US3012834A

US3012834A - Escapement mechanism

Info

Publication number: US3012834A
Authority: US
Inventors: Mayall William
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-02-16
Filing date: 1959-02-16
Publication date: 1961-12-12
Anticipated expiration: 1978-12-12

Description

Dec. 12, 1961 w. MAYALL ESCAPEMENT MECHANISM Filed Feb. 16, 1959 INVENTOR WILL/AM MAY/1L1. BY-% ATTORNEYS I I I I United States Patent fiice 3,012,834 Patented Dec. 12, 1961 This invention relates to a mechanism for interrupting continuous rotation of a driven shaft, and is particularly concerned with a mechanism for interrupting the rotation of a shaft forming part of a dispensing device for paper towelling in continuous ribbon form in which the shaft is rotated by the ribbon of paper towelling as it is withdrawn from a supply of towelling within the device.

Numerous mechanisms for this purpose have been proposed in the past, but those mechanisms have suffered from the disadvantage that they are of complicated construction, and in consequence are expensive to manufacture.

An object of this invention is to provide a simple and inexpensive mechanism for interrupting rotation of a driven shaft after the shaft has been rotated through a determined angle.

Another object of the present invention is to provide an improved mechanism for metering lengths of paper towelling from a supply of paper towelling in the form of a continuous ribbon contained within a dispensing device, and for interrupting the withdrawal of the sheet material from the dispensing device after a predetermined length of sheet material has been released.

According to the invention, the mechanism for interrupting continuous rotation of a driven shaft comprises a snail cam journalled for rotation about its axis and arranged in driven relationship with the driven shaft, the snail cam having a camming surface which includes a starting portion and a terminal portion spaced radially of the starting portion, the snail cam carrying a stop for coaction with a blocking means with which the camming surface coacts, the stop being spaced circumferentially of the terminal portion of the cam to an extent permitting the blocking means to be passed between the terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extent of the blocking means measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be rotated when the blocking means is adjacent the starting portion, the stop coacting with the blocking means to prevent rotation of the snail cam until such time as the blocking means is adjacent the starting portion and clear of said stop, and a one-way acting dashpot associated with the blocking means and permitting the blocking means to be moved in a substantially unconstrained manner by the camming surface asthe snail cam is rotated, and, after the blocking means has left said terminal portion, acting to return the blocking means towards said starting portion and clear of the stop in a controlled manner and at a determined rate.

According to a further feature of the invention, the mechanism of the'invention forms part of a device for dispensing metered lengths of a continuous ribbon of sheet material, with a determined time delay between the delivery or availability of successive ones of said metered lengths.

According to further features of the invention, the snail cam is mounted directly on the driven shaft for it to be rotated in unison therewith, and the dashpot is comprised by a container which is partially filled with a liquid, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a peripheral "side wall which is spaced from a juxtaposed side wall of the container and having an internal cavity which is open at an upper surface of the piston to permit the liquid to flow freely into the cavity when the piston is submerged, the piston having a perforation located at a position below the upper surface and which communicates with the cavity, operating means associated with the piston and the blocking means and operable by the blocking means to move the piston to submerge it in the liquid, and a guide restricting movement of the piston to movement in a substantially vertical plane, the specific weight of liquid being greater than the effective specific weight of the piston so that the piston will rise in the liquid when it is freed to do so by the blocking means.

The foregoing and further objects and advantages of the invention will-become apparent from a study of the following specification, when taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of one form of mechanism according to the invention for preventing continuous rotation of a driven shaft, the mechanism being shown in the environment of a dispensing device for a ribbon of paper towelling, a cover of the device being shown partially broken away to expose the internal structure; and,

a dashpot forming part of the mechanism of FIGURE 1, the section being taken on the line. 2--2 of FIGURE 1.

The dispensing device shown in the drawing includes a vertical structural wall 10 by means of which the device may be attached to a supporting structure such as a wall, a horizontal structural wall 11, and a pair of parallel side walls 12 which extend forwardly of the wall 10 and between which is journalled a shaft 13.

The horizontal wall 11 provides a support for a roll of paper towelling 14, the towelling passing upwardly from the rear of the roll in the drawing, over a cylinder 13:; fast with the shaft 13, and then downwardly for it .to pass between a plate 15 carried by the side walls 12 and a cover 16 of the device.

The cylinder 13a is secured to the shaft 13 in any convenient manner and is surfaced with a material having a high coefficient of friction, such as latex, to minimize v slippage of the towelling over the cylinder as the towelling of other manners.

is withdrawn from the device.

The cover 16 of the device is pivoted at 17 to the wall 10so that the cover may be swung downwardly toexpose the interior of the device, any convenient means being provided for locking the cover in place when it isv in its raised condition. I

The shaft 13 is carried by the walls 12 for rotation about its longitudinal axis, and at one of its ends is removably connected to the rotation interrupting mechanism; of the invention so that the shaft 13 and cylinder 13a may be removed as a unit to facilitate loading of the device with a fresh supply of towelling.

Therotation interrupting mechanism of the invention includes a snail cam, indicated generally at 20, which is journalled on the adjacent side wall 12 for rotation about its axis, the cam being comprised by a disc 21 and a strip 22 which is secured to that face of the disc remote from the shaft 13 and which is coiled into the form of a single turn of an Archernedean spiral. The outer periphery of .the strip 22 provides the camrning surface of the cam 20, that surface including a starting portion 22a and a terminal portion 2212, the carnming surface progressively increasing in radius from the starting portion to the terminal portion. I j

Obviously, the snail cam Zli may he formed in a variety For example, the camming surface may be formed directly on the edge. of the disc v 21, or the snail cam can be provided by acasting or moulding of metal or of plastics material in which the camming sur-' face is provided by the peripheralcontour of the-casting FIGURE 2 is a longitudinal sectional elevation through or moulding, or in which the ca-mming surface is provided by a track extending axially from a radial face thereof or by a continuous groove formed in a radial face thereof. Cooperating with the camming surface of the strip 22 is a blocking means 24 provided by a post of determined length and having a rounded upper end or first part for coaction with the cumming surface and a lower end or second part for coaction with a stop as hereinafter described. The blocking means is carried by a cranked portion of'a lever 25 which is pivotally mounted at 26 on the side wall 12 of the device, and which is provided with a laterally-extending bracket 27 adjacent its cranked end. The bracket 27 is for coaction with a piston rod 28 of the dashpot 29, hereinafter described in detail.

Extending axially of the disc adjacent its periphery is a stop 30, which may be formed independently of the disc and secured thereto in any convenient manner, or which may be formed integrally with the disc. The stop 30 is spaced circumferentially of the terminal portion 22b to an extent permitting the blocking means 24 to pass between the portion 22b and the stop 30, and is spaced radially of the starting portion 22a to an extent which is greater than the extent of the blocking means 24 measured in a direction radially of the axis of rotation of the cam, so that when the rounded upper end of the blocking means 24 is in engagement with or closely adjacent to the starting portion 22:: of the carnming surface, the lower end of the blocking means 24 is clear of the stop 30 and the snail cam is free to rotate in the direction of the arrow 20a, i.e., in a counter-clockwise direction.

The operation of the rotation interrupting mechanism is as follows:

As the snail cam is rotated in a counter-clockwise direction, the camming surface acts on the upper end of the blocking means 24 to depress it and in turn to depress the lever and the piston rod 28 of the dashpot 29.

The blocking means 24, as it is left by the terminal portion 22b of the catnming surface, lies in the path of the stop and is engaged by the stop and thus prevents further rotation of the cam 20 until such time as blocking means 24 is moved upwardly for it to lie in a position in which the stop 20 can pass beneath the blocking means, at which time the blocking means is positioned between the starting portion 22a of the snail cam and the stop 30.

In this way it is ensured that the snail cam 29, and thus the shaft 13 is only permitted to make one complete revolution before the rotary motion is interrupted, the time delay of the interruption being determined by the time taken to move the blocking means 24 fromthe position it is left in by the terminal portion 22b to the position it occupies when clear of the stop 30 and adjacent the starting portion 22a.

This time delay is dependent on the rate of return of the piston of the dashpot 29, which is preferably constructed and operates in the following manner.

Referring now to FiGURE 2, the dashpot 29 is comprised by a cylindrical container 32 which is partially filled with a liquid and positioned within the container is a piston 34 which is formed of a material having a specific weight which is less than that of the liquid.

Conveniently, a liquid such as mercury may be used as the dashpot liquid, in which case the piston may be formed of a metal which is insusceptible to amalgamation with the mercury. Alternatively, the piston may be formed of cork or of a foamed plastics material provided with a liquid impervious skin, in whichcase a liquid such as ethylene glycol may be used. Mercury and ethylene glycol are similar in the respects that they bothhave a low change in viscosity due to temperature change, are insusceptible to evaporation, and are not subject to oxidation to any material extent. V

The piston 34 is formed. as a hollow cup-shaped them her having a horizontalbottom wall 34a and a peripheral side Wall 3412 which extends upwardly of the wall 34a and-which is spaced from the juxtaposed side wall of the container 32, the wall 34a being provided with a perforation 35 of a determined size.

The difference in specific weights of the piston 34 and the liquid 33 is arranged such that the piston 34 will float on the liquid and will support the weight of the piston rod and that of the lever 25 and blocking means 24, normally to maintain the blocking means in or near contact with the starting portion 22a of the camming surface.

The cylinder 32 is provided with a closure cap 32a having a tubular member 32b which acts as a guide for the piston rod 28 and which additionally dispenses with the need to provide a seal between the cap 32a and the piston rod to prevent leakage of the fluid from the dashpot in the event that the device is inverted. To accomplish this, the tubular member 32b is formed of such a length that when the dashpot is inverted the depth of the liquid overlying the cap 32a and surrounding the member 32b is of less depth than the height of the tubular member.

The operation of the dashpot is as follows:

As the snail cam 20 is rotated and the blocking means 24 is depressed by the carnming surface, the lever 25 is pivoted downwardly about its pivot 26 to depress the piston rod 28, and thus to force the piston 34 downwardly into the liquid until it is submerged in the liquid.

As the piston 34 is forced downwardly into the liquid a certain amount of liquid will pass through the perforation 35, but the majority of the liquid will be displaced upwardly to flow between the peripheral side wall 34b of the piston and the wall of the container 32, and will flow over the upper edge of the wall 34b to flood the cavity in the piston defined by the bottom wall 34a and side wall 34b.

As the terminal portion 22b of the camming surface passes over the blocking means 24, the blocking means is freed to rise, and thus the piston 34 is freed to rise in the liquid 33. Due to the difference in specific weights of the dashpot liquid and the material from which the piston 34 is formed, when the piston 34 is so freed it will rise quickly in the liquid 33 until such time as the upper edge of the wall 3% is aligned with the upper surface of the liquid in the container, and the piston will continue to rise upwardly until such time as the effective weight of the piston plus the weight of the liquid trapped in its cavity is equal to the weight of the liquid displaced by the piston.

. This results in a relatively fast initial upward travel of the piston 3d with the result that the blocking means 24 is moved quickly to a small extent towards the starting portion 22a of the camming surface for the blocking means to iie in the path of the terminal portion 22b, thus preventing any possible reverse rotation of the snail cam and of the shaft 13 and the roll of towelling.

As soon as the upper edge of the side wall 341) rises above the level of the liquid in the container, the weight of the body of liquid trapped within the cavity of the piston is added to the weight of the piston assembly and acts to increase the effective specific weight of the piston, and thus to reduce the apparent difference in specific weights of the piston and the liquid. The piston then assumes a position in the liquid such that the weight of liquid displaced-by the piston is equal to the effective weight of the piston, i.e., the combined weight of the piston assembly and the liquid trapped in its cavity.

fHowever, at this time the level of'the liquid trapped within the cavity 34c is above the level of the liquid in the container, and the liquid in the cavity commencest-o flow through the perforation 35 at a'rate controlled by the viscosity of the liquid and the frictional drag imposed on the flow by the perforation 35.

As the liquid in the cavity flows through the perforation 35 the effective weight of the piston is progressively decreased, and thus the effectivedifierence between the apparent specific weight of the piston and that of the liquid in the container is progressively increased, thus causing the piston torise'in'the iiquid iu'the cylinder in a controlled manner and at a determined rate. dependent on the rate of flow of liquid through the perforation 35.

It will thus be seen that the rate of ascent of the piston can be controlled by forming the perforation 35 to be of a determined size for the particular liquid employed. It will also be noted that the dashpot includes no valves or moving part other than the piston, and includes no springs or similar devices which can age or become defective.

As will be observed, the rotation interrupting mechanism is axially narrow, and may conveniently be positioned within the confined space between the side wall 11 and the juxtaposed side wall of the cover 16 without materially increasing the overall Width of the device over that necessary to accommodate the roll of paper towelling.

It will be appreciated that various modifications of the structure shown in the drawing may be made without departing from the scope of the invention defined in the appended claims, and that the construction hereinbefore described is to be taken by way of example only of one of the many forms which the mechanism according to the invention may take. For example, it may be found convenient to locate the cam 20 in a position in which its axis of rotation is other than aligned with the axis of the shaft 13, in which case any convenient form of gear or friction drive may be employed between the shaft and cam for transmitting rotary motion of the shaft to the snail cam.

What I claim as my invention is:

1. In a despensing device for a ribbon of sheet material, a mechanism for metering and interrupting the withdrawal of said sheet material from'a supply of said material, the mechanism including a shaft driven by the sheet material as it is withdrawn from the dispenser, a snail cam journalled for rotation about its axis and arranged in driven relationship with the driven shaft, blocking means actuated by a camm ing surface of the snail cam, said camming surface including a starting portion and a terminal portion spaced radially of the starting portion, a stop carried by the snail cam and for coaction with the bolcking means, the stop being spaced circumferentially of said terminal portion, in a direction measured opposite to the direction of rotation of the cam, to an extent permitting the blocking means to be passed between said terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extent of the blocking means measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be rotated when the blocking means is adjacent said starting portion, the stop coacting with the blocking means to prevent rotation of the snail cam until such time as the blocking means is adjacnt said starting portion and clear of said stop, and a one-way acting dashpot acting on the blocking means, the dashpot permitting the blocking means to be moved in a substantially unconstrained manner by the camming surface as the snail cam is rotated, and, after the blocking means has left said terminal portion, acting to return the blocking means towards said starting portion and clear of said stop in a controlled maner and at a determined rate.

2. In a dispensing device for a ribbon of sheet material, a mechanism for metering and interrupting the withdrawal of said sheet material from a supply of said material, the mechanism including a shaft driven by the sheet material as it is withdrawn from the dispenser, a snail cam mounted on the driven shaft for rotation in unison therewith, a blocking memberactuated by a camming surface of the snail cam, said camming surface including a starting portion and a terminal portion spaced radially of the'starta ing portion, a stop carried by the snail cam and for coaction with the blocking member, the stop being spaced circumferentially of said terminal portion, in a direction measured opposite to the direction of rotation of the cam, to an extent which is greater than the extent of the blocking member measured in a circumferential direction, thus permitting the blocking member to be moved radially between said terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extent of the blocking member measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be rotated when the blocking member is adjacent said starting portion, the stop coacting with the blocking member to prevent rotation of the snail cam until such time as the blocking member is adjacent said starting portion and clear of said stop, and a one-way acting dashpot acting on the blocking member, the dashpot permitting the blocking member to be moved in a substantially unconstrained manner by the camming surface as the snail cam is rotated, and, after the blocking member has left said terminal portion, acting to return the blocking member towards said starting portion and clear of said stop in a controlled manner and at a determined rate.

3. In a dispensing device for a ribbon of sheet material, a mechanism for metering and interrupting the withdrawal of said sheet material from a supply of sadi material, the mechanism including a shaft driven by the sheet material as it is Withdrawn from the dispenser, a snail cam journalled for rotation about its axis and arranged indriven relationship with the driven shaft, a blocking member actuated by a camming surface of said snail cam and mov-. able substantially in a vertical plane, said camming surface including a starting portion and a terminal portion spaced radially of the starting portion, a stop carried by the snail cam and for coaction with the blocking member, the stop being spaced circumferentially of the terminal portion, in a direction measured opposite to the direction of rotation of the cam, to an extent which is greater than the extent of the blocking member measured in a circumferential direction, thus permitting the blocking member to be moved radially between said terminal portion and the stop, and being spaced radially outward of the starting portion to an extent which is greater than the extent of the blocking member measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be rotated when the blocking member is adjacent said starting portion, the stop coacting with the blocking member to prevent rotation of the snail cam until such time as the blocking member is adjacent said starting portion and clear of said stop, a lever pivotally suported from a stationary structural member of the mechanism, the lever supporting the blocking member, and a one-way acting dashpot acting on the lever, the dashpot permitting the blocking member to be moved in a substantially uncon: strained manner by the camming surface as the snail cam is rotated, and, after the blocking member has left said terminal portion, acting to return the blocking member towards said starting portion and clear of said stop in a controlled manner and at a determined rate.

4. In a dispensing device for a ribbon of sheet material, a mechanism for metering and interrupting the withdrawal of said sheet material from a supply of said material, the mechanism including a shaft driven by the sheet material as it is withdrawn from the dispenser, a snail cam journalled for rotation about its axis and arranged in driven relationship with the driven shaft, blocking means actuated by .a camming surface of the snail cam, said camming surface including a starting portion and a terminal portion spaced radially of the starting portion, a stop carried by the snail cam and for coaction with the blocking means, the stop being spaced circumferentially of said terminal portion to an extent permitting the blocking means to be passed between said terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extentiof the block ing means measured in a direction radially of the axis of rotation of the snail earn to permit the snail cam to be rotated when the blocking means is adjacent said starting portion, the stop coacting with the blocking means to prevent rotation of the snail cam until such time as the blocking means is adjacent said starting portion and clear 7 of said stop, and a one-way acting dashpot associated with the blocking means, the dashpot comprising a container for a liquid, a quantity of liquid within and partly filling the container, 3. piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a side Wall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at an upper surface of the piston to permit the liquid to flow freely into the cavity when the piston is submerged and having a perforation located at a position below said upper surface and which communicates with the cavity, operating means associated with the piston and the blocking means and operable by the blocking means to move the piston to submerge it in said liquid, and a guide restricting movement of the piston to movement in a substantially vertical plane, the apparent weight of the piston being sufficiently less than the Weight of liquid displaced by the piston when the piston is submerged in said liquid, so that the piston will rise in the liquid when permitted to do so by the blocking means, said dashpot permitting the blocking means to be moved in a substantially unconstrained manner by the camming surface as the snail cam is rotated, and, after the blocking means has left said terminal portion, acting to return the blocking means towards said starting portion and clear of said stop in a controlled manner and with a determined time delay.

5. in a dispensing device for a ribbon of sheet material, a mechanism for metering and interrupting the withrdrawal of said sheet material from a supply of said material, the mechanism including a shaft driven by the sheet material as it is withdrawn from the dispenser, a snail cam mounted on the driven shaft for rotation in unison therewith, a blocking member actuated by a camming surface of the snail cam, said camming surface including a starting portion and a terminal portion spaced radially of the starting portion, a stop carried by the snail cam and for coaction with the blocking member, the stop being spaced circumferentially of said terminal portion to an extent permitting the blocking member to be passed between said terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extent of the blocking member measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be rotated when the blocking member is adjacent said starting portion, the stop coacting with the blocking member to prevent rotation of the snail cam until such time as the blocking member is adjacent said starting portion and clear of said stop, a lever pivtotally supported from a stationary structural member of themechanism, the lever supporting the blocking member, and a one-way acting dashpot associated with the lever, said dashpot comprising a container for a liquid, a quantity of liquid within and partially filling the container, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a side wall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at an upper surface of the piston to permit the liquid to flow freely into the cavity when the piston is submerged and having a perforation located at aposition below said upper sur face and Which communicates with the cavity, a piston rod connected to the piston and in contact with the lever for moving the piston to submerge it in said liquid, and a guide restricting movement of the piston to a substantially vertical plane, the apparent weight of the piston being less .than the weight of liquid displaced by said piston when said piston is completely submerged in said liquid so that the piston will rise in the liquid when permitted to do so by the blocking means, the dashpot permitting the blocking member to be moved in a substantially unconstrained manner by the camming surface as the snail cam is rotated, and, after the blocking member has, left said terminal portion, acting to' return the blocking member towards said starting portion and clear of said stop in a controlled manner and at a determined rate.

6. A mechanism for preventing continuous rotation of a driven shaft, comprising a snail cam journalled for rotation about its axis and arranged in driven relationship with the driven shaft, blocking means for coaction with a camming surface of the snail cam, said ca-mming surface including a starting portion and a terminal portion spaced radially of the starting portion, a stop carried by the snail cam and for coaction with the blocking means, the stop being spaced circumferentially of said terminal portion to an extent permitting the blocking means to be passed between said terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extent of the blocking means measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be rotated when the blocking means is adjacent said starting portion, the stop coacting with the blocking means to prevent rotation of the snail cam until such time as the blocking means is adjacent said starting portion and clear of said stop, ane a one-way acting dashpot associated with the blocking means, the dashpot com-prising a container for a liquid, a quantity of liquid within and partly filling the container, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a side wall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at an upper surface of the piston to permit the liquid to flow freely into the cavity when the piston is submerged and having a perforation located at a position below said upper surface and which communicates with the cavity, operating means associated with the pison and the blocking means and operable by the blocking means to move the piston to submerge it in said liquid, and a guide restricting movement of the piston to movement in a substantially vertical plane, the apparent weight of the piston being sufiiciently less than the weight of liquid displaced by the piston when the piston is submerged in said liquid so that the piston will rise in the liquid when permitted to do so by the blocking means, said dashpot permitting the blocking means to be moved in a substantially unconstrained manner by the camming surface as the snail cam is rotated, and, after the blocking means has left said terminal portion, acting to return the blocking means towards said starting portion and clear of said stop in a controlled manner and with a determined time delay,

7. A mechanism for interrupting continuous rotation of a driven shaft, comprising a snail cam mounted on the driven shaft for rotation in unison therewith, a blocking member actuated by a cumming surface of the snail cam, said camming surface including a starting portion and a terminal portion spaced radially of the starting portion, a stop carriedby the snail cam and for coaction with the blocking member, the stop being spaced circumferentially of said terminal portion to an extent permitting the blocking member to be passed between said terminal portion and the stop, and being spaced radially of the starting portion to an extent which is greater than the extent of the blocking member measured in a direction radially of the axis of rotation of the snail cam to permit the snail cam to be' rotated when the blocking member is adjacent said starting portion, the stop coacting with the blocking member to prevent rotation of the snail cam I until such time as the blocking member is adjacent said starting portion and clear of said stop, a lever pivotally supported from a stationary structural member of the mechanism, the lever supporting the blocking member, and a one-way acting dashpot associated with the lever, said dashpot comprising a container for a liquid, at quantity of liquid within and partially filling the container, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a side wall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at an upper surface of the piston to permit the liquid to flow freely into the cavity when the piston is submerged and having a perforation located at a position below said upper surface and which communicates with the cavity, a piston rod connected to the piston and operable by the lever for moving the piston to submerge it in said liquid, and a tubular guide surrounding the piston rod and restricting movement of the piston to movement in a substantially vertical plane, the apparent weight of the piston being less than the weight of liquid displaced by said piston when said piston is completely submerged in said liquid so that the piston will rise in the liquid when permitted to do so by the bloclc'ng means, the dashpot permitting the blocking member to be moved in a substantially unconstrained manner by the camming surface as the snail cam is rotated, and, after the blocking member has left said terminal portion, acting to return the blocking member towards said starting portion and clear of said stop in a controlled manner and at a determined rate.

8. A mechanism as claimed in claim 1 wherein the dashpot comprises a container for liquid, a quantity of liquid within and partially filling the container, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a sidewall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at the upper surface of the piston and having a perforation located at a position below said upper surface and which communicates with the cavity, a piston rod acting on the piston, the upper end of the piston rod acting on the blocking means, and a guide restricting movement of the piston rod to movement in a substantially vertical plane.

9. In a dispensing device for a ribbon of sheet material, a mechanism for metering and interrupting the withdrawal of said sheet material from a supply of said material, the mechanism including a shaft driven by the sheet material as it is withdrawn from the dispenser, a snail cam journalled for rotation about its axis and arranged in driven relationship with the driven shaft, said cam having a camming surface which includes a starting portion and a terminal portion spaced a determined distance radially of the starting portion, a stop carried by the cam, blocking means supported independently of the cam and including a first part actuated by the camming surface, the blocking means also including a second part fast with the first part and for engagement with the stop, the first part of the blocking means being movable radially between the starting and terminal portions of the camming surface, the second part of the blocking means extending from the first part into engagement with the stop when the first part is in a position substantially in line circumferentially with the starting portion of the camming surface and is clear circumferentially of the terminal portion of the camming surface and is radially in line with said terminal portion, the second part thus preventing rotation of the cam, the radial extent of the contacting surface between the stop and the second part of the blocking means being less 10 than the radial distance between the starting and terminal portions of the camming surface, so that when the first part of the blocking means moves from the terminal portion to the starting portion of the camming surface the second part moves out of engagement with the stop to allow rotation of the cam, and a one-way acting dashpot acting on the blocking means, the dashpot permitting the first part to be moved in a substantially unconstrained manner radially from the starting portion to the terminal portion as the camming surface is rotated, and after the first part has left the terminal portion and the second part is in engagement with the stop, acting to return the first member towards the starting portion and the second member clear of the stop in a controlled manner and at a predetermined rate.

10.A mechanism as claimed in claim 9 wherein the dashpot comprises a container for liquid, a quantity of liquid within and partially filling the container, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston having a sidewall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at the upper surface of the piston and having a perforation located at a position below said upper surface and which communicates with the cavity, a piston rod acting on the piston, the upper end of the piston rod acting on the blocking means, and a guide restricting movement of the piston rod to movement in a substan tially vertical plane.

11. A mechanism as claimed in claim 9 wherein the dashpot comprises a container for liquid, a quantity of liquid within and partially filling the container, a piston within the container and arranged to move up and down with its longitudinal axis substantially vertical, the piston being of less specific weight than the liquid and having a side wall which is spaced from a juxtaposed side wall of the container, the piston having an internal cavity which is open at an upper surface of the piston to permit the liquid to flow freely into the cavity when the piston is submerged and having a perforation located at a position below said upper surface and which communicates with the cavity, operating means associated with the blocking means, the operating means acting on the piston and being operable by the blocking means for forcing the piston downwardly into the liquid, and a guide restricting movement of the piston to movement in a substantially vertical plane, the combined weight of the piston, operating means, and blocking means being less than the weight of liquid displaced by the piston when the piston is submerged in the liquid, whereby the piston and operating means and blocking means will be displaced upwardly by the liquid when such movement is permitted by the blocking means.

References Cited in the file of this patent UNITED STATES PATENTS 998,289 Hall July 18, 1911 1,016,270 Johnson Feb. 6, 1912 2,067,760 Harvey Jan. 12, 1937, 2,135,767 Price et a1. Nov. 8, 1938 2,202,183 Wooster May 28, 1940 2,277,051 West Mar. 24, 1942