Machinery's Handbook, 31st Edition
2680 O-RINGS extrusion of the O-ring into the surrounding clearance gap is likely to occur, reducing the life of the O-ring. If conditions of use fall to the left of the curve, no extrusion should occur, in which case the O-ring can be expected to perform adequately. As the graph indicates, high-pressure applications require smaller clearances and harder O-rings for effective sealing. As an example, in an O-ring application with a 0.004 in. (0.1 mm) diametral clearance and 2500 psi (17.24 MPa) pressure, extrusion will occur with a 70 Shore A durometer O-ring but not with an 80 Shore A durometer O-ring. Published gland dimensions nor- mally apply to standard (70 Shore A) hardness O-rings and may need to be adjusted for use with harder materials. Backup Rings.— Also referred to as anti-extrusion rings, backup rings are strongly rec- ommended to prevent O-ring extrusion and nibbling in applications with pressures above 1500 psi (10.3 MPa), where pressure reversals are common, or where large clearance gaps are necessary. ( Nibbling occurs when the O-ring extrudes into the clearance gap and then is pinched as the clearance gap opens and closes dynamically.) Backup rings are installed in the gland on one or both sides of the O-ring as shown in Fig. 5. Backup rings may be made of leather, hard rubber, synthetic polymers such as phenolic or PTFE, various metals, or other hard materials. Two backup rings are required if pres- sure reversals will occur. (If space permits only one backup ring in an application with constant pressure direction, it should be placed so the O-ring is pushed into it when pres- sure is applied.) Fig. 5. Preferred High-Pressure Configuration Including Backup Rings Groove Width and Slope.—Groove widths are standardized to create a gland that ap- proaches 85 percent filled when the O-ring is installed, assuming no swell, straight (per - pendicular) walls, and no radii or edge breaks. If one or more backup rings are used, groove width must be increased as needed. If nonstandard gland dimensions are used in a customized application, gland fill must be evaluated to ensure groove width is sufficient. Groove widths for many O-ring sizes are shown in Table 2 and Table 3. Perpendicular groove walls are best for preventing extrusion or nibbling, but sloped walls often facilitate machining and may be specified as a cost-saving measure. Groove walls sloped outward up to 5 degrees from perpendicular generally are acceptable in low- pressure applications (less than 1500 psi or 10.34 MPa). O-Ring Stretch, Cramping, and Cross Section Reduction.— Stretch occurs when the inner diameter of an installed and squeezed O-ring is smaller than that of the hous- ing groove or rod. When an elastomeric O-ring is stretched, its cross-sectional area decreases. Cramping, also called cramming, occurs when the outer diameter of an in- stalled and squeezed O-ring is larger than that of the housing groove or bore. Interference caused by stretch or cramping provides some zero-pressure sealing force, stabilizes the O-ring, and works to hold it in place. Stretch and cramping allowance often is traded against the economic value of using standard stock and tooling sizes. Excess stretch or cramping can be successful; it can be found in the gland dimensional standards for some O-ring sizes. (Relevant stretch and cramping equations can be found for each gland type beginning on page 2682.)
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online