Accumulating conveyor
FIELD OF THE INVENTION 
     The present invention relates to accumulating conveyors. More specifically, an accumulating conveyor is provided for accumulating relatively heavy articles such as pallets in both forward and reverse directions by using a series of cam actuated lifting platforms disposed along the conveyor path for lifting pallets to be accumulated from a driving chain in a controlled fashion. 
     BACKGROUND OF THE INVENTION 
     Conveyors for accumulating articles and thereafter discharging said articles in a controlled fashion are known in the art. For example, U.S. Pat. No. 4,120,393 to Motooka et al. and U.S. Pat. No. 3,934,707 to Bowman both show devices for accumulating articles on a conveyor by blocking or lifting these articles from the conveying surface. The Motooka et al. and Bowman references are said to be particularly useful for relatively soft and flexible articles, such as those packed in plastic or metal foil wrappers or articles with insufficient or weakened bottom areas. U.S. Pat. No. 3,251,451 to Conway et al. also shows the concept of lifting an article from a conveyor to temporarily halt its progress. 
     U.S. Pat. No. 3,930,573 to Wyman describes a drive chain that can be raised to drive or lowered to cease driving selected pallets disposed along a conveyor to thereby provide accumulating ability. 
     Accumulating conveyors are also known for selectively activating and deactivating groups of rollers in variously defined zones to provide the accumulating function. See, e.g., U.S. Pat. Nos. 4,042,101 to Krammer et al.; 4,266,659 to Meyer et al.; 4,362,238 to Rivette; 4,453,627 to Wilkins; and 4,534,462 to Hoover et al. For example, U.S. Pat. No. 3,718,248 to Muller shows an accumulation roller conveyor in which the direction of article movement is reversible. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the preferred embodiment of the present invention, an accumulating conveyor is provided that is particularly useful for accumulating relatively heavy articles such as pallets. The operator can determine whether the conveyor will accumulate such articles in either a forward or reverse mode, such as for example either in a left to right direction or vice versa. A plurality of zones are defined between opposite ends of the conveyor, and an endless driving member such as a chain or chains passes through said zones to propel articles along the conveyor path in a chosen direction, either forward or reverse. A series of platforms is disposed along said path, there being one platform per zone. A dual cam wheel-follower combination for each platform is activated by a pneumatic cylinder and a pneumatic control system to lift the platform with appropriate timing established by a pneumatic control system and thereby halt an article in that zone. The cam wheel has a shaped edge for lifting articles in a particularly smooth and stable manner. Once a control system begins the accumulation of articles at either one end or the other end of the conveyor, the platforms are activated in successive upstream zones to accumulate articles as they are sensed therein. 
     Further in accordance with the particularly preferred embodiment, the dual cam wheel-follower combination uses a cam wheel having a shaped edge defining upward displacement H of the platform as a function of an angular displacement θ of said cam about an axis of rotation. 
     Also provided in accordance with the present invention is a method of controlling the operation of an accumulating conveyor having a plurality of zones between opposite ends of the conveyor, each zone including at least one selectively operable means for displacing an article out of engagement with an article driving means, the method comprising the steps of operating the displacing means of either endmost zone to cause any article sensed in said endmost zone to be halted; sensing the presence or absence of articles in each zone between said endmost zones; and operating the displacing means of each of said zones between said endmost zones to cause any article sensed in said zone to be halted only when an adjacent downstream zone, which can be in either a first or second direction depending upon the direction of conveyor movement, has an article halted therein. 
     It is an object of the present invention to provide a method of and apparatus for accumulating relatively heavy articles on a conveyor and subsequently discharging accumulated articles in either a forward or a reverse mode. 
     It is a further object of the present invention to provide an accumulating conveyor that is particularly well suited to accumulate relatively heavy loaded pallets. 
     It is a further object of the present invention to provide an accumulating conveyor that has a lifting action to disengage articles from a driving means, said lifting action to be relatively smooth to provide suitable stability for relatively heavy loads. 
     Further objects and advantages of the present invention will become apparent from the following description of the drawings and the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the preferred embodiment of the accumulating conveyor of the present invention; 
     FIG. 1A is an expanded schematic diagram of the accumulating conveyor of FIG. 1 accumulating in a first or forward mode; 
     FIG. 2 is a schematic diagram of the conveyor of FIG. 1A accumulating in a second or reverse mode; 
     FIG. 2A is a schematic diagram of the conveyor of FIG. 2 discharging articles in the reverse mode; 
     FIG. 3 is a top view of one of the platforms of the accumulating conveyor of the present invention; 
     FIG. 4 is a side view of the dual cam wheel-follower mechanism for raising the platform of FIG. 3; 
     FIGS. 5 and 6 are partial side views with partial cutaway of one cam wheel-follower combination with the associated platform in the lowered and raised positions respectively; 
     FIG. 7 is an end view with partial cutaway of the cam wheel-follower combination of FIGS. 5 and 6; 
     FIGS. 8 and 8A are side views of the cam wheel of the present invention; and 
     FIG. 9 is a schematic diagram of the pneumatic controller circuit for the preferred embodiment of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the accumulating conveyor 10 of the present invention. Conveyor base 11 has been partially broken away to show cam wheel pairs 13, which are operated by double acting pneumatic cylinders 15 to raise and lower platforms 17 under the direction of control system 34 in response to article detector pairs 21. By lifting articles from the driving surface such as chain 19, such articles are halted and with the proper platform timing can thereby be accumulated. Conveyor 10 receives articles from and/or conveys articles to companion conveyors 23 and 25, depending upon whether conveyor 10 is running in the forward or the reverse mode as determined by an OPER selector 27, there being an OPER 1  selector for end 14 of the conveyor and an OPER k  selector for conveyor end 12, the &#34;k&#34;  indicating the kth or last zone, with k being any integer indicating the total number of zones. The OPER selectors are connected to controller 34 by suitabe control lines. 
     It is understood that conveyors 23 and 25 can be replaced by any source and/or receiving area for pallets, such as for example forktrucks or transfer cars. 
     Conveyor 10 can be conveniently thought of as being divided into zones Z 1 , Z 2 , . . . Z n , . . . Z k  as shown in FIG. 1, where there are k number of zones and Z n  is the general expression for any one of the k zones. A n  is therefore the general expression for an article in zone Z n  at any moment in time. 
     Selectors OVR 1  and OVR k  indicated generally at 29 are connected to controller 34 by suitable control lines and provide an operator with control capability for accumulation and/or discharge of articles at either end of the conveyor, as described more fully in detail below. 
     As shown in the expanded schematic view of FIG. 1A, the preferred embodiment of the accumulating conveyor 10 of the present invention has opposing ends 12 and 14 with a plurality of zones Z 1 , Z 2 , Z 3 . . . Z n-1 , Z n , Z n+1 , . . . , Z k  indicated generally at 16 therebetween with broken lines 17 separating adjacent zones, wherein k can be any integer indicating the chosen number of zones. Conveyor 10 has a continuously operable drive means such as chain 19, which in the preferred embodiment comprises a pair of conventional endless conveyor chain runs 20 and 22 as shown in FIG. 3. A single chain of the pair is also indicated at 24 in the phantom side view in FIG. 7. The return portion of chain 19 is removed in FIGS. 1A-2A to more clearly show additional structural details as discussed below. Chain 19 is moved along chain guides 26 and 28 (see FIGS. 3 and 7) by routine motor means such as indicated at 30 and 32, which are under the direction of controller 34 through control lines 46 and 48. Chain 19 is disposed for engaging the bottoms of articles or pallets such as articles 36 and 38 to propel said articles through the zones Z 1 , . . . , Z k  along a conveyor path indicated generally by bi-directional arrow 40. Conveyor 10 can propel articles such as articles 36 and 38 in either a first direction towards end 14 as indicated by arrow 42 or in a second, opposite, direction towards end 12 as indicated by arrow 44. It is understood that the terms &#34;first direction&#34; and &#34;second direction&#34; as well as similar terms like &#34;forward&#34; and &#34;reverse&#34; or &#34;left&#34; and &#34;right&#34; are used here solely for convenience of description. The terms &#34;upstream&#34; and &#34;downstream&#34; are relative; that is, regardless of whether the conveyor is being operated in the forward or reverse mode, any articles on the conveyor will tend to move &#34;downstream&#34; from a more &#34;upstream&#34; position. 
     Controller 34 controls motors 30 and 32 through suitable control lines 46 and 48, and controller 34 includes routine means for responding to operator inputs through OPER 1  and OPER k  to select in which direction conveyor 10 is to be driven by motors 30 and 32. 
     Articles such as articles 36 and 38 are driven along path 40 of conveyor 10 and are typically pallet-loaded articles having a weight in the range of approximately 50-6,000 pounds each. Although the conveyor 10 of the present invention is designed to accommodate heavy loads, it is understood that lighter articles could be accumulated as well so long as the size of each article was appropriate for interaction with the sensors and the platforms to be described in more detail below. The term A n  shown at 52 in FIG. 1A and as used in FIGS. 2 and 2A is used to describe the article in any zone Z n  shown at 54 at any given time; A n+1  describes any article in adjacent zone Z n+1  at 56 at any given time; A n-1  at 60 describes any article in the other adjacent zone Z n-1  at 62, and so on. It is understood that the subscript &#34;n&#34; is used generically to indicate a zone, article, platform, or sensor. The subscripts &#34;n+1&#34; and &#34;n-1&#34; are used generically to indicate adjacent zones, articles, platforms, or sensors in either the first or second directions as indicated in FIGS. 1A, 2, and 2A. 
     The platforms P 1 , P 2 , P 3 , . . . P n-31  1, P n , P n+1 , P k  indicated at 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, and 84 respectively provide means for disengaging articles from chain 19 by lifting articles away from chain 19, therefore halting such an article&#39;s progress. For example, in FIG. 1A articles A n  at 52 and A n-1  at 60 have been lifted from chain 19 by platforms P n  at 74 and P n-1  at 72 respectively shown in a raised position. By contrast, articles 36 and 38 have not been lifted, as platforms 78 and 80 are in a lowered position. A more detailed description of the platforms P n  will be provided below with reference to FIG. 3. Platforms P n  are operable by associated means M n  shown generally at 86 for activating or lifting platforms P n . In the preferred embodiment, pneumatic actuator means or cylinders are used under the direction of controller 34 through trunk line 47 and are described in more detail below with reference to FIGS. 4-7. Trunk 47 of course contains a number of separate control lines and is shown for simplicity&#39;s sake as a single line in FIGS. 1A, 2, and 2A. The cylinders act upon cam wheel-follower combinations under the direction of controller 34 through the pneumatic control circuit described in FIG. 9. It is understood that the control circuit is not limited to pneumatic or air logic devices, but could be any suitable control system such as for example an electrical or electronic system. 
     There is one platform P n  associated with each zone Z n , such that the progress of an article A n  can be halted in that zone Z n  from proceeding further in the direction of travel of chain 19, be it forward or reverse. 
     Routine sensor devices, such as for example mechanical trip levers operatively connected to pneumatic limit valves, provide means S n  associated with each zone Z n  for sensing the presence or absence of articles A n  within the zone Z n  of each sensing means S n . Each sensor S 1 , S 2 , S 3 , . . . , S n-1 , S n , S n+1 , . . . , S k  preferably includes a detector pair of routine detectors. These sensors are shown only conceptually in FIGS. 1A, 2, and 2A. With reference to the further detail of FIG. 1, it is seen that each sensor includes a pair of detectors, with DF n  indicating the detector towards one end such as the front and DR n  the other detector towards the other end such as the rear of a zone Z n . For example, zone Z n  at 54 shows S n  at 88 as comprising two detectors 90 and 92, detector 90 being DR n  for the &#34;rear&#34; and detector 92 being DF n  for the &#34;front&#34; of zone Z n  as seen in more detail in FIG. 1. Detectors 90 and 92 provide signals that the leading edge of an article is present in either the front or rear portion of their associated zone. In the preferred embodiment, the signal is pneumatic and the detectors are routine devices operating routine limit valves, although the signal could be electrical or optical for a different system. As described below, the closing of a limit valve and the subsequent air flow therethrough will provide an air pressure signal indication of the presence of an article, while the opening of a limit valve and the cessation of air flow therethrough and the absence of an air pressure signal will indicate the absence of an article. Sensors S n  communicate with controller 34 through trunk 47. The sensors S n  are shown below chain 19 in FIGS. 1A, 2, and 2A for convenience only, the exact vertical position of the sensors S n  being unimportant for the purposes of this invention. 
     The controller 34, which includes an air logic system as described more fully below with respect to FIG. 9, controls the activating means M n , such as the pneumatic cylinders and associated cam wheels described below with reference to FIGS. 4-8, to lift and lower platforms P n  in accordance with the following protocol. When the accumulating conveyor 10 is in the &#34;forward&#34; mode indicated by arrow 42 in FIG. 1A, the operator may make a decision to begin accumulating articles at end 14. In response to an appropriate input to controller 34 from OVR 1  and OPER 1 , zone Z 1  at 94 and detector DF 1  at 93 are enabled. When article 96 has proceeded along conveyor 10 in the direction of arrow 42 and starts to pass through zone Z 1 , detector DF 1  will detect the leading edge 98 of article 96, which will now be called article A 1 , because it is in zone Z 1 . When DF 1  detects the presence of A 1  , an air pressure signal is generated which through the operation of the air logic system embodied in controller 34 and results in the activation of cylinder M 1  at 100, which raises platform P 1  at 64 to lift article A 1  from chain 19 and thereby halt its progress. Thereafter, zone Z 2  at 102 and detector DF 2  at 104 are enabled by the air pressure signal generated by DF 1 , such that when article A 2  at 106 appears in zone Z 2  and is detected by DF 2 , platform P 2  at 66 is activated by cylinder 108 and lifted to halt article A 2 . Accordingly, platforms P n  are activated in successive zones upstream of any zone where an article has been halted as the presence of articles in said upstream zones is sensed. 
     The accumulating conveyor 10 of the present invention has the ability to accumulate articles in the reverse direction as well, as depicted in FIG. 2, where through appropriate operator inputs to controller 34 through OVR k  and OPER k  the conveyor chain 19 now propels articles toward end 12 and away from end 14. The same sequence of events as described above for FIG. 1A now takes place, except that this time accumulation begins in zone Z k  at 110. When article A k  at 112 is sensed by detector DR k  at 114, M k  at 116 is activated to raise platform P k  at 118 and halt article A k . Zone Z k-1  at 120 and its detector DR k-1  at 122 are thereby enabled to halt article A k-1  at 124 when its presence is detected by DR k-1 . This process is carried out in successive zones upstream of Z k-1  until all articles as they appear have been halted in successive upstream zones. 
     Articles that have been accumulated are discharged by the conveyor 10 in either direction as well, regardless of the direction in which they have been accumulated. For example, FIG. 2A depicts articles being discharged that were previously accumulated as shown and described in FIG. 2. By appropriate operator inputs to controller 34 through OVR k  and OPER k , articles can be discharged. For example, FIG. 2A depicts articles that were accumulated as shown in FIG. 2 being discharged toward end 12. Article 112 is not shown in FIG. 2A because it has already moved off of conveyor 10, article 124 is now in zone Z k , and articles 126, 128, and 130 are in the three successive zones upstream of zone Z k  and moving downstream. 
     The manner of lowering platforms for discharging of articles is discussed, by way of example only, with respect to zone Z n  at 132 and article A n  therein. When the adjacent downstream zone Z n+1  is cleared of its article, sensor S n+1  generates a signal that in accordance with control of controller 34 directs cylinder M n  to be deactivatable to lower platform P n  and replace halted article A n  into engagement with chain 19. The platform P n  is deactivatable in response to either a sensing means S n+1  associated with said immediately adjacent zone Z n+1  in one direction sensing the absence of an article, or if the discharging is to take place in the opposite direction, if the sensing means S n-1  associated with the immediately adjacent zone in the other direction senses the absence of an article. That is, in the discharge mode, each activating means M n  successively deactivates each disengaging means P n  in successive zones upstream of any zone where an article has been halted and then replaced onto the driving chain 19. 
     The Conveyor Structure 
     The conveyor 10 itself includes a base indicated generally at 11 in FIG. 1 in a partial cutaway and in more detail in FIGS. 3-7, where the base weldment includes a pair of opposing side bodies 136 and 138 joined by cross members 140, 142, 144, 146, and 148. Base 11 can be any structure suitable for routine mounting on an appropriate support surface, such as for example a concrete floor or support legs, brackets, and/or hangers. 
     Mounted between side bodies 136 and 138 for vertical movement up and down between chains 20 and 22 is a platform or lifting frame, indicated generally at 152, which is formed from two longitudinally extending members 154 and 156 with treads 158 and 160 thereon. In FIG. 3, treads 158 and 160 have been partially cut away to reveal members 154 and 156 therebelow, which are joined by transverse members 170 and 172. Legs such as legs 174 and 176 join lower transverse members 170 and 172 to the upper transverse members and longitudinal members of the lifting frame 152. Transverse members 162, 164, 166, and 168 are part of the base. 
     Platform 152 has length L indicated at 175 between first and second ends 176 and 178 of platform 152, length L being chosen to support any articles to be lifted in a relatively stable manner. It is understood that the actual platform structure can be varied from the length L, so long as adequate support is provided for the articles to be lifted. 
     Detectors 180 and 182 can either be mounted as suitably chosen, such as for example on a side plate such as 138, or alternatively at 184 and 186 toward the center of platform 152 on cross members 168 and 170. Detectors 180 and 182 as a pair make up S n  for each platform P n  and zone Z n . One detector 180 is mounted towards or adjacent end 178, while the other detector is mounted towards or adjacent end 176, thereby providing one &#34;forward&#34; detector DF n  and one &#34;reverse&#34; detector DR n  for each platform P n . Detectors 180 and 182 can be any suitable transducers for detecting the presence or absence of an article, and in the preferred embodiment described herein include roller levers that operate pneumatic valves as described below. Accordingly, when an article A n  being propelled along on chains 20 and 22 passes over platform 152 from end 178 towards end 176, detector 180 is disabled but detector 182 detects the presence of article A n  when the leading edge of A n  mechanically trips suitable member 188. The reaction time of controller 34 and pneumatic cylinder M n  is such that, if zone Z n  is enabled and ready to halt an article, platform 152 is lifted by M n  quickly enough to lift article A n  off the chains 20 and 22 and halt its progress. Replacing of the article A n  on chains 20 and 22 occurs when controller 34 activates cylinder M n  in the opposite direction causing P n  to lower and return A n  into contact with chains 20 and 22. 
     Also shown on FIG. 3 are top views of the cam wheel-follower combinations at 190 and 192, pneumatic cylinder 194 and pull chain 196, the operation of which will now be described further in detail below with respects to FIGS. 4 and 5. 
     FIG. 4 shows the dual cam operated lifting mechanism of the present invention with part of platform 152 cut away to show the mechanism in more detail. FIG. 5 shows one cam wheel-follower combination in further detail in a lowered condition, FIG. 6 shows the same view but in a raised condition, and FIG. 7 shows further detail in a cross sectional view. 
     With reference now to FIGS. 4 and 7, cam wheel 198 is fixedly mounted such as by welding axially onto roller 200 which extends between side plates 138 and 136 and is fixedly mounted by axle bolt means 201 to plate 138. Toothed sprocket wheel 202 is likewise fixedly mounted axially on roller 200, such as by welding. Wheel follower 204 is rotatably mounted by bolt means 205 to longitudinal member 154 of platform 152 for reacting to rotation of cam wheel 198 to raise platform 152 in the direction indicated by arrow H. 
     A corresponding cam wheel 206, toothed sprocket wheel 208, and follower 210 is similarly mounted at the other end of platform 152. 
     Double acting pneumatic cylinder 194 is mounted on suitable support means 214 under platform 152 and approximately between cam wheels 198 and 206 and is operatively connected to a chain 196 that loops in a circuit around both sprocket wheels 202 and 208 with suitable routine adjusting and connecting links 220 and 222. 
     Accordingly, when cylinder 194 is activated to the left in FIG. 5, chain 196 and cams 198 and 206 are in the position shown in FIG. 5, and platform 152 is in the lowered position of FIG. 5. When an opposite air pressure is applied to cylinder 194 under direction of controller 34, cylinder 194 is activated to the right, and chain 196 is pulled to the left as shown by arrow 197 in FIG. 6, thereby rotating sprocket wheel 202, roller 200, and cam wheel 198 counterclockwise to lift platform 152 vertically as indicated by arrow H in FIG. 6. Once air pressure is applied in the opposite direction to cylinder 194, cam wheel 198 will be rotated in the opposite or lowering position--here clockwise--to return to the condition of FIG. 5. 
     The preferred shape of cam wheel 198 is shown in FIG. 8, where it is seen that wheel 198 has a shaped edge 224 with a radius that varies from r 0  at one point 226 on the edge to a radius r 1  at a second point 228 on the edge. Edge 224 is in effect generated by boring an aperture 230 in disc 232 on the centerline 234, but off center to achieve the camming action. Accordingly, r 0  corresponds to the desired lowered position and r 1  to the desired raised position of height H. Also as shown in FIG. 8A, the cam wheel 198 can be formed by drilling an appropriate offset bore 230 in disc 232 and then cutting the disc in half along center line 234 to form halves 236 and 238, comprising two cam wheels 236 and 238 to be welded to their respective rollers. 
     Also as shown in FIGS. 5 and 6, leg 172 has associated therein a slide member 173 that slidingly abuts a guide member 175 fixedly attached to crossmember 142. Members 173 and 175, as shown by the lowered and raised positions of FIGS. 5 and 6 respectively, cooperate to restrict platform 152 against moving with any horizontal component lifted by the cam action. Such restrictions are important to ensure stability of any articles being lowered or raised by platform 152. 
     The Pneumatic Control System and System Operation 
     The operation of the present invention is described with respect to the control system of FIG. 9. The OPER control at 364 is set to &#34;forward&#34; and OVR 1  at 323 is set to accumulate articles. FIG. 9 shows an OPER control 364 only at one end, but it is understood that this control can be duplicated at the other end as well. The system is therefore being operated in a &#34;forward&#34; or first direction, for accumulation of articles proceeding from left to right. Under initial conditions, control air source 300 is &#34;on&#34; and power air source 302 is &#34;on&#34;. All the lifting frames P 1  through P k  are lowered, and the detecting means DF 1  through DF k  comprising limit valves LV-1 through LV-k are open, or non passing. Chain 19 is running from zone Z k  towards zone Z 1 . 
     During the filling or accumulating cycle, a pallet adjacent zone Z k  at 306 advances onto conveyor 10 and proceeds downstream to the furthermost open zone 308, here Z 1 . Detector DF 1  detects the presence of the pallet, and LV-1 at 310 closes. Control air passes from line 312 through valve LV-1 to line 314, through shuttle valve 316 for locking out any reverse signals, then to line 318 to relay valve RV-1 at 320, which has been initialized by admission of control air to line 321 by override valve OVR1 at 323 having been set by the operator to position &#34;A&#34;, to be set in position B and let control air pass therethrough to line 322, thereby operating power valve PV-1 at 324 to position B. Power air on line 326 from power air source 302 is thereby admitted to cylinder M 1  at 328, which raises platform P 1  to stop the pallet and fill zone Z 1 . 
     In addition to activating M 1 , limit valve LV-1 at 310 also enables the next upstream zone Z 2  by transmitting control air on line 330 through shuttle valve 332 to set relay valve RV-2 at 334 to position &#34;B&#34;, which will now allow control air to pass through to power valve PV-2 at 336 when control air becomes available through LV-2 at 338. In this fashion, zone Z 2  has been enabled or armed by zone Z 1  &#39;s having halted an article. 
     When the next article travels down conveyor 10, this article will proceed no further than zone Z 2 , because LV-2 at 338 will be tripped to send control air from line 312 on line 340, through shuttle valve 342, then line 344, then RV-2 at 334, then line 346 to shift PV-2 at 336 to position &#34;B&#34; and through line 348 to activate cylinder M 2  at 350 to raise platform P 2  and halt the article. 
     This process continues successively upstream for each zone Z n  at 352, which has its own LV-n, RV-n, PV-n, M n , and P n  until the last zone Z k  at 306 is filled. At this point, a warning light or lockout device may stop the supplying of further pallets to the conveyor, which is now full and can accept no more pallets for accumulation until at least one zone is emptied. 
     To unload or discharge the pallets in this forward direction, the operator will issue a signal requiring a pallet to now advance off of the conveyor 10 into a receiving area adjacent zone Z 1 , such as a transfer car (not shown). Such a signal is issued by shifting override valve OVR 1  at 323 to position &#34;B&#34;, thereby sending control air through line 356, and shuttle valve 358, and line 360 to shift PV-1 to position &#34;A&#34;, thereby venting M 1  and permitting P 1  to lower and replace article A 1  in engagement with chain 19. It is to be noted that RV-1 is also reset thereby to position &#34;A&#34;, such that control air cannot be further delivered to PV-1 to shift PV-1 to position &#34;B&#34;  when LV-1 is repeatedly opened and closed as pallets pass thereover to be discharged. RV- 1 will remain in this unenabled position until either OVR 1  is returned to position A, or zone Z 1  is armed by zone Z 2  when articles are being accumulated the reverse direction. 
     When pallet or article A 1  is substantially completely off zone Z 1 , LV-1 opens and control air now passes on line 362 through shuttle valve 364 to set RV-2 to position A and send control air on line 366 to move PV-2 to position A and vent cylinder M 2 . This process continues upstream until all desired accumulated pallets have been off loaded through the downstream empty zones one at a time. This procedure will continue until conveyor 10 is empty. The conveyor likewise has the capacity of emptying or filling at the same time. 
     Accumulation and discharge of articles in either direction is controlled by override valves OVR 1  and OVR k  at each end of the conveyor 10. There can also be selector switches OPER 1  and OPER k  at either end of conveyor 10 for selecting either forward or reverse modes. 
     Accumulation of articles in the reverse direction proceeds in the same manner, except that in general detectors DR n  are used to activate reverse limit valves LVR-n. For example, override valve OVR k  at 362 is set to position A and operator valve 364 is set to position B, thereby introducing control air to line 366 through OVR k  to line 368, then through shuttle valve 370 and line 372 to set valve RV-k at 374 to position B. When an article trips LVR-k at 376, control air passes through line 378, shuttle valve 380 and line 382 through RV-k and line 384 to set PV-k at 386 to position B, thereby activating M k  at 387. The tripping of LVR-k also sends control air on line 388 to arm or enable the next upstream zone, and so on. Discharge of articles takes place by moving OVR k  to position B, just as described above for the forward direction. 
     It should be understood that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention, and it is therefore intended that such changes and modifications be covered by the following claims.