US3138976A

US3138976A - Hydraulic forging presses

Info

Publication number: US3138976A
Application number: US862258A
Authority: US
Inventors: Robra Helmut
Original assignee: Schloemann AG
Current assignee: Schloemann AG
Priority date: 1958-12-27
Filing date: 1959-12-28
Publication date: 1964-06-30
Anticipated expiration: 1981-06-30

Description

June 30, 1964 H. ROBRA 3,138,976

HYDRAULIC FORGING PRESSES Filed Dec. 28, 1959 5 Sheets-Sheet l lNl/E N TOR JiRObra/ June 30, 1964 H. ROBRA HYDRAULIC FORGING PRESSES .3- Sheets-Sheet 2 Filed Dec.8, 1959 INVENTOR United States Patent 3,138,976 HYDRAULIC FURGING PRESSES Hclmut Rohra, Mulheim (Ruhr), Germany, assignor to Schloemann Alrtiengeseilschaft, Dusseldorf, Germany Filed Dec. 28, 1959, Ser. No. 862,258 Claims priority, application Germany Dec. 27, 1958 6 Claims. (Cl. 78-413) The subject of a prior application, Serial No. 812,249, of May 11, 1959, now Patent Number 3,031,903, is a control of a hydraulic forging press with a control member moved by the platen, in which, in dependence upon the position of the platen, voltage vectors produced by Way of a remote-control member, are compared, in a phase-detector circuit, with a constant voltage vector, and a direct voltage, which is positive or negative in relation to the Zero position of the platen, is supplied to a changeover relay, which in its turn, by way of a time relay and a lifting magnet, controls the pressure-water admission valve, or, directly by way of a further lifting magnet, the pressure-Water outlet valve.

The present invention consists in a hydraulic forging press the controlling of which is designed according to that patent application Serial No. 812,249, and which is driven directly by a plurality of pumps, the delivery quantities of which are so distributed that for rack-forging (coarse forging), the quantities delivered by all the pumps are employed, whereas for finish-forging with small strokes, only the quantitiy delivered by a portion of the pumps is used, and in effecting the control by means of a single main control known in itself, with electro-hydraulic stroke drive.

According to a further feature of the invention, the de livery quantity employed for smoothing (finish-forging) can be switched to and fro between the press cylinder and the retracting cylinders. Further in accordance with the invention, in racking (:rack-forging), only the working stroke is traversed with the delivery quantity of all the pumps, while for the return stroke, a portion of the pumps is shut off.

In the retraction pipe a pre-stressed valve and a nonreturn valve acting in the reverse direction are provided. The pre-stressed valve is pre-stressed to a pressure corresponding to the weight of the movable parts of the press.

The retractions may be controllable in a manner known in itself.

Two constructional examples of the control according to the invention are illustrated in the accompanying drawing, in which:

FIGURE 1 shows a forging press with electrical controlling means according to application Serial No. 812,249 and direct pump drive with constantly switched retractions; while FIGURE 2 shows a similar forging press with direct pump drive, but with controllable retractions, wherein the electrical controlling means according to the application Serial No. 812,249 have been omitted.

In FIGURE 3, three valve-lifting diagrams a, b and c are represented.

In FIGURE 1, 1 denotes a forging press generally, the essential details of which are marked as follows: by 2 the platen, which carries retraction plungers 4 dipping into retraction cylinders 3, and an upper forging saddle 5; by 6 the lower forging saddle, and by 7 the press cylinder. The platen 2 carries a switching rod 8, which is provided at its lower end with a toothed rack 9. The function of this switching rod 8 will be hereinafter explained.

For the driving of the forging press 1, two pumps It and 11 are provided, which are in communication, by way of

pipes

12 and 13, with an elevated tank 14.

A pipe 15 leads from the pump to a main control 3,133,976 Patented June 30, 1964 16, and a pipe 17 leads from the pump 11 to the main control 16. The main control 16 contains the following valves: a press outlet valve 20, a press admission valve 21, two non-return valves 22 and 23, and a by-pass valve 24. From the main control 16 a pipe 25 leads to the press cylinder 7, and a further pipe 26, which branches by way of a non-return valve 27 and a pre-stressed valve 28, leads through a distributor 29 to the retraction cylinders 3. An exhaust pipe 31 leads back from the main control 16, through a shock-absorber 30, to the elevated tank 14.

The valves 20, 21 and 24 are actuated by electro-hydraulically moved lifting pistons 35, which are each controlled by a piston slide valve 36. The actuating of the piston valves 36 is here effected by lifting magnets 37a, 37b and 370. The piston valves 36 are in communication, by way of branch pipes, with a pressure-oil pipe 38 and with an exhaust-oil pipe 39. The lifting pistons 35 are likewise connected by branch pipes with the pressure-oil pipe 38.

The switching rod 8 mentioned above, which is secured to the platen 2 of the forging press 1, meshes by its teeth 9 with a pinion 5%, which is connected by way of a shaft 51 and an electro-magnetic clutch 52 with an inductive remote-control member 53. The inductive remote-control member 53 is connected by electrical conductors 54 with a further inductive remote-control member 55, which is connected, adjustably by means of a control motor 56, and by way of electrical conductors 57 with network conductors 58. The inductive remote-control member 53 is connected by way of leads 59 with a phase-detector circuit 60, which has electrical conductors 63 leading to a winding 61 of a change-over relay 62. The change-over relay 62 is connected by electrical conductors 64 with the Winding of the lifting magnet 37a, actuating, through one of the piston valves 36 and one of the lifting pistons 35, the press outlet valve 20, and by electrical conductors 65 With the winding of the lifting magnet 37c actuating the by-pass valve 24. The reversing relay 62 is furthermore connected by electrical conductors 66 to a. time relay 67, which is connected by electrical conductors 68 with the lifting magnet 37b actuating the press admission valve 21. Electrical conductors 69 lead from the time relay 67 to a potentiometer 70, which is adjustable from a control desk 71. The control motor 56 of the remote-control member 55 is likewise switchable from the control desk 71 by way of electrical conductors '72, as is also the electro-magnetic clutch 52, by way of electrical conductors 73. The electrical conductors 64, 66 and 68 are connected with the mains 74.

The successions of movements in normal forging and in rapid forging are described hereunder.

In normal forging the forging press 1 is driven by the two pumps 10 and 11, the individual delivery quantities of which are unequal, and are determined according to the different finishing and racking speeds, as well as according to the size of the press. The pump It) delivers through the pipe 15 and the non-return valve 23, opened by the pressure, and through the press admission valve 21 opened by its associated electro-hydraulic lifting appliance 35/36/3712, by way of the pipe 25, into the press cylinder 7. The pump 11 delivers through the pipe 17 and the non-return valve 22 opened by the pressure, likewise through the opened press admission valve 21 and by way of the pipe 25, into the press cylinder 7. The bypass valve 24 at this time remains closed. The water expelled from the retraction cylinders 3 in the pressing position is led back to the main control 16 by way of the distributor 29, the pipe 26 and the pro-stressed valve 2%, opened by the pressure, and is here taken up into the circuit extending from the pump 10 to the press cylinder 7. Since the cross-sectional area of the press plunger 7 is much greater than the aggregate cross-sectional area of the retraction plungers 4, the gross force urging the press plunger 7 downwards, when all the plungers are subjected to the same intensity of pressure, is far greater than the gross force urging the plungers 4 upwards. At this juncture the switch finger of the change-over relay 61 is inclined to the right.

The retraction of the platen 2 is effected, even in normal forging, with only one of the two pumps, here with the pump 10. In the retraction position therefore, the

press outlet valve 20 and the by-pass valve 24 are opened,

while the press admission valve 21 is closed. The changeover relay 61 is now in the left-hand full-line position. The pump 10 now delivers through the pipe into the pipe 26, likewise opening into the main control 16, and, by way of the non-return valve 27 opened by the pressure, into the retraction cylinders 3, so that the platen 2 moves upwards. The pump 11 now revolves idly, impelling water through the pipe 17 and the opened by-pass valve 24, and through the exhaust pipe 31 leading to the elevated tank 14. When the press is at a standstill, all the electro-hydraulically actuated valves of the main control 16 are open. The pump 10 then revolves idly impelling water by way of the open press admission valve 21 and the likewise open press outlet valve and the exhaust pipe 31, while the idle running of the pump 11 by way of the open by-pass valve 24 is the same as that described above for the retraction position. The platen 2 with the upper forging saddle 5, rests upon the retraction water columns which are subject to the pressure established by the pre-stressed valve 28. The pre-stressed valve 28 is adjusted to correspond to the Weight of the movable parts of the forging press.

The delivery quantities of the two pumps 10 and 11 are so distributed, according to the requirements of normal forging and of rapid forging, that for racking both pumps are needed, whereas only one of the pumps is required for the finishing. In the constructional example of FIGURE 1, the pump 11 may therefore be shut off during the finishing, or allowed to run idly by way of the by-pass valve 24. In the smoothing pressing position the pump 10 delivers by way of the pipe 15 and the nonreturn valve 23 opened by the pressure, through the press admission valve 21 opened by its associated electrohydraulic lifting drive 36/ 37b, and the pipe 25, into the press cylinder 7. The volume displaced from the retraction cylinders 3 is forced, by way of the prestressed valve 28 and the pipe 26, into the pump flow to the press cylinder 7.

With the change-over to retraction, with the commencement of closing of the press admission valve 21 there coincides the commencement of opening of the press outlet valve 20. The valve movements overlap one another, thereby yielding a smooth Working of the press. In this overlapping, at about half the valve lift, the press admission valve 21 and the press outlet valve 20 are half opened, and the pump 10 runs idly by way of these two valves and the waste-water pipe 31. The press stands stationary upon the water column of the retraction cylinders 3, loaded by the loading valve 28. At the end of closure of the press admission valve 21, the entire delivery quantity of the pump 10 is supplied by way of the pipe 26 and the non-return valve 27 to the retraction cylinders 3. The delivery quantity of the pump 10 is therefore switched to and fro between the press cylinder 7 and the retraction cylinders 3. The retraction plungers are so dimensioned that the speed of retraction amounts to about four to five times the pressing speed,

since the area of the working surface of the press plunger in the cylinder 7 is four or five times as great as the aggregate area of the working surfaces of the plungers in the retraction cylinders 3.

In the standstill position of the press, the press admission valve 21 and the press outlet valve 20 are opened,

4 so that the delivery quantity of the pump 10 runs idly, as in the overlapping position of valves in reversing to retraction already described, and the movable weights of the press are counterbalanced by the initial pressure of the valve 28.

The actuation of the valves 20, 21 and 24 arranged in the main control is effected electro-hydraulically by exciting the lifting magnets 37a, 37b and 370 and displacing the piston valves 36, which are in communication with the pressure-oil pipe 38, fed by an oil pump, not shown. For normal forging, manual actuation with electrical contact-making is provided in the control panel 71 for the lifting magnets 37a, 37b and 370, but for smooth or rapid forging automatic switching as hereinbefore described is employed. Normal forging may however also be effected automatically. The automatic switching works in the following manner: When the platen 2, with the upper forging saddle 5, has reached its lower deadcenter position, determined by the size of the forging, a change is made to finish-forging, with the pump 10 running and the pump 11 stopped, the electro-magnetic clutch 52 being at the same time switched on from the control desk 71 by way of the electrical conductors 73, and the pinion 50, actuated by the teeth 9 of the switching rod 8, being thereby coupled to the inductive remote control member 53. The inductive remote-control member 53 produces a voltage vector which is proportional to the position of the platen 2, and which is supplied to the phase-detector 60 through the electrical conductors 59, and through a transferrer or transformer to the grids of the thermionic valves or

tubes

81, 82, 83 and 84, is amplified, and is compared with a constant voltage vector supplied by way of the electrical conductors 85 and a transferrer or transformer 86. Upon agreement of the two vectors, the output magnitude zero is yielded, which corresponds to a definite zero position of the platen 2. Above the zero position the comparison of vectors yields a positive direct voltage, and below it a negative direct voltage. The direct voltage is tapped off between the

reference points

87 and 88. Its sign is dependent upon the upward or downward movement of the platen 2, relative to its zero position. The direct voltage is supplied to the winding 61 of the polarised change-over relay 62 through the electrical conductors 63, which in its turn actuates, through the electrical conductors 66, the electrical time relay 67, which stops the platen 2 upon reaching a definite height, and, after the delay adjusted by means of a potentiometer 70 from the control panel 71, excites, by way of the electrical conductors 68, the winding of the lifting magnet 37b associated with the press admission valve 21. This lifting magnet 37a, by way of the piston valve 36 allocated to it, actuates the lifting piston 35 of the press admission valve 21 and opens it, whereby the press cylinder 7 is supplied with pressure water and the descent of the platen 2 is initiated. The inductive remote-control member 53 is thus moved in the opposite direction, and generates a voltage vector, which, compared with the constant voltage vector, yields a direct voltage of opposite sign in relation to the zero position of the platen 2 in the phase-detector 60. The direct voltage is supplied through the electrical conductors 63, to the winding 61 of the polarized change-over relay 62, which, through the electrical conductors 64, excites the winding of the lifting magnet 37a pertaining to the press outlet valve 20. This lifting magnet 37a, by way of the piston valve 36 associated with it, actuates the lifting piston 35 of the press outlet valve 20, whereby this is opened, and the platen 2 is moved into the upper dead position by the retraction plungers, which are under pressure at all times.

After the first finishing strokes, if the required dimension of the workpiece has not yet been obtained, the control motor 56 of the inductive remote control member 55 is switched on, from the control panel 71, through the electrical conductors 72, whereby the inductive remote control member 55 is displaced with respect to the inductive remote control member 53, so that the central position of the stroke of the platen or crosshead 2 is altered and fine adjustment of the working level is obtained. The rotary movement of the remote control member 55 causes a change in the exciting voltages supplied to the remote control member 53 by Way of the conductors 54-, and a consequent change in the voltage vector supplied to the phase detector 6t? by (the remote control member 53.

In the case of automatic normal forging with the pumps 11 and 11 both switched on, the lifting magnet 37c associated with the by-pass valve 24 is connected in parallel, by way of conductors 65 and a switch 65a, with the lifting magnet 37a actuating the press outlet valve 20. During the pressure stroke both pumps are acting, but during the retraction stroke of the platen only the pump 10 is acting, the delivery of the pump 11 being by-passed to exhaust through the valve 24.

This control system permits the forging press to be adjusted in several different Ways during operation. By the introduction of opposing voltages into the emitter circuit, the stroke height of the oscillatory movement can be changed, that is to say, the reversal points of the crosshead '2 are moved further apart and the oscillatory stroke is thereby lengthened. In order to permit con tinuous advance towards the Working level, an intermediate stroke position is provided. This permits advance towards any working level without the so-called addition strokes. For this purpose, the electromagnetic clutch 52 and associated automatic rapid forging apparatus is switched off, and the press inlet and outlet valves are controlled directly by means of pressure switches.

For changing the stroke rate while keeping the stroke height constant, a variable standstill period, controlled by the time relay 67, is introduced. During this time the output of the pump 11 and also the output of the pump 11 in certain cases, circulates by way of the opened press inlet and outlet valves.

The valve lift diagrams in FIGURES 3 show the over lap of the press inlet valve (PE) and press outlet valve (PA) with and Without the standstill period. The ordinate shows the valve lift of the two valves for press inlet and press outlet, with respect to time (abscissa). In the valve opening diagram (at) there is no standstill period. The press outlet valve overlaps the press inlet valve on about half the valve stroke. In the opening diagram (5), a standstill period is introduced at the top dead point of the press; the length of this period can be read from the interval between the beginning of the opening of the inlet to the beginning of the closing of the outlet. By Variation of the standstill periods it is possible to vary the stroke rate independently of the stroke height.

The embodiment illustrated in FIGURE 2 shows a forging press driven by two pumps, as in FIGURE 1, but with controlled retracting apparatus. The electrical circuit corresponds to that of FIGURE 1 and is not repeated in FIGURE 2.

Two pumps 11 and 11 are fed through

pipes

12 and 13 from an elevated tank 14, and have different outputs. As in the first embodiment, the outputs are so distributed that during rapid forging only the pump 10 is used for driving the press, whereas both pumps 10 and 11 drive the press during normal forging. A duct 15 leads from the pump 11 to a main control 102, and a duct 17 leads from the pump 11 to the main control 1G2. The main control contains five electrohydraulically controlled valves, that is to say, a press outlet valve 163, a press inlet valve 104, a retraction inlet valve 105, a retraction outlet valve 1%, a bypass valve 107 and two non-return valves 1118 and 109. From the main control 162 a duct leads to the press cylinder 7 of a forging press 1 having a crosshead 2 and retracting cylinders 3. An exhaust or water discharge pipe 31 leads from the main control 102 through a shock absorber 30 and back to the elevated tank 14. The retracting cylinders 3 communicate with the main control 102 through a duct 117 in which a safety Valve 118 and a distributor 29 are in terposed. The valves 193, 104, 1115, 1% and 107 are actuated by electrohydraulic reciprocatory driving means comprising lifting

magnets

37a, 37b, 37c 37d and 37e, piston slide valves 36 and lifting pistons 35. The piston slide valves 36 and the lifting pistons 35 communicate with a pressure oil duct 38 and an oil discharge duct 39 through branch ducts.

The platen or crosshead 2 of the forging press 1 is provided, as in the first embodiment, with a switching rod 8 which has teeth 9 and is adapted to be coupled to an inductive remote control member 53 by way of a pinion 5i and an electro-magnetic clutch 52. The remote control member 53 is connected to an electrical control circuit which corresponds to that of FIGURE 1 and is not repeated in FIGURE 2.

During normal forging, both pumps 11) and 11 drive the forging press 1. In the pressing position the pump 10 delivers through the duct 15 and the opened press inlet valve 1114 and through the duct 25 into the press cylinder 7. The pump 11 delivers through the duct 17, and through the non-return valve 1% opened by the pressure, into the current flowing from the pump 10 to the press inlet valve 194. In the pressing position the retraction inlet valve 166 is open, so that the liquid driven out of the retraction cylinders 3 by the downward movement of the crosshead 2 can flow back through the distributor 29 and the duct 117 to the water discharge pipe 31 and thus through the shock-absorber 31) into the elevated reservoir 14. In the press position, the other valves of the main control 162 are closed. In the retraction position, the press outlet valve 163 and the retraction inlet valve and the by-pass valve 1t)? are open. The press inlet valve 1114 and the retraction outlet valve 106 are closed. During its return stroke the crosshead 2 is driven only by the pump 11 delivering through the duct 15 and the open retraction inlet valve 1135 into the duct 117 leading to the retraction cylinders 3. The pump 11 idles, i.e. delivers through the opened by pass valve 107 and the Water discharge pipe 31. The discharged Water displaced from the press cylinder 7 by the upward movement of the crosshead 2 flows through the open press outlet valve 103 and the water discharge duct 31 back into the elevated tank 14. In the press standstill position, the press outlet valve 103, the press inlet valve 1%4 and the bypass valve 107 are open. The retraction inlet valve 105 and the retraction outlet valve 1% are closed. The pump 10 now delivers through the open press inlet valve 1614 and the open press outlet valve 103, while the pump 11 idles, i.e. delivers through the open bypass valve 107, as in the previous instance.

Apart from the pressing position, the valve movements during rapid forging are the same as during normal forging. During rapid forging, in the pressing position only the pump 10 acts on the press cylinder '7 while the pump 11 either idles, i.e. delivers through the open bypass valve 107, or is switched ofli.

The advantage of the embodiment shown in FIGURE 2 over that shown in FIGURE 1 lies in the fact that losses of pressing force are avoided by controlled retracting apparatus. The embodiment illustrated in FIGURE 1, on the other hand, provides simpler and therefore more rapid control.

Both embodiments have the advantage of permitting the use of pumps with constant output, that is to say comparatively simple pumps, and non-reversing pumps, which operate Without reversal losses. The direct pump drive also has the advantage of adaptation of the pressure to the resistance at any given moment, and consequently the advantage of high efficiency.

Within the scope of this invention more than two pumps may be employed, one or more or all of which can be supplying pressure water to the said pressure-water supply pipe, at least one additional pump capable of supplying pressure water to the said pressure-water supply pipe, a non-return valve interposed between this additional pump and the pressure-water supply pipe, a main control including admission valves interposed in the piping connecting the pumps with the press cylinder, an exhaust valve for exhausting Water from the press cylinder, and valve-lifting electro-magnets which open these valves when required, a remote control member, gearing rotating the remote control member in one direction or the other according to the direction in which the platen of the press is moving, a phase-detector circuit, so connected with the remote control member as to transmit controlling impulses in accordance with the direction of rotation of the remote control member, and a change-over relay the switching position of which is determined by the impulses transmitted to it by the phase-detector circuit, the circuits of the valve-lifting electro-magnets being controlled by the change-over relay.

2. An electro-hydraulically controlled hydraulic forging press as claimed in claim 1, comprising at least one retraction cylinder and retraction plunger sliding therein, the effective area of the retraction plunger being substantially less than the efiective area of the press plunger, and the retraction cylinder being in open communication with the delivery pipe of the first pump when this pump is in operation.

3. An electro-hydraulically controlled hydraulic forging press as claimed in claim 1, the main control including a second non-return valve interposed between the first pump and the press admission valve.

4. An electro-hydraulically controlled hydraulic forging press as claimed in claim 3, the forging press including at least one retraction cylinder, and the control system comprising a third non-return valve, interposed between the first pump and the retraction cylinder, to pass pressure water from the first pump to the retraction cylinder.

5. An electro-hydraulically controlled hydraulic forging press as claimed in claim 4, the control system further comprising a pre-stressed valve in parallel with the third non-return valve, this pre-stressed valve being so loaded as to permit the return of water from the retraction cylinder to the delivery pipe of the first pump when the first pump is delivering water to the press cylinder, but not to permit such return when the water in the retraction cylinder is simply supporting the weight of the platen.

6. An electro-hydraulically controlled hydraulic forging press as claimed in claim 1, further comprising: a by-pass valve which when opened by one of the valvelifting electro-magnets allows the liquid delivered by the additional pump to escape to exhaust, a switch interposed in the electrical connection between the change-over switch and the said valve-lifting electro-magnet, this electrical connection being in parallel with that between the changeover switch and the electro-magnet that opens the exhaust valve from the press cylinder, so that if the additional pump is running while water is being exhausted from the press cylinder, the by-pass valve can be opened by closing this switch.

References Cited in the file of this patent UNITED STATES PATENTS 1,814,857 Rosle July 14, 1931 1,912,184 Ferris May 30, 1933 2,387,307 Stone Oct. 23, 1945 2,410,283 Garr Oct. 29, 1946 2,652,691 Hartemann Sept. 22, 1953 2,735,405 Hipple Feb. 21, 1956 2,886,010 Hayos et al May 12, 1959 3,031,903 Billen May 1, 1962 FOREIGN PATENTS 782,948 Great Britain Sept. 18, 1957

Claims

1. AN ELECTRO-HYDRAULICALLY CONTROLLED HYDRAULIC FORGING PRES, COMPRISING: A PIPE SUPPLYING PRESSURE WATER TO THE PRESS CYLINDER AND PLUNGER, A FIRST PUMP CAPABLE OF SUPPLYING PRESSURE WATER TO THE SAID PRESSURE-WATER SUPPLY PIPE, AT LEAST ONE ADDITIONAL PUMP CAPABLE OF SUPPLYING PRESSURE WATER TO THE SAID PRESSURE-WATER SUPPLY PIPE, A NON-RETURN VALVE INTERPOSED BETWEEN THIS ADDITIONAL PUMP AND THE PRESSURE-WATER SUPPLY PIPE, A MAIN CONTROL INCLUDING ADMISSION VALVES INTERPOSED IN THE PIPING CONNECTING THE PUMPS WITH THE PRESS CYLINDER, AN EXHAUST VALVE FOR EXHAUSTING WATER FROM THE PRESS CYLINDER, AND VALVE-LIFTING ELECTRO-MAGNETS WHICH OPEN THESE VALVES WHEN REQUIRED, A REMOTE CONTROL MEMBER, GEARING ROTATING THE REMOTE CONTROL MEMBER IN ONE DIRECTION OR THE OTHER ACCORDING TO THE DIRECTION IN WHICH THE PLATEN OF THE PRESS IS MOVING, A PHASE-DETECTOR CIRCUIT, SO CONNECTED WITH THE REMOTE CONTROL MEMBER AS TO TRANSMIT CONTROLLING IMPULSES IN ACCORDANCE WITH THE DIRECTION OF ROTATION OF THE REMOTE CONTROL MEMBER, AND A CHANGE-OVER RELAY THE SWITCHING POSITION OF WHICH IS DETERMINED BY THE IMPULSES TRANSMITTED TO IT BY THE PHASE-DETECTOR CIRCUIT, THE CIRCUITS OF THE VALVE-LIFTING ELECTRO-MAGNETS BEING CONTROLLED BY THE CHANGE-OVER RELAY.