US1734913A - Method and apparatus for electrically heating articles - Google Patents

Description

1929. H. E. NORMAN 1,734,913

METHOD AND APPARATUS FOR ELECTRICALLY HEATING ARTICLES Fil y 1927 -2 Sheets-Sheet 1 IN VEN TOR.

A TTORNEYS.

m 6%W1 W s ja 5 Nov. 5, 1929. NORMAN 1,734,913

METHOD AND APPARATUS FOR ELECTRICALLY HEATING ARTICLES File July 192 2 Sheets-Sheet 2 INVENTOR.

ATTORNEYS.

Patented Nov. 5, 1929 UNHTED STATES PATENT "OFFICE HORACE E. NORMAN, OF SPRINGFIELD, MASSACHUSETTS, ASSIGNOR TO NATIONAL EQUIPMENT COMPANY, OF SPRINGFIELD, MASSACHUSETTS, A CORPORATION OF MASSACHUSETTS METHOD AND APPARATUS FOR ELECTRIOALLY HEATING ARTICLES Application filed July 14,

This invention relates'to a method and ap- .paratus for electrically heating metals, of the class in which the resistance increases with'the temperature thereof.

This method and apparatus has been developed particularly for use in electrically heating articles preparatory to forging but obviously the character of the operation, which ensues after the articles have been heated, is largely immaterial. The invention will be disclosed herein, by way of illustrative example, as adapted to heat one end of an article preparatory to an upsetting or heading operation.

The heating is effected by placing the article, or that portion thereof which is to be heated, between contacting electrodes in the usual or any suitable way. One of these electrodes has a larger area of contact with the article than the other and is usually cooled, so that heating is initiated adjacent the other electrode, being the point of highest resist- The latter electrode usually engages the article at the end which is to be upset and the heating therefore starts at such end and progresses inwardly. VVha-t is needed for proper heating of the article for the upsetting operation is a so-called soaking heat which penetrates into the article a substantial distance from the end engaged by the last named electrode. That is, a certain length of the article, measured from one end thereof, mustbe heated with substantial uniformity. This result, however, is not readily obtained. The article heats first at point of highest resistance, namely, at one end, and as this end gets hotter its resistance increases and thereby heats still more rapidly, making worse conditions which are already bad as tending to produce non-uniform heating. The action, if allowed to continue, will cause burning ofthe article at one end before the rest of the portion of the article which requires heating is properly heated.

To overcome this tendency to heat the article nonuniformly, in the case of machines having a manually operable electrode, the operator will withdraw the electrode from contact with the hot end of the article Inomentarily, allowing the latter to 0001 some- 1927. Serial No. 205,728.

what at such end and allowing the heat to All end of the article has cooled somewhat and its resistance has thereby been lowered. The

electrode may be removed and reapplied several times during the heating of one article. In the case of machines wherein the electrodes are applied to the article by automatic mecha'nism, the above scheme is not feasible and the method then resorted to is to apply a relatively low voltage to the article for a relatively long time. The voltage can be so chosen as to avoid burning of the hot end of the article but at a distinct sacrifice of time. The very reason for the provision of the automatic machine is to save all the time possible so that production can be boosted to a maximum. Consequently, it is most important, if not vitally necessary, to secure proper heating of the work in the shortest possible time.

The problem, to the solution of which this invention is directed, is the electrical heating of metals of the class described, properly and without burning, in a much shorter time than has been possible with any of the former methods of which I am aware.

My invention offers as a solution of this problem, and has for its object, the cutting down of the voltage impressed on the article to be heated after the heating has progressed to a certain stage. This is done to offset the rise in voltage which I have found occurs as a consequence of the heating of the article, for reasons which I shall hereinafter explain in detail.

This rise in voltage is what-causes the burning of the end of the blank. It also explains T the use of relatively low initial voltages which are purposely chosen so low'that they will not rise high enough tocause burning. However, I have found that if I ofiset the rise in voltage, I can use higher initial voltages and heat the Work properly and Without burning in less than half the time which would otherwise be required. I

The stage in the heating of the article at which the voltage is cut down may be deter mined in various ways. The operator, by watching the heating of the work, may judge the time at which the change is to be effected and may manually effect the change. The change may also be automatically effected by any suitable means which will respond to a rise in voltage across the terminals of the article to be heated. Both these methods and exemplary apparatus, by means of which the methods may be practiced, will be disclosed herein, but it is within the province of this invention to determine the time when the change in voltage should be made by any other suitable means,tl1e essential feature being a reduction in voltage at the proper time to compensate for the rise in voltage which necessarily follows as the article is heated.

These and other objects will appear in the following descriptions and be pointed out in the appended claims.

The invention will be disclosed with ref erence to the accompanying drawings, in which z- Fig. 1 is a diagrammatical view of one apparatus by which the method may be practiced, arranged for manual control; and

Fig. 2 is a similar view of another apparatus by which the method may be practiced under automatic control.

Referring first to Fig. 1, (1 represents the article to be heated and for the upsetting operation it is necessary to include only a cer tain portion, such as w, in circuit between two electrodes 6 and e. The electrode 6, as shown, contacts with one end face 7" of the article while the electrode 6 contacts with part of the side of the article and at a distance from said end. The electrode 6 may, and often does, consist of more than one part and the manner in which this and the other electrode engage the work may be varied as desired,that shown being purely exemplary.

The electrodes 6 and e are connected, usually by heavy copper leads Z and 7/ to the secondary s of a suitable step-down heating transformer, the core of which is designated a and the primary p. The latter is supplied from an auto-transformer t, which is connected by wires 5 and 6 and a double pole switch 7 to a supply line 8 of suitable voltage. The switch 7 is electromagnetically operated, as by the plunger 9 and solenoid 10 which is included in circuit with a supply line 11, usually of lower voltage than line 8. One of the line wires 11 leads directly to one terminal of solenoid 10. The other wire 11 leads to one terminal of a switch 12. The other terminal of the latter is connected by a wire 13 to a contact 14 on the lever 15 of a regulating switch, designated generally as r. This lever 15 can swing to the right or to the left of the illustrated position so as to move the contact 14 into engagement with either of two fixed contacts 16 and 17. Both of the latter contacts are connected by a wire 18 to the other terminal of solenoid 10. The lever 15 also carries a second contact, marked 19, which is insulated from the contact 14 and is adanted to engage either of a pair of contacts 20 and 21, according to whether the lever 15 is swung to the right or to the left of the illustrated position. Contacts 20 and 21 are connected by wires 22 and 23, respectively, to different taps 24 and 25, respectively, of the auto-transformer t,the con tact 21 being connected to a tap of lower voltage than that to which contact 20 is connected. The contact 19 is connected by a wire 26 to one terminal of primary 7? and the other terminal of the latter is connected by a wire 27 to wire 5.

The lever 15 of the regulating switch is, at the start of the heating operation, positioned so that contact 19 is engaged with contact 20 and so that contact 14 is engaged with contact 16. The switch 12 is the usual switch provided on the forging machine to initiate the electric heating operation. It usually is operable by movement of and with the electrodes, being arranged to close only after the electrodes have made contact with the work and to open only after the electrodes have been disengaged with the work. Assuming then that lever 15 is positioned as described and that the electrodes are applied to the article a, the closing of switch 12 causes energization of solenoid 10 which in turn causes the closing of switch 7. Current then flows in the heating circuit and the upper end f of article (4 starts to heat. The heating starts here because this is the point of highest resistance in the heating circuit,-the electrodes 0 having a greater area of contact with blank 4 and beingusually also cooled. As the end f gets hot, the operator, judging the time by the appearance of the heated article, throws lever 15 to the left, and thereby moves contact 19 out of engagement with contact 20 and. into engagement with contact 21. This ca ses the primary 2) of the heating transformer to be connected to the tag 25 of lower voltage whereby the voltage in the secondary or heating circuit will be correspondingly reduced. The above described mov'zancnt of lever 15 also results in disengagement of contacts 14 and 16, whereby the supply line switch 7 is opened, and this action preferably occurs just before the contacts 19 and 20 disengage so that there w ll be no arcing at these contacts. The switch 7 is thereafter closed again by reason of the engagement of contacts 14 and 17 and this preferably occurs just after contacts 19 and 21 have been engaged and for the above reason.

Referring now to Fig. 2, the heating transformer and the heating circuit including its electrodes are the same as before. Also the primary p is supplied from the auto-transsupply to auto-transformertis controlled by the electroanagnetic switch 7 which is itself controlled by switch 12 from .a low'voltage circuit 11. The differences relate entirely to the manner in which the taps 24 and 25 are successively connected in the'order named to primary p.

To effect this result, I make use of a selector switch, marked SS; a current relay, marked CR; and a potential relay, marked PR. The selector switch includes two switch -arms'30 and 31, which are connected by wires 32 and 33 to the'taps 24:and 25,'respectively. Arms 30 and 31 are biased to normally remainin open position, as shown. Arm 31 has a plunger 34 for cooperation with a solenoid 35 whereby, when the latter .is energized,

arm 31 will be swung to the .left and into Similarly,

a solenoid 38 whereby, when the latter is energized, arm 30will be caused to swing to the left vand into engagement with a contact 39. Contacts 36 and 39 are connected together and to one terminal of the solenoid 40 of current relay CR by a wire 41. The other terminal of solenoid 40 is connected by a wire 42 to one terminal of primary p, the other terminal of the latter being connected by the wire 27 as before, to wire 5. The

plunger 37 differs from plunger 34 in that it carries a contact 43 adapted normally to engage a fixed contact 44. The energization ofsolenoid 38, which serves to close switch '30, also serves to separate the contacts 43,

and 44.

The current relay CR comprises, in addition to solenoid 40, a plunger 45 which carries a contact 46, adapted on energization of the solenoid to engage a fixed contact 47. The potential relay PR includes a solenoid 48 having a plunger 49 carrying a contact 50 which normally engages the lower contact 51 of a pair of fixed contacts, the other and upper contact being marked 52. plunger 49 is adapted to be lifted when sole The noid 48 is energized, whereby contact 50 is first disengaged from contact 51 and then engaged with contact 52. The lifting of plunger 49 is, however, retarded by a dash pot, conventionally indicated at 53, whereby there is a short time interval between the disengagement of contacts50 and 51 and the subsequent engagementof contacts 50 and 52. Solenoid 48 is connected by a wire 54 to switch 12 and by a wire '55 to the proper line wire 11, whereby the solenoid will be energized when and while switch 12 is closed. Contacts 47 and 50 are connected by a wire 56 and the latter is connected to wire 54. Contacts 52 and 44 are connected 'by a wire 57. Contact 43 is connected by a wire 58 to one terminal of solenoid '35 and the other and connect the primary 7) to the high Voltterminal of the latteris connected -by;a wire of switch 7 as before, and also energizes solenoid 38, causing arm 30 to engage contact 39 agetap 24 ofauto-transformer2f. The circuit for solenoid 38 ,may be traced as follows:

from switch 12 by wire 54 to wire 56, thence by wire 56 to contact 50, which is initially engaged with contact 51, thence by wires 62 and 61'to the-solenoid, returning by wires and 59 to the proper low voltage supply wire 11.

Current cannot flow through solenoid 35 because contacts 50 and 52 are disengaged. Current does flow through solenoid 40 of the current relay and causes contacts 46 and .47 to engage, whereby a second path from wire 54 to solenoid 38 is completed. This is necessary in order to keep the solenoidenergized and switch 30 closed because the former path, which includcdcontacts 50 and 51, will be interrupted as plunger 49 starts slowly upwardly under the force exerted by solenoid 48. The latter, of course, is energized coinciden-tally with solenoid 38. As the solenoid 38 was energized to close switch 30, the contacts 43 and 44 became separated. Consequently, when the potential relay PR eventually causes contact 50 to engage contact 52, the circuit which is adapted to connect soleno'id 35 to line 11 will still :be open. Such circuit may be traced as follows: from switch 12 by wire 54 to wire 56 and contact 50,

thence from contact 52 by wire 57 to contact .105

44, thence from contact 43 by wire 58 to solonoid 35, returning by wire 59 to the proper .line Wire 11. Thus, by reason of the separation of contacts 43-and 44, solenoid 35 will not 'be energ1zed when contacts 50 and 52 engage.

article heated, as will appear, whereby the current relay may also be'said to respond to a rise in voltage across said terminals. Upon such a rise in voltage and the consequent decrease in current in the heating circuit, there is a proportional decrease in current in the circuit of primary p. lVhen the current in the latter decreases below a predetermined de gree, solenoid 40 is no longer able to hold the plunger 45 in raised position and it falls,

causing separation of contacts 46 and 47. It will be recalled that it was the engagement of these contacts which was relied on to keep solenoid 88 energized. Consequently, when they become separated, solenoid 38 will be cut off from its supply circuit and switch 30 will open. The opening of this switch results in the engagement of contacts -13 and a l, whereby solenoid is energized and switch arm 31 is swung into engagement with contact 36. Thus, the primary p is disconnected from the high voltage tap 2d and connected, after a momentary interruption, to a lower voltage tap 25. The heating then continues until switch 12 is opened, whereupon the supply to solenoics 35 and is interrupted and the parts controlled thereby return to the illustrated positions. The solenoid 38 has already been de-cnergized, wherefore the parts controlled thereby have already returned to illustrated position, as have also those controlled by current relay CR.

It has been stated that a rise in voltage across the terminals of the article to be heat ed occurs as the heating progresses. This rise in voltage is slower than the increase in resistance of that part of the article being heated and not in the same proportion thereto. The rise in voltage occurs for several reasons. First, the increase in resistance of the article being'heated causes a greater drop in voltage across its terminals, which is one factor productive of the rise in voltage referred to. Second, the increased resistance of the heating circuit means decreased current, if the secondary voltage of the heating transformer remains constant. This decrease in current means a lesser drop in voltage in the leads Z and Z and thus a greater drop in voltage across the terminals of the article being heated. Third, as the current decreases, the load on the heating transformer decreases as does also the load on the supply line 8, and these decreases in load result in voltage rises. These various factors then cause a rise in voltage across the terminals of the article to be heated as the heating progresses and such rise is accompanied by a decrease in current, which is made use of to operate the current relay and cause a reduction in the voltage applied to the primary ;0 and thus also to the heating circuit and article a.

As illustrative of a typical case and without in any sense setting up ny hard and fast limits, a voltage of 1.3 was initially applied the terminals of the article to be heated. As the heating progressed, the voltage across said terminals rose to 1.75 volts, whereupon the voltage of the primary circuit was reduced so that the voltage across said terminals was reduced to .9 volts. The heating was then continued and the voltage rose to 1.2 at the end of the heating interval. The article was heated properly and without burning in 10 seconds. Contrasting this with prior practice, a low initial voltage. say .7 5 volts is used in order that the final voltage may not rise too high and burn the work and the heating of the work under these conditions required 25 seconds. This case is given, merely as an illustrative example, to show the relative saving in time which may be made by the use of my invention.

In the heating operation, the heating commences at the end face 7 of the article, being the point of greatest resistance. Heating progresses from face f towards electrode e and the penetration of the heat to the desired distance from said face is largely a question of time and applied voltage. If a low enough voltage is applied across the terminals of the article, it can be heated satisfactorily but more time is taken than would be required if higher voltages were applied. The higher voltages, however, are likely to cause burning of the worn unless provisions made to prevent such action. This *ill be apparent when one considers that the heat initiated at the point 7" increases the resistance at this point, resulting in the work getting hotter and hotter near one end untilitbu 5. By cutting down the voltage after the hea..-ng nears the stage at which the work would burn under former methods, the heating effect at the hot end is diminished to compensate for the tendency to overheat, and "he heat has time to spread along the blank by conduction before the metal can burn at point 7 My method enables the use of higher initial voltages, which means more rapid heating, by cutting down the voltage after the heating has progressed close to the stage at which burning might otherwise result. I believe that I am the first to conceive the method ierein set forth, and the apparatus by which the method may be practiced, and I desire to claim my invention in the broadest possible legal manner.

lVhat I claim is:

1. The method of electrically heating articles of the class in which the resistance increases with the temperature, which consists in applying an initial voltage to the articles and. after the voltage across the tcrminals of the article has risen to a predetermined degree, reducing the voltage applied thereto.

2. The method of electrically heating articles of the class in which the resistance increases with the temperature. which con sists in applying an initial voltage to the articles, allowing the voltage across the terminals of the articles to rise a predetermined degree, and then reducing the applied voltage and continuing the operation under the last applied voltage.

3. In combination, an electric heating circuit including electrodes for contacting with the article to be heated and for impressing a certain initial voltage thereon, and compensating means responsive to a rise in voltage across the terminals of said article to decrease the voltage of said circuit.

4. Electric heating apparatus comprising, a heating transformer, electrodes connected to the secondary of the latter and to which the article to be heated is adapted to be connected, a source of E. M. F. to supply the primary of said transformer, and means responsive to a rise in voltage across the electrodes to decrease the voltage applied to said primary.

5. Electric heating apparatus comprising, a heating transformer, electrodes connected to the secondary of the latter and to which the article to be heated is adapted to be connected, an auto-transformer having taps to supply various voltages to said primary, a switch by means of which the tap of higher voltage is initially connected to said primary, a second switch adapted to connect a tap of lower voltage to said primary, and means responsive to a. rise in voltage across the terminals of said electrodes to open the first switch and close the second.

In testimony whereof I have affixed my signature.

HORACE E. NORMAN.

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