document: This invention relates to conveyor systems and particularly to power and free conveyor systems. 
       BACKGROUND OF THE INVENTION 
       In power and free conveyor systems wherein carriers are moved along predetermined paths by engagement with conveyors, it is often necessary to transfer the carriers from one predetermined path to another. In one type of system, this transfer is achieved by providing a transfer conveyor that engages the carrier and transfers it between the predetermined paths. In another type of system, the carrier is moved from one path toward the other and then pushed through the transfer area to the second path. 
       The invention is also applicable to the transfer of a carrier from one powered conveyor to another, either from a faster to a slower conveyor or from a slower conveyor to a faster conveyor or between conveyors moving at the same speed. 
       In U.S. Pat. Nos. 3,640,226, and 3,662,873, there is shown a power and free conveyor system wherein each of the carriers has a second dog that is normally urged to operative carrier position but is held by the track out of operative position. At a transfer point, a portion of the track is cut away to permit the second dog to move to operative carrier transferring position. Such a system effectively provides for transfer of the carrier without any change in elevation of the power and free tracks. However, where the pushers on the conveyor chain are spaced apart greater distances or where the system includes transfer across short and long distances, a loss of efficiency in transfer may occur either because of the time delay in awaiting a pusher on the conveyor chain or minimum length between the first and second dogs. 
       It has also been heretofore suggested to provide a third dog on the carrier which has the same configuration as the second dog and is normally held out of operative position by the track but can be brought to operative position to transfer the carrier across a greater distance. 
       Among the objects of the present invention are to provide a power and free conveyor system wherein a carrier is transferred from one predetermined path to another by pushing across a transfer zone in a minimum period of time and wherein such system is achieved with minimum cost and maintenance; and wherein selective transfer of the carrier can be achieved over short and long distances as desired. 
       SUMMARY OF THE INVENTION 
       In accordance with the invention, a third dog is provided in longitudinally spaced relation to the first and second dogs and is normally urged to an operative carrier pushing position but is held by the track out of operative position. The second and third dogs include cam projections thereon that engage portions of the track which hold the second and third dogs out of operative position. The cam projection on the second dog extends oppositely to the cam projection on the third dog. As the carrier moves to a transfer zone one or the other of the second and third dogs is successively moved to operative position. By providing selectively operable second and third dogs, it is possible to transfer across short or long distances thereby accommodating various systems wherein the transfer varies between short and long distances. 
      
      
       DESCRIPTION OF THE DRAWINGS 
       FIG. 1 is a fragmentary side elevational view of a conveyor system embodying the invention. 
       FIG. 2 is a fragmentary elevational view on an enlarged scale of a portion of the system shown in FIG. 1. 
       FIG. 3 is a fragmentary sectional view taken along the line 3--3 in FIG. 2. 
       FIG. 4 is a fragmentary side elevational view similar to FIG. 2 showing the parts in a different operative position. 
       FIG. 5 is a fragmentary view similar to FIGS. 2 and 4 showing the parts in different operative position. 
       FIG. 6 is a fragmentary sectional view taken along the line 6--6 in FIG. 2. 
       FIG. 7 is a fragmentary plan view of the system. 
       FIG. 8 is a fragmentary plan view of a modified form of conveyor system. 
       FIG. 9 is a fragmentary plan view of a further modified form of conveyor system. 
       FIG. 10 is a fragmentary plan view of another modified form of conveyor system. 
       FIG. 11 is a fragmentary plan view of a further modified form of conveyor system. 
       FIG. 12 is a fragmentary plan view of a further modified form of conveyor system. 
      
      
       DESCRIPTION 
       Referring to FIG. 1, the invention relates to a conveyor system wherein a power chain is adapted to selectively engage trolleys and move them in predetermined paths. The trolley motion is from right to left. Specifically, conveyor chain 10 is supported by chain trolleys 11 for movement along a track 12. The chain 10 includes longitudinally spaced pushers 13 that are adapted to engage carriers, as presently described, to move them. As shown, a carrier 15 may comprise longitudinally spaced trolleys 16,17,18 pivotally interconnected by tie bars 19,20. The trolleys include wheels 22 that engage the lower flanges of spaced inwardly facing C-shaped channels of track 21 (FIGS. 3 and 6). The foremost trolley 16 includes an actuating lever 23 that is operatively connected to a pusher dog 24 so that when lever 23 engages an obstacle or a projection 25 on the rear trolley 18 of a preceding carrier, the lever 23 is swung clockwise as viewed in FIG. 1 to lower the pusher dog 24 out of the path of a pusher 13. Such an arrangement is known as an accumulating conveyor system. The foremost trolley 16 further includes a pivoted holdback dog 26 in accordance with conventional practice. 
       As further shown in FIG. 7, the conveyor system is shown in connection with an exit switch which includes a second track 27 that extends at an angle from the track 21 and a switch tongue 28 that is operated to selectively guide the carrier 15 into the second track 27. The switch tongue 28 is controlled by signal devices such as are well known in the art, for example, as shown in the U.S. Pat. No. 2,868,139. A second power chain 29 is provided in overlying relation to a portion of the track 27 and is adapted to pick up the carrier and move it along the track 27. 
       Referring more specifically to FIG. 1, succeeding trolleys 17, 18 of the carrier 15 are each provided with a pivoted pusher dog 30,31 respectively, each of which is pivoted intermediate to its ends to its respective trolley and counterweighted so that the pusher dog end 30,31 thereof is urged normally to operative pushing position. However, the width of each pusher dog 30,31 is such that the top edges of projections 30a,31b respectively, normally engage the underside of the upper horizontal flanges 21a,21b  of the track so that the pusher dogs 30,31 are normally in the position shown in FIG. 4, namely, out of the path of pushers 13. Projection 31b extends beneath the flange 21b while projection 30a extends in the opposite direction beneath the flange 21a. As shown in FIG. 7, in the case of a transfer of the carrier from the track 21 to the track 27 by positioning of the switch tongue 28, a portion of the flange 21a is cut away as at 34a, to permit the pusher dog to swing counterclockwise under the action of the counterweight to the position shown in FIGS. 2 and 5 and thereby be in position for engagement with a pusher 13 which will push the carrier 15 through the switching or transfer area into position for engagement of the leading power dog 24 with the pusher of a secondary chain 29 along second track 27. 
       When the switch tongue 28 is actuated to divert the carrier, the pusher 13 which is in engagement with the pusher dog 24 will disengage from the pusher dog 24 as the foremost trolley 16 of carrier 15 is diverted to the track 27. The carrier 15 will then be momentarily stopped. However, by this time, the pusher dog 30 will have been moved upwardly through track opening 34a so that the succeeding pusher 13 of the power chain 10 will engage and cause the carrier 15 to be moved further along the track 27 sufficiently to permit dog 24 to be picked up by a pusher of the second power chain 29. As the carrier is moved along track 27, the upper flange 21a of track 27 is provided with a cam down opening 35a which will engage projection 30a of the pusher dog 30 to pivot it down out of the path of the pusher of chain 29. In order to cause downward movement of the pusher dog 30a, a portion 35a of the flange 21a is bent upwardly. Through this transfer, dog 31 is held down by flange 21b out of operative position. 
       Referring to FIG. 8, there is shown an exit system where the transfer is across a greater gap than in FIG. 7. In FIG. 8 when the switch tongue 28 is actuated to divert the carrier, the pusher 13 which is in engagement with pusher dog 24 will disengage from the pusher dog 24 as the foremost trolley 16 of the carrier 15 is diverted to the track 27&#39;. The carrier 15 will then be momentarily stopped. However, by this time, the pusher dog 31 will have been moved upwardly through track opening 34b so that a succeeding pusher 13 of the power chain 10&#39; will engage pusher dog 31 and cause the carrier 15 to be moved further along the track 27&#39; sufficiently to permit dog 24 to be picked up by a pusher of the second power chain 29&#39;. As the carrier is moved along track 27&#39;, the upper flange 21b of track 27&#39; is provided with a cam down opening 35b which will engage projection 31a of the pusher dog 31 to pivot it down out of the path of the pusher of chain 29&#39;. In order to cause downward movement of the pusher dog 31, a portion 35b of the flange 21b is bent upwardly. Through this transfer dog 30 is held down by flange 21a in inoperative position. 
       More specifically, as shown in FIGS. 7 and 8, the dog 30 can be used for a short distance of transfer (FIG. 7) while dog 31 can be used for a long distance of transfer as when the chain 29&#39; is a greater distance from chain 10&#39; (FIG. 8). 
       Cam down openings 35a, 35b may normally be both provided, as shown. Although not used to effect transfer to: (1) simplify design and construction; and (2) return dog 30,31 to inoperative position which may have not been used to effect transfer but was moved to the operative position while passing through the switch area. 
       It can thus be seen that in accordance with the invention, a third dog is provided in longitudinally spaced relation to the first and second dogs and is normally urged to an operative carrier pushing position but is held by the track out of operative position. As the carrier moves to a transfer zone, one or the other of the second and third dogs is successively moved to operative position. As a result, it is possible to transfer selectively across a greater or shorter distance thereby accommodating various systems wherein the transfer varies between short and long distances. 
       In both forms of FIGS. 7 and 8, a carrier which is not switched off is pushed all the way through the switching area by engagement of its front dog 24 with a dog 13 of the power chain so that the trailing dogs 30,31 move through this area without being in contact with a dog 13. As a result, dogs 30,31 are free to swing upward to their operating position at the start of the track flange cut-outs 34a,34b  and are forced down again at 35a,35b. 
       In a track configuration as in FIGS. 7 and 8, but with the carriers and chains moving in the opposite direction as shown in FIGS. 9 and 10, a carrier would move either from left to right on the straight tracks 21 or would enter this track from track 27. No cut-outs 34a,34b  are needed in tracks 21 in this case, but only in track 27 to move dog 30,31 to operative position to enable a pusher dog 13 on power chain 29 to advance the carrier far enough to place the front dog 24 into the path of the chain 10 and pusher dogs 13. Cam down openings 35a, 35b will normally be located on track 21 to return dog 30,31 to inoperative position after transfer. 
       More specifically, referring to FIG. 9, wherein the carrier is to be moved across the shorter gap from track 27, the dog 30 is permitted to move upwardly through opening 34a&#39; into the path of a succeeding pusher of chain 29 which will, in turn, push the carrier to bring dog 24 into the path of a pusher on chain 13 moving along track 21. Further movement of the carrier along track 21 will bring the projection on pusher dog 30 into engagement with cam down portion 35a&#39; to pivot dog 30 downwardly. 
       Similarly, where the gap to be traversed is greater as in FIG. 10, the cut out portion 34b&#39; permits dog 31 to pivot upwardly so that a succeeding pusher on chain 29 will push the carrier to bring its dog 24 into the path of a pusher 13&#39; on the chain moving along track 21&#39;. As the carrier is moved along track 21&#39;, cam down portion 35b&#39; will pivot dog 31 downwardly. 
       Throughout the portion of the system wherein the transfer is achieved, the relative positions vertically of the power track 12 and carrier tracks 21,27 remain constant and are not changed. 
       In another type of system, the carrier is moved from one path toward the other and then is pushed through the transfer area to the second. Specifically, as shown in FIG. 11, the carrier is moved along a track 40 in a portion between spaced power conveyors 41,42, each of which has pushers 43,44. As the conveyor 41 moves over its sprocket 45, the pusher 43 thereon which is in engagement with the pusher dog 24 of the carrier will disengage from the pusher dog 24 and will be momentarily stopped. At this point, cut-away portion 46a, along the track 40 will have permitted the second pusher dog 30 of the carrier to pivot upwardly into the path of a succeeding pusher 43 which then pushes the carrier across the gap between the conveyors 41,42 bringing the leading pusher dog 24 into a position of engagement with a pusher dog 44 of the succeeding conveyor 42. As the carrier is pulled across the space by pusher 44 between the conveyors 41,42, the cam projection 30a on the second pusher dog 30 engages the cam down portion 47a which pivots dog 30 downwardly returning it to an inoperative position. 
       A greater gap to be traversed than FIG. 11, is shown in FIG. 12. This arrangement is such that the third dog 31 is permitted to pivot upwardly. Specifically, the carrier is adapted to move along a track 40&#39; in a portion between spaced power conveyors 41&#39;,42&#39;, each of which has pushers 43&#39;, 44&#39;. As the conveyor 41&#39; moves over its sprocket 45&#39;, the pusher 43&#39; thereon which is in engagement with the pusher dog 24 of the carrier will disengage from the pusher dog 24 and will be momentarily stopped. At this point, cut-away portion 46b along the track 40&#39; will have permitted third pusher dog 31 of the carrier to pivot upwardly into the path of a succeeding pusher 43&#39; which then pushes the carrier across the gap between the conveyors 41&#39;, 42&#39; bringing the leading pusher dog 24 into a position of engagement with a pusher 44&#39; of the succeeding conveyor 42&#39;. As the carrier is pulled across the space by pusher 44&#39; between the conveyors 41&#39;,42&#39;, the cam projection 31b on the third pusher dog 31 engages the cam down portion 47b which pivots dog 31 downwardly returning it to an inoperative position. 
       Thus, the dog 30 can be used to transfer across a short distance (FIG. 11) and the dog 31 can be used to transfer across a long distance (FIG. 12). 
       In addition to selectively permitting either dogs 30 or 31 to move upwardly for transfer, the system can be operated also by moving the dogs upwardly simultaneously or sequentially as may be required. 
       For example, where it is desired to minimize the number of pushers on the conveyor, it might be desirable to cut away both flanges of the track at appropriate points to permit both dogs 30,31 to be simultaneously moved upwardly. In this manner, the pusher 13 which is nearest to one of the dogs 30,31 will move the carrier forwardly. Thus, the number of pushers on the conveyor chain can be minimized thereby reducing costs and still retaining a minimum lapse of time in transfer. 
       The dogs might also be operatively positioned sequentially such as in a situation wherein a long transfer is desired. The first dog 30 is permitted to move upwardly to permit a succeeding pusher 13 to move the carrier partway and then a second dog 31 is permitted to move upwardly to permit a succeeding pusher 13 to move the carrier a further distance. Such an arrangement might also be advantageous where it is desired to depress the first-mentioned dog 30 for clearance or other purposes.
summary: A conveyor system including a first load supporting track and a second load supporting track with an intermediate transfer portion. A powered conveyor is provided in association with each of the first and second tracks. A plurality of carriers are provided. Each of the carriers has a first dog that is in position for normal engagement with the pusher member of the conveyor and longitudinally spaced second and third dogs that are normally urged to an operative carrier pushing position but are held by the track out of operative position. The second and third dogs include cam projections thereon that engage portions of the track which hold the second and third dogs out of operative position. The cam projection on the second dog extends oppositely to the cam projection on the third dog. At a transfer point, an appropriate portion of the track is cut away to permit either the second or third dogs to move to operative carrier transferring position.