GB2058289A - Piston for internal combustion engine - Google Patents

Abstract

A piston body (1) and a crown portion (3) are connected together by the insertion of anchoring means into a bore (12) in the piston body, the anchoring means comprising at least one member (7) that is rigidly connected axially at one end (9) to a portion of the body, the temperature of which, at maximum operating temperature is such that loosening of the member in that portion is prevented, and is rigidly connected axially at its other end (8) to the crown portion, thereby being loaded both in tension and compression by the forces acting between the piston body and crown portion in operation. The rigid connections may be screw threaded connections with preloaded or locked threads. The anchoring member (7) may comprise a bolt or stud that is threaded in respective holes (13, 15) in both portions and is locked in each individually e.g. by means of a lock nut (16) or set screw, or by axial abutment between the head or end (11) of the bolt or stud and a respective one of said portions, or by a change in thread pitch. The threads in said holes may be pre-formed, in which case the thread starts may have to be matched, or may be formed by the bolt or stud itself. <IMAGE>

Description

SPECIFICATION Piston for internal combustion engine This invention relates to pistons for internal combustion engines.

It is known to construct pistons for internal combustion engines so that they have a separate crown portion which is attached to the body of the piston. For example, it is known to provide a crown portion formed with a combustion chamber or to provide a ring-shaped insert to form the lip of a combustion chamber in the crown of a piston.

The crown portion or insert is preferably composed of a material particularly suited to the thermal and gas pressure conditions to which the crown is exposed in operation.

The crown portion or inset may be attached to the body of the piston by casting the body around or against it so that the two are keyed together. A technique of this type is used in the piston disclosed in our copending United Kingdom Patent Application No. 12916/76 in which an insert is provided with legs that are embedded in the body of the piston by casting the body around the insert and legs. Furthermore the legs are such as to extend from the insert into a portion of the body, the temperature of which, at maximum operating temperatures, is such that loosening of the member in that portion is prevented.

Alternatively, the crown portion or insert may be attached to the pre-formed body of the piston using separate attachment means such as bolts or studs. In all of the known examples, the bolts or studs serve to clamp the crown portion or insert against the body of the piston. However, this clamping action is dependent upon the bolts or studs being held in tension by the reaction force between the inter-engaging surfaces of the crown portion and piston, and is therefore impaired by relative expansion and thermal yielding of said interengaging surfaces. There is therefore a tendency for the bolts or studs to loosen, which in turn results in damage or destruction of the piston.

An object of the present invention is to provide a piston in which a crown portion or insert is attached to the body of the piston securely whilst avoiding the aforesaid disadvantage of the known clamping technique.

A piston according to the invention comprises a piston body and a crown portion that are connected together by insertion into a bore in the piston of anchoring means comprising at least one member that is rigidly connected axially at one end to a portion of the body, the temperature of which, at maximum operating temperature is such that loosening of the member in that portion is prevented, and is rigidly connected axially at its other end to the crown portion, thereby being loaded both in tension and compression by the forces acting between the piston body and crown portion in operation.

The respective rigid axial connections between the anchoring member and crown portion and body portion ensure that said portions are secured together without using the anchoring member as a tensile clamping member thereby avoiding any loosening of the connections by relative expansion of the anchoring member or yield of any interengaging surfaces of said portions.

The rigid connections, especially that between the anchoring member and body portion, are preferably such as to allow preloading that will maintain effective axial location under operating conditions.

The rigid connections may be made in many different ways but generally screw threaded connections with preloaded or locked threads are preferred as being simple and inexpensive. For example, the anchoring member may comprise a bolt or stud that is threaded in respective holes in both portions and is locked in each individually by means of a lock nut or set screw, or by axial abutment between the head or end of the bolt or stud and a respective one of said portions, or by a change in thread pitch. The threads in said holes may be pre-formed, in which case the thread starts may have to be matched, or may be formed by the bolt or stud itself.

The rigid connection between the anchoring member and the piston body is located in a portion of the piston body which, in use, is at a temperature such that its creep strength, which caries inversely with temperature, is sufficient to resist the stresses resulting from the inertia and gas forces.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is half an axial section through a piston according to one embodiment of the invention; Figure 2 is half an axial section through a piston according to a second embodiment of the invention; Figure 3 is a scrap view showing a modification to the anchoring means of the piston of Figure 2; Figure 4 shows a further modification to the anchoring means of the piston of Figure 2; Figure 4 is half an axial section through a piston according to a third embodiment of the invention; Figures 6 and 7 show detailed views of the anchoring means in Figure 5; Figure 8 is a scrap view showing a modification to the piston of Figure 5;; Figures 9 to 1 5 each show an axial section of a piston according to an alternative embodiment of the invention, the difference in each case consisting in the nature of the anchoring means.

The piston illustrated in Figure 1 comprises a piston body 1 cast out of aluminium-silicon alloy and having a recess 2 in the crown to receive an annular insert 3 of heat resistant material. The crown of the piston has a combustion chamber4 formed by a recess 5 in the crown of the piston body and the central bore of the insert 3. The insert 3 is formed with an inwardly projecting annular lip 6 so as to give the combustion chamber a re-entrant profile. The insert is composed of a heat resistant material so as to give the lip 6 the necessary strength to withstand the combustion chamber operating conditions. For example, the insert may be cast from a copper alloy including 0.5% beryllium and 2.5% cobalt.

The insert is secured in place in the recess 2 by two or more threaded studs 7 that extend through the piston body parallel to the piston axis and are located on diametrically opposite sides of said axis. Each stud has a threaded portion 8, 9 at each end and an intermediate plain portion 10 of a reduced diameter. One threaded end portion 8 has a tapered tip 11 and, during the assembly operation, it is inserted from the underside of the piston through a plain bore 12 in the piston body and engages a blind bore 13 in the underside of the insert. The bore 13 is of a smaller diameter than the end portion 8 of the stud and the stud is rotatably driven so that it is threaded into the bore 13, forming a cooperating thread in the wall of the bore 13 in the process.The stud is driven in this manner until its tapered tip 11 abuts the tapered end 14 of the bore 13, the loading reaction then producing a locking action between the threads.

The threaded portion 9 at the other end of the stud has a larger diameter than the end portion 8, and engages a corresponding radially enlarged portion 15 of the bore 12 at its lower end. This enlarged portion 15 of the bore is of a smaller diameter than the threaded end portion 9 of the stud, and thus the end portion 9 is threaded into the bore 15 by forming a thread therein as the end portion 8 is threaded into the bore 13 in the insert.

Preferably, the longitudinal dimensions of the different portions of the stud and bore in the piston body are such that the upper threaded portion 8 commences forming a thread in the insert before the lower threaded portion 9 commences forming a thread in the piston body.

The threads on the portions 8 and 9 are both of the same pitch.

Once the stud is fully inserted a locknut 1 6 is threaded onto the lower threaded portion 9, a loading reaction being produced between it and the underside of the piston to lock the cooperating threads of the portion 9 and bore 1 5.

It will be appreciated that in the final assembled state, the insert is rigidly connected to one end of the stud and the piston body is rigidly connected to the other end of the stud. Consequently there need be no initial clamping force in the stud. In use, the stud acts alternately as a tie and strut in resisting the operating forces applied to the piston.

A piston having a similar piston body 1 and insert 3 to that of the piston of Figure 1, is shown in Figure 2. Again, a pair of threaded studs 7 connect the insert to the piston body. Each stud is of a constant diameter, is threaded throughout its length and terminates in a tapered tip 11 at one end and in a recessed head 17 at the other end.

Each stud is rotatably driven into a bore 12 in the piston body so as to form a cooperating thread throughout its length. The tip of the stud then engages a blind bore 13 in the underside of the insert 3, of the same diameter as the bore 12, and the stud proceeds to thread into the bore 13.

Abutment of the tapered tip 11 with the tapered end of the bore 13 serves to lock the cooperating threads of the stud 7 and bore 13, as in the embodiment of Figure 1.

The stud 7 is of a length such that its recessed head 1 7 lies within the bore 12 when fully inserted, the recess 18 in the head being engaged by a suitable tool such as an hexagonal key to drive the stud home. After insertion of the stud 7, a locking grub screw 1 9 is screwed into the bore 12 and abuts the head of the stud to lock the cooperating threads of the stud and bore 12.

As an alternative to using the grub screw 19, a set screw 20 can be used as shown in Figure 3, to lock the cooperating threads of the stud 7 and bore 12.

In another alternative embodiment of the invention, the stud 7 in Figure 2 is replaced by a longer bolt with a similar tapered tip 11. The bolt forms threads in the bore 12 in the piston body and in the bore 13 in the insert, as described above, but the head of the bolt is spaced away from the bottom of the piston body when the bolt is fully inserted. In orderto lock the bolt in the bore 12, a lock nut 20 is provided on the bolt to be loaded against the underside of the piston body, as shown in Figure 4.

Another embodiment of the invention employing bolts as the anchoring means, is illustrated in Figures 5 to 7. The piston body 1 and insert 3 can be considered to be basically the same as in the preceding embodiments but the insert is formed with a recess 21 to receive the head 22 of a bolt 23 that is threaded from the top of the piston down through the aligned bores 12, 13 in the piston body and insert 3. The bores 12, 1 3 are formed with a smaller diameter than the bolt 23 and the bolt is rotatably driven to form cooperating threads in the walls of the bores. The head of the bolt is loaded against the base of the recess 21 so as to lock the cooperating threads of the bolt and bore 13 in the insert as shown in Figure 6.A lock nut 24 is threaded onto the lower end of the bolt after insertion and is loaded against the lower face of the piston body to lock the cooperating threads of the bolt and bore 12 in the piston body, as shown in Figure 7.

In the embodiments of Figures 1,4 and 5 to 7, the bolt 7 is tensioned by a locknut during assembly and this serves to load the interengaging faces of the insert and piston body. This can be avoided by providing a gap between these two faces using a suitable jig or a shim. If the shim is composed of a soft material or one that yields rapidly with rise in temperature, it may be left in place after tightening the lock nut.

The embodiments of Figures 5 to 7 may be modified by using shorter bolts 22 and locking each to the piston body using a grub screw or lock screw. Figure 8 illustrates such a modification in which a lock screw 25 is used. This is of the same diameter as the bolt 22 and forms a cooperating thread in the bore 12 in the same manner. This lock screw 25 applies an upwards force to the bolt to lock it to the piston body. This upwards force is in the same direction as the upwards reaction on the head of the bolt 22 that locks its threads in the bore 1 3 in the insert. This fact is indicated by the arrows representing the locking forces entering between the flanks of the threads in Figures 6 to 8.

In all of the preceding embodiments where a bolt or screw or stud has been threaded in the insert or piston body this has been achieved by driving the bolt screw or stud so as to form a cooperating thread. However, it will be appreciated that in all cases pre-formed threads may be provided instead. The only difficulty will be to ensure that successive threads engaged by the bolt, screw or stud are started in the correct relationship to one another.

Further, when using a pre-formed thread in a bore in the insert of piston body it is possible to change or vary the pitch of the thread on the bolt so as to obtain an automatic locking action of the bolt when threaded into the bore. For example, the lock nut 20 in the embodiment of Figure 4 may be omitted and a preformed thread with a change or variation of pitch pre-formed on the bolt 7 to be received in the bore 12.

It should also be appreciated that, some of the preceding embodiments can be modified so that the two connections of the anchoring means to the insert and piston body can be interchanged or inverted.

Other alternative embodiments of the invention are illustrated in Figures 9 to 17 in which bolts or pins 26 are inserted through aligned plain bores 12, 13 in the piston body insert 3, and are connected to the insert and piston body by means other than direct threaded engagements.

In all cases, the heads of the bolts or pins 26 engage recesses 27 in the insert and are welded or brazed in place. This allows the insert to be thinner and lighter than when threading studs or bolts into the insert.

In the embodiment of Figure 9, the bolt 26 is formed with a thread at its lower free end and projects into an internally threaded recess 27 in the lower face of the piston body. A nut 28 is threaded onto the end of the bolt and abuts the piston body. A second nut 29 is then threaded into the recess so as to be loaded against the end of the bolt and lock the cooperating threads of the bolt and nut 28. The threads of the two nuts 28, 29 are of opposite hand and a locking washer30 is provided between them. Alternatively, the threads of the two nuts 28, 29 could be of the same hand and locked by other means.

In order to avoid unloading of the bolt 26 due to thermal expansion, a spring member, e.g., a Belleville or dished washer, may be provided between the bolt 26 and the nut 29. Alternatively, a longer bolt 26 could be used and the nut 29 modified as shown in Figure 10 so that it acts resiliently on the end of the bolt.

In the embodiment of Figure 11, the bolt 26 is formed with a thread at its lower free end and projects into a frusto-conical, internally threaded recess 31 in the lower face of the piston body. A nut 32 having an outer profile of a matching frusto-conical shape and formed with an external thread of the same pitch as its internal thread, is threaded onto the bolt and simultaneously engages the internal thread of the recess, eventually locking in place in the recess. The thread starts on the bolt and in the recess have to be accurately positioned to match the thread starts on the nut 32. The nut 32 may be slit, at least partially, to allow radial gripping movement of the threads.

In the embodiment of Figure 12, the lower end of pin 26, after insertion through the bores 12, 13, is swaged open within a recess 34 formed in the lower face of the piston body. The recess is threaded internally and a set screw 35 with a conical end 36 is threaded into the recess so that the conical end clamps the swaged portions of the pin against the bottom of the recess. Spring means such as a Belleville washer may be inserted between the screw 35 and swaged end of the pin 26.

In the embodiment of Figure 13, the lower end of the pin 26 projects into internally threaded recess 40 in the lower face of the piston body. An olive 41 is inserted over the pin and a nut 42 is threaded into the recess over the end of the pin and acts upon the drive to deform it axially against the bottom of the recess and cause it to grip the pin. Preferably, the end of the pin is knurled or serrated so as to allow more positive gripping by the olive. Spring means such as a Belleville washer may be inserted between the nut 42 and the olive 41.

In the embodiment of Figure 14, the pin 26 is formed with a tubular shank 43 and a threaded end portion 44 of reduced diameter with a shoulder 45 therebetween that engages an annular ledge 46 within the lower end of a dual diameter bore 47 formed through the piston body to receive the pin. A nut 48 is threaded onto the end portion of the bolt and tightened against the lower face of the piston body so that the shoulder 45 is clamped against the ledge 46 and a compressive pre-load applied to that portion of the piston between the shoulder 45 and nut 48 that locks the nut 48. Because this clamping action is located at a cooler region of the piston, it is more resistant to thermal loosening. The tightening action of the nut is also used to pre-load the tubular shank 43 of the bolt in tension.

In the embodiment of Figure 15, the pin 26 is tubular and is formed with an internal thread to receive a stud 49 that is engaged from the underside of the piston body. This arrangement is equivalent to that of Figure 1 6 in that the stud 49 acts like the nut 48 to clamp the lower end of the.

pin at 45 against an annular ledge 46 within the dual diameter bore 47 formed through the piston body.

In all of the aforesaid embodiments, the piston body and insert and manufactured as separate components and then assembled together.

However, in other embodiments it is possible that the piston body may be cast around the insert or vice versa and the anchoring means fitted afterwards to connect them together. For example, holes and recesses could be bored in the assembly to receive any of the stud, bolt or pin type anchoring means illustrated in the drawings.

Claims (27)

1. A piston for an internal combustion engine comprising a piston body and a crown portion that are connected together by the insertion of anchoring means into a bore in the piston, the anchoring means comprising at least one member that is rigidly connected axially at one end to a portion of the body, the temperature of which, at maximum operating temperature is such that loosening of the member in that portion is prevented, and is rigidly connected axially at its other end to the crown portion, thereby being loaded both in tension and compression by the forces acting between the piston body and crown portion in operation.

2. A piston as claimed in claim 1 in which at least one of said connections between the anchoring member and the piston body and crown portion is a screw threaded connection with preloaded or locked threads.

3. A piston as claimed in claim 2 in which said anchoring member is a headed bolt that is rigidly connected axially to one of said crown or body portions by being screw threaded therein and by abutment of the head with said one portion to lock the engaging threads.

4. A piston as claimed in claim 3 in which the bolt is screw threaded in the other of said crown or body portions and the threads locked.

5. A piston as claimed in claim 4 in which said thread of the bolt is locked to the thread of said other portion by a lock nut, set screw or grub screw.

6. A piston as claimed in any one of claims 3 to 5 in which the head of the bolt engages a recess in the crown portion and abuts the bottom of the recess.

7. A piston as claimed in claim 2 in which the anchoring means is a stud that is rigidly connected axially to one of said crown or body portions by being screw threaded in a blind hole therein and by abutment of the leading end of the stud with the end of said hole.

8. A piston as claimed in claim 7 in which the stud is screw threaded in the other of said crown or body portions and the threads locked.

9. A piston as claimed in claim 8 in which said thread of the bolt is locked to the thread of said other portion by a lock nut, set screw or grub screw.

10. A piston as claimed in any one of claims 7 to 9 in which the stud is screw threaded in a blind hole in the underside of the crown portion.

11. A piston as claimed in any one of claims 2 to 10 in which each of said screw threaded connections between the anchoring member and the crown or body portion is formed using a threaded anchoring member in said crown or body portion during insertion therein.

1 2. A piston as claimed in claim 2 in which said screw threaded connection is produced between cooperating pre-formed threads that are locked by a difference in pitch between the two threads.

1 3. A piston as claimed in claim 1 in which the anchoring member is rigidly connected axially at one end to one of said crown and body portions by a nut that is threaded over said one end of the anchoring member so as to abut the bottom of a recess in said one portion, and that, is loaded against said bottom by a member threaded in said recess so as to abut said one end of the anchoring member.

14. A piston as claimed in claim 1 in which the anchoring member is rigidly connected axially at one end to one of said crown and body portions by a frusto-conical shaped nut that is threaded on said one end of the anchoring member and is received in a correspondingly frusto-conical shaped recess in said one portion.

1 5. A piston as claimed in claim 1 in which the anchoring member is rigidly connected axially at one end to one of said crown and body portions by a member that is threaded in a recess in said one portion and clamps a portion at said one end of the anchoring member against the bottom of said recess.

16. A piston as claimed in claim 1 5 in which said portion at said one end of the anchoring member is a flared portion.

17. A piston as claimed in claim 15 in which said portion at said one end of the anchoring member is an olive.

18. A piston as claimed in claim 1 in which the anchoring member is rigidly connected axially at one end to one of said crown and body portions by a threaded member that is in threaded engagement with a portion of the anchoring member and abuts said one portion to urge a shoulder on the anchoring member against an abutment on said one portion.

19. A piston as claimed in claim 18 in which the threaded member is a nut threaded on a reduced diameter end portion of the anchoring member, said shoulder being formed by the change in diameter of the anchoring member.

20. A piston as claimed in claim 19 in which the anchoring member is tubular throughout the larger diameter portion of its length.

21. A piston as claimed in claim 18 in which the threaded member is a bolt that is threaded into the end of the anchoring member, said shoulder being the free end of said anchoring member

22. A piston as claimed in any one of claims 1 8 to 21 in which said one portion is the body portion and the anchoring member is received in a bore in the body portion having an internal shoulder to form said abutment.

23. A piston as claimed in any one of claims 1 to 17 in which said one portion is the body portion.

24. A piston as claimed in any one of claims 13 to 21 in which said one portion is the body portion and the anchoring member is formed with an end portion that is received in a recess and is welded or brazed in place.

25. A piston as claimed in any one of the preceding claims in which the crown portion is an insert having an inner peripheral lip that forms the lip of a combustion chamber.

26. A piston substantially as herein described with reference to Figures 1,2,3,4 or 5 to 7,8,9, 10,11,12,13, 14or 15.

27. An internal combustion engine provided with a piston, as claimed in any one of the preceding claims.