Redundant pivot trip latch

A trip mechanism having a hatchet plate disposed on a pivot pin assembly having a pivot pin member with at least three pivoting surfaces. First and second pivoting surfaces are located where the pivot pin member engages the supporting side plates. Thus, the pivot pin assembly may rotate in the traditional manner, i.e., both the hatchet plate and the pivot pin assembly rotate between the side plates. An additional pivoting surface is located where the hatchet plate engages the pivot pin assembly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical switching apparatus(es) such as protective devices and switches used in electric power distribution circuits carrying large currents. More particularly, it relates to a trip mechanism having at least two modes of rotation about at least three pivoting surfaces.

2. Background Information

Electrical switching apparatus for opening and closing electric power circuits typically utilize an energy storage device in the form of one or more large springs to close the contacts of the device into the large currents which can be drawn in such circuits. Such electrical switching apparatus includes power circuit breakers and network protectors which provide protection, and electric switches which are used to energize and deenergize parts of the circuit or to transfer between alternative power sources. These devices also include an open spring or springs which rapidly separate the contacts to interrupt current flowing in the power circuit. As indicated, either or both of the close spring and open spring can be a single spring or multiple springs and should be considered as either even though the singular is hereafter used for convenience. The open spring is charged during closing by the close spring which, therefore, must store sufficient energy to both overcome the mechanical and magnetic forces for closing as well as charging the open springs. Moreover, the close spring is required to have sufficient energy to close and latch on at least 15 times the rated current.

Both tension springs and compression springs have been utilized to store sufficient energy to close the contacts and to charge the open spring. The tension springs are easier to control, but the compression springs can store more energy. In either case, a robust operating mechanism is required to mount and control the charging and discharging of the spring. The operating mechanism typically includes a manual handle, and often an electric motor, for charging the close spring. It also includes a latch mechanism for latching the close spring in the charged state, a release mechanism for releasing the stored energy in the close spring, and an arrangement, a pole shaft for example, for coupling the released energy into the moving conductor assembly supporting the moving contacts of the switch.

The latch mechanism includes a hatchet plate that was fixed to a pivot pin. The pivot pin extended between, and was disposed within aligned openings in, two side plates. The pivot pin was structured to rotate within the aligned openings. While this configuration performs the desired function, if the pivot pin becomes fixed in one position, the hatchet plate may be prevented from rotating. For example, if, over an extended period of time, vibration caused the pivot pin openings to become deformed, the pivot pin may not rotate properly. This disadvantage could be overcome if the hatchet plate had more than one mode of rotation about the longitudinal axis of the pivot pin.

There is, therefore, a need for a pivot pin assembly that allows for more than one mode of rotation of a hatchet plate about the pivot pin.

There is a further need for a pivot pin assembly having at least three pivoting surfaces.

There is a further need for a pivot pin assembly that allows for more than one mode of rotation of a hatchet plate about the pivot pin which can be installed in existing circuit breakers.

SUMMARY OF THE INVENTION

These needs, and others, are met by the present invention which provides for a latch mechanism having a hatchet plate disposed on a pivot pin assembly having a pivot pin member with at least three pivoting surfaces. First and second pivoting surfaces are located where the pivot pin member engages the supporting side plates. Thus, the pivot pin may rotate in the traditional manner, i.e., both the hatchet plate and the pivot pin rotate between the side plates. An additional pivoting surface is located where the hatchet plate engages the pivot pin. Thus, if the pivot pin were to become unable to rotate, the hatchet plate could still rotate about the third pivoting surface. Additionally, because the hatchet plate is rotating about the axis of the pivot pin, the nature of pivoting motion is essentially identical to the motion create when the pivot pin rotates.

The pivot pin member may include additional elements, such as a hatchet plate bearing and a hatchet plate race. In this embodiment, the third pivoting surface is the outer surface of the hatchet plate bearing that engages the hatchet plate or the hatchet plate race. The hatchet plate race is, preferably a torus coupled to the hatchet plate. However, the hatchet plate race may be free to rotate, thus defining a fourth pivoting surface. Additionally, the hatchet plate bearing may also be a torus disposed on a cylindrical pivot pin member. In this configuration, the hatchet plate bearing may rotate on the pivot pin member, thus the inner surface of the hatchet plate bearing defines a fifth pivoting surface. Similarly, the pivot pin assembly may include side plate races disposed between the pivot pin member and the side plates. Where the side plate races are fixed to the side plates, the pivot pin member first and second pivoting surfaces engage the side plate races. The side plate races may, however, be free to rotate within the side plates. Thus, the outer sides of the side plate races define a sixth and seventh pivoting surfaces.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described as applied to a power air circuit breaker; however, it also has application to other electrical switching apparatus for opening and closing electric power circuits. For instance, it has application to switches providing a disconnect for branch power circuits and transfer switches used to select alternate power sources for a distribution system. The major difference between a power circuit breaker and these various switches is that the circuit breaker has a trip mechanism which provides overcurrent protection. The invention could also be applied to network protectors which provide protection and isolation for distribution circuits in a specified area.

This invention may be used with the apparatus disclosed in U.S. Pat. No. 6,072,136, which is incorporated by reference. U.S. Pat. No. 6,072,136 provides a full description of the charging mechanism, as well as various other components of the circuit breaker, which are not relevant to the present invention.

Referring toFIG. 1, the power air circuit breaker1of the invention has a housing3which includes a molded front casing5and a rear casing7, and a cover9. The exemplary circuit breaker1has three poles10with the front and rear casings5,7forming three, pole chambers11. Each pole10has an arc chamber13which is enclosed by a ventilated arc chamber cover15.

Circuit breaker1has an operating mechanism17which is mounted on the front of the front casing5and is enclosed by the cover9. The operating mechanism17has a face plate19which is accessible through an opening21in the cover. The operating mechanism17includes a large close spring18which is charged to store energy for closing the circuit breaker. Face plate19mounts a push to close button23which is actuated to discharge the close spring for closing the circuit breaker1, and a push to open button25for opening the circuit breaker. Indicators27and29display the condition of the close spring and the open/closed state of the contacts, respectively. The close spring18is charged by operation of the charging handle31or remotely by a motor operator (not shown).

The common operating mechanism17is connected to the individual poles by a pole shaft33with a lobe35for each pole10. As is conventional, the circuit breaker1includes an electronic trip unit37supported in the cover9which actuates the operating mechanism17to open all of the poles10of the circuit breaker1through rotation of the pole shaft33in response to predetermined characteristics of the current flowing through the circuit breaker1.

FIG. 2is a vertical section through one of the pole chambers11. The pole10includes a line side conductor39which projects out of the rear casing7for connection to a source of ac electric power (not shown). A load conductor41also projects out of the rear casing7for connection typically to the conductors of the load network (also not shown).

Each pole10also includes a pair of main contacts43that include a stationary main contact45and a moveable main contact47. The moveable main contact47is carried by a moving conductor assembly49. This moving conductor assembly49includes a plurality of contact fingers51which are mounted in spaced axial relation on a pivot pin53secured in a contact carrier55. The contact carrier55has a molded body57and a pair of legs59(only one shown) having pivots61rotatably supported in the housing3.

The contact carrier55is rotated about the pivots61by the operating mechanism17which includes a drive pin63received in a transverse passage65in the carrier body57through a slot67to which the drive pin63is keyed by flats69. The drive pin63is fixed on a drive link71which is received in a groove73in the carrier body. The other end of the drive link71is pivotally connected by a pin75to the associated lobe arm35on the pole shaft33similarly connected to the carriers (not shown) in the other poles of the circuit breaker1. The pole shaft33is rotated by the operating mechanism17.

A moving main contact47is fixed to each of the contact fingers51at a point spaced from the free end of the finger. The portion of the contact finger51adjacent the free end forms a moving arcing contact or “arc toe”77. A stationary arcing contact79is provided on the confronting face of an integral arcing contact and runner81mounted on the line side conductor39. The stationary arcing contact79and arc toe77together form a pair of arcing contacts83. The integral arcing contact and runner81extends upward toward a conventional arc chute85mounted in the arc chamber13.

The contact fingers51are biased clockwise as seen inFIG. 2on the pivot pin53of the carrier55by pairs of helical compression springs87seated in recesses89in the carrier body57. The operating mechanism17rotates the pole shaft33which, in turn, pivots the contact carrier55clockwise to a closed position (not shown) to close the main contacts43. To open the contacts, the operating mechanism17releases the pole shaft33and the compressed springs87accelerate the carrier55in a counterclockwise direction to an open position (not shown). As the carrier55is rotated clockwise toward the closed position, the arc toes77contact the stationary arcing contacts79first. As the carrier55continues to move clockwise, the springs87compress as the contact fingers51rock about the pivot pin53until the main contacts43close. Further clockwise rotation to the fully closed position (not shown) results in opening of the arcing contacts83while the main contacts43remain closed. In that closed position, a circuit is completed from the line conductor39through the closed main contacts43, the contact fingers51, flexible shunts91, and the load conductor41.

To open the circuit breaker1, the operating mechanism17releases the pole shaft33so that the compressed springs87accelerate the carrier55counterclockwise as viewed inFIG. 2. Initially, as the carrier55moves away from the line conductor39, the contact fingers51rock so that the arcing contacts83close while the main contacts43remain closed. As the carrier55continues to move counterclockwise, the main contacts43open and all of the current is transferred to the arcing contacts83which is the condition shown inFIG. 2. If there is a sizeable current being carried by the circuit breaker1such as when the circuit breaker1trips open in response to an overcurrent or short circuit, an arc is struck between the stationary contacts79and the moveable arcing contacts or arc toes77as these contacts separate with continued counterclockwise rotation of the carrier55. As the main contacts43have already separated, the arcing is confined to the arcing contacts83which preserves the life of the main contacts43. The electromagnetic forces produced by the current sustained in the arc push the arc outward toward the arc chute85so that the end of the arc at the stationary contact79moves up the integral arcing contact and runner81and into the arc chute85. At the same time, the rapid opening of the carrier55brings the arc toes77adjacent the free end of the arc top plate93as shown in phantom inFIG. 2so that the arc extends from the arc toes77to the arc top plate93and moves up the arc top plate93into the arc plates94which break the arc up into shorter sections which are then extinguished.

The operating mechanism17is a self supporting module having a cage95. As shown inFIG. 3, the cage95includes two side plates97which are identical and interchangeable. The side plates97are held in spaced relation by four elongated members99formed by spacer sleeves101, and threaded shafts103and nuts105which clamp the side plates97against the spacer sleeves101. Four major subassemblies and a large close spring18make up the power portion of the operating mechanism17. The four major subassemblies are the cam assembly107, the rocker assembly109, the main link assembly111and a close spring support assembly113. All of these components fit between the two side plates97. Referring toFIGS. 3 and 4, the cam assembly107includes a cam shaft115which is journaled in a non-cylindrical bushing117and a spring clutch collar222(SeeFIG. 12) which are seated in complementary non-cylindrical openings119in the side plates97. The bushing117has a flange121which bears against the inner face123of the side plate97, and the cam shaft115has shoulders125which position it between the bushing117and the collar222so that the cam shaft115and the bushing117are captured between the side plates97without the need for fasteners. Similarly, a rocker pin127of the rocker assembly109has shoulders129which capture it between the side plates97as seen inFIGS. 3–5. Flats131on the rocker pin127engage similar flats133in openings135in the side plates97to prevent rotation of the rocker pin127. The cam shaft115and rocker pin127add stability to the cage95which is self-aligning and needs no special fixturing for alignment of the parts during assembly. As the major components are “sandwiched” between the two side plates97, the majority of the components need no additional hardware for support. As will be seen, this sandwich construction simplifies assembly of the operating mechanism17.

The close spring18is a common, round wire, heavy duty, helical compression spring87closed and ground flat on both ends. A compression spring87is used because of its higher energy density than a tension spring. The helical compression close spring18is supported in a very unique way by the close spring support assembly113in order to prevent stress risers and/or buckling. In such a high energy application, it is important that the ends of the close spring18be maintained parallel and uniformly supported and that the spring be laterally held in place. As illustrated particularly inFIGS. 4 and 6, and also inFIGS. 8–11, this is accomplished by compressing the helical compression close spring18between a U-bracket137which is free to rotate and also drive the rocker assembly109at one end, and a nearly square spring washer or guide plate139which can pivot against a spring stop or support pin141which extends between the slide plates97at the other end. The close spring18is kept from “walking” as it is captured between the two side plates97, and is laterally restrained by an elongated guide member143that extends through the middle of the spring, the guide plate139and the brace145of the U-bracket137. The elongated guide member143, in turn, is captured on one end by the support pin141which extends through an aperture147, and on the other end by a bracket pin149which extends through legs151on the U-bracket137and an elongated slot153in the elongated member143.

The rocker assembly109includes a rocker155pivotally mounted on the rocker pin127by a pair of roller bearings157which are captured between the side plates97and held in spaced relation by a sleeve159as best seen inFIG. 5. The rocker155has a clevis161on one end which pivotally connects the rocker155to the U-bracket137through the bracket pin149. A pair of legs163on the other end of the rocker155which extend at an obtuse angle to the clevis161, form a pair of roller devises which support rocker rollers165. The rocker rollers165are pivotally mounted to the roller devises161by pins167. These pins167have heads169facing outwardly toward the side plates97so that they are captured and retained in place without the need for any snap rings or other separate retainers. As the rocker155rocks about the rocker pin127, the guide plate139rotates on the spring support pin141so that the loading on the close spring18remains uniform regardless of the position of the rocker155. The close spring18, guide plate139and spring support pin141are the last items that go into an operating mechanism17so that the close spring18can be properly sized for the application.

The U bracket pin149transfers all of the spring loads and energy to the rocker clevis161on the rocker155. The translational loads on the rocker155are transferred into the non-rotating rocker pin127and from there into the two side plates97while the rocker155remains free to rotate between the side plates97.

Referring toFIGS. 4–11, the cam assembly107includes, in addition to the cam shaft115, a cam member171. The cam member171includes a charge cam173formed by a pair of charge cam plates173a,173bmounted on the cam shaft115. The charge cam plates173a,173bstraddle a drive cam175which is formed by a second pair of cam plates175a,175b. A cam spacer177sets the spacing between the drive cam plates175a,175bwhile spacer bushings179separate the charge cam plates173a,173bfrom the drive cam plates173a,173b,175a,175band from the side plates97. The cam plates173a,173b,175a,175bare all secured together by rivets181extending through rivet spacers183between the plates. A stop roller185is pivotally mounted between the drive cam plates175aand175band a reset pin187extends between the drive cam plate175aand the charge cam plate173a. The cam assembly107is a 360° mechanism which compresses the close spring18to store energy during part of the rotation, and which is rotated by release of the energy stored in the close spring18during the remainder of rotation. This is accomplished through engagement of the charge cam plates173a,173bby the rocker rollers165. The preload on the close spring18maintains the rocker rollers165in engagement with the charge cam plates173a,173b. The charge cam173has a cam profile189with a charging portion189awhich at the point of engagement with the rocker rollers165increases in diameter with clockwise rotation of the cam member171. The cam shaft115and therefore the cam member171is rotated either manually by the handle31or by an electric motor (not shown). The charging portion189aof the charge cam profile189is configured so that a substantially constant torque is required to compress the close spring18. This provides a better feel for manual charging and reduces the size of the motor required for automatic charging as the constant torque is below the peak torque which would normally be required as the spring approaches the fully compressed condition.

The cam profile189on the charge cam173also includes a closing portion189bwhich decreases in diameter as the charge cam173rotates against the rocker rollers165so that the energy stored in the close spring18drives the cam member171clockwise when the mechanism is released.

The drive cam175of the cam member171has a cam profile191which, in certain rotational positions, is engaged by a drive roller193mounted on a main link195of the main link assembly111by a roller pin197. The other end of the main link195is pivotally connected to a drive arm199on the pole shaft33by a pin201. This main link assembly111is coupled to the drive cam175for closing the circuit breaker1by a trip mechanism203which includes a hatchet plate205pivotally mounted on a hatchet pivot pin assembly207supported by the side plates97, as described in greater detail below, and biased counterclockwise by a spring219. A banana link209is pivotally connected at one end to an extension on the roller pin197of the main link111and at the other end is pivotally connected to one end of the hatchet plate205. The other end of the hatchet plate205has a latch ledge211which engages a trip D shaft213when the shaft is rotated to a latch position. With the hatchet plate205latched, the banana link209holds the drive roller193in engagement with the drive cam175. In operation, when the trip D shaft213is rotated to a trip position, the latch ledge211slides off of the trip D shaft213and the hatchet plate205passes through a notch215in the trip D shaft213which repositions the pivot point of the banana link209connected to the hatchet plate205and allows the drive roller193to float independently of the drive cam175.

The sequence of charging and discharging the close spring18can be understood by reference toFIGS. 8–11. It should be understood that there are two conditions for two components; the close spring18which may be charged or discharged, and the main contacts43which may be open or closed. Thus,FIGS. 8–11show the four combinations of these conditions. That is, inFIG. 8, the main contacts43(not shown) are in the open position and the close spring18is discharged. InFIG. 9, the close spring18is charged and the main contacts43(not shown) remain open. InFIG. 10, the close spring18has been discharged to close the main contacts43(not shown). Finally, inFIG. 11, the main contacts43(not shown) remain closed and the close spring18has been charged. A detailed description of the sequence to charge the close spring18, close the main contacts43, and charge the close spring18again follows.

InFIG. 8the mechanism is shown in the discharged open position, that is, the close spring18is discharged and the main contacts43are open. It can be seen that the cam member171is positioned so that the charge cam173has its smallest radius in contact with the rocker rollers165. Thus, the rocker155is rotated to a full counterclockwise position and the close spring18is at its maximum extension. It can also be seen that the trip mechanism203is not latched so that the drive roller193is floating although resting against the drive cam175. As the cam shaft115is rotated clockwise manually by the handle31or through operation of the charge motor (not shown) the charge portion189aof the charge profile on the charge cam173which progressively increases in diameter, engages the rocker roller165and rotates the rocker155clockwise to compress the spring18. As mentioned, the configuration of this charge portion189aof the profile is selected so that a constant torque is required to compress the spring18. During this charging of the close spring18, the driver roller193is in contact with a portion of the drive cam profile191which has a constant radius so that the drive roller193continues to float.

Moving now toFIG. 9, as the close spring18becomes fully charged, the drive roller193falls off of the drive cam profile191into a recess217. This permits the reset spring219to rotate the hatchet plate205counterclockwise until the latch ledge211passes slightly beyond the trip D shaft213. This raises the pivot point of the banana link209on the hatchet plate205so that the drive roller193is raised to a position where it rests beneath the recess215in the drive cam175. At the same time, the rocker rollers165reach a point just after 170° rotation of the cam member171where they enter the charge portion189bof the charge cam profile189. On this portion189bof the charge cam profile189, the radius of the charge cam173in contact with the rocker rollers165decreases in radius with clockwise rotation of the cam member171. Thus, the close spring18applies a force tending to continue rotation of the cam member171in the clockwise direction. However, a close prop (not shown inFIG. 9) which is part of a close prop mechanism, described fully in U.S. Pat. No. 6,072,136, engages the stop roller185and prevents further rotation of the cam member171. Thus, the close spring18remains fully charged ready to close the main contacts43of the circuit breaker1.

The main contacts43of the circuit breaker1are closed by release of the close prop. With the close prop disengaged from the stop roller185, the spring energy is released to rapidly rotate the cam member171to the position shown inFIG. 10. As the cam member171rotates, the drive roller193is engaged by the cam profile191of the drive cam175. The radius of this cam profile191increases with cam shaft rotation and since the banana link209holds the drive roller193in contact with this surface, the pole shaft33is rotated to close the main contacts43as described in connection withFIG. 2. At this point the latch ledge211engages the trip D latch213and the main contacts43are latched closed. If the circuit breaker1is tripped at this point by rotation of the trip D shaft213so that this latch ledge211is disengaged from the trip D shaft213, the very large force generated by the compressed contact springs87(seeFIG. 2) exerted through the main link195pulls the pivot point of the banana link209on the hatchet plate205clockwise downward as the hatchet plate205rotates about the hatchet pin assembly207(SeeFIG. 8) and the drive roller193drops free of the drive cam175allowing the pole shaft33to rotate and the main contacts43to open. With the main contacts43open and the close spring18discharged the mechanism would again be in the state shown inFIG. 8.

Typically, when the circuit breaker1is closed, the close spring18is recharged, again by rotation of the cam shaft115either manually or electrically. This causes the cam member171to return to the same position as inFIG. 9, but with the trip mechanism203latched, the banana link209keeps the drive roller193engaged with the drive cam profile191on the drive cam175as shown inFIG. 11. If the circuit breaker1is tripped at this point by rotation of the trip D latch213so that the hatchet plate205rotates clockwise, the drive roller193will drop down into the recess215in the drive cam175and the circuit breaker1will open.

As shown in greater detail inFIG. 12, the trip mechanism203includes a hatchet plate205pivotally mounted on a hatchet pivot pin assembly207supported by a first side plate97A and a second side plate97B. Each side plate97A,97B includes a pivot pin opening98A,98B (respectively). The hatchet pivot pin assembly207includes a pin member230having a diameter and a longitudinal axis, as well as, at least a first pivoting surface232, a second pivoting surface234, and a third pivoting surface236. The hatchet pivot pin assembly207may further include a hatchet plate bearing240, a hatchet plate race242, a first side plate race244and a second side plate race246. The hatchet plate205includes an opening250sized to allow the pin member230to pass therethrough.

In one embodiment, the pin member230is rotatably coupled to the first and second side plates97A,97B by passing through the pivot pin openings98A,98B. In this embodiment, the pivot pin openings98A,98B are sized to securely, but rotatably, fit about the pivot pin member230. That is, the pivot pin openings98A,98B are sized to be just larger than the pin member230diameter. In this embodiment the pin member230is pivotally coupled to the first side plate97A at the first pivoting surface232and pivotally coupled to the second side plate97B at the second pivoting surface234. Thus, the pin member230may pivot about the longitudinal axis. The hatchet plate opening250is also sized to securely, but rotatably, fit about the pin member230. The hatchet plate205is then rotatably disposed on the pin member230at the third pivoting surface236. In this configuration, the hatchet plate205has at least two modes of rotation about a single axis, the pivot pin longitudinal axis. The modes of rotation include the hatchet plate205pivoting about pivot pin member230at the third pivoting surface236and both the hatchet plate205and the pivot pin member230pivoting, as a unit, at the first and second pivoting surfaces232,234.

In another embodiment, where the pivot pin assembly207includes a hatchet plate bearing240and a hatchet plate race242, the hatchet plate bearing240is an integral portion of the pivot pin member230having an increased diameter. The hatchet plate bearing240is longitudinally positioned on said pivot pin, and sized, to engage the hatchet plate opening250. Preferably, the hatchet plate race242is disposed in the hatchet plate opening250. Thus, the hatchet plate opening250will be sized to accommodate both the hatchet plate race242and the hatchet plate bearing240. The hatchet plate bearing240has an outer surface241that is the third pivoting surface236. The hatchet plate race242is, preferably a torus260having an inner surface262and an outer surface264. The hatchet plate bearing240is sized to securely, but rotatably, fit within the hatchet plate race242. Thus, the hatchet plate bearing240outer surface241, that is, the third pivoting surface236, engages the hatchet plate race inner surface262. Alternatively, the hatchet plate bearing240may be sized with a diameter smaller than the hatchet plate race242and include a plurality of raised portions243. The raised portions243are sized to securely, but rotatably, fit within the hatchet plate race242. Thus, the total surface area making contact between the hatchet plate bearing240and the hatchet plate race242is reduced, thereby reducing the amount of friction during rotation. Additionally, the hatchet plate bearing240and the hatchet plate race242may have a corresponding taper.

In the preferred embodiment, the hatchet plate race242is fixed to the hatchet plate205. Thus, the hatchet plate205has at least two modes of rotation about a single axis, the pivot pin longitudinal axis. The modes of rotation include the hatchet plate205and hatchet plate race242pivoting about pivot pin member230at the third pivoting surface236, i.e., at the hatchet plate bearing240, as well as, both the hatchet plate205and the pivot pin member230pivoting, as a unit, at the first and second pivoting surfaces232,234. Alternatively, the hatchet plate race242may be free to rotate in the hatchet plate opening250. Thus, the hatchet plate race outer surface264defines a fourth pivoting surface266. The hatchet plate race242may include a double flange (not shown) to trap the hatchet plate race242on the hatchet pate205, or, as shown, the hatchet plate205may form an indented pocket206about the hatchet plate opening250. Thus, the hatchet plate race242may be trapped inside the pocket206by a cap208. In this configuration, the hatchet plate205has three modes of rotation about the pivot pin member230axis, the two modes identified above, as well as the hatchet plate205pivoting about the hatchet plate race242.

In another embodiment, the hatchet plate bearing240A is a separate element from the pivot pin member230. In this embodiment, the hatchet plate bearing240A is a torus252having an inner surface254and an outer surface256. The hatchet plate bearing torus252is sized to securely, but rotatably, fit within the hatchet plate race242with the hatchet plate bearing torus outer surface256acting as the third pivoting surface236. Further, the pivot pin member230is sized to securely, but rotatably, fit within hatchet plate bearing torus252. Thus, the hatchet plate bearing torus252is structured to pivot about said pivot pin member230and the hatchet plate bearing torus inner surface254defines a fifth pivoting surface258.

In another embodiment, where the pivot pin assembly207includes a first side plate race244and a second side plate race246. The first side plate race244is a torus270having an inner side272and an outer side274. Similarly, the second side plate race246is a torus276having an inner side278and an outer side280. The first side plate race244is disposed in the first side plate pivot pin opening98A and the second side plate race246is disposed in the second side plate pivot pin opening98B. Thus, the first and second pivot pin openings98A,98B are sized to accommodate both the pivot pin member230and the first side plate race244and second side plate race246, respectively. The first side plate race244and second side plate race246are preferably fixed to the first and second side plates97A,97B. Thus, the first side plate race inner side272engages the first pivoting surface232and the second side plate race inner side278engages the second pivoting surface234.

In an alternate embodiment, the first side plate race244and second side plate race246are free to rotate in the first and second pivot pin openings98A,98B. Thus, the first side plate race outer side274acts as a sixth pivoting surface290and the second side plate race outer side280acts as a seventh pivoting surface292. Preferably, the first side plate race244and second side plate race246each include opposing flanges296that are disposed on opposite sides of the first and second side plates97A,97B, respectively. In this embodiment, the hatchet plate205and pivot pin assembly207have an additional mode of rotation about the pivot pin member230axis. That is, should the pivot pin member230become locked to the first side plate race244and second side plate race246, the hatchet plate205and pivot pin assembly207may rotate as a complete unit, including the first side plate race244and second side plate race246, within the first and second pivot pin openings98A,98B.