Abstract:
Disclosed is a drive unit operable to translate and rotate a shaft, the drive unit including: (i) a hollow electric motor having a rotor, (ii) a fluid cylinder, (iii) means for connecting the shaft to the rotor of the hollow electric motor, (iv) means permitting the shaft to move lengthwise coupled with the fluid cylinder, and (iv) means connecting the fluid cylinder to the shaft, whereby the shaft may be rotated by the hollow electric motor and moved lengthwise by the fluid cylinder, and wherein the fluid cylinder has an outer wall coupled with the rotor of the hollow electric motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation patent application of prior U.S. patent application Ser. No. 10/873,288, which now U.S. Pat. No. 7,316,553 was filed 23 Jun. 2004 (Applicant reference number H-723-0-US). This divisional patent application also claims the benefit and priority of U.S. patent application Ser. No. 10/873,288, filed 23 Jun. 2004. 
    
    
     TECHNICAL FIELD 
     This invention relates to drive apparatus for rotating and translating a shaft. The invention is particularly useful for driving a plasticating screw of an injection-molding machine. More specifically, the invention relates to drive apparatus for rotating and reciprocating a plasticizing screw of an injection-molding machine wherein the screw is rotated by a hollow electric motor and reciprocated by a hydraulic piston. 
     BACKGROUND 
     The use of hollow motors and hydraulic pistons to drive and rotate plasticating screws is known. However, none of the known systems suggests combining the advantages of hollow motors to rotate the plasticating screw while using a hydraulic piston to move it lengthwise. 
     U.S. Pat. No. 4,105,147 to Stubbe describes a screw extruder rotated by a gear drive from an electric motor and moved lengthwise by a hydraulic piston. The screw has a splined shaft end to permit sliding of the shaft through the gear drive. 
     The U.S. Pat. No. 4,895,505 to Fanuc Ltd. describes a linear motor for moving an injection screw linearly. The linear motor includes a series of permanent magnets attached to the motor armature that react with the alternating current supplied to the surrounding stator windings to cause linear movement of the armature and the screw shaft attached to the armature. The patent describes the use of a hollow motor to move a screw shaft linearly. 
     The U.S. Pat. No. 5,540,495 issued Jul. 30, 1996 to Krauss-Maffei describes an extruder screw drive that includes a first motor for translating movement of the screw and a second motor for rotating the screw. The described embodiment shows two hollow motors. The drive means for translating the screw and the slide means for rotating the screw fit partially within one another. 
     U.S. Pat. No. 5,645,868 to Reinhart describes a drive apparatus for an injection unit that includes a hollow electric motor that engages the screw shaft through three clutches. One clutch provides rotation of the screw, a second enables forward movement of the screw and a third prevents the screw from rotating while it is being moved forward. No hydraulic units are used. 
     U.S. Pat. No. 5,747,076 to Jaroschek et al describes an injection-molding machine that uses a hydraulic piston to assist an electric motor driving a rack and pinion mechanism to advance the screw. 
     The U.S. Pat. No. 5,804,224 issued Sep. 8, 1998 to Fanuc Ltd. describes an arrangement where a ball screw is integrally formed on the rotor shaft. A motor positioned coaxially with it rotates the ball screw. 
     The U.S. Pat. No. 5,891,485 issued Apr. 6, 1999 to Sumitomo describes an injection apparatus that includes two hollow shaft electric motors. One motor is intended to rotate the screw shaft while the other moves it lengthwise. The rotors of the two motors are coupled to the shaft. Each rotor is located in a separate chamber. 
     U.S. Pat. No. 6,068,810 to Kestle et al describes an injection unit having a quill inside a piston to enable retraction and extension of the screw by the application of hydraulic pressure. A motor rotates the quill, which is connected to the piston through a spline to thereby rotate the screw. The motor attaches to the end of the quill. 
     U.S. Pat. No. 6,108,587 to Shearer et al describes an injection molding system that includes a motor for driving gears to rotate the screw and a hydraulic piston for translating the screw. 
     U.S. Pat. No. 6,478,572 to Schad describes an injection unit that uses a single electric motor to rotate an extruder screw and charge a hydraulic accumulator. The charge in the accumulator is directed to stroke the extruder screw. 
     U.S. Pat. No. 6,499,989 describes a device for removing disks from a mold. In the described embodiments a hollow electric motor is used to rotate the take-out shaft and a linear electric motor is used to move the shaft linearly. The hollow motor drives the shaft through a gearbox that enables the speed of the shaft to be varied. As an alternative, the patent suggests that a pneumatic or hydraulic cylinder could be used to move the shaft linearly. In the embodiments described, the linear actuator is located outside the rotary actuator. This provides an assembly that is larger and less cost effective. 
     U.S. Pat. No. 6,517,336 to Emoto et al and European Patent No. 0967064 A1 to Emoto disclose an injection molding system having a hollow electric motor that rotates a screw shaft and at the same time causes the shaft to advance by means of a connection to a ball screw shaft/spline shaft unit. A separate metering motor rotates the screw to load the screw with resin. Rotational movement is provided through a belt and pulley arrangement that can rotate the screw independently of the rotor on the hollow motor. The rotor on the hollow motor is attached to a splined portion of the screw shaft and is used to rotate the splined portion, which, in turn, rotates a ball screw to drive a ball nut and thereby move the shaft lengthwise. 
     U.S. Pat. 6,530,774 to Emoto describes an injection molding system using an electric motor and gear train to rotate the screw and a hollow shaft electric motor to move the screw lengthwise by driving a ball screw shaft through a splined shaft connection. 
     U.S. Patent Application No. 2002/0168445 A1 to Emoto et al describes an injection system that also includes a metering motor and a hollow shaft motor to rotate the screw and move the screw lengthwise, respectively. 
     The European Patent application 1162053 published Dec. 12, 2001 to Krauss-Maffei describes a two motor system where one motor provides rotational movement of the screw shaft and the other motor provides translational movement of the screw shaft. Clutch arrangements are used to enable the motors to operate separately or together. 
     The Japanese Patent 61266218 published Nov. 25, 1986 to Sumitomo describes a two motor injection system using hollow motors, a ball drive mechanism and splined shafts. 
     German Patent DE 10 135 443 discloses an injection unit for a plastics injection molding machine, featuring a plasticizing screw with a rotary drive and an electric motor with a stationary component mounted on a frame and an output component providing the linear axial screw motion for injecting the plastic melt into a mold. In known injection units of this kind, the output component of the electric motor is located in the power train between the drive component and the screw. The electric motor can be a linear or a rotary type whose rotary motion is converted into the linear motion of the output component. In both cases, the force that can be applied through the output component is limited. 
     PCT Patent Application WO 03/046388 A1 discloses an actuator (1) having a housing (1a) in which a driving shaft (2) is accommodated provided with a through bore (7), which through bore series as an actuated cylinder (3), at least one piston (4) mid a piston shaft (5) extending axially in said through bore, said at least one piston and said piston shaft being rotatable together with said driving shaft. Based on the known prior art, it is the aim of the invention to provide an improved actuator according to the above preamble, which obviates the described drawbacks, and which allows aa accurate and stable positioning of the piston shaft in axial direction. As a technical solution for this object the actuator according to the invention is characterised in that said at least one piston and said piston shaft is movable back and forth in both directions within said through horn hy actuating said cylinder. This allows an accurate positioning of the piston shaft relative to the driving shaft enabling the accurate operation of the actuator under various operating conditions. 
     While these references describe many combinations of electric and hydraulic driving systems for a screw of an injection-molding machine, they fail to describe a system combining the unique advantages of better control of the positioning of the screw with a hollow electric motor and the high injection power provided by a hydraulic injection unit. The present invention provides a compact injection unit having the unique advantages of both electric and hydraulic driving systems. 
     SUMMARY 
     The present invention provides a drive unit operable to translate and rotate a shaft, the drive unit including: (i) a hollow electric motor having a rotor, (ii) a fluid cylinder, (iii) means for connecting the shaft to the rotor of the hollow electric motor, (iv) means permitting the shaft to move lengthwise coupled with the fluid cylinder, and (iv) means connecting the fluid cylinder to the shaft, whereby the shaft may be rotated by the hollow electric motor and moved lengthwise by the fluid cylinder, and wherein the fluid cylinder has an outer wall coupled with the rotor of the hollow electric motor. 
     According to one general aspect of the present invention, the drive unit is a part of an injection unit for an injection-molding machine with a hollow electric motor to rotate the injection screw and a hydraulic piston to reciprocate the screw. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional sketch of a basic drive unit in accordance with the invention; 
         FIG. 2  is a cross-sectional side view of a preferred embodiment of the drive unit for an injection molding machine where the drive unit is in an extended position; 
         FIG. 2A  is a cross-sectional view of the piston head for the drive unit shown in  FIG. 2 ; 
         FIG. 2B  is a partial sectional view illustrating a hydraulic supply channel to the piston of the drive unit shown in  FIG. 2 ; 
         FIG. 2C  is a cross-sectional view of a portion of the piston and spline insert; 
         FIG. 2D  is a cross sectional view of the timing belt and encoder; 
         FIG. 3  is a cross-sectional side view of the preferred embodiment of the drive unit for an injection molding machine where the drive unit is in a retracted position; 
         FIG. 4  is a perspective view of the piston and spline insert of the preferred drive unit; 
         FIG. 5  is a cross-sectional sketch of another embodiment of the invention; 
         FIGS. 6A and 6B  are cross-sectional views of another embodiment of the invention having the driving cylinder surrounding the hollow motor; 
         FIG. 7  is a cross-sectional side view of a further embodiment of the invention; 
         FIG. 7A  is a sectional view of the embodiment shown in  FIG. 7  taken along the section line  7 A- 7 A. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates the invention in a simple form. As shown in  FIG. 1 , a hollow shaft motor  45  has a housing  61 , a stator  46  and a rotor  47 . Stator  46  is shown mounted on a wall of housing  61 . The rotor  47  is fixed onto cylinder  48 . Cylinder  48  has a spline portion  49  formed on its interior surface. An insert fitted onto the cylinder  48  could replace the spline portion  49 . The spline portion  49  engages splines  62  (one shown) on a piston  50 . A shaft (not shown) integral with or attached to the piston  50  is rotated by motor  45  through the interconnection between the rotor  47  and the piston  50 . 
     The shaft attached to the piston  50  is moved lengthwise by applying fluid pressure to either side of the head of the piston  50  through openings  51  and  52  in the wall of cylinder  48 . When the drive unit is being used in an injection-molding machine, the fluid might be hydraulic oil or a water-based graphite solution. Piston  50  slides on spline portion  49  and rotates in bearings provided by wear rings  53   a  and fluid seals  53   b . The entire assembly of rotor  47 , cylinder  48  and piston  50  is rotatably supported and axially located in bearings  63  and  64 . 
     While  FIG. 1  illustrates a rudimentary sketch of the invention, those skilled will be able to make any minor modifications necessary to the construction of an acceptable drive unit in accordance with the invention. For example, means other than a spline shaft could be provided to permit sliding of the shaft while keeping the shaft rotatable. A single key sliding along a keyway could be used. 
     The drive unit will now be described with reference to a plasticating screw for an injection-molding machine. The invention is particularly suited to use in such a system where it is necessary to rotate the screw to melt the injection material and move the screw lengthwise with significant driving force to inject the material into a mold. 
     Referring to  FIGS. 2 and 3 , a screw  1  resides in a barrel  2  and can rotate and move axially therein. Injection material, such as plastic pellets, is fed to screw  1  through opening  4 . Barrel  2  is mounted in injection housing  3  and kept in place by means of a barrel retaining plate  5 . The slot  6  is designed to receive a tool to hold the screw  1  in place while the piston  23  is rotated to unscrew the piston  23  from the screw  1  at the threaded connection  29 . Piston stop  7  is designed to prevent rotation of the tool when the piston  23  is being retracted from the screw  1  and determines the fully extended position of the piston  23 . This facility is provided to enable removal and replacement of the screw  1  when necessary. 
     The forward portion of piston  23  contacts the cylinder wall  18  through piston rings  45 . The piston  23  moves axially along the wall  18  as the screw  1  is advanced and retracted. Spline slots  17  slide in spline insert  15  to enable the piston  23  to move lengthwise. 
     The hollow motor  30  rotates piston  23  and thereby screw  1 , which is attached to piston  23 . Connector box  8  provides power to the motor  30  through wire channel  9 . Stator  12  is energized to rotate the rotor  16 . The motor  30  preferably has a permanent magnet rotor, however, any hollow electric motor could be used to rotate the piston  23  and screw  1 . The rotor  16  is shrink fitted to the cylinder wall  18 . The rotor  16  can be attached in any other way to the wall  18  so long as the rotor  16  and wall  18  move as a single unit. Spline insert  15  is connected to cylinder wall  18  by means of bolts  44 . Spline insert  15  engages slots  17  (best shown in  FIG. 4 ) on the exterior wall of piston  23 . Thus, when rotor  16  rotates, cylinder wall  18  and piston  23  also rotate so there is no relative rotational motion between the cylinder wall  18  and the piston  23 . 
     Cooling channels  10  are provided in motor housing  11  to enable cooling of the motor  30 . Piston head  24  is attached to the rearward end of piston  23  by bolts  31  and includes a plurality of channel openings  37  (see  FIGS. 2A and 4 ) between regions  32  and  33 . This enables the piston  23  to be of minimal thickness. Piston head  24  rotates and slides on cylinder wall  22  by means of piston rings  45   a . Hydraulic fluid such as hydraulic oil is supplied to regions  32  and  33  through hydraulic fluid channel  25  in rear housing  26  to propel piston  23  and screw  1  forward to inject material into a mold. 
     The piston  23  and attached screw  1  are retracted by means of the build-up of material at the head of the screw  1  in a manner well known in the art. To prevent voids in the melt, a low pressure is applied through the region  32  to the bore side of the piston  23 . Slots  38  (See  FIG. 2C ) are provided in spline insert  15  to ensure fluid communication between regions  34  and  35 . 
     The cylinder wall  18  is supported in roller bearing races  13  and  14  to facilitate rotation of the assembly with minimal friction losses. Roller bearing race  13  is supported in end piece  41  and ball bearing race  14  is supported by ring  89 . 
     Dowels  27  extend from motor housing  11  into end piece  41  and cylinder ring  36 . The dowels  27  prevent any tendency for the end piece  41  and cylinder ring  36  to rotate relative to the motor stator  12  as a consequence of rotational pressures created by the rotation of the rotor  16  and piston  23 . 
     Dowels  28  extend from rear housing  26  into cylinder wall  22  to prevent any tendency of the cylinder wall  22  to rotate in response to rotation of piston head  24 . 
     Cylinder wall  22  is in sealing engagement with cylinder ring  36  and rear housing  26 . As these seals are only subject to radial stress, they are less likely to leak or rupture than seals that are subjected to both radial and axial stresses. 
     Tie rods  19  extend from the rear housing  26  to the barrel retaining plate  5  and housing  3  to clamp the entire drive assembly together. 
     Temposonic rod  20  is attached to rear housing  26  and extends through an opening in piston head  24 . A magnet assembly  21  on piston head  24  responds to movement of piston head  24  to send a signal through rod  20  that indicates the position of piston head  24  and consequently screw  1  in a manner well understood by those skilled in injection-molding. 
     The rotational speed and position of screw  1  is determined by means of a timing belt  39  and encoder  40  in a manner well understood in the art of servomotor control. 
     In operation, the region  32  is pressurized through port  25 . This forces piston  23  and the attached injection screw  1  to move forward. Plastic in front of the screw  1  is injected into a mold cavity. At the end of the injection, region  32  is retained at a lower pressure for a short duration. The region  32  is then depressurized and region  35  pressurized so that piston  23  retracts a short distance. The hollow motor  30  turns on to rotate the piston  23  and the attached screw  1  to melt plastic pellets supplied to the screw  1  through opening  4 . During this interval, it may be necessary to keep a relatively low pressure in region  32  to prevent voids and bubbles from forming in the melt. The motor  30  is stopped when the screw  1  retracts to a predetermined position. Further retraction of the screw  1  may occur to relieve the melt pressure. After the screw  1  has fully retracted, the next injection cycle is initiated and the injection process is repeated to provide melt to the mold cavity. 
       FIG. 5  illustrates schematically a further embodiment of the invention. In this embodiment the rotor  54  is firmly attached to a piston  55  and has a width at least as wide as the combined length of the stroke of the piston  55  and the width of the stator  56 . Piston head  57  reciprocates in cylinder  58 . 
     Cylinder  58  is shown with a single fluid inlet  159 . A second inlet could be provided, however, in some applications a second inlet may not be required. For example, in the case of a plasticating screw for an injection-molding machine the build-up of plastic injection material at the end of the screw may provide sufficient pressure on the screw to move the piston back to its injection position. 
     This embodiment has the advantages of keeping the entire motor out of the hydraulic portion of the drive and removes the need for a spline shaft connection since the piston  55  is free to rotate and translate on the bearings  59  and  60 . 
     The embodiment shown in  FIG. 5  could be further modified to make the stator  56  longer and the rotor  54  shorter. The drive unit would operate in the same manner but the reduced size of the rotor  54  would reduce the weight on the piston  55  and reduce the cost of the motor. 
     In the embodiment of the invention shown in  FIGS. 6A and 6B , the drive cylinder surrounds the hollow motor. Stationary cylinder housing  70  supports a non-rotating piston  71  on bearings  72  and  73 . The bearings  72  and  73  permit piston  71  to move lengthwise. Housing  70  and piston  71  form a piston chamber  74 . A toroidal piston face  75  extends from piston  71  to provide a driving surface for lengthwise movement of the assembly. Piston face  75  is surrounded by piston rings  88 . 
     The stator  76  of a hollow motor is attached to an inner surface of piston  71  in operating relationship with rotor  77  of the motor. Rotor  77  is attached to the shaft  78 . 
     With this arrangement, rotor  77  of the hollow electric motor is rotated to thereby rotate the shaft  78 . The shaft  78  is supported by and rotates in bearings  79 . 
     Providing fluid pressure on either side of piston face  75  moves the entire assembly of the piston  71 , stator  76 , rotor  77  and shaft  78  lengthwise. 
       FIG. 6A  shows the shaft  78  in a retracted position.  FIG. 6B  shows the shaft  78  in its extended position. 
     The arrangement shown in  FIGS. 6A and 6B  has the advantage of being of short length but does require a larger part of the assembly to move lengthwise. This embodiment also removes the requirement for a spline shaft or equivalent means. 
       FIGS. 7 and 7A  show a modification to the embodiment shown in  FIGS. 6A and 6B  where, instead of having a single toroidal piston, two separate pistons are provided. In this embodiment the pistons are fixed and the cylinder translates. 
     As shown in  FIG. 7 , shaft  80  is supported by and rotates on bearings  81  and  82 . Stator winding  83  is fixed to housing  84 . Housing  84  also encloses pistons  85  and  86  in cylinders  187  and  188 , respectively. Fluid connections (not shown) are provided to the cylinders  187  and  188  to drive the pistons  85  and  86  in a manner well understood in the art. The rotor  87  of the hollow electric motor is fixed to the shaft  80 . 
     In operation, energization of the stator  83  causes the rotor  87  to rotate and thereby rotate the shaft  80 . Providing fluid pressure to the pistons  85  and  86  forces the housing  84  to move lengthwise. The lengthwise motion of the housing  84  forces the stator  83 , rotor  87  and shaft  80  to also move in a lengthwise direction. 
     The embodiment shown in  FIG. 7  is compact and does not require a single large toroidal cylinder or a spindle drive. However, it does require the entire housing assembly including the hollow motor and the cylinders to move lengthwise. 
     The selection of an appropriate embodiment of the invention would be determined by the requirements of the application being addressed. For example, if limited length was available, the embodiment shown in  FIGS. 6A and 6B  or  FIG. 7  might be selected whereas if weight on the shaft were a concern other embodiments may be better suited. 
     It is to be understood by persons skilled in the art that the invention is not limited to the illustrations described herein, which are deemed to illustrate the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts and details of operation. The invention is intended to encompass all such modifications, which are within its spirit and scope as defined by the claims.