Field of invention:

The present invention relates to a manufacturing method for a pinion drive. In particular, it relates to method of producing as-forged near net shape pinion drive having as forged gear teeth with continuous grain flow lines conforming to the profile of the teeth, using sequential forging operations, and a finisher die assembly to facilitate finish forging.

Background of invention:

Typically, a pinion drive is used in automobile and industrial applications. The main function of a pinion drive is power transmission. A typical pinion drive is shown in Figure 1. The pinion drive consists of four parts - head (1), teeth (2), stem (3), and tail (4). The part carrying teeth (2) is roughly conical in shape and has teeth which are either helical or spiral in configuration. In a typical transmission system of a vehicle, the crown wheel and pinion are used for the transfer of rotational motion from one axis to another which is typically perpendicular to the first axis.

Conventionally, pinion drives are manufactured using multistep processes such as casting or forging, followed by various machining processes. Depending on the final geometry, a pre-machined shape is prepared using forging or casting. Sequential machining operations like turning, precision gear cutting, finish machining are applied to the pre-machined shape to produce the final shape using specialized equipment. Conventional methods of pinion manufacture have following shortcomings:

1. They incur significant wastage of material and resources which leads to higher cost of production.

2. The machining process requires precision tooling and arrangements which consume high cost and time.

3. The machining of teeth from the forged part leads to cutting of the grain flow lines. Due to this, the grain flow lines are neither continuous nor follow the profile of the teeth; the result is that pinions manufactured using the conventional processes have lower strength and life which will increase cost of replacement in very early stage. The above drawbacks can be overcome if the teeth (spiral or helical) can be forged in the hot forging process itself. This will ensure continuous grain flow following the profile of the teeth. Continuous grain flow will enhance the strength of the teeth. Further making near net shaped teeth will reduce the machining time and cost. However, the forging of helical or spiral teeth is a difficult process as the shape of the teeth or teeth profile makes the die cavity negative, and the negative die cavity makes ejection of the forged part from the dies very difficult. Therefore, a hot forging process for manufacturing near net shaped pinion gear with helical/ spiral teeth has not been established in the industry. Many prior art documents have displayed methods to overcome this problem with ejection of the forged part. US patents 4050283A and 6041640 disclose method of ejection which is designed specifically for ejecting the spiral or helical gear teeth after forging. Both these methods involve design and manufacture of a complex ejector device with many moving parts. The ejection system is complicated, hence prone to failure due to breaking and/or wearing of any of the multiple parts of the ejection system. US patent 4050283 A entails some modification on the top surface of the pinion which needs to be machined off after the forging process. These ejection systems are also part specific and different pinion drives having different spiral/helix angle need different ejection system altogether. US patent 5787753 A talks about a modification in either the top die or the bottom die in order to restrict the axial motion of the forged part while allowing its rotational motion. This mechanism allows the ejection of the part through unscrewing type motion of the part. The drawback of this invention is that a negative impression which has to be made in the die restricts the axial motion of the part during the ejection process. The negative impression region leads to sticking of the forged part in one of the dies; consequently, the ejection force required is very large. A special ejection mechanism is also required. Moreover, the necessity of the ejection mechanism makes the design of dies more complicated. Due to the sticking force, there is increased wear of the negative impression area, which leads to early failure of the dies, ultimately reducing the batch size of production.

Due to the above drawbacks in the prior art, the production of pinion with as- forged helical or spiral teeth has hitherto not been possible economically. Thus, there remains a very real and substantial need for manufacturing pinion drives having improved mechanical properties through innovative forging process which reduces cost, resources, time and increases the life through formation of as forged teeth in the pinion.

Summary of invention:

Present invention discloses a forging method and a finisher die assembly (5) to manufacture pinion drives with as-forged teeth for use in automobile industry and other industrial applications.

The process of manufacturing of pinion drives disclosed here uses a stepwise forging operation to produce pinion drive with as-forged teeth. Forging is carried out in a stepwise operation to complete the deformation of material into deep die cavities without any forging defects such as under-fills, cold-shuts, laps and folds. The necessary steps of die and tooling design has to be taken for every stage which ensures complete material fill up. The resulting forged component has better quality, wear resistance, higher strength to weight ratio and continuous/ directional grain flow lines. The process starts with a preform made from a cylindrical billet and follows through with upsetting followed by finisher forging and flash trimming operation. The invention overcomes the problem of ejection of the helical or spiral gear teeth from the dies through an innovative die design methodology which allows easy and automatic removal of the pinion from the die.

The invention also discloses finisher die assembly (5) with a novel aspect in that a gap (12) is introduced between the top surface of the top finisher die (10) and corresponding inside surface (11) of the top finisher die holder of the assembly (5). During the forging process, the gap allows easy ejection of the forged part.

Brief Description of Figures:

Figure 1 shows the perspective view of finish forged pinion drive

Figure 2 shows the stepwise forging process of pinion drive using the invention Figure 3 shows the arrangement of finisher die assembly (without the part to be forged) of the invention

Figure 4 shows a flowchart of the process of invention

Figure 5 shows the continuous grain flow lines conforming to the teeth profile Objects of the Invention:

An object of the present invention is to provide a method for forging of a pinion drive. It is another object of the present invention to manufacture a pinion drive with as- forged condition teeth using a stepwise forging process which avoids the rough machining to the gear teeth profiles.

It is yet another object of the present invention to provide better mechanical properties and continuous grain flow lines along the contours of the teeth profile.

It is still another object of the invention to reduce the machining (gear teeth cutting) time required during the manufacturing of the pinion drive.

It is a further object of the invention to reduce the input material required during the manufacturing of the pinion drive. It is another objective of the present invention to provide a finisher die assembly which allows ejection of the forged pinion having as forged helical or spiral teeth from the forging dies.

Yet another objective of the present invention is to reduce material, resources and energy costs of manufacturing a pinion drive.

List of Parts:

1. Head Tail

2. Teeth Finisher die assembly

3. Stem Top finisher die holder 6A. First impression 11. Bottom face of the first

7. Bottom finisher die holder impression

7A. Second impression 12. Gap

8. Bottom finisher die 13. Upset preform

8A. Third impression 14. Finish forged pinion

9. Top finisher die 15. Top upsetting die

9A. Fourth impression 16. Bottom upsetting die

10. Top face of top finisher die 17. Bottom ejector

Detailed description of invention:

The present invention is related to a method of manufacturing pinion drive used in automobile and industrial application and a finisher die assembly (5) to facilitate easy ejection of the forged part.

Throughout this specification, the terms pinion and pinion drive are used interchangeably to refer to the component indicated in Figure 1.

As shown in Figure 4, the process of manufacturing the forged pinion drive according to the present invention involves the following steps:

1. Billet cutting into flat-faced billets and preparing billets for preform formation.

2. Preform making to required size by upsetting dies.

3. Forming a finish forged pinion drive having as-forged teeth using finisher forging step. 4. Post forging treatment of the finish forged part

5. Nominal machining of the part to produce the final part.

Figure 1 shows a pinion drive having the head portion (1), the teeth (2) on head portion, stem (3) and tail (4).

The details of each manufacturing step are provided below.

1. Billet Heating:

- A forged cylindrical shape solid billet of required chemistry, size, identification is provided

The input billet is inspected for dimensional compliance, for material cleanliness and soundness criteria using ultrasonic testing.

The inspected billet is heated in an oil or gas fired furnace in the temperature range of 1150 C to 1280 C for sufficient soaking time to achieve uniform temperature in the heated billet.

The output of this operation is a heated billet

2. Upsetting:

- Any type of forging equipment such as mechanical, hydraulic press capable of applying sufficient amount of force is used for preform making.

The preform is manufactured using upsetting process.

The upsetting operation involves two dies, namely a top upsetting die (15) and a bottom upsetting die (16), with preforming cavities/impressions provided in both dies. - Both the dies are properly lubricated.

The heated billet from the furnace is subjected to upsetting operation. Billet is upset after ensuring that it is kept in proper position.

The output of this process is an upset preform (13).

3. Finisher Formation by Forging:

- Any type of forging equipment such as mechanical, hydraulic press capable of applying sufficient amount of force is used for final or finisher forging.

The finisher forging operation involves providing a finisher die assembly (5). The finisher die assembly (5) consist of two dies, namely a top finisher die (9) and a bottom finisher die (8), and two die holders, namely a top finisher die holder (6) and a bottom finisher die holder (7). The top finisher die (9) has a head portion cavity (9 A) that consists of plurality of helical or spiral or curved teeth segments in a negative tooth configuration. In one aspect of the invention, the outer shape of the top finisher die (9) is made conical and the top finisher die holder (6) has a conical shaped cavity or impression (6A) in which the top finisher die (9) sits.

The bottom finisher die (8) has a cavity (8A) corresponding to the stem (3) and tail (4) portion of the forged pinion such that a near-net shaped forged component is obtained at the end of the forging operation. The bottom finisher die (8) sits in a bottom finisher die holder (7). The bottom finisher die (8) is cylindrical in shape which sits in a cylindrical cavity (7 A) provided in the bottom finisher die holder (7).

The upset preform is positioned on the third impression (8A) of the bottom finisher die (8) such that the axes of the upset preform (13) and the finisher dies (8, 9) match with each other. It is ensured that, both the top and bottom dies of the finisher die assembly are properly lubricated.

The finisher operation is done such that both the finisher die impressions (8A, 9A) completely fill up while excess material comes out in the form of flash after forging.

The output of this operation is finish forged pinion (14).

The finish forged pinion is ejected from the top finisher die (9) under the influence of self-weight of the forged pinion. The details of the finisher die assembly (5) and its working - such that easy ejection of the forged part is facilitated - are explained in details in the section titled 'Finisher die assembly'.

Trimming operation:

The finish forged pinion is transferred to trimmer die for flash trimming operation. The hot job (forged product) is located properly on trimmer impression and flash trimming is carried out.

The trimming operation is carried out in any mechanical or hydraulic press having the required load capacity.

The output of this operation is a trimmed finish forged pinion. 5. Post forging operation:

After the finisher operation, post forging operations are conducted on the trimmed finish forged pinion leading to a treated finish forged pinion. These operations include shot blasting, heat treatment, crack detection etc.

6. Machining operation:

The treated finish forged pinion is subjected to a machining process to obtain the final pinion drive.

The method of invention forms the pinion teeth in an as-forged condition. The as- forged teeth have a shape which is very close in form/profile to that of the final machined teeth but with some machining allowance. Thus, during the machining operation, the grain flow lines are not cut and they conform to the teeth profile and remain continuous. Due to the presence of these continuous grain flow lines conforming to the profile of the teeth, the invented forging process sequence enhances component strength and life.

Finisher die assembly:

As explained previously, one of the biggest problems associated with the formation of as forged helical or spiral gear teeth arises from the requirement that negative impression needs be provided in the finisher die. As discussed earlier, the negative impression does not allow easy removal of the finish forged pinion from the finisher die after the completion of the finisher forging operation.

The above problem is overcome in this invention through an innovative finisher die assembly design and assembly which allows for the easy and automatic removal of the finish forged pinion (14) from the top finisher die (9).

The present invention discloses a finisher die assembly (5) for forging the near net shape component from a solid billet. The assembly (5) is shown in the Figure 3. The assembly comprises a top finisher die holder (6) and bottom finisher die holder (7) with pockets or impressions (6A, 7A) respectively having shapes of die blocks for holding top finisher die (9) and bottom finisher die (8). The finisher dies (8, 9) are preferably removable and can be replaced with other dies into the finisher die holders (6, 7).

The impressions in the finisher top and finisher bottom dies are designed such that the head (1) and teeth (2) are formed in the top finisher die (9) while stem (3) and tail (4) are formed in the bottom finisher die (8). A cavity or first impression (6A) is provided in the top finisher die holder (6) to accommodate the top finisher die (9), and a cavity or second impression (7 A) is provided in the bottom finisher die holder (7) to accommodate the bottom finisher die (8). A cavity or third impression (8A) is provided in the bottom finisher die (8) to correspond to the stem (3) and tail (4) portions of the finish forged pinion drive (14), and a cavity or fourth impression (9 A) is provided in the top finisher die (9) to correspond to the head (1) portion of the finish forged pinion drive (14).

The top finisher die (9) is assembled in the top finisher die holder (6) while the bottom finisher die (8) is assembled in the bottom finisher die holder (7).

As explained previously, the helical or spiral teeth in the top finisher die (9) require negative impression, and hence, in the conventional processes it becomes difficult to eject the finish forged pinion (14) from the top finisher die (9). In the conventional methods, the finish forged pinion can be ejected out if it is given a rotational (unscrewing like) motion along with the axial motion. So the ejection of the part requires two simultaneous motions - an axial motion along with rotational (unscrewing like) motion. After the completion of the finisher stroke in the press, the finisher top die (9) and top finisher die holder (6) move upwards. Due to the negative impression, the finish forged pinion (14) also moves along with it thus, getting removed from the bottom finisher die (8). Once the finish forged pinion is free from the bottom finisher die (8), only the self-weight acts on it. During the finish forging operation, the material flows to completely fill up the cavities (8A, 9A) of the finisher dies (8, 9). Due to this, in the conventional processes, the complete surface of the finish forged pinion (14) is in contact with the surface of the impression (8A, 9A) in the finisher dies (8, 9). This complete contact leads to a magnitude of friction between the two surfaces (the outer surface of the forged part and surface of the impressions in the finisher dies) which cannot be overcome by the axial force generated by the self-weight. Hence, with conventional design of dies and die holder, the finish forged pinion does not eject under self-weight and remains stuck in the finisher top die (8), and thus remains hanging. To overcome this issue an innovative finisher die and die holder design has been proposed in the present invention. According to this (see Figure 3), the external shape of top finisher die (9) is made conical as against the conventionally used cylindrical shape. The conical shaped top finisher die (9) sits inside a conical shaped first impression (6A). As a novel aspect of the finisher die assembly (5), the depth (h2, see Figure 3) of the first impression (6 A) is kept more than the height (hi, see Figure 3) of the top finisher die (9), such that after the finisher die assembly (5) is complete, there is gap (12) between the bottom face (11) of the first impression (6A) and the top face (10) of the top finisher die (9). The gap (12) at the start of the finisher forging operation is kept as small as practically feasible, typically maintained between 0.5 mm and 5 mm. (It should be noted that no such gap is present in the conventional die assembly designs, where the top finisher die is cylindrical in shape and the top face of the top finisher die and bottom face of the impression in the top finisher die holder touch each other such that the top die rests on this face.)

In one aspect the invention, the gap (12) is non-uniform. The presence of gap (12) has the following effect. During the finish forging operation, when the material is deforming, reaction forces are exerted on the top finisher die (9). Since during this time the motion of the top finisher die (9) is in downward direction, the reaction forces are upward in direction. Due to these forces, the top finisher die (9) deforms and there is a relative motion between the top finisher die (9) and top finisher die holder (6). The top finisher die (9) gets compressed such that the initially provided gap (12) reduces, and consequently the compression contracts the impression (9 A) (which forms the teeth profile in the pinion) of the top finisher die (9). However, this contraction is elastic in character. Once the forging operation is complete and, when upon removal of the forging forces, the top finisher die (9) and top finisher die holder (6) start moving away from each other, the reaction forces become zero. Due to this, the top finisher die (9) expands again to regain its original shape/size and position relative to top finisher die holder (6) (elastic recovery). In other words, the original gap (12) is restored substantially. Consequently, the impression (9 A) in the finisher top die (9) also expands. The contraction and subsequent expansion of the impression in the top finisher die (9) results in reduction of the overall contact area between the finisher die inner surface and outer surface of the finish forged part. This reduction in contact area drastically reduces the friction between the top finisher die (9) and the finish forged part. Consequently, the forged pinion becomes loose from the top finisher die (9), thus letting the forged part automatically both to rotate and eject under its self-weight. A bottom ejector (17) is provided in the bottom finisher die (8) to facilitate the part removal in the case where the loosened pinion drive falls into the bottom finisher die (8).

Thus, the invented die design and assembly allows the easy and automatic ejection of the finish forged part (14) from the finisher top die (9) under self- weight without requirement of any complex ejection systems as disclosed in prior arts.

The above described design and assembly of the finisher die with finisher die holder also has a very positive effect on the die life. Due the contraction of the top finisher die during the forging operation, the die is under compressive stresses during the forging process. The presence of the compressive stresses nullifies the tensile stresses produced during the forging process thus, improving the die life significantly.

Advantages of the invention:

1. The invented step wise forging process will make high strength pinion drive having as-forged teeth with improved machining productivity, manufacturing cost and better performance in its service life.

2. As-forged teeth forging produces continuous grain flow lines conforming to the teeth profile thus improving the strength of the part and also avoid rough machining cost.

3. Stepwise forging process will make good quality and economical product. The invented die and die holder design and assembly allows the easy and automatic ejection of the finish forged pinion drive under self- weight without requirement of any complicated ejection mechanisms.

The invented die and die holder assembly also improves the die life by producing compressive residual stresses in the die during the forging operation.

It is evident from the foregoing discussion that the invention has a number of embodiments:

1. A finisher die assembly (5) for making a pinion drive, said assembly (5) comprising two finisher dies (8, 9), namely a top finisher die (9) and a bottom finisher die (8), and two finisher die holders (6, 7), namely a top finisher die holder (6) and a bottom finisher die holder (7), characterized in that said two finisher dies (8, 9) are together capable of producing a pinion drive (14) which is near-net shaped, wherein

- said top finisher die (9) has a top face (10) and a fourth impression (9 A) for the head (1) portion of said pinion drive (14) that incorporates a plurality of helical or spiral or curved teeth segments and a negative tooth configuration;

- said bottom finisher die (8) has a third impression (8A) for stem (3) and tail (4) portions of said pinion drive (14);

- said top finisher die holder (6) has a first impression (6 A) which itself has a bottom face (11), and wherein shape of said first impression (6 A) corresponds to the external shape of said top finisher die (9), further wherein in the fully assembled state of said assembly (5) there is a gap (12) provided between said bottom face (11) and said top face (10).

A finisher die assembly (5) as disclosed in embodiment 1, characterised in that the magnitude of said gap (12) is between 0.5mm to 5 mm, preferably 1mm.

A finisher die assembly as disclosed in any of embodiments 1 or 2, characterised in that the outer shape of the said top finisher die (9), and shape of said first impression (6A) are conical.

A finisher die assembly as disclosed in any of embodiments 1 to 3, wherein said gap (12) is non-uniform.

A forging process for making a pinion drive (14), characterised in that said process comprises the steps of near-net forging the gear teeth having a helical or spiral shape, and ejecting the forged part under its self-weight.

forging process for making a pinion drive as disclosed in embodiment 5, characterised in that said steps of near-net forging the gear teeth comprise the sub-steps of:

- cutting a billet into a flat-faced billet and preparing it for formation of an upset preform (13);

- making an upset preform (13) to required size by upsetting the heated billet in a set of upsetting dies (15, 16);

- forming a finish forged pinion drive (14) having as-forged teeth of helical or spiral shape using a finisher forging step;

- machining nominally said finish forged pinion. A forging process for making a pinion drive as disclosed in embodiment 6, characterised in that said steps of billet cutting and preparing it for formation of upset preform (13) comprises the sub-steps of

- before cutting it, ensuring said billet is a solid billet that is materially clean and devoid of defects, followed by cutting it to required size;

- heating said clean and defectless cut billet in a furnace at a temperature between 1150° C and 1280° C.

A forging process for making a pinion drive as disclosed in any of embodiments 6 to 7, characterized in that said step of making an upset preform (13) comprises the substeps of:

- deploying preform-making forging equipment such as mechanical, hydraulic press capable of applying sufficient amount of force required for preform making;

- upsetting said heated billet between a set of two pre-form making dies, namely a top upsetting die (15) and a bottom upsetting die (16), both dies provided with respective preforming impressions, said upsetting dies (15, 16) being lubricated prior to placement of said heated billet between them;

- forging said heated and upset billet into said upset preform (13).

A forging process for making a pinion drive as disclosed in any of embodiments 6 to 8, characterized in that said step of forming a finisher pinion drive (14) comprises the substeps of: - deploying finisher-making forging equipment such as mechanical, hydraulic press capable of applying sufficient amount of force to said upset preform to produce said finish forged pinion drive (14), wherein said finisher-making forging equipment employs said finisher die assembly (5) as claimed in claim 1;

- positioning said upset preform (13) on an impression provided on said bottom finisher die (8), wherein said impressions (8A, 9A) of said both top and bottom finisher dies are lubricated prior to placing said upset preform (13) between them;

- applying a forging force and completely filling up said impressions (8 A, 9 A) of both finisher dies (8, 9) so that excess material comes out in the form of flash;

- ejecting the finish forged pinion drive (14) automatically under its own weight.

10. A pinion drive (14) manufactured using a forging process as disclosed in any one of the embodiments 5 to 9 and using said finisher die assembly (5) as disclosed in any of embodiments 1 to 4, characterised in that said finish forged pinion drive (14) has continuous grain flow lines in its as-forged helical or spiral teeth (2).

11. A pinion drive (14) as disclosed in embodiment 10, characterised in that said pinion drive (14) has continuous grain flow lines in said as- forged helical or spiral teeth (2). 12. A pinion drive (14) as disclosed in embodiments 10 or 11, characterised in that said pinion drive (14) is of near-net shape.

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.