Vertically movable workpiece support device

Abstract

A vertically movable workpiece support device has a ratchet wheel with a larger diameter than the outer diameter of a driving sprocket wheel and is concentrically attached to an end surface of a boss portion in the driving sprocket wheel. A pair of left and right bearing members are provided side by side, on a side on which the free end side of a drive chain is horizontally extended with respect to the driving sprocket wheel, so as to sandwich a horizontally extending path section of the drive chain. A movable pawl is pivotally supported between the pair of left and right bearing members and engages with the ratchet wheel to block the rotation of the ratchet wheel caused by the movement of the drive chain when the workpiece support base is lowered. An actuator switches the movable pawl to a non-action position with respect to the ratchet wheel.

Claims

1. A vertically movable workpiece support device, comprising: a raising/lowering drive means that drives a workpiece support base supported so as to be parallelly and vertically movable to raise and lower the workpiece support base; the raising/lowering drive means having a drive chain that is bendable to only one side and a driving sprocket wheel driven by a motor that engages with the drive chain, in which the drive chain supports the workpiece support base in a vertically linear path section, where the drive chain is extended from one end portion coupled to the workpiece support base to a portion engaging with the driving sprocket wheel and in which a part of the drive chain on a free end side from the driving sprocket wheel is extended horizontally through a lower side of the driving sprocket wheel; a ratchet wheel having a larger diameter than an outer diameter of the driving sprocket wheel is concentrically attached to an outer side of the driving sprocket wheel, a pair of left and right bearing members are provided side by side, on a side on which the free end side of the drive chain is horizontally extended with respect to the driving sprocket wheel, so as to sandwich a horizontally extending path section of the drive chain; a movable pawl which engages with the ratchet wheel so as to block rotation of the ratchet wheel caused by a movement of the drive chain when the workpiece support base is lowered is pivotally supported between the pair of left and right bearing members; an actuator operable to switch the movable pawl to a non-action position with respect to the ratchet wheel; an upper end portion of the movable pawl is pivotally supported at a height of a circumferential edge of the ratchet wheel, an expanded part of the movable pawl between the upper end portion and a lower end pawl portion is expanded to a side away from the ratchet wheel, and gravity acting on the expanded part causes a rotation force on the movable pawl in such a direction that the lower end pawl portion biased toward and engages with a circumferential edge of the ratchet wheel; the expanded part of the movable pawl has a notched concave portion having a rear side and lower side which are open on a side opposite to the ratchet wheel, engagement holes are provided in both left and right side wall portions of the notched concave portion, a tip end of a drawing-in shaft of the actuator is disposed in the concave portion, a tip end of a drawing-in shaft of the actuator is disposed in the notched concave portion, and both end portions of a drive pin attached to the tip end of the drawing-in shaft are fitted into the engagement holes, and each engagement hole has a shape and size to allow certain movement of the drive pin therein such that an engagement/disengagement swing movement of the movable pawl caused by the rotation of the ratchet wheel when the workpiece support base is raised is not transmitted to the drive in, and when the drawing-in shaft of the actuator is made to perform a drawing-in operation, the movable pawl is held in a state where the movable pawl is disengaged from the ratchet wheel via the dove pin and the engagement holes.

2. The vertically movable workpiece support device according to claim 1, further comprising: the ratchet wheel having a number of pawls each having a height; a second ratchet eel made of a resin is rotated integrally and concentrically with the ratchet wheel, and the second ratchet wheel has the same number of pawls as the ratchet wheel; and the second ratchet wheel having pawls which are taller in height than the pawls of the ratchet wheel such that an engagement of the movable pawl between the pawls of the ratchet wheel is allowed but that when the movable pawl rides over the pawls of the ratchet wheel, the movable pawl is floated up from the pawls of the ratchet wheel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is side view in a state where a workpiece support base is raised to a raising limit height.

(2) FIG. 2 is side view in a state where the workpiece support base is lowered to a lowering limit height.

(3) FIG. 3 is a plan view of a lower half on the side of the workpiece support base of FIG. 2

(4) FIG. 4 is a vertical cross-sectional elevational view when viewed from the side of a fixed support shaft in a parallel link mechanism in a state where the workpiece support base is at an intermediate height.

(5) FIG. 5 is a vertical cross-sectional elevational view when viewed from the side of a movable support shaft in the parallel link mechanism in a state where the workpiece support base is at the lowering limit height.

(6) FIG. 6 is a side view of a main part in a state where an anti-drop means according to the present invention is attached.

(7) FIG. 7 is a vertical cross-sectional side view showing a state where the anti-drop means according to the present invention is removed from FIG. 6.

(8) FIG. 8 is a partial horizontal cross-sectional plan view of FIG. 6.

(9) FIG. 9A is a partial vertical cross-sectional side view showing the anti-drop means according to the present invention, and FIG. 9B is a cross-sectional view taken along line X-X of FIG. 9A.

(10) FIG. 10A is a partial horizontal cross-sectional plan view of FIG. 9A, and FIG. 10B is a partial vertical cross-sectional side view showing a state where the movable pawl of FIG. 9A is switched to a retraction position.

(11) FIG. 11A is a partial cutout side view of a main part showing another embodiment of the present invention, and FIG. 11B is a partial cutout plan view of the same main part.

DETAILED DESCRIPTION OF THE INVENTION

(12) A vertically movable workpiece support device according to the present invention will be described below with reference to drawings. As shown in FIGS. 1 to 5, in the workpiece support device, on a horizontal frame 1, via a parallel link mechanism 2, a workpiece support base 3 is supported so as to be parallelly and vertically movable while keeping a horizontal posture. The parallel link mechanism 2 uses a pair of left and right crosslinks 4A and 4B, and the respective crosslinks 4A and 4B have a laterally symmetrical structure and are respectively formed by overlapping two link units 5a and 5b of the same length and are pivotally supported with and integrated by a support shaft 6 so as to freely open and close at a center position.

(13) The lower ends of the pair of left and right link units 5a positioned on the inner side of both the crosslinks 4A and 4B are pivotally supported with mutually-concentric horizontal support shafts 8a and 8b to a pair of left and right fixed bearings 7a and 7b which are additionally provided on a rear end portion of the horizontal frame 1, and the upper ends of both the link units 5a are pivotally supported with mutually-concentric horizontal support shafts 11a and 11b to movable bearings 10a and 10b which engage with guide rails 9a and 9b in a forward/backward direction laid in parallel to each other to the lower side of a front end side of the workpiece support base 3 so as to freely slide horizontally forward and backward thereon. The upper ends of the pair of left and right link units 5b positioned on the outer side of both the crosslinks 4A and 4B are pivotally supported with mutually-concentric horizontal support shafts 13a and 13b to a pair of left and right fixed bearings 12a and 12b which are additionally provided on the lower side of a rear end portion of the workpiece support base 3, and the lower ends of both the link units 5b are pivotally supported with mutually-concentric horizontal support shafts 16a and 16b to movable bearings 15a and 15b which engage with guide rails 14a and 14b in the forward/backward direction laid in parallel to each other on the upper end side of the horizontal frame 1 so as to freely slide horizontally forward and backward thereon.

(14) The workpiece support base 3 supported with the parallel link mechanism 2 as described above can be moved parallelly and vertically between a raising limit position shown in FIG. 1 and a lowering limit position shown in FIG. 2 while keeping a horizontal posture, and when the workpiece support base 3 is present in the lowering limit position shown in FIG. 2, four support members 17a to 17d which individually support two left and right parts of the front end portion and two left and right parts of the rear end portion of the workpiece support base 3 are provided so as to stand on the horizontal frame 1. The respective support members 17a to 17d support a receiving board 20 on the upper ends of column main bodies 18 via screw shafts 19 such that the height can be freely adjusted.

(15) A raising/lowering drive means 21 which drives the workpiece support base 3 to raise and lower the workpiece support base 3 is additionally provided. The raising/lowering drive means 21 is formed with a pair of left and right drive chains 22a and 22b, a pair of left and right driving sprocket wheels 23a and 23b which engage with the respective drive chains 22a and 22b, and a speed reducer equipped motor 24 which synchronously drives both the driving sprocket wheels 23a and 23b. As shown in FIG. 7, the drive chains 22a and 22b are formed with roller chains in which coupling pins 26 that couple a pair of left and right outer link plates 25a and a pair of left and right inner link plates 25b in a position displaced to one side and that are arranged at regular intervals are made to support rollers 27 between the inner link plates 25b so as to feely idle the rollers 27 and which are freely bendable to one side as conventionally known.

(16) In the pair of left and right drive chains 22a and 22b configured as described above, one ends thereof are coupled via chain end portion coupling tools 28a and 28b to the lower side on a rear end side of the workpiece support base 3 such that the bendable sides of the drive chains 22a and 22b, that is, the side on which the rollers 27 protrude is directed so as to be positioned on the front end side of the workpiece support base 3. The drive chains 22a and 22b are suspended vertically downward from the workpiece support base 3, are engaged with the rear side of the pair of left and right driving sprocket wheels 23a and 23b pivotally supported on the rear end side on the horizontal frame 1 in a mutually concentric manner, and thereafter extended horizontally forward through the lower side of the driving sprocket wheels 23a and 23b. That is, in the drive chains 22a and 22b, between the workpiece support base 3 and the driving sprocket wheels 23a and 23b a vertically linear path section 22X which supports the workpiece support base 3 and a horizontally extending path section 22Y which is extended horizontally forward from the lower side of the driving sprocket wheels 23a and 23b are formed. In the speed reducer equipped motor 24, a speed reducer 24b is integrally coupled to the front end side of a motor main body 24a directed in the forward/backward direction, the speed reducer equipped motor 24 includes an electromagnetic brake, the speed reducer 24b is installed on the horizontal frame 1 so as to be positioned at an intermediate part of the pair of left and right driving sprocket wheels 23a and 23b and the driving sprocket wheels 23a and 23b are attached to a pair of left and right output shafts 24c and 24d which protrude from the speed reducer 24b to both the left and right sides in a mutually concentric manner. Therefore, by energizing the motor main body 24a so as to be operated, the electromagnetic brake is released, and it is possible to synchronously drive the pair of left and right driving sprocket wheels 23a and 23b in the same direction at the same speed.

(17) In the pair of left and right driving sprocket wheels 23a and 23b, chain guide means 29a and 29b for guiding the respective drive chains 22a and 22b are additionally provided. As shown in FIGS. 6 to 8, the chain guide means 29a and 29b are attached via spacers 31a and 31b to the outer sides of column members 30 which are provided so as to stand on the horizontal frame 1, and are formed with an inner roller guide plate 32 which enters between the inner link plates 25b of the drive chains 22a and 22b from behind and guides the rollers 27 so as to freely roll the rollers 27, and an outer roller guide plate 33 which is attached to a support member 31c attached to the front sides of the column member 30 and guides the rollers 27 of the drive chains 22a and 22b so as to freely roll the rollers 27. The inner roller guide plate 32 includes a linear guide side 32a which guides the rollers 27 in a vertically linear path section positioned higher than the driving sprocket wheels 23a and 23b of the drive chains 22a and 22b, and an arc-shaped guide side 32b which guides the rollers 27 of the drive chains 22a and 22b which are turned along a range of approximately 90 degrees of the driving sprocket wheels 23a and 23b. The outer roller guide plate 33 includes a linear guide side 33a which guides the rollers 27 at a position higher than the driving sprocket wheels 23a and 23b and between the linear guide side 33a and the linear guide side 32a of the inner roller guide plate 32.

(18) Furthermore, chain guide rails 34a and 34b which support and guide the horizontally extending path sections 22Y of the respective drive chains 22a and 22b that are extended horizontally forward from the lower side of the driving sprocket wheels 23a and 23b are laid on the horizontal frame 1. The chain guide rails 34a and 34b include a horizontal linear guide side 35 which enters between the inner link plates 25b of the drive chains 22a and 22b from below and which supports and guides the rollers 27 of the drive chains 22a and 22b so as to freely roll the rollers 27, and the beginning end portion (rear end portion) thereof is connected to the free end of the arc-shaped guide side 32b of the inner roller guide plate 32 in the chain guide means 29a and 29b.

(19) In the workpiece support device configured as described above, an anti-drop safety mechanism 36 for the workpiece support base 3 is provided. The anti-drop safety mechanism 36 is additionally provided in the driving sprocket wheel 23a, a pair of left and right driving sprocket wheels 23a and 23b which form the raising/lowering drive means 21 of the workpiece support base 3, and is formed with a ratchet wheel 38 which is concentrically attached to the outer side of the driving sprocket wheel 23a, a movable pawl 39 which engages with the ratchet wheel 38, and an actuator 40 which operates the movable pawl 39. Although the ratchet wheel 38 is attached with one bolt 38a screwed to a screw hole provided in the center of the outer end surface of the output shaft 24c in the speed reducer 24b to which the driving sprocket wheel 23a is attached, in order to prevent the ratchet wheel 38 from being rotated around the bolt 38a, a plurality of anti-rotation pins 38b in a circumferential direction are used which are fitted in parallel to the axis center between both a boss portion 37 of the driving sprocket wheel 23a that is externally fitted and keyed to the output shaft 24c and the ratchet wheel 38.

(20) In the movable pawl 39, an upper end portion 39e thereof is pivotally supported with a support shaft 42 which is placed laterally in a horizontal direction between upper end portions of a pair of left and right bearing plates 41a and 41b provided to stand on the horizontal frame 1 so as to sandwich, from both the left and right sides, the rear sides (the sides on which the drive chains 22a and 22b are extended horizontally from the lower side of the driving sprocket wheels 23a and 23b) of both the driving sprocket wheel 23a and the ratchet wheel 38, and a lower end pawl portion 39a positioned directly below the support shaft 42 engages between rear side pawls at the height of the axis center of the ratchet wheel 38. The lower end pawl portion 39a of the movable pawl 39 engages between the rear side pawls at the height of the axis center of the ratchet wheel 38, and thereby the rotation (the counterclockwise rotation of the ratchet wheel 38 shown in FIG. 9A) of the ratchet wheel 38 (the driving sprocket wheel 23a) when the workpiece support base 3 is lowered is blocked. The movable pawl 39 has an expanded part 39b having a wide width. The expanded part 39b is located between the upper end portion 39e pivotally supported with the support shaft 42 and the lower end pawl portion 39a directly therebelow. The expanded part 39b is expanded forward away from the ratchet wheel 38, and it is configured such that, by gravity acting on the expanded part 39b, a rotation force is exerted on the movable pawl 39 in such a direction that the lower end pawl portion 39a is biased toward and moved close to the side of the ratchet wheel 38 such that the lower end pawl portion 39a engages the ratchet wheel.

(21) The actuator 40 is adhered so as to be fixed to an inner side surface in the vicinity of the upper end of the bearing plate 41b, of the pair of left and right bearing plates 41a and 41b, on the inner side of the driving sprocket wheel 23a, and is attached horizontally to the outer side surface of an attachment plate 43 which is extended forward away from the driving sprocket wheel 23a, and a drawing-in shaft 40a which protrudes forward is coupled to the movable pawl 39 in an interlocking manner. That is, in the wide width shaped part 39b of the movable pawl 39, a notched concave portion 39c whose rear side and lower side are opened is provided, and oval shaped engagement holes 39d are provided in an up/down direction in both left and right side wall portions of the notched concave portion 39c. In the drawing-in shaft 40a of the actuator 40, a drive pin 40b which protrudes to both the left and right sides is attached to a tip end portion that is loosely fitted into the notched concave portion 39c of the movable pawl 39, and both the end portions of the drive pin 40b are loosely fitted into the oval shaped engagement holes 39d provided in both the left and right side wall portions of the notched concave portion 39c. The oval shaped engagement holes 39d are formed so as to have such a size that when the movable pawl 39 performs a forward/backward swing movement around the support shaft 42 between an action posture where the lower end pawl portion 39a engages between the pawls of the ratchet wheel 38 and a non-action posture where the lower end pawl portion 39a rides on the outer side of the pawl of the ratchet wheel 38, the oval shaped engagement holes 39d do not interfere with the drive pin 40b at the tip end of the drawing-in shaft 40a which is positioned at the protrusion limit position of the actuator 40.

(22) By operating the speed reducer equipped motor 24 so as to rotationally drive the pair of left and right driving sprocket wheels 23a and 23b in the clockwise direction in FIG. 7, the drive chains 22a and 22b are pulled from the top of the chain guide rails 34a and 34b, and pushed upward vertically through the lower side of the drive chains 22a and 22b. At this time, the inner roller guide plate 32 of the chain guide means 29a and 29b enters between the inner link plates 25b so as to regulate the positions of the drive chains 22a and 22b leftward and rightward in the horizontal direction, and simultaneously, between the linear guide side 32a of the inner link plate 25b and the linear guide side 33a of the outer roller guide plate 33, the rollers 27 of the drive chains 22a and 22b are sandwiched in the forward/backward direction thereof, with the result that the drive chains 22a and 22b are raised vertically upward from the upper end of the chain guide means 29a and 29b while keeping the linear state. Although by the pushing up action of the pair of left and right drive chains 22a and 22b, the workpiece support base 3 is moved upward, since the workpiece support base 3 is supported by the parallel link mechanism 2 such that the workpiece support base 3 can only be raised and lowered in parallel while keeping the horizontal posture, the workpiece support base 3 in which the two left and right parts of the rear end portion thereof are pushed upward by the pair of left and right drive chains 22a and 22b is consequently moved upward in parallel while keeping the horizontal posture.

(23) When the workpiece support base 3 is moved upward as described above, the ratchet wheel 38 which is rotated together with the driving sprocket wheel 23a is rotated in the clockwise direction in FIG. 9, and as described previously, the movable pawl 39 performs the forward/backward swing movement around the support shaft 42 against gravity acting on the wide width shaped part 39b. However, due to the size of the oval shaped engagement holes 39d of the movable pawl 39 described previously, even when the drawing-in shaft 40a of the actuator 40 is left at the protrusion limit position, there is no possibility that a force is applied to the drawing-in shaft 40a of the actuator 40 due to the forward/backward swing movement of the movable pawl 39.

(24) When the raised workpiece support base 3 is lowered, as shown in FIGS. 10A and 10B, the actuator 40 is energized so as to be operated to forcibly draw in the drawing-in shaft 40a, and, via the drive pin 40b at the tip end thereof and the oval shaped engagement holes 39d on the side of the movable pawl 39, the movable pawl 39 is forcibly made to swing forward around the support shaft 42 against gravity acting on the wide width shaped part 39b, and thus the movable pawl 39 is switched to the non-action state where the lower end pawl portion 39a does not interfere with the pawls of the ratchet wheel 38. Consequently, since the driving sprocket wheel 23a integrated with the ratchet wheel 38 can be rotated in the counterclockwise direction in the state shown in FIG. 7, the speed reducer equipped motor 24 is operated to make the pair of left and right driving sprocket wheels 23a and 23b rotationally driven in the counterclockwise direction in FIG. 7, and thus the drive chains 22a and 22b supporting the workpiece support base 3 are lowered. At this time, the drive chains 22a and 22b are guided by the inner roller guide plates 32 and the outer roller guide plates 33 of the chain guide means 29a and 29b, turned in a range of approximately 90 degrees around the driving sprocket wheels 23a and 23b from a vertical path section supporting the workpiece support base 3, guided onto the chain guide rails 34a and 34b through the lower side of the driving sprocket wheels 23a and 23b, and moved horizontally forward on the chain guide rails 34a and 34b.

(25) Although in the above description, when the workpiece support base 3 is moved upward, the movable pawl 39 is engaged and disengaged with respect to the pawls of the ratchet wheel 38 without operating the actuator 40, when the speed reducer equipped motor 24 is operated so as to make the workpiece support base 3 perform the raising/lowering movement, the actuator 40 may be constantly energized so as to be operated, and thus the movable pawl 39 may be switched to the non-action state where the movable pawl 39 is disengaged from the pawls of the ratchet wheel 38. In any case, the actuator 40 is controlled such that, when the raising/lowering movement of the workpiece support base 3 by the operation of the speed reducer equipped motor 24 is completed, a state where the movable pawl 39 can engage with the pawls of the ratchet wheel 38 by gravity, that is, a state where the drawing-in shaft 40a is returned to the protrusion limit position without energizing the actuator 40, is achieved.

(26) When, as described above, the speed reducer equipped motor 24 is operated so as to raise and lower the workpiece support base 3, and the workpiece support base 3 reaches a predetermined height, the energization of the motor main body 24a of the speed reducer equipped motor 24 is interrupted such that the drive of the driving sprocket wheels 23a and 23b is stopped. The workpiece support base 3 reaching the predetermined height is then supported by the vertically linear path section 22X of the pair of left and right drive chains 22a and 22b extending from the driving sprocket wheels 23a and 23b to the workpiece support base 3. When, after the workpiece support base 3 is driven to be raised or lowered to the intended height, the energization of the speed reducer equipped motor 24 is interrupted, the electromagnetic brake included in the speed reducer equipped motor 24 is operated, braking is applied to the output shafts 24c and 24d and the driving sprocket wheels 23a and 23b are locked. Therefore, the workpiece support base 3 is prevented from being unexpectedly lowered from the predetermined height, as a result of idling of the driving sprocket wheels 23a and 23b in the direction in which the workpiece support base 3 is lowered (the counterclockwise direction in FIG. 7) due to the fact that the drive chains 22a and 22b receive gravity on the side of the workpiece support base 3.

(27) However, even if an abnormal event is encountered in which the electromagnetic brake included in the speed reducer equipped motor 24 does not normally operate due to failure and therefore the workpiece support base 3 is unexpectedly lowered, when the workpiece support base 3 is not driven by the speed reducer equipped motor 24 so as to raise or lower the workpiece support base 3, the movable pawl 39 is in a state where the movable pawl 39 engages with the pawls of the ratchet wheel 38 by gravity as described above. Therefore, the rotation (the rotation in the counterclockwise direction in FIG. 7) of the driving sprocket wheel 23a when the workpiece support base 3 is lowered is instantaneously blocked by the engagement of the movable pawl 39 with the pawls of the ratchet wheel 38, with the result that it is possible to prevent abnormal lowering of the workpiece support base 3, that is, an accidental drop.

(28) Although in the embodiment described above, in the configuration in which the workpiece support base 3 is driven by the pair of left and right drive chains 22a and 22b to be raised and lowered, only in the driving sprocket wheel 23a driving the drive chain 22a on one side, the anti-drop safety mechanism 36 including the ratchet wheel 38, the movable pawl 39 and the actuator 40 is additionally provided, it is assumed that in this configuration, a load on the side of the dropping workpiece support base 3 can be safely received with one ratchet wheel 38 and one movable pawl 39, in a case where the receiving strength of the ratchet wheel 38 and movable pawl 39 is insufficient, it suffices to additionally provide the anti-drop safety mechanism 36 in the driving sprocket wheels 23a and 23b each driving the pair of left and right drive chains 22a and 22b. Also, in the embodiment described above, the extending direction in which the drive chains 22a and 22b are extended horizontally from the lower side of the driving sprocket wheels 23a and 23b is set to the side on which the parallel link mechanism 2 supporting the workpiece support base 3 is present, and in this case, the horizontally extending part of the drive chains 22a and 22b can be positioned not on the inner side of the pair of left and right crosslinks 4A and 4B in the parallel link mechanism 2 but on the outer side thereof or can be positioned on the lower side of the crosslinks 4A and 4B. As a matter of course, the extending direction in which the drive chains 22a and 22b are extended horizontally from the lower side of the driving sprocket wheels 23a and 23b may be set to a side opposite to the side on which the parallel link mechanism 2 supporting the workpiece support base 3 is present.

(29) Furthermore, in a case where the pair of left and right anti-drop safety mechanisms 36 are provided as described above, the attachment phases of the mutually same ratchet wheels 38 in the respective anti-drop safety mechanisms 36 to the driving sprocket wheels 23a and 23b can be displaced only by a half pitch of the pawls, or in a case where, as in the embodiment described above, one anti-drop safety mechanism 36 is provided, the mutually same two ratchet wheels 38 can be attached to the driving sprocket wheel 23a such that the attachment phases to the driving sprocket wheel 23a are displaced from each other only by a half pitch of the pawls, and the two movable pawls 39 which engage and disengage with respect to the respective ratchet wheels 38 can be concentrically and pivotally supported, for example, with the common support shaft 42. According to these configurations, it is possible to halve the drop distance of the workpiece support base 3 until the workpiece support base 3 which drops as a result of the driving sprocket wheels 23a and 23b becoming free is received by the engagement of the movable pawl 39 with the ratchet wheel 38.

(30) FIGS. 11A and 11B show another embodiment of the present invention, and as shown in the figures, in the ratchet wheel 38 described above, a second ratchet wheel 44 which is rotated integrally with the ratchet wheel 38, which has the same number of pawls and which is made of a resin such as an MC nylon can be provided concentrically and side by side. The second ratchet wheel 44 includes pawls 44a which are tall in height such that the engagement of the movable pawl 39 between the pawls 38c of the ratchet wheel 38 is allowed but when the movable pawl 39 rides on the pawls 38c of the ratchet wheel 38 so as to pass through them, the movable pawl 39 is floated up from the pawls 38c of the ratchet wheel 38. In the figures, in a state where the second ratchet wheel 44 is concentrically abutted against the outer side of the ratchet wheel 38 which is attached to the boss portion 37 of the driving sprocket wheel 23a with one bolt 38a and a plurality of the anti-rotation pins 38b, the second ratchet wheel 44 is adhered to the ratchet wheel 38 with a plurality of bolts 45 in the circumferential direction so as to be fixed thereto. In this case, as shown in the figures, in the center portion of the second ratchet wheel 44, a through hole into which the head portion of the attachment bolt 38a in the ratchet wheel 38 is fitted may be provided or a counterbored hole into which the head portion of the attachment bolt 38a is fitted may be provided in the center portion of the ratchet wheel 38. As a matter of course, the ratchet wheel 38 and the second ratchet wheel 44 may be integrated with an appropriate means so as to be attached to the boss portion 37 of the driving sprocket wheel 23a.

(31) According to another embodiment described above, when, in a state where the movable pawl 39 can engage with the pawls 38c of the ratchet wheel 38 by gravity, the driving sprocket wheels 23a and 23b are rotationally driven in such a direction (the clockwise direction in FIG. 11A) that the workpiece support base 3 is pushed upward by the drive chains 22a and 22b, the movable pawl 39 swings around the support shaft 42 while making sliding contact with the pawl 44a (the pawl 44a made of the resin) whose height is the higher of the pawl 38c of the ratchet wheel 38 and the pawl 44a of the second ratchet wheel 44. Therefore, the generation of intermittent noise, as in the case where the second ratchet wheel 44 is not present, when the movable pawl 39 made of metal swings around the support shaft 42 while making sliding contact with the pawl 38c of the ratchet wheel 38 made of metal, can be significantly suppressed. As a matter of course, when the movable pawl 39 is forcibly switched by the actuator 40 to the action position in order to block the dropping of the workpiece support base 3 when the driving sprocket wheel 23a becomes free, it is possible to make the movable pawl 39 reliably engaged between the pawls 38c of the ratchet wheel 38, without being influenced by the presence of the pawls 44a of the second ratchet wheel 44, to block the dropping of the workpiece support base 3.

(32) The vertically movable workpiece support device of the present invention can be utilized as a highly safe workpiece support base in which it is possible to prevent an accidental drop of the workpiece support base when a driving sprocket wheel that drives a drive chain (push chain) raising and lowering the workpiece support base is unexpectedly brought into a free state due to, for example, a failure on the side of a motor.