GB1581042A

GB1581042A - Deceleration of high-inertia bodies

Info

Publication number: GB1581042A
Application number: GB3332476A
Authority: GB
Original assignee: Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1977-08-01
Filing date: 1977-08-01
Publication date: 1980-12-10

Description

(54) IMPROVEMENTS RELATING TO THE DECELERATION OF HIOH-INERA BODIES (71) We, METAL BOX LIMITED of Queens House, Forbury Road, Reading RG1 3JH, a British company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a protective device for use in association with a movable flexible body having high inertia, the device (hereinafter called "a device of the kind hereinbefore specified") being such as to control the rate of deceleration of said body when the body, being in motion, is subjected to a sudden arresting force; to a said flexible body in combination with a said device; and to ovens for curing or drying coatings on articles, such oven including a conveyor and a said device associated therewith.

The problem of bringing to rest a massive flexible moving body, when some point of the body is subjected to a sudden decelerating force, is a vital one in the case of, for example, chain conveyors, particuiarly when the conveyor is carrying a plurality of articles. Forces causing sudden deceleration can take many different forms; for example, if a bearing seizes up, or an element of a chain conveyor breaks, or an article carried by the conveyor becomes jammed between the conveyor and some associated equipment or the conveyor frame. In general the problems in question will tend to arise when some part of the flexible body (such as a chain conveyor) is subjected to a sudden restraining force. In the case of a conveyor or other flexible body being stopped because of failure of its power supply, there will not normally be any problem in decelerating it because it is usually posible to allow deceleration to occur naturally, the body being allowed to continue moving freely until stopped by friction or other forces. If, however, the body has a sufficiently high inertia, and if frictional, gravitational or windage forces are also significant, there will be a liklihood, even on normal stopping, for the body to come to rest too rapidly and/or with substantial impact. If the body is made - up of individual components, or if, as with a conveyor, it is carrying loose articles, the much smaller inertia of these articles or components may cause them to separate from the body.

It is well known -to provide protective devices for producing controlled decelera tion of a solid body subjected to sudden arresting impacts. Such devices are adapted to absorb energy rapidly from the body, and are typified by the automatic friction brakes and hydraulic cushions used for arresting falling lifts or elevators. Such devices are however unsuitable when the body has very high inertia, is itself flexible and, as in the case of a long and heavy chain conveyor, is also of considerable length. Here the need is to permit the whole length of the body to come to rest as smoothly as possible but not too rapidly and without appreciable change taking place in its -tension; and thus even a device merely producing controlled deceleration of one part of the body will not satisfy this requirement, because it will tend to reduce tension in one portion of the body whilst correspondingly increasing it in another. Such an arrangement therefore gives rise to a risk of breaking the flexible body.

Variations in tension in a high-inertia flexible body during deceleration can also lead to vibration problems; and in the case of a chain conveyor carrying a large number of articles, the latter thereby run the risk of being shaken off or damaged, whilst the conveyor itself may become damaged by such vibration, or subjected to sudden impact and fracture due to being thrown off its sprockets.

There is therefore a need for a simple and reliable protective device, which can preferably be arranged at one point along the path of a high-inertia flexible movable body, and which is capable of controlling the rate of deceleration of the said flexible body when the latter, being in motion, is subjected to a sudden arresting force. As will be seen from: the foregoing, it is necessary for such a device to operate without any significant change occurring in the tension of-the said flexible body.

According to the invention, in a first aspect, a device of hte kind hereinhefore specified comprises: an enclosed air cylinder; a piston movable axially in the cylinder; a body-engaging element freely rotatable about its own axis for rolling engagement with an elongate flexible element of a said high-inertia flexible body, so as to permit substantially equal tension in the portions of said flexible element at either ide of the body-engaging element, said axis being movable with said piston; air supply means for supplying the cylinder with air at a predetermined pressure on the side of the piston proximal to the body-engaging element; and orifice means, adapted to permit air to escape therethrough at sonic velocity and arranged to be connected with said cylinder on said proximal side of the piston in response to movement of the piston such as to increase the cylinder air pressure to a value above said predetermined pressure, whereby the velocity of said piston movement is limited to a constant maximum value governed by exhaust of air through said orifice means at sonic velocity.

According to the invention, in a second aspect, there are provided, in combination, a high-inertia flexible body including an elongate flexible element, and a device according to said first aspect of the invention mounted at a fixed location, said flexible body being arranged to move past said location with said elongate flexible element engaging part-way around the body-engaging element of said device, whereby an increase in tension in said flexible element will cause the piston to undergo said movement to increase cylinder air pressure, and whereby said movement causes the effect tive length of said flexible element to increase at a constant rate.

In the case of a flexible body such as a chain conveyor, a chain of the conveyor passes around the said body-engaging element which will typically be a sprocket, so that if some part of the conveyor is sud denly arrested, an increase in tension transmitted along the chain to the sprocket increases the force balancing the air pressure in the cylinder, and therefore moves the - piston in a direction such that two events occur simultaneously. The first is that the~sprocket is moved so as to increase the effective length of the chain, thus tending to restore the original tension in the chain. The second of the two events is that the piston movement exhausts air from the cylinder at sonic velocity..

At sonic velocity the exhaust orifice is choked; and because of the movement of the sprocket to relieve the tension increase in the chain, this tension, and therefore the load applied to the piston by the chain tends to remain substantially constant.

Under these circumtances, and provided the chain tension is at least equal to a predetermined value which can be calculated for any particular application, the air pressure in the clylinder remains constant; and, more importantly, the rate at which the piston moves is constant, irrespective of the value of either the air pressure in the cylinder or the tension in the chain.

By virtue of this last-mentioned feature, viz. a piston adapted to move at substantially constant velocity, it will be seen that the invention provides a simple device for limiting and controlling deceleration of the chain or other movable flexible body, since, although initial movement df the piston is effected by a change in the load applied thereto by the chain, subsequent movement of the piston, whereby the chain is paid out to enable it to come to rest steadily, is controlled by the limiting condition of the choked exhaust orifice.

It will be realised that a protective de-.

vice according to the invention need only be provided at one point along the length of the flexible body to be controlled, since it is responsive to forces transmitted in either direction along the flexible body to the device from the point at which the flexible body has been interrupted, which may or may not be the point at which driving forces are normally transmitted to the flexible body.

It is also important to note that the deceleration imposed by a protective device according to the invention is a steady one and can still be quite rapid, the constant factor being the rate of increase in effective length of the chain or other flexible body. Thus, even if one point on the flexible body is arrested suddenly, for example bv sudden failure of the power supply, or seizure of a bearing on a shaft carrying part of the flexible body, the protective device can be made such as to enable the latter to come to rest over its whole length smoothly but not so quickly as to set up undue impact forces.

The invention is susceptible of a wide variety of applications. Examples include haulage cables for hauling vehicles up and down'inclines, where the inertia of the cable can be said - to- include that of the vehicles; or cable or belt conveyors. A particular example of an embodiment, in which an endless conveyor is employed, is an oven for curing or drying coatings, for example paint, lacquer or print applied to articles which are slowly moved through the oven on the conveyor. In particular.

some ovens of this kind involve a very long conveyor, carrying loose and relatively light articles which could become easily dislodged (with consequent danger or damage or further damage to the conveyor or oven) in the event of too sudden a deceleration of the conveyor.

Accordingly, the invention, in a third aspect, provides an oven for curing or drying coatings on a plurality of articles, wherein said oven includes a combination, within the scope of the said second aspect of the invention, of an endless chain conveyor, whereof a chain constitutes the said elongate flexible element, and a said device mounted at a fixed location at one end of the oven.

Typical examples of such ovens are those in which the conveyor is of serpentine configuration, for instance a peg oven for curing coatings on metal can bodies, the can bodies being loosely carried on upstanding or projecting pegs which in turn are carried by the conveyor which extends in a large number of succesive vertical runs through the oven.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: -- Figure 1 is a diagrammatic sectional elevation of a peg oven having a protective device according to the invention; Figure 2 is an enlarged, simplified elevation showing part of one end of .the same oven with the protective device; and Figure 3 is a diagram showing in simplified form an electro-pneumatic control system of or for said protective device.

A printing and coating machine, not shown in the drawings, for printing and lacquering metal can bodies, delivers the can bodies with their lacquer coatings in a wet condition to an oven feeder, also not shown. The feeder delivers the can bodies to the input or right-hand end 11 (Figure l) of a peg oven 10, in which the coatings on the can bodies are cured. The peg oven is of the generally-conventional kind having a chain conveyor 12 which extends in serpentine manner ion a succession of vertical runs 13, alternately up and down.

In operation, the chain conveyor 12 is driven through the oven 10 in the direction indicated by arrows in Figure 1, by a motor (not shown). The conveyor is maintained in continuous motion. At the delivery end 14 of the oven, the can bodies, with their laequer coatings now fully cured by heat to which they are exposed in the oven, are delivered for washing, inspection and packing.

The chain conveyor 12 is of the endless kind, having a return run 15. It comprises support chains l6, Figure 2, which extend over a large number of sprockets, such as the sprockets 17 in Figure 1, in the oven 10. Each can is supported internally but losely on one of a large number of pegs (not shown) projecting from and-spaced along the conveyor 12.

The chain conveyor 12 is, by virtue of its configuration, extremely long, and has a very high mass and therefore a high inertia. Should any- part of the chain conveyor be stopped suddenly, for example by seizure of a bearing on the shaft of one of the sprockets 17, or failure of the power supply or by breakage of some component of the conveyor or of any other part of the oven, the inertia of the conveyor, combined with friction at the many sprockets and between other relatively moving components, will tend to cause a rapid rise in tension in the chains 16, leading almost immediately to chain fracture. The consequences of this can be disastrous. For one thing, all the can bodies being carried by the chain will be rendered useless; it will be realised that under these circumstances mot or all of them will be shaken off their pegs and will fall into parts of the oven where they may foul and damage the mechanisms and where, with their inflammable coatings, there is a serious risk of the cans causing a fire.

Even if the can - bodies do not cause serious damage to the oven directly or indirectly; howeyer, the tension in the conveyor chains is such that if one of the chains should fracture in tension, the damage that may be done to the oven as a consequence is quite likely to wreck it.

Adjacent to the inlet end 11 of the peg oven, and attached to the outside of the oven housing 18, is a bracket 19 carrying a number of sprockets around which there extends a loop of the conveyor 12. The bracket 19 also carries the principal parts of a protective device 20, which is provided at this fixed location for controlling the rate of deceleration of the chain conveyor 12 in the event that the latter, when in motion, is subjected suddenly to an arresting force in such a manner as to tend to cause - an increase in tension in the conveyor chains for example as discussed above.

The components carried by the bracket 19 are shown more' clearly in Figure 2.

Referring therefore to that Figure, it will be seen that the bracket 19 carries two coplanar sprockets 22 and 23 spaced apart in the line of the return run 15 of one of the conveyor chains 16. The chain 16 extends therefrom over further sprockets 24 (Figure 1) and 25, to the station of the oven at which the can bodies are received by the conveyor 12.

The protective device 20 comprises a single-acting ram 26 in the form of an enclosed air cylinder 27, anchored by a pivot 28 at its rear end to the oven frame 29.

The ram 26 is shown diagrammatically in Figure 3, and includes a piston 30, slidable axially within the cylinder 27 and having, pivoted to the outer end of its piston rod 31, a rigid carrying arm 32. The arm 32 is pivoted near its free end to the piston rod 31, and at its other end, by means of a pivot 33 defining a fixed axis, to the bracket 19. The free end of the arm 32 carries a body-engaging element in the form of a sprocket wheel 34 freely rotatable about its own axis for rolling engagement with the chain 16, which - extends from the sprocket 22 to the- sprocket 23 via this sprocket 34, extending as shown part-way around the latter.

The piston 30 is normally in the retracted position indicated in Figure 3, in which condition the arm 32 is in the normal or working position shown in full lines in Figure 2.

in the electro-pneumatic circuit shown diagrammatically in Figure 3, the pneu mastic circuit is indicated by double lines and the electric circuit by single lines. Air under pressure is supplied by a compressor (not shown) to an inlet 35 and thence to the cylinder 27 via a non-return valve 36, a combined air filter and pressure regulator 37, a three-part double solenoid operated valve 38, an air line 42, and a quickexhaust valve 39. The quick-exhaust valve 39 has one branch 40 connected to the cylinder 27 at the outer or free end of the latter, in front of the piston 30. The valve 39 also has an exhaust orifice nozzle 41 of a predetermined size, and is of a known kind arranged normally to hold the air line 42 in communication with the cylinder 27. in the event of a rise in pressure in the branch 40, however, the valve 39 quickly vents to atmosphere through the nozzle 41, thus restoring the pressure in the branch 40. The valve 39 includes a nonreturn element 43 at its inlet side.

The piston rod 31 is arranged to operate a trip switch 44 (not shown in Figure 2).

The trip switch 44 is of the two-gang type, having a first pair and a second pair of contacts 45, 46 respectively; these are of the normally-open and normally-closed kind respectively. The normally-closed contacts 46 are connected in the emergency stop circuit of the printing and coating machine mentioned above. The normallyopen switch contacts 45 are connected in the power supply circuit of a trip solenoid 47 and a reset solenoid 48 of the solenoid valve 38. The solenoids 47 and 48 are electrically connected in parallel with each other and with a chain overload warning light and/or audible device 49, and are operable selectively by a key-type selector switch 50.

In normal operation, with the piston 30 in its normal or retracted position as shown, it will be seen from Figure 2 that the chain 16 defines a loop 51 around the sprocket 34 of the protective device 20. In this condition the trip switch 44 is held down with the contacts 45 open and the contacts 46 closed. Also in this condition, air, under constant pressure P, is supplied to the front of the piston 30 (i.e. on the side of the piston proximal to the sprocket 34) from the inlet 35 via the circuit shown in Figure 3.

The chain 16, guided by the sprockets 22, 34 and 23, passes in the direction shown by the arrows in Figure 1 at a substantially constant chain tension Tc (Figure 3) which is substantially equal in both arms of the loop 51. So long as this tension remains contant, it applies a constant axial load to the piston 30, balancing the constant air presure P in the cylinder 27 and so maintaining the piston in equilibrium in its retracted position.

If the power supply to the motor driving the conveyor 12 fails, or the conveyor is subjected to a sudden arresting force for any other reason, as discussed above, the chain tension To will tend to increase, so that the piston 30 begins to rise. This has three effects. Firstly, it releases the trip switch 44, and this opens the switch contacts 46 and stops the operation of the printing and coating machine, so halting the supply of further can bodies to the oven 10. Secondly, release of the trip switch 44 closes the cantacts 45 of the latter, so energising the warning device 49. However, the most important effect is that as the piston 30 rises, the sprocket 34 and its arm 32 are moved towards their position shown in chain-dotted lines in Figure 2. This reduces the length of the loop 51 of the chain 16 and thus increases the effective length of the conveyor 12. The effect of this is to reduce the chain tension Tc SO that this tension tends to be returned to its normal value. The action of the protective device 20, as will shortly be seen, is in practice quick enough for the chain tension to be maintained at substantially its normal value throughout the process of sudden deceleration. In maintaining this tension without allowing any significant increase therein, the protective device permits the conveyor 12 to decelerate and come smoothly to rest at a controlled rate with the minimum of jarring or vibration.

This control is achieved by virtue of the fact that the rate at which the piston 30 moves, and therefore the rate at which the chain loop 51 decreases in length, is substantially constant, as will now be explained.

As the piston 30 rises, the resulting increase in presure P in front of the piston, when it reaches a predetermined value slightly above the normal or equilibrium pressure, causes the quick-action valve 39 to exhaust to atmosphere the air displaced from the cylinder 27 by the piston 30. The size of the nozzle 41 is so chosen that the velocity of the air so exhausted therethrough is that of sound' under normal ambient conditions (Mach 1). In this condition the nozzzle 41 is choked and, as is well known, the velocity at which the air can escape through the nozzle cannot then increase above this value. However, due to this escape of air the piston 30 continues to rise at a constant linear velocity governed by the constant flow rate of air through the nozzle 41, so maintaining the chain tension Tc and the cylinder air pressure P substantially constant. The loop 51 is de- creased in length at one-half of the linear velocity of the piston 30.

Movement of the piston 30 continues until it reaches the end of its travel, with the arm 32 in the position shown by chaindotted lines in F gure 2. Since the linear velocity of the piston is governed only by the size of the nozzle 41, the latter is so chosen that the chain 16 will come to rest when, or just before, the piston completes its stroke. The valve 39 closes automatically when the air pressure falls at this time.

To restore the system to normal working, the solenoid valve 38 (which will have operated to vent the air from the inlet 35 to atmosphere upon operation of the quickexhaust valve 39 to exhaust the air from the cylinder 27 to atmosphere) is reset by operation of the key selector switch 50.

This energises the reset solenoid 48 through a pair of electrical contacts 52.

The other position of the key switch 50 operates the solenoid 47 through a pair of electrical contacts 53, to provide a manually operated test facility for the protective device 20 by cutting off the supply of air under pressure to the cylinder 27 and so cause the latter to vent to atmosphere through the nozzle 41.

WHAT WE CLAIM IS: 1. A device of the kind hereinbefore specified, comprising: an enclosed air cylinder; a piston movable axially in the cylinder; a body-engaging element freely rotatable about its own axis for rolling engagement with an elongate flexible element of a said high-inertia flexible body, so as to permit substantially equal tension in the portions of said flexible element at either side of the body-engaging element, said axis being movable with said piston; air supply means for supplying the cylinder with air at a predetermined pressure on the side of the piston proximal to the body-engaging element; and orifice means, adapted to permit air to escape therethrough at sonic velocity and arranged to be connected with said cylinder on said proximal side of the piston in response to movement of the piston such as to increase the cylinder air pressure to a value above said predetermined pressure, whereby the velocity of said piston movement is limited to a constant maximum value governed by exhaust of air through said orifice means at sonic velocity.

2. A device according to Claim 1, wherein said body-engaging element is a sprocket wheel.

3. A device according to Claim 1 or Claim 2, wherein said piston has a piston rod, the said body-engaging element being mounted on the piston rod.

4. A device according to Claim 3, wherein the said body-engaging element is mounted on the piston rod by means of a carrying arm arranged to be pivoted on a fixed axis, the body-engaging element being carried by, and the piston rod coupled to, said carrying arm at a portion thereof remote from said fixed axis.

5. A device of the kind hereinbefore specified, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, Figure 3 of the accompanying drawings.

6. A device according to Claims 4 and 5, constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, Figure 2 of the accompanying drawings.

7. In combination, a high-inertia flexible body including an elongate flexible element, and a device according to any one of the preceding claims mounted at a fixed location, said flexible body being arranged to move past said location with said elongate flexible element engaging part-way around the body-engaging element of said device, whereby an increase in tension in said flexible element will cause the piston to undergo said movement to increase cylinder air pressure, and whereby said movement causes the effective length of said flexible element to increase at a constant rate.

8. A combination according to Claim 7, wherein said elongate flexible element is a chain.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. therein, the protective device permits the conveyor 12 to decelerate and come smoothly to rest at a controlled rate with the minimum of jarring or vibration. This control is achieved by virtue of the fact that the rate at which the piston 30 moves, and therefore the rate at which the chain loop 51 decreases in length, is substantially constant, as will now be explained. As the piston 30 rises, the resulting increase in presure P in front of the piston, when it reaches a predetermined value slightly above the normal or equilibrium pressure, causes the quick-action valve 39 to exhaust to atmosphere the air displaced from the cylinder 27 by the piston 30. The size of the nozzle 41 is so chosen that the velocity of the air so exhausted therethrough is that of sound' under normal ambient conditions (Mach 1). In this condition the nozzzle 41 is choked and, as is well known, the velocity at which the air can escape through the nozzle cannot then increase above this value. However, due to this escape of air the piston 30 continues to rise at a constant linear velocity governed by the constant flow rate of air through the nozzle 41, so maintaining the chain tension Tc and the cylinder air pressure P substantially constant. The loop 51 is de- creased in length at one-half of the linear velocity of the piston 30. Movement of the piston 30 continues until it reaches the end of its travel, with the arm 32 in the position shown by chaindotted lines in F gure 2. Since the linear velocity of the piston is governed only by the size of the nozzle 41, the latter is so chosen that the chain 16 will come to rest when, or just before, the piston completes its stroke. The valve 39 closes automatically when the air pressure falls at this time. To restore the system to normal working, the solenoid valve 38 (which will have operated to vent the air from the inlet 35 to atmosphere upon operation of the quickexhaust valve 39 to exhaust the air from the cylinder 27 to atmosphere) is reset by operation of the key selector switch 50. This energises the reset solenoid 48 through a pair of electrical contacts 52. The other position of the key switch 50 operates the solenoid 47 through a pair of electrical contacts 53, to provide a manually operated test facility for the protective device 20 by cutting off the supply of air under pressure to the cylinder 27 and so cause the latter to vent to atmosphere through the nozzle 41. WHAT WE CLAIM IS:

1. A device of the kind hereinbefore specified, comprising: an enclosed air cylinder; a piston movable axially in the cylinder; a body-engaging element freely rotatable about its own axis for rolling engagement with an elongate flexible element of a said high-inertia flexible body, so as to permit substantially equal tension in the portions of said flexible element at either side of the body-engaging element, said axis being movable with said piston; air supply means for supplying the cylinder with air at a predetermined pressure on the side of the piston proximal to the body-engaging element; and orifice means, adapted to permit air to escape therethrough at sonic velocity and arranged to be connected with said cylinder on said proximal side of the piston in response to movement of the piston such as to increase the cylinder air pressure to a value above said predetermined pressure, whereby the velocity of said piston movement is limited to a constant maximum value governed by exhaust of air through said orifice means at sonic velocity.

9. A combination according to Claim 7

or Claim 8, wherein said high-inertia flexible body is a conveyor.

10. In combination, an endless chain conveyor and a device according to Claim 6.

arranged at a fixed location with respect to the path of the conveyor, substantially as hereinbefore described with reference to, and as illustrated in, Figure 2 of the accompanying drawings.

11. An oven for curing or drying coatings on a plurality of articles, wherein said oven includes a combination, according to Claim 10, of an endless chain conveyor, whereof a chain constitutes the said elongate flexible element, and a said device mounted at a fixed location at one end of the oven.

12. An oven according to Claim 11, being a peg oven adapted for curing or drying coatings on metal containers.

13. A peg oven, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

14. A coated article whose coating has been cured or dried in an oven according to any one of Claims 11 to 13.

15. A coated metal container whose coating has been cured or dried in a peg oven according to Claim 12 or Claim 13.