Method and apparatus for shaping a bell end on a pipe of termoplastic material

A method of, and a device for, forming a bell end on a^ipe, and pipe comprising a bell end when manufactured by the method.

The invention relates to a method of the kind which forms a bell end on a pipe of thermoplastic material by shaping the preheated pipe end by means of a shaping mandril’!, the pipe end being subjected to an upsetting operation so that a bell end comprising an annular sealing chamber is obtained, the inner diameter of the bell end, viewed in the direction of the open end of the bell, increasing in a step-wise manner to a maximum value, and decreasing again in a step-wise manner, the wall thickness of the bell end being at least equal to the wall thickness of the pipe,part of the inner wall of the sealing chamber serving as a sealing face.

A method of the aforesaid kind is known from Netherlands Patent Application 72 13 924; according to this known method in the formation of a bell end on a pipe of thermoplastic material, the upsetting operation compensates for the reduction of the wall thickness of the enlarged bell end caused by the radial widening thereof, so that there is no need to provide in advance a reinforcement sleeve on the end of the pipe, or to thicken in advance the pipe end, in a separate operation, as was done in prior methods. When use is made of this known method, the wall thickness will be substantially the same over the entire length of the enlarged bell end.

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A bell end with a wall thickness varying along the bell is obtained, if the wall thickness of the tube end to be shaped has already been increased previously, as proposed in the British Specification 997,552, According to U.S, Patent 3,205,535 the wall thickness of the tube ends to be shaped is increased by locally upsetting the tube during the extrusion thereof. During the shaping operation the previously increased wall thickness is reduced again, the strongest reduction invariably taking place at the portion of the bell having the largest diameter resulting in a minimum wall thickness of the bell at said portion? however, the location of. the minimum wall thickness at the largest diameter portion of the bell is not necessarily wanted.

In all these known methods given bell portions where maximum stress does not occur, become overproportioned, because the minimum wall thickness of the bell end is calculated for the highest stress which occurs. Furthermore, because at a given wall thickness of the various bell end. portions and at a given pressure of a fluid in the pipe, the wall stress caused by the fluid pressure is directly proportional to the different inner diameters of said bell end portions, an irregular stress distribution will occur in the wall, if the wall thickness is not adapted to said diameters.

One object of the present invention is to provide a method which enables a bell end to be formed which has a controlled wall thickenss distribution such that, when the bell end is loaded by the pressure of a fluid, the stress in the wall is constant over substantially the entire length of the bell end, the bell end having a pressure resistance which at least equals that of the relvevant pipe.

According to the invention, there is provided a method of the aforesaid kind in which the upsetting operation is performed so that, viewed in the direction of the open end of the bell, the wall thickness of the bell end is increased, the maximum wall thickness provided being ϊ.β&.βίϊβ.εΙ at the bell portion comprising the sealing face.

As a result, a bell end can be formed which has an optimum wall thickness distribution; the very critical upsetting being limited to the necessary minimum; superfluous upsetting of for any given bell end portions, and all attendant risks, such as, creasing , are thus reduced.

The maximum wall thickness will occur at the bell end portion subjected to the maximum load, notably the bell portion whose inner wall acts, in cooperation with a sealing ring to be inserted, as the sealing face. The relevant bell end portion is loaded by the pressure of a fluid flowing through the pipe as well as by the force of the compressed a ־

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8.12.75 sealing ring. It has been empirically demonstrated that the maximum wall thickness ensuring adequate pressure resistance of the bell end can be determined by means of the formula:

c _ P . D

0׳ - P wherein:

S = wall thickness in cm

P = nominal pressure in kg/cm²D = inner diameter in cm

C? - permissible stress in kg/cm².

The wall thickness of the various bell end portions is directly proportional to the inner diameter of these bell end portions. However, the wall thickness of the bell end portion comprising the sealing face should be such that this portion not only has the required pressure resistance but also adequate shape stability in order to prevent an increase of the diameter due to cre/page and to ensure reliable sealing for at least the service life of the pipe; the required shape stability is achieved by permitting a stress ($ in the wall portion comprising the sealing face which is lower than that in the remainder of the bell end, which results in a wall thickness which is larger than would be required solely on the basis of pressure resistance. Because according to this method overproportioning of given bell end phk 17 ו

8.12.75

<td></td><td> portions is prevented, a bell end having optimum pressure resistance and shape stability is obtained using a minimum quantity of material. A preferred embodiment of the method accord-</td>

<td> 5</td><td> ing to the invention is characterized in that the upsetting operation is performed so that the wall thickness of the bell end decreases again from the bell end portion of maximum wall thickness in the direction of the open end of the bell. Over-proportioning</td>

<td> 10</td><td> of the bell end is thus prevented and a further material saving is realized. According to a further embodiment of the method according to the invention, the upsetting operation is performed exclusively in the direction of the open</td>

<td> 15</td><td> end of the bell, the successive bell end portions being successively upset in the direction from the pipe side towards the open end of the bell, the upsetting of the wall of the sealing</td>

a ן . . phk 11.7 ΐψ.8.1975

ί chamber being effected via conical transition portions, the angle of inclination thereof with respect to the centre line of the bell end being smaller than 45° and larger than 120°, respectively. It was thus found to be possible to per, 5 form the upsetting operation in a controlled and reproducible j 1 manner in a single direction, without irregularities such as

I creases occurring. Because the upsetting operation takes

I place in the direction of the open end of the bell, moreover, j a device having a comparatively simple control can be used.

I ! 10 Thanks to the said angles of inclination, the desired wall thickness distribution is obtained in spite of the varying diameter of the bell end, A uniform wall thickness is also realized over the circumference of the bell end, in spite of 1 possible irregularities in the wall thickness of the ׳ original pipe in the circumferential direction. Differences in wall thickness of individual pipes with respect to each i other are also compensated for. Irregularities are generally eliminated by the upsetting operation according to the invention, resulting in improved proportioning of the 20 sealing chamber and particularly of the sealing face.

In a further preferred embodiment of the method according to the invention, the upsetting operation is performed such that the maximum wall thickness is obtained at the^toell end portion wliich has an inner diameter which is 25 ו smaller than the maximum and larger than the minimum inner ; diameter, the upsetting of the bell end portion having the maximum inner diameter being effected via a conical transition ^!,. portion enclosing an angle of inclination of less than 45° with the centre line of the bell end. This results in a bell phk 7 ו ,ו

14.8.1975 end whose sealing chamber has a step-wise varying diameter.

This sealing chamber serves to accommodate a special sealing ring, i_.e_. a ring having a scaling lip; the portion of the sealing chamber of smaller diameter serves as the sealing face, the portion of larger inner diameter serving as the locating portion. Use can then be made of a sealing ring which is optimally located as well as sealed.

A final preferred embodiment of the method according to the invention is characterized in that the wall thickness of the bell end portion comprising the scaling face is increased by at the most 30% with respect to the original wall thickness of the pipe by the upsetting operation.

Shapes and dimensions which are optimal for upsetting usually are not optimal for obtaining proper sealing. !Due to the described step, the largest wall thickness with a smooth inner surface is obtained for the bell end portion comprising the sealing face, so that proper sealing is also ensured. It was found that the upsetting process becomes critical if the wall thickness is increased by more than 3θ%.

A pipe comprising a bell end manufactured according to the invention, wherein the inner diameter of the bell end, viewed in the direction towards the open .end of the bell, increases in a step-wise manner to a maximum

ג value and decreases again in a step-wise manner, the wall thickness of the bell end being at least equal to the wall thickness of the pipe and the bell end comprising an annular sealing chamber with a sealing. face, is characterized in that the wall thickness increases in the direction of the open end of the bell, the maximum wall thickness occurring

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20.1.1976 ./ ' at the area of the sealing face. The bell end portion of such a pipe has an equal pressure resistance over substantially the entire circumference and length, the pressure resistance being at least equal to that of the pipe wall. The maximum wall thickness occurs at the bell portion which is loaded most, i.e. the portion comprising the sealing face.

In a preferred embodiment of the pipe comprising a bell end according to the invention, the wall thickness of the bell end decreases in the direction from the bell end portion comprising the sealing face towards the open end of the bell. The pipe comprising a bell end according to the invention thus clearly differs from other pipes comprising bell ends with varying wall thickness for which a reinforcement or a thickened portion has been provided during manufacture; therefore, the bell end of such known pipes has a wall thickness at the end portion of the bell which exceeds that at the area of the sealing face where the maximum wall thickness is desirable.

A further preferred embodiment of the pipe comprising a bell end according to the invention is characterized in that it comprises conical transition portions between the sealing chamber and the adjoining bell end portions, the said transition portions enclosing an

ג angle of inclination of less than 45° and more than 120° with the centre line of the bell end. The pipe comprising a bell end according to 'the invention is characterized by a uniform wall thickness and a regular sealing face in the circumferential direction.

In another preferred embodiment of the pipe comprising a bell end according to the invention, the bell end

PHK 1.17

14.8.1975 portion of maximum wall thickness has an inner diameter which is smaller than the maximum and larger then the minimum inner diameter, the transition between the bell end portion of maximum wall thickness and the bell end portion of maximum

J) inner diameter being in the form of a conical transition portion enclosing an angle of inclination of less than 45° with the centre line of the bell end. Such a shape of the bell is typical for the application of a sealing ring comprising a sealing lip. The bell end portion of maximum wall thickness whose inner wall also serves as the sealing face, is the most vulnerable part of the pipe comprising a bell end; this bell end portion is protected against damage during transport, storage and assembly, by the bell end portion of maximum inner and outer diameter.

yet

In/another preferred embodiment yet of the pipe comprising a bell end according to the invention, the bell end portion comprising the sealing face has a wall thickness which is at the most 30% larger than the original wall thickness of the pipe.

The invention also relates to a device for performing the method according to the invention, comprising a shaping mandril having a metal core portion and an elastic shaping ring, a shaping sleeve, a pipe clamp and an upsetting

ג member, the shaping mandril, shaping sleeve and pipe clamp being displaceable relative to each other, the shaping mandril and the shaping sleeve being concentrically arranged with respect to each other in the operating position and bounding an annular shaping space, the outer diameter of the shaping mandril and the inner diameter of the shaping sleeve first

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14.8,1975

<td></td><td> increasing in a step-wise manner, viewed from the front end of the shaping mandril facing the pipe clamp, via conical transition portions, to a maximum value at the area of the shaping ring, and decreasing in a step-wise manner; according</td>

<td> 5</td><td> to the invention, in this device the height of the shaping space, viewed from the front end of the shaping mandril increases to a maximum value at the area of the shaping ring. Using a mandril with the described annular shaping space, a bell end of varying wall thickness can be formed in a</td>

<td> 10</td><td> simple, accurate and reproducible manner, without separate preparation of the pipe end to be treated being required. In a preferred embodiment of the device according to the invention, the height of the shaping space decreases again from the portion of maximum height in the</td>

<td> 15</td><td> direction of the rear end of the shaping mandril. Once the shaping mandril and the shaping sleeve are in the operating position, the ultimate shaping space has been formed and the profile and the wall thickness distribution of the bell end to be formed are determined, without a further relative</td>

<td> 20</td><td> displacement of shaping mandril and/or shaping sleeve being required. In a further preferred embodiment of the device according to the invention, the shaping ring comprises conical transition portions which adjoin the core portion, the</td>

<td> 25</td><td> transition portions facing the front end and the rear end of the mandril extending at an angle of less than 45° and less than 60°, respectively, with respect to the centre line of the shaping mandril. Due to these steps, the upsetting operation can be performed in a controlled manner,without</td>

PHK 117 creasing and with a high dimensional accuracy of the bell end thus formed, in spite of the step-wise varying diameter of the shaping space.

In a further preferred embodiment of the device according to the invention, the shaping ring comprises two cylindrical surfaces, the diameter of the cylindrical surface directed towards the front end of the shaping mandril being smaller than the diameter of the other cylindrical surface and larger than the diameter of the adjoining part of the core portion, the shaping ring having a conical transition portion between the two cylindrical surfaces, which encloses an angle of inclination of less than 45° with the centre line of the shaping mandril. Using this device, a bell end can be formed with a portion of maximum wall thickness, the inner wall thereof serving as the sealing face, which is not situated at the bell portion of maximum outer and inner diameter.

Another preferred embodiment yet of the device according to the invention is characterized in that the height of the shaping space at the area of the cylindrical surface of the shaping ring directed towards the front end of the shaping mandril exceeds at the most 30% the height of the shaping space at the area of the smallest cylindrical V portion on the front end of the shaping mandril.

In a further preferred embodiment yet of the device according to the invention, the upsetting member is formed by an annular radial abutment face on the rear of the shaping mandril. Because of this structurally very simple step in making a separate, displaceable upsetting member superfluous

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14.8.'1975 the upsetting operation can be initiated, as soon as the pre- j heated pipe end contacts the abutment face.

A final preferred embodiment of the device according to the invention is characterized in that a cooling ring is provided on the front end of the shaping sleeve directed towards the pipe clamp. Due to the cooling, the pipe portion adjoining the bell end is cooled so as to prevent creases, ridges or grooves on the pipe at the area of the end face of the shaping sleeve facing the pipe clamp.

The inventiai will be described in detail hereinafter with reference to the drawing.

Fig. 1 is a longitudinal sectional view of an embodiment of the device according to the invention, together with a pipe to be treated;

Fig, 2 shows the device in the operating position prior to the upsetting operation of the pipe;

Fig. 3 shows the device after the upsetting operation of the pipe;

Fig. 4 shows, on an enlarged scale, a pipe comprising a bell end formed using the method and the device according to the invention.

Because the device is of a rotation-symmetrical construction, the drawing only shows the upper half. This device 1 mainly comprises a multi-part cylindrical shaping mandril 3, a two-part shaping sleeve 5» and a pipe clamp 7» having an inner diameter which substantially equals the outer diameter of the pipe to be treated. The shaping mandril 3 and the pipe clamp 7 are slidably journalled on a frame, only the portions 8 and 10 thereof being shown. The shaping mandril

PHK. 1 17

14.8.1975 is connected, by way of a vertical plate 6 and a piston rod to the piston 11 of an hydraulic cylinder 1J on the frame portion 8. The pipe clamp 7 is longitudinally divided into two pivotably connected shells and is connected, by way of a piston rod 15, to the piston 17 of an hydraulic cylinder 19 on the frame portion 10. The shaping mandril 3 is composed of a metal core portion 21 and a shaping ring 23 of elastically deformable material which is slidable on the core portion 21; a sleeve-shaped end portion 25, forming part of the core portion 21, is slidable on the front end of the core portion 2 which faces the pipe clamp 7, and can be locked in known manner on the core portion 21 by hydraulic means not shown. The shaping sleeve 5 comprises a shaping cylinder 27 which is longitudinally divided into two radially displaceable half shells, and also a closing ring 29 which is slidable on the core portion 21. The closing ring 29 is connected, by way of a piston rod 3/ to the piston 33 of an hydraulic cylinder 35 on the plate 6. The displacement of the shaping mandril in the direction of the pipe clamp is limited by an adjustable nut 37 on a rod 39 secured on the plate 6. The core portion 21, the shaping cylinder 27 and the ring 29 are provided with chambers 41, 43 and 45, respectively for the circulation of heating and cooling fluids. Λ gap 44 partly separates the cooling ring 42 from the shaping cylinder 27. The cooling ring 42 is also split into two shells, and comprises a cooling chamber 46. The inner diameter of the cooling ring 42 substantially equals the outer diameter of the pipe to be .treated

To form a bell end on a pipe 47, the pipe end is preheated to a temperature such that the pipe material is deformable. Meanwhile, the shaping mandril 3, the shaping

PHK.117

14.8.19 75

<td></td><td> cylinder 27 and the ring 29 are also heated by circulation of a heating fluid, such as steam or hot oil, in the chambers 41, 43 and 45» whilst the cooling ring 42 is cooled by circulation of a cooling fluid in the cooling chamber 46. The</td>

<td> 5</td><td> shaping mandril 3 is then situated in a retracted position (not shown) at the left in the drawing. The two half shells of the shaping cylinder 27 and of the cooling ring 42 have been radially displaced from each other; the two shells of the pipe clamp 7 have also been opened. The pipe 47 is</td>

<td> 10</td><td> arranged in the pipe clamp 7 such that the end of the pipe projects a predetermined distance from the pipe clamp. After the pipe clamp has been closed, the pipe is positioned in the axial and the radial direction. Subsequently, the two half shells of the shaping cylinder 27 and of the cooling rin£</td>

<td> 15</td><td> 42 are radially displaced towards each other. By means of the hydraulic cylinder 13» The shaping mandril 3 is introduced into the heated pipe end as far as the position shown in Fig. 1. The introduction of the shaping mandril is facilitatec by a conical face 5θ ^on the end portion 25 of the shaping</td>

<td> 20</td><td> mandril. The shaping ring 23 is then secured on the core portion 21 by means of the end portion 25. The preheated, *. plastically deformable pipe end slides over the outer circumference of the shaping mandril during the relative displacement of shaping mandril and pipe, and assumes the</td>

<td> 25</td><td> profile of the shaping mandril, the pipe end thus being preshaped. This situation is shown in Fig. 1. The end face of the pipe end just engages an annular abutment face 49 on the rear of the shaping mandril. Finally, the ring 29 is displaced in the direction of the shaping cylinder 27 by the</td>

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14.8.1975 piston 33 as far as the position shown in Fig. 2. In this position a shaping space 51 is formed by the outer circumference of the shaping mandril 3 and the inner circumference of the shaping sleeve 5, th® height H of the said shaping space first increasing, viewed from the front end of the shaping mandril, to a maximum value, and subsequently decreasing again, the minimum height being at least equal to the wall thickness S of the. pipe 47. The shaping space 51 is fully filled by an upsetting operation, so that a bell end 53 is formed whose wall thickness W, in accordance with the height H of the shaping, space 51 and viewed in the direction towards the end of the bell, first increases to a maximum r value and subsequently decreases again. The upsetting operatio is performed in a simple manner in that the pipe clamp 7 with the pipe kf is displaced in the direction of the shaping mandril. The shaping space is then filled from the front end of the shaping mandril to the rear end, successive portions of the preshaped pipe end of different diameter being successively upset. Fig. 3 shows the situation after' completion of the upsetting operation. Subsequently, the heating fluid circulating in the chambers 41, 43 and 45 is replaced by a cooling fluid. After the shaping mandril and the shaping sleepke as well as the pipe end now converted into a bell end 53 have cooled down sufficiently, the core portion 21 and the ring 29 are displaced to the left, the shaping ring 23 and the end portion 25 then remaining in the bell end. The elastic shaping ring 23 is removed from the bell end by hand or by means of a tool, after which the end portion 25 of the mandril can also be removed. After the opening of the ριικ 117

14.8.1975 shaping cylinder 27 of the cooling ring 42 and of the pipe clamp 7, the pipe 47 comprising the bell end thus obtained can be removed from the device. After the shaping ring 23 and the end portion 2ע have been secured again on the core portion 21 and after the pipe clamp 7 has been returned to the original axial position, and after the shaping portions have been heated to the desired temperature again, the device , will be ready again for a further operation.

In the closed position of the device shown in the Figs. 2 and 3, the shaping mandril and the shaping sleeve are concentrical relative to each other. The outer diameter of the shaping mandril 3 and the inner diameter of the shaping'sleeve 5 vary step-wise, the diameter first increasing in three steps, and subsequently decreasing again in one step. The outer diameter A of the shaping mandril substantially equals the diameter Di of the pipe 47; the diameter

B substantially equals the outer diameter De of the pipe 47, and the diameter C lias a value between the diameter B and a maximum diameter D. The diameter E at the end portion oJ? the bell end is substantially equal again to the outer diameter De of the pipe. The transition between successive cylindrical portions of different diameter is effected via conical transition portions T, X, Y, Z (Fig. 2) at an acute arg-^e of inclination with respect to the centre line F-F of the shaping mandril; the angle a of the transition portion T equal. 15°, the angle b of the transition portion X equals 30°, the angle c of the transition portion Y also equals 30°, and the angle d of the transition portion Z equals 37.5°. The inner circumference of the shaping sleeve 5 is parallel to the outer ־15PHK 117

14.8.1975 circumference of the shaping mandril 3. The height H of the shaping space is minimal at the portion having the diameter A, and equals the thickness S of the pipe 47; at the portion having the diameter B, the height H increases, has a maximum value at the portion having the diameter C, and decreases again at the portions having the diameters D and E. This will be clarified with reference to Fig. 4, which shows on an enlarged scale the bell end 53. The reference 55 denotes a sealing ring which is provided with a sealing lip 57 and which is fixed inside the bell end by a retaining ring 59 at the area of the bell portion having the maximum inner diameter D. The reference 61 denotes the wedge end of a pipe to be inserted into the bell end 53. The transition between successive bell end portions of different inner diameter is effected, in accordance with the already described profile of the shaping mandril and viewed in the direction towards the open end of the bell, via conical transition portions Τ', X., Y' and Z¹ which enclose an angle a of 15°> an angle b of 30°₍ an׳, angle c_ of 30° and an angle d¹ of 142.5° with tl centre line G-G of the bell end. In accordance with the variation of the height H of the shaping space, the wall thickness distribution W is such that WSUXV » the A ΰ C' D' K waif' thickness being equal to the wall thickness S of the pipe. The bell end has a maximum wall thickness W = W c max at the area of the sealing lip 57. The maximum wall thickness ^max ^Can i^{ncrease a} given value which is 3θ% larger than the wall thickness S of the pipe, in dependence of the diameter C. The inner wall of this bell end portion also serves as the sealing face 63. At this most critical part,

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8.12.75 loaded not only by the internal pressure of a fluid but also by the compressed sealing ring, the wall thickness is determined not only by the pressure resistance but also by the required shape stability.

The sealing ring of the type shown has excellent sealing properties and can be very reliable secured in the sealing chamber. Moreover, the portion of maximum wall thickness of the bell end, also comprising the sealing face, is protected against damage during transport, storage and assembly by the adjoining bell end portion of maximum diameter.