Field of the Invention:

The present invention relates to machining of a crown gear used in a differential assembly. Particularly it relates to grinding of the crown gear teeth profile and face in the assembled condition with differential housing/casing.

Background of the invention:

A differential is used in an automobile to drive a pair of wheels such that it allows the wheels to rotate at same or different speeds. At different speeds, the differential allows the outer wheels of an automobile to rotate faster than the inner wheels during a turn. A crown gear is an important part of the differential assembly which transmits the torque from the engine of an automobile to its wheels through the differential. It is important to manufacture the teeth profile of crown gear with the accuracy of ISO 1328-1 1995 Grade 5. The lead and the run out of the crown gear along the length, OD and the profile twist needs to be maintained as per the accuracy of the grade. Conventionally, the crown gear teeth profile and the differential housing are machined individually as separate components within the prescribed tolerances and then assembled together by bolting. This conventional process of machining the components individually and assembling them after machining, leads to the crown gear profile being out of tolerances when inspected in assembled condition with differential housing.

There is therefore a need to provide a method of manufacturing whereby the crown gear profile is within the tolerance limits when inspected in assembled condition with differential housing.

Summary of the Invention:

Present invention relates to the machining or grinding of the crown gear profile in an assembled condition with the differential housing. The invented process elaborates the machining or grinding of crown gear teeth profile in assembled condition with the differential housing. In this invention, a special enclosed type of fixture is manufactured for grinding of crown gear teeth profile to avoid the grinding dust or burrs to enter differential housing sub-assembly.

Objects of Invention

Some of the objects of the present invention, which at least one embodiment herein satisfies, are now disclosed. It is an object of the present invention to produce crown gear with grinded teeth profile and face with the accuracy of ISO 1328-1 1995 Grade 5. It is another object of the present invention to provide a method for grinding the teeth profde and face of crown gear in the assembled condition with differential housing. It is yet another object of the invention to provide specially developed fixtures to be used during crown gear teeth profile grinding in assembled condition with the differential housing

It is still another object of the present invention to avoid the grinding dust or burr to enter into differential housing sub-assembly during grinding of crown gear teeth profile.

Brief Description of Figures:

The following non-limiting figures illustrate the method of grinding the teeth profile and face of crown gear in assembled condition with differential housing in accordance with the preferred embodiments of the present invention.

Figure 1 shows a sub-assembly of typical crown gear and differential housing Figure 2 shows conventional manufacturing process of crown gear

Figure 3 shows the Fixture- 1 clamped with sub-assembly of crown gear and differential housing in accordance with one implementation of the present invention

Figure 3a shows the sectional and isometric views of Fixture- 1 Figure 4 shows the Fixture-2 clamped with sub-assembly of crown gear and differential housing in accordance with one implementation of the present invention

Figure 4a shows the isometric view of Fixture-2 in accordance with one implementation of the present invention

Figure 5 shows steps involved in manufacturing process of crown gear in accordance with one implementation of the present invention

Figure 6 shows sub-steps involved in crown gear face and teeth grinding in accordance with one implementation of the present invention

List Of Parts:

1 - Differential Housing 11B - Face of Fixture-2 supporting

2 - Crown Gear 25 crown gear

3 - Crown gear teeth profile 11C - Fixture-2 ID

4 - Crown Gear Flank 11D - Bolting holes for mounting

5 - Crown Gear OD Fixture-2

6 - Crown Gear ID 11 E - Locator of Fixture-2

7 - Resting face 30 12 - Differential housing OD

8 - Crown gear bore 13 - Crown gear top face

9 - Crown gear face 14 - Top side OD of differential

10 - Fixture- 1 housing

11 - Fixture-2 15 - Bottom side OD of differential

11A - Fixture-2 enclosure 35 housing 16 - Crown gear bottom face 19 - Step OD of Fixture- 1

17 - Mandrel of Fixture- 1 5 20 - Tailstock Centre

18 - Differential Housing ID 21 - Sleeve of Fixture- 1

Detailed Description of the Invention:

The present invention is focused on eliminating the stated drawbacks/limitations of conventional manufacturing processes. This is achieved by providing the grinding method for crown gear teeth profde (3) and bottom face (16) in the assembled condition with differential housing (1) with help of Fixture-1 (10) and Fixture-2 (11). The key inventive feature of the present invention is the sequence of manufacturing method and special fixtures (10, 11) used in order to grind the teeth (3) and bottom face (16) of crown gear (2) in assembled condition with differential housing (1) within the ISO 1328-1 1995 Grade 5 tolerances. In an embodiment of present invention, there is provided a sub-assembly of crown gear (2) and differential housing (1), as shown in Figure 1, of a typical automobile; the said crown gear (2) comprises array of gear teeth profile (3), Gear Flank (4), OD (5), ID (6), Resting face (7), bore (8), Crown gear face (9), Crown gear top face (13), Crown gear bottom face (16); the said differential housing (1) comprises Differential housing OD (12), Top side OD (14) of differential housing (1), Bottom side OD (15) of differential housing (1), and the ID (18) of the Differential Housing (1).

Figure 2 shows the conventional manufacturing method to produce any typical crown gear (2) of differential. The major steps involved in this are as follows.

Turning the forged blank/billet/disc to produce a turned disc

PCD Hole Drilling on said turned disc to produce a drilled disc

Gear Hobbing on said drilled disc to produce a hobbed disc

Teeth Flank Chamfering on said hobbed disc to produce a chamfered disc

- Heat Treatment, preferably case carburizing, of the chamfered disc to produce a heat treated disc

Bore and face hard turning of the heat treated disc to produce a crown gear Teeth profile grinding the hard turned disc to produce a final crown gear Inspection of the crown gear

- Assembly of the crown gear and differential housing

Inspection on gear tester

The typical problem which arises during this conventional process, of machining the crown gear (2) individually and assembling them after machining with differential housing (1), is that the crown gear teeth profile (3) is out of tolerances when inspected in assembled condition. To eliminate this problem, in one aspect of the present invention, the crown gear (2) is assembled with the differential housing (1) before performing the face (16) and teeth profde (3) grinding on the crown gear (2). The invented manufacturing process is as show in Figure 5. The steps of process are given below.

a. Turning: Turning is a forged blank/billet/disc to produce a turned disc b. Hole Drilling: Next, PCD hole drilling is carried out on the turned disc to produce a drilled disc.

c. Gear Hobbing: Next, gear hobbing is performed on the drilled disc to produce a hobbed disc. In this operation, rough teeth profde is generated on crown gear using a gear hobbing machine to achieve good accuracy in rough condition.

d. Chamfering: Next, teeth flank chamfering is performed on the hobbed disc to produce a chamfered disc.

e. Heat Treatment: Heat treatment, preferably case carburizing, is done on the chamfered disc to produce a heat treated disc. Heat treatment is carried out to achieve the required case depth and hardness on the crown gear flanks (4), crown gear OD (5), crown gear ID (6).

f. Hard Turning: Hard turning the bore and face of the heat treated disc is done to produce crown gear (2). The squareness of the crown gear bore (8) and resting face (7) is maintained within 0.020 mm in hard turning process. g. Cleaning: Next, a cleaning machine is used to ensure the washing of the crown gear and the differential housing in such way that no machined burrs are present. h. Assembly: In the next step, the crown gear (2) and the differential housing (1) are sub-assembled by press fitting and bolting with required torque. i. Bottom face grinding: Next, grinding of the bottom face (16) of the crown gear (2) is carried out, while in assembled condition with the differential housing (1), using Fixture-1 (10).

j . Washing-1 : Next, Washing of the sub-assembly is done to remove grinding dust or burrs produced during the bottom face grinding operation.

k. Teeth grinding: Next, Teeth grinding is performed on the Crown gear (2), while in assembled condition with differential housing (1), with the help of Fixture-2 (11). The critical gear teeth parameters like Profile Slope Deviation (±5 microns) and Helix Slope Deviation (±8 microns) are maintained in this grinding operation.

l. Washing-2: Next, Washing of the sub-assembly is done to remove grinding dust or burrs produced during the teeth grinding operation.

m. Inspection: As a last step, Inspection of the crown gear (2) in assembled condition with the differential housing (1) is performed on a gear tester to check whether all the critical dimensions of the sub- assembly are within tolerance specifications or not. In another aspect of the invention, Fixture- 1 (10) is used to grind the bottom face (16) of the crown gear (2). As shown in the Figure 3, the crown gear (2) and differential housing (1), in assembled condition, are mounted on the Fixture- 1 (10). Fixture-1 (10) consists of hydraulically actuated mandrel (17) which clamps the differential housing ID (18) with the mandrel OD (17) in order to accommodate the sub-assembly of crown gear (2) and differential housing (1) onto the Fixture- 1 (10). To start this grinding operation, the mandrel (17) is loaded on the machine between centres by using driving carrier at step OD (19). Runout of the bottom face (16) of the crown gear (2) is a very important parameter in order to achieve the crown gear teeth parameters during the subsequent crown gear teeth profde grinding operation. Runout of the bottom face (16) of the crown gear (2) is maintained below 0.01 mm during this side face grinding operation using Fixture-1 (10). Perpendicularity or squareness of the bottom face (16) of the crown gear (2) with respect to the differential housing OD (12) is also maintained within 0.01 mm during this operation.

Figure 3a shows the sectional and isometric view of special Fixture- 1 (10). This Fixture-1 (10) is used during the grinding of crown gear bottom face (16). Special features of this fixture are as follows:

- Hydraulic actuation of the mandrel (17) to hold the differential housing ID (18).

- Hydraulic actuation of Fixture- 1 will ensure the concentricity between differential housing ID (18) and bottom side OD of differential housing (15) after the grinding of this OD.

The said Fixture-1 (10) used for the grinding of crown gear side face (16) comprises mandrel (17), Sleeve (21) and Step OD (19). The mandrel (17) has cylindrical shape while the inner side of sleeve (21) is cylindrical while the outer side can be cylindrical or conical or polygonal. The diameter of Step OD (19) and mandrel (17) of Fixture- 1 (10) is same. The sleeve (21) and mandrel are joined to each other by any joining means preferably by bolting. The mandrel (17) is in contact with differential housing ID (18). The bottom side OD (15) of differential housing is located in the inner diameter of sleeve (21). The hydraulic actuation of Fixture- 1 (10) locates the mandrel (17) into the differential housing ID (18) thereby maintaining the concentricity between differential housing ID (18) and bottom side OD of differential housing (15). The Step OD (19) is used to load the Fixture-1 (10) on the grinding machine.

In another aspect of the invention, Fixture-2 (11) is used to grind the crown gear teeth profde (3). The Fixture-2 (11) along with sub-assembly of crown gear (2) and differential housing (1) loaded on it is as shown in Figure 4. This Fixture-2 (11) is having two parts i.e. Fixture-2 enclosure (11A) and locator (11E). This Fixture-2 (11) is mounted on the grinding machine by bolting it to the base through bolting holes (11D). The perpendicularity or squareness between Face of Fixture-2 (11B) supporting crown gear and Fixture-2 ID (11C) is maintained within 0.01 mm. Tailstock centre (20) of grinding machine is located on the Fixture-2 enclosure (11A) of Fixture-2 (11). Runout of this tailstock centre (20) is checked and verified within 0.005 mm. The crown gear (2) and differential housing (1) sub-assembly as shown figure 1 is mounted on this Fixture-2 (11) after caring out the grinding operation of crown gear bottom face (16). Bottom side OD of differential housing (15) is clamped into the Fixture-2 ID (11C) by means of hydraulic clamping. After placing the crown gear (2) and differential housing (1) sub-assembly into Fixture-2 (11), Runout of Crown gear top face (13) and Top side OD of differential housing (14) is verified in order to ensure the proper clamping of this sub-assembly. The Fixture-2 enclosure (11E) is provided on this sub-assembly to avoid the dust or burr to enter into this sub-assembly during the crown gear teeth profile (3) grinding.

Figure 4a shows the sectional and isometric view of Fixture-2 (11). This Fixture-2 (11) is used during the crown gear teeth profile (3) grinding. Some special features of this fixture are as follows,

Hydraulic actuation of Fixture-2 ID (11C) to hold the bottom OD of differential housing (15).

Hydraulic actuation will ensure the perpendicularity or squareness of Fixture-2 ID (11C) and Face of Fixture-2 (11B) supporting crown gear where the crown gear (2) is going to rest during crown gear teeth grinding. Fixture-2 (11) is provided with Fixture-2 enclosure (11A) to avoid the entering of dust or burr during gear teeth profile grinding.

The said Fixture-2 (11) used for the grinding of crown gear teeth profile (3) comprises Fixture-2 enclosure (11A), Face of Fixture-2 (11B) supporting crown gear, Fixture-2 ID (11C), bolting holes for mounting Fixture-2 (1 ID) and Locator (11 E). The details of the sub-steps performed on a crown gear (2) in an assembled condition with differential housing (1) during grinding of crown gear bottom face (16) and teeth profile (3) are explained in line with Figures 3, 4, 5 and 6. The sequential sub-steps for grinding crown gear face and teeth profile are explained as follows:

i. Fixture- 1 (10) is used to perform grinding on the bottom face (16) of the crown gear (2) in assembled condition with differential housing (1). This Fixture- 1 (10) is clamped on base body of gear grinding machine with the help of Step OD (19) of Fixture-1 (10).

ii. The sub-assembly of crown gear (2) and differential housing (1) is fitted on the mandrel (17) of Fixture- 1 (10) by means of hydraulic clamping as shown in figure 3.

iii. Perpendicularity or squareness of the crown gear bottom face (16) which will rest on the Fixture-2 during teeth profile (3) grinding process and squareness of differential housing OD (12) is maintained within 0.01 mm by grinding the bottom face (16) of the crown gear (2).

iv. Sub-assembly washing is performed to ensure its cleanliness.

v. Locator (11E) of crown gear teeth grinding Fixture-2 (11), as shown in Figure 4, is loaded on the base body of gear grinding machine as vertically as possible. Locator (11E) of Fixture-2 (11) as shown in Figure 4 is clamped as vertically as possible on the base of gear grinding machine with the help of bolts. vi. The run out of the Fixture-2 ID (11C) and Fixture-2 face (11B) is kept within 0.010 mm and checked by using dial indicator.

vii. In the first alignment, the bottom side OD (15) of the differential housing (1) is clamped on this Fixture-2 ID (11C) by means of hydraulic clamping. It is shown in figure 4.

viii. In the second alignment, the bottom face (16) of the crown gear (2) of the sub-assembly is resting on the top face of Fixture-2 (1 IB). It is shown in Figure 4.

ix. The enclosure (11A) is placed as shown in Fig.4 on the top face (13) of the crown gear to avoid entering of grinding dust or burr into differential housing sub-assembly, which essentially comprises the crown gear (2) bolted on the differential housing (1).

x. Run out of top side OD (14) of the differential housing (1) and the top face (13) of the crown gear as shown in Figure 4 is verified and it is found within 0.01.

xi. Special fine grade grinding wheel is used to grind the crown gear teeth profile (3) to achieve the desired quality and surface finish less than Ra 0.3.

xii. CNC program is developed for grinding crown gear teeth profile (3). CNC program input is provided to gear grinding machine. The grinding of crown gear teeth profile (3) is performed in assembled condition with differential housing (1). xiii. Final Inspection is done on the gear tester machine. It was found that all crown gear profile geometry are within the tolerances as per the quality standard ISO 1328-1 1995 Grade 5.

It is evident that the invention has a number of embodiments. In its preferred embodiment, the invention discloses a method for manufacturing crown gear of differential assembly and grinding its teeth profile (3). The method has the following steps:

a. turning a forged blank/billet/disc to produce a turned disc;

b. PCD holes drilling on said turned disc to produce a drilled disc;

c. gear hobbing on said drilled disc to produce a hobbed disc, wherein rough teeth profile is generated on said crown gear (2) using a gear hobbing machine;

d. teeth flank chamfering on said hobbed disc to produce a chamfered disc; e. giving heat treatment, preferably case carburizing, to said chamfered disc to produce a heat treated disc;

f. bore and face hard turning of said heat treated disc to produce said crown gear (2);

g. washing of the said crown gear (2) and a differential housing (1) individually using cleaning machine to eliminate machining burrs;

h. assembling together said cleaned crown gear (2) and said cleaned differential housing (1) by press fitting and bolting with required torque to produce the sub-assembly; i. grinding of the bottom face (16) of said crown gear (2) carried out, while in assembled condition with the differential housing (1), using Fixture-1 (10); j . washing of said sub-assembly of said crown gear (2) and differential housing (1) to remove grinding dust or burrs produced during the bottom face (16) grinding operation;

k. grinding of teeth profde (3) of said crown gear (2) in assembled condition with differential housing (1) on a grinding machine with the help of Fixture- 2 (H);

l. second washing of said sub-assembly of said crown gear (2) and differential housing (1) to remove grinding dust or burrs produced during the teeth profde (3) grinding operation;

m. Inspection of the crown gear (2) in assembled condition with the differential housing (1) performed on a gear tester. In an embodiment of the method, said grinding of said bottom face (16) of said crown gear (2) in assembled condition with differential housing (1) is carried out using Fixture- 1 (10) of said step i, further wherein said Fixture- 1 (10) comprises a mandrel (17), sleeve (21) and step OD (19), wherein said mandrel (17) is cylindrical in shape and the inner side of said sleeve (21) is cylindrical while the outer side of said sleeve (21) is either cylindrical or conical or polygonal; also wherein the diameter of said step OD (19) and that of said mandrel (17) of Fixture-1 (10) is equal; and wherein said sleeve (21) and mandrel (17) are joined to each other by any joining means such as bolting; wherein said mandrel (17) is in contact with differential housing ID (18) of said differential housing (1); wherein the bottom side OD (15) of said differential housing (1) is located in the inner diameter of sleeve (21); wherein said mandrel (17) is hydraulically actuated; and wherein the hydraulic actuation of Fixture- 1 (10) locates the mandrel (17) into the differential housing ID (18) of said differential housing (1) thereby maintaining the concentricity between differential housing ID (18) and bottom side OD (15) of said differential housing (1).

In another embodiment of the method, grinding of teeth profile (3) of said crown gear (2) in assembled condition with differential housing (1) is carried out using said Fixture-2 (11) of said step k, wherein said Fixture-2 (11) comprises an enclosure (11A) and a locator (11E); wherein the said Fixture-2 (11) is mounted on said grinding machine by bolting it to the base of said grinding machine through bolting holes (1 ID); further wherein tailstock centre (20) of said grinding machine is located on said Fixture-2 enclosure (11A) of Fixture-2 (11); and wherein said crown gear (2) and differential housing (1) sub-assembly is mounted on said Fixture-2 (11) after carrying out the grinding operation of the bottom face (16) of the crown gear (2); and wherein said bottom side OD (15) of said differential housing (1) is clamped into the Fixture-2 ID (11C) by means of hydraulic clamping.

In a further embodiment of the method, said step i has the following sequential sub-steps: Clamping the Fixture- 1 (10) on base body of gear grinding machine with the help of Step OD (19) of Fixture-1 (10);

Fitting the sub-assembly of crown gear (2) and differential housing (1) on the mandrel (17) of Fixture- 1 (10) by means of hydraulic clamping;

- Grinding the bottom face (16) of the crown gear (2), wherein during the grinding the squareness of the crown gear bottom face (16) with respect to differential housing OD (12) is maintained within 0.01 mm.

In yet another embodiment of the method, said step k has the following sequential sub-steps:

- loading and clamping said Locator (11E) of said Fixture-2 (11) on the base body of gear grinding machine as vertically as possible;

- checking the run out of said Fixture-2 ID (11C) and Fixture-2 face (11B) by using dial indicator and keeping said runout within 0.010 mm;

- carrying out first alignment, wherein the bottom side OD (15) of the differential housing (1) is clamped by means of hydraulic clamping on said Fixture-2 ID (11C);

- carrying out second alignment, wherein the bottom face (16) of the crown gear (2) of the sub-assembly is rested on the top face of Fixture-2 (1 IB); - verifying the run out of top side OD (14) of the differential housing (1) and the top face (13) of the crown gear and maintaining it within 0.01; - placing the enclosure (11A) on the top face (13) of the crown gear (2) of the sub-assembly to avoid entry of grinding dust or burrs into differential housing sub-assembly;

- using the grinding wheel of special fine grade for grinding the crown gear teeth profile (3) to obtain desired quality and surface finish less than Ra 0.3;

- providing developed CNC program as an input to gear grinding machine and performing grinding of crown gear teeth profile (3) in assembled condition with differential housing (1).

- finally inspecting the crown gear (2) on the gear tester machine for tolerances as per the quality standard ISO 1328-1 1995 Grade 5.

In another embodiment of the method, the alignment between Fixture-2 ID (11C) of said Fixture-2 (11) and bottom OD (15) of said differential housing (1) is achieved using hydraulic means. In yet another embodiment of the method, the squareness of Fixture-2 ID (11C) and Face of Fixture-2 supporting crown gear (11B) is maintained by hydraulic actuation of Fixture-2 (11).

In a still further embodiment of the method, said Fixture-2 (11) is provided with Fixture-2 enclosure (11A) to avoid the entry of dust or burrs during gear teeth profile grinding.

In a yet further embodiment of the invention, an apparatus to carry out the method is disclosed. The apparatus comprises Fixture- 1 and Fixture-2. The said Fixture- 1 (10) for carrying out grinding of the Bottom face (16) of said crown gear (2) in assembled condition with differential housing (1) further comprises a mandrel (17), sleeve (21) and step OD (19), wherein said mandrel (17) is in cylindrical shape and the inner side of said sleeve (21) is cylindrical while the outer side of said sleeve (21) is either cylindrical or conical or polygonal; also wherein the diameter of said step OD (19) and that of said mandrel (17) of Fixture- 1 (10) is equal; and wherein said sleeve (21) and mandrel (17) are joined to each other by any joining means such as bolting; wherein said mandrel (17) is in contact with differential housing ID (18) of said differential housing (1); wherein the bottom side OD (15) of said differential housing (1) is located in the inner diameter of sleeve (21); and wherein said mandrel (17) is hydraulically actuated which will ensure the concentricity between differential housing ID (18) and bottom side OD

(15) of said differential housing (1). The said Fixture-2 (11) for carrying out grinding of teeth profile (3) of said crown gear (2) in assembled condition with differential housing (1) characterised in that said Fixture-2 further comprises an enclosure (11A) and a locator (11E); wherein the said Fixture-2 is mounted on said grinding machine by bolting it to the base of said grinding machine through bolting holes (11D); further wherein tailstock centre (20) of said grinding machine is located on said enclosure (11A); and wherein said crown gear (2) and differential housing (1) sub-assembly is mounted on said Fixture-2 (11) after carrying out the grinding operation of the bottom face

(16) of the crown gear (2); and wherein said bottom side OD (15) of said differential housing (1) is clamped into the Fixture-2 ID (11C) by means of hydraulic clamping.

Advantages of the invention:

The process of grinding of crown gear teeth profile (3) and face (16) in assembled condition with the differential housing (1) has several advantages in terms of quality of the crown gear (2). The parameters like crown lead variation, crown gear run out along length and OD and crown gear profile twist are achieved within the tolerances as per the quality standard ISO 1328-1 1995 Grade 5. The enclosed design of the special fixture used to clamp the sub-assembly, prevent the grinding dust or burr to enter into the differential housing sub-assembly.

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.