CA1067769A

CA1067769A - Light metal pistons

Info

Publication number: CA1067769A
Application number: CA283,390A
Authority: CA
Inventors: Frederick E. Cockcroft
Original assignee: Hepworth and Grandage Ltd
Current assignee: Federal Mogul Bradford Ltd
Priority date: 1976-07-24
Filing date: 1977-07-22
Publication date: 1979-12-11
Also published as: IN148225B; IT1079329B; BR7704827A; FR2359334B1; ES460997A1; FR2359334A1; JPS5314218A; AU507668B2; NZ184707A; AU2725577A; DE2733269A1

Abstract

ABSTRACT
A light metal piston, for an internal combustion engine or a compressor, has a crown with a skirt forming with the crown a closed internal cavity (i.e. with no transverse slot between the crown and the skirt).
Inserts of lower coefficient of expansion than the light metal have part of their inner faces open to the interior cavity of the piston, and break through around the thrust faces into the lower piston ring groove, whereby to separate the light metal of the skirt from that of the crown. The inserts are substantially symmetrical on each side of the thrust axis, having their free ends extending towards the respective gudgeon pin bosses, and terminating outside or only marginally within the bosses.

Description

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77f~9 : . ` , ~ This invention relates to light-metal pistons for : reciprocating combustion engines or for reciprocating compressors, that is pistons made predominantly of aluminium, magnesium, or similar metals and their alloys.
Pistons having expansion control inserts, usually of steel, to control the thermal expansion of the piston ` skirt are well known. Such pistons may either have a ~;~ transverse slot between the ring groove band and the piston skirt on one or both thrust faces of the piston, ~,; ' i' ,, or the ring groove band and the skirt may be integral.
If the pistons have a transverse slot one or both ~: i thrust faces, compressive forces are unsupported in this region and, due to the variation in pressure loading on the piston crown during each cycle of operation, the slot opens and closes slighly on such cycle. This ultimately , . ~
leads to fatigue failure, usually at the ends of the slot.

' On the other hand, the transverse slot allows an expansion ;.; .
~ control insert to exert good control of the thermal .~
~ expansion of the upper end of the skirt, because the skirt ,. j .. .
` 20 is not rigidly connected to the ring band.
;" ~ -~; i( When the ring groove band and the skirt are integral, .. ..
~; compressive forces are supported and fatigue failure from the cuase referred to is unlikely. But the control of the ,;" ~ ~
thermal expansion of the top end of the skirt by means of ,~
the insert is adversely affected, because the top end of the skirt is rigidly connected to the ring band and tends ; ~. I
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, :: 1067769 to expand with it. The problem is made worse because the ring band in operation usually becomes hotter than the ~ skirt.
- Moreover, hitherto expansion control inserts have been embedded in, and extend on each side of, the gudgeon pin bosses, there normally being two inserts, one associated , with each boss.
Typical example of pistons with expansion control inserts are found in German Patentschrift 1,003,990 ; 10 Patentschrift 1,041,293, Auslegeschrift 1,284,761 and ; Auslegeschrift 1,284,762. The latter clearly shows a , piston having a transverse slot.
It is clearly difficult to reconcile good thermal expansion control of the top of the skirt with a pistoh ~; 15 which is strong in compression and resistant to fatigue , ' failure. Moreover, we have found that the practice of embedding the expansion control insert in the gudgeon pin ", boss tends to weaken the boss, which is one of the most highly-stressed regions of the piston.
v, l 20 The object of the present invention is to provide k~
designs of light-metal piston which have good thermal expansion control of the top of the skirt, are simultan-eously strong in compression, and have inserts which do ~,:.;: . : ~ .
not weaken the gudgeon pin bosses. It will also be well 25 understood that the piston designs must be capable of .'7 i~

manufacture on a production scale.

, ,According to the invention, a light-metal piston ,ifor a combustion engine or a compressor has a crown . '' :'

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~ `` 1067769 i including an annular band in which piston ring grooves are formed, a skirt ~ .
around the opposing thrust faces of the piston in a manner to provide with - the crown a closed internal cavity, gudgeon pin bosses having bores opening into said cavity, and two expansion control inserts made of a material of r .
; lower coefficient of thermal expansion than the light metal, part of the inner ~: face of each insert being open to the interior cavity of the piston, each .
. insert having an upper part breaking through into the lower piston ring - groove, each insert having a lower part with the lower part of each insert ` being at a greater distance from the open end of the skirt than the part of the gudgeon pin boss nearer to the open end, with the inserts separating the . light metal of the piston skirt from that of the annular band around the ,~.; opposite thrust faces of the piston, each insert having an upper elongated band and a lower elongated band and at least one interconnecting portion between the upper and lower bands, the inserts being each substantially :.:. symmetrically disposed on each side of the thrust axis of the piston, and having free ends extending towards the respective gudgeon pin bosses, the free ends of each insert being disposed on the same side of the gudgeon pin axis, 1'."~'4' and the free ends of each insert terminating at most only marginally within . the gudgeon pin bosses.
A number of embodiments of the invention will now be described with reference to the accompanying drawings, of which:-Figure lA is a view of a first design of piston showing a cross-:
.~ section on the thrust axis, and Figure lB is a view of the same piston :~ showing a cross-section on the gudgeon pin axis.
Figures 2A and 2B show cross-sections on the lines ;;:, ,.. .
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IIA - IIA and IIB -IIB respectively of Figures lA and IB, Figure 3 is a developed view of the insert of the piston shown in Figures lA, lB, ZA and 2B, -.
.. . 5 Figures 4A, 4B, 5A and 5B are views of a second ~: design of piston, Figure 4A corresponding to Figure lA, . Figure 4B corresponding to Figure lB, Figure 5A being a :~
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section on VA - VA, Figure 5B being a section on VB -VB, and Figure 6 being a developed view of the insert, Figures 7A, 7B, 8A and 8B are views of a third design of piston, Figures 7A and 7B corresponding respec-tively to Figures lA and lB, Figure 8A being a section on .::
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VIIIA - VIIIA and Figure 8B being a section on VIIIB -.,.~
` VIIIB, Figure 9A, 9B, lOA and lOB are views of a fourth design of piston, Figures 9A and 9B corresponding respec-tively to Figures lA and lB, Figure lOA being a section ;
: on XA - XA and Figure lOB being a section on XB - XB, ~: Figures llA, llB and 12 are views of a fifth design 20 of piston, Figures llA and l.lB corresponding respectively .
to Figures lA and lB, and Figure 12 being a section on ~ :
XII - XII, ~ .; . .
: Figures 13A and 13B are views of a sixth design of .; piston, corresponding respectively to Figures lA and lB, .. ,, . ~
. 25 Figures 14, 15 and 16 are views of a seventh design -of piston, Figure 14 corresponding to Figure lA, Figure 15 being a section on XV - XV, and Figure 16 being a .; . developed view of the insert of this design. ~.
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Figure 17 is a view of an eighth design of piston, corresponding to Figure lA, and ~- Figures 18 to 23 are views of a ninth design of piston, Figure 18 being a cross-section on the line XVIII
; 5 - XVIII of Figure l9, Figure l9 being a cross-section on " , ~ the line XIX - XIX of Figure 18, Figure 20 being a cross-~`.' ., , section on the line XX - XX of Figure 19, Figures 21, 22 ; and 23 being views of the insert, Figure 21 looking on -~ arrow XXI of Figure 23, Figure 22 looking on arrow XXII of Figure 21, and Figure 23 looking on arrow XXIII of Figure 12.
Referring to Figures lA, lB, 2A, 2B and 3, the piston ; comprises a crown 20, an annular band 21 in which piston ring grooves 22 are formed, a skirt 23 integrally connected with the annular band, a pair of gudgeon pin bosses 24 formed integral with the skirt 23 and having ~oaxially bores, thrust faces 25 on the thrust axis of the skirt, which axis is at right angles to the gudgeon pin axis, and pillars 26 extending from the bosses 24 to the crown 20.
~; ! 20 The piston is made of light alloy (e.g. aluminium or its alloy) and is provided with a pair of inserts of a ~; material having a lower coefficient of thermal expansion jl ..:,, ; than the light alloy (e.g. of sheet steel).
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In the embodiment each insert 27 has an upper elongated band 28, a lower elongated band 29, and a i,; . ~ .
, connecting portion 30. It will particularly be noted ,.. . .
I ~ that contrary to the usual practice, the inserts 27 are . .. . .

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~ symmetrical about the thrust axis of the piston, the con- -:,.;,,, : ' ~; necting portions 30 laying on the thrust axis and the bands 28, 29, extending towards the respective gudgeon pin , . ii .- .
bosses. Part of the inner face of both the upper bands 28 and the lower bands 29 is open to the interior cavity of the piston, but the upper bands 38 break through into . ~ .
the lower piston ring groove and separate the light metal ~, of the piston skirt 23 from that of the annular band 21 ,.
~;; around the opposite thrust faces of the piston. The free ends of the bands 28, 29, may terminate either marginally "~
within (as shown) or outside the gudgeon pin boss 24, and ~ -~; in this embodiment the upper bands are of the same length as the lower bands. -~ . i .. .
:i:~:, Referring now to Figures 4A, 4B, SA, 5B and 6 the piston is similar to the first design except for the inserts. Moreover, the materials of the piston body and of the insert are as specified above. ~`
In this e~bodiment, each insert 37 has an upper elongated band 38 and a lower elongated band 39, and these bands are joined together at both ends of each insert by connecting portions 40. The inserts 37 are symmetrical about the thrust axis of the piston; part of the inner ~
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? face of both the upper band 38 and the lower band 39 is '~ open to the interior cavity of the piston, but the upper .... .
! 25 band 38 breaks through into the lower piston ring groove "'.~; :
and separates the light metal of the piston skirt 23 from that of the annular band 21 around the opposite thrust : faces of the piston. The connection portions 40 marginally ~' 6 . , .
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~; 1067769 ~ overlap the gudgeon pin bosses 24.
~ , ~ Referring to Figures 7A, 7B, 8A and 8B, the piston d. ~
~- and inserts are exactly similar to those of the first x design, except that the lower elongated bands 49 are shorter than the upper elongated bands 48.
- Figure 8A also shows a modification which may be ~ ,, 4s ' applied to any of the embodiments described herein. In this modification the distance between the two ends of the inserts 47 is reduced, and the light alloy is provided with fillets 51 between the thrust faces 25 and the ,~
gudgeon pin bosses 24 in which at least part of the upper bands 48 are embedded.
Referring to Figures 9A, 9B, lOA and lOB, the fourth design of piston and inserts are again similar to those , .
' 15 of the first design except for the omission of the lower elongated bands of each insert. Thus the inserts 57 i; have elongated bands 58 with portions 60 lying on the thrust axis, forming a T shape. The bands 58 break ~ ..; i ` through into the lower piston ring groove and are in all respects similar to bands 28. The insert 57 may have a .~ ,.................................................................... .
r.~ hole 61 on the thrust axis to assist in locating the ., ~ . .
insert in the die for casting.
In Figures llA, llB and 12, the inserts 67 are . . ~ :
~` identical with those of the fourth design, except that, unlike portions 60, the sides of which are parallel to the longitudinal axis of the piston, the portions 70, .', , ` from which elongated bands 68 extend towards the gudgeon ~, . o -pin bosses, have their sldes at approximately 45 to the --" . . ~ . , ;'~`- 7.
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longitudinal axis.
., Moreover, Figure 12 also shows a modification which may be applied to any of the embodiments described herein. ~ -¦, In this modification the upper faces of bands 68 are ~,. 5 stepped, so that they break through into the lower piston ', ring groove for a distance on each side of the thrust axis, but not towards their ends remote from the thrust c axis. It will be understood that a small part of each insert may be removed during the machining of the lower ,~ lO piston ring groove. ~ ' ' In the sixth embodiment shown in Figures 13A and ... . .
13B, each insert 77 is composed of a single band 78, which is symmetrical about the thrust axis; a notch 81 .
may be provided on or adjacent to the thrust axis to assist in locating the insert in the die.
;~ Referring now to Figures 14 - 16, the piston is again as described above except for the inserts 87. The latter are again symmetrical about the thrust axis, but each has two connecting portions 90 between the upper elongated band 88 and the lower elongated band 89. The ;........................................................................ . .
i' bands 88, 89, are otherwise as described with reference ;~ to the first design.

~: The eighth design of piston, shown in Figure 17, is ~, the same as that of the second design (shown in Figures 4A, 4B, 5A, 5B and 6) except that the connecting portions ~b"' 100 are extended away from the piston crown to provide ~" anchorage faces 101, and the upper faces of bands 98 are ~'~ .................................................................... .
s stepped as described with reference to Figure 12.
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1067'769 , In the embodiment of Figures 18 to 23, each insert ~4 107 has an upper elongated band 108, a lower elongated . ' .:
band 109 and connecting portions 110 at the ends of the bands and 111 at their centre.
, 5 It will be noted that the inserts 107 are symmetri-cal about the thrust axis of the piston, the connecting portions 111 laying on the thrust axis and the bands 108, 109, extending towards the respective gudgeon pin bosses where they are joined at their ends by connecting portions 110. Part of the inner face of the upper bands 108 and ~, of the lower bands 109 is open to the interior cavity of ,, the piston, but the upper bands 108 break through into the lower piston ring groove and separate the light metal of the piston skirt 23 from that of the annular band 21 around the opposite thrust faces of the piston. The ends ~..
of the inserts (formed by connection portions 110) may terminate either marginally within (as shown) or outside '; the gudgeon pin boss 24. ~'' It will be noted that in each case the transverse s 20 slot commonly provided between the annular band and the skirt is omitted, and that the inserts separate the skirt from the annular band around the opposite thrust faces by ....
'; ~ breaking into the lowest piston ring groove.

;~ Pistons having inserts in accordance with the present invention have been found to have greater stiffness on the ''.
thrust face than pistons having inserts which are centred ' on the gudgeon pin boss and extend partway round the .~ circumfer`ence on each side of the gudgeon pin boss.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A light metal piston for a combustion engine or a compressor having a crown including an annular band in which piston ring grooves are formed, a skirt around the opposing thrust faces of the piston in a manner to provide with the crown a closed internal cavity, gudgeon pin bosses having bores opening into said cavity, and two expansion control inserts made of a material of lower coefficient of thermal expansion than the light metal, part of the inner face of each insert being open to the interior cavity of the piston, each insert having an upper part breaking through into the lower piston ring groove, each insert having a lower part with the lower part of each insert being at a greater distance from the open end of the skirt than the part of the gudgeon pin boss nearer to the open end, with the inserts separating the light metal of the piston skirt from that of the annular band around the opposite thrust faces of the piston, each insert having an upper elongated band and a lower elongated band and at least one interconnecting portion between the upper and lower bands, the inserts being each substantially symmetrically disposed on each side of the thrust axis of the piston, and having free ends extending towards the respective gudgeon pin bosses, the free ends of each insert being disposed on the same side of the gudgeon pin axis, and the free ends of each insert terminating at most only marginally within the gudgeon pin bosses.

2. A light metal piston as claimed in claim 1, wherein said inter connecting portion between the upper band and the lower band lies on the thrust axis of the piston.

3. A light metal piston as claimed in claim 1, comprising intercon-necting portions between the upper band and the lower band only at or near the opposite ends of the insert.

4. A light metal piston as claimed in claim 1, in which the upper faces of the inserts are stepped so that they break through into the lower piston ring groove for a distance on each side of the thrust axis but not towards their ends remote from the thrust axis.

5. A light metal piston as claimed in claim 1, in which the piston is provided with fillets between the thrust faces and the gudgeon pin bosses in which part of the inserts are embedded.