CONTROL AND OPERATION OF A CLUTCH DRIVE SYSTEM

Abstract: A motor-driven drive shaft (12) is coupled to a grinding mill (4) or other unbalanced rotatable member through a clutch (5) arranged to accommodate misalignment, the clutch being so controlled during start-up as to be initially engaged to effect a limited preliminary rotation of the mill in the drive direction, then disengaged to permit the mill to rotate under its own weight through a first rotation in the direction reverse to the drive direction and through a second rotation in the drive direction, and then to be re-engaged during the second rotation of the member in the drive direction. Clutch orbiting is thus reduced if not eliminated.

Description: CONTROL AND OPERATION OF A CLUTCH DRIVE SYSTEM This invention relates to the control and operation of a clutch drive system, and particularly, but not exclusively, to the drive systems of grinding mill equipment. A typical grinding mill installation, as used in the treatment of ores, comprises an electric motor which drives the actual mill itself through a set of gears. In order to minimise starting current drawn by the motor, when the grinding mill is set in operation, a clutch is included in the drive system. The mill with its unbalanced load is very heavy and to maintain precise alignment between the clutch drive and driven shafts would be a difficult, lengthy and costly exercise, because of dynamic movements occurring in operation. Moreover, if the shafts are aligned in cold conditions they tend to be misaligned after a few hours of running because of differential thermal movements. Also, the gear set generates forces which can combine to yield associated bearing forces that have not only different levels, but also different directions. The clutches employed in grinding mill installations thus of a kind capable of accommodating misalignment but such clutches can readily exhibit unacceptable orbiting. It would accordingly be very advantageous to provide a method of and a means for coupling a rotary drive to an unbalanced driven member through a clutch accommodating misalignment between the drive and the load, such that clutch orbiting is reduced or eliminated. The invention accordingly provides a method of operating an installation in which a rotary drive is applied from a motor through a clutch to a driven member, for example a grinding mill, the method comprising a start-up procedure in which the rotary drive is applied to the driven member with the member already rotating in the drive direction. The unacceptable clutch orbiting results from failure of the clutch to accommodate itself to misalignment where the shafts, or one of the shafts, which it connects lack adequate stiffness to enforce such accommodation. By effecting clutch engagement with the driven clutch part already rotating in the drive direction the accommodation required of the clutch is reduced to a level at which the necessary adjustment readily ensues. The preliminary rotation of an unbalanced driven member can be conveniently obtained by initial application of the rotary drive to impart a movement to the member in the drive direction, the drive being discontinued to allow the member to rotate back beyond its initial position and then forward again, so as to then have the desired rotation for again receiving the rotary drive, after which normal operation follows. The invention accordingly also provides a control system for an installation in which a rotary drive is applied from a motor through a clutch to an unbalanced driven member, for example a grinding mill, the control system comprising means responsive to a start signal to engage the clutch so as to apply the rotary drive to the unbalanced driven member, disconnecting the drive from the member after rotation thereof through a predetermined angle, and again applying the drive after the member has rotated back beyond the starting position and has resumed rotation in the driven direction. The periods during which the clutch is initially engaged and then disengaged can be preset, one or both preset periods being preferably selectively adjustable, or one or both can be condition responsive, as to the sensed angular position of the unbalanced driven member. The invention is further described below, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a grinding mill installation in accordance with the invention; Figures 2A & 2B are schematic side views of a clutch incorporated in the installation of Figure 1 in different respective positions; and Figure 3 graphically displays operation of the clutch of Figures 2A and 2B during start-up. In Figure 1 of the drawings there is illustrated a grinding mill installation 1 comprising an electric motor 2 of which the rotary output drive is applied to a grinding mill 4 successively through a clutch 5 and gearing 6. The operation of the installation 1 is under control of a controller 9 by which energization of the motor 2 and actuation of the clutch 5 is effected. The clutch 5 comprises a driving hub 11 mounted on the drive shaft 12 of the electric motor 2. The hub 11 carries two, or typically more, annular driving plates 14 by means of a splined connection 15, such that the plates are capable of limited movement along the hub axis and also radially of the axis. The clutch 5 also comprises a driven outer ring 16 mounted on a shaft 17 by way of a spider member 19. The ring 16 carries at least one annular driven friction disc 20 by a splined connection 21 at the outer edge of the disc, which permits limited axial and radial movement of the disc relative to the axis of the shaft, which may not be aligned with the common axis of the driving hub 11 and shaft 12. The or each driven friction disc 20 is received between an adjacent pair of the driving plates 14. The shaft 17 is secured to a pinion of the gearing 6 by which the rotational drive is applied to the grinding mill 4. The clutch 5 is engaged by supplying pressure air from a source 25 to pneumatic actuators 26 which effect axial compression of the stack of driving plates and friction discs against a return spring. On operation of a start-up press-button 30, the controller 9 first effects energization of the electric motor 2 to set the shaft 12 and the driving hub 11 into rotation. As the driving hub 11 rotates, the driving plates 14 tend to centralise about the driving hub axis under centrifugal force, moving from the positions of Figure 1 to those of Figure 2A. The controller 9 now begins engagement of the clutch by operation of the actuators 26, and the friction discs 20 are gripped by the driving plates 14 with the discs in a position offset from the driven ring axis, as shown in Figures 2A. As the rotational drive begins to be applied to the grinding mill 4, the friction discs 20 exert a radial force on the outer ring 16 and a reaction force on the driving hub 11, which forces would reach a maximum if the friction discs were rotated through 180°, as illustrated in Figure 2B f and would return to a minimum if a complete 360° rotation were to be effected. These cyclic forces would be resisted by the stiffnesses of the driving and driven shafts 12 & 17 which, if sufficient, would enforce radial slippage of the friction discs 20. If the shaft stiffnesses were insufficient to occasion this radial slip, the driving hub 11 and the driven ring 16 force each other to deflect in a cyclic manner, that is, clutch orbiting ensues, in an amount dependent on the dimensional relationships of the clutch parts. Because of the substantial weight of the grinding mill, the torque needed to start the mill moving is very large so that correspondingly high forces are generated within the clutch 5. In a typical grinding mill, the pneumatic pressure may reach 1.57 bar, corresponding to a gripping force of 110,000 Newtons. In accordance with the invention therefore, the controller 9 causes the clutch 5 to be uncoupled after the mill 4 has been rotated initially through a small angle, preferably within the range of 10 to 20°. The mill 4 is of course unbalanced because the load it contains occupies its lowest region, so the mill will begin to rotate in the reverse direction when the clutch 5 is released. The inertial effect of the load will cause the mill to turn back beyond its starting position and then again rotate in the drive direction. As the mill reaches the start position, the controller 9 causes air pressure to be applied to the actuators 26 to effect clutch engagement again. As a consequence of this procedure, the second engagement of the clutch 5 takes place with the driven friction discs 20 already rotating at a reasonable speed. The radially acting forces when the friction discs 20 have rotated through 180° is substantially reduced. Accordingly, these factors allow the radial slip required between the driving plates 14 and the friction discs 20 to be minimised and to occur without orbiting. An operational sequence is illustrated in Figure 3, in which the rotational speed of the driven friction discs 20, and the air pressure within the clutch actuators 26 are plotted against time. It will be seen that, on start up, the clutch pressure rises to around 20 psi in the first two seconds and then falls back to zero after three seconds. The friction disc rotational speed changes approximately similarly, but the reverse rotational movement begins before four seconds from the start time and ends just before eight seconds from the start time. At just before 10 seconds from the start time, the mill is rotating through its start position in the driving direction. The clutch 5 is then engaged and driven rotation of the mill in the driving direction follows. Although the rate of increase of the air pressure applied to the actuators 26 is shown as being uniform, the controller 9 could be arranged to provide for an initial application of this clutch air pressure at a lower rate, as indicated in Figure 3 by the chain dotted line. The controller 9 can operate in accordance with a predetermined cycle , but at least one of the periods of initial clutch engagement and subs equent temporary disengagement is preferably selectively adjustable , as by adjustment input means 31 for the controller 9 , in dependence on the grinding mill load for example . The or each period can instead be dependent on sensed load or performance characteristics of the grinding mill. The angular position of the mill 4 can for example be sensed by a sensor 35 the output of which is supplied to the controller 9 . The invention can of course be embodied in a variety of ways other than as specifically described and illustrated.

Claims: -1- CLAIMS 1. A method of coupling a rotating drive shaft to an unbalanced rotatable member through a clutch arranged to accommodate misalignment , the method comprising the steps of : initially engaging the c lutch to ef f ect a limited preliminary rotation of the member in the drive direction, disengaging the clutch to permit the member to rotate under the weight thereof through a f irst rotation in the direction reverse to the drive direction and through a second rotation in the drive direction, and re-engaging the clutch during the second rotation of the member in the drive direction. 2. A method as claimed in claim 1 wherein the initial engagement effects the limited preliminary rotation through an angle in the range of 10 to 20° . 3. A method as claimed in claim 1 or 2 wherein at least one of the steps of disengaging and re-engaging the clutch is in response to a sensed angular position of the member. 4. A method as claimed in claim 1 , 2 or 3 wherein the clutch is re-engaged when the rotary member is at the position thereof at the initial engagement of the clutch. 5. A method of applying a rotary drive to a rotatable member by way o f a c lutch c apab l e o f ac c ommodating misalignment between the axes of rotation of the rotary drive and the rotatable member, in which the clutch is engaged to apply the rotary drive when the rotatable member is rotating in the direction of the rotary drive . 6. A method as claimed in claim 5 wherein the rotation of the rotatable member in the direction of the rotary drive is effected by the preliminary steps of initially engaging the clutch to ef fect an initial rotation of the rotatable member in the drive direction from an initial position of the rotatable member , and disengaging the clutch bef ore the rotatable member has rotated through 180 ° to allow return rotation of the rotatable member under gravity followed by the rotation in the rotary drive direction. 7. A method as claimed in claim 6 comprising engaging the clutch during rotation of the rotatable member through the initial position. 8. A clutch drive system in which a motor drive shaft is selectively coupled to and de-coupled from an unbalanced rotatable load by engagement and disengagement of a clutch arranged to accommodate misalignment between the drive shaft and the rotatable load, control means for the clutch responsive to a start-up signal successively to engage the clutch to effect limited rotation of the load from a starting position thereof in a drive direction, to disengage the clutch, and to re-engage the clutch when the load is inertially rotating in the drive direction after having rotated back beyond the starting position. 9. A clutch drive system as claimed in claim 8 wherein the control means is arranged to engage and disengage the clutch for fixed predetermined periods. 10. A clutch drive system as claimed in claim 8 having means for selectively adjusting at least one of the periods of engagement and disengagement of the clutch. 11. A clutch drive system as claimed in claim 8 having sensor means providing an output dependent on the angular position of the load, and wherein the control means is responsive to the sensor means output to commence at least one of the disengagement and the re-engagement of the clutch. 12. A clutch drive system as claimed in claim 8, 9, 10 or 11 wherein the clutch comprises first annular clutch discs carried by and around the drive shaft for limited axial and radial movement, a ring drivingly connected to the load, and second annular clutch discs carried by and within the ring for limited axial and radial movement, the second clutch discs being interleaved with the first clutch discs and frictionally engageable therewith on engagement of the clutch. 13. A clutch drive system as claimed in any one of claims 8-12 wherein the unbalanced load comprises a grinding mill and material charged therein, and comprising reduction gearing between the clutch and the grinding mill. 14. A clutch drive system as claimed in any one of claims 8-13 wherein the clutch includes pneumatic actuator means operable to effect engagement of the clutch, and wherein the control means effects operation of the actuator means at an initial lower pressure on re-engagement of the clutch.

Inventors: WALTER DAVID THOMAS, WALTER, DAVID, THOMAS
Applicants: TI INTERLOCK LTD, TI INTERLOCK LIMITED
Application year: 1992
Main classification: B02C17
IPC codes: B02C17, F27B7