GB2041179A - Roller hearth furnace - Google Patents

Abstract

In a roller hearth furnace having a high temperature furnace chamber, the rollers 22 of the roller hearth for supporting and transporting workpieces to be heated in the chamber are each supported at each end by a rotary sleeve 36 having an inner diameter larger than the outer diameter of the roller 22. Rotary movement of the sleeves 36 causes the rollers to rotate by means of frictional engagement where the rollers are supported in the sleeves. The oversized sleeves provide axial and radial clearance between the rollers and the sleeves to allow for roller dimensional variation during various furnace operating conditions. <IMAGE>

Description

SPECIFICATION Roller hearth furnace This invention relates to high temperature furnaces and particularly to such furnaces having rollers which form a hearth for supporting and transporting through the furnace workpieces to be treated.

One of the limiting factors in high temperature furnace operation is the temperature level which the components inside the heated furnace chamber can withstand. Typically, furnaces are mesh belts, alloy rollers, pushers, or other similar devices to support and transport workpieces through the furnace, but these devices are unable to withstand the desired high temperature leveis associated with some processes.

For example, in sintering processes associated with the metallurgical techniques used in the manufacture of certain metal parts, a temperature level above that which conventional metal alloy furnace rollers can withstand is desirable. In a common technique, powdered metal is compressed into a desired shape and then sintered to fuse the powder into a unitary piece without melting it. Generally, the higher the temperature to which the workpieces can be brought without melting, the better will be their strength characteristics and overall quality. Such workpieces frequently withstand higher temperature levels without melting than the furnace transport mechanism can withstand without melting or otherwise deteriorating to such an extent that its operation is impaired.

With the aim of providing a furnace with a roller hearth which is capable of withstanding constantly high furnace temperatures, according to the invention a roller hearth furnace has a roller hearth comprising at least one cylindrical roller which is supported at each end by a retaining sleeve having an open end into which the end of the roller projects, said open end of each sleeve having an internal cylindrical surface loosely surrounding the corresponding end of the roller and forming a slip joint therewith, and drive means for rotating each sleeve and attached to the end of the sleeve opposite its open end into which the roller projects, the arrangement being such that frictional engagement between the ends of the roller and the internal surfaces of the sleeves which loosely surround and support the ends of the roller causes the roller to rotate when the sleeves are rotated.

Usually the roller is one of a plurality of such rollers which form the hearth in the furnace chamber and which are all supported in a similar manner to each other transversely of the direction of travel of work along the hearth. With this arrangement a roller hearth can be provided having sufficient strength to support a substantial weight and in which the rollers are supported in such a way as to compensate easily for roller deformation during operation of the furnace, such as thermal expansion and uneven circularity which may develop at high temperatures.

Preferably the open end of each roller supporting sleeve defines a cylindrical cavity having a diameter larger than the outer diameter of the roller end which is received and supported in the open end, and the open ends of the sleeves at opposite ends of each roller include means for permitting limited travel of the roller along its longitudinal axis, the open ends of the sleeves thereby permitting thermal expansion of the roller in all directions. Preferably the means for permitting limited travel of the roller along its longitudinal axis comprises a pair of abutments confronting the opposite ends of the roller and spaced apart by a distance slightly in excess of the maximum length of the roller.

For high temperature furnace operation, such as that associated with powder metallurgy, the rollers need to be made of a refractory material to withstand the desired temperatures inside the furnace. The rollers may be constructed of ceramic or other suitable refractory materials. Ceramic rollers may be formed of numerous materials, including, but not limited to, aluminium oxide, silicon carbide, zirconium oxide, and these compounds forming a base with other constituents. The roller and sleeve construction used in this invention avoids difficulties associated with a rigid ceramic-metal joint under high temperature and mechanical stress conditions.

Preferably the roller supporting sleeves are drivingly connected to each other so that all of the sleeves are rotated simultaneously, continuously, and in unison by the drive means. The resulting continuous rotation of the rollers tends to minimize uneven heat distribution throughout the rollers thereby minimizing nonsymmetrical temperature induced deformation such as roller sag. In the case of-a high temperature furnace the roller driver mechanism is operated continuously so that the constantly rotating rollers will experience uniform heat distribution throughout.

One example of a roller hearth furnace in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a partially sectioned top view of the furnace; Figure 2 is a sectional side view of the furnace taken along the line 2-2 in Fig. 1; Figure 3 is a partially sectioned and broken away view of one of the rollers of the furnace, together with its support and driving means; Figure 4 is a top plan view illustrating a number of rollers of the type shown in Fig. 3 supporting a workpiece thereon; and Figure 5 is a sectional view taken on the line 5-5 in Fig. 4.

Figs. 1 and 2 illustrate a high temperature sintering furnace for sintering compressed powdered metal parts to fuse the powdered metal into a unitary piece without melting it.

The furnace comprises a primary heating chamber 10, a preheating chamber 12, a cooling chamber 14, and a holding chamber 1 6. The chamber 1 2 is provided with heating elements 15, and the chamber 10 is provided with heating elements 17 and 1 9. The heating elements may be of any suitable type, for example electrically powered silicon carbide heating elements. The walls 1 8 of the furnace are of a suitable, relatively thick, refractory material through which the rollers 20, 22 and 24 extend for connection to an external driving means 34, which will be described later.

The primary heating chamber 10 is separated from the preheating chamber 1 2 by projecting wall sections 26 and 28, and is separated from the cooling chamber 1 4 by a heat shield door 30 which opens and closes a port 32.

The door 30 is reciprocally mounted in a slot 31 and is operated by a lifting mechanism 29.

The rollers 20 and 24 are conventional metal alloy rollers, but may be of any convenient type, and are located in the preheating chamber 12 and the cooling chamber 14 respectively. The rollers 22 are made of refractory material and are located in the primary furnace chamber 10. The rollers are arranged to be rotated in unison by the roller drive means 34, which has the capability of continuously driving all of the rollers 20, 22, and 24 during operation of the furnace.

The rollers 22 and the drive means 34 will now be described in detail. The rollers 22 can best be understood by reference to Figs. 3, 4 and 5. In Fig. 3 there is illustrated a cylindrical roller 22 associated at each end with a supporting and driving sleeve 36 having an open end defining a cylindrical cavity which receives the end of the roller and which has an inner diameter larger than the outer diameter of the end of the roller. The sleeve 36 forms with the roller 22 a slip joint. The slip joint provides for movement of the roller relative to the sleeve in both the axial and the radial directions relative to the roller. This provides a junction between the roller and its supporting and driving mechanism which has the flexibility required to compensate for expansion, contraction and deformation of the roller during operation of the furnace.Extending from a closed end of the sleeve 36, remote from the open end which receives the roller, is a drive shaft 40 having thereon journals 42 and 44. As the sleeves 36 at opposite ends of the roller are rotated by their drive shafts 40, they impart to the roller 22 the desired rotary motion. As will be understood from reference to Figs. 3 and 5, the sleeves 36 drive the roller 22 through frictional engagement between the inwardly facing annular surface 46 of each sleeve 36 and the outer circumference of each end portion 48 of the roller 22. These end portions 48 function as mounting sections for the roller 22. Techniques to improve the frictional engagement can be used, although in practice these have not been found essential.

Within each sleeve 36 an abutment 38 is formed for limiting travel of the roller 22 in the direction of its longitudinal axis. The drive shaft 40 mounts the sleeve 36 so that the abutment 38 confronts the terminal end 50 of the roller 22, and so that the abutments 38 of opposed sleeves are displaced from each other by a distance which is slightly in excess of the length of the roller 22. In this way the slip joints formed between the sleeves 36 and the roller 22 allow the requisite freedom of movement and compensate properly for expansion, contraction and deformation of the roller. The gap between the terminal ends 50 of the roller and the abutments 38 can vary according to the design of the system. However, the space separating the pair of abutments 38 should be at least slightly in excess of the length of the roller 22 at the maximum anticipated operating temperature.Insulating material may be used between the shafts 40 and the terminal ends 50 of the roller 22 to reduce heat losses through the slip joints.

As tolerance between the mounting end sections 48 of a roller and the surrounding surfaces 46 of the support sleeves increases, the ability to compensate for roller expansion, contraction and deformation increases and the driving force between the sleeves 36 and the roller decreases. Correspondingly, as the tolerances between the sections 48 and the surfaces 46 decrease, the driving force increases and the ability to compensate for expansion, contraction and deformation decreases.

Fig. 4 illustrates an array of rollers 22 driven by an associated array of sleeves 36 and forming a hearth for supporting and transporting either a workpiece or a tray on which several workpieces may be placed.

Opposed side walls 51 of the primary furnace chamber 10 are provided with cylindrical bores 52 each of sufficiently large diameter to receive the outer diameter of a cylindrical sleeve 36 but small enough to provide a minimum tolerance between the sleeve 36 and the inner diameter of the bore 52 to reduce heat loss from the furnace chamber.

The rollers 22 extend across the chamber 10, their ends projecting into the sleeves which are mounted in the bores 52 in the side walls 51. The retaining sleeves 36 are supported within the bores 52 by their journals 42 and 44 respectively. The fact that the rollers 22 stay entirely within the furnace chamber and the furnace walls lovers the thermal gradient along the rollers and thereby increases their resistance to high temperature.

As mentioned earlier, the drive means 34 is arranged to drive the rollers in all three fur nace chambers 10, 1 2 and 14. The rollers in the furnace chambers 1 2 and 14 are conventional rollers and are driven from one end only, but the refractory rollers 22 in the primary chamber 10 are driven from both ends. The rollers 20 are provided with drive shafts 58 at one end and are supported at the other end by bores 60 in the furnace wall.

Similarly, the rollers 24 are driven at one end by shafts 62 and are supported at the opposite end by bores 64. The drive shafts 58, 62 and 40 are each provided with two sprockets, an outer sprocket 66 and an inner sprocket 68. In addition, the roller, which is of conventional construction, at the junction between the preheating chamber 1 2 and the primary chamber 10 has primary drive sprockets 70 mounted on its drive shafts (the roller having a drive shaft at each end) outwardly of the bearings 56. The sprockets 70 form the primary drive for the entire system of rollers, and the roller with which the sprockets 70 are associated will be termed the driving roller.

Each drive shaft is interconnected with the adjacent drive shaft or shafts on the same side of the furnace by a series of sprocket-driving chains 72. Each roller is thus drivingly connected to each adjacent roller. For example, in a series of rollers a first roller and a second roller may be connected by a chain 72 drivingly engaging their respective sprockets 68; the second roller and a third roller may be connected by a chain 72 drivingly engaging their sprockets 66; the third roller and a fourth roller may be connected by a chain 72 drivingly engaging their sprockets 68, etc. in alternating fashion. The driving roller extends across the furnace to drive both ends of the rollers 2, and one end of the rollers 20.The rollers 20 are driven by connection between the first roller 24 in the cooling chamber 14 and the last roller 22 in the primary chamber 1 0. All of the sprockets are sized to produce uniform rotation of all of the rollers. However, if certain rollers are to rotate faster than others, the sprocket design may be adjusted to accomplish this.

To facilitate loading and unloading of the furnace, various work moving devices are provided. Adjacent the preheating chamber 1 2 an elevator 74 is associated at its upper position with a pusher 76. The elevator lifts a work supporting tray 78 to the level of the hearth formed by the rollers 20, and the pusher 76 slides the work supporting tray 78 from its position on the elevator onto the rollers 20. A pushacross 80 associated with the cooling chamber 14 is arranged to slide the work supporting tray 78 from a position on the rollers 24 into the holding chamber 16, from which it is ejected from the furnace by a pushoff 82.

The drive sprockets 70 are powered by a drive motor 71 adapted to provide relatively slow rotational movement to the rollers. The motor 71 may be a variable drive motor to variably determine the transit time of work in the primary heating chamber 1 0. Typically, this time would be variable between 10 and 30 minutes. The rotary motion of the rollers advances a tray 78 from the preheating chamber 12, through the primary heating chamber 10, and then to the cooling chamber 1 4.

If it is desired to maintain the work in either of the chambers 1 2 and 14 for a longer period of time than would be required for the material to travel therethrough on the roller hearth, its passage is interrupted to provide the necessary residence time. For this purpose magnetic clutches 84 and 86 are provided for selectively terminating rotation of the rollers 20 and 24 respectively while permitting continuous rotation of the rollers 22 in the primary heating chamber 1 0.

In the preferred example illustrated and described herein, the preheat chamber 1 2 is maintained at approximately 982"C and the primary heating chamber at approximately 1370"C. When a tray with powdered metal workpieces thereon is to be retained in the preheating chamber 12, the operator releases the magnetic clutch 84 so that the sprocket 66 associated therewith is disengaged from the associated shaft 58 and the rollers 20 do not turn.When sufficient residence time in the preheating chamber 1 2 is accomplished, the clutch 84 is re-engaged and the tray of work is transmitted to rotating rollers 22 in the primary heating chamber 1 0. Upon nearing the end of the chamber 10, a radiamatic pyrometer means 88 senses the presence of the tray and causes the lift mechanism 29 to raise the door 30 so that the tray may be fed onto the rollers 24 in the cooling chamber 14. The door 30 is thereafter closed automatically. If the tray is to be retained in the cooling chamber 14, sensing means 90 associated with the cooling chamber provides a signal in response to which the clutch 86 disengages its associated sprocket 66 from the associated shaft 62. The rollers 24 are then stationary for an interval sufficient to permit the work to cool.After sufficient cooling time is provided, the pushacross 80 discharges the work from the rollers 24 to the holding chamber 16. The clutch 86 may be re-engaged to drive the rollers 24 upon discharge of the work, or, if desired, these rollers may be re-engaged only in response to a signal given when the door 30 is re-opened.

When the primary chamber 10 is operated at temperatures of 1370 C and above, it is desirable for the refractory rollers 22 to rotate continuously to provide even heat distribution therethrough. At this high temperature, conventional roller materials tend to fail and, under some circumstances, the refractory rollers will exhibit sag if they are subjected to uneven heat distribution.

The rollers 22 may be manufactured from any suitable refractory material. Ceramic materials are preferred and in this example high purity aluminium oxide rollers have been used with considerable success. Alternatives include rollers of aluminium oxide mixed with various compounds such as silicon oxide and magnesium oxide. Generally, greater strength is obtained with a very high purity aluminium oxide. A product sold under the trade name Mullite by McDanel Refractory Porcelain Company, 510 Ninth Avenue, Beaver Falls, Pennsylvania, U.S.A. is one commercially available material which can be used. The rollers might also be formed of a material having as its base silicon carbide or zirconium oxide. The sleeves 36 may be made of any appropriate material which resists oxidation and provides the necessary strength at the desired operating temperature. Nickel-iron alloys are satisfactory.When the furnace chamber 10 shown in Fig. 2 operates at 1370"C the sleeves 36 are insulated by the furnace walls so that they experience a temperature in the region of 1038 C. The aluminium oxide rollers could withstand a temperature of up to 1760"C. At this temperature other things being the same, the sleeves would experience a temperature higher than 1038"C. Generally, the sleeves may be protected from heat by providing insulation, by cooling, and/or by locating the sleeves further away from the high temperature zone.

Rollers of high purity aluminium oxide, 3.00 inches (7.62 cms) in outer diameter, 2.50 inches (6.35 cms) in inner diameter and 3.75 feet (1.143 metres) long supported in sleeves of 3.04 inches (7.72 cms) internal diameter which are spaced approximately 8.00 inches (20.32 cms) apart will support a tray load of approximately 30 Ibs/sq. ft (146.5 kgs/sq.m.) at a furnace temperature of 1315"C. In a similar arrangement under similar conditions an array of rollers 4.00 inches (10.16 cms) outer diameter, 3.50 inches (8.89 cms) inner diameter and 3.00 feet (0.914 metres) long could support approximately 125 Ibs/sq.ft. (612.6 kgms/sq.m.).

Claims (11)

1. A roller hearth furnace in which the roller hearth comprises at least one cylindrical roller which is supported at each end by a retaining sleeve having an open end into which the end of the roller projects, said open end of each sleeve having an internal cylindrical surface loosely surrounding the corresponding end of the roller and forming a slip joint therewith, and drive means for rotating each sleeve and attached to the end of the sleeve opposite its open end into which the roller projects, the arrangement being such that frictional engagement between the ends of the roller and the internal surfaces of the sleeves which loosely surround and support the ends of the roller causes the roller to rotate when the sleeves are rotated.

2. A furnace according to claim 1, which is operable at temperatures of at least 1370"C and the roller is made of refractory material.

3. A roller hearth furnace according to claim 2, in which the roller is made of a ceramic material which is resistant to a tem- perature of at least 1370 C and is one of a plurality of such rollers which are all supported in a similar manner to each other and which are arranged transverse to the direction of travel of work along the roller hearth, and the drive means for each sleeve comprises a shaft extending from and drivably connected to the end of the sleeve opposite its open end, a journal on the shaft, bearing means mounting the journal for rotary movement, transmission means connected to the shaft and drivably connecting together the shafts of all the sleeves so that the shafts, and hence the sleeves all rotate in unison, and means for continuously driving the transmission means.

4. A furnace according to claim 3, in which the open end of each sleeve defines a cylindrical cavity having an internal diameter larger than the outer diameter of the roller end which is received and supported in the open end, and the open end includes an abutment confronting the end of the roller, the abutments of the sleeves at opposite ends of each roller being spaced apart by a distance slightly in excess of the maximum length of the roller, and the open ends of the sleeves thereby permitting thermal expansion of the rollers in all directions.

5. A furnace according to claim 1, in which the open end of each sleeve defines a cylindrical cavity having a diameter larger than the outer diameter of the roller end which is received and supported in the open end, and the open ends of the sleeves at opposite ends of the roller include means for permitting limited travel of the roller along its longitudinal axis, the open ends of the sleeves thereby permitting thermal expansion of the roller in all directions.

6. A furnace according to claim 5, in which the means for permitting limited travel of the roller along its longitudinal axis comprises a pair of abutments confronting the opposite ends of the roller and spaced apart by a distance slightly in excess of the maximum length of the roller.

7. A roller hearth furnace according to claim 1 or claim 2, in which there is a primary heating chamber for heat-treating workpieces which are carried through the chamber by the roller hearth, the chamber having a pair of opposed walls of refractory material, and means for furnishing heat to the primary heating chamber, and in which the roller is made of heat resistant ceramic material and is one of a plurality of such rollers which are all supported in a similar manner to each other and which extend across the chamber to define the hearth within the chamber, the sleeves which receive and support the ends of the rollers being mounted in cylindrical bores extending through the opposed refractory walls of the chamber, and the sleeves being drivingly connected together so that all of the sleeves are rotated in unison about their longitudinal axes by the drive means.

8. A furnace according to claim 7, in which the open end of each sleeve defines a cylindrical cavity having an internal diameter larger than the outer diameter of the roller end which is received and supported in the open end, and the open end includes an abutment confronting the end of the roller, the abutments of the sleeves at opposite ends of each roller being spaced apart by a distance slightly in excess of the maximum length of the roller for permitting limited travel of the roller along its longitudinal axis, and the open ends of the sleeves thereby permitting thermal expansion of the rollers in all directions.

9. A furnace according to claim 7 or claim 8, in which the ceramic rollers are made of aluminium oxide, and the primary heating chamber is capable of sintering workpieces of powdered metal at temperatures of at least 1370"C.

1 0. A furnace according to any one of claims 7 to 9, in which there is a preheating chamber adjacent one end of the primary heating chamber for preheating workpieces prior to their entry into the primary chamber, means for providing heat to the preheating chamber, a cooling chamber adjacent the primary heating chamber at its end remote from the preheating chamber for receiving and cooling heated workpieces from the primary chamber, a first set of rotatable metal alloy rollers in the preheating chamber, a second set of rotatable metal alloy rollers in the cooling chamber, each of the metal alloy rollers of the first and second sets being mounted substantially parallel to the ceramic rollers in the primary chamber and having a drive shaft at one end thereof, means connecting the drive shafts in each set so thal she rollers of each set rotate in unison, means connecting a drive shaft af each set to the drive means of the ceramic rollers in the primary chamber so that the metal alloy rollers rotate in unison with the ceramic rollers, and a pair of magnetic clutches for disconnection of the drive to the sets of metal alloy rollers from the drive means of the ceramic rollers so that rotation of either or both sets of metal alloy rollers can be terminated while permitting continuous rotation of the ceramic rollers.

11. A furnace according to claim 10, including a heat shield door reciprocally mounted in a slot between the primary heating chamber and the cooling chamber, radiamatic pyrometer means which is mounted in the primary chamber near the heat shield door and which is sensitive to the presence of a tray of workpieces to cause the door to open to permit entry of the tray into the cooling chamber, and means in the cooling chamber for sensing the presence of a tray and providing a signal to disengage the magnetic clutch associated with the second set of metal alloy rollers whereby rotation thereof is terminated.

1 2. A furnace according to claim 1, substantially as described with reference to the accompanying drawings.