Abstract:
An oscillating disc cutter including a cutting disc and a drive mechanism. The drive mechanism includes a drive shaft to effect eccentric oscillation of the cutting disc and a radial bearing disposed to permit relative rotation between the drive shaft and the cutting disc. The cutter further including a hydrostatic axial bearing disposed to react axial forces while accommodating induced rotation of the cutting disc when operatively engaged and to induce a rotational drag thereby limiting rotational peed of the cutting disc when free running. A water pressurized fluid bearing induces a predetermined axial load in the hydrostatic bearing such that a predetermined maximum running clearance in the hydrostatic bearing is maintained.

Description:
CROSS REFERENCE TO PRIORITY APPLICATIONS 
   This application is the U.S. National Phase of International Application No. PCT/AU03/00473, filed in Australia on 22 Apr., 2003, which designated the U.S., and claims priority to Australian Application No. PS 1868, filed 22 Apr. 2002, each incorporated by reference in its entirety. 
   FIELD OF THE INVENTION 
   This invention relates to an oscillating disc cutter with speed controlling bearings and has been devised particularly though not solely to prevent high speed rotation of a disc cutter when the cutting disc is disengaged from a rock face. 
   BACKGROUND OF THE INVENTION 
   Oscillating disc cutters of the type described in international patent specification WO 00/46486 (the contents of which are incorporated herein by way of cross reference) have the general requirement that a mechanism is provided to prevent the cutting disc from rotating at a high speed when the cutter is not engaging the rock face. It should be noted that the reference to international patent specification WO 00/46486 is not an admission that this publication forms part of the common general knowledge in Australia or in any other territory. 
   In normal cutting mode, when the disc cutter is presented to the cutting face the disc naturally rotates at about 30-40 rpm in the opposite direction to the shaft due to the rubbing friction caused by displacement difference between the diameter of the cutting disc and oscillating path diameter. It will be appreciated that this low speed rotation in the cutting mode is advantageous because it provides for even wear of the cutting disc and prevents temperature build-up at one point on the cutter. 
   However, during free running mode, when the cutter is not in contact with the rock face, torque transmitted to the disc from the shaft through bearing  609  (shown in FIG. 7 of WO 00/46486 and reproduced here as  FIG. 5 ), causes the disc cutter to rotate in the same direction as the shaft. Without some degree of control, the cutter would speed up to around the same speed as the shaft, i.e. around 3000 rpm. 
   Reapplying the cutter to the rock face causes an almost instantaneous acceleration of the disc from around 3000 rpm in one direction to around 30-40 rpm in the opposite direction. This can cause significant wear and damage to the cutting edge. In international patent specification WO 00/46486, a solution is proposed of using a gear arrangement shown generally  616  in  FIG. 5 , ( FIG. 7  of that specification). 
   Such a gear arrangement is heavy, prone to wear, maintenance issues, and causes additional drag when the cutter is engaged with the rock face. 
   It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. 
   SUMMARY OF THE INVENTION 
   Accordingly the present invention provides an oscillating disc cutter including a cutting disc and a drive mechanism, the drive mechanism including a drive shaft to effect eccentric oscillation of the cutting disc and a radial bearing disposed to permit relative rotation between the drive shaft and the cutting disc, the cutter further including a first axial bearing disposed to react axial forces while accommodating induced rotation of the cutting disc when operatively engaged and to induce a rotational drag thereby limiting rotational speed of the cutting disc when free running. 
   Preferably, the cutter further includes a second bearing to induce a predetermined axial load in the first bearing. 
   Preferably, the second bearing substantially reacts the axial forces induced by the first bearing. 
   Preferably, the first bearing is a oil operated hydrostatic bearing and the second bearing is a fluid pressurised and lubricated bearing. 
   Preferably, pressure in the fluid bearing is maintained at a level such that a predetermined maximum running clearance in the hydrostatic bearing is maintained thereby inducing shear forces in the oil of the hydrostatic bearing. Preferably, the shear forces cause rotational drag in the bearing thereby limiting the rotational speed of the cutting disc in when free running. 
   Preferably, the fluid bearing takes the form of a water-pressurised annulus. 
   Preferably, the limited rotational speed of the cutting disc is 0 to 100 rpm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Notwithstanding any other forms that may fall within its scope, one preferred form of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  is a cross sectional elevation through an oscillating disc cutter incorporating the present invention; 
       FIG. 2  is cross sectional view of a variation of the disc cutter shown in  FIG. 1 ; 
       FIG. 3A  is a partial view of a hydrostatic bearing face in accordance with the invention; 
       FIG. 3B  is cross sectional view of the bearing face shown in  FIG. 3A ; 
       FIG. 4A  is a reproduction of  FIG. 1  from WO 00/46486 and shows a part cross-sectional view of an oscillating disc cutting device taken; 
       FIG. 4B  is a reproduction of  FIG. 2  from WO 00/46486 and is an enlarged view of the cutting device of  FIG. 1 ; 
       FIG. 4C  is a reproduction of  FIG. 3  from WO 00/46486 and is a schematic view of the action of the cutting device in excavating a rock face; and 
       FIG. 5  is a reproduction of  FIG. 7  from WO 00/46486 and shows a part cross-sectional view of an oscillating disc cutting device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4A  is a cross-sectional view of an oscillating disc cutting device according to WO 00/46486. The cutting device  10  includes a mounting assembly  11  and a rotary disc cutter  12 . The mounting assembly  11  includes a mounting shaft  13  which is rotatably mounted within a housing  14 , that can constitute or be connected to a large mass for impact absorption. The housing  14  thus, can be formed of heavy metal or can be connected to a heavy metallic mass. The shaft  13  is mounted within the housing  14  by a bearing  15  mounted against a stepped section  16 . 
   The housing  14  can have any suitable construction, and in one form includes a plurality of metal plates fixed together longitudinally of the shaft  13 . Such an arrangement is shown in  FIG. 4B , and with this arrangement, applicant has found that a plurality of iron plates  17   a  and a single lead plate  17   b  provides effective impact absorption based on weight and cost considerations. 
   The shaft  13  is mounted for rotating motion about a central longitudinal axis AA. The shaft  13  includes a driven section  18  and a mounting section  19 . The driven section  18  is connected to drive means  20  at the end thereof remote from the mounting section by any suitable connectors, such as heavy duty threaded fasteners  21 , while a seal  22  is applied between the facing surfaces of the mounting section and the drive means. 
   The drive means  20  can take any suitable form and the means shown in  FIG. 4A  is a shaft that may be driven by a suitable engine or motor. The drive means  20  is mounted within the housing  14  by bearings  23 , which are tapered roller bearings. The bearings  23  are mounted against a stepped section  24  of the drive means  20  and against a mount insert  25  which is also stepped at  26 . The mount insert  25  is fixed by threaded connectors  27  to the housing  14  and fixed to the mount insert  25  by further threaded connectors  28  is a sealing cap  29  which seals against the drive means  20  by seals  30 . The sealing cap  29  also locates the outer race  31  of the bearings  23  by engagement therewith at  32 , while a threaded ring  33  locates the inner race  34 . 
   The mounting section  19  is provided for mounting of the disc cutter  12  and is offset from the axis AA of the driven section  18 . In this particular embodiment, the mounting section is also angularly offset from axis AA. The axis BB of the mounting section  19  is shown in  FIG. 1  and it can be seen that the offset angle α is in the order of a few degrees only. The magnitude of the offset between axis AA and BB determines the size of the oscillating movement of the disc cutter  12  whilst the magnitude of the angle α determines the degree of nutating movement. In other embodiments, the axes AA and BB may be offset parallel such that the angle α is zero. Such a configuration provides only oscillation and no nutation. 
   The disc cutter  12  includes an outer cutting disc  35  that is mounted on a mounting head  36  by suitable connecting means, such as threaded connectors  37 . The outer cutting disc  35  includes a plurality of tungsten carbide cutting bits  38  which are fitted to the cutting disc in any suitable manner. Alternatively, a tungsten carbide ring could be employed. The outer cutting disc can be removed from the cutting device for replacement or reconditioning, by removing the connectors  37 . 
   The disc cutter  12  is rotatably mounted on the mounting section  19  of the mounting shaft  13 . The disc cutter  12  is mounted by a tapered roller bearing  39 , that is located by a step  40  and a wall  41  of the mounting head  36 . An inclined surface  42  of the mounting head  36  is disposed closely adjacent a surface  43  of a mounting insert  44 . The surfaces  42  and  43  are spaced apart with minimum clearance to allow relative rotating movement therebetween and in this nutating embodiment, the surfaces have a spherical curvature, the centre of which is at the intersection of the axes AA and BB. 
   The disc cutter  12  is rotatably mounted to the mounting section  19  of the mounting shaft  13  by the tapered roller bearing  39  and by a further tapered roller bearing  53 . The bearing  53  is far smaller than the bearing  39  for the reason that the large bearing  39  is aligned directly in the load path of the disc cutter and thus is subject to the majority of the cutter load. The smaller bearing  53  is provided to pre-load the bearing  39 . 
   The oscillating movement of the disc cutter applies an impact load to the rock surface under attack, that causes tensile failure of the rock. With reference to  FIG. 4C , it can be seen that the motion of the disc cutter  12  brings the cutting tip or edge  58  into engagement under the oscillating movement at point  59  of the rock  56 . Such oscillating movement results in travel of the disc cutter  12  in a direction substantially perpendicular to the axis AA. Future chips are defined by the dotted lines  61 . The action of the disc cutter  12  against the under face  59  is similar to that of a chisel in developing tensile stresses in a brittle material, such as rock, which is caused effectively to fail in tension. 
   The direction S of impact of the disc cutter against the rock under face  59  is reacted through the bearing  39  and the direction of the reaction force is substantially along a line extending through the bearing  39  and the smaller bearing  53 . 
   The mass of the disc cutter is relatively much smaller than the mass provided for load absorption purposes. The load exerted on the disc cutter when it engages a rock surface under the oscillating/nutating movement, is reacted by the inertia of the large mass, rather than by the support structure. 
   The oscillating disc cutter of the present invention is generally similar in configuration to that described above. More particularly, the disc cutter shown in  FIGS. 1 and 2  is generally similar in configuration to that shown in  FIG. 7  of international patent specification WO 00/46486, and reproduced here as  FIG. 5 . Like numbers refer to the components in that drawing as described in the description of the international patent specification. 
   Instead of the bearings  605  and  606  being water lubricated, only bearing  605  in the present invention is water lubricated. Bearing  606  is replaced by a hydrostatic bearing  700  supplied with high pressure oil through an annular passageway  701  inside a demountable ring  702 , to which oil is supplied under pressure via nipple  703 . The bearing  700  contains pockets  800  in the normal manner of hydrostatic bearings. 
   As can clearly be seen in  FIG. 3A , these pockets may be in the form of a concentric grid pattern on the casing body opposing the disc  603 , however, in alternative embodiments they may take on any form as is known in the art of hydrostatic bearings. In this embodiment there are ten pockets  800  evenly disposed in a circular array around the bearing. Each pocket&#39;s extremity is defined by a peripheral groove  801 . A further oil channel groove in the form of a cross  802  dissects each pocket into four lands  803 . Referring to  FIG. 3B , these lands are at substantially the same height as the bearing surface between the pockets. Many hydrostatic bearings do not include these lands and the pockets are merely depressions in the bearing surface. However, in this embodiment, the lands effectively function to reduce the clearance gap between the bearing surfaces over a greater area thereby increasing the shear in the oil and enhancing the viscous drag characteristics of the bearing. 
   Oil is feed into the centre of each cross through a respective flow control orifice  706 . Each respective orifice regulates the oil in each of the pockets of the bearing as is common in hydrostatic bearings. 
   Referring to  FIG. 2 , oil exiting the bearing is able to seep either directly into the body of the device between bearings  609  and  610  or into outer drain channel  705  at the periphery of the bearing. 
   Providing a set minimum load on the hydrostatic bearing is fluid bearing  605 . This fluid bearing maybe considered simply as a pressurised annulus, however, is referred to throughout as a fluid lubricated bearing. The fluid bearing surfaces include an annular plate portion of the disc  603  and a corresponding portion of the cutter housing opposing the annular plate. These bearing surfaces are separated by an annular gap into which water is introduced at pressure through a series of passageways  607 . A hose and hose fittings (not shown) may be used to transport pressurised water from a pressure pump (not shown). In this embodiment the water is en-route to the cooling jets for the cutting edge of the cutter however, in other embodiments, separate cooling water and bearing water systems may be used. In still further embodiments, different fluids may be used for cooling and pressurizing the fluid bearing. 
   The pressurised water provides a force on the plate thereby maintaining clearance between the bearing surfaces and providing an opposing force to the hydrostatic bearing. It will be appreciated that by regulating the pressure of the water, the magnitude of opposing force may also be regulated. Accordingly, by carefully controlling the water pressure in the fluid bearing and the oil pressure in the hydrostatic bearing, the clearance between the faces of the hydrostatic bearing can be set. 
   It will also be appreciated that the fluid bearing allows for a minute amount of axial yaw if the cutter head is differentially loaded. Such differential loading is accommodated and resisted by the hydrostatic bearing. 
   The fluid bearing surfaces may be covered with an antifriction material, as a safety measure should the bearing surfaces contact, for instance, as a result of failed water supply or during transport. 
   Typical values for the oil pressure supplied to the hydrostatic bearing and water pressure supplied to the fluid bearing are 14,000 kPa and 800 kPa respectively. 
   In operation, the cutter is powered by a 2-pole induction motor which, with a power supply at 50 Hz, rotates the dive shaft  612  at a speed of around 3000 rpm. Of course, alternative power supplies and a range of cutting speeds may be used. 
   However, it will be appreciated that drag inherent in the fluid and hydrostatic bearings provides a balancing torque to counter the rotation of the disc. By carefully selecting an appropriate pressure level in the fluid bearing, the clearance between the faces of the hydrostatic bearing are such that the rate of shear of the oil will rise with increasing speed of the disc. The friction developed due to the shear in the oil balances the rotation causing torque thereby limiting the free running speed of the disc to a desired value. 
   It will be appreciated that as well as rotation speed and clearance in the hydrostatic bearing, the frictional forces developed will also depend upon the design of the hydrostatic bearing surfaces and oil pockets and viscosity of the oil used. In turn, oil temperature will affect oil viscosity and therefore bearing performance. In this embodiment, standard hydraulic fluid is used however, other appropriate oils may be used as a replacement. The relationship between the viscosity of the oil selected and temperature is critical when selecting the oil. 
   Accordingly, the pressure of water supplied to the water lubricated bearing, the oil type, and the oil viscosity, temperature and pressure in the hydrostatic bearing are all carefully selected and controlled where appropriate to ensure correct function of the bearing and to avoid damage to the parts. In this regard the oil is passed through a heat exchanger of sufficient capacity to control the oil temperature. 
   An additional retardation force may be applied by drag inherent in the fluid bearing. Disengaging the cutter from the rock face reduces the axial load on the hydrostatic bearing which in turn causes the disc  603  to be forced closer to the water lubricated bearing surface  605 . This may provide for an increase in drag thereby preventing the disc  603 , to which the disc cutter  602  is bolted, from rotating at a high speed when the cutter is not engaging the rock face. 
   In this embodiment, the free running speed is selected to be about 30-40 rpm. While this is in the reverse direction to the operational speed, the difference is small enough to prevent damage and substantial wear to the cutter disc. However, in alternative embodiments, the parameters of the system may be selected to provide for virtually any free running speed desired in the direction of the shaft. 
   Accordingly, the drag in each axial bearing combines to eliminate the need for the gear arrangement  616  referred to in the description of  FIG. 7  in international patent specification WO 00/46486. 
   In alternative embodiments of the invention, other types of axial load bearings known in the art may replace the hydrostatic and fluid lubricated bearings. For instance, the hydrostatic bearing may be replaced by a Michell bearing and the fluid lubricated bearing may take to form of a mechanical, hydrodynamic, electromagnetic or other type of bearing able to withstand and/or provide an axial load. In such embodiments, one or other of the bearings may have a more significant effect in controlling the speed of the cutter disc when free spinning. 
   Although the cutting device is of the type generally described in WO 00/46486, it will be appreciated that various types of similar cutting devices may be used, with or without the nutating feature described in that patent specification. 
   It will be appreciated that the invention provides an effective means for limiting the speed of the cutter disc when in free running mode without the use of mechanical parts which are comparatively higher wearing. 
   Thus, in essence, the water lubricated bearing  605  and the hydrostatic bearing function as drag brakes on the rotation of the disc  603  and hence of the cutter  602 . 
   Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.