Planet Carrier And A Process And Apparatus To Manufacture It

Abstract

The present invention is directed to an integral planet carrier with no joints, a method of manufacturing it and an apparatus for doing so. Such integrally manufactured components have better strength than a conventionally produced multi-piece jointed planet carriers or integral planet carriers made from casting process. The present invention provides a hot forging process which can be used for the manufacturing of the planet carrier. The manufacturing process comprises of forward extrusion of a billet followed by backward extrusion. This is followed by bending operation, which is in turn followed by a flattening operation. Post-forging heat treatment and other treatments such as shot blasting follow. Finally machining is carried out to arrive at the final integrally formed planet carrier. The forward extrusion, backward extrusion, bending and flattening operations are done on a press or hammer having sufficient energy and load capacity. Preferably these operations are performed on a hydraulic press in order to achieve the required accuracy and precision.

Description

FIELD OF INVENTION

The present invention relates to a planet carrier. Particularly, the present invention relates to the planet carrier used in the power transmission system of a vehicle. More particularly the present invention relates to an integral planet carrier and a hot forging process to manufacture the same.

Introduction

A planet carrier is used in epicyclic gear systems. Epicyclic or planetary gear systems consist of three types of gears as given below:

• • 1. Sun gear (normally only one)
  • 2. Planet gears (more than one)
  • 3. Ring gear.

This system of gear works in a fashion similar to the planetary system of sun and various planets in the solar system. The sun gear is at the center of the system (similar to sun in the solar system). The planet gears (generally two or more) are in mesh with the sun gear and revolve around it. The ring gear meshes with the planet gears. The rotary motion is transferred from the sun gear to the planet gear to the ring gear or the other way around.

Planet gears are normally mounted on a movable part which is called a carrier. The rotary axes of sun gear and the planet carrier are same but they can rotate independent of each other. The rotational axes of all the gears in the epicyclic gear system are generally parallel to each other (they can be at an angle for some specific cases).

The planetary or epicyclic gear system is used when very high torque transmission or very high transmission ratio is required. During the motion transmission, multiple planet gears are transferring the rotary motion from sun gear to ring gear (or vice versa). The torque or load of transmission is equally distributed among the planet gears. The planetary or epicyclic gear systems are designed to provide high power density in comparison with the standard parallel axis gear trains and are suitable for use in transmission systems of very large dump trucks, tippers etc.

A planet carrier, as explained previously, carries all planet gears. It has two cylindrical parts/sections. As shown in FIG. 1, the larger cylindrical section (1A) is hollow with both its ends closed or covered. One of the closed ends has a central hole through which a shaft can pass. On the other end, a shaft type extension (2) is provided. The hollow cylindrical part (1A) has windows (3) on its side surface. Planet gears are mounted in the hollow cylindrical part (1A) through the windows (3). The sun gear is mounted on a shaft passing through the central hole of the planet carrier. The axes of planet carrier and sun gear are the same while the axes of individual planet gears and the sun gear are parallel to each other.

The planet carrier is used in the transmission system of large tippers or dumpers etc. Due to the complex geometry of the planet carrier, it is normally manufactured in two parts (denoted as part 1 and part 2 in FIG. 1A and FIG. 1B), typically by forging, and then joined together to achieve the final shape. Individual parts that are manufactured separately (Part 1 and Part 2) are then joined together either by welding or bolting. In some cases sheet metal forging process is also used for the manufacturing of individual parts (Part 1 and Part 2).

A number of patents or patent applications disclose planet carriers. However, none discloses an integral planet carrier made using a hot forging method and for use in automotive industry.

Patents CN103615525A and CN104148797B and all disclose methods of manufacturing the planet carrier in two parts and then joining them by EBW or welding or bolting.

Patents U.S. Pat. Nos. 3,667,324, 3,842,481, 4,043,021, 5,558,593, 7,214,160, and 4,721,014 all disclose planet carriers made using sheet metal forming and typically the two plate members that form the planetary carrier are joined together using a method of joining (welding, bolting etc.). None of these patents disclose an integrally formed planetary carrier.

U.S. Pat. No. 9,702,451 discloses a planet carrier that is open in structure and will need closing. Moreover the patent does not disclose a method of manufacture of the planet carrier.

Chinese patent CN103769825 uses a cold-forging method of manufacture of planet carriers which do not have closed structure—it will require a separate closing cover.

Chinese patents CN102829171, CN203809666 and CN204327937 all disclose integrally formed planet carriers that are made using casting. The forged parts have more desirable properties that are not obtained using casting technology.

The Chinese patent 103963233 discloses an integral planet carrier made out of plastic using injection moulding technology. It cannot be applied to metal planet carriers.

Finally, the German patent application DE102011011438 discloses a method of manufacture using Roll Forming Operation that uses incremental forging technique. The planet carrier is not made integrally and involves joining of different parts.

The traditional method as disclosed in prior art has the following drawbacks associated with it:

• • 1. Since the carrier is made in two parts and then joined, its strength is inherently less when compared to an integral part as the strength of the weld depends upon the quality of welding and its location.
  • 2. The manufacturing process becomes lengthy as the two parts have to be manufactured separately and then a joining process has to be carried out.
  • 3. When Casting method is used for the manufacture of an integral planet carrier then it will have “as cast” microstructure which has many casting defects and lower mechanical properties. The microstructure of this part will consist of dendrites and casting defects like voids, cracks, micro porosities etc.

Thus, there exists a room for advancement over the existing technology in that an integral planet carrier and a method of manufacturing the same through hot forging process would not only increase the strength of the part but also reduce manufacturing cycle time.

OBJECTS OF INVENTION

Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:

It is an object of the present invention to provide an integral (single piece) planet carrier.

It is another object of the present invention to provide manufacturing method for the integral planet carrier through hot forging process.

It is still another object of the present invention to provide an integral planet carrier which has better strength to weight ratio.

Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

FIGS. 1A and 1B shows a typical planet carrier of prior art

FIGS. 1C and 1D shows an integral planet carrier of the invention

FIG. 2 illustrates the flow diagram for forging process of the invention

FIG. 3A shows the schematic diagram of set-up used for the forward extrusion process

FIGS. 3B and 3C illustrate the first preform obtained after the forward extrusion process

FIG. 4A shows the schematic diagram of set-up used for the backward extrusion process

FIGS. 4B and 4C illustrate the second preform obtained after the backward extrusion process.

FIG. 5 illustrates the forged integral planet carrier obtained after the flattening operation—FIG. 5A showing the front view while FIG. 5B showing the section view of the forged integral planet carrier.

List of parts
1 - Planet Carrier
1A - Cylindrical part of the planet
carrier
2 - Shaft type extension of the planet
carrier
3 - Window on the side surface of
the cylindrical part (1A)
4 - First preform
4A - Cylindrical part or head of the
first preform
4B - Small diameter cylindrical part
or a shaft of first preform
5 - Second preform
5A - Walled hollow part or blind
hole of the second preform
5B - Small diameter cylindrical part
or a shaft of second preform
6 - Third preform
7 - As forged integral planet carrier
7A - Flattened part of as forged
integral planet carrier
7B - head portion of the as forged
integral planet carrier
7C - Treated integral planet carrier
7D - final integral planet carrier
FT - Forward extrusion top die
FB - Forward extrusion bottom die
BT - Backward extrusion top die
BB - Backward extrusion bottom die
DT - Bending/deformation top die
DB - Bending/deformation bottom
die
FLT - Flattening top die
FU1 - upper cavity in bottom die for
forward extrusion (first upper cavity)
FL1 - lower cavity in bottom die for
forward extrusion (first lower cavity)
S1 - upper cavity in bottom die for
backward extrusion (first subcavity)
S2 - lower cavity in bottom die for
backward extrusion (second
subcavity)
C1 - upper cavity in bending bottom
die (second upper cavity)
C2 - lower cavity in bending bottom
die (second lower cavity)
R - Ram
R1 - Recess

SUMMARY OF INVENTION

The present invention is directed to an integral planet carrier with no joints and a method of manufacturing the same. Such integrally manufactured components have better strength than a conventionally produced multi-piece jointed planet carrier or an integral planet carrier made from casting process.

The present invention provides a hot forging process which can be used for the manufacturing of the planet carrier. The manufacturing process comprises of forward extrusion of a billet (not shown) followed by backward extrusion. This is followed by bending operation, which is in turn followed by a flattening operation.

The forward extrusion, backward extrusion, bending and flattening operations are done on a press or hammer having sufficient energy and load capacity. Preferably these operations are performed on a hydraulic press in order to achieve the required accuracy and precision.

DESCRIPTION OF THE INVENTION

The present invention relates to the manufacturing of a planet carrier (1) for planetary gear system using the hot forging process. The key inventive feature of this invention is the design and development of the integral planet carrier and the manufacturing process to make the same.

According to the present invention, the manufacturing process starts with a forged billet as a raw material. The raw material is heated to the required temperature in a furnace and then put in forward extrusion dies in a press. As can be seen from FIG. 1, the planet carrier has a large cylindrical section (1A) and a smaller cylindrical section (2). This difference in the diameter between the two sections is produced in the billet during the forward extrusion process (denoted by 4A & 4B in FIG. 3). Thus, the forward extrusion operation gives the required material distribution to the raw material. The forward extruded billet is then transferred to the backward extrusion dies where backward extrusion operation is completed.

The backward extruded preform is then cooled down to room temperature. This preform is heated and then transferred to a forging press where bending operation followed by flattening operation is performed. This operation brings the part to its final form. In another embodiment, the backward extruded preform is transferred directly to bending and flattening stations and the complete process is performed without any intermediate reheating.

The hot forging is followed by a heat treatment process which is followed by the machining process.

Planet Carrier Manufacturing Process

As shown in FIG. 2, the invented manufacturing process typically involves the following steps:

1. Billet or Raw Material Heating

• • A forged billet of the required material chemistry is used for this process. The section and length of the billet taken for this operation is predefined based on the material requirement for the part to be produced. The cross section of the billet may be either round, or rounded cross section (RCS). Preferably a round cross section billet is used. The billet is heated in an oil/gas fired or electrical furnace in the temperature range of 1150-1280° C. for sufficient soaking time to achieve uniform temperature in the heated billet. The output of this process is a heated billet.

2. Forward Extrusion

• • Forward extrusion is a process where the billet is forced to flow in the same direction as the ram being used to apply pressure. The forward extrusion process is done using a combination of a forward-extrusion-top-die (FT) and bottom die (FB) (see FIG. 3A). The external surface of the forward-extrusion-top-die connected to a ram is close fitting to the bottom die cavity consequently preventing material extruding past it and controlling the flow of the material in the same direction as the ram.
  • This operation is carried out on the heated billet at a forward extrusion station using forging equipment having required energy and load capacity. The heated billet is placed on a bottom die (FB) aligned concentrically along the central longitudinal axis of said forward-extrusion-top-die (FT) and subjected to forward extrusion using the top die-bottom die combination. The forward extrusion top die (FT) is moved axially and concentrically towards the said heated billet till said first preform is produced. This operation imparts the required material distribution for the backward extrusion step. The output of forward extrusion operation is a first preform (4), which has a solid upper cylindrical section or a head (4A), and a solid lower cylindrical section or a shaft (4B), the diameter of the solid head (4A) portion being larger than that of the shaft (4B).
  • The forward-extrusion-top-die (FT) used in the forward extrusion step is cylindrical in section and has a diameter equal to or slightly less than the diameter of the heated billet.
  • The bottom die (FB) used in this operation has a cavity with two diameters. The upper part (or first upper cavity FU1) of the cavity has a diameter equal to the required diameter of the solid cylindrical head section 4A and its length/depth is more than the length of the heated billet by at least 1.5 times. Further it should be noted that the diameter of the heated billet used as input to this operation is also equal or slightly less than the diameter of the upper cavity, so that the heated billet is easily able to be inserted into the first upper cavity (FU1). The lower part (or first lower cavity FL1) of the cavity has a diameter which conforms with the required diameter of the shaft 4B and its length/depth is 1 mm to 50 mm more than the required length of the shaft 4B.

3. Backward Extrusion

• • The forward extruded first preform (4) is next subjected to the backward extrusion operation. It is important to minimize the time of transfer of the first preform (4) to a backward extrusion station so that the temperature of the first preform does not reduce any more than 5-10% of its temperature at the end of the forward extrusion process. Backward extrusion, as the description suggests is the opposite of forward extrusion and is where metal is forced to flow in the direction opposite to that of the ram. The side surface of the backward extrusion top die (BT) (attached to ram) is recessed circumferentially from its bottom face up to a length L1 to create a mirror image of the recess (or a hollow—R1) in the first preform as required. The material of the head (4A) of first preform flows into this gap or recess (R1) under the force applied by the movement of backward-extrusion-top-die (BT) to produce a walled hollow part (5A) having an open end (second preform—5).
  • In the preferred embodiment, the bottom dies used for the forward and the backward extrusion steps are the same physical dies.
  • However, it is possible to use a physically different bottom die. In this scenario, the bottom die has a cavity which is made of two subcavities—each subcavity having a different diameter than the other. The diameter of the first subcavity (S1) is equal to the required external diameter of the walled hollow part (5A). The length of the first subcavity (S1) is greater than the length of the walled hollow part (5A) i.e. L. The diameter of the second subcavity (S2) is so as to accommodate the solid cylindrical shaft (5B). The length of the second subcavity is more than the length of the solid cylindrical shaft (5B) by 1 mm to 50 mm. In another embodiment, the length of the second subcavity is equal to the length of the solid cylindrical shaft (5B).
  • In the case where a physically different bottom die is used for the backward extrusion operation, the first preform is placed centrally on the second subcavity such that the central longitudinal axis of second subcavity and that of the first preform are aligned.
  • As the ram pushes the backward-extrusion-top-die (BT), the material of the solid cylindrical head (4A) flows into the recess (R1)—i.e. gap between the internal wall of the bottom die (BB) and the backward-extrusion-top-die (BT). The backward extrusion operation produces a second preform (5) having a walled hollow part (5A) having an external length L and an internal length L1, and a solid cylindrical shaft (5B) (see FIGS. 4B and 4C).
  • The length of the circumferential recess (R1) provided in the backward-extrusion-top-die (BT) is substantially the same as the internal length (L1) of the walled hollow part (5A).
  • During the backward extrusion the smaller diameter of the preform does not change, i.e. diameter of shaft 4B is equal to diameter of the shaft 5B. This operation can be done on any forging equipment having sufficient load and energy capacity. Preferably this operation is done on the hydraulic press. The output of this process is second preform (5).

4. Heating of the Second Preform

• • It is important to maintain the temperature of the second preform to above a minimum forging temperature at any stage. Preferably the minimum forging temperature is 900° C. If the temperature of the second preform falls below this value, the second preform is heated to the temperature range of 1150-1280° C. It is also possible to simply heat the walled hollow part (5A) to the temperature range of 1150-1280° C. while not heating the shaft 5B. The heating of the second preform is done using induction heater or oil or gas fired furnace. In the case the second preform is heated, the output of this operation is heated second preform.

5. Bending

• • The bending operation can be done in any forging equipment having sufficient energy and load capacity. Preferably it is done on a hydraulic press. The output of this operation is a third preform (6). In this operation, some portion of the hollow part or blind hole (5A) of second preform or heated second preform, near its open end, is deformed (or bent or tilted) using a bending-top-die towards the central longitudinal axis of the preform to form an angle with respect to the central longitudinal axis of the second preform resulting in a tilted wall.
  • The second preform is placed in a bending bottom die. The bending bottom die has a cavity (C) with two different diameters so as to fully accommodate the shaft portion (5B) of the second preform and partly accommodate the walled hollow part (5A).
  • The lower section (C1)—second lower cavity—of this cavity (C) conforms to the diameter of cylindrical region 5B of heated second preform. Further the length of this cavity is equal to or greater than the length of 5B.
  • The upper section (C2)—second upper cavity—of the bending bottom die cavity has a diameter conforming to the outer diameter of the walled hollow part (5A). The length of the upper cavity (C2) is about 30 to 80% the length (L) of the walled hollow part (5A). Hence, the hollow part (5A) of second preform projects above the top surface of the bending bottom die by a length of projection which is in an amount of 20 to 70% of the external length L (0.2L to 0.7L—refer to FIG. 4A) of the walled hollow part (5A). It is the part of the hollow part (5A) which projects out of the bending bottom die which is bent/tilted inwards (that is towards the central longitudinal axis) as explained above.
  • Deformation is carried out in the bending operation by moving the bending-top-die towards the bottom die. The bending-top-die which has an internal cavity of conical shape with a surface that is inclined at an angle with respect to the central longitudinal axis of the bending top die—is pushed towards the second preform. The angle is less than 90°, preferably between 15° to 55° when measured from the central longitudinal axis. As it comes in contact with the second preform, the part of the second preform that sticks out above the bending bottom die deforms under the force applied due to the movement of the top die. The output of this operation is third preform (6). The remaining non-deformed part of the hollow part (5A) remains substantially parallel to the longitudinal axis of the third preform (6).
  • The smaller diameter part of the third preform (6) remains undeformed in this operation also.

6. Flattening

• • The bottom die for the flattening operation is same as that used for the bending operation. Only the top die is replaced for the flattening operation by a flattening-top-die which has an annular second recess or ring shaped cavity. The outer diameter of this ring shaped cavity is equal to the outer diameter of the head portion (7B) of as forged integral planet carrier (7). The inner diameter of this ring shaped cavity is equal to the inside diameter D (as shown in FIG. 5B) of 7A. In the flattening operation the deformed/bent/tilted wall portion of third preform (6) is further deformed to flatten it to form a flattened part (7A). The flattened surface is substantially perpendicular to the longitudinal axis. The smaller diameter part of the preform remains undeformed in this operation also. The output of this operation is the hot-forged integral planet carrier (7) as shown in FIG. 5.

7. Heat Treatment and Post Forging Operation

• • The hot-forged integral planet carrier (7) thus produced is then heat treated to achieve the required mechanical properties. Post-forging operations like shot blasting etc. are also carried out as appropriate on the part. The output of this operation is treated integral planet carrier.

8. Machining

• • Sections of the hot forged integral planet carrier are carved out and machined so that a final integral planet carrier as shown in FIGS. 1(C and D) is produced.

The process disclosed herein thus produces a planet carrier (1) which is an integral or integrally formed object, devoid of joints and is made using hot forging technique.

As another aspect of the invention, the apparatus to produce an integrally formed planet carrier is disclosed. It comprises the following tools placed sequentially:

• • a billet heating station for heating the billet;
  • a forward extrusion station to forward extrude said billet into the first preform (4);
  • a backward extrusion station to backward extrude said first preform (4) into the second preform (5);
  • a bending station to deform the walled hollow part (5A) of said second preform (5) to produce a third preform (6) having a deformed or bent or tilted walled hollow part;
  • a flattening station for flattening said deformed or bent or tilted walled hollow part to produce the as-forged integral planet carrier (7);
  • a heat treatment and post-forging treatment station for treating said as-forged integral planet carrier into the treated integral planet carrier (7C);
  • a machining station to machine said treated integral planet carrier (7C) into said final integral planet carrier (7D).

It is evident from the foregoing discussion that bending station disclosed here requires a forging equipment, preferably a hydraulic press, in which are placed a bending top die (DT) and a bending bottom die (DB), both dies placed co-axially, and wherein said bending top die (DT) has a conical cavity that faces said bending bottom die (DB), wherein when said second preform (5) is placed on said bending bottom die (DB), said bending top die (DT) is capable of moving towards said bending bottom die (DB) so as to accommodate the projection of said second preform (5). Further, the surface of said conical cavity has an angle less than 90 degrees when measured from its central longitudinal axis, and preferably between 15 and 55 degrees. In another aspect of the apparatus disclosed here, the flattening station has a flattening top die (FLT) having an annular second recess or a ring shaped cavity. Finally, a heating station is provided between the backward extrusion stations and the bending station for heating said second preform (5), if necessary, to ensure that the temperature of the second preform doesn't fall below the minimum required forging temperature.

The benefits of this invention are as follows:

• • 1. In this invention a manufacturing process has been proposed which allows the manufacture of the planet carrier using bulk/hot forging method without any joints. Thus, the output of the process is an integral planet carrier with excellent product properties achieved due to hot forging process (shown in FIGS. 1C & D).
  • 2. The integral nature of the part improves the strength of the part which increases its life.
  • 3. The forged structure of the part (i.e. continuous grain flow lines, equiaxed grains/microstructure, directional properties and absence of any voids, micro porosities or cracks) also improves the mechanical and fatigue properties of the part and hence, increases its life.
  • 4. The inventive feature of the forging process flow is the sequential forging steps and the forging dies which lead to formation of the enclosed or covered structure which is formed in as forged condition (shown in FIG. 5B) eliminating the need for use of any joining method like welding, bolting etc.
  • 5. The apparatus disclosed here allows production of a planet carrier which is an integral or integrally formed object, devoid of joints and is made using hot forging technique

While the above description contains much specificity, these should not be construed as limitation in the scope of the invention, but rather as an exemplification of the preferred embodiments thereof. It must be realized that modifications and variations are possible based on the disclosure given above without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

Claims

1. A planet carrier, characterised in that said planet carrier is an integral planet carrier (7D), devoid of joints and is made using hot forging, said planet carrier (1) comprising a cylindrical part or a head (7B) which has a flattened part (7A), and a shaft (5B), and wherein sections of said planet carrier (1) are carved out and machined to produce a final integral planet carrier (7D).

2. A planet carrier as claimed in claim 1 characterised in that it has continuous grain flow lines.

3. A process of making an integral planet carrier (7D), characterised in that said process comprises the steps of:

heating of a billet to produce a heated billet;

forward extruding said heated billet to produce a first preform (4) having a head (4A) and a shaft (4B);

backward extruding said first preform (4) to produce a second preform (5) having a walled hollow part (5A) and a shaft (5B);

ensuring that the temperature of the second preform (5) is maintained at or above a minimum forging temperature;

bending or deforming walled hollow part (5A) of said second preform (5) to produce a third preform (6) having a head with a bent or deformed or a tilted wall portion;

flattening said bent or deformed or tilted wall portion to produce a hot forged integral planet carrier having a head (7B) with a flattened part (7A);

heat treating said hot forged integral planet carrier (7) having a flattened head (7A) followed by shot blasting it to produce treated integral planet carrier (7C);

machining of said treated integral planet carrier (7C) to produce final integral planet carrier (7D);

said process thus leading to an integrally formed planet carrier (7D).

4. A process as claimed in claim 3, characterised in that in the step of heating, said billet is heated in a furnace to a temperature of 1150 to 1280° C.

5. A process as claimed in claim 3, characterised in that said forward extruding step is carried out at a forward extrusion station using a combination of a forward-extrusion-top-die (FT) and a bottom die (FB), wherein said heated billet is placed on said bottom die (FB) aligned concentrically along the central longitudinal axis of said forward-extrusion-top-die (FT) and said forward extrusion top die (FT) is moved axially and concentrically towards the said heated billet till said first preform (4) is produced which has a solid upper cylindrical section or a head (4A), and a lower cylindrical section or a shaft (4B), wherein the diameter of the head (4 A) portion is larger than that of the shaft (4B).

6. A process as claimed in claim 3, characterised in that said backward extruding step is carried out in an hydraulic press using a combination of a backward-extrusion-top-die (BT) and a backward extrusion bottom die (BB), wherein the side surface of the backward extrusion top die (BT) is recessed circumferentially from its bottom face up to a length (L1) thereby creating a recess or gap (R1) between said side surface and the internal face of the upper cavity of said backward extrusion bottom die (BB), and wherein material of said head (4A) flows into said gap (R1) under the force applied by the movement of said backward-extrusion-top-die (BT) to produce a walled hollow part (5A) having an open end as a part of said second preform (5).

7. A process as claimed in claim 3, characterised in that in the step of ensuring that said second preform (5) remains over a minimum forging temperature, second preform (5) is heated if its temperature falls below said minimum forging temperature, heating being carried out preferably to a temperature between 1150 and 1280° C.

8. A process as claimed in claim 3, characterised in that said bending step to produce said third preform (6) is carried out on a forging equipment, preferably a hydraulic press using a bending top die (DT) and a bending bottom die (DB), wherein when the second preform (5) or heated second preform (5), as the case may be, is placed on said bending bottom die (DB), such that there is a projection of said walled hollow part (5 A) that projects above the top surface of said bending bottom die (DB).

9. A process as claimed in claim 8, characterised in that said bending top die (DT) has an internal cavity of conical shape with a surface that is inclined at an angle with respect to the central longitudinal axis of said second preform (5).

10. A process as claimed in claim 8, characterised in that said angle is less than 90 degrees.

11. A process as claimed in claim 8, characterised in that said angle is between 15 and 55 degrees.

12. A process as claimed in claim 8, characterised in that the length of projection is 20% to 70% of the external length of said walled hollow part (5A).

13. A process as claimed in claim 8, characterised in that said bending-top-die (DT) is pushed towards the second preform (5) until said projection is substantially within said conical cavity, thereby forming a third preform (6) that has said bent or tilted wall portion.

14. A process as claimed in claim 3, characterised in that said flattening operation performed on said bent or tilted wall portion of said third preform (6) is carried out using the same bottom die as used in said bending step, and wherein a flattening top die (FLT) having an annular or ring shaped cavity is used for carrying out the flattening of said tilted wall portion to produce a hot-forged integral planet carrier (7).

15. A process as claimed in claim 3, characterised in that said hot-forged integral planet carrier (7) is heat treated and subjected to post-forging treatments such as shot-blasting to produce a treated integral planet carrier (7C).

16. A process as claimed in claim 3, characterised in that said treated integral planet carrier (7C) is machined to produce a final integral planet carrier (7D).

17. A process as claimed in claim 3, characterised in that said minimum forging temperature is 900° C.

18. A process as claimed in claim 3, characterised in that in the step of ensuring that said second preform remains over a minimum forging temperature, the walled hollow part (5A) of said second preform (5) is heated if the temperature of said second preform (5) falls below said minimum forging temperature, heating being carried out preferably to a temperature between 1150 and 1280° C.

19. A hot forging apparatus to make a planet carrier (7D), characterised in that said apparatus comprises the following tools placed sequentially:

a billet heating station for heating said billet;

a forward extrusion station to forward extrude said billet into said first preform (4);

a backward extrusion station to backward extrude said first preform (4) into said second preform (5);

a bending station to deform said walled hollow part (5 A) of said second preform (5) to produce a third preform (6) having a deformed or bent or tilted walled hollow part;

a flattening station for flattening said deformed or bent or tilted walled hollow part to produce said as-forged integral planet carrier (7);

a heat treatment and post-forging treatment station for treating said as-forged integral planet carrier into a treated integral planet carrier (7C); a machining station to machine said treated integral planet carrier (7C) into said final integral planet carrier (7D).

20. A hot forging apparatus as claimed in claim 19, characterised in that said bending station comprises a forging equipment, preferably a hydraulic press, in which are placed a bending top die (DT) and a bending bottom die (DB), both dies placed co-axially, and wherein said bending top die (DT) has a conical cavity that faces said bending bottom die (DB), wherein when said second preform (5) is placed on said bending bottom die (DB), said bending top die (DT) is capable of moving towards said bending bottom die (DB) so as to accommodate the projection of said second preform (5).

21. A hot forging apparatus as claimed in claim 19, characterised in that surface of said conical cavity has an angle less than 90 degrees when measured from its central longitudinal axis.

22. A hot forging apparatus as claimed in claim 19, characterised in that said angle is between 15 and 55 degrees.

23. A hot forging apparatus as claimed in claim 19, characterised in that said flattening station has a flattening top die (FLT) having an annular second recess or a ring shaped cavity.

24. A hot forging apparatus as claimed in claim 19, wherein a heating station is provided between said backward extrusion station and said bending station for heating said second preform (5).