U.S. patent application number 12/442645 was filed with the patent office on 2010-01-14 for method for the production of a piston for internal combustion engines and piston for an internal combustion engine.
Invention is credited to Heraldo Carlos Furquim, Joao Lester Garcia.
Application Number | 20100006055 12/442645 |
Document ID | / |
Family ID | 37808215 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100006055 |
Kind Code |
A1 |
Garcia; Joao Lester ; et
al. |
January 14, 2010 |
Method for the production of a piston for internal combustion
engines and piston for an internal combustion engine
Abstract
A method for producing a piston for internal combustion engines
includes the following steps: a first part is pre-fabricated by hot
forging and a recess which has an undercut is formed in the first
part during pre-fabrication by forming on the first part a
projection, to which projection a lateral force is applied to form
the undercut; a second part is pre-fabricated by hot forging and a
projection is formed on this second part whose dimensions are
matched to the dimensions of the recess; the two parts are joined
together so that the projection on one part engages in the recess
in the other part; and a compressive force is applied to the two
parts which is sufficiently large and so aligned that the material
of the projection on one part flows into the recess in the other
part and completely fills it to connect the parts by positive
fit.
Inventors: |
Garcia; Joao Lester; (Sao
Paulo, BR) ; Furquim; Heraldo Carlos; (Sao Paulo,
BR) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Family ID: |
37808215 |
Appl. No.: |
12/442645 |
Filed: |
September 25, 2007 |
PCT Filed: |
September 25, 2007 |
PCT NO: |
PCT/EP07/60158 |
371 Date: |
July 14, 2009 |
Current U.S.
Class: |
123/193.6 ;
29/888.042; 72/352; 92/255 |
Current CPC
Class: |
B21K 25/00 20130101;
F02F 2200/04 20130101; B23P 15/10 20130101; B21K 1/185 20130101;
F02F 3/003 20130101; Y10T 29/49252 20150115 |
Class at
Publication: |
123/193.6 ;
29/888.042; 72/352; 92/255 |
International
Class: |
F02F 3/00 20060101
F02F003/00; B23P 15/10 20060101 B23P015/10; B21J 5/00 20060101
B21J005/00; F16J 1/00 20060101 F16J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2006 |
EP |
06121267.6 |
Claims
1. Method for producing a piston for internal combustion engines in
which a first part is pre-fabricated by hot forging and a recess
which has at least one undercut is formed in the first part during
pre-fabrication by forming on the first part, by means of a shaping
tool, a projection which is directed substantially in an opposite
direction to that in which the shaping tool acts, to which
projection a lateral force which is directed in the direction of a
receptacle is then applied to form the at least one undercut, a
second part is pre-fabricated by hot forging and a second
projection is formed on the second part whose dimensions are
matched to the dimensions of the recess in such a way that the
second projection can be slid freely into the recess of the first
part and a volume of the second projection is at least equal to a
volume of the recess, the first and second parts are joined
together so that the second projection of the second part engages
in the recess of the first part, and a compressive force is applied
to the first and second parts which have been joined together which
is sufficiently large and so aligned that material of the second
projection of the second part flows into the recess in the first
part and completely fills it to connect the first and second parts
by positive fit.
2. Method according to claim 1, wherein the recess and the second
projection are formed at respective end-faces of the first and
second parts.
3. Method according to claim 1, wherein, the first part forms a
head of the piston and the second part forms the skirt of the
piston.
4. Method according to claim 3, wherein the second projection is
associated with the skirt part of the piston and the recess with
the head of the piston.
5. Method according to claim 1, wherein the recess has a circular
opening.
6. Method according to claim 1, wherein an opening of the recess is
of an area which is smaller than a projected floor area of the
recess, which projected floor area is situated opposite the
opening.
7. Method according to claim 6, wherein, to form the at least one
undercut, at least a portion or portions of a circumferential
surface surrounding the recess are aligned, starting from the floor
area, to be inclined towards the area of the opening.
8. Method according to claim 1, wherein, when force is applied to
make the positive fit connection between the first and second
parts, the second part which is provided with the second projection
is heated to forging temperature at least in the region of the
second projection.
9. Method according to claim 1, wherein the direction in which the
compressive force acts is aligned to be co-axial with a
longitudinal direction of the second projection and the recess.
10. Method according to claim 1, wherein the volume of the second
projection of the second part is of a size such that, taking into
account thermal expansion of the first and second parts, material
of the second projection completely fills the recess of the first
part even in a cooled-down state.
11. Method according to claim 10, wherein the shape of the
circumference of the second projection on the second part is
matched to the shape of the opening of the recess in the first part
in such a way that the second projection is able to be slid into
the opening when it is in the a heated state where it is heated to
hot-forging temperature, and wherein the height of the second
projection is greater than the depth of the recess.
12. Piston for internal combustion engines, produced from a first
part and a second part produced by hot forging which are connected
together by positive fit, wherein a receptacle is formed in the
first part which is surrounded by a circumferential wall and, in
the receptacle, a recess which is surrounded by an encircling
collar portion which is aligned to be inclined at an angle to a
longitudinal axis of the piston in such a way that at least one
undercut is formed which is substantially completely filled in
order to bring about the positive fit of material of a projection
which is formed on the second part, an encircling free space being
left between an outer circumferential surface of the collar portion
and an inner circumferential surface of the circumferential
wall.
13. Piston according to claim 12, wherein the recess and the
projection are formed at respective end-faces of the first and
second parts.
14. Piston according to claim 12, wherein the first part forms a
head of the piston and the second part forms a skirt of the
piston.
15. Piston according to claim 14, wherein the projection is
associated with the skirt of the piston and the recess with the
head of the piston.
16. Piston according to claim 12, wherein the recess has a circular
opening.
17. Piston according to claim 12, wherein a shoulder is formed at a
transition from the projection to a main portion of the second part
projection.
18. Piston according to claim 12, wherein material of the second
part which is provided with the projection surrounds at least a
portion or portions of the collar portion.
19. Piston according to claim 12, wherein the first part is
composed of a first material and the second part is composed of a
second material which is different from the first material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a National Phase Application of
International Application No. PCT/EP2007/060158, filed on Sep. 25,
2007, which claims the benefit of and priority to European patent
application no. EP 06 121 267.6, filed on Sep. 26, 2006. The
disclosures of the above applications are incorporated herein by
reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a method of producing a piston for
internal combustion engines from two pre-fabricated parts which,
having been pre-fabricated, are connected together to form the
piston. As well as this, the invention also relates to a piston
which is produced in a corresponding manner from two parts.
BACKGROUND
[0003] Pistons for internal combustion engines are usually produced
by casting or forging processes. Production by casting has the
advantage that it allows pistons of complex shape and low weight to
be produced. However, what has to be accepted at the same time is
that the production involves considerable cost and complication.
This is particularly true when a steel material is to be used as
the material for producing pistons able to withstand especially
high stresses.
[0004] Depending on their size and intended purpose, forged steel
pistons may both be of a one-piece form and may also be composed of
two or more parts. In the case of multi-piece pistons assembled
from two or more parts, the individual parts are usually connected
together, by suitable joining processes, by friction, bonding, or
positive fit in such a way that they will withstand the forces
acting on them in practical use. What is suitable for this purpose
is for example welding or screwing together of the separate parts
of the piston.
[0005] An example of a multi-piece piston for an internal
combustion engine is known from DE 102 44 513 A1. This piston has,
on the one hand, a head part which is forged from steel and
integrally formed in which are formed dishing for the combustion
chamber, an annular wall and a cooling passage in the form of a
groove. On the other hand the piston has a skirt part which carries
the head part of the piston and in which are formed bosses to
receive a piston pin which connects the piston to the connecting
rod. To produce this piston, the head part and skirt part of the
piston are pre-shaped by forging in separate operations and are
then machined by stock-removing machining to finish them. The
finish machining of the head part of the piston also includes in
this case the stock-removing machining of the portions of wall
adjoining the cooling passage, by means of which portions of wall a
joint is then made to the skirt part of the piston by physical
union by welding or brazing.
[0006] It is true that multi-piece construction of this kind allows
the piston which is formed from the two parts to be made of a
complex shape. However, apart from the problems relating to
load-bearing capacity which arise from its multi-piece nature, the
cost and complication its production involves are considerable.
[0007] Disadvantages of the production of one-piece pistons are the
high weight of the blank for the piston, as a result of which
processing and handling equipment of particularly large dimensions
is required, and the expense involved in the mechanical
post-processing which is inevitably required in present-day
practice. Despite the advantages that one-piece pistons have, as
far as their load-bearing capacity is concerned, the disadvantages
mentioned mean that when production is conventional one-piece
pistons can only be produced at increased production costs.
[0008] One possible way of connecting together, by forging, a
piston formed from two previously manufactured parts is known from
JP 03-267552 A. In this piece of prior art, a piston-skirt blank
whose basic shape is that of a cylinder is produced by sintering a
metal powder. A projection which is of a circular disc-like shape
is produced on the end-face of the piston-skirt blank when this is
done.
[0009] In addition to the skirt part of the piston, what is also
produced by the known method is a head part of the piston which is
likewise of a disc-like basic shape. The diameter of the skirt part
of the piston corresponds to the diameter of the head part of the
piston in this case. Formed in the end-face of the head part of the
piston is a recess whose opening is so defined by an encircling
portion which projects into the recess that an undercut is formed
between the said portion and the floor area of the recess. To allow
the skirt part and head part of the piston to be joined together,
the head part of the piston is first placed in a die whose inside
diameter corresponds to the outside diameter of the skirt part and
head part of the piston. The recess in the head part of the piston
faces towards the opening of the die while the said head part of
the piston is supported at its other end-face by means of a punch.
The piston-skirt blank is then introduced into the die until its
projection is seated in the recess in the head part of the piston.
The head part of the piston then has a forging force applied to it
by means of a shaping punch, which force causes the material of the
skirt part of the piston to flow into the recess in the head part
of the piston and to fill the undercut which is formed in the
latter. The skirt part of the piston is given its cup-like final
configuration at the same time.
[0010] The piston which is produced by the method from JP 03-267552
A is of an outside shape which is, in essence, completely
cylindrical. Formed in the circumferential surface of the skirt
part of the piston in this case, closely adjacent to the head part
of the piston which is carried by the skirt part of the piston, are
grooves for piston rings. Neither the skirt part of the piston nor
the head part of the piston have, in this case, any additional
configurational features which would make them suitable for a
modern-day internal combustion engine. In particular, the known
piston does not have any special shaping of the head part of the
piston of the kind which is nowadays required if optimum use is to
be made of the energy from the fuel which is burnt in the given
internal combustion engine. It is also found that simple designs of
piston of the kind described in JP 03-267552 A are not equal to the
thermal demands which arise in modern-day internal combustion
engines.
[0011] Comparable possible ways of producing pistons from two parts
by means of a positive fit between the parts produced by forging
are known from DE 725 761 C, JP 54-021945 A, GB 2 080 485 A or U.S.
Pat. No. 3,075,817 A1. However, what all these pieces of prior art
have in common is that the pistons which are assembled in a known
manner from two parts are each of a simple shape which no longer
meets the modern-day demands that are made of pistons for internal
combustion engines.
SUMMARY OF THE INVENTION
[0012] Against the background of the prior art explained above, an
aspect underlying the invention is therefore to provide a method
which makes possible the inexpensive production even of pistons of
complex shape for internal combustion engines. Another aspect is
also to specify a piston for internal combustion engines which can
be produced inexpensively with great accuracy of manufacture
despite its being of complex shape.
[0013] In accordance with the invention, the connection between the
two parts of the piston is made by means of a mechanical connection
in which the material of the projection on one part is clamped by
the material surrounding the recess in the other part in such a way
that the two parts are indissolubly connected together. For this
purpose, there is formed in the region of the recess in one part an
undercut which, on the two parts being compressed, is filled by the
material of the projection which flows into it. There is formed in
this way a mechanical locking system which operates in essence by
positive inter-engagement and which ensures that the two parts of
the piston produced in accordance with the invention are held
solidly together in a durable way. A major advantage of the
invention lies in this case in the fact that the individual parts
from which the piston is assembled, which are composed of a steel
material for example, can be preformed in a completely finished
form and the connection between the parts can be made without any
additional connecting members such as screws or bolts. The
mechanical connection which is provided in accordance with the
invention, which is made by material of the two parts interlocking
by positive fit, makes it possible in this case for the at least
two individual parts from which a piston according to the invention
is assembled to be accurately pre-shaped. When they are put
together to form the piston, they are therefore of a minimized
weight, which means that only low forces have to be applied to
handle the workpieces. What is more, due to the joining process
according to the invention, there is no change in the basic shape
of the piston and a consequence of this is that, at least as a
rule, only a very much reduced amount of mechanical post-processing
of the fully joined piston is utilized.
[0014] Something which proves to be particularly advantageous in
this connection is that the way in which the two parts of the
piston are connected in accordance with the invention makes it
possible for the piston to be produced by hot-forging operations
alone. In this way, as well as the two parts of the piston being
pre-fabricated by hot forging, the undercut which is formed in one
part may also be produced by hot-forging steps.
[0015] For this purpose, a projection is first formed on the first
part by means of a shaping tool, which projection is directed
substantially in the opposite direction from that in which the tool
acts. A lateral force which is directed in the direction of the
receptacle is then applied to this projection to form the undercut.
When the undercut is produced in such a way in two stages, a
projection which has no undercut and from which the forging tool
can be separated again by a simple lifting movement is first formed
on the first part by means of a suitable tool. Then, by the lateral
application of force, the projection is sloped in the direction of
the receptacle of the first part in such a way that the projection
makes an angle of less than 90.degree. between its free end and the
bottom of the receptacle. Any additional stock-removing machining
to make the undercut can be avoided in this way.
[0016] What is more, with the manner of production in accordance
with the invention there is no longer any need for the parts of the
pistons to be heated to their melting point locally. With a piston
according to the invention there is likewise no longer any risk of
changes in microstructure or of stresses arising in the piston
which such heating involves.
[0017] Another significant aspect of the invention is that the at
least two parts are connected together by a simple operation
comparable to a forging step. The apparatus required for this
purpose can be designed to be simple and hence inexpensive because
a special die or comparable aids which determine the flow of the
material and prevent the components from deforming are not required
in the region of the connecting zone and instead the desired
filling of the recess in one part of the piston by the material of
the projection on the other part of the piston is ensured by the
fact that the projection is plugged into the recess in the other
part and the flow of material which then occurs when pressure is
applied is determined by the shape of the recess itself.
[0018] The outcome is that the invention thus makes available a
method which, in a simple and inexpensive way, makes it possible
for pistons for internal combustion engines to be produced which
are very accurately shaped and, at the same time, able to carry
high stresses. Their configuration is selected in such a way in
this case that they can be joined together from two parts with
simple means without the need for expensive and complicated
apparatus or excessively high forces. An embodiment of the
invention which is particularly right for practical requirements is
characterized in that the recess and the projection are formed at
respective end-faces of the parts respectively associated with
them. In this embodiment, all that is required to cause the desired
flow of material is a compressive force acting in the direction of
the longitudinal axis of the piston which is to be produced. At the
same time, what is ensured in the case of this arrangement is a
connection which is optimum with regard to the stresses which occur
in practical use.
[0019] A particularly simple form for the parts of the piston and a
variant of the method according to the invention which can be
carried out in an equally simple way are obtained when one part
forms the head of the piston to be produced and the other part
forms the skirt thereof.
[0020] Basically, it is immaterial to the success of the invention
which of the parts the projection and recess are respectively
associated with. In this way, in cases where one part forms the
piston skirt to which the given connecting rod is coupled in
practical use and which guides the piston in the bore of the
cylinder and where the other part forms the piston head in whose
end-face remote from the piston skirt a dishing for the combustion
chamber is usually formed, it is possible for the projection to be
formed on the head part of the piston and the recess to be formed
in the skirt part thereof. However, from the production point of
view it has proved to be particularly practical for the projection
to be associated with the skirt part of the piston and the recess
with the head part thereof.
[0021] Something which also makes a contribution to simplifying
that part of the piston which is provided with the recess in the
course of previous manufacture is for the recess concerned to have
a circular opening.
[0022] The undercut which is provided in accordance with the
invention in the region of the recess can easily be produced by
making the opening of the recess an area which is smaller than the
projected floor area of the recess, which projected floor area is
situated opposite the opening. With sizing of this kind, the area
of the opening is always smaller than the floor area when the
latter is projected into the plane of the area of the opening. What
this means is that, when the floor area is seen in plan, at least a
portion or portions of the edge of the opening are arranged to be
offset from the edge of the floor area towards the center of the
floor area, which means that an undercut is necessarily formed at
the portions in question as the edge of the opening changes to the
edge of the floor area. The undercut may be formed in this case by,
starting from the floor area of the recess, aligning at least a
portion or portions of the circumferential surface surrounding the
recess to be inclined towards the area of the opening.
[0023] Basically, it is conceivable for the parts which together
form the piston to be connected together by cold forming. However,
a considerable simplification of the complication which this kind
of forming involves can be achieved by, when force is applied to
make the positive fit connection between the first and second
parts, heating the part which is provided with the projection to
forging temperature at least in the region of the projection. When
this is the case, the first, cold, part acts, by means of its
receptacle, as a die for the forming of the projection on the
second part, which projection is inserted into the receptacle and
is at forging temperature, which means that there is an assurance
of even and complete filling of the undercut region of the
receptacle by the material of the projection in the course of the
deformation of the projection which occurs as a result of force
being applied.
[0024] The support which one part of the fully assembled and joined
piston has on its other part may be boosted by forming a shoulder
at the transition from the projection to the main portion of the
part associated with the projection. The other part is able to
support itself on this shoulder at least by the wall which defines
its recess.
[0025] Something that has proved particularly apt for practical
requirements is an embodiment of the invention in which at least a
portion or portions of the recess are defined by a freely projected
collar portion. This collar portion on the one hand forms the
shaping element by which the undercut which is filled with the
material of the projection on the other part is formed in the
region of the recess. On the other hand, the flow of material which
occurs in the course of the application of pressure can be steered
in such a way that the collar portion ensures that the two parts
are clamped together reliably, securely and durably by engaging
comparatively deeply into the material of the part of the piston
which is provided with the projection and by virtue of the fact
that the material of the part provided with the projection
surrounds at least a portion or portions of the collar portion.
[0026] The security with which the two parts of the piston
according to the invention are held together even under the
heating-up which occurs in operation can be optimized, while at the
same time not changing the simple assembly process, by making the
volume of the projection on one part of a size such that, taking
into account the thermal expansion of the two parts, the material
of the projection completely fills the recess in the other part
even in the cooled-down state. For this purpose the shape of the
circumference of the projection on one part may be matched to the
shape of the opening of the recess in the other part in such a way
that the projection is able to be slid into the opening when it is
in the state where it is heated to hot-forging temperature, and in
such a way that the height of the projection is greater than the
depth of the recess.
[0027] A significant advantage of the invention lies in the fact
that the manner in accordance with the invention of producing a
piston allows the respective materials which are selected for the
two parts from which the piston is assembled to be ones which are
optimally matched to the stresses which act on the respective parts
in operation. In this way, the invention makes it possible, when
selecting the respective materials, for allowance to be made not
only for the respective mechanical stresses but also for stresses
which arise as a result of, for example, thermal or chemical
effects to which a piston according to the invention is exposed in
practical use.
[0028] It is therefore proposed in a particularly advantageous
embodiment of the invention that one part of a piston according to
the invention be manufactured from a first material and that the
other part be manufactured from a second material which is
different from the first material. As a function of the particular
area of use, the first part for example may therefore be previously
manufactured from steel of a first grade and the second part from
steel of a second grade, or the first part from a grade of steel
and the second part from another metallic material and in
particular a light metal, or the first part from a ceramic and the
second part from a metallic material. As well as hot forging being
used as a method of previous manufacture for forgeable materials,
previous manufacture by sintering may also be used in accordance
with the invention at least for the head part of the piston. The
starting material for the head part of the piston is then powdered
metal for sintering.
[0029] The invention also allows the individual parts from which a
piston is assembled in the manner according to the invention to be
differently heat-treated or differently treated in some other way
to allow for the stresses which act on the respective parts in
practice.
[0030] The production and configuring in accordance with the
invention of a piston for internal combustion engines thus provides
a wide range of possible means of optimization which allow pistons
of this kind each to be matched to their respective intended uses
in the optimum way.
[0031] The piston according to the invention is so designed that,
while being able to be produced easily, it meets the demands made
of modern-day pistons. In this way, it is assembled from two parts
produced by hot forging which are connected together by positive
fit. At the same time however, in the region of the transition
between the head of the piston and the skirt of the piston, an
encircling free space which is known per se by means of which the
heat which arises in practical use is dissipated in terms of a
cooling passage. To achieve this, there is formed, in accordance
with the invention, on one part a receptacle which is surrounded by
a circumferential wall and, in this receptacle, a recess which is
surrounded by an encircling collar portion which is aligned to be
inclined at an angle to the longitudinal axis of the piston in such
a way that at least one undercut is formed which is substantially
completely filled in order to bring about the positive
inter-engagement of material of a projection which is formed on the
other part, the said free space being left between the outer
circumferential surface of the collar portion and the inner
circumferential surface of the circumferential wall.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The invention is explained in detail below by reference to
drawings which show an embodiment. In the drawings, which are each
schematic longitudinal sections:
[0033] FIG. 1 shows a piston assembled from two parts.
[0034] FIG. 2 shows the parts from which the piston shown in FIG. 1
is assembled.
[0035] FIG. 3 and FIG. 4 show two of the operating steps which are
performed when the first part of the piston is being produced.
DESCRIPTION
[0036] The piston 1 is assembled from a first, head part 2 of the
piston which forms its head and a second, skirt part 3 of the
piston which forms its skirt, which parts are connected together by
positive fit and friction in the region of a joint zone 4 which is
formed between the head part 2 of the piston and the skirt part 3
thereof. The head part 2 of the piston, the skirt part 3 of the
piston and also the connection by friction and positive fit between
the said two parts 2, 3 are produced in this case by hot-forging
operations.
[0037] The head part 2 of the piston is produced from a steel blank
by hot forging and is of a disc-like basic shape. Formed in that
end-face 5 of the head part 2 of the piston which is associated in
practical use with a combustion chamber (not shown) in an engine
block (not shown likewise) is a dishing 6 for the combustion
chamber. Following on from the end-face 5 there is a
circumferential wall 7 which points in the direction of the skirt
part 3 of the piston and which surrounds a receptacle 9 which is
formed in that end-face 8 of the head part 2 of the piston which is
associated with the skirt part 3 of the piston. The area at the
bottom of the receptacle 9 is situated opposite the skirt part 3 of
the piston and formed in it is a recess 10.
[0038] To produce the head part 2 of the piston, a preform (not
shown) is first produced by simple upsetting from a steel blank
(not shown likewise) which is heated to a forging temperature of
approximately 1050.degree. C., from which preform a piston-head
blank 2a whose basic shape already corresponds to that of the head
part 2 of the piston is then produced by means of a forging tool
(also not shown). The recess 10, in its rough shape, has already
been formed in this case in the piston-head blank 2a by means of
the forging tool. At the same time, a non-undercut projection 12a
has been formed on the piston-head blank 2a by the forging tool,
which projection 12a surrounds the recess 10 in an annular form and
is aligned in the opposite direction to that direction R in which
the forging tool (not shown) acts. In the case of the piston-head
blank 2a, the face of the inner wall of the recess 10 surrounded by
the projection 12a is thus substantially cylindrical.
[0039] The calibration of the piston-head blank 2a then takes place
in a further forging operating step. For this purpose, the
piston-head blank 2a is placed in a two-piece calibrating tool K
whose bottom part K1 associated with the end-face 5 of the
piston-head blank 2a copies the finished shape of the dishing 6 for
the combustion chamber in the head part 2 of the piston. The top
part K2 of the calibrating tool K has, by contrast, on its side
associated with the bottom part K1 of the tool, a projection V
which extends round in an annular shape and which is carried by a
plate E.
[0040] This projection V is so arranged that, when a piston-head
blank 2a is lying on the bottom part K1 of the tool by its end-face
5, the said projection V points into the annular gap S which is
present between the projection 12a and the circumferential wall 7
of the piston-head blank 2a. Starting from the free end of the
projection V, the inner circumferential surface U thereof makes an
obtuse angle .beta. of 115-120.degree. with the underside of the
plate E which carries the projection V, and the projection V is
thus thicker in cross-section in the region of its root which
adjoins the plate E than in the region of its free tip. At the same
time, the outer circumferential surface of the projection V extends
parallel to the inner surface of the circumferential wall 7.
[0041] When the calibrating tool K2 is lowered, the projection V
engages in the annular gap S and its inner circumferential surface
U impacts on the projection 12a on the piston-head blank 2a. In
this way, a lateral force Q directed into the recess 10 is exerted
on the projection 12a and the material of the projection 12a is
displaced by this lateral force Q towards the recess 10.
[0042] As soon as the calibrating tool K2 has reached its lowest
position, at which the tip of its projection V is seated against
the bottom of the annular gap S, the projection 12a on the
piston-head blank 2a has been shaped into the collar portion 12,
which is now arranged in a position where it is inclined at an
angle .alpha. of approximately 25-30.degree. to the longitudinal
axis L of the head part 2 of the piston.
[0043] In this way, the circular opening 11 of the recess 10 is
surrounded by the encircling collar portion 12 which projects
freely into the receptacle 9 and which, starting from the likewise
circular floor area 13 of the recess 10, is aligned towards the
longitudinal axis L of the head part 2 of the piston. In this way,
the floor area 13 is larger than the area occupied by the opening
11. At the same time, an undercut 14 is formed in the region of the
angle .alpha. which is made between the floor area 13 and the
collar portion 12 which is arranged to be inclined, which undercut
14 cannot be obtained by a movement which only takes place parallel
to the longitudinal axis L.
[0044] The skirt part 3 of the piston is likewise produced from a
cylindrical steel blank by a plurality of hot-forging operations.
For this purpose, the blank (not shown) was placed in the die of a
forging apparatus (not shown likewise) in which, starting from one
end-face of the blank and by means of a punch, a recess 15 in the
skirt part 3 of the piston was then formed in a first forging step,
which recess 15 is at the rear relative to the head part 2 of the
piston in the fully assembled state. At the same time, a
cylindrical projection 16 and a shoulder 17 which follows on
without a step from projection 16 and encircles it were formed in
the region of the other end-face of the blank, the shapes of which
cylindrical projection 16 and shoulder 17 were preset by the die of
the forging apparatus. The blank which had been pre-contoured in
this way was then fully shaped in a second forging step. Apart from
minor differences, the geometrical dimensions of the skirt part 3
of the piston which is obtained in this way correspond to the final
size which is required and there are thus only small amounts of
mechanical post-processing which have to be carried out (near net
shape production).
[0045] On the skirt part 3 of the piston which is brought to a
finished state in this way, the projection 16 which merges into the
main portion 18 of the skirt part 3 of the piston without a step
via the shoulder 17 is formed on the end-face situated opposite the
recess 10. The main portion 18 comprises in essence an encircling
wall in which are formed, amongst other things, the mounting
openings (not visible here) for a connecting rod of the internal
combustion engine for which the piston 1 is intended. Except that
it is undersized, the diameter D of the projection 16 corresponds
in this case to the diameter of the opening 11 of the recess 10 in
the head part 2 of the piston, thus enabling the projection 16 to
be introduced into the recess 10 in the head part 2 of the piston
with a small amount of clearance. The transition from the
projection 16 to the end-face 19 is formed to be continuous and
free of any steps, i.e. is formed not to have a right-angled
shoulder. This configuration makes it easier for the projection 16
to be introduced into the recess 10.
[0046] To simplify the introduction of the projection 16 to an
additional degree and at the same time to make it possible for the
head part 2 of the piston and the skirt part 3 thereof to be
aligned with particular accuracy, the projection 16 may be formed
to taper slightly, starting from the shoulder 17, in the direction
of its free end-face 19.
[0047] The height H of the projection 16 is larger in this case
than the depth T of the recess 10. This being so, the dimensions of
the projection 16 on the skirt part 3 of the piston are thus
matched, overall, to the dimensions of the recess 10 in the head
part 2 of the piston, while allowing for a proportion Vk by which
the volume of the projection 16 shrinks as it cools down after the
skirt part 3 of the piston has been connected to the head part 2
thereof. Where the skirt part 3 and head part 2 of the piston are
produced from steel and where that volume of the recess 10 which is
to be filled by the material of the projection 16 is V1, this extra
volume Vk works out as Vk=V1.times.0.014.
[0048] To ensure that there is a connection between the parts 2 and
3 which is lastingly solid under all conditions of temperature, the
volume V2 of the projection 16 is therefore V2=V1+Vk, the
additional volume Vk being formed particularly in the region of the
projection 16, which projection 16 is associated with the collar
portion 12 of the head part 2 of the piston after the joining of
the skirt part 3 and head part 2 of the piston
[0049] To connect the head part 2 of the piston to its skirt part
3, the skirt part 3 of the piston is first heated to a forging
temperature of approximately 1050.degree. C. while the head part 2
of the piston remains at room temperature.
[0050] The two parts 2, 3 are then positioned in suitably shaped
receptacles in a compressing apparatus (not shown) in such a way
that their longitudinal axes L are in line with one another and the
projection 16 on the skirt part 3 of the piston and the recess 10
in the head part 2 of the piston are facing towards one another.
The parts 2, 3 are then moved towards one another until the free
end-face 19 butts against the floor area 13 of the recess 10. A
compressive force P acting in the direction of the longitudinal
axis L is then exerted on the head part 2 of the piston and/or on
the skirt part 3 thereof. This force is sufficiently large for the
material M of the projection 16 on the skirt part 3 of the piston,
which has been heated to forging temperature, to flow into the
space in the recess 10 which had, up till then, been free in the
region of the undercut 14.
[0051] The compressing process is continued until the free edge of
the collar portion 12 is seated in the hollow 20 at which the
projection 16 merges into the adjoining shoulder 17 on the skirt
part 3 of the piston. In this state, the steel material of the
projection 16 completely fills the recess 10 including the undercut
14. The head part 2 of the piston is now connected to the skirt
part 3 by positive fit by the material of the projection 16 which
fits behind the collar portion 12.
[0052] The overfilling of the recess 10 which occurs as a result of
the additional volume Vk of the projection 16 is compensated for by
elastic deformation of the collar portion 12. The collar portion
12, having been deformed in this way, moves back towards its
original shape as it cools down and the positive inter-engagement
which is created by the filling of the recess 10 is thus
supplemented by a frictional engagement which is caused by the
interlocking and elastic return of the material of the projection
16 and of the collar portion 12, which latter is not, or not fully,
deformed plastically.
[0053] Because the edge region of the collar portion 12 penetrates
slightly into the material of the skirt part 3 of the piston, the
head part 2 of the piston is, at the same time, supported on the
shoulder 17 by means of the collar portion 12 in such a way that,
even when the stresses in the region of the dishing 6 for the
combustion chamber are adversely distributed, it is ensured that
forces will be evenly transmitted from the head part 2 of the
piston to the skirt part 3 thereof.
[0054] Between the outer circumferential surface of the collar
portion 12 and the inner circumferential surface of the
circumferential wall 7 there is left, in this case, an encircling
free space 21 of a channel-like form which is available in
practical use to dissipate the heat from the head part 2 of the
piston, particularly in the region of the highly stressed
circumferential wall 7.
[0055] For the head part 2 of the piston to be connected to its
skirt part 3, it is, basically, possible for both parts to be
heated to hot-forging temperature. It is however enough for only
the skirt part 3 of the piston, or even only the projection 16 on
the skirt part 3 of the piston, to be heated to hot-forging
temperature while no deliberate increase is made in the temperature
of the head part 2 of the piston. Regardless of whether the
projection 16 is heated on its own or together with the entire
skirt part 3 of the piston, the recess 10 in the head part of the
piston acts in this case as a forming die for the reshaping of the
projection 16 on the skirt part 3 of the piston which is required
to connect the skirt part 3 and head part 2 of the piston together.
The head part 2 of the piston can then be left in the bottom part
K1 of the tool in this reshaping step. In this way, the bottom part
K1 of the tool can be used not only to calibrate the blank 2a of
the head part 2 of the piston but also as a tool for connecting the
head part 2 of the piston to its skirt part 3. The tooling costs
can be reduced in this way and there is also no need for the
forging tool to be changed between the individual operations,
which, all in all, has a beneficial effect on the costs of
production.
REFERENCE NUMERALS
[0056] 1 Piston [0057] 2 Head part of piston [0058] 2a Piston-head
blank [0059] 3 Skirt part of piston [0060] 4 Joint zone [0061] 5
End-face of the head part 2 of the piston and of the piston-head
blank 2a [0062] 6 Dishing for combustion chamber [0063] 7
Circumferential wall of the head part 2 of the piston and of the
piston-head blank 2a [0064] 8 Second end-face of the head part 2 of
the piston [0065] 9 Receptacle in the head part 2 of the piston
[0066] 10 Recess [0067] 11 Opening of the recess 10 [0068] 12
Collar portion [0069] 12a Projection of the piston-head blank 2a
[0070] 13 Floor area of the recess 10 [0071] 14 Undercut [0072] 15
Recess at rear of the skirt part 3 of the piston [0073] 16
Projection [0074] 17 Shoulder [0075] 18 Main portion of the skirt
part 3 of the piston [0076] 19 End-face of the projection 16 [0077]
20 Groove at the transition from the projection 16 to the shoulder
17 [0078] 21 Free space [0079] .alpha., .beta. Angles [0080] D
Diameter of the projection 16 [0081] E Plate [0082] H Height of the
projection 16 [0083] K1 Bottom part of calibrating tool K [0084] K2
Top part of calibrating tool K [0085] K Calibrating tool [0086] L
Longitudinal axis of the piston 1 and of the parts 2, 3 [0087] M
Material of the projection 16 [0088] P Compressive force [0089] R
Direction in which the forging tool acts [0090] T Depth of the
recess 10 [0091] V Projection on part K1 of the tool [0092] S
Annular gap [0093] U Inner circumferential surface of the
projection V [0094] Q Force
* * * * *