HK1088866A - Drive assembly for rotating and translating a shaft - Google Patents

Description

Drive assembly for rotating and translating a shaft

Technical Field

The present invention relates to a device for rotating and translating a shaft. The invention is particularly useful for driving the extrusion screw of an injection molding machine. More particularly, the present invention relates to a drive for rotating and reciprocating an extrusion screw of an injection molding machine, wherein the screw is rotated by a hollow motor and reciprocated by a hydraulic piston.

Background

It is known to use a hollow motor and hydraulic piston to drive and rotate an extrusion screw. However, none of the known systems suggest the combination of the advantages of using a hollow motor to rotate the extrusion screw and a hydraulic piston to move the screw longitudinally.

U.S. patent No.4105147 to stubbbe describes a screw extruder that is rotated by a motor through gear drives and moved longitudinally by hydraulic pistons. The screw has a splined shaft end to allow the shaft to slide through the gear drive.

Us patent 4895505 to Fanuc ltd describes a linear motor for moving an injection screw in a straight line. The linear motor includes a series of permanent magnets fixed to the motor poles which react with alternating current supplied to the surrounding stator windings to produce linear motion of the armature and the screw shaft mounted on the armature. This patent describes the use of a hollow motor to move the screw shaft linearly.

U.S. patent 5540495 issued on 30/7/96 and to Krauss-Maffei describes an extrusion screw drive device including a first motor for translational movement of the screw and a second motor for rotating the screw. The described embodiment shows two hollow motors. The sliding means for translating the screw and the driving means for rotating the screw are fitted into each other.

U.S. patent No.5645868 to Reinhart describes a drive device for an injection unit that includes a motor engaged with a screw shaft through three clutches. One clutch provides rotation of the screw, the second clutch moves the screw forward, and the third clutch prevents rotation of the screw as it moves forward. No hydraulic unit is used.

U.S. patent No.5747076 to jarochek et al describes an injection molding machine that employs a hydraulic piston to assist a motor that drives a rack and pinion mechanism to advance a screw.

Us patent 5804224 to Fanuc ltd describes a structure in which a rolling screw is integrally formed on the rotor shaft. A motor placed coaxially with it rotates the rolling screw.

U.S. patent 5891485 issued on 6/4/99 to Sumitomo describes an injection device that includes two hollow shaft motors. One motor is designated for rotating the screw shaft, while the other motor moves the screw shaft in the longitudinal direction. The rotors of both motors are engaged with the shaft. Each rotor is located in a separate chamber.

U.S. patent No.6068810 to Kestle et al describes an injection unit having a sleeve in a piston that allows a screw to retract or extend due to hydraulic pressure. A motor rotates the sleeve, which is splined to the piston, thereby rotating the screw. The motor is mounted on the end of the sleeve.

U.S. patent No.6108587 to Shearer et al describes an injection molding system that includes a motor for driving a gear to rotate a screw and a hydraulic piston for translating the screw.

U.S. patent No.6478572 to Schad describes an injection unit that employs a separate motor to rotate the extrusion screw and pressurize a hydraulic accumulator. The pressurization in the accumulator is intended to reciprocate the extrusion screw.

Us patent No.6499989 describes a device for removing a tray from a mould. In the described embodiment, a hollow motor is used to rotate the take-off shaft, while a linear motor is used to linearly move the shaft. The hollow motor drives the shaft through a gearbox which varies the speed of the shaft. As an alternative, the patent suggests that a pneumatic or hydraulic cylinder may be used to directly move the shaft. In the described embodiment, the linear actuator is located outside the rotary actuator. This provides a relatively large and inexpensive assembly.

U.S. patent No.6517336 to Emoto et al and european patent No.0967064 a1 to Emoto disclose an injection molding system having a hollow motor that rotates a screw shaft and simultaneously advances the shaft via a connection with a rolling screw shaft/spline shaft unit. A separate metering motor rotates the screw to load the screw with resin. The rotary motion is provided by a belt and pulley arrangement which is rotatable independently of the rotor of the hollow motor. The rotor of the hollow motor is mounted on the splined portion of the screw shaft and is used to rotate the splined portion, which in turn rotates a rolling screw to drive a rolling nut to move the shaft longitudinally.

U.S. patent 6530774 to Emoto describes an injection molding system that uses a motor and a gear train to turn a screw and a hollow shaft motor connected via a splined shaft to move the screw longitudinally by driving a rolling screw shaft.

U.S. patent application No.2002/0168445 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 longitudinally, respectively.

European patent application 1162053 to Krauss-Maffei, issued 12.12.01, month and 12, describes a dual motor system in which one motor provides rotational motion of a screw shaft and the other motor provides translational motion of the screw shaft. Clutch means are provided for operating the motors separately and together.

Japanese patent to Sumitomo, issued 11/25/86, describes a two motor injection system that employs two hollow motors and a rolling screw drive mechanism and a splined shaft.

While these references describe a number of combinations of electrical and hydraulic drive systems for the screw of an injection molding machine, they do not describe a system that combines the unique advantages of better control of the positioning of the screw by a hollow motor, with high injection power provided by a hydraulic injection unit. The present invention provides a compact unit that has the unique advantages of both electric and hydraulic drive systems.

Summary of the invention

The invention provides a new drive unit for translation and rotation of a shaft. The unit comprises a hollow electric motor and at least one hydraulic cylinder, and means for connecting at least a portion of the shaft to the rotor of the motor. The coupling means comprises a means for allowing the shaft to move in the longitudinal direction. The hydraulic cylinder is connected to the shaft, whereby the shaft can be rotated by the electric motor and moved in the longitudinal direction by the hydraulic cylinder.

In accordance with one general aspect of the present invention, a drive unit is part of an injection unit for an injection molding machine having a hollow motor for rotating an injection screw and a hydraulic piston for reciprocating the screw.

More specifically, the present invention provides an injection unit for an injection molding machine, the injection unit comprising: a hollow motor and a hydraulic cylinder, a first cylinder wall of the hydraulic cylinder being coupled to the rotor of the hollow motor, a second cylinder wall of the hydraulic cylinder being coupled to the stationary portion of the hollow motor; a piston slidable along the inner surfaces of the first and second cylinder walls, a first end of the piston engaging the first cylinder wall and a second end of the piston engaging the second cylinder wall; a rotating device fixed to the rotor to rotate the piston, the rotating device allowing the piston to slide along the cylinder wall; a first passage means for supplying hydraulic fluid to drive the piston in a forward direction; and a second passage means for supplying hydraulic fluid to drive the piston in the opposite direction, and an injection screw fixed to one end of the piston.

In a preferred embodiment, the hydraulic unit is located at least partially within the hollow motor to thereby provide a smaller and more compact assembly.

Brief description of the drawings

FIG. 1 is a schematic cross-sectional view of a basic drive unit according to the present invention;

FIG. 2 is a side cross-sectional view of a preferred embodiment of a drive unit for an injection molding machine, with the drive unit in an extended position;

FIG. 2A is a cross-sectional view of a piston head for the drive unit shown in FIG. 2;

FIG. 2B is a partial cross-sectional view showing a hydraulic supply passage 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 a timing belt and encoder;

FIG. 3 is a side cross-sectional view of a preferred embodiment of a drive unit for an injection molding machine, wherein 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 schematic cross-sectional view of another embodiment of the present invention;

FIGS. 6A and 6B are cross-sectional views of another embodiment of the present invention having a drive cylinder surrounding a hollow motor;

FIG. 7 is a side cross-sectional view of yet another embodiment of the present invention;

figure 7A is a cross-sectional view of the embodiment shown in figure 7 taken along section line 7A-7A.

Detailed description of the preferred embodiments

Figure 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. The stator 46 is shown mounted on the wall of the housing 61. The rotor 46 is fixed to a cylinder 48. The cylinder 48 has a spline portion 49 formed on its inner surface. An insert fitted to the cylinder 48 may replace the splined portion 49. Splined portion 49 engages splines 62 (one shown) on piston 50. A shaft (not shown) integral with or fixed to the piston 50 is rotated by the motor 45 via the interconnection between the rotor 47 and the piston 58.

The shaft fixed to the piston 50 is moved longitudinally by fluid pressure supplied to either side of the head of the piston 50 through bores 51 and 52 in the wall of the cylinder 48. When the drive unit is used in an injection molding machine, the fluid may be hydraulic oil or a water-based graphite solution. The piston 50 slides over the splined portion 49 and rotates in bearings provided by the anti-friction ring 53a and fluid seal 53 b. The entire rotor 47, cylinder 48 and piston 50 assembly is rotatably supported in bearings 63 and 64 and axially located therein.

Although fig. 1 shows a basic schematic of the invention, experts are able to make necessary minor modifications to the structure of an acceptable drive unit according to the invention. For example, means other than a splined shaft may be provided to allow the shaft to slide while keeping the shaft rotatable. A single key that slides along a keyway may also be used.

The drive unit will now be described with reference to an extrusion screw for an injection molding machine. The present invention is particularly suitable for use in systems where it is necessary to turn the screw to melt the injected material and move the screw longitudinally with sufficient drive force to inject the material into the mold.

Referring to fig. 2 and 3, a screw 4 is located in a barrel 2 and is rotatable therein and axially movable. Injection material, such as plastic pellets, is fed into the screw 1 through the opening 4. The cylinder 2 is mounted in the injection seat 3 and is held in place by a cylinder holding plate 5. The groove 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 joint 29. The piston stop 7 is designed to prevent rotation of the tool when the piston 23 is retracted from the screw 1 and determines the fully extended position of the piston 23. This means is provided to enable the screw 1 to be removed and replaced if necessary.

The front portion of the piston 23 is in contact with the cylinder wall 18 by means of a piston ring 45. The piston 23 moves axially along the wall 18 as the screw 1 advances or retracts. Spline teeth 17 slide in spline insert 15 to enable piston 23 to move longitudinally.

The hollow motor 30 rotates the piston 23 and thus the screw 1, which is fixed to the piston 23. The terminal block 8 provides power to the motor 30 through the wire channel 9. The stator 12 is energized to rotate the rotor 16. The motor 30 preferably has a permanent magnet rotor, however, any hollow motor can be used to rotate the piston 23 and screw 1. The rotor 16 is shrink fitted to the cylinder wall 18. The rotor 16 may be secured to the wall 18 in any other manner so long as the rotor 16 and the wall 18 are movable as a unit. The spline insert 15 is attached to the cylinder wall 18 by screws 44. Spline insert 15 engages spline teeth 17 (best shown in fig. 4) on the outer wall of piston 23. Thus, when the rotor 16 rotates, the cylinder wall 18 and the piston 23 also rotate, and thus there is no relative rotational movement between the cylinder wall 18 and the piston 23.

A cooling passage 10 is provided in the motor housing 11 to cool the motor 30.

Piston head 24 is fixed to the rear end of piston 23 by screws 31 and includes a number of passage holes 37 between areas 32 and 33 (see fig. 2A and 4). This allows the piston 23 to have a minimum thickness. Piston head 24 rotates and slides on cylinder wall 22 by means of piston 45 a. Hydraulic fluid, such as hydraulic oil, is supplied to the areas 32 and 33 through hydraulic fluid passages 55 in the rear seat 26 and pushes the piston 23 and the screw 1 forward to inject the material into the mold.

The piston 23 and the fixed screw 1 are retracted by the addition of material to the head of the screw 1 in a manner known in the art. In order to prevent cavitation in the melt, a low pressure via region 32 acts on the bore side of piston 23. Grooves 38 (see fig. 2C) are made in the spline insert 15 to ensure fluid communication between the regions 34 and 35.

The cylinder wall 18 bears against the rolling bearing races 13 and 14 to facilitate rotation of the assembly with minimal frictional losses. The rolling bearing race 13 is supported on the end piece 41, while the rolling bearing race 14 is supported by the ring 89.

The dowel pins 27 extend from the motor housing 11 into the end piece 41 and the cylinder ring 36. The pins 27 prevent any tendency of the end piece 41 and cylinder ring 36 to rotate relative to the motor stator 12 due to the rotational pressure created by the rotation of the rotor 16 and piston 23.

A locating pin 28 extends from rear seat 26 into cylinder wall 22 to prevent any tendency of the cylinder wall to rotate in response to rotation of piston head 24.

The cylinder wall 22 is in sealing engagement with the cylinder ring 36 and the rear seat 26. Because these seals are only radially stressed, they are less likely to leak or crack than seals that are simultaneously radially and axially stressed.

A rod 19 extends from the rear seat 26 to the cylinder retaining plate 5 and seat 3 to clamp the entire drive assembly together.

Temposonic rod 20 is secured to rear seat 26 and extends through an opening into piston head 24. Magnetic assembly 21 on piston head 24 sends a signal through rod 20 in response to movement of piston head 24, which indicates the position of piston head 24 and thus screw 1 in a manner well understood by those skilled in the injection molding art.

The rotational speed and position of the screw 1 are determined by means of a timing belt 39 and an encoder 40 in a manner well understood by those familiar with servomotor control.

In operation, region 32 is pressurized through port 25. It pushes the piston 23 and the attached injection screw 1 forward. Plastic is injected into the mold cavity ahead of the screw 1. At the end of the injection, the region 32 is kept at a lower pressure for a short time. Thereafter, zone 32 is depressurized and zone 35 is pressurized to retract the screw a short distance. The hollow motor 30 is switched on, rotating the piston 23 and the attached screw 1 to melt the plastic granules supplied to the screw 1 through the opening 4. During this time, it may be necessary to maintain a low pressure in the region 32 to prevent voids and bubbles from forming in the melt. When the screw 1 is retracted to a predetermined position, the motor 30 is stopped. Further retraction of the screw 1 may occur to release the melt pressure. After the screw 1 is fully retracted, the next injection cycle is started and the injection step is repeated, supplying melt to the mold cavity.

Fig. 5 schematically shows another embodiment of the invention. In this embodiment, the rotor 54 is fixedly secured to the piston 55 and has a width at least as wide as the combined stroke length of the piston 55 and the width of the stator 56. The piston head 57 reciprocates in the cylinder 58.

Cylinder 58 is shown having a single fluid inlet 159. A second inlet may be provided, however, in some applications the second inlet may not be required. For example, in the case of an extrusion screw for an injection molding machine, the addition 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 advantage that the entire motor is kept outside the hydraulic part of the drive and no splined shaft connection is required, since the piston 55 is free to rotate and translate on the bearings 59 and 60.

The embodiment of fig. 5 may be further modified to make the stator 56 longer and the rotor 54 shorter. The drive unit operates in the same manner, but the reduced size of the rotor 54 will reduce the weight on the piston 56 and reduce the cost of the motor.

In the embodiment of the invention shown in fig. 6A and 6B, the drive cylinder surrounds the hollow motor. Stationary cylinder block 70 supports a non-rotating piston 71 on bearings 72 and 73. The bearings 72 and 73 allow the piston 71 to move in the longitudinal direction. The seat 70 and the piston 71 form a piston chamber 74. An annular piston face 75 extends from the piston 71 to provide a driving surface for longitudinal movement of the assembly. Piston face 75 is surrounded by piston rings 88.

The stator 76 of the hollow motor is secured to the inner surface of the piston 71 in operative relationship with the rotor 77 of the motor. The rotor 77 is fixed to a shaft 78.

With this structure, the rotor 77 of the hollow motor rotates, thereby rotating the shaft 78. The shaft 78 is supported by and rotates in bearings 79.

Fluid pressure is provided on either side of the piston face 75 to longitudinally move the entire assembly of piston 71, stator 76, rotor 77 and shaft 78.

Fig. 6A shows the shaft 78 in a retracted position. Fig. 6B shows the shaft 78 in its extended position.

The arrangement shown in figures 6A and 6B has the advantage of having a short length, but does not require a substantial part of the assembly to be moved longitudinally. This embodiment also does not require a spline shaft or equivalent device.

Fig. 7 and 7A show a modification of the embodiment shown in fig. 7A and 7B, in which instead of a single annular piston, two separate pistons are provided. In this embodiment, the piston is fixed and the cylinder translates.

As shown in fig. 7, the shaft 80 is supported by and rotates on bearings 81 and 82. The stator winding 83 is fixed to the housing 84. Housing 84 also encloses pistons 85 and 86 located in cylinders 187 and 188, respectively. Fluid connections (not shown) are provided on 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 motor is fixed to the shaft 80.

In operation, energization of the stator 83 rotates the rotor 87 and thereby the shaft 80. Providing fluid pressure to pistons 85 and 86 forces housing 84 to move longitudinally. The longitudinal movement of the housing 84 forces the stator 83, the rotor 87 and the shaft 80 to also move in the longitudinal direction.

The embodiment shown in fig. 7 is compact and does not require a separate large toroidal cylinder or spline drive. However, it requires the entire housing assembly including the hollow motor and cylinder to be moved in the longitudinal direction.

The choice of a suitable embodiment of the invention is determined by the requirements set forth in the use aspect. For example, if only a limited length is available, the embodiment shown in fig. 6A and 6B or fig. 7 may be selected, while other embodiments may be more suitable where weight on the shaft is of importance.

It is to be understood by those skilled in the art that the present invention is not limited to the illustrations described herein, which are deemed to be the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. It is intended that the invention be construed as including all such modifications which fall within the spirit and scope of the appended claims.

Claims (53)

1. An injection unit for an injection molding machine, said injection unit comprising: a hollow motor and a hydraulic cylinder, a first wall of said hydraulic cylinder being coupled to a rotor of said hollow motor, a second wall of said hydraulic cylinder being connected to a stationary portion of said hollow motor; a piston slidable along the inner surfaces of said first and second cylinder walls, a first end of said piston engaging said first cylinder wall and a second end of said piston engaging said second cylinder wall; a plurality of splines extending along an intermediate portion of said piston and a spline insert, said insert fixedly attached to said rotor and engaging said splines; first passage means for supplying hydraulic fluid to drive said piston in a forward direction; and second passage means for supplying hydraulic fluid to drive said pistons in opposite directions; and means for securing an injection screw to said piston, whereby said piston slides longitudinally along said spline insert when hydraulically driven, and said piston is rotated by said spline connection when said motor is activated to rotate said rotor.

2. An injection unit for an injection molding machine, said injection unit comprising: a hollow motor and a hydraulic cylinder, a first wall of said hydraulic cylinder being coupled to a rotor of said hollow motor, a second wall of said hydraulic cylinder being connected to a stationary portion of said hollow motor; a piston slidable along the inner surfaces of said first and second cylinder walls, a first end of said piston engaging said first cylinder wall and a second end of said piston engaging said second cylinder wall; rotating means secured to said rotor for rotating said piston, said rotating means permitting said piston to slide along said cylinder wall; a first passage means for supplying hydraulic fluid to drive said piston in a forward direction; and second passage means for supplying hydraulic fluid to drive said pistons in opposite directions; and means for securing an injection screw to said piston, whereby said piston slides longitudinally along said cylinder wall when hydraulically driven, and said piston is rotated by said rotating means when said motor is activated to rotate said rotor.

3. An injection unit as defined in claim 2, wherein said rotating means comprises said first cylinder.

4. An injection unit as defined in claim 1, 2 or 3, wherein said second cylinder wall has a larger diameter than said first cylinder wall.

5. An injection unit as defined in claim 1, 2 or 3, further comprising means for detecting the rotational position and rotational speed of said piston.

6. An injection unit as defined in claim 1, 2 or 3, further comprising means for detecting the rotational position and the rotational speed of said piston, characterized in that said means for detecting comprises a timing belt (timing belt) and a coding means.

7. An injection unit as defined in claim 1, 2 or 3, further comprising a temposonic rod fixed to said first and second ends to detect the longitudinal position of said piston in said cylinder.

8. An injection unit as defined in claim 1, 2 or 3, further comprising a dowel pin (dowel) arrangement between said second cylinder wall of said hydraulic cylinder and said rear seat, said dowel pin arrangement preventing rotation of said second cylinder wall relative to said rear seat.

9. An injection unit as defined in claim 1, 2 or 3, further comprising a dowel arrangement between a housing member of said motor and a cylinder ring supporting said housing member to prevent rotation of said stator relative to said cylinder ring.

10. An injection unit for an injection molding machine as claimed in claim 1, 2 or 3, wherein said rotor is fitted on said first cylinder wall.

11. An injection unit for an injection molding machine as claimed in claim 1, 2 or 3, wherein said first cylinder wall rotates on a ball race.

12. An injection unit for an injection molding machine, said injection unit comprising: a hollow motor and a hydraulic cylinder, a first wall of said hydraulic cylinder being coupled to a rotor of said hollow motor, a second wall of said hydraulic cylinder being connected to a stationary portion of said hollow motor; a piston slidable along the inner surfaces of said first and second cylinder walls, a first end of said piston engaging said first cylinder wall and a second end of said piston engaging said second cylinder wall; a rotating means secured to said rotor for rotating said piston, said rotating means allowing said piston to slide along said cylinder wall; means for providing hydraulic fluid to drive said piston along said cylinder wall, and means for securing an injection screw to said piston, whereby said piston slides longitudinally along said cylinder wall when hydraulically driven, and said piston is rotated by said rotating means when said motor is activated to rotate said rotor.

13. An injection unit for an injection molding machine as defined in claim 12 wherein said rotating means includes a spline insert engaging splines on said piston.

14. An injection unit for an injection molding machine, said injection unit comprising: a hollow motor and a hydraulic cylinder, a first wall of said hydraulic cylinder being coupled to a rotor of said hollow motor, a second wall of said hydraulic cylinder being connected to a stationary portion of said hollow motor; a piston movable along the inner surfaces of said first and second cylinder walls, a first end of said piston engaging said inner surface of said first cylinder wall and a second end of said piston engaging said inner surface of said second cylinder wall; a rotating means secured to said rotor for rotating said piston, said rotating means permitting said piston to move along said cylinder wall; means for providing hydraulic fluid to drive said piston along said inner wall, and means for securing an injection screw to said piston, whereby said piston moves longitudinally along said cylinder wall when hydraulically driven, and said piston is rotated by said rotating means when said motor is activated to rotate said rotor.

15. An injection unit for an injection molding machine as defined in claim 14 wherein said rotating means includes a spline insert engaging splines on said piston.

16. An injection unit as defined in claim 12, 13, 14 or 15 wherein said rotating means comprises said first cylinder.

17. An injection unit as defined in claim 12, 13, 14 or 15 wherein said second cylinder wall has a larger diameter than said first cylinder wall.

18. An injection unit as defined in claim 12, 13, 14 or 15, further comprising means for detecting the rotational position and speed of said piston.

19. An injection unit as defined in claim 12, 13, 14 or 15, further comprising means for detecting the rotational position and speed of said piston, wherein said means for detecting comprises a timing belt and a coding means.

20. An injection unit as defined in claim 12, 13, 14 or 15, further comprising a temposonic rod secured to said first and second ends to detect the longitudinal position of said piston in said cylinder.

21. An injection unit as defined in claim 12, 13, 14 or 15, further comprising a dowel arrangement between said second cylinder wall of said hydraulic cylinder and said rear seat, said dowel arrangement preventing rotation of said second cylinder wall relative to said rear seat.

22. An injection unit as defined in claim 12, 13, 14 or 15, further comprising a dowel arrangement between a housing member of said motor and a cylinder ring supporting said housing member to prevent rotation of said stator relative to said cylinder ring.

23. An injection unit as defined in claim 12, 13, 14 or 15 wherein said rotor is engaged on said first cylinder wall.

24. An injection unit as defined in claim 12, 13, 14 or 15 wherein said first cylinder wall rotates on a ball race.

25. A drive unit for translating and rotating a shaft, said unit comprising: a hollow motor and at least one fluid cylinder, means for connecting at least a portion of said shaft to the rotor of said motor, means for allowing longitudinal movement of said shaft, and means for connecting said fluid cylinder to said shaft, whereby said shaft can be rotated by said motor and moved longitudinally by said fluid cylinder.

26. A drive unit for translating and rotating a shaft, said unit comprising: a hollow motor and at least one fluid cylinder, means for connecting at least a portion of said shaft to the rotor of said motor, said connecting means comprising means for allowing longitudinal movement of said shaft, and means for connecting said fluid cylinder to said shaft, whereby said shaft can be rotated by said motor and moved longitudinally by said fluid cylinder.

27. A drive unit for translating and rotating a shaft, said unit comprising: a hollow motor and at least one fluid cylinder surrounding said hollow motor, means for connecting at least a portion of said shaft to the rotor of said motor, said connecting means comprising means for allowing said shaft to move longitudinally, and means for connecting said fluid cylinder to said shaft, whereby said shaft can be rotated by said motor and moved longitudinally by said fluid cylinder.

28. A drive unit for translating and rotating a shaft, said unit comprising: a hollow motor and at least one fluid cylinder, means for connecting at least a portion of said shaft to a rotor of said motor, said rotor surrounding said fluid cylinder, said connecting means comprising means for allowing longitudinal movement of said shaft, and means for connecting said fluid cylinder to said shaft, whereby said shaft can be rotated by said motor and moved longitudinally by said fluid cylinder.

29. A drive unit as defined in claim 25 wherein said means for allowing said shaft to move longitudinally comprises a splined insert on the wall of said cylinder and splines on said shaft.

30. A drive unit as defined in claim 26 or 28 wherein said connecting means comprises a splined insert on the wall of said cylinder and splines on said shaft.

31. A drive unit as defined in claim 29 or 30 wherein said cylinder includes a piston secured to one end of said shaft.

32. A drive unit as defined in any one of claims 25, 26, 28, 29, 30 or 31 wherein said rotor of said electric motor is fixedly secured to an outer wall of said cylinder.

33. A drive unit as defined in claim 29 or 30 wherein said spline insert is on an inner wall of said cylinder.

34. A drive unit as defined in claim 25 or 26 wherein said motor includes a stator and said rotor has a width substantially greater than said stator.

35. A drive unit as defined in claim 34 wherein said rotor has a width substantially equal to the combined width of the travel of said stator and the piston in said cylinder.

36. A drive unit as defined in claim 34 or 35 wherein said rotor is fixedly secured to said piston and moves longitudinally with said piston.

37. A drive unit as defined in claim 25 or 26 wherein said motor includes a stator and said stator has a width substantially greater than said rotor.

38. A drive unit as defined in claim 37 wherein said stator has a width substantially equal to the combined width of the travel of said rotor and said piston in said cylinder.

39. A drive unit as defined in claim 37 or 38 wherein said rotor is fixedly secured to said piston and moves longitudinally with said piston.

40. A drive unit as defined in claim 25, 26, 28, 29, 30, 31, 32 or 33 wherein said cylinder rotates on bearings on a stationary motor housing.

41. A drive unit as defined in claim 40 wherein said cylinder includes a piston, said cylinder, piston and rotor rotating as a single unit and said piston moving longitudinally in said cylinder.

42. A drive unit as defined in claim 25 wherein said unit includes a motor housing and said cylinder is secured to an outer wall of said housing.

43. A drive unit as defined in claim 42 wherein said motor includes a stator and said rotor has a width substantially greater than said stator.

44. A drive unit as defined in claim 43 wherein said rotor has a width substantially equal to the combined width of the travel of said stator and the piston in said cylinder.

45. A drive unit as defined in claim 44 wherein said rotor is fixedly secured to said piston and moves longitudinally with said piston.

46. A drive unit as defined in claim 40 wherein said motor includes a stator and said stator has a width substantially greater than said rotor.

47. A drive unit as defined in claim 40 wherein said stator has a width substantially equal to the combined width of the travel of said rotor and said piston in said cylinder.

48. A drive unit as defined in claim 25 wherein said at least one cylinder comprises a plurality of cylinders.

49. A drive unit as defined in claim 25 wherein said at least one cylinder comprises two cylinders.

50. A drive unit as defined in claim 48 or 49 wherein said shaft rotates on bearings on the motor housing.

51. A drive unit as defined in claim 48 or 49 wherein said unit includes a housing and said cylinder is disposed along a peripheral wall of said housing.

52. A drive unit as defined in any of claims 48 to 51 wherein said rotor is fixedly secured to said shaft and moves longitudinally therewith.

53. A drive unit as defined in any one of claims 1 to 26, 29, 34 to 39, 42 to 45 and 48 to 52 wherein said cylinder is located at least partially within the confines of said hollow motor.