstands temperature changes and compression. The Lawson part number is 90286, and it is available from Lawson Products Co. in Reno, Nevada, (702-856-1381). Loctite 404 also works but does not survive temperature variations as well. It is available at NAPA Auto Parts Stores. Shelf life is critical. Always secure a new supply when a gasketing job is started; never use an old bottle that has been on the shelf since the last job.
Maximum horizontal fill of the groove should be 75 to 85%, as explained above in the circular gasket section. However, it is not necessary to fill the groove fully to 75% to obtain a good seal. Choose the width of ribbon that comes close to, but does not exceed, 75 to 80%. If one standard ribbon width fills only 70% of the groove and the next size standard width fills 90%, choose the size that fills 70%. As in the circular groove explained above, place the gasket so that expansion space is equal on both sides. The key point is that the cross-sectional area of the gasket remains the same while the cover is tightened; the thickness decreases, but the width increases. See below and figure 45.
Nitrile (buna N) is a synthetic rubber compound and, as cover bolts are tightened, the gasket is compressed. Thickness of the gasket is decreased, and the width is increased. If a gasket is too large, rubber will be pressed into the void between the cover and the sealing surface. This will prevent a metal-to-metal seal, and a leak will result. It is best if the cross-sectional area of the gasket is a little smaller than the groove cross-sectional area. As cover bolts are tightened, the
thickness of the gasket decreases, but the width increases so that cross-sectional area (thickness times the width) remains the same. Care must be taken to ensure that the gasket cross-sectional area is equal to or slightly smaller (never larger) than the groove cross-sectional area. This will provide space for the rubber to expand in the groove so that it will not be forced out into the metal-to-metal contact area (see figure 45). If it is forced out into the “metal-to-metal” seal area, a leak generally will be the result. When this happens, our first response is to tighten the bolts, which bends the cover around the gasket material in the metal-to-metal contact area. The leak may stop (more often, it will not); but the next time the cover is removed, getting a proper seal is almost impossible because the cover is bent. Take extra care to correctly size the gasket to prevent these problems.
Figure 45 – Cross Section of Gasket Remains Constant Before Tightening and After. wxd = gw x gt
On some older bushings used on 15-kV voltages and above, it is necessary to install a semiconductive gasket. This type bushing (such as GE type L) has no ground connection between the bottom porcelain skirt flange and the ground ring. The bottom of the skirt is normally painted with a conductive paint, and then a semiconductive gasket is installed. This allows static electric charges to bleed off to the ground. The gaskets are typically a semiconductive neoprene material. Sometimes, the gasket will have conductive metal staples near the center to bleed off these
charges. When replacing this type gasket, always replace with like material. If like gasket material is not available, use cork-neoprene.
Failure to provide a path for static electric charges to get to the ground will result in corona discharges between the ground sleeve and the bushing flange. The gasket will be rapidly destroyed, and a leak will result.
When bonding the ends of ribbon together, the ends should be cut at an angle (scarfed) at about 15 degrees. The best bond occurs when the length of the angle cut is about four times the thickness of the gasket (see table 5). With practice, a craftsperson can cut 15-degree scarfs with a utility knife. A jig can also be made from wood to hold the gasket at a 15-degree angle for cutting and sanding. The ends may be further fine-sanded or ground on a fine bench grinder wheel to match perfectly before applying glue.
Thin metal conductive shim stock may be folded over the outer perimeter around approximately one-half the circumference. These pieces of shim stock should be evenly spaced around the circumference and stick far enough in toward the center so that they will be held when the bolts are tightened. As an example, if the gasket is 8 inches in diameter, the circumference would be πD or 3.1416 times 8 inches = 25.13 inches in circumference. Fifty percent of 25.13 is about 12½ inches. Cut 12 strips 1-inch wide and long enough to be clamped by the flange top and bottom when tightened. Fold them over the outside edge of the gasket, leaving a little more than a 1-inch space between, so that the shim stock pieces will be as evenly spaced as possible around the circumference.