Improved ceramic tipped pivot rod and method for its manufacture

Abstract: Pivot rods, such as push rods of the type found in fuel injector and engine cylinder valve drive trains, wherein a pivot element (15, 20) is formed of a ceramic material that is joined to a nonceramic mounting shaft (5) as a tip portion thereof, and a method for its manufacture are improved by utilizing an attachment sleeve (10) to interconnect the pivot element to the mounting shaft in an end-to-end abutting fashion. The attachment sleeve (10) is secured to a portion of the pivot insert (15, 20) by a first interference fit securement and to an end portion of the mounting shaft (5) by a second interference fit securement. The combined axial length of the first portion of the pivot insert and the end portion of the mounting shaft is greater than the axial length of the attachment sleeve for preventing load transmission between the mounting shaft and the pivot insert via the attachment sleeve and the diameters thereof are coordinated to the maximum tensile principle stress of the pivot insert and the thickness and material of the attachment sleeve to result in deformation of the attachment sleeve during creation of the interference fit securements without exceeding the maximum tensile principle stress of the ceramic and, preferably, so as to result in the attachment sleeve being deformed along its full length to a substantially constant outer diameter that is equal to that of the outer diameter of a second portion of the pivot insert (which projects axially from the receiving space of the attachment sleeve) and the body portion of the mounting shaft.

Description: The present invention relates to a pivot rod with a pivot insert formed of a ceramic material according to the pre-characterizing part of claim 1 as well as to a method for its manufacture according to the pre-characterizing part of claim 6. In the prior art forming the starting point of the invention (JP-A-57-13203) the mounting shaft of the pivot rod is made of a carbon fiber reinforced resin material, pivot inserts of a ceramic material having a maximum tensile principal stress forming the rod ends. The pivot insert has a recess formed in a stem or first portion thereof. An attachment sleeve having a hollow interior receiving space is formed by a metal pipe. An end edge of the metal pipe forming the attachment sleeve is engaged in the recess formed on the first portion of the pivot insert by caulking. In other words the metal of the metal sleeve is radially deformed into a circumferential indentation on the periphery of the first portion of the pivot insert. As can be seen from the drawing in the prior art-document the end edge of a metal pipe has no specific position in the circumferential indentation on the periphery of the ceramic pivot insert, on the left side its position is different from the right side. This is only logic, because caulking cannot produce an exact position of such end edge. The result is that, although the second portion of the pivot insert should not directly contact the attachment sleeve, the tolerances involved cannot guarantee that this does not happen from time to time (as on the left side of the drawing). Therefore it is possible that at least partly a butt joint between the attachment sleeve and the pivot insert may occur which increases the danger of cracking of the ceramic material. The mounting shaft of plastic is attached to the attachment sleeve only by a friction fit, i. e. it is fittingly inserted into the attachment sleeve formed by the metal pipe without any deformation of the metal pipe. In order to produce such pivot rod this is done after caulking of the metal pipe on the pivot insert has taken place. From a further prior art reference (EP-A-0 282 714) there may be obtained a pivot rod with a first interference fit securement of the first portion of a pivot insert to a hollow mounting shaft or a massive mounting shaft. Nevertheless, here the difficult problems of the butt joints formed when a hollow mounting shaft is used, are not disclosed. From above discussion of the prior art it will be clear that there is a problem that the second portion of the pivot insert and the attachment sleeve could come in direct contact which might result in damage, in particular cracking of the pivot insert. Above defined problem is solved with a pivot rod with the features of the pre-characterizing part of claim 1 by means of the features of the characterizing part of claim 1. First of all, the pivot insert is secured to the attachment sleeve not by caulking but through a first interference fit securement as specifically described. Such interference fit securement with plastical deformation of the peripheral wall of the attachment sleeve allows for exactly defining the distance between the second portion of the pivot insert and the top end of the attachment sleeve, thus securing safely that the second portion is not directly contacting the attachment sleeve. Further, the mounting shaft is secured to the attachment sleeve likewise through a second interference fit securement. This is done so that the mounting shaft is in abutting relation with an end face of the first portion of the pivot insert inside the receiving space of the attachment sleeve. Therefore, in operation of such pivot rod it is absolutely sure that axial loading is directly transmitted between the ceramic pivot insert and the mounting shaft without any effect of the attachment sleeve. Since by this mounting technique the possibility of a butt joint between the pivot insert and the attachment sleeve is positively excluded, the likelyhood of cracking or other damages of the pivot insert is drastically reduced. Further improvements of the inventive pivot rod are discussed in claims 2, 3, 4, 5. Claim 4 relates to a mounting shaft with a stepped diameter with a body portion having an outer diameter greater than the diameter of the end portion. Of course here the sum of the axial length of the first portion of the pivot insert and the axial length of the end portion of the mounting shaft is greater than the axial length of the attachment sleeve. Otherwise the possibility of a butt joint between the pivot insert and the attachment sleeve could not be positively excluded. Claim 5 gives a preferred relationship of the axial length of both parts. As to the method of manufacturing an improved ceramic tipped pivot rod claim 6 is relevant. Improved method-steps are further discussed in claims 7, 8, 9. The present invention takes advantage of relationships between principle tensile stress and diametral interference that have previously been determined in above mentioned prior art from EP-A-0 282 714. Thus, for the sake of brevity, the details of these relationships will not be reiterated in this application. Instead, the reader's attention is directed to this prior art reference. Hereafter follows a detailed description of a preferred embodiment of the present invention according to the accompanying drawings. In the drawings Fig. 1 is a perspective view of a pivot rod in accordance with a preferred embodiment of the present invention Fig. 2 is a schematic representation depicting the dimensional relationships between a ceramic pivot insert, attachment sleeve, and mounting shaft of the preferred embodiment pivot rod, prior to assembly thereof, for purposes of explaining the method in accordance with the present invention and Figs. 3 and 4 are schematic representations, respectively, of a cylinder head valve and fuel injector drive train incorporating a pivot rod in accordance with the present invention. Fig. 1 illustrates a pivot rod 1 in accordance with a preferred embodiment of the present invention. Such a pivot rod 1, as noted later, may serve as a push rod of the type finding particular utility in drive trains as described relative to Figs. 3 and 4. Pivot rod 1 is comprised of a mounting shaft 5, a pair of attachment sleeves 10, and a pair of pivot inserts 15, 20. Each of the pivot inserts 15 is formed of a ceramic material, such as silicon nitride, while the mounting shaft 5 is a metal rod such as a piece of standard steel rod stock or a cast steel piece. Additionally, each of the sleeves 10 may be formed of a piece of "off the shelf" tubing, such as steel tubing of a standard size, tolerances, and wall thickness. As can be seen from Fig. 2, the mounting shaft 5 of pivot rod 1 has reduced diameter end portions 5a at each of opposite ends of a body portion 5b onto which attachment sleeve 10 is interference fitted. Similarly, a reduced diameter first portion 15a of the pivot insert 15 (or a corresponding reduced diameter portion 20a of pivot insert 20, only a portion of which is visible in Fig. 1) is press fit secured within a receiving space 10' defined by the peripheral wall of attachment sleeve 10, after the attachment sleeve 10 has been secured to mounting shaft 5 for the reasons noted in the following paragraph. In this regard, the thickness t and composition of the attachment sleeve 10, as well as the extent of the interference (which corresponds to the difference between the diameter D of the attachment sleeve 10 and the diameter D I of the end portions 5a and the corresponding diameter of the first portions 15a, 20a of the pivot inserts 15, 20) are selected in accordance with the above-referenced relationships disclosed in EP-A-0 282 714, so as to result in the peripheral wall of attachment sleeve 10 being plastically deformed by the first portion 15a, 20a of the pivot insert 15, 20 and by the end portion 5a of the counting shaft 5 during formation of the interference fits without exceeding the maximum tensile principle stress of the ceramic material of the pivot inserts 15, despite variations in the degree of diametral interference existing between the internal diameter of the peripheral wall circumscribing the receiving space 10' of the attachment sleeve 10 and the external diameter of the first portion 15a of the pivot insert 15 resulting from manufacturing tolerances. Additionally, in accordance with the present invention, the combined length composed of the axial length H PI of the first portion 15a, 20a of the respective pivot insert 15, 20 and the axial length H SI of the end portion 5a of the mounting shaft 5 is made to be greater than the axial length L of the attaching sleeve 10 to prevent damage to the brittle ceramic material of the pivot insert 15, 20, both during formation of the joint between it and the mounting shaft 5 and to prevent fracturing during use due to the transmission of axial loads between the pivot insert 15, 20 and the mounting shaft 5 via the attachment sleeve 10. That is, the first, (insertion) portion 15a, 20a is joined to a second (pivot surface) portion 15b (which is convexly curved) or 20b (which is provided with a concavely curved recess 20d) via a shoulder 15c, 20c, respectively, to enable the pivot surface to be maximized while achieving the other benefits noted in the following paragraph. However, such a shoulder 15c, 20c may be damaged if it were to impact against the facing end surface 10a of the attachment sleeve 10 during creation of the interference fit securement. Also, brittle cracking leading to failure of the ceramic pivot insert 15, 20 could be experienced due to overstressing of this shoulder 15c, 20c or the fillet interconnecting it to the first portion 15a, 20a, as could occur if the parts were assembled in a fashion permitting axial loads to be transmitted between the shoulders 5c of the mounting shaft and the respective shoulder 15c, 20c of the pivot insert 15, 20 via the attachment sleeve 10. It is also noted that the lengths H PI and H SI may be equal, but are not necessarily so, as they may be varied to make one relatively longer or shorter than the other as best suited to manufacturing considerations, so long as neither portion 5a or 15a, 20a is made so long as to impose bending stresses of an extent great enough to cause failure of the attachment, or so short as to provide an inadequate length for a secure fastening of the attachment sleeve 10 thereto. Still further, it is desirable for aesthetic and other reasons, to produce a pivot rod 1 which gives the appearance, as much as is possible, of a one-piece rod and does not either increase the maximum diameter of the pivot rod 1 or reduce the diameter of the pivot surfaces (keeping in mind that, in practice, such pivot rods will have to pass through bores of the engine structure with which it is associated as part of an overall drive train). Therefore, the diameter D I of the end portion 5a of the mounting shaft 5 and of the insertion portion 15a, 20a of the ceramic pivot inserts 15, 20 is reduced relative to the outer diameter of body portion 5b of the mounting shaft 5 and of the second portion 15b, 20b of the pivot insert 15 at shoulder 5c, 15c, 20c, by an amount that is coordinated to the thickness t of the attachment sleeve 10 and the extent to which the attachment sleeve 10 is deformed as a result of the interference securements produced so as to result in the attachment sleeve 10 being deformed throughout its axial length to a substantially equal extent and so as to have a deformed outer diameter that is substantially constant and equal to the outer diameter of the second portions 15b of the pivot inserts 15 and body portion 5b of the mounting shaft 5. In this regard, it is also pointed out that while, in the drawings, a noticeable gap is shown between the end face 10a of attachment sleeve 10 and the shoulder 15c, 20c of the pivot inserts 15, 20, respectively, such is for purposes of illustration only. In practice, such a gap will be less than one-half of a millimeter so as to be virtually indistinguishable from the gapless interface between shoulder 5c and the opposite end of attachment sleeve 10. To facilitate the creation of the interference fit interconnections of the mounting shaft 5 and pivot inserts 15 with the attachment sleeve 10, the inner rim 10b of the attachment sleeve 10 is chamfered and a radiused or chamfered peripheral edge r provided on end portion 5a of the mounting shaft 5 and insertion portion 15a, 20a of the pivot inserts 15, 20. In this way, the larger portion to be inserted can be guided into the receiving space of the attachment sleeve 10 which will be enlarged thereby. Even though the pivot rod 1 described above is formed with a convex surface at one end and a concave surface at the opposite end, such need not necessarily be the case, as can be seen with reference to Figs. 3 and 4, which schematically show drive trains of the type which may be significantly improved through the utilization of ceramic ball and socket joints to increase the compressive loads to which such joints may be subjected and the wear life thereof. For example, Fig. 3 depicts an engine cylinder head valve drive train wherein ball and socket joints 30 are created at each of opposite ends of the push rod 1 to transmit movement produced by a cam 32 to a valve rocker lever 34 that is used to seat and unseat valves 36 with respect to valve seat inserts 38 via a cross bridge 39. However, this drive train also utilizes a pivot rod 1' wherein only a single pivot insert is provided at one end thereof, this being a convex pivot insert. Furthermore, in the case of the fuel injector drive train of Fig. 4, in addition to being provided with joints 30 for transmission of force from the cam 32 to the rocker arm 34 via pivot rods 1, 1', ball and socket joints 40 are provided by a further modified pivot rod 1'' that acts between the rocker arm 34 and an injector piston 42. This pivot rod 1'' is provided with convex surfaced pivot inserts at each of opposite ends thereof. Likewise, for other applications, it is possible to produce pivot rods in accordance with the present invention which have only a single concave pivot surface at one end or which have concave pivot inserts at each of opposite ends thereof. A pivot rod produced in accordance with the foregoing has been found to achieve all of the benefits of significantly increased wear life and the simplification of production associated with a pivot rod as produced in accordance with EP-A-0 282 714 without creating any potential areas for stress failure of the ceramic material of the pivot inserts or of the joint between the inserts and the mounting shaft during use or as a result of operational stresses. Likewise, a pivot rod produced in accordance with the present invention is not prone to stress failure or loosening due to thermal effects.

Claims: A pivot rod (1) comprising a mounting shaft (5), an attachment sleeve (10) having a hollow interior receiving space (10'), a pivot insert (15) formed of a ceramic material having a maximum tensile principle stress, this pivot insert (15) comprising a first portion (15a) and a second portion (15b) projecting axially therefrom and having an outer diameter greater than the first portion (15a), wherein the first portion (15a) is disposed within said receiving space (10') to secure the pivot insert (15) to the attachment sleeve (10) with the second portion (15b) not directly contacting the attachment sleeve (10), and wherein an end portion (5a) of the mounting shaft (5) is disposed within the interior receiving space (10') of the attachment sleeve (10) to secure the mounting shaft (5) to the attachment sleeve (10), characterized in that the first portion (15a) of the pivot insert (15) is secured to the attachment sleeve (10) through a first interference fit securement of the first portion (15a) disposed within the receiving space (10') to a peripheral wall of the attachment sleeve (10) circumscribing the receiving space (10'), this interference fit securement being constructed as a means for preventing the maximum tensile principle stress of the ceramic material from being exceeded, despite variations in the degree of diametral interference existing between an internal diameter (DA) of the peripheral wall circumscribing the receiving space (10') and an external diameter of the first portion (15a) of the pivot insert (15) resulting form manufacturing tolerances of the wall having been plastically deformed by the first portion (15a) of the pivot insert (15) during formation of the interference fit securement through coordination of the thickness and material composition of the peripheral wall with the maximum tensile principle stress, the end portion (5a) of the mounting shaft (5) is secured to the attachment sleeve (10) through a second interference fit securement, and the end portion (5a) of the mounting shaft (5) is in abutting relation with an end face of the first portion (15a) of the pivot insert (15) to secure the necessary distance between the second portion (15b) of the pivot insert (15) and the attachment sleeve (10), for preventing load transmission between said mounting shaft (5) and said pivot insert (15) via said attachment sleeve (10). A pivot rod according to claim 1, characterized in that said pivot insert (15) has a convexly-shaped contact surface on said second portion (15b) or said pivot insert (15) has a concavely-shaped contact surface in said second portion (15b). A pivot rod according to claim 1 or 2 characterized in that a pivot insert (15) is mounted to each of opposite ends of the mounting shaft (5) by an interference fit securement between the respective attachment sleeve (10) and the respective pivot insert (15). A pivot rod according to claim 1, 2 or 3, characterized in that the end portion (5a) of the mounting shaft (5) has an outer diameter that is reduced relative to the outer diameter of a body portion (5b) of the mounting shaft (5) by an amount coordinated to the thickness of the attachment sleeve (10) and the extent to which the attachment sleeve (10) is deformed as a result of the first and second interference fit securements and resulting in the attachment sleeve (10) being of a substantially constant outer diameter along its length that is equal to the outer diameter of the second portion (15b) of the pivot insert (15) and the body portion (5b) of the mounting shaft (5). A pivot rod according to claim 4, characterized in that the axial length (H PI ) of the first portion (15a) of the pivot insert (15) is approximately equal to the axial length (H SI ) of the end portion (5a) of the mounting shaft (5). A method of manufacturing a pivot rod (1) having a mounting shaft (5) and a pivot insert (15) of a ceramic material, with a given maximum tensile principle stress, said pivot insert (15) being positioned with a first portion (15a) thereof disposed within a hollow interior receiving space (10') of an attachment sleeve (10) that is secured on an end portion (5a) of the mounting shaft (5) and with a second portion (15b) of the pivot insert (15) having an outer diameter greater than the first portion (15a) and projecting axially from said receiving space (10'), characterized by the steps of: a) coordinating the thickness and material composition of a peripheral wall of the attachment sleeve (10) that circumscribes the receiving space (10') with the maximum tensile principle stress of the ceramic material so that said peripheral wall will plastically deform under a stress below said maximum tensile principle stress, b) securing said first portion (15a) of the pivot insert (15) to said peripheral wall of the attachment sleeve (10) so that said second portion (15b) does not directly contact said attachment sleeve (10) by an interference fit without exceeding the maximum tensile principle stress of the ceramic material, despite variations in the degree of diametral interference existing between an internal diameter of the peripheral wall and an external diameter of said first portion (15a) resulting from manufacturing tolerances of the mounting shaft (5) and pivot insert (15), by producing plastic deformation of said peripheral wall by said first portion (15a) of the pivot insert (15) during formation of said interference fit, and c) interference fit securing the end portion (5a) of the mounting shaft (5) within the interior receiving space (10') of the attachment sleeve (10) in abutting relation with an end face of said first portion (15a) of the pivot insert (15), d) so as to secure the necessary distance between the second portion (15b) of the pivot insert (15) and the attachment sleeve (10), for preventing load transmission between said mounting shaft (5) and said pivot insert (15) via said attachment sleeve (10). A method according to claim 6, characterized in that step c) is performed before step b) so as to prevent axial contact between the second portion (15b) of the pivot insert (15) and a facing end surface (10a) of the attachment sleeve (10), thereby ensuring that the second portion (15b) is not damaged during step b). A method according to claim 6 or 7, characterized in that steps a), b), c) are performed so as to result in the attachment sleeve (10) being deformed throughout its axial length to a substantial equal extent so as to have a deformed outer diameter that is substantially constant and equal to the outer diameter of the second portion (15b) and preferably, of a body portion (5b) of the mounting shaft (5). A method according to claim 6, 7 or 8, characterized in that said steps a), b), c) are performed for each of opposite ends of the mounting shaft (5).

Inventors: BENTZ JOSEPH C, BENTZ, JOSEPH C.
Applicants: CUMMINS ENGINE CO INC, CUMMINS ENGINE COMPANY, INC.
Application year: 1989
Main classification: F01L1
IPC codes: F02M57, B21D39, F01L1, F02M59, B23P11