BRIEF DESCRIPTIONS OF THE DRAWINGS
FIG. 1 is an exploded partially sectionalized view of a pipe joint in a plastic pipeline showing the sealing ring located within the female pipe end and the mating male pipe end.
FIG. 2 is a schematic representation of the problem of overinsertion of the male plastic pipe end within the mating female pie end in a plastic pipe system.
FIG. 3 is a partial, cross-sectional view of a portion of a pipe joint showing how the problem of overinsertion occurs.
FIG. 4 is a graphical representation of the forces involved in making up a pipe joint showing the peak in the stress curve.
FIG. 5 is a simplified schematic of a pipe joint showing the relevant contact angles of the male and female pipe ends which can be modified to lessen the possibility of overinsertion.
FIG. 6 is a graphical analysis of the Von Mises stress levels in the spigot and the bell under 100 kN overinsertion force.
DETAILED DESCRIPTION OF THE INVENTION
Turning to FIG. 1, there is shown an exploded view of a plastic pipe joint in which a belled female pipe end 10 is provided with an annular groove 12 for receiving an elastomeric sealing gasket 14. The annular sealing gasket 14 is a ring shaped member which, in cross section, has a compression seal region 16 and a trailing seal region 18. The gasket may be reinforced with a steel ring 20 which circumscribes the gasket body at one circumferential location. The sealing regions 16, 18 contact the exterior surface 22 of the mating male pipe section 24 upon assembly of the joint. During the assembly process, the male pipe end 24 travels to the left along the longitudinal axis 28 of the female, bell pipe end 10. Both of the pipe sections 10, 24 are formed of PVC. In the example illustrated in FIG. 1, the mating male pipe end 24 has a chamfered lip region 26. The sealing gasket is preferably made of a resilient elastomeric, thermoplastic material. For example, the sealing gasket may be formed of natural or synthetic rubber, such as SBR, or other elastomeric materials which will be familiar to those skilled in the plastic pipe arts such as EPDM or nitrile rubber. As will be apparent from the description which follows, any number of specialized sealing rings can be utilized in order to optimize the sealing function of the assembly.
The belled pipe end 10 may be formed by the so called “Rieber” process, familiar to those skilled in the waterworks industries. In the early 1970's, a new technology was developed by Rieber & Son of Bergen, Norway, referred to in the industry as the “Rieber Joint.” The Rieber system employed a combined mold element and sealing ring for sealing a joint between the socket end and spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, the elastomeric gasket was installed within a simultaneously formed internal groove in the socket end of the female pipe during the pipe belling process. The provision of a prestressed and anchored elastomeric gasket during the belling process at the pipe factory provided an improved socket end for a pipe joint with a sealing gasket which would not twist or flip or otherwise allow impurities to enter the sealing zones of the joint, thus increasing the reliability of the joint and decreasing the risk of leaks or possible failure due to abrasion. The Rieber process is described in the following issued United States patents, among others: U.S. Pat. Nos. 4,120,521; 4,061,459; 4,030,872; 3,965,715; 3,929,958; 3,387,992; 3,884,612; and 3,776,682.
FIG. 2 of the drawings is a simplified illustration of the forces at work in a typical plastic pipeline installation which can lead to the problem of “overinsertion.” The PVC pipe joint shown in FIG. 2 is made up of a female, belled pipe section 10 and a male, spigot pipe end 24, as described with reference to FIG. 1. When the spigot is “stabbed” into the mating socket to make the connection, the pipes are assembled by a thrust force “Q.” At the present time in the industry, the male pipe has a “witness mark” on its exterior surface. This mark theoretically ensures that the backhoe operator will not overinsert the male pipe into the female, belled pipe end. However, any carelessness or inadvertence on the part of the backhoe operator may result in an excessive longitudinal thrust force “Q” being applied by the spigot against the female bell. If the connection is tight, internal pressure cannot reach the gasket. As a result, internal pressure fluctuations on the spigot cause undesirable concentrated stresses against the bell. Further, if the spigot is “jammed” into the throat of the bell during assembly of the joint, allowable joint deflection is reduced by approximately one half. With reference to FIG. 2, the longitudinal thrust “Q” imposes a radial force “q” on the 45° surface illustrated, which wedges the bell end outwardly and tends to shear the bell from the pipe, the radial force being:
q=Q/πD
where “D” is the bell diameter at that point.
FIG. 3 is another simplified illustration of the assembly forces encountered during the make up of a plastic pipe connection. When the beveled end 26 of the male, spigot pipe end reaches the bottom wall of the socket (generally at 28 in FIG. 3), the spigot acts upon the socket as a wedge. In FIG. 3, the bottom wall 28 forms an angle α of approximately 15° with respect to the internal diameter of the pipe wall 29. With a typical 15° angle between the taper of the male pipe and the bottom of the bell, the wedge effect is almost a factor of four. This means that, if a net force (after that which is taken out by seal friction) reaches the bottom of the socket pipe end, the resulting radial force which is attempting to force the socket open will be approximately four times greater, e.g., 3.9 and 3.7, respectively, in FIG. 3. This may be enough force to damage the bell pipe end and compromise the connection.
As briefly mentioned, current practice is to use a “witness mark” on the exterior surface of the male, spigot pipe end in order to lessen the possibility of overinsertion during joint make up. However, in practice, even if the male pipe is only installed up to the witness mark, overinsertion can occur on the joints immediately behind the first joint. This is due to the fact that there is a peak in the assembly force during make up, illustrated graphically in FIG. 4. As shown in FIG. 4, this peak is typically more than twice the final assembly force. When the joint reaches this peak, the force transmitted to the trailing pipes is greater than the resistance from the installed sealing gaskets. While a certain force is applied to overcome peak resistance from the sealing gasket, if the receiving pipe is not anchored, all of this force is transmitted to the joint behind. The seal in the joint behind is fully installed, so it will take out at most about 50% of this force by friction. The remainder of the force is the overinsertion force.
FIG. 5 is a schematic illustration of a typical belled pipe end 10 and mating male, spigot pipe end 24 illustrating a seal with a sustained assembly force. Theoretically, if the assembly force is sustained after it reaches the peak illustrated in FIG. 4, then the joints behind will offer at least the same resistance as the joint being assembled. This effect should reduce the incidence of overinsertion. In the present invention, the problem of overinsertion is addressed in FIG. 5 of the drawings by modifying the internal geometry of the belled end 10. FIG. 5 illustrates the approach to the problem in which the female pipe belled end 10 forms an internal socket with a socket bottom wall 23, and wherein an interface angle β exists between the nose of the male pipe end and the socket bottom wall 23, the interface angle being increased by a predetermined amount in order to provide the control mechanism for preventing overinsertion of the male pipe within the female pipe opening. This could be accomplished by modifying the belling mandrel so that it will render a sharp angle at the bottom surface of the socket, thereby reducing the wedge effect. For example, with reference to FIG. 5, if the interface angle β between the spigot and the bottom of the socket 23 is increased from 15° to 60° (i.e., the surface 23 forms a sharper angle), then the wedge effect would become about six times smaller.
FIG. 6 depicts the Von Mises stress of the spigot and the bell under 100 kN overinsertion force. The stress level on the spigot and the bell becomes lower as the interface angle (β in FIG. 5) is increased. In the present invention, the interface angle is preferably increased above 15°, more preferably in the range from about 20° to 40° and is most preferably about 30°. An increase to a 30° angle is feasible to reduce the incidence of damage caused by overinsertion. Overall stress on the socket is reduced by about 40% in the worst case condition, where there is initial overinsertion, a thermal expansion and no internal pressure. Further increases in the interface angle would not produce significant improvements due to contact stress concentration and it would become more difficult to manufacture in the belling process.
The present invention provides several advantages. The possible problem of overinsertion of the male, spigot pipe end within the female, bell pipe end is avoided by simple changes in the geometry of the bell end internal surfaces. The change in the angle at which the taper of the male pipe end contacts the bottom wall of the bell end opening can be adjusted to reduce the incidence of a “wedge effect” during joint make-up. The change in angle can be accomplished during the pipe belling generation at the pipe manufacturing plant by changes to the exterior of the pipe belling mandrel.
While the invention has been shown in one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.
Patent Application
20080018017
