DESC:Racking Mechanism for Draw-out Circuit Breaker

Field of the invention

The present invention relates to protection devices such as circuit breakers and more particularly, to a racking mechanism in a cradle arrangement for a draw-out circuit breaker.

Background of the invention

In a draw out type of circuit breakers, the circuit breaker is racked-in and racked-out to and from a cradle assembly to occupy any of 'Disconnected/Isolated', 'Test' and 'Connected/ Service' position. When the breaker is completely racked-in to the 'Connected' position, breaker terminals are in contact with cradle terminals through cradle jaws. In this condition, both power (main) circuit and control circuit are connected and the breaker can be switched ON. When the breaker is racked-in to the ‘Test’ position, control circuit contacts of the breaker and the cradle gets connected. In the 'Test’ position, the main circuit is not connected but the control circuit is connected. When the breaker is completely racked out to the 'Disconnected' position, both the main circuit and the control circuit are disconnected. The breaker can also be drawn out to a ‘Maintenance’ position where it can be lifted off the cradle for servicing and maintenance activities.

The racking operation of the circuit breaker is accomplished by a racking mechanism of the cradle assembly. The racking mechanisms of the prior arts use a power screw to transmit force to a sliding rack of a rack and pinion arrangement. The translational motion of the sliding rack is transformed to rotational motion of a pinion of the rack and pinion arrangement. The pinion is rigidly coupled to a cradle cam shaft which in turn is responsible for racking of the circuit breaker.

However, such power screw based racking mechanisms are less efficient. Further, there exists a possibility of back drive that causes wearing out of a power screw thread and eventually reduces mechanical life reliability. Moreover, the racking mechanisms of the prior arts do not have any provision for torque amplification and hence, user may experience discomfort when performing the racking operation of the circuit breaker if force is higher than ergonomic design constraints.

Accordingly, there is a need of a racking mechanism for a draw-out circuit breaker that overcomes the above mentioned drawbacks of the prior art.

Objects of the invention

An object of the present invention is to reduce transmission losses while changing a direction and type of motion within a racking arrangement of a draw-out circuit breaker (herein after ‘the circuit breaker’).

Another object of the present invention is to amplify torque for racking the circuit breaker with minimal user effort.

Yet another object of the present invention is to eliminate a back drive by using a worm gear pair that provides better wear characteristics of a worm wheel and ease of replacement in case of damage.

Still another object of the present invention is to provide ease of adaptability of the racking arrangement in case of accommodating different worm wheel pairs to increase / decrease racking torque as per requirement.

Summary of the invention

Accordingly, the present invention provides a racking mechanism for a draw-out circuit breaker. The draw-out circuit breaker is mounted on a cradle assembly for moving between 'Disconnected’, 'Test' and 'Connected’ positions. The cradle assembly includes a pair of side plates, a pair of rail assembly, a base plate, a drop plate assembly and a cam shaft. The cam shaft includes a cam and a camshaft pinion fitted thereon. The racking mechanism comprises a support plate, a worm gear, a worm wheel, an auxiliary pinion and a rack.

The support plate is mounted on the base plate of the cradle assembly. The worm gear is pivoted on the support plate. The worm gear is capable of undergoing rotation about an axis thereof upon application of an input torque thereon. The worm gear undergoes a clockwise rotation upon application of a clockwise input torque thereon for achieving the ‘Connected’ position of the draw-out circuit breaker. The worm gear undergoes a counter clockwise rotation upon application of a counter clockwise input torque thereon for achieving the ‘Disconnected’ position of the draw-out circuit breaker.

The worm wheel is mounted on a pivot shaft for undergoing rotation thereabout in response to the rotation of the worm gear thereby resulting in torque amplification. The pivot shaft is mounted on the base plate. The pivot shaft is mounted on the base plate. The pivot shaft includes a restraining screw fitted therein for restricting the rotation of the worm wheel thereabout.
The auxiliary pinion is rigidly coupled to the worm wheel. The auxiliary pinion includes a plurality of first tooth configured thereon for receiving and transmitting rotational motion from the worm wheel.

The rack is a sliding flat rack made up of a sheet metal. The rack includes a plurality of second tooth and at least two screws configured thereon. The plurality of second tooth is adapted for meshing with corresponding plurality of first tooth for converting the rotational motion to translational/sliding motion of the rack. The at least two screws is adapted for guiding the translational motion of the rack.

The camshaft pinion converts the translational motion of the rack into the rotational motion of the cam shaft and hence of the cam thereby pushing or pulling the drop plate assembly and eventually the rail assembly thereby resulting in movement of the draw-out circuit breaker on the rail assembly to any of the 'Disconnected' position, the 'Test' position and the 'Connected’ position.

Brief description of the drawings

The objectives and advantages of the present invention will become apparent from the following description read in accordance with the accompanying drawings wherein,

Figure 1 shows a racking mechanism for a draw-out circuit breaker in ‘Connected’ position, in accordance with the present invention;

Figure 2 shows the racking mechanism for the draw-out circuit breaker in ‘Disconnected’ position, in accordance with the present invention; and

Figure 3 shows the racking mechanism for the draw-out circuit breaker in ‘Test’ position, in accordance with the present invention.

Detail description of the invention

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiment.

The present invention provides a racking mechanism for a draw-out circuit breaker. The racking mechanism uses a gear based transmission system to reduce transmission losses while changing a direction and type of motion within a racking arrangement. The racking mechanism uses a torque amplifying arrangement that improves user’s comfort during racking operation of the draw-out circuit breaker. The racking mechanism eliminates a back drive by using a worm gear pair that provides better wear characteristics of a worm wheel and ease of replacement in case of damage. The racking mechanism is adaptable to accommodate different worm wheel pairs to increase / decrease racking torque as per requirement.

The present invention is illustrated with reference to the accompanying drawings, wherein numbers indicated in the bracket represent the components of the invention throughout the description.

Referring now to figures 1 to 3, in one aspect, a racking mechanism (100) for a draw-out circuit breaker (not shown) in accordance with the present invention is shown. Specifically, the draw-out type of circuit breaker (hereinafter “the circuit breaker”) is an air circuit breaker. The circuit breaker is mounted on a cradle assembly (not numbered). The circuit breaker is movable between 'Disconnected/Isolated', 'Test' and 'Connected/ Service' positions with respect to the cradle assembly.

The cradle assembly includes a pair of side plates (110), a pair of rail assembly (120) (herein after ‘the rail assembly (120)’), a base plate (130), a drop plate assembly (140) and a cam (160). The cam (160) is fitted on a cam shaft (150). The cam shaft (150) includes a camshaft pinion (145) fitted thereon. In an embodiment, the cam (160) is welded on the cam shaft (150) and the cam shaft (150) includes the camshaft pinion (145) welded thereon. The pair of side plates (110) has flanges (not numbered) which guide and constrain the rail assembly (120) allowing only a single degree of freedom to move to and fro within the cradle assembly. The rail assembly (120) is an integral part of the cradle assembly and serves as guide ways for mounting and supporting the circuit breaker on the cradle assembly. Specifically, the rail assembly (120) includes slots (not numbered) configured therein for allowing projections (not shown) of a housing (not shown) of the circuit breaker to rest thereon for facilitating a draw-out feature of the circuit breaker.

The base plate (130) is bolted rigidly to the pair of side plates (110). The base plate (130) provides a base for mounting the racking mechanism (100) thereon. The racking mechanism (100) is responsible for moving the circuit breaker with respect to the cradle assembly in a racking operation thereof. The input of the racking operation from the racking mechanism (100) is taken and sent as an output by the cam shaft (150). The cam (160) through a surface profile (not shown) thereof drives the drop plate assembly (140) via a cam-follower type arrangement. The oscillatory motion of cam shaft (150) is transmitted into translational motion of the drop plate assembly (140). The drop plate assembly (140) is rigidly coupled to the rail assembly (120) by means of rivets (not shown).

The racking mechanism (100) is mounted on the base plate (130). The racking mechanism (100) is responsible for moving the circuit breaker to and fro the 'Disconnected’ position, the 'Test' position and the 'Connected’ position with respect to the cradle assembly in a racking-in operation and a racking-out operation thereof. The racking-in operation moves the circuit breaker from the Disconnected-Test-Connected positions and the racking-out operation moves the circuit breaker in the Connected-Test-Disconnected positions. The racking mechanism (100) comprises a support plate (10), a worm gear (30), a worm wheel (50), an auxiliary pinion (70) and a rack (90).

The support plate (10) is mounted on the base plate (130) of the cradle assembly. The worm gear (30) is pivoted on the support plate (10) so as to allow free rotation about an axis (not numbered) thereof. The worm gear (30) is capable of undergoing rotation about the axis thereof upon application of an input torque thereon. In an embodiment, the input torque is applied on the worm gear (30) manually. The racking mechanism (100) transmits force required for racking the circuit breaker by means of the worm gear (30). The worm gear (30) undergoes a clockwise rotation upon application of a clockwise input torque thereon for achieving the racking-in operation of the circuit breaker means the ‘Connected’ position of the circuit breaker. The worm gear (30) undergoes a counter clockwise rotation upon application of a counter clockwise input torque thereon for achieving the racking-out operation of the circuit breaker means the ‘Disconnected’ position of the circuit breaker. The worm gear (30) rotates to mesh with the worm wheel (50) for transmitting rotational motion thereto.

The worm wheel (50) is mounted on a pivot shaft (40). The pivot shaft (40) is mounted on the base plate (130). The pivot shaft (40) includes a restraining screw (35) fitted therein. In an embodiment, the worm wheel (50) is mounted on the pivot shaft (40) by a pin (not shown) and the retaining screw (35) is thread fitted on the pivot shaft (40). The worm wheel (50) is adapted for undergoing free rotation about the pivot shaft (40) in response to the rotation of the worm gear (30) thereby resulting in torque amplification. The torque amplification is based on a gear ratio of a pair of the worm gear (30) and the worm wheel (50). In a preferred embodiment, the gear ratio can be adjusted as per the input torque for ease in the racking operation. The rotational motion of the worm wheel (50) is constrained from side thrust produced in the pair of the worm gear (30) and the worm wheel (50) by the restraining screw (35) that allows the worm wheel (50) to rotate only about the pivot shaft (40) and no other degree of freedom.

The auxiliary pinion (70) is rigidly coupled to the worm wheel (50). In an embodiment, the auxiliary pinion (70) is rigidly coupled to the worm wheel (50) by means of locking profiles (not shown). The auxiliary pinion (70) includes a plurality of first tooth (not numbered) configured thereon. The plurality of first tooth is adapted for receiving the rotational motion from the worm wheel (50) and transmitting the rotational motion therefrom to the rack (90).

Specifically, the rack (90) is a sliding flat rack made up of a sheet metal. However, it is understood that any other rack made up of a suitable material known in the art can be used in the racking mechanism (100) in other alternate embodiments. The rack (90) includes a plurality of second tooth (not numbered) and at least two screws (85) configured thereon. The plurality of second tooth is adapted for meshing with corresponding plurality of first tooth on the auxiliary pinion (70) for converting the rotational motion to translational/sliding motion of the rack (90). The at least two screws (85) is adapted for guiding the translational motion of the rack (90).

The translational motion of the rack (90) is converted into the rotational motion of the cam shaft (150) by the camshaft pinion (145). Specifically, the camshaft pinion (145) forms a rack and pinion arrangement (not numbered) with the rack (90) to convert the translational motion of the rack (90) into the rotational motion of the cam shaft (150). The camshaft pinion (145) being welded onto the cam shaft (150) transfers the translational motion of the rack (90) to the rotational motion of the cam shaft (150). The rotation of the cam shaft (150) causes the cam (160) welded thereon to undergo the rotational motion. The cam (160) rotates to push or pull the drop plate assembly (140) connected thereto and eventually the rail assembly (120) connected to the drop plate assembly (140). As a result, the circuit breaker being mounted on the rail assembly (120) moves thereon to any of the 'Disconnected' position, the 'Test' position and the 'Connected’ position.

Advantages of the invention

1. The racking mechanism (100) replaces inefficient power screws with the gear train arrangement that achieves a change in direction and type of motion within the racking arrangement with optimal utilization of input energy and improved transmission efficiency even at low speeds due to reduced transmission losses.
2. The racking mechanism (100) replaces complex and expensive existing rack with simple and inexpensive flat sheet metal rack (90) to enable reliable operation and cost effective component manufacturing.
3. The racking mechanism (100) includes a self-locking back drive feature that holds the circuit breaker securely in case of any fault condition such as short circuit conditions.
4. The racking mechanism (100) eliminates back drive due to characteristic of the worm gear pair (30, 50) and thus achieves better wear characteristics of the worm wheel (50).
5. The racking mechanism (100) incorporates the torque amplifying arrangement that amplifies torque based on the worm gear ratio (ratio between the pair of the worm gear (30) and the worm wheel (50)) for racking the circuit breaker with minimal user effort.
6. The adaptable worm wheel (50) is capable of being replaced easily in case of damage as well as modified to adjust gear ratio as per user input torque requirement for ease in the racking operation.
7. Ease of adaptability of the racking arrangement in case of accommodating different worm pairs (30, 50) enables to increase / decrease racking torque as per requirement.
8. User may, at times encounter excessive racking force, which may be ergonomically inappropriate. The novel racking arrangement makes provision for an adjustable torque mechanism by means of the replaceable worm wheel (50) to adjust gear ratio.
9. The racking mechanism (100) is highly robust and withstands enormous forces during short circuit conditions. Minimal exertion of input energy is a pre requisite hence it should be efficient to avoid frictional losses.

The foregoing objects of the invention are accomplished and the problems and shortcomings associated with prior art techniques and approaches are overcome by the present invention described in the present embodiment. Detailed descriptions of the preferred embodiment are provided herein; however, it is to be understood that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure, or matter. The embodiments of the invention as described above and the methods disclosed herein will suggest further modification and alterations to those skilled in the art. Such further modifications and alterations may be made without departing from the spirit and scope of the invention.
,CLAIMS:We claim:

1. A racking mechanism (100) for a draw-out circuit breaker, the draw-out circuit
breaker being mounted on a cradle assembly for moving between 'Disconnected’, 'Test' and 'Connected’ positions, the cradle assembly having a pair of side plates (110), a pair of rail assembly (120), a base plate (130), a drop plate assembly (140) and a cam (160) fitted on a cam shaft (150) having a camshaft pinion (145) fitted thereon, the racking mechanism (100) comprising:
• a support plate (10) mounted on the base plate (130);
• a worm gear (30) pivoted on the support plate (10), the worm gear (30)
being capable of undergoing rotation about an axis thereof upon application of an input torque thereon;
• a worm wheel (50) mounted on a pivot shaft (40) for undergoing rotation
thereabout in response to the rotation of the worm gear (30) thereby resulting in torque amplification, the pivot shaft (40) having a restraining screw (35) fitted therein for restricting the rotation of the worm wheel (50) thereabout, the pivot shaft mounted on the base plate (130);
• an auxiliary pinion (70) rigidly coupled to the worm wheel (50), the
auxiliary pinion (70) having a plurality of first tooth configured thereon for receiving and transmitting rotational motion from the worm wheel (50); and
• a rack (90) having,
a plurality of second tooth configured thereon for meshing with
corresponding plurality of first tooth for converting the rotational motion to translational/sliding motion, and
at least two screws (85) configured thereon for guiding the
translational motion of the rack (90),
wherein, the camshaft pinion (145) converts the translational motion of the rack (90) into the rotational motion of the cam shaft (150) and hence of the cam (160) thereby pushing or pulling the drop plate assembly (140) and eventually the rail assembly (120) thereby resulting in movement of the draw-out circuit breaker on the rail assembly (120) to any of the 'Disconnected' position, the 'Test' position and the 'Connected’ position.

2. The racking mechanism (100) as claimed in claim 1, wherein the worm gear (30)
undergoes a clockwise rotation upon application of a clockwise input torque thereon for achieving the ‘Connected’ position of the draw-out circuit breaker.

3. The racking mechanism (100) as claimed in claim 1, wherein the worm gear (30)
undergoes a counter clockwise rotation upon application of a counter clockwise input torque thereon for achieving the ‘Disconnected’ position of the draw-out circuit breaker.

4. The racking mechanism (100) as claimed in claim 1, wherein the rack (90) is a
sliding flat rack made up of a sheet metal.