Conveyors
United States Patent Robert Apr. 1, 1975 CONVEYORS 3,092,038 6/1963 [76] Inventor: Jean Robert, 7 Square Grange,  
 Pans. France 3,420,188 1/1969 Oct. 9, wakabayashl S [21] APPL 797367 Primary Examiner-Lloyd L. King Assistant Examiner-Robert Saifer [3Q] Fo -gig A li ti p i i D t Attorney, Agent, or Firm-Stevens, Davis, Miller &amp;  
 Oct. 14. 1969 France 69.35159 Moshe [52] US. Cl 104/172 S, 104/252 [57] ABSTRACT [51] Int. Cl. B65g 17/42 A f I d 581 Field 61 Search 104/172 s 96 178 252 5 a 0 i 1 trolley dnven by an endless cham. The cham has dogs engaging retractable books on the trolley. The inven- [56] References Cited tion provides an improved hook-control system for I t UNITED STATES PATENTS stoppmg he trolley 3.070.037 12/1962 Burrows 104/172 s 12 Claims 11 Drawing Figures l F- F &#34;1 r l 1 I r I f f k x. x I  
  /&#39;v 2 19/9, m ZZZ/82620 l5 s 4 6 2a 3 CONVEYORS This invention relates to a conveyor comprising: a number of trolleys on which loads are hung for conveyance and which can move on a runway; and an endless drive chain which extends substantially parallel to the runway and which has drive dogs co-operating with re tractable trolley-mounted hooks. Consequently, when unretracted a trolley hook engages with a drive dog and the trolley is moved along its runway, but when the trolley hook is retracted the chain drive dogs cannot en gage with the hook and the trolley remains stationary somewhere on the runway.  
  The hook control systems for stopping the trolleys are complicated in known conveyors of this kind, particularly since when the stationary trolleys are loaded, the between-trolleys spacing must be large enough for overhanging loads on two consecutive trolleys not to touch one another; however, when unloaded the trolleys can be much closer together.  
  Conveyors of this kind are known wherein each retractable hook is disposed on an arm pivotable by a sensor which is disposed at the front of the trolley (as considered in the direction of trolley movement) for actuation by an abutment disposed on the path of the sensor. Upon encountering the abutment the sensor pivots the hook to retract the same against resilient means.  
  There are disadvantages in a system of this kind, for retractable stop means must be provided along the runway not only to serve as abutments for the sensor but also to urge the same strongly against the particular abutment concerned to ensure maximum hook retraction because, since the hook is always biased by the re silient means, it might project sufficiently to be abutted by the consecutive drive dogs of the chain, with the result of unwanted noise and the likelihood of damage. Another result of the continuous biasing of the hook against its retraction direction by the resilient means is that a trolley encountering an obstacle tends because of the resilient means to recoil, only to be returned to the obstacle by the drive dog and so on, so that the trolley hunts around its stop position. To obviate this hunting, the stopping means must be complicated still further to provide very rapid stoppage of the trolley.  
  This invention obviates the disadvantages of the known conveyors.  
  According to the invention, a conveyor comprises: a number of trolleys which can move on a runway and on which loads can be hung for conveyance&#34;, and an endless chain which extends substantially parallel to the runway and which has drive dogs each adapted to cooperate with a hook rigidly secured to a pivot arm on each trolley, pivoting of the arm being effected by a sensor adapted to move relatively to the trolley against resilient means, the pivoting occurring when the sensor co-opcrates with releasable abutments disposed along the conveyor path, the reaction of the resilient means causing the pivot arm to take up unstable positions between a retracted end position and a projecting operative end position with a tendency to return from the retracted end position towards the operative end position. The device is characterized in that in both end positions the arm is located by senser-controlled means, the sensor itself having moved into its end positions.  
  Since the pivot arm is normally biased towards its operative position, the sensing element is, with advantage,  
 a rockablc lever biased by a tension spring, one end of which co-operates with the stop abutment and the other end of which cooperates with the pivot arm. Ac cording to the invention, that end of the rockable lever which co-operates with the pivot arm and which in shape resembles, with advantage, a rounded apex, com prises two flanks adapted to co-operate alternately with two facets of the pivot arm to lock the same in its two end positions and the pivot arm has between the two facets a ramp with which the last-mentioned end ofthe rockable lever co-operates to enable the resilient means to return the pivot arm from an unstable retracted position to its operative position. It then becomes possible for a trolley to be stopped by an abutment acting on the sensor either to move the same into the opposite end position as compared with the end position of the sensor before abutting the abutmentin which case the arm is locked in the retracted position-or to cause only a partial oscillation ofthe sensor so that, the intermediate ramp remaining in contact with the sensor end, the pivot arm returns to its operative position and immediately the stop abutment is retracted.  
  According to a feature of the invention. these two possibilities are used for storing empty and/or loaded trolleys respectively. Accordingly, in an advantageous form of the invention, a recess in which a finger rigidly secured to the pivot arm engages when the same moves from its operative position to a retracted position is as sociated with a stop abutment. This recess can be so deep that the pivot arm can reach its fully retracted position and therefore be locked therein by the sensor (empty trolley storage position), or else the recess can be so shallow that the pivot arm can move only as far as a position in which it is retracted enough to prevent co-operation between the drive dogs and the hooks but not enough to be locked by the sensor (loaded trolleys storage position).  
  Preferably, the recesses take the form of recesses in edging which extends parallel to the runway; if the pivot arm has two symmetrical fingers, the edging can be disposed on either side of the runway, Advantageously, the fingers can bear rollers for ready entry into and exit from the recesses. So that the trolleys can be stopped in a definite position, the recess width is slightly greater than finger width. For instance, to stop a number of loaded trolleys, a number of abutments are associated with a number of shallow recesses, each abutment/recess pair being disposed at a predetermined distance from the immediately previous and immediately following pair to allow for trolley loading. The abutments can be controlled individually for their retraction or they can be under the-control of the immediately previous trolley. For empty trolley storage the abutment can always be the immediately previous trolley, in which event only the first trolley need be associated with a shallow recess, the pivot arm of such trolley not being locked.  
  To release the pivot arm of a trolley locked by its sensor, the pivot arm comprises, preferably at the front, an extension which extends beyond the sensor and which, when the pivot arm moves into its retracted end position, comes into a position to co-operate with a ramp inclining upwardly and rearwardly from the immediately previous trolley. The forward movement of the immediately previous trolley therefore moves the pivot arm far enough towards its operative position to release such arm from locking by the sensor and enables the arm to be moved into its operative position by the resilient means.  
  Advantageously, the abutment for the sensor of the immediately following trolley and the ramp associated with the extension of the pivot arm of the lastmentioned trolley are combined to form a single hookshaped member disposed at the rear of each trolley. This enables a row of empty trolleys to be stopped, by immobilization of an abutment acting just on the first trolley of the row, in a position in which the trolleys are not just contacting one another but partly overlapping; conversely, by enabling the first trolley to be driven by the chain all the trolleys are driven by the chain consecutively.  
  A conveyor of this kind may have disadvantages be cause of the recesses and because the pivot arm is in an unstable retracted position, and not locked in a retracted position, when the drive dog releases from the book because of the sensor striking a stop abutment, for if the finger rigidly secured to the pivot arm is not at that time exactly above a recess, the pivot arm does not reach its locked retracted position and tends to return to its operative locked position, the finger resting on the edging which is formed with the recesses. Consequently, the drive dogs following the drive dog which co-operated with the trolley strike the hook and cause noise, with the likelihood of damage to the trolley and disturbances in conveyor operation. These disadvan tages may occur more particularly in connection with the end trolleys of a long row to be stored as a result of inevitable tolerances and clearances in manufacture and in the positioning of the recesses. Similar considerations apply to fast-moving and heavily loaded trolleys. Also, the presence of edging or edgings with recesses do not allow displacing manually a given trolley when necessary.  
  To obviate these disadvantages, therefore, according to a feature of the invention. at the time when the sensor of any trolley has encountered a stop abutment so that the chain drive dog which was driving the trolley releases from the trolley hook, the pivot arm of such trolley is already in its locked retracted position. This feature obviates the disadvantages mentioned and makes it possible to obviate the recessed edging, since to stop a number of loaded trolleys all that is required is a number of releasable abutments each operated by a trolley to stop the immediately following trolley, each abutment being disposed at a predetermined distance from the immediately previous abutment and from the immediately following abutment to allow for trolley loading. Advantageously, the abutments take the form of a rocker, one end of which is pushed aside by the immediately previous trolley so that the other end is brought on to the path of the immediately following trolley, return means acting on the rocker.  
  Preferably, that end of the rocker which is pushed aside by the immediately previous trolley is pawlshaped to prevent the trolley recoil, for instance, due to swaying of the load suspended on the trolley.  
  So that the pivot arm of any trolley can be released when the immediately previous trolley is restarted by a drive dog and, therefore, the abutment between the two trolleys has been retracted, there is provided along the runway a number of unlocking rockers, one end of each such rocker comprising a ramp and being pushed aside by the immediately previous trolley as the same passes by, so that the other end of the rocker is moved to release the pivot arm of the immediately following trolley, the rockers returning to their initial position after the trolley has passed by.  
  Preferably, to obviate premature release of the immediately following trolley, that end of the releasable abutment rocker which is disposed on the side of the immediately previous trolley has an extension to ensure that such abutment is retracted only when the immediately previous trolley has travelled a predetermined dis tance.  
  in the case of empty trolley storage, the releasable abutment can, as in the embodiment previously described, always be the immediately previous trolley in the row, each pivot arm also having an extension which extends beyond the sensor and which moves into a position for co-operation with a ramp on the immediately previous trolley. Consequently, a row of loaded or empty trolleys can be stopped e.g. by the electromagnetic actuation of an abutment which acts on the first trolley of the row; conversely, when the first trolley is allowed to be driven by the chain, all the other trolleys are moved off by the chain seriatim.  
  In the conveyor according to the invention, trolleys can be uncoupled from or coupled with the chain,  
 merely by interaction between trolleys, whatever the spacing between them.  
  The invention can be clearly understood from the accompanying drawings wherein:  
  FIG. 1 is a partly sectioned view on the line ll of FIG. 2 showing a trolley ofa conveyor according to the invention with its pivot arm in the operative position;  
 FIG. 2 is a plan view of the trolley of FIG. 1;  
  FIG. 3 is a view similar to FIG. I with the pivot arm in an intermediate position and with the loaded trolley stationary;  
  FIG. 4 is a view in partial elevation to show the parking of empty trolleys, the pivot arm thereof being in the retracted position;  
  FIG. 5 shows an alternative form of trolley for a conveyor according to the invention, the trolley pivot arm being shown in the position which it takes up when the chain drive dog releases from the hook mounted on the pivot arm;  
  FIGS. 6-10 show the storage of loaded trolleys as shown in FIG. 5, FIGS. 7 and 9 being sections on the lines Vll-Vll and lX--lX of FIGS. 6 and 8 respectively, while FIG. 10 is a diagrammatic plan view to show how the release rockers of the pivot arms operate, and  
 FIG. ll shows empty trolley storage.  
  The conveyor shown partially in the drawings comprises an endless chain 1 (shown in chain-dotted lines) which is of known kind and which moves continuously in the direction indicated by an arrow F and which has drive dogs 2. The conveyor also comprises a known runway 3 which extends substantially parallel to chain 1 and on which trolleys 4 run on running rollers 5 and guide rollers 6 which co-operate with runway 3. Loads can be hung on the trolleys 4 by means of carrier rods 7, which are shown only partially.  
  Connected to each trolley 4 for pivoting around a pivot 8 is a pivot arm 9 which has at the front a downwardly curved extension 10 and which has two facets 11 and 11 and, therebetween, a ramp [1,; the facets correspond to the operative and retracted end positions respectively and the ramp is associated with the unstable intermediate positions.  
  A lever 14 has an end 13 which resembles a rounded wedge and which co-operates with the facets and ramp 1],, 11 I1 lever I4 serves as a sensing member and is pivotable around a spindle I5 which is disposed at the front of the trolley 4 and which can, with advantage, be used as an axle for a pair of rollers 5. Surfaces 13a, 13b of end 13 co-operate with facets II, and 11;, respectively. Lever I4 is biased by a tension spring 16 which tends to return the lever bottom end forwardly; movement in this forwards direction is limited by abutments 12, which are rigidly secured to lever 14, abutting the side edges of trolley 4. Arm 9 has a step I7 forming a hook which, when arm 9 is in its operative position, engages with one of dogs 2 to drive a trolley 4 in the direction indicated by the arrow F.  
  Devices 18 in the form of roller cams are disposed on either side of arm 9; one of the devices I8 is adapted to engage in any of the deep recesses 19 or shallow recesses 19a in an edging 20 which can be disposed along runway 3 on either side thereof to suit convenience. As will become apparent hereinafter, the deep recesses I9 are for stopping empty trolleys, the pivot arm 9 being in the end retracted position, and the shallow recesses 19a are for stopping loaded trolleys, the pivot arm 9 being maintained in an unstable intermediate position on the base of the shallow recesses 19a.  
  Pivotable on spindle 8 is a pawl 21 which is pulled by a spring 22 so that its rear end tends to be engaged with a resilient abutment 26 rigidly secured to part 9a of arm 9. A resilient abutment 27 which is also rigidly secured to part 9a operates when arm 9 changes over from its retracted position to its operative position. The purpose of pawl 21 is to prevent the trolley from overtaking its drive dog. At its rearward end the trolley 4 is shaped to form a hook 23.  
  When a dog 2 has engaged between step 17 and pawl 21 in the manner visible in FIG. 1 and the corresponding trolley is being driven, and when an abutment is then placed on the path of the sensing member I4, the same pivots against spring I6 and the end 13 slides along ramp 11,. so that arm 9, if opposite a recess, can pivot around its spindle 8 by gravity and move the pawl 21. The dropping of arm 9 can be stopped by a roller I8 abutting the base of a shallow recess 19a, in which event end I3 remains in engagement with the ramp 11 in the manner visible in FIG. 3. Opposite a deep recess 19, however, the arm 9 drops further, in the manner visible in FIG. 4, and flank I317 of arm I3 engages with facet 1],, so that spring I6 cannot contract and raise arm 9.  
  In both cases the step 17 releases from the dog 2 with which it was previously in engagement and is lowered far enough for the following dogs 2 not to strike the arm 9 nor the lever 21.  
  In the case shown in FIG. 3, however, if the abutment 24 which caused the sensing member 14 to pivot is retracted, the senser l4 pulled by the spring 16 raises the pivot 9 by its end I3 so that roller I8 releases from recess I9a. Arm 9 then returns to its operative position in which step I7 is disposed on the path of the dogs 2, and the first such dog to come along can move the trol- Iey 4.  
  A number of pairs each comprising a recess 19a and an abutment 24 can be provided, at any required spacing, along a parking section of the runway. The spacing can therefore be large enough for the parking of loaded trolleys. If required, the abutments 24 can be placed on the path of any trolley by the action of the trolley which passed by immediately previously, to which end the abutments 24 can take the form of a rocker, one end of which is raised by the immediately previous trolley so that the other end descends on to the path of the immediately following trolley, the rocker being acted on by return or restoring means.  
  An advantageous feature of the invention for empty trolley storage is that stored trolleys can contact one another and even partly overlap one another to keep down parking length. Also, the trolleys can interact with one another. To this end, there is disposed on the path of the first trolley 4, to be stored (see FIG. 4) an abutment (not shown) which is similar to the abutment 24 of FIG. 3 and which is associated with a shallow recess 19a so that pivot arm of trolley 4, takes up an unstable retracted position such that arm end I3 bears on facet I1 Arm 9 is not locked in this position, and unless roller 18 was engaged in recess 19:: and bearing on the steep end face 19, of recess 19a, spring 16 might return arm 9 to its top position. When the immediately following trolley 4 encounters hook 23 of trolley 4,, senser 14 of trolley 4 is pushed aside and pivot arm 9 of trolley 4 pivots into its end retracted position, its roller 18 dropping into a deep recess 19 in edging 20. The following trolleys stop and are locked in the same way.  
  When the abutment associated with trolley 4, is re tracted, arm 9 of trolley 4, is biased into its operative position by spring 16 and trolley 4, is ready to be driven by the first dog 2 to come along. When it starts to move, trolley 4, releases arm 9 of trolley 4 from its end retracted position by co-operation between ramp 23a of hook 23 of trolley 4, with extension 10 of trolley 4 The same is therefore also ready to be driven by a dog 2 since its arm 9 has risen as a result of co-operation between end l3 of sensor 14 and ramp II and as a result of the action of spring I6; trolley 4 releases, when it starts, the next trolley, and so on.  
  To preclude the hunting previously referred to when the rollers 18 of the trolleys 4 engage in the recesses in edging 20, the width of such recesses is very slightly greater than the diameter of the rollers I8. Also, as can be seen in FIG. I. the rear wall I9, of the recesses is steep. For ready egress of the rollers I8 from the recesses, the top part of the recess front walls I9 is inclined. as can be seen in FIG. I.  
  In the embodiment shown in FIGS. 5-11, the trolleys 4 have rollers 6a, which co-operate with runway 3, and rollers 6b, which are coaxial with the rollers 6a and which are adapted to operate the abutments 24 and 31 to be mentioned hereinafter. In FIGS. 5-II like elements have the same references as in FIGS. 1-4.  
  Roller camming devices 18 combining a finger and a roller are disposed on both sides of pivot arm 9; one device I8 is adapted to co-operate with any of the release rockers 30 disposed along edging 20 which can be disposed along the runway 3 on either side thereof to suit installation convenience. As will become apparent hereinafter, the release rockers 30 are adapted to cooperate with abutment rockers 31 disposed below runway 3 for storage and removal from storage of loaded trolleys.  
  When a drive dog 2 has engaged between step l7 and pawl 21 in the manner shown in FIGS. 6 and II and when the corresponding trolley is moving, and when an abutment 24 (see FIG. is then placed on the path of the sensor 14, the same pivots against the force of its biasing spring and its end 13 slides along ramp 11. so that arm 9 can be pivoted around its spindle 8 by the thrust of the drive dog 2 which, as it advances the trolley, urges the pivot arm 9 downwardly, since the pivot spindle of pivot arm 9 is at the rear bottom part of the trolley. The pivoting of arm 9 causes pawl 21 to pivot, Arm 9 descends and is locked, flank 13b of arm 13 engaging with facet 11 and preventing the spring from contracting. and thus from raising arm 9, until drive dog 2 releases from book 17. Arm 9 is then low enough for the following drive dogs 2 not to strike the arm 9 nor the lever 21.  
  A number of abutments 24 can be disposed. at any spacing from one another, along a parking section of the runway, The spacing can be large enough for the parking of loaded trolleys. If required, the abutments 24 can be moved on to the path of any trolley by the action of the trolley which passes by immediately previ ously Accordingly, the abutmcnts 24 can take the form of rockers 31 pivotable around vertical spindles 32 connected to runway 3; one end of rocker 31 is pushed aside by the immediately previous trolley so that the other end is moved on to the path of the immediately following trolley (see FlGS. 6l0), return means (not shown for the sake of clarity) acting on the rockers.  
  So that loaded trolleys can unlock one another after a period in storage, unlocking rockers are disposed along runway 3. As can be seen in FIG. 6, the rockers 30 have a ramp 33 at one end and a blade 34 at the other. either gravity or resilient means normally keeping the blade 34 lower than the ramp 33. Ramp 33 is adapted to co-operate with roller 18 of the immediately previous trolley and blade 34 is adapted to cooperate with the roller 18 of the immediately following trolley.  
  Consequently, when arm 9 of a preceding trolley 4, (sec FlGS. 8-10) is unlocked (in a manner to be described hereinafter) and the rocker 31 which was locking it has pivoted out of its path, a dog 2 engages with its hook 17 and moves it in the direction of the arrow F. The roller 18 at the top of trolley 4, co-operates with ramp 33 of a corresponding rocker 30 to pivot the same, the blade 34 of such rocker being disposed below roller 18 of the trolley 4 immediately behind. Blade 34 therefore forces arm 9 of trolley 4 to rise, and flank 13a of arm 13 of trolley 4 engages with the corresponding facet 11,. Since at its departure trolley 4 retracted the rocker 31 disposed between the trolleys 4 and 4 and since trolley 4 has been unlocked through the agency of rocker 30, the first dog 2 to come along can move trolley 4 A number of loaded trolleys in storage can therefore release one another.  
  To ensure that the rockers 31 are retracted only when the preceding trolleys have travelled some distance (the amount necessary to release the following trolleys), each rocker 31 has a front extension which co-operates with the trolleys 4 to keep the rockers 31 in the locking position until a trolley 4 has passed beyond the extension 35. Also, to prevent recoil of any trolley 4 abutting an obstructing rocker 31, the front part of the rockers 31 is shaped to form a pawl 36. Consequently, if any trolley 4 abuts a rocker 31 after having pivoted the previous rocker 31 (as considered in the direction of trolley movement), such trolley is trapped between the two consecutive rockers 31. Resilient abutments 37 are provided between the rockers 30 and the edging 20 to reduce clicking noises as the rockers pivot.  
  FIG. 6 shows a trolley 4; which is beginning its departure and which has not yet released the immediately following trolley 4 FIG. 11 shows the storage and removal from storage of empty trolleys.  
  When a trolley 4 encounters the hook 23 of the immediately previous trolley 4 sensor 14 of trolley 4 is pushed aside and its pivot arm pivots right into its end retracted position. The following trolleys stop and are locked in the manner described.  
  When a previous trolley 4 starts to be driven again by a dog 2 (after unlocking in the manner just de scribed), it releases arm 9 of the immediately following trolley 4 from its end retracted position by ramp 23a of its hook 23 co-operating with extension 10 of trolley 4 The same is therefore ready to be driven by a dog 2 because of its arm 9 being raised by co-operation between end 13 of senser 14 and ramp 11 and because ofthe action of the spring biasing the sensor, and upon its departure trolley 4 in turn releases the next trolley 4 and so on.  
  Of course, for empty and full trolley storage an abutment (not shown) operated eg by an electromagnet and adapted to release the trolley is placed on the path of the first trolley of a series to be put in storage.  
  In the removal of empty trolleys from storage, to ensure that a released trolley does not pull behind it one or more trolleys which may not have been released, a rocker 38 is disposed along the runway near the first trolley in storage; rocker 38 is, for instance, of similar construction to the rockers 31 and forms a removable abutment acted on by return means (not shown) which move temporarily into the path of the first of the remaining trolleys when the first trolley moves away. To unlock the first of the remaining trolleys, the rocker 38 acts not on lever 14 but on the actual trolley body (see FIG. 11).  
 I claim:  
 1. A conveyor apparatus. comprising:  
 a runway for trolleys;  
 releasable abutments disposed along the path of the conveyor;  
 a plurality of trolleys movable on said runway and from which loads may be hung; each trolley comprising a pivot arm having a drive hook rigidly attached thereto, a sensor comprising a rockable lever pivotally mounted on said trolley and biased by a tension spring, one end of which co-operates with a stop abutment and the other end of which co-operates with the pivot arm, that end of the rockable lever which co-operates with the pivot arm being in the shape of a rounded apex and having two flanks adapted to co-operate alternately with two facets of the pivot arm to lock the same in its two end positions, and the pviot arm having between said two facets a ramp with which the lastmentioned end of the rockable lever co-operates for enabling the tension spring to return the pivot arm from its retracted position to its operative posi tion, said trolley also comprising sensor-controlled means for moving the pivot arm into two corresponding end positions when the sensor is in its end positions; and an endless chain extending substantially parallel to said runway, said chain having drive dogs thereon. which dogs are adapted to co operate with the drive hook on each trolley so that when a trolley engages a stop abutment the drive dog is released from the trolley drive hook and the pivot arm of the trolley is locked in its retracted position,  
 2. A conveyor apparatus, comprising:  
 a runway for trolleys;  
 a plurality of trolleys movable on said runway and from which loads may be hung, each trolley comprising a pivot arm having a drive hook rigidly attached thereto, a sensor pivotally mounted on said trolley and biased by resilient means, a portion of said sensor engaged with said pivot arm, said sensor adapted to cooperate with the immediately preceding trolley so that engagement of a sensor with said immediately preceding trolley will cause pivoting of said pivot arm to a position between a retracted end position and a projecting operative end position, said resilient means biasing said sensor in a di rection from its retracted end position toward its operative end position, the said pivot arm comprising a forward extension which extends beyond the sensor which extension, when the pivot arm moves h into its retracted end position, comes into a position to co-operate with a ramp inclining upwardly and rearwardly from the immediately previous trolley, each trolley also comprising sensor-controlled means for moving the pivot arm into two corresponding end positions when the sensor is in its end positions; and an endless chain extending substantially parallel to said runway, said chain having drive dogs thereon. which dogs are adapted to cooperate with the drive hook on each trolley so that when a trolley engages a stop abutment the drive dog is released from the trolley drive hook and the pivot arm ofthe trolley is locked in its retracted position.  
  3. A conveyor according to claim 2 wherein the abutment for the sensor of an immediately following trolley and the ramp associated with the extension of the pivot arm of the immediately preceding trolley are combined to form a single hook-shaped member disposed at the rear of each trolley,  
  4. A conveyor according to claim 1 wherein, at the time when the trolley sensor has encountered a stop abutment so that the chain drive dog is released from the trolley hook, at flank of the end of the rockable lever and a facet of the pivot arm are engaged with one another locking the pivot arm in its retracted position.  
  5. A conveyor according to claim 4 comprising a number of releasable abutments each actuatable by any trolley to stop the trolley immediately behind, each abutment being disposed at a predetermined distance from the immediately previous abutment and from the immediately following abutment to allow for trolley loading, wherein the conveyor comprises along the runway a number of unlocking rockers, one end of each such unlocking rocker comprising a ramp for pushing said rocker aside by the immediately previous trolley as it passes by, so that the other end of the rocker is moved to release the pivot arm of the immediately following trolley, the unlocking rockers returning to their initial position after the trolley has passed by.  
  6. A conveyor according to claim 5, wherein the abutments take the form of a rocker. mounted so that one end thereof may be pushed aside by the immediately previous trolley so that the other end is brought on to the path of the immediately following trolley, and return means acting on the rocker.  
  7. A conveyor according to claim 5, wherein that end of the rocker which is pushed aside by the immediately previous trolley is pawl-shaped to prevent trolley recoil when the trolley has pushed aside such rocker end.  
  8. A conveyor according to claim 6, wherein that end of the releasable abutment rocker which is disposed on the side of the immediately previous trolley has an extension for ensuring that such abutment is retracted only when the immediately previous trolley has travelled a predetermined distance,  
  9. A conveyor according to claim 5, wherein the pivot arm has at least one finger, wherein that end of a release rocker which is disposed opposite the ramp comprises a blade which engages below the finger.  
  10. A conveyor according to claim 1, having near a first stored trolley a releasable auxiliary abutment adapted to be placed temporarily in the path of the immediately following trolley.  
  ll. A conveyor according to claim 10, wherein the auxiliary abutment acts on the trolley body.  
  12. A conveyor according to claim 10, wherein the releasable auxiliary abutment is a rocker which is biased by return means and which is operable by a passing trolley to move it into the path of the immediately following trolley.