GB2059557A

GB2059557A - Tunnel furnaces

Info

Publication number: GB2059557A
Application number: GB7933367A
Authority: GB
Original assignee: Chloride Silent Power Ltd
Current assignee: Chloride Silent Power Ltd
Priority date: 1979-09-26
Filing date: 1979-09-26
Publication date: 1981-04-23
Also published as: GB2059557B

Abstract

An improved method of and apparatus for feeding ceramic articles into and through a pass-through sintering furnace utilises a pusher 26 (Figure 2) which is driven forward at uniform speed by a lead screw 23 driven by a motor 30 via a reduction gear 31 and clutch 32 and which pushes a freely rotatable pusher element 24 pushing a cylindrical ceramic element 22 supported on parallel rollers 20, 21 through the furnace. The rollers 20, 21 are driven to rotate the ceramic element 22 as it moves through the furnace. Operation of microswitch 60 at the end of the forward stroke causes pusher 26 to be withdrawn at high speed by driving the lead screw 23 in reverse direction by motor 40 via two-position clutch 41. Just before the end of the return stroke a microswitch 63 operates clutch 41 to connect the lead screw 23 to be driven more slowly via a reduction belt drive 50, 51. <IMAGE>

Description

SPECIFICATION Pass-through sintering furnace This invention relates to pass-through sintering furnaces and is concerned more particularly with a method of the apparatus for feeding ceramic articles through such furnaces.

Pass-through sintering furnaces are used for firing ceramic articles and find particular application where the duration of firing and the firing conditions must be carefully controlled.

Such furnaces are used for example in the manufacture of beta-alumina ceramic articles for use as a solid electrolyte in electrochemical cells and energy conversion devices. As is described in British Patent Specifications Nos. 1297373, 1375167,1458221 and 1458222, beta alumina ceramic articles, such as tubes, may be formed as a green shape and sintered by passing them rapidly through a furnace having a sintering zone which may be short compared with the length of each article. The article may be passed through such a furnace relatively rapidly so that the sintering time is typically less than 2 minutes. If the length of the sintering zone is less than the length of the article, it is important that the article, if it is to be uniformly treated over its whole length, should be moved at a substantially uniform speed through the sintering zone of the furnace.

For tubular articles, the furnace preferably has a rotatable firing tube through which the articles pass, these articles being moved longitudinally through this tube but allowed to roll as the furnace tube rotates; this rotation of the furnace tube gives improved uniformity of sintering conditions and improved straightness of the articles being fired.

With such a technique, the articles cannot be carried through the furnace on a conveyor system but they have to be pushed into and through the furnace. To give uniform firing, a succession of articles, possibly separated with spacers, can be passed through the furnace but since these articles are pushed through the furnace, they must be fed into the furnace at a uniform speed.

Typically, beta-alumina ceramic tubes, such as are used in sodium sulphur cells as a solid electrolyte, might be fed into and through the furnace at 80 mm per minute and it is undesirable that a tube undergoing sintering should be at rest for more than about 6 seconds. If each tube is to be pushed by the following tube, then the pushing is necessarily interrupted each time a new green tube has to be put into the line of tubes. This requires that the successive tubes are fed into the furnace in such a manner that the line of tubes being pushed into the furnace has the new green shape for firing introduced into the line and the pushing of the tubes actually in the furnace resumed with an interval of less than 6 seconds.

The green shapes are fragile and have to be handled carefully. Heretofore the practice has been to put each green shape manually onto a moving conveyor system in contact with the preceding shape or in contact with a separating spacer so that the conveyor system carries the line of tubes forwardly to move off the conveyor system and into the furnace in a continuous stream. Such a technique can be used for small batches of tubes such as are made for experimental purposes but is not satisfactory for continuous quantity production.

It is an object of the present invention to enable a succession of articles to be fed through a pass through sintering furnace with each article pushing the preceding article either directly or via a spacer and in which any interruption of the movement to enable a further article to be introduced into the feed device does not exceed a predetermined time duration.

According to one aspect of this invention, a method of feeding ceramic articles into and through a pass-through sintering furnace comprises moving a pusher forwardly at a uniform speed to push a succession of articles or of articles and spacers into and through the furnace, rapidly withdrawing the pusher a sufficient distance to introduce a fresh article or article and spacer at the rear end of said line and then moving the pusher forwardly again at said uniform speed to push the newly-introduced article into engagement with the preceding articles or articles and spacers and to continue to move the line forwardly.The rearward movement of the pusher is made only just sufficient to introduce the fresh article (or article and spacer) into the line with the necessary tolerances between the article and the pusher and the preceding article so that the forward movement of the pusher and the freshlyintroduced article causes the line of articles in the furnace to resume movement with the minimum of further delay.

Typically, for firing beta-alumina tubes for use as solid electrolyte material, the rearward movement of the pusher may be 200 times or more faster than the forward movement.

Means may be provided for introducing the fresh article between the pusher and line of articles automatically in synchronism with the rearward movement of the pusher.

In the sintering of tubular articles, and in which the furnace tube is rotated, means may be provided for rotating the tubular articles in said line before they enter the furnace. This may be done for example by pushing said tubular articles along a guideway having rollers frictionally engaging the tubular articles, the rollers being rotated to turn said articles at the required speed of rotation in the furnace or by feeding the articles through a first rotating tube before they enter the furnace.

The invention furthermore includes within its scope apparatus for feeding a line of ceramic articles or ceramic articles and spacers through a pass-through sintering furnace comprising a pusher for pushing a line of articles or of articles and spacers into and through the furnace, means for driving the pusher forwardly at a uniform speed, means operative, when the pusher has reached a predetermined limit at the forward end of its movement to retract the pusher rearwardly at a much higher speed than the forward movement and then to move forwardly again at the aforementioned predetermined speed. The return speed is preferably at least 200 times as fast as the forward speed. Because of the fast return speed, means may be provided for slowing down the return motion just before the limit of the return travel is reached at which limit a brake is applied.

Conveniently the pusher movement is effected by means of a lead screw. Forward and return drives with separate clutches may be provided together with means for automatically engaging one clutch and disengaging the other at the limits of travel. These clutches may be electrically operated and controlled by means of microswitches operated by the pusher or by an element moving with the pusher.

The forward drive conveniently is a variable speed electric drive through a reduction gear. The return drive may also be an electric drive and, as explained above, provision may be made for reducing the speed just as the pusher is reaching the final limit of its movement. For this purpose the return drive may include speed-reducing means, e.g. a reduction gear or belt drive, together with clutch means operative to couple the speedreducing means into the drive to the lead screw.

These clutch means may be constituted by the same clutch as is employed for disengaging the return drive, the clutch having a disengaged position together with two alternative engaged positions, one for the fast return and the other for the slower return speed just before the final limit of return movement.

As previously explained, the invention finds particular application in feeding tubular articles into a sintering furnace having a rotatable furnace tube. In this case, preferably the articles are rotated as they enter into the furnace. Provision may be made therefore for rotating each article fed into the furnace and in this case preferably the pusher includes a free-running pushing element for engaging the line of tubular articles. Preferably means are provided for rotating the pushing element with said tubular articles. The articles may be rotated by resting on parallel spaced drive rollers arranged parallel to the axis of the tubular articles entering the furnace. In this case the freerunning element on the pusher conveniently also frictionally engages the drive elements.

The invention furthermore includes within its scope the combination of a pass-through sintering furnace with means for feeding beta-alumina ceramic tubes into the furnace as described above.

Furthermore the invention includes a method of sintering beta-alumina ceramic tubes by firing in a pass-through furnace into which the tubes are fed as described above.

The invention also includes within its,scope a beta-alumina ceramic tube sintered by this method.

In the following description, reference will be made to the accompanying drawings in which Figure 1 is a diagrammatic illustration of an induction furnace for sintering beta-alumina ceramic electrolyte tubes; Figure 2 is an exploded view showing in further detail part of the support means for a tube being fed into the furnace, and the drive means for pushing the tube into and through the furnace; and Figure 3 is a diagrammatic plan view showing the drive for a lead screw in the feed mechanism of Figure 3.

Referring to Figure 1 there is shown diagrammatically an induction furnace for zone sintering of beta-alumina ceramic tubes which are passed through the furnace. This furnace comprises an insulated furnace box 10 with a graphite susceptor block 11. An induction coil 14 energised from a radio frequency source 1 5 is arranged around the susceptor block 11. Passing through the susceptor block 11 and furnace box 10 is a furnace tube 12, typically formed of recrystallised alumina, which furnace tube is continuously rotated at uniform speed by drive rollers 1 3. Typically a rotation speed of 1 5 to 60 r.p.m. is used. At the entry end of the furnace there is an entry tube 1 7 with a flared outer end to form a guide tube.The tube 17 in this embodiment is a short tube and is fixed. it may however be rotated, particularly if short articles are to be passed through the furnace. The furnace tube 12 slopes upwardly at an angle of 730 from the entry end to the exit end to give a convective air flow through the furnace towards the exit end where an outlet tube 1 8 is rotated by rollers 1 9 so that the sintered tubes continue to rotate as they leave the furnace tube 12. The green ceramic tubes to be sintered are fed into the entry tube 17 and furnace tube 12 by sliding along a pair of rollers 20, 21 (See Figure 2) which have their axes parallel and parallel to the axis of a tube to be fired, such as tube 22 which rests on the rollers 2G, 21.The-two rollers 20, 21 are driven at the same uniform speed in the same direction and, by frictional engagement with the tube 22, they rotate this tube 22. The rollers 20, 21 are driven so that their peripheral speed is the same as the required peripheral speed of the tube 22 as it passes through the rotating furnace tube 12. The tubes such as tube 22 are pushed along the rollers 20, 21 by means of a cylindrical pusher element 24 (Figure 1) having elastomeric O-rings 25 engaging the rollers 20, 21. In Figure 2, the pusher 26 and pusher element 24 are, for clarity, shown spaced away from the end of the tube. This pusher element 24 is mounted for free rotation about its axis on a linearly movable pusher 26 so that the cylindrical element 24, by engagement with the rollers 20, 21 is rotated at the same speed as the tube 22. The pusher 26 is moved linearly by means of a lead screw 28 which is driven so that the tubes are pushed through the furnace at a uniform linear speed of typically 80 mm/min.

Periodically the pusher 26 is withdrawn rapidly for a distance which is one or two mm greater than the length of the tube to be sintered, a fresh green tube is inserted between the pusher 26 and tube 22 previously being pushed and the slow forward drive at 80 mm/min. is resumed. Typically it might be required that the tubes in the furnace should not be left at rest for more than say 6 secs.

Thus the withdrawal of the pusher and the reengagement after insertion of a fresh green tube has to take place within a time period less than 6 secs. Because the withdrawal movement is only one or two mm longer than the length of the tube, the pusher 26 in a matter of a second or so engages the end of the newly inserted tube and forces it forwardly to bear against the previously pushed lines of tubes so that the forward movement of the tubes through the furnace is rapidly resumed.

Figure 3 illustrates the drive mechanism for the lead screw to achieve this required motion.

Referring to that figure, the lead screw is shown at 23 with the pusher 28 shown diagrammatically.

Forward movement of the lead screw is effected by means of a drive motor 30 with a reduction gear and variable speed drive 31 driving the lead screw shaft via an electromagnetically operated clutch 32 and a belt drive 33 giving a further speed reduction.

The fast return motion of the lead screw is effected by a drive motor 40 whicn, via an electromagnetic clutch 41, directly drives an output shaft 42 at a high speed. This output shaft is coupled to the lead screw via a belt drive 44 giving a small speed reduction. In this particular example the motors are arranged to give a forward drive speed of 80 mm/min. and a return drive speed of 500 mm/sec.

Because of the very high return drive speed, provision is made for reducing the return drive speed to about 50 mm/sec. just before the end of the return travel. For this purpose, the motor 40 via two speed reduction belt drives 50, 51 drives a hub 52 surrounding the output shaft 42. The aforementioned clutch 41 is a two-position clutch having a neutral mid position in which the output shaft 42 is totally disengaged. In the direct drive position for the fast return, an output shaft 45 from the motor 40 is coupled by the clutch 41 directly to the output shaft 42. For the slower return speed, the clutch 41 couples the output shaft 42 to the aforementioned hub 52 instead of to the motor shaft 45 so that the output shaft 42 is driven at the reduced speed of the hub 52.

When the fast return speed is engaged, the hub rotates freely at its slow speed without affecting the drive operation whereas when the slow return speed is required, the hub 52 drives the output shaft 42 independently of the input to the clutch from the motor 40.

Thus for the fast return speed, there is direct drive from the motor to the output shaft via the clutch whilst for the slow return speed, the drive from the motor to the output shaft is via the two belt drives.

The two clutches 32, 41 may be operated automatically by means of microswitches engaged by the pusher 28 or by an element driven by the lead screw 23. This is illustrated diagrammatically in Figure 3 by a first microswitch 60 at the forward end of the travel of the pusher 28 or other element on the lead screw 23. Operation of this microswitch disengages the clutch 32 for the forward drive and engages the clutch 41 for the fast return drive. Just before the end of the return motion, a microswitch 63 changes over the clutch 41 to reduce the speed of the return drive from fast return to slow return. This occurs just before a third microswitch 64 is operated to disengage the return drive, by putting clutch 41 in its neutral position, and applying a brake to stop the motion accurately, and also automatically to operate the clutch 32 to engage the forward drive.

It will be seen that the drive mechanism operates automatically to give a slow forward movement to the pusher and a fast return with slow-down at the end of the return travel just before the interruption of the reverse drive and restarting of the forward movement. The cyclic operation is continuous and, by adjustment of the position of the microswitches 60, 63, 64, the extent of reverse movement can be predetermined. The microswitches 63, 64 can also provide the required signals for automatically controlling a delivery mechanism for releasing a green tube onto the rollers 20, 21 just as the pusher reaches the end of its return movement.

Claims

1. A method of feeding ceramic articles into and through a pass-through sintering furnace comprising moving a pusher forwardly at a uniform speed to push a succession of articles or of articles and spacers into and through the furnace, rapidly withdrawing the pusher a sufficient distance to introduce a fresh article or article and spacer at the rear end of said line and then moving the pusher forwardly again at said uniform speed to push the newly-introduced article into engagement with the preceding articles or articles and spacers and to continue to move the line forwardly.

2. A method as claimed in claim 1 wherein the rearward movement of the pusher is made only just sufficient to introduce the fresh article into the line with the necessary tolerances between the article and the pusher and the preceding article so that the forward movement of the pusher and the freshly-introduced article causes the line of articles in the furnace to resume movement with the minimum of further delay.

3. A method as claimed in either claim 1 or claim 2 wherein the rearward movement of the pusher is 200 times or more faster than the forward movement.

4. A method as claimed in any of the preceding claims wherein the fresh article is introduced between the pusher and lines of articles automatically in synchronism with the rearward movement of the pusher.

5. A method as claimed in any of the preceding claims and for feeding tubular articles into a furnace tube wherein the tubular articles are rotated in said line before they enter the furnace,

6. A method as claimed in claim 5 wherein said tubular articles are pushed along a guideway having rollers frictionally engaging the tubular articles, the rollers being rotated to turn said articles at the required speed of rotation in the furnace.

7. A method as claimed in claim 5 wherein said tubular articles are fed through a first rotating tube before they enter the furnace.

8. Apparatus for feeding a line of ceramic articles or ceramic articles and spacers through a pass-through sintering furnace comprising a pusher for pushing a line of articles or of articles and spacers into and through the furnace, means for driving the pusher forwardly at a uniform speed, means operative, when the pusher has reached a predetermined limit at the forward end of its movement to retract the pusher rearwardly at a much higher speed than the forward movement and then to move forwardly again at the aforementioned predetermined speed.

9. Apparatus as claimed in claim 8 wherein the return speed is at least 200 times as fast as the forward speed.

10. Apparatus as claimed in either claim 8 or claim 9 wherein means are provided for slowing down the return motion just before limit of the return travel is reached.

11. Apparatus as claimed in any of claims 8 to 10 wherein the pusher movement is effected by means of a lead screw.

12. Apparatus as claimed in claim 11 wherein forward and return drives with separate clutches are provided together with means for automatically engaging one clutch and disengaging the other at the limits of travel.

1 3. Apparatus as claimed in claim 1 2 wherein the clutches are electrically operated and controlled by means of microswitches operated by the pusher or by an element moving with the pusher.

14. Apparatus as claimed in any of claims 8 to 13 wherein the forward drive is an electric drive through a reduction gear and wherein the return drive is also an electric drive and wherein provision is made for reducing the speed just as the pusher is reaching the final limit of its movement.

1 5. Apparatus as claimed in claim 14 wherein the return drive includes speed-reducing means together with clutch means operative to couple the speed-reducing means into the drive to the lead screw.

16. Apparatus as claimed in claim 1 5 as appendant to claim 12 wherein said clutch means is constituted by the same clutch as is employed for disengaging the return drive, the clutch having a disengaged position together with two alternative engaged positions, one for the fast return and the other for the slower return speed just before the final limit of return movement.

1 7. Apparatus as claimed in any of claims 8 to 16 and for feeding tubular articles into a sintedng furnace having a rotatable furnace tube wherein means are provided for rotating each article as it enters into the furnace.

18. Apparatus as claimed in claim 1 7 wherein the pusher includes a free-running pushing element for engaging the line of tubular articles.

1 9. Apparatus as claimed in claim 1 8 wherein means are provided for rotating the pushing element with the tubular articles.

20. Apparatus as claimed in claim 1 9 and having parallel spaced drive rollers arranged parallel to the axis of the tubular articles entering the furnace and on which the articles rest to be driven thereby.

21. A method of feeding beta-alumina ceramic tubes into and through a pass-through sintering furnace substantially as hereinbefore described with reference to the accompanying drawings.

22. Apparatus for feeding beta-alumina ceramic tubes into and through a pass-through sintering furnace substantially as hereinbefore described with reference to the accompanying drawings.

23. The combination of apparatus as claimed in any of claims 8 to 20 or 22 with a pass-through sintering furnace.

24. A method of sintering beta-alumina ceramic tubes by firing in a pass-through sintering furnace into which the tubes are fed into the furnace by the method of any of claims 1 to 7 or claim 21.

25. A beta-alumina ceramic tube sintered by the method of claim 24.