US2014142A - Pressure control system for wood grinders - Google Patents

Description

Sept. 10, 1935. E. A. MAHANNAH PRESSURE CONTROL SYSTEM FOR WOOD GRINDERS I 3 Sheets-Sheet 1 Filed Feb. 8, 1928 INVENTOR. E? W flaw/14 ATTORNEY-J:

Sept. 10, 1935. E. A. MAHANNAH PRESSURE CONTROL SYSTEM FOR WOOD GRINDERS Filed Feb. 8, 1928 I5 Sheets-Sheet 2 INVENTOR.

ATTORNEY-J1 Sept. 10, 1935. E. A. MAHANNAH Y 5 3 PRESSURE CONTROL SYSTEM FOR WOOD GRINDERS Filed. Feb. 8, 1928 5 Sheets-Sheet 3 IN V EN TOR.

MEANS h ATTORNEY. 5,

Patented Sept. 10, 1935 PATENT"- OFFICE PRESSURE CONTROL SYSTEM FOR WOOD GRINDERS Everett A. Mahannah, New York, N. Y., assignor to International Paper 00., a corporation of New York Application February 8, 1928, Serial No. 252,692

16 Claims.

'Ih's invention relates to'rotary grinders for production of wood pulp to be used inthe manufacture of paper, certain types of such machines being characterized by a grindstone carried by a driving shaft mounted in suitable bearings at the opposite sides of the stone, and the wood in the form of short logs or billets is positively fed to the stone through a pocket or magazine, the feeding means being in some cases in the form of hydraulically operated pressure shoes or plungers and in other cases endless traveling chains equipped with lugs or spurs are employed to feed the wood, which chains may be driven in various ways, as for instance, by an electric motor, or by hydraulic means, or by other forms of power. In the operation of such grinders, due to the unequal and excessive pressure to which the stone is sometimes subjected in the grinding operation, and the inability to properly control such pressure and maintain it at a safe and uniform value, the stone frequently bursts, which results in serious loss to both life and property, and interrupts the operation of the grinder; and by reason of the variation in the pressure of the wood against the stone, other objectionable results develop resulting in the production of a pulp'of non-uniform character and grade and consequently reduced value.

It is the aim of the present invention to ob- 30 viate this objectionable action and results, and the invention consists of means of improved form and construction for regulating and controlling the pressure to which the stone is subjected in the grinding operation, so that said pressure will be uniform and not subject to dangerous changes or fluctuations, and in carrying out the invention I utilize as the controlling means, the pressure which is exerted on the film of oil between the grindstone shaft and its bearing, which pressure is proportional to the speed of rotation of the shaft, the viscosity and temperature of the oil, and the load which the shaft carries, and which pressure is brought about by the slight radial movements of the shaft in its bearing This pressure in the embodiment of the invention shown by way of example, is employed to control, through suitable electrically operated controlling means, an electric motor by which the wood feeding mechanism is operated, the operaton of this controlling means being such that on an increase of pressure of the oil film, the feed motor will be slowed down and the pressure of the wood on the stone will be reduced, and on a decrease of pressure of the oil film, the feed motor will be speeded up and. the pressure of the wood on the stone will be increased, these changes being effected automatically and resulting in the maintenance of the pressure of the wood on the stone at the proper and desired degree. 5

In the specification tolfollow the invention will be described in detail, and the novel features thereof will be pointed out in the appended claims.

In the accompanying drawings:

Fig. 1 is an end elevation of a form of wood grinder shown by way of example as having my invention applied thereto, the pipe line leading from the oil film being shown, but the controlling mechanism operated by the oil pressure in said pipe line, being omitted.

Fig. 2 is a longitudinal section through the grindstone and its carrying shaft and bearings, showing the pipe line leading from the two bearings,. and showing diagrammatically the wood feeding motor and the electrically operated controlling mechanism therefor.

Fig. 3 is a longitudinal section on an enlarged scale through one of the shaft bearings showing how the pipe line is tapped in the bearings to communicate with the oil films.

Fig. 4 is a transverse section through the same on the line 4-4 of Fig. 3.

Fig. 5 is a diagrammatic view of a modification of the invention.

Referring to Figs. 1 to 4 of the drawings:

The wood grinder to which the invention is shown by way of example as applied, comprises a suitable base frame 5 provided atopposite ends with shaft bearings 2, in which is mounted for rotation a horizontal driving shaft 3 to which is fixed, between the bearings, a grind stone 4, which shaft is adapted to be driven by a suitable power means not shown.

Extending vertically above the stone at its center is a magazine 5 to hold the wood to be ground, through which extends at its opposite sides, the inner stretches of two endless traveling wood feeding chains 6, furnished with spurs or lugs not shown, to engage the body of wood in the magazine, which chains travel at their upper ends around sprocket wheels I mounted on a magazine frame and at their lower ends around sprocket wheels 8 mounted on said frame adjacent the grindstone. The said chains are driven in the present instance by an electric driving motor 9 mounted on the magazine frame at its base, and geared to the lower sprocket wheels 8 by suitable reduction gearing III, II and I2.

The feed motor is supplied with current from a generator l3 by electric conductors i4, and the generator is driven by a three phase A. C. motor I5 supplied with current from a power line I511. The field i6 and the field rheostat ll of the generator are connected in series with a D. C. power line l8 by conductors l9 and and likewise the field 2| and the field rheostat 22 of the feed driving motor 9 are connected by conductors 23 and 24 with said power line l8.

In accordance with my invention, the pressure of the oil film between the shaft and its bearings as before alluded to, is utilized through the medium of piping leading from said oil films, to operate electrically a mechanism to vary the voltage of the generator l3 and consequently the speed of the feed motor 9 and therefore the degree of pressure of the wood against the stone. As best shown in Figs. 2, 3, and 4, vertical pipes 26 are tapped through the bottoms of the two bearings so that the oil will enter the upper ends of the pipes from the oil films, and to these pipes are detachably connected. as by screw threads, the inner ends of branch pipes 21 which are removably connected by unions 28 with short pipe sections 29 extending outwardly through the ends of the bearings, where these short pipe sections are connected with pipes 30 leading to a common pipe 3|. Pipes 32 and 33 branch of! from pipe 3|, the pipe 32 leading to a pressure gauge 34, and the pipe 33 leading to a pressure actuated member to control an electric controlling circuit, which actuating member, in the present instance, is in the form of a Bourdon tube 33, see Fig. 2, with which the pipe 33 communicates. The Bourdon tube is jointed as at 35:: to a contact lever 31 fulcrurned as at 38 to a fixed bracket 38, and the lever has connected with it a conductor 4|) leading to one of the leads of the D. C. power line l8. The contact lever carries a contact point 311: which on the movement of the lever by the expansive action of the Bourdon tube, is adapted to make electric contact with a contact point 4| on a lever 42 fulcrumed as at 43, the said lever 42 having connected with it a conductor 44 connected with the other lead. of the power line l8.

Branch conductors 45 and 46 lead from the respective conductors 48 and 44 to the solenoid coils of a relay 41, so that when the circuit through the conductors 40 and is closed, the relay coils will be short circuited and the coils will be deenergized, and when the said circuit is opened, the relay coils will be energized from the power line l8, the purpose of which arrangement and action of the parts will presently appear.

An armature lever 48 is fulcrumed as at 49 and is adapted, when the relay coils are energized, to be moved and make electric contact with a contact point 49a, and conductors 50 and 5| are connected respectively with the lever 48 and with contact point 49a and are connected to the opposite sides of the generator field rheostat l1, with the result that when said circuit is closed by the relay, the generator field rheostat II will be short circuited.

The form and relation of the parts described are such that with the pressure of the wood acting on the stone at the proper and desired degree, the circuit through conductors 40 and 44 will be open at the contact points 31:: and 4| and the relay coils will be energized from the power line I 8, and the circuit through conductors 50 and 5| will be closed at contact points 48 and 49a, whereby the generator field rheostat will be short circuited. This is the position and condition of the system under normal operation of the grinder.

, If now conditions arise in the operation of the grinder which will cause an increase oi the pressure of the wood on the stone, say even of a pound more or less per square inch, the pressure of the oil film increasing, the Bourdon tube will actuate contact lever 31 and close the circuit at contact points 311: and 4|. This action will short circuit the relay 41, and the relay coils being deenergized, the circuit through conductors 50 and 5| will be opened, thereby causing the current from the power line I8 to flow through the rheostat ll of the generator, by which action the voltage of the generator will be reduced and the speed of the feed motor also reduced, and con sequently the pressure of the wood against the stone will be likewise reduced, whereby the pressure on the stone will be brought back to its normal and proper degree. As the pressure thus decreases, the circuit at the contacts 31:: and 4| will be opened by the Bourdon tube, and the relay coils being energized, the relay control circuit will be closed and the generator rheostat will be short circuited as before, and the voltage of the generator being increased, the feed motor will be speeded up and the pressure of the wood on the stone will be increased. In such operation of the parts, a constant regulating action will take place, with the result that the pressure of wood against the stone will be maintained at the particular degree for which the contacts 31:: and 4| may have been initially set.

In order to prevent "hunting" in the action of V the contact lever 31 in controlling the operation of the relay, there is connected with the lever 42 a solenoid core 58a actuated by solenoid coils lla connected by means of conductors S2 and 83 with the conductors .|4 supplying current to the feed motor from the generator, an expansion spring 54 acting on the lever 42 in opposition to the attractive force of the coils. The lever 42 is moved more or less against the counter force of the spring, according to the change in voltage of the supplying generator l3 with the result that the circuit closed at the contact points 312:, and 4| by the movement of the contact lever 31, will be instantly opened, and then again quickly closed due to the change of voltage resulting from the opening action, and in this way the circuit con trolling action of the lever 31 will take place without extreme movements of the same.

The particular degree of pressure exerted by the wood on the stone will be determined by the relative position of the contact points 31:1: and 4|, 55

and I propose to provide for the adjustments oi. the distance between these contact points in order that the working pressure of the wood in the grinding action may be set at the desired point. This object may be conveniently eflected by mounting the lever 42 and the solenoid coils 5|a associated with said lever, on a movable plate or frame 55 which plate has swiveled to it a feed screw 58 threaded in a fixed nut 51, so that by turning the screw in one direction or the other, the contact point 4| may be adjusted closer to or further from the contact point 31:1: on the contact lever 31, according to the requirements.

In Fig. 5 I have shown diagrammatically a modification of the invention, which modification includes means whereby in the event of an interruption in the feed of the wood to the stone, as for instance by the jamming of the wood in the magazine, the feed motor and consequently the action or the feeding mechanism, will be auto- .zine and thus break up the jam.

From this figure of the drawings it will ,be seen that the Bourdon tube 35a connected as before to the piping from the oil film, is arranged to operate a contact lever 31a, which lever on an increase of pressure of the oil film is adapted to make contact with a contact point 80 whereby electric circuits will be controlled which will reduce the speed of the feed motor, and which lever on an undue decrease of the pressure of the oil film, such as would result from a total cessation of the feeding action as by the jamming of the woo-d, will make contact with a contact point GI and thereby control circuits which will reverse the direction of feed of the feeding mechanism, all of which will be described in detail later on.

ihe feeding mechanism is operated as before by a feed motor 62 supplied with current from a generator 83 through conductors 8d and 65, the conductor 65 having connected in it a relay coil 66 of a current relay 86a operative on a core fil carrying a contact plate 88 adapted when the current reaches a predetermined value to make contact with contact points 88 and iii for the purpose presently to be described. The generator 63 may be driven from any suitable source of power, as by the three phase motor l?- shown in Fig. 2 for driving the generator in that form of the invention.

A counter E. M. F. generator H is provided, the action of which is to generate a voltage in the opposite direction from that supplied to it from the power line, and this generator is connected in the power line by the following conductors:

, Starting with the positive bus 12 of the power line, the current flows through conductor 13 through contact points 14 and 15 of a double pole, double throw, reversing contactor l6, thence by conductor H to contact making Wattmeter l8 and by conductor 19 through adjustable field rheostat 88, thence by conductor 8| to the field '81 of the generator 63, and leaving the generator field the current passes through conductor 83 and contact points 84 and 85 of the reversing contactor 16, thence by conductor 86 to the armature of the generator H, and finally by conductor 81a .to the negative bus 88 of the power line.

The contacts 14 and 85 of the reversing contactor 16 are carried by a core 89 acted on by a solenoid coil 90, which when deenergized will permit said contacts 14 and 85 connected respectively, with the conductors l3 and 86 to make contact with the contact points 15 and 84 of the reversing contactor and cause the current of generator II to pass in one direction through the field 81 of the generator 83, the said coil 98 when energized, as will be presently described, causing the contacts '14 and 85 to make contact with contact points 14a and 85a of the reversing contactor connected respectively with conductors 83 and 11, thereby reversing the current in the field '81 and consequently reversing the direction of the current generated by the generator 63, and thereby reversing the direction of the feed of the wood.

The field 9| of the generator H is connected to the power line, and the current thereto, starting with the positive bus conductor 12, fiows through conductor 92 branching from the bus conductor to resistance 93 thence by conductor 93a to the field 9| and leaving the field, the current flows to the negative bus conductor 88 through conductor 94.

The contact lever 31a is connected to the positive bus conductor 12 by means of a conductor 85 and when said lever makes contact with contact point 80 when the pressure of the oil film increases,

a circuit will be closed which will shunt the resistance 93 which circuit will be through the following conductors: Q

Positive bus conductor 12, branch conductor 95, contact lever 31a, contact point 60, conductor 98 connected with point 6!), adjustable resistance 95a, conductor 91, conductor 93a, generator field When the contact lever 31a makes contact with contact point 6|, the coil 90 of the reversing contactor will be energized and the current of gen erator H will be reversed, the current of the circuit closed by this action pursuing the following course andfiowing through a time delay relay A. Starting with the positive bus conduct-or 12, the current fiows through conductor 95, contact lever 31a, contact point 6!, conductor 98, current relay contact points 69 and TE! and bridging contact plate 88, conductor 99, terminal me on the time relay A, conductors l8! and m, magnet coil E83, and terminal tilt of said relay, the current leaving the time relay by conductor W5 and flowing to the negative bus conductor. On the closure of this circuit the relay will act to close a circuit from the bus line through conductor Hi8, relay terminal till, contact points Hi8, I88, till, and ill, conductor i it, relay terminal l l3, conductor l M, contactor coil 98 and conductor H5 connecting said coil with the negative bus conductor. On

the closure of this circuit through the coil 98, the

rent of the generator H and consequently causing areversal of the current in the field 81 of generator 63 and a reversal of the current deliverd to the motor 82 and finally a reversal of the motor so that the feeding mechanism will be operated in reversed direction.

scribed, a circuit will be completed through contact points I08, and H39 through its actuating motor, and when the motor has revolved a suffiperiod of delay, the relay will act to open the motor circuit and the motor will come to rest, which action will open the circuit through the contactor relay coil 90, whereupon the contact points 14 and 85 of the reversing contactor will be disengaged from contact points Ma and 85a and engaged with contact points 15 and 84. This will reverse the current of the generator I! to its former direction, and the feed of the wood to the stone by the feed motor will beresumed.

A resistance I I8 is connected to the conductors l1 and 19 around the contact making watt meter 18, and the action of the watt meter is such that when the current of the feed motor reaches a In the action of the time delay relay, as decient number of turns to give the desired definite dangerous value, determined by the capacity of tance H8. This will tend to lower the value of the voltage generated by the generator 63, and will reduce the power required-to drive the feed motor 62. When the reduction reaches a point which will permit the watt meter contacts to again close, the contact lever 31a operated by the Bourdon tube will act as before to hold a constant pressure on the feed motor.

From the foregoing description it will be understood that normally with the instrument set to maintain a predetermined pressure of the wood on the stone, the contact lever 31a will stand in a neutral position between the contacts and GI out of engagement with both, the contacts 14 and 85 of the reversing contactor will be in contact with the contacts 15 and M and the current relay 66a will hold the circuit controlled by it open at the points 69 and III. In this position of the parts the resulting current from the counter E. M, F. generator II and the supplying power line will flow through the field 81 of the generator 63 in a direction to cause the feed motor to be operated to feed the wood to the stone, the field of the generator "H being supplied with current from the power line through a circuit including the resistance 83. When now there is an increase of pressure of the wood on the stone, the rise 01' pressure of the oil film will act on the Bourdon tube and cause contact lever 31a to make contact with contact point 50. This will shunt the resistance 53 and will increase the voltage of the counter E. M. F. generator H, which will reduce the effective component of the power line voltage thus reducing the current flowing in the field coil 81 which will decrease the voltage generated by the generator 63, which in turn will reduce the speed of the feed motor 62 and consequently the pressure of the wood on the stone. The pressure of the oil film being thus reduced, the Bourdon tube will cause the contact lever to open the circuit previously closed by it and this will again permit the current feeding the field SI of the counter E. M. F. generator to flow through the resistance 93, with the result that the voltage of the counter E. M. F. generator will be reduced and the voltage or the generator 53 increased and the motor will be speeded up again. This regulating action will, of course, take place continuously and automatically and will operate to maintain the pressure of the wood on the stone practically constant at the desired degree.

If now there occurs a great reduction oi. pres-- sure of the wood on the stone such as would result froma jam 0! the wood in the magazine and a practical cessation of the feed of the wood, the pressure of the oil film being also greatly reduced, the Bourdon tube will cause the contact lever 31a to make contact with the point BI and the feed motor current supplied by the generator 63 increasing due to overload on the motor, the current relay will cause its contact plate 68 to engage with contact points 69 and 10, thereby clos ing the circuit at this point and closing the circuit through the coil I03 of the time delay relay A and it will act to close the circuit including the reversing contactor coil 90. This action will cause the reversing contactor core 89 to reverse the resulting current from the counter E. M. F. generator and the power line flowing to the field 81 of the generator 63, with the result that the direction of the current flowing to the feed motor will be reversed and the feed motor likewise reversed, thereby jumbling up the wood in the magazine and breaking up the jam. Due to the action of the time delay relay the motor will operate in reversed direction for a predetermined period of time, whereupon the relay will act to open the circuit at contact points I I0 and l l I through the coil 90 of the reversing contactor, and the contactor will act to reverse the current from the counter E. M. F. generator H and cause a resumption of the feeding action of the wood to the stone. This will cause the pressure of the oil film to rise, and the Bourdon tube will cause the contact lever to disengage the contact point BI and assume'its former neutral position ready to again resume its controlling action in maintaining a uniform pressure on the stone.

It will be understood that the circuit controlled by the contact lever 31a when it contacts with point BI, is normally open at contact points 69 and 70 01 the current relay a, since this relay will not become effective to cause its contact plate 68 to bridge the gap between the said contact points 59 and 10 and thus close the circuit, until the current supplied by the generator 83 rises to the capacity of the motor which it drives, which rise would occur, for instance, in the event of an interruption in the feed of the wood as by Jam of the wood in the magazine as hereinheiore described. The contact lever 37a will not therefore act to reverse the direction of the wood feeding means on every reduction of pressure of the oil film sufficient to cause the contact lever to make contact with contact point Bl, but only-in cases where the inability of the feed motor to advance the wood causes the current in the iced motor armature circuit to build up to a dangerous degree. Under such conditions, the core t? of the relay will be energized and will cause the contact plate 58 to engage with the contact points 68 and ill and thus close the circuit.

It will be apparent therefore, that the reversing contactor will be operated to reverse the current supplied to the field of the motor driving generator 53, only upon a rise of the current of the iced motor 62 suificient to actuate the current relay liBa, such as would result from a total or practically total suspension of the feed or" the wood and a continued operation of the feeding mechanism in the attempt to overcome the interruption in the feed.

"The controlling mechanism in the two forms of the invention hereinbefore described, constitutes in effect an automatically operating self-regulatring means which will act to maintain the pres: sure of wood against the stone at a uniform unvarying value for all practical purposes, thereby preventing undue pressure being applied to the stone and avoiding stone breakage, the said regulating means acting also to automatically break up jams of wood in the magazine which would otherwise interrupt the grinding operation.

Having thus described my invention, what I claim is:

1. In a wood grinder, the combination of a grindstone mounted for limited radial movement, means for feeding wood against the stone to exert pressure thereon, which pressure is subject to variations, a motor for driving the feeding means, a generator for supplying current to said motor, means for driving the generator, a power line connected in series with the held and the field rheostat oi. the generator, a circuit connected around said field rheostat and adapted when closed to shunt the same, and means operated by the said radial movement or said grindstone for controlling said shunt circuit.

2. In a wood grinder, the combination of a grindstone mounted for limited radial movement, means for feeding wood against the stone to exert pressure thereon, which pressure is subject to variations a motor for driving the feeding means, a generator for supplying current to the motor, means for driving the generator, a power line connected in series with the field and the field rheostat of the generator, a normally closed circuit connected around the field rheostat to shunt the same, and means operated by a radial movement of said grindstone upon an increase of pressure of the wood against the stone for opening said shunt 1 circuit; whereby the voltage of the generator will be reduced and the feed motor slowed down.

3. In a wood grinder, the combination of a grindstone mounted for limited radial movement, means for feeding wood against the stone to exert pressure thereon, which pressure is subject to variations, a motor for driving the feeding means, a generator for supplying current to said motor, means for driving the generator, a power line connected in series with the field and the field rheostat of the generator, a circuit connected around said field rheostat and adapted when closed to shunt the same, a relay fed from the power line and controlling said shunt circuit, a circuit connected around the relay to shunt the same. and means operated by radial movements of the grindstone resulting from variations of the pressure of the wood on the stone for controlling the last mentioned circuit.

4. In a wood grinder, the combination of a grindstone, its carrying shaft and bearings, means for feeding wood against the stone. to exert pressure thereon, an oil film between the shaft and bearings subject to said pressure, an electric motor for driving the feeding means, a generator for supplying current to the motor, a powerline connected in series with the field and the field rheostat of the generator, a circuit connected around said field rheostat to shunt the same, and

a movable element for controlling said circuit, said element being responsive to the pressure of the oil film.

5. In a wood grinder, the combination of a grindstone mounted forlimited radial movement, means for feeding the wood against the stone to exert pressure thereon which pressure is subject to variations, an electric motor to drive the feeding means, a motor driving generator to supply current to the motor, a power line connected in a circuit with the field of said generator, a counter E. M. F. generator in said circuit, a reversing contactor in said circuit between the counter E. M. F. generator and said generator field, a normally open circuit connected to the power line, a time delay relay in said normally open circuit, a current relay in the circuit supplying current to the motor and adapted when I the current is increased to a predetermined value to close the normally open circuit, and a movable element responsive to the radial movement resulting from the pressure of the wood on the stone to control said normally open circuit.

6. In a wood grinder, the combination of a grindstone, its carrying shaft and bearings thereautomatically by said radial movements on an undue decrease of said pressure, to reverse the direction of action of the feeding means.

7. In a wood grinder, the combination with a grindstone, its carrying shaft and bearings therefor, in which the shaft is movable radially, of means for feeding the wood against the stone, operating means for said feeding means whereby the stone and its carrying shaft will be subjected to pressure acting to move them radially varying degrees according to the variations'of pressure of the wood against the stone, and automaticallyopcrating, self-regulating, pressure controlling means including a device adapted to be operated by said radial movements of said shaft, and acting on an undue decrease of the said pressure to reverse the direction of the feeding means.

8. In a wood grinder, the combination of a I grindstone, its carrying shaft and bearings therefor in which the shaft is movable radially, means for feeding the wood against the stone, operating means for said feeding means whereby the stone and its carrying shaft will be subjected to pres. sure acting to move them radially varying degrees according to the variations of pressure of the wood against the stone, a device associated with said shaftand operated by said radial movements thereof and coacting with said operating means for feeding the wood against the stone, whereby the stone and its carrying shaft will be subjected to pressure acting to move them radially varying degrees in accordance with the variations of the pressure of the wood against the stone, an electric motor for driving the feeding means, a movable element associated with said shaft and operated by said radial movements thereof, an electric circuit including said movable element to be controlled thereby, said circuit being adapted to be closed on an undue decrease of the voltage of the motor driving current, and means actuated on the closure of said circuit-to reverse the direction of rotation of the feed motor.

10. In a wood grinder, the combination of a grindstone, its carrying shaft and bearings therefor in which the shaft is movable radially, means for feeding the wood against the stone, whereby the stone and. consequently its carrying shaft will be subjected to pressure acting to move them' radially varying degrees in accordance with the variations of the pressure of the wood against the stone, an electric motor for driving the feeding means, means for supplying driving current to the motor, a movable element associated with said shaft and operated by said radial movements thereof, and automatically operated means controlled by said movable element and said current supplying means which becomes operative on an undue decrease of the pressure, to reverse the current fed to the feed motor.

11. In a wood grinder, the combination of a grindstone, its carrying shaft and bearings therefor, means for feeding the wood against the stone' to exert pressure thereon, operating means for said feeding means, said feeding means being reversible in action to relieve the pressure of the wood on the stone, an oil film between the shaft and its bearings subject to said pressure, and automatically operating means controlled by variations of the pressure of the oil film and operative on an undue decrease of the pressure of the wood against the stone, to reverse the direction of action of the feeding means.

12. In a wood grinder, the combination of a grindstone mounted for limited radial movement,

connected with said movable means and said power line to operate on an undue decrease oi the pressure of the wood on the stone, to reverse the current through the E. M. F. generator and thereby reverse the current through the motor driving generator; whereby the direction of feed of the wood will be reversed.

13. In a wood grinder, the combination oi a grindstone, its carrying shaft and'bearings therefor in which the shaft is movably radially, means including an electric motor for feeding the wood against the stone, whereby the stone and consequently the shaft will be subjected to pressure acting to move them radially varying degrees in accordance with the variations of pressure of the wood against the stone, a generator for supplying current to the motor, means operated automatically by said radial movement of the shaft to vary the voltage of the generator, a power line circuit including the field of said generator, a resistance connected in said circuit, and a contact making wattmeter connected in a normally closed branch circuit around said resistance, said wattmeter being adapted, on rise of load on the feed motor to a dangerous point under extreme variations in pressure of the wood on the stone, to open the branch circuit and cause the generator field current to flow through the resistance, whereby the voltage of the generator and consequently that-of the motor will be reduced.

14. In a wood grinder, the combination of a grindstone, its carrying shaft and bearings therefor in which the shaft is movable radially, means including an electric motor for feeding the wood against the stone, whereby the stone and consequently the shaft will be subjected to pressure acting to move them radially varying degrees in accordance with the variations of pressure of the wood against the stone, a generator for supplying current to the motor, means operated automatically by said radial movement of the shaft to vary the voltage or the generator, and means including a contact making wattmeter adapted to insert resistance in the field circuit of the generator when the load on the feed motor reaches a dangerous value under extreme variations in pressure of the wood on the stone to reduce the generator voltage and hence the voltage applied to the feed motor.

15. In a wood grinder, the combination oi a grindstone having a carrying shaft and bearings therefor and mounted for radial movement, means including an electric motor for feeding wood against the stone whereby the stone and shaft will be subjected to pressure tending to move them in a radial direction dependent upon the variation of pressure of the wood against the stone, a generator for supplying current to the motor, means associated with said generator field and operated automatically by said radial movement of the shaft to vary the voltage of the supplying generator, and means including a contact making wattineter connected across a resistance in the field of said generator operative when the load on the feed motor reaches a dangerous value under extreme variations of pressure of the wood on the stone to cause insertion of said resistance into the field of the generator whereby voltage of the generator and consequently that applied to the motor will be reduced.

16. A Ward Leonard control system for grind- 5 ing machines for the production of wood pulp having a grindstone and a feeding means for urging wood and the like against said stone, comprising a motor for driving said feeding means, a

generator arranged to supply current to said motor thereby to control said feeding means, a field winding provided on said generator, means for controlling the energization of said field winding to cause said generator normally to supply current to said motor in a direction and of sufficient magnitude to urge the wood against said stone under a predetermined pressure, reversing means for said motor means responsive to a predetermined reduction in said pressure for operating said reversing means to reverse the.

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