FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE OF THE INVENTION A DETENT PLUNGER
APPLICANTS
TATA MOTORS LIMITED, an Indian company
having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
INVENTORS
Mr. Anand Swichalappa Damami, Mr. Ashak Rudragoud Biradar, Mr. Amit Suhas Barve, Mr, Prafulia Vijay Wankhede, and Mr. Girish Jadhav.
All Indian nationals
working in TATA MOTORS LIMITED,
an Indian company having its registered office
at Bombay House, 24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in
which it is to be performed.

Field of invention:
The present invention related to a detent mechanism used in gear boxes.
Background of Invention:
A transmission has a selection of gear ratios to match the internal combustion engine characteristics to requirements at the wheels. In a manual transmission the driver has to select the appropriate gear in which the vehicle is to be driven based on his torque and speed requirements. Once the gear is changed the driver must get the signal to that effect. One of the ways of providing this signal is by detent mechanism.
Detents are used in all sorts of applications. They are used within the gear box to let the driver know that he has shifted from one gear to the another. This requires the rotary motion (or linear motion) to be divided in to discrete increments. This is usually accomplished by a spring loaded plunger paired with a notched wheel (or notched bar, in the situation where linear motion is to be divided in to discrete increments).
The detent mechanism usually consists of a notched wheel. A spring loaded plunger gets assembled in one of the notches of the toothed wheel; thus holding the wheel at a particular position unless some external torque is provided to overcome the detent force. The notches are so arranged that rotary motion will be in discrete increments. These increments of rotary motion correspond to distance to be traveled for selecting the required gear.
The addition of a detent introduces additional friction into the shift system. The aim is usually to keep friction forces due to addition of detent to the minimum. At the same time the detent feel must be felt by the driver. Consequently the spring must have a certain minimum stiffness and the notches in the toothed wheel must have a certain profile. The only component which allows some flexibility in design is the detent plunger. In most of the detent designs plunger design is varied in order to get minimum friction without compromising on the detent feel.

As the toothed wheel of a detent rotates it applies a force on the spring loaded plunger. The plunger moves In by overcoming the spring force which is constantly trying to push it out. As the plunger moves a friction force gets generated at the point of contact of plunger and toothed wheel. This friction force imposes resistance to motion of toothed wheel. This friction force combined with the spring load needs to be applied by the driver to accomplish gear shifting. Higher friction and higher spring loads lead to hard shifting and are to be avoided. Detent feel is determined by the spring load and detent profile. Hence optimum amount of spring load is required. However friction is a by product and needs to be kept to the minimum.
There are different types of plunger designs in use. Fig (2) shows a single piece plunger. It is shaped in the form of a bullet (having a spherical head 10 on top of the cylindrical base 9). This construction gives rise to sliding friction at the plunger-toothed wheel interface, consequently friction is very high and may lead to hard gear shifting. Fig (3) shows a plunger widely used in industry. It consists of a ball 6 free to roll on a number of small balls 5. The ball 6 and the small balls 5 are housed inside the housing 4. This construction gives rise to rolling friction at the plunger-toothed wheel interface, consequently friction is very low and gives a very good shift feel.
Although the plunger defined in FIG (3) gives very good performance it is costly on account of its construction and the number of parts involved. The proposed invention looks to provide performance in line with the plunger defined in FIG (3) at a lower cost by simplifying the construction and reducing the number of parts.
Summary of this invention:
The primary objective of this invention is to provide a low cost plunger design which adds minimum amount of friction to the shift system in which the detent is used.
According to present invention detent plunger comprises of, a plunger with a ball seat and a bait disposed therein supported on a spring located partially within a

cylindrical member, the spring biasing the plunger into engagement with a detent profile on a toothed wheel, the ball seat having a cylindrical base and a head portion extending from said cylindrical base, the head portion having an partial spherical cavity for locating said ball and acting as the ball seat, and plurality of slits extending partially along the length of said head portion. The diameter of open end of the cavity is less than the diameter of the ball. The open end of said cavity is having an entry chamfer for inserting said ball.
In another embodiment of invention a detent plunger comprises of, a plunger supported on a spring and located partially within a cylindrical member, the spring biasing the plunger into engagement with a detent profile on a toothed wheel, the plunger is having a cylindrical base and plurality of curved fingers extending from the cylindrical base to form a partial spherical cavity to locate a ball. The fingers are terminating at same plane to create an opening for the spherical cavity. Further the finger end is provided with an entry chamfer and the opening diameter of said fingers is less than the diameter of said ball. The diameter of said partial cavity is higher than the diameter of said ball. The ball is partially projecting out of said cavity. The spherical cavity is partially formed in said cylinder base.
Brief description of the Drawings:
FIG (1) shows cross sectional view of a detent.
FIG (2) shows the construction of a single piece plunger.
FIG (3) is a section view, showing the construction, of a conventional
plunger.
FIG (4) is a section view of the proposed invention.
FIG (5) is an exploded view of ball seat arrangement.
FIG (6) shows the section view of the ball-seat.
FIG (7) shows the top view of the ball-seat.
FIG (8) shows the section view of the ball-seat.

Detailed description of the Preferred Embodiment:
While the present invention is susceptible of embodiment in various forms, there is shown in the drawings and will hereinafter be described a preferred embodiment of the invention. This is with the understanding that the present disclosure is to be considered as setting forth an exemplification of the invention which is not intended to limit the invention to the specific embodiment illustrated.
Referring now to the drawings, wherein like reference numerals indicate like parts throughout several views, the present invention is arranged in combination with and after being installed, forms an integral part of detent used in a gear-box. FIG (1) shows the cross sectional view of a typical detent. The detent typically consists of a toothed wheel 1, the spring 2 and the plunger 3. The plunger 3 with ball seat and a ball disposed therein supported on a spring located partially within a cylindrical member. The spring 2 biasing the plunger 3 into engages with a detent profile on the toothed wheel 1. A unique construction of plunger 3 is the topic of this invention. FIG (4) shows a section view of the proposed invention and FIG (5) shows its exploded view. The plunger is made up of two parts the ball 8 and the ball seat 7. FIG (6) shows a section view of the ball seat 7. It has a cylindrical base 16 and a head 15 which is in the shape of frustum of a cone. It further has a partial spherical cavity 12 which opens at the top. Plurality of radial slits II extending partially along the length are provided in the top portion of the ball seat (Refer to Fig 6 & 7). Referring to FIG (8), according to another embodiment of the present invention, plurality of finger extensions 17 are provided on the cylindrical base 16. The fingers 17 are curved inside to form a partial spherical cavity 12 to locate a ball. The fingers are terminating at particular plane to create an opening to access the spherical cavity.
An entry chamfer 13 is provided in the top portion of the partial spherical cavity 12. The ball 8 is snap fitted in to the partial spherical cavity 12 of the ball seat 7. A portion of the ball 8 will protrude out of the ball seat 7 on assembly as shown in FIG (4). The diameter 14 where the partial spherical cavity opens at the top is kept less than the diameter of the bail 8. This feature ensures that the bail 8 does not fall out of the ball seat 7 even when subjected to heavy vibrations. Further the diameter of the partial spherical cavity 12 is kept larger than the

diameter of the ball 8. This feature allows the ball 8 to rotate freely in the partial spherical cavity 12 of the ball seat 7. This keeps the friction at the plunger-toothed wheel interface to a minimum.
The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

We claim:
1. A detent plunger comprises of, a plunger with a ball seat and a ball disposed therein supported on a spring located partially within a cylindrical member, said spring biasing the plunger into engagement with a detent profile on a toothed wheel, said ball seat having a cylindrical base and a head portion extending from said cylindrical base, said head portion having an partial spherical cavity for locating said ball and acting as said ball seat, and plurality of slits extending partially along the length of said head portion.
2. The detent plunger as claimed in claim 1 wherein the diameter of open end of said cavity is less than the diameter of said ball.
3. The detent plunger as claimed in claim 1 wherein said open end of said cavity having an entry chamfer for inserting said ball.
4. A detent plunger comprises of, a plunger supported on a spring and located partially within a cylindrical member, said spring biasing the plunger into engagement with a detent profile on a toothed wheel, said plunger having a cylindrical base, plurality of curved fingers extending from said cylindrical base to form a partial spherical cavity to locate a ball.
5. The detent plunger as claimed in claim 7 wherein said fingers are terminating at same plane to create an opening for said spherical cavity.
6. The detent plunger as claimed in claim 7 wherein said finger end is provided with an entry chamfer.
7. The detent plunger as claimed in claim 7 wherein the opening diameter of said fingers is less than the diameter of said ball.

8. The detent plunger as claimed in any one of the preceding claims wherein the diameter of said partial cavity is higher than the diameter of said ball.
9. The detent plunger as claimed in any one of the preceding claims wherein said ball is partially projecting out of said cavity.
10. The detent plunger as claimed in any one of the preceding claims wherein said cavity is partially formed in said cylinder base.