US2253552A - Snap-acting device - Google Patents

Description

Aug 26, 1941. L. w. BURCH SNAP-ACTING DEVICE 2 Sheets-Sheet 1 Filed June 25, 1938' ATTO RN KY5 Aug. 26, 1941. w, BURCH 2,253,552

SNAP-ACTING DEVICE Filed June 2:5, 1938 2 Sheets-Sheet'Z g w iyglca l m t M! m ATTORNEY Patented 26, 1 941 s PAT NT- OFFICE sms-scrum nsvics Lyn n The W.Burch,Bronxvilie,N.Y.,allignorto Wilcolator Company, a corporation Del- Application 2:, 193a Serial No. uses:

3Claims.

This invention relates to actuators for switches, valves, and the like, and has particular reference to a novel actuator of the snap-acting type which is 01! a simple and durable construction and is positive and reliable in operation. The new actuator may be used to particular advantage in thermostats and accordingly will be specifically described in connection with such a device, although it is to be understood that its use is not limited to thermostats. v

Snap-acting thermostats 01 various types have been devised heretofore, all ofv which include a member movable abruptly from one position to another at a critical temperature. One form of these thermostats comprises a dish-shaped. bimetallic disc made up 0! two sheets of metal having diiIerent thermal coeflicients of expansion and superimposed one on the other. Due to the unequal expansions of the two sheets upon a rise in temperature, internal stresses are in the disc which tend to change it to a shape of opposite curvature. However, the configuration of the disc is such that a change of shape is normally resisted until the internal stresses developed by the expansion attain a critical magnitude, whereupon the disc changes abruptly to a shape of opposite curvature. Thermostats of this form are objectionable for the reason that their operation is dependent on the use of bimetallic members, which are expensive. Also, since the bimetallic disc is operated by internal forces, it does not lend itself to cold checking, that is, to calibration at room temperature to determine in advance the critical temperature at which it operates.

Another form of snap-acting thermostat devised heretoforederives its snap-action from a generally trough-shaped element made of sheet metal and adapted to buckle abruptly when stressed. In these devices, the stress may be imposed by an external source, such as a bimetallic element or a thermo-responsive bellows, or it may be imposed internally by making the troughshaped element of bimetal. In either case, ,the stress is built up gradually without substantially changing the shape 01 the element, until the stress overcomes the bending resistance of the element due to its trough-shape, whereupon the element buckles along a line transverse to the direction of the trough. so that it becomes substantially straight transversely at the region of buckling. When the stress is released suiilciently,

the element returns to its initial form with .a.

snap-action. These devices, as previously made,

have not entirely satisfactory, because when developed the trough-shaped element a made of bimetal ahas the same disadvantages as the bimetallic disc, and when the element is made of a single metal it requires the use. 01 a bellows or other thermoffsponsive meansacting on the element to buckle Thepresent invention, therefore, is directed to the provision at a novel snap-acting device oi a simple construction which may be made thermoresponsive without the use oi a bellows, a bimetallic element, or the like. In addition, the new device is adapted for manufacture at low cost, and readily lends itself to cold checking.'.'

A device made in accordance with the present invention comprises a flexible sheet bent into a trough-shape and disposed between confining means adapted to engage the sheet at each end near its longitudinal axis. The sheet is compressible endwise between the confining means, but by reason of the transverse curvature of the trough, the sheet may be compressed up to a critical amount without substantially changing 'its shape. However, when the compression exthermo-expansible metal so that a rise in temperature, causes it to expand against the confining means, or the confining means maybe thermo-responsive so that a change in temperaturecauses it to compress the opposite ends of the sheet, or, if desired, both of these expedients may be employed. In any case, the flexible sheet is subjected to endwise compression between the confining means and when the compression reaches a critical amount, the sheet buckles abruptly and, at the region oi'buckling, undergoes a rapid movement which may be used to actuate a switch, a valve, or the like. I

In the preferred form of the new device, the

' trough-shaped element is retained at its ends in suitable seats mounted in a frame or casing and,

when the device is used in a tht, is expansible by heat against'the seats to subject it tocompression. The seat at one end is adjustable toward or away from the opposite seat by suitable means; such as a handle, and by operat ingthehandletheinitial compressioninthesheet maybe adjusted. Theswitchorotherdeviceto be actuated may be disposed intermediate the ends of the sheet, so that the throw of the sheet incident to its buckling is transmitted to the device. Preferably, the amplitude of the motiontransmitted from the sheet to the device to be actuated is adjustable by means operable to move the sheet and the device relatively toward or away from each other.

It will be apparent that in the new actuator the flexible sheet, by reason of its spring action and trough shape, constitutes a quick-acting element which is movable abruptly at its intermediate portion by endwise compression thereof. Accordingly, the sheet may be unimetallic, instead of bimetallic, its buckling action being independent of the unequal expansions of different metals. Since the buckling of the sheet is effected by endwise compression thereof, the snap action may be obtained by employing a simple thermo-expansible sheet, whereby the use of a bellows or other independent thermo-responsive means may be avoided. Also, since the sheet may be operated entirely by compression, the device may be readily cold checked by determining the coefficients of expansion of the sheet and its confining means, measuring the amount of compression necessary to buckle the sheet at room temperature, and calculating from these factors the increase in temperature required to produce the necessary critical compression.

For a better understanding of the invention ref erence may be had to the accompanying drawings in which- .Fig. 1 is a longitudinal section through one form of the new actuator;

Fig. 2 is a rear view of the device shown in Fig. 1;.

Fig. 3 is a view similar to Fig. 2, with the snapacting element removed;

Figs. 4, and 6 are sections on the lines 4-4, 55 and, 6-6, respectively, in Fig. 1;

Fig. 7 is an enlarged longitudinal section through the snap-acting element of the device.

shown in Fig. 1;

Fig. 8 is a perspective view of a modified form of the snap-acting element;

Figs. 9 and 10 are front and rear views, respectively. of another form of the new actuator;

Figs. 11 and 12 are sections on the lines i l-,-H and l2l2, respectively, in Fig. 9;

Fig. 13 is a front view of the snap-acting element shown in Fig. 9, with the element in its normal position; 7

Figs. 14; and 15 are sections on the lines i4- and lB-ll, respectively, in Fig. 13;

Fig. 16 is a view similar to Fig. 13, showing the snap-acting element in its buckled position, and

Figs. 17 and 18 are sections on the lines ll----l'l and ll-II, respectively, in Fig. 16.

In the drawings, I have shown two forms of my new actuator used in connection with a switch, although it will be understood that the actuator is not limited to this use but may be used to operate a valve or other member.

Referring to Figs. 1 toil, inclusive, the device there shown comprises a casing 20 made of a strong material having a low coefllcient of expansion and which will not be weakened when subjected to high temperatures. I have found that a material suitable for the casing is a compound of alumina, silica and magnesium, such as Alsimag. The casing is formed in back with a longitudinal recess 2| and near one end has a passage 2i extending to the front of the casing from the recess. At its front end, the passage 1 l is enlarged to receive a plate 22 seated in the passage and secured to the casing by screws 23 (Fig. 5). 'The plate 22 is slotted, as shown at 22, and a double shouldered screw 24 is fitted in the slot with the shoulders of the screw engaging the opposite sides of plate 22, whereby the screw is held against axial movement relative to the plate. The screw 24 is threaded through one end of a mounting element 25 movable longitudinally in the passage 2|, the mounting element being formed at its rear end with a V- shaped portion 28, the apex of which is seated against the casing at one end of the recess 2|. By turning the screw 24, the mounting element 25 may be moved transversely of the casing in the recess.

The opposite end of the casing is formed with a longitudinal slot 28 in which a bell crank lever 29 is pivoted on a transverse pin 30. The pin and lever are inserted in the casing from the front with the pin disposed in a recess 30' in the front of the casing (Fig. 6), and the pin is held against the end wall of the recess by a plate ll, one end of which is .bent inwardly and extends into the recess 30' into contact with the pin. The plate 3| is seated against the front wall of the casing and'is held in position by a bushing 32 loosely fitted in a passage 32' in the casing and extending through the plate, the bushing having a head 33 seated against a suitable shoulder in the passage. A nut 33' is threaded on the front end-of the bushing and screwed tightly against the plate 3|.

Within the bushing 32 is a shaft 34 provided with threads 34 mating with internal threads in the bushing. At its front end, the shaft 34 carries an adjustment handle 35 which may be provided with a pointer (not shown) cooperating with a suitable scale of temperature graduations. The inner end of shaft 34 extends through the head of the bushing and bears against a horizontal face 35 of the bell crank lever constituting one arm of the lever. The other arm of the lever is disposed in recess 2! and formed with a V- shaped notch 31.

Extending between the bell crank lever 29 and mounting element 25 is a generally trough-shaped member made of sheet metal or the like, such as a nickel-steel alloy, the member 40 being substantially straight in the direction of its longi-- tudinal axis but curved in a transverse direction so that it forms a cylindrical segment (Fig. 4). The member 40 is doubled over at its ends, as shown at 40, and has a clip 4| folded around one end and a clip 42 folded around the opposite end, the clips being pressed tightly against the ends of the member. The clips 4| and 42 are bowed slightly (Figs. 5 and 6) and are made of a suitable material, such as Inconel, which will withstand high temperatures and maintain its shape indefinitely. I find that the member 40 may be made by forming the clips 4i and 42 into an arc shape and forcing them over the ends of a pre-formed sheet of metal, or by forcing the arc-shaped clips over the ends of a normally flat sheet of metal which is bent from its normal shape to receive the clips. In the latter instance, the clips 4|, 42 hold the sheet 40 in the shape of a trough, although the intermediate portion of the sheet is curved less than the end portions. In either case. the clips maintain the member 40 in its trough shape indefinitely and form a protective bearing for the member which prevents wearing of the ends thereof. Preferably, the

The end clips 4| and 42 are provided with slots 7 4| and 42', respectively, and the member 4.

is mounted in the casing with the V-shaped portion 25 of the mounting element 25 seated in the slot of clip 4|, and the notched portion 31 of the bell crank lever seated in the slot in clip 42. A screw 45 in the front wall of the casing extends through one leg of a generally U-shaped spring member 41 in the casing and through the corresponding leg 'of a similarly shaped conducting member 48 within the spring member. The members 41 and 48 are held against the front wall of the casing by a plate 49 extending through one side of the casing and through openings in the members 41 and, the plate being clamped against the front legs of the members by the screw 46. The rear legs of members 41 and 4| extend transversely across the recess 2| and carry at their outer ends a pin 50 extending therethrough and provided with a head forming a contact, the pin being held by its head and a shoulder 55 against axial movement relative to the members 41 and 48. At its rear end, the pin 50 extends into a central bore in an insulating button 52 seated against the rear. legs of members 41 and 48 and engaging the front face of member 40. Another plate 53 extends through the opposite side ofthe casing into recess 2| and is secured by a screw 54, the two plates 49 and 53 being separated and insulated from each other. At its inner end, the plate 53 carries a contact 55 coacting with the contact 5|. The plates 49 and 53 are provided at their outer ends with screws 45' and 53', respectively, to which lead wires from an electric device to be controlled may be connected.

In operation, the snap-acting member 4||-'normally engages the button 52 and holds contact 5| in engagement with the contact 55. When the temperature increases, the member 40 expands against ,the mounting element 26 and lever 29, so

' that an endwise compression is imposedon the member. By reason of its trough shape, the member 40 resists the tendency to buckle away from the contacts during the initial compressing action, and the button 52 as well as the normal resistance of member 40 to buckling prevent it from buckling toward the contacts. However,

when the endwise compression of the member sufficient to overcome its resistance to a change in form, the member buckles as uptly away from the button 52 and allows the contact 5| to snap away'from contact 55 under the action of the sp i companied by a bending thereof transversely at a region intermediate its ends and by a consequent flattening of the intermediate part of the mem her. When the temperature decreases, the snapacting member contracts so that the compression thereorris gradually released, and when the compression is reduced to an amount at which the tendency of the snap-acting member to assume its normal trough shape can assert itself, the member snaps abruptly to its initial position in which it holds the contact 5| against contact 55.

The critical temperature at which the snapacting member moves back to its normal contact closing position is somewhat lower than the critical temperature at which it buckles upon a rise in temperature, this lagging? being due to the elastic deformation of the member during compression which causes it to elongate slightly when it buckles and relieves the compression. Before The buckling of member 40 is acthe member can snap to its initial position, it must then cool to a temperature below the critical buckling temperature, until it contracts an amount equal to this elongation.

The critical temperature at which the snap-'- acting element buckles under compression may be varied by operating the handle 55. That is, by turning the handle in one direction, the shaft 34 is screwed inwardly against the horizontal surface 35 of the bell crank lever and urges the lever in a clockwise direction about its pivot 3|! 4 (Fig. 1). The notched portion 31 of the lever is thus moved toward the snap-acting member and decreases the spacing between the confining means therefor, whereby the member 40 does not have to expand to as great an extent to acquire the requisite compression for buckling it and will buckle at a lower critical temperature. Conversely, when the handle is turned in the opposite direction, the inner end of shaft 34 is moved outwardly away from the bell crank lever, .so that an appreciable expansion of the snap-acting member is required before the bell crank lever is forced against the end of shaft 34. By properly calibrating the instrument, the critical temperature at which the snap-acting element buckles forany adjustment of the handle will be indicated by the position of the handle relative to the scale of graduations- (not shown).

The device may be cold checked" by measuring at room temperature the distance through which the notched portion 31 of the bell crank lever must be moved toward the mounting 26, for any given setting of the handle to buckle the snap-acting member. The coefficients of expansion of member and casing 20 being known, the increase in temperature above room temperature necessary to expand the member against its confining means 26 and 31 an amount equal to the distance measured, may be readily computed.

The casing 20, being made of a material having a low coefficient of expansion, does not expand to any appreciable extent when the temperature rises, and, therefore, the effect of the expansion of the casing on the operation of the snap-acting member is negligible. As the temperature rises and expands the member 4|! against its confining means, the beads 43 are flexed by the compression and store potential energy which is suddenly released when the member buckles, whereby the sions thereof. By reason of the and clips 4| and 42, the snap-acting member is centered with respect to its confining means 25, 31 and is held against lateral, edgewise shifting between the confining means. In addition, the end clips prevent wearing of the ends of r the member 40, so that the effective length thereof remains constant.

The amplitude of the motion transmitted from the snap-acting member to the contact 5| may be varied by the adjustment screw 24 to suit different-requirements. When it is desired to decrease the amplitude, the screw 24 'is turned soas to move the mounting element 25 forwardly in the passage 2|, thereby moving the snap-acting member forwardly and increasing its pressure on the button 52 which is backed by the closed contacts. The snap-acting member is thus bent slightly outwardly by the button 52 in the direction in which the member buckles. Accordingly, when the compression on the snap-acting memher is sumcient to buckle it, the intermediate portion thereof moves a shorter distance away from the contact N and decreases the rearward movement of the button under the action of spring 41. When it is desired to increase the amplitude, the screw 24 is turned in the opposite direction so as to move the mounting element 25 and snapacting member ll rearwardly and decrease the pressure exerted by member 40 against button 52. The intermediate portion of the snap-acting member then moves away from the fixed contact 55 a greater distance when the element buckles and permits a longer movement of button 52 and spring 41.

While the snap-acting member 40 is prevented from buckling rearwardly by reason of its normal engagement with button II, it is not necessary that the member be backed by a stop to prevent reverse buckling. I have found that by making the member 40 in the form described and subjecting it to compression substantially along its longitudinal median, the compressive force resulting from an increase intemperature tends to buckle it only in the forward direction. Apparently, the reason for this is that the application of compression along the median line results in a loading of the trough member which is eccentric with respect to the true axis of the member, the latter being spaced some distance outwardly from the median line and in front of the concave face of the member. This eccentric loading of the trough member is a significant feature of the new device, because it renders the member capable of buckling under the action of the endwise compression. More particularly, if the trough member were compressed along its true axis instead of an eccentric axis, it would crumple or resist buckling to such an extent that the device would not operate in the manner described. Also, without an eccentric loading of the trough member, it would be difficult to control the direction in which the member buckles. Thus, the eccentric loading not only allows the member to buckle in the proper manner, but also insures that the buckling will take place in the desired direction. Also, while I have illustrated a device in which a pair of closed contacts is opened when the temperature rises to a certain degree, it will be understood that the snap-acting mem ber may be mounted with its concave face adjacent the button 52, whereby the contacts are 1. normally open and are closed by buckling of the.

member 40.

vIt will be apparent that the new thermostat may be manufactured at low cost, since the parts are simple and inexpensive and may be readily assembled. The snap-acting member 0, because of its trough shape and inherent spring action, not only buckles abruptly when compressed endwise a suillcieni; amount, but also snaps back to its normal position when the compression is sufficiently released. Accordingly, I have provided an actuating device which operates abruptly in either direction without the use of bimetallic elements and independently of separate thermo-responsive means, such as a bellows.

The force with which the central portion of the member 40 moves upon buckling depends on the thickness of the sheet metal, the material from which it is made, its normal transverse curvature, and other factors. By suitable selection of the material for the member 40 and proper bend-- ing thereof, the operating characteristics of the device may be made to suit the requirements of the particular use to which it is put.

A modified form of the snap-acting member is shown in Fig. 8, the member there shown comprising a sheet of metal I? bent into the form of a trough. The ends of the sheet are bent forwardly at substantially right angles to the body thereof and form reinforcing end portions I8 and 58' which maintain the sheet in its trough form. A clip 58 is seated on the end portion ll between the sides thereof and is formed with a part 59 adapted to fit against the mounting element 28 in place of the adjacent end of member 40. The clip is located on the sheet 51 by integral legs 50 extending through slots cut in the side edges of the end portion 58. At the opposlte end of sheet 5! is a clip 8! seated on the end portion 58' and formed with a part 82 adapted to fit in the notched portion 31 of lever 29. Integral legs 63 on the clip 8| extend through suitable slots in the side edges of end portion 58' and prevent lateral displacement of the clip.

In the device shown in Figs. 9 to 18, inclusive, the snap-acting element is mounted in a distortible frame adapted to contract endwise and compress the element in response to a rise in temperature. The device comprises a generally diamond-shaped frame Bl formed by a strip of metal, such as Invar, having a low coefficient of expansion, the strip being bent with the ends thereof overlapped at one end of the frame, where they are secured together by means which will be described presently. A cross member 81 of high expansion metal, such as aluminum, extends across the back of the frame andis provided at its ends with forwardly extending legs 68 (Fig. 12). The legs 58 are of greater width than the cross member and are formed to fit against the inner surface ofstrip 66 at opposite corners of the frame, where the legs are secured by screws 59.

Projecting from the cross member 61 and preferably integral therewith are two oppositely extending arms 10 and H. A sheet of insulating material 12 is disposed on the front face of the arms 10, II and extends from the upper end of arm H to the lower end of arm Ill. Mounted on arm 10 near the lower end thereof is anupwardly extending contact arm 13, the lower end of which is clamped between the head of a bolt H and a spacing washer 15 on the bolt. The contact arm 13 carries on its front face a contact member 15 disposed substantially midway between the ends of cross member 61, and on its rear face a stop 13'. A'second contact arm 11 is mounted on arm H and is clamped near its upper end between the head of a bolt 18 and a spacing washer 19. The contact arm 11 extends downwardly and carries on one side of its free end a contact member coacting with the contact member 16, and on the opposite side an insulated projection 11.

The bolts 14 and 18 extend rearwardly through openings in the sheet 12 and through insulating sleeves l4 and I8 fixed in-the arms ill and-II, respectively. Insulating washers 8i and 82. are mountedon the threaded ends of bolts 14 and 78, respectively, and are held in engagement with the back of arms 10 and ii bywashers 8i and 82' backed by nuts 83 and 84, respectively, threaded on the bolts. Preferably, an adjustment screw 85 is mounted in a threaded opening in the cross member 61 with the end of the screw in engagement with the back of insulating sheet 12 opposite the stop I3. By turning the screw 85, the central portion of insulating sheet I! is moved toward or away from cross member '1,

and 11 and accordingly will be under the control of contacts 16 and 80.

The overlapping portions of the strip are provided with aligned openings in which is threaded one end of a hollow stem 61. The stem 61 is formed with a shoulder 61' which bears against the outer face of strip 66 and is locked on the frame by a nut 68 screwed tightly on the threaded inner end of the stem. The stem 81 extends outwardly from the frame and is internally threaded to receive a screw shaft 66 which carries at its outer end a handle 66 having a pointer 66'. The handle and pointer cooperate with a temperature scale. (not shown) arranged on a panel 6| which is secured against rotation in any suitable manner. A pin 62 is slidable in the inner end of stem 61 and rests against the corresponding end of shaft 66, the free end of the pin being formed with a transverse slot 63.

The end of frame 66 opposite the pin 62 is flattened, as shown, and has a block 66 secured to' the inner face thereof by rivets 65'. A pin 66 is threaded into the block 66 and strip 66 and has an enlarged head provided with a transverse slot 61. I

Seated at its opposite ends in the transverse slots 66 and 61 is a flexible sheet of high expansion metal 66, preferably unimetallic. As shown particularly in Figs. 13, 14 and 15, the sheet 66 is elongated and is bent transversely into a trough shape, the sheet being preferably substantially straight in the direction of its length. The back of the sheet, as shown in Fig. 11, normally engages the projection 11' on contact arm 11 and thereby maintains the contacts 16 and 66 closed against the spring action of arm 11.

when the device is subjected to heat, the cross member 61 expands against the opposite sides of the relatively inexpansible frame 66, thereby causing the pins 62 and 66 to move together and compress the flexible sheet 66 endwise. At the same time, the sheet66 expands against pins 62, 66 under theaction of the heat, and this expansion adds to the endwise compression of the sheet. The sheet, due to its configuration, resists the force tending to buckle it away from the contacts during the initial compressing action,-

and is prevented from buckling toward the contacts by engagement with the projection 11'. The contacts are thus held by the sheet in a closed position against cross arm 11. However, when the endwise compression of the sheet reaches a critical amount, its resistance to a change in shape'is overcome so that the sheet buckles away from the contacts (Figs. 16, 17 and 18). The buckling of the sheet is. accompanied by a bending thereof transversely at a region substantially intermediate its ends and by a consequent flattening of the intermediate part of the sheet. The sheet thus releases the projection 11' and allows the contacts 16, 66 to snap to their normally open positions.

When the temperature decreases, the cross arm 61 contracts and draws the sides of frame 66 inwardly, and as a result the ends of the frame are moved outwardly toward their initial positions, thereby gradually releasing the compression on the flexible sheet 66. Simultaneously, the sheet 66 contracts and further decreases the compression thereof. When the compression is reduced to a critical amount at which the tendency of the sheet to assume its normal position can assert itself, the sheet snaps abruptly to its initial position in which it holds the contacts 16, together against the insulating sheet 12.

By manipulating the handle 66, the inner end of shaft 86 may be moved toward or away from the sheet 66 so as to vary the amount of expansion of the sheet necessary to effect its buckling, and as the handle is turned, the pointer 66 indicates on the panel 6| the critical temperature at which the sheet will buckle. Also, by adjusting the set screw 85, the central portion of insulating sheet 12 and stop 11' may be forced outwardly against the sheet 66, so that the latter is bent slightly toward its buckled position and exerts a greater pressure on the contacts. With this adjustment, it will be apparent that the intermediate portion of the sheet 66 moves through a shorter distanceduring'operation of the thermostat, whereby the amplitude of the movement of arm 11 is decreased. While the sheet 66, as shown. is made. of a thermo-expansible metal, such as a nickel-steel alloy, it may be made of a material having a negligible coeflicient of expansion sothat the compression thereof is effected substantially entirely by the frame.

I claim:

'1. An actuating device comprising a sheet of flexible material, curved members secured to the ends of the sheet for holding it in a generally trough shape, and means for subjecting the sheet to endwise compression, said curved members having recesses therein for receiving said means to locate the sheet relative to said means, said sheet being operable to buckle abruptly when compressed a predetermined amount and to return abruptly to its original position upon release of the compression.

2. An actuating device comprising a pair of bowed clips having recesses therein, a sheet of flexible material having its ends held by the clips, said clips and sheet forming a generally trough-shaped element, and means at the ends of said element engaged in said recesses for subjecting the element to endwise compression, said element being operable to buckle abruptly when compressed a predetermined amount and. to return abruptly to its original position upon release of the compression.

3. An actuating device comprising a snap element, control means coacting with the snap element intermediate its ends, curved members secured to the ends of the snap element for holding it in a trough shape and having slots therein, movable seats for the snap element at the opposite ends thereof for compressing the element and buckling it, the seats being engaged in said slots to locate the sheet relative to the seats, adjustment means for moving one of the seats toward the other seat to vary the compression in the snap element, and adiustment means for moving the second seat laterally relative to the first seat and in the direction of buckling to vary the throw of the snap element relative to the control means. LYNDON W. BURCH.

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