DESC:FORM 2
The Patent Act 1970
(39 of 1970)
&
The Patent Rules, 2005

COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

TITLE OF THE INVENTION
“FORK BOLT FOR FRONT FORK SUSPENSION ASSEMBLY
OF A VEHICLE”

Endurance Technologies Limited
E-92, M.I.D.C. Industrial Area, Waluj,
Aurangabad – 431136, Maharashtra, India

The following specification particularly describes and ascertains the nature
of this invention and the manner in which it is to be performed.

Field of Invention

The present invention relates to the front fork suspension assembly for a two wheeled vehicle. More particularly, the present invention relates to a fork bolt for front fork suspension assembly wherein said fork bolt has a hybrid structure to enhance its strength.

Background of the Invention

Fork bolts are used to seal the fork pipes of the suspension system from the top. Despite serving the critical function of keeping the shock absorber safe from environmental conditions, the fork bolts have not been explored for technical improvements by the industry experts as of now. The function of the fork bolt is not only sealing the fork pipe from its top end but providing the abutment for the main spring housed in the fork pipe.

Since, the fork bolts are mounted at the top of the fork pipes, the fork bolt experiences compressive load during working condition of the fork assembly. By considering the vehicle level safety view point, the fork assembly is required to meet good tensile load capacity and compression load capacity. The compressive and tensile load requirements may reach up to 13 KN during operation of a vehicle. The fork bolt of the fork assembly plays a vital role for sustaining these loads. Owing to the aggressive load requirements, the fork bolts are currently being made from EN8M carbon steel. These fork bolts of the prior art thus impose the issues of increased weight and extra machining time leading to high cost of the fork bolt. Therefore, there is a long pending unmet requirement to overcome the above mentioned lacunae of the existing fork bolts and provide an efficient and economic fork bolt for the suspension assembly of a vehicle.

Objectives of the Present Invention

The main object of the present invention is to provide an efficient, light weight and cost effective fork bolt for front fork suspension assembly of a two wheeled vehicle.

Another objective of the present invention is to provide a fork bolt for fork assembly of a vehicle wherein the fork bolt is configured to have a hybrid structure with a metal insert injection molded in the plastic body of the fork bolt.

Another objective of the present invention is to provide a hybrid fork bolt which is light weight, durable and sustain the rigorous loading conditions during the operation of the vehicle.

Yet another objective of the present invention is to provide a hybrid fork bolt of plastic with a metallic insert therein requiring no extra machining to be performed to deliver finished fork bolt.

Yet another objective of the present invention is to provide a hybrid fork bolt that reduces the production time without compromising the tensile and compressive load requirements and provides an economic solution of fork bolt.

Brief Description of Drawings

This invention is illustrated in the accompanying drawings, throughout which like reference letters and numerals indicate corresponding parts in the various figures. The embodiment/s herein and advantages thereof will be better understood from the following description when read in conjunction with the following drawings, wherein

Figure 1 shows cut sectional view of a conventional fork bolt (10) made in metal and assembled within the fork pipe (30).

Figure 2 presents cut sectional view of a fork bolt of the present invention assembled/fitted within the fork pipe in accordance with an embodiment of the present invention.

Figure 3 shows isometric view of the fork bolt in accordance with an embodiment of the present invention.

Figure 4 shows cut sectional view of the fork bolt in accordance with an embodiment of the present invention.

Figure 5 presents a perspective view of a metallic insert of the fork bolt in accordance with the present invention.

Figures 6 and 7 show top view and sectional view, respectively of a metallic insert of the fork bolt as per the present invention.
Figure 8 shows cut sectional view of a fork bolt of the invention with another embodiment of the metallic insert.

Figures 9 and 10 show top view and sectional view, respectively of another embodiment of metallic insert of the fork bolt as per the present invention.

Detailed Description of the Present Invention

Referring to Fig. 1, a conventional fork bolt (10) made of solid metal body is fitted at the top open end of a fork pipe (30) in such a way that the lower face of the fork bolt (10) arrests a main spring (40) placed inside the fork pipe (30). The fork bolt (10) is getting locked in the fork pipe (30) with the help of a circlip (C1) and is provided with a groove (G1) on its outer circumferential surface for accommodating an O-ring (20). The O-ring (20) is placed in the groove of the fork bolt (10) thereby sealing the interference between the inner surface of the fork pipe (30) and outer surface of the fork bolt (10).

Referring to Figs. 2 and 4, the fork bolt (500) of the present invention is configured to have a fork bolt body (50) made in plastic, a skirt portion (50S) and a metallic insert (100) is insert molded in the fork bolt body (50) forming the hybrid structure of the fork bolt (500). The fork bolt body (50) and the skirt portion (50S) together is an integral unit made up of plastic selected from the group of thermoplastic materials and preferably polyamide. This integral unit of fork bolt body (50) and the skirt portion (50S) made by insert molding process is configured to form a frustum shaped cavity (70) at the inner peripheral surface of the skirt portion (50S). The most preferred material for the fork bolt body (50) and its skirt portion (50S) is polyamide with glass filled so as to effectively compensate with the mechanical properties required for the purpose. The fork bolt body (50) along with its skirt portion (50S) of the fork bolt (500) of the present invention is formed by insert molding process and said skirt portion (50S) is configured to have at least a pair of oil seal fins (55) formed on its lower outer circumferential surface and an O-Ring projection (60) formed above the oil seal fins (55). The oil seal fins (55) helps in evenly distributing the oil pressure within the fork pipe and the O-Ring projection (60) helps in avoiding the oil leakage therefrom.

The bottom face (F) of the skirt portion (50S) of the fork bolt (500) is configured to provide the abutment to the main spring (40). Further, the fork bolt (500) is configured to have a groove (50G) on its outer circumferential surface at the interfacing area of said fork bolt body (50) and the skirt portion (50S). The groove (50G) is configured to accommodate an O-ring (20) therein so as to seal the interference between the inner surface of the fork pipe (30) and the outer peripheral surface of the fork bolt (500) and thereby completely eliminating the leakage of oil, if any. The said groove (50G) is positioned in between the base of the metal insert (100) in the fork bolt body (50) and the oil seal fins (55) on the skirt portion (50S) of the fork bolt (100).

Referring to Figs. 4 to 7, the metallic insert (100) is configured to have an inverted U-profile comprising of a base portion (105), a wall portion (110) and disc portion (115) in such way that the disc portion (115) is integrally connected with the base portion (105) through the wall portion (110) forming frustum shaped cavity (130) with the base portion (105). The material of the metallic insert (100) is selected from a group of metals viz. steel, Aluminum (Ferrous & Non-Ferrous), Magnetic-Nonmagnetic materials, etc. The most preferred material for the metallic insert (100) is a low alloy steel (E34 steel) which is extensively preferred for automotive applications.

During the process of insert molding to form the fork bolt (500) of the present invention, the positioning of the base portion (105) of the metallic insert (100) is intelligently done at ‘y’ in between the ‘x’ and ‘z’ portion of the total height (H) of the fork bolt (500). Thus, the total height (H) of the fork bolt corresponds to the below mentioned proportions in order to impart the desired mechanical strength of the hybrid nature / structure of the fork bolt,
H = x + y + z
Wherein, x = c1*y and y = c2*z
wherein, c1 = 2.0 to 5.0, and c2 = 0.1 to 0.5
where,
x = distance of base portion of metallic insert from the top surface of the fork bolt
y = thickness of the base portion of the metallic insert, and
z = distance of base portion of the metallic insert from the bottom surface of the fork bolt

Further, the fork bolt (100) has an outer diameter D wherein the diameter is calculated in accordance with the below mentioned equations so as to impart the required load bearing capacity during the operation of the front fork suspension assembly.

s=4P/pD2+(Mb* y)/I+UTS/FOS
where,
s = combined stress on the fork bolt;
P = tensile load on the fork bolt;
Mb = bending load on the fork bolt;
D = outer diameter of the fork bolt;
I = moment of inertia of the fork bolt;
y = perpendicular distance to the neutral axis;
UTS = ultimate tensile stress; and
FOS = factor of safety.

Thus, the ratio of the outer diameter D of the fork bolt (500) to the total height (H) of the fork bolt (500) is set to be in a range of 1.2 - 1.5 to impart the required mechanical strength during the operation of the same. In other words, the ratio said total height (H) of the fork bolt (500) to the outer diameter D of the fork bolt (500) is in the range of 0.60 to 0.85 to impart the required mechanical strength during the operation of the same.

Further, the disk portion (115) of the metallic insert (100) is configured to have at least three thorough openings (120) spaced at an equal angular distance and these openings (120) are provided so as to rigidly lock the metallic insert (100) in the fork bolt body (50) of the fork bolt (500) and thereby make an integrated hybrid structure of the fork bolt (500). These thorough openings (120) are positioned on the disc portion (115) of the metallic insert (100) in such way that the center of each of the through openings (120) maintains an equiangular relation with each other from the geometric center of the disk portion (115). The center of each of the thorough openings (120) lies on the imaginary circle (IC1) having radius Rb, and this radius Rb is maintained at specific proportion with respect to the radius Ra of the disc portion (115) of the metallic insert (100) so as to ensure seamless flow of molten plastic through these thorough openings (120) during the process of insert molding to form an integrated structure of the fork bolt (500). The empirical relation between Rb and Ra is as given below:
Rb = k1*Ra
Where, k1 = 0.5-0.6; and
Ra = the radius of the disk portion (115) of the insert

The Figs. 8 to 10 discloses another embodiment of the metallic insert (200) of the fork bolt (500) within the scope of the invention. The metallic insert (200) is configured to have an inverted U-shaped profile having a base portion (205), wall portion (210) and a disk portion (215). The disk portion (210) is configured to have at least three flared bosses (220) having a thorough opening (225) on the top surface of the disk portion (215). The disc portion (215) of the metallic insert (200) is integrally connected with the base portion (205) through the wall portion (210) forming frustum shaped cavity (230) with the base portion (205).

The flared bosses (220) are configured to be formed in a manner such that the axis of the flared bosses (220) forms an angle with the central axis of the metallic insert (200). The said angle is not more than 15 degree. The most optimized range of the to have strong bonding of metal insert (200) with the fork bolt body (50) during injection molding varies from 6 to 15 degrees. Further, assuming an imaginary base circle (IC2) of radius Rc, the center of each of the flared bosses (220) lies on the imaginary circle. In order to carry out the invention i.e. to ensure seamless flow of molten plastic from the flared bosses (220), an empirical relation was established between the Rc and Rd which is:
Rc = k2*Rd
Where, k2 = 0.6-0.75; and
Rd is the radius of the disk portion (215)

The fork bolt (500) of the present invention was rigorously tested for its mechanical properties as per the HMSI 5140Z-GHA-6700 standard and the encouraging and at par results were found as tabulated below.
Sample Acceptance criteria as per the standard Breaking load of conventional metallic fork bolt Breaking load of fork bolt of the invention
Sample 1 11.8 kN 30.5 kN 27.1 kN
Sample 2 11.8 kN 31.7 kN 24.9 kN
Sample 3 11.8 kN 26.4 kN 29.9 kN

The above test result table makes it crystal clear that the fork bolt of the present invention with the uniquely profiled metallic insert therein impart superior result in terms of its mechanical properties in light the recommended standard.

The fork bolt (500) in accordance with the disclosed embodiments imparts following technical advantages that contributes to the advancement of technology and thereby establishes the inventive step:
It provides the fork bolt with improved tensile strength, such that the tensile strength of the hybrid fork bolt is at par with the conventional steel fork bolt.
It provides a light weight, compact and durable fork bolt for a two/three wheeler suspension leading to drastic reduction in weight by 60%.
It provides a cost effective solution of fork bolt for the two/three wheeler suspension without compromising the functionality and mechanical properties of the fork bolt leading to drastic reduction in the cost of the fork bolt by 40%.
It reduces the manufacturing time and eliminate the machining operation and time required therefor thus reducing the overall cost of the fork bolt.
It provides excellent durability against the water contamination and corrosion etc.

The disclosed invention hence overcomes the limitation of the systems forming state of the art. As would be clear to a person skilled in the art, the metallic insert (100, 200) can be configured to have the different profiles in place of the openings (120) and the flared bosses (220) provided to facilitate the flow of molten plastic around the metallic insert (100, 200) during injection molding. Therefore, any change pertaining to profile of the metal insert and/or the openings (120, 220) of said metallic insert must not be construed to be taking a resulting system out of the scope of the claimed invention.

It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
,CLAIMS:We Claim

1. A fork bolt (500) for front fork suspension assembly of a vehicle comprising of a fork bolt body (50), a skirt portion (50S) and a metallic insert (100, 200),
wherein
- the fork bolt body (50) and the skirt portion (50S) are made from thermoplastic and are configured to form an integral unit by insert molding process forming a frustum shaped cavity (70) at the inner peripheral surface of the skirt portion (50S);
- the metallic insert (100, 200) is configured to have an inverted U-profile comprising of a base portion (105, 205), a wall portion (110, 210) and disc portion (115, 215); and said disc portion (115, 215) is integrally connected with the base portion (105, 205) through said wall portion (110, 210) forming a frustum shaped cavity (130, 230) with the base portion (105, 205); and
- said metallic insert (100, 200) is insert molded in the fork bolt body (50) forming the hybrid structure of the fork bolt (500).

2. The fork bolt (500) for front fork suspension assembly as claimed in claim 1, wherein the thermoplastic of fork bolt body (50) and the skirt portion (50S) is selected form glass filled polyamide; and the metallic insert (100, 200) is made from a low alloy steel (E34 steel).

3. The fork bolt (500) for front fork suspension assembly as claimed in claim 2, wherein
- the disk portion (115) of the metallic insert (100) is configured to have at least three thorough openings (120) spaced at an equal angular distance from the geometric center of the disc portion (115) so as to rigidly lock the metallic insert (100) inside the fork bolt body (50) of the fork bolt (500);
- the center of each of said thorough openings (120) forms an imaginary circle having radius Rb,; and
- said radius Rb is configured to maintain a specific relation with the radius Ra of the disc portion (115) of the metallic insert (100) so as to ensure seamless flow of molten plastic through these thorough openings (120) during the process of insert molding to form an integrated structure of the fork bolt (500).

4. The fork bolt (500) for front fork suspension assembly as claimed in claim 3 wherein the radius Rb of an imaginary circle (IC1) is 0.5 to 0.6 times the radius Ra of the disk portion (115) of the metal insert (100).

5. The fork bolt (500) for front fork suspension assembly as claimed in claim 2, wherein
- the disc portion (210) of the metallic insert (200) is configured to have at least three flared bosses (220) on the top surface of the disk portion (215) and said bosses (220) do have a thorough opening (225) there through;
- each of the flared bosses (220) is configured to form an angle with the central axis of the metallic insert (200);
- the center of each of said flared bosses (220) forms an imaginary circle having radius RC; and
- said radius RC is configured to maintain a specific relation with the radius Rd of the disc portion (215) of the metallic insert (200) so as to ensure seamless flow of molten plastic through these thorough the flared bosses (220) during the process of insert molding to form an integrated structure of the fork bolt (500).

6. The fork bolt (500) for front fork suspension assembly as claimed in claim 5, wherein the angle of the flared bosses (220) with the central axis of the metallic insert is limited to 15 degree.

7. The fork bolt (500) for front fork suspension assembly as claimed in claim 6, wherein the radius RC of an imaginary circle (IC2) is 0.60 to 0.75 times the radius Rd of the disk portion (215) of the metal insert (200).

8. The fork bolt (500) for front fork suspension assembly as claimed in any of the claims 4 and 7, wherein
- the metallic insert (100, 200) is positioned in the fork bolt body (50) to have thickness ‘y’ between the ‘x’ portion of the fork bolt body (50) and the ‘z’ portion of the total height (H) of said fork bolt (500); wherein the ‘x’ is 2.0 to 5.0 times ‘y’, and said thickness ‘y’ of the metallic insert (100, 200) is 0.1 to 0.5 times ‘z’; and
- the ratio said total height (H) of the fork bolt (500) to the outer diameter D of the fork bolt (500) is in the range of 0.60 to 0.85 to impart the required mechanical strength during the operation of the same.

9. The fork bolt (500) for front fork suspension assembly as claimed in claim 8, wherein
- said fork bolt (500) is configured to have a groove (50G) on its outer circumferential surface at the interfacing area of the fork bolt body (50) and the skirt portion (50S) to accommodate an O-ring (20) therein; and
- said groove (50G) is positioned in between the base of the metal insert (100, 200) in the fork bolt body (50) and the oil seal fins (55) on the skirt portion (50S) of the fork bolt (100).

10. The fork bolt (500) for front fork suspension assembly as claimed in claim 9, wherein
- the skirt portion (50S) is configured to have at least a pair of oil seal fins (55) formed on its lower outer circumferential surface and an O-Ring projection (60) formed above said oil seal fins (55); and
- the bottom face (F) of the skirt portion (50S) of the fork bolt (500) is configured to provide the abutment to the main spring (40).

Dated this 26th day of July 2024

Sahastrarashmi Pund
Head – IPR
Endurance Technologies Ltd.

To,
The Controller of Patents,
The Patent Office, at Mumbai.