CA2607433A1 - Piston ring assembly for an injection unit - Google Patents

Abstract

A piston assembly for injection unit of a molding machine is provided. The piston assembly includes a piston housing defining a piston chamber that has apertures at opposing ends. An injection piston is slidably located at least partially within the piston chamber and extends through both the apertures. The injection piston is movable between a retracted position and an extended position by selective pressurization of the piston chamber. Support structures for the injection piston are located within the piston housing proximate the first aperture and the second aperture, the support structures operable to maintain pressure in the piston chamber.

Description

PISTON RING ASSEMBLY FOR AN INJECTION UNIT
TECHNICAL FIELD

The present invention relates, generally, to injection units for molding machines. More specifically, the present invention relates to a support structures for a piston assembly.

BACKGROUND OF THE INVENTION

The injection molding process typically comprises preparing a polymeric (or sometimes metal) material in an injection unit of a molding system, injecting the now-melted material under pressure into a closed and clamped mold, solidifying the material in its molded shape, opening the mold and ejecting the part before beginning the next cycle. The molding material typically is supplied to the injection unit from a hopper in the form of pellets or powder. The injection unit transforms the solid material into a molten material (sometimes called a"melt"), typically using a feed screw, which is then injected into a hot runner or other molding system under pressure from the feed screw or a plunger unit. A shut off valve assembly is typically provided to stop and start the flow of molten material from the barrel to the molding system.

Some examples of known molding systems having such an injection unit are: (i) the HyPETTM
Molding System, (ii) the QuadlocTM Molding System, (iii) the HylectricTM
Molding System, and (iv) the HyMetTM Molding System, all manufactured by Husky Injection Molding Systems Ltd.

SUMMARY OF THE INVENTION
According to a first aspect of the invention, the present invention provides a piston assembly for injection unit of a molding machine, comprising:
a piston housing, defining a piston chamber and a first aperture and a second aperture on opposing ends of the piston chamber, the piston housing being selectively pressurizable;
an injection piston, slidably located at least partially within the piston chamber and extending through both the first aperture and the second aperture, the injection piston being movable between a retracted position and an extended position by selective pressurization of the piston chamber; and support structures for the injection piston located within the piston housing proximate the first aperture and the second aperture, the support structures operable to maintain pressure in the piston chamber.

A piston assembly for injection unit of a molding machine is provided. The piston assembly includes a piston housing defining a piston chamber that has apertures at opposing ends. An injection piston is slidably located at least partially within the piston chamber and extends through both the apertures. The injection piston is movable between a retracted position and an extended position by selective pressurization of the piston chamber. Support structures for the injection piston are located within the piston housing proximate the first aperture and the second aperture, the support structures operable to maintain pressure in the piston chamber.

BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 is a sectional view of an injection unit in accordance with an aspect of the present invention;
Fig. 2 is a perspective view of the injection unit shown in Fig. 1, shown in a pivoted position;
Fig. 3 is a cross sectional view of a barrel connector for the injection unit shown in Fig. 1;
Fig. 4 is an exploded view of a screw connector for the injection unit shown in Fig. 1;
Fig. 5 is a partial-cutaway view of an alternative piston assembly for the injection unit shown in Fig. 1;
Fig. 6 is a front cross sectional view of a drive assembly for the injection unit shown in Fig. 1 taken along lines D-D;
Fig. 7 is a perspective view of a barrel collar for the injection unit shown in Fig. 1;
Fig. 8 is a cross sectional view of a piston assembly for the injection unit shown in Fig. 1; and Fig. 9 is a cross sectional view of a drive assembly for the injection unit shown in Fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Referring now to Fig. 1, an injection unit for a molding machine in accordance with a non-limiting embodiment of the invention is shown generally at 20. Injection unit 20 is mounted on a base structure 22 and includes a barrel assembly 24, a nozzle assembly 26, a piston assembly 27 and a drive assembly 28.

Referring additionally to Fig. 2, base structure 22 typically sits on vibration pads (not shown) on the factory floor and typically houses the controls, electronic cabinets, hydraulic tanks, and other common equipment (none shown) as is known to those of skill in the art. A
swivel plate 30 is pivotally mounted to a planar surface 32 on base structure 22 along the longitudinal axis of base structure 22. When swivel plate 30 is pivoted, a large cross-section of the swivel plate is still supported by the planar surface 32, so that the load of injection unit 20 is distributed across base structure 22.

A rail 34 is fixedly mounted along the longitudinal axis of swivel plate 30.
As will be described in greater detail below, the barrel assembly 24 is slidably mounted to the rail 34 via a pair of rail carriages 60, and operable to translate between a fully-forward position, i.e., towards a mold, and a fully-back position, i.e., away from a mold (not shown). While a single, wide rail 34 is preferred, two narrower rails could also be used to mount injection unit 20.

An axle 36 defines a mounting interface for swivel plate 30. Swivel plate 30 pivots about the axle 36, which is located by the swivel plate at or near the centre of gravity for injection unit 20.
Preferably, swivel plate 30 includes at least two mounting interfaces for axle 36 in order to locate the injection unit at different positions relative to base structure 22.

During normal use, swivel plate 30 is immobilized and prevented from pivoting by at least one fastener 38, and preferably a plurality of fasteners 38 that extends through aligned apertures in both swivel plate 30 and base structure 22. Preferably, fasteners 38 are jack screws, and can thus adjust the vertical angle of injection unit 20 relative to the primary melt channel in a runner system (not shown). When injection unit 20 is down for servicing, the fasteners 38 are removed, permitting swivel plate 30 to pivot. Slots 39 are provided in the sides of swivel plate 30. Pins 41 extending from base structure 22 are located within slots 39 and delimit the degree of rotation permitted for injection unit 20. Thus, the injection unit 20 can be pivoted away from the mold (not shown), permitting easier serving or replacement of components such as barrel assembly 24 or drive assembly 28.

Referring back to Fig. 1, barrel assembly 24 includes a barrel 40 and a barrel housing 42. Barrel housing 42 can be integrally formed (such as through casting) or it can be assembled from smaller pieces for ease of manufacturing. A screw 46 is rotatably and slidably located within a channel 50 in barrel 40. Injection material, such as plastic pellets is stored in a hopper 54 that is mounted over barrel housing 42. The injection material is fed to channel 50 through an integrally-formed feed throat 56, where it is plasticized by screw 46. Heater bands 48 are provided along the exterior of barrel 40, and help to melt the injection material. The plasticized material is expelled through an orifice 52 in nozzle assembly 26 into a mold (not shown). The flow of the plasticized material can be metered by a valve 58, which is moved between an open and closed position by an actuator 66.

The barrel housing 42 is slidably mounted to rail 34 via rail slides 60. The barrel 40 extends through a central aperture 62 defined by barrel housing 42 and extends into a chamber 64 in barrel housing 42. The sidewalls of central aperture 62 provide a wide stance for the barrel mounting. In the presently-illustrated embodiment, barrel housing 42 is cast as a single component, but it is contemplated that multi-component versions could also be manufactured for ease of assembly. An access window 67 (best seen in Figs 2 and 5) provides egress into chamber 64.
A control box 68 provides power to heater bands 48 on the barrel 40.

Referring additionally to Fig. 3, a locating groove 70 is defined on a portion of the exterior surface of barrel 40 within chamber 64, proximate an end 72 of the barrel (Fig. 1). A
barrel coupler 74, comprising a pair of semi-annular half-couplers 81 (Fig. 7) that cooperate together to retain barrel 40 within barrel housing 42 and prevent its withdrawal. The barrel coupler 74 is located around the end 72 on barrel 40. Fasteners (not shown) are threaded through aligned apertures 75 (best seen in Fig. 5 and Fig. 7) defined in each the two half-couplers to clamp the two parts together. Bolts 77 are threaded through aligned apertures 79 (partially seen in Fig. 7) to mount the barrel coupler 74 to barrel housing 42 and prevent its rotation via that of the screw 46. On a first end 76 of the barrel coupler 74, an annular tab 78 depends radially inwards, extend into locating groove 70 and is sized so that it abuts against the sidewalls of the locating grove 70, and further abuts against a sidewall 80 of chamber 64. A second end 82 of barrel coupler 74 extends to the end 72 of barrel 40 so that the two cooperatively define an end surface 84. Optionally, second end 82 and end 72 can be substantially co-planar. As is described in greater detail below, end surface 84 defines a piston stop for an injection piston 85 of piston assembly 27.

Referring now to Fig. 8, the piston assembly 27 is described in greater detail. Piston assembly 27 includes a piston housing 44 and the injection piston 85. The piston housing 44 can be integrally formed (typically through casting) or it can be assembled from smaller components for ease of manufacturing. Optionally, piston housing 44 can be integrally formed along with barrel housing 42.
Piston housing 44 includes a piston chamber 86 that includes a pair of apertures on opposing ends of piston chamber 86. A first aperture, namely piston aperture 200 is coaxial and in communication with chamber 64 in barrel housing 42. A second aperture, namely shaft aperture 120, is coaxially and in communication with in drive assembly 28. A portion of the injection piston 85, namely piston cylinder 90 is sized to be in slidable engagement within piston chamber 86 and chamber 64, so that the injection piston 85 can move between a retracted position and an extended position (each described in greater detail below).

A first end 92 of piston cylinder 90 includes a screw connector 94 (described in greater detail below) for coaxially mounting an end 98 of screw 46. When piston cylinder 90 is in the extended position, first end 92 of the piston cylinder 90 abuts against end surface 84, stopping the forward motion of screw 46. A second end 104 of piston cylinder 90 includes a piston shaft 112 that extends coaxially out through a shaft aperture 120 in piston housing 44 into drive assembly 28 (and is described in greater detail below). Piston shaft 112 can be integrally formed as part of piston cylinder 90, or can be separately attached. When piston cylinder 90 is in the retracted position, the second end 104 abuts against an endwall 124 of piston chamber 86, stopping the return motion of screw 46.

Referring now to Figs. 3 and 4, screw connector 94 is shown in greater detail.
A recess 96 is coaxially defmed on the first end 92 of the piston cylinder 90. Recess 96 includes a opening portion 100, a frusto-conical portion 102, and a base portion 106 sized to fit a collet 108. A collet 108 for retaining screw 46 is adapted to be inserted within recess 96, and includes a flange portion 110 sized to fit within opening portion 100 and a tapered portion 116 sized to fit within frusto-conical portion 102 of recess 96. Collet 108 further includes a central bore 114 sized for screw 46 to pass therethrough. Apertures 118 are concentrically distributed around flange portion 110 and align with apertures 122 in collet 108.

To connect screw 46, collet 108 is placed within recess 96 so that tapered portion 116 is located within frusto-conic portion 102. A screw base 126 of screw 46 is inserted through central bore 114 so that it bottoms out against the end of base portion 106. Fasteners 128 are inserted through aligned apertures 118 and 122 to secure collet 108 to piston cylinder 90. As the fasteners 128 are tightened, a wedging action between tapered portion 116 and frusto-conic portion 102 causes the tapered portion 116 to securely grip the screw base 126 of screw 46. While the presently-illustrated embodiment shows only a single collet 108, it is contemplated that two or more collets 108, sized to fit within recess 96 could also be used. Alternatively, a portion of the collet 108 could extend outside of recess 96, provided that the wedging action occurred substantially within recess 96.

The inventors have determined that the piston design in previous injection units, such as in the one described in the 2005/0048162 application, are prone to misalignment as they can include long unsupported portions when fully extended. In contrast, the present invention provides fixed, non-telescopic support structures for the piston 85 at each end of piston housing 44. Referring back to Fig. 8, a first support structure, namely wear collar 130, is located in a wear niche 132 at the edge of piston chamber 86 adjacent to barrel support chamber 42. A second support structure is defined by sidewalls 135 of shaft aperture 120. Both the wear collar 130 and the sidewall 135 provide a tight-fit engagement which maintains the piston 85 in coaxial alignment with screw 46 over the full range of travel without sagging. Additionally, both wear collar 130 and the sidewall 135 include a number of sealing niches 134 for locating 0-rings in order to maintain fluid pressure within piston chamber 86.
Additional piston seal niches 134 are defined within a piston guiding structures 136 defined on the exterior surface of by piston cylinder 90. The guiding structures 136 are concentric portions of piston cylinder 90 having a wider diameter than the rest of the cylinder.
Preferably, the gap between guiding structures 136 and the sidewall of piston chamber 86 is slightly greater than that between either wear collar 130 or the sidewall 135 and the adjacent portion of the piston 85 so as to facilitate the insertion of piston 85 into and through piston housing 44 while the wear collar 130 is removed from the wear niche 132.

During assembly of injection 20 (or reassembly after servicing, such as replacing the 0-rings), the piston housing 44 is detached from barrel housing 42 to expose piston aperture 200. Wear collar 130 is also removed. The piston 85 is inserted into piston chamber 86, piston shaft 112 first. As the piston shaft 112 is threaded through shaft aperture 120, the guiding structure 136 helps maintain alignment of the injection piston 85. Once the injection piston 85 is in place, the wear collar 130 can be concentrically mounted around the piston cylinder. 90 within wear niche 132. Remounting the wear collar 130 will also help to correct the alignment of the injection piston 85. The piston housing 44 can now be connected (or reconnected) to barrel housing 42.

Thus the present invention provides non-telescoping support for the piston cylinder 90 over a major portion of its length (while either extended or retracted), so that alignment with screw 46 is readily maintained both during operation and during regular maintenance.

Injection unit 20 can be configured to use a first piston, namely a dual action injection piston 85A
(shown in Fig. 1), or a second piston, namely single action injection piston 85B (shown in Fig. 5).
As known to those of skill in the art, dual action injection pistons use hydraulic pressure to both extend and retract the piston. The piston cylinder subdivides the piston chamber into two fluid-tight portions, and forward or rearward motion is effected on the injection piston by pressurizing one portion or the other single action injection pistons do not subdivide the piston chamber, and are moved to the extended position via hydraulic pressure in the piston chamber, but are returned to the retracted position by depressurizing the chamber and the plasticizing of the resin to push back the screw. Generally speaking, dual action cylinders provide higher performance (i.e., lower cycle times), but an increased complexity and cost. Unless explicitly noted, the following features are common to both dual action injection pistons 85A and single action injection pistons 85B.

For a dual-action injection piston 85A, a first piston cylinder, namely piston cylinder 90A (i.e., configured for dual action) has an outer diameter adapted for a fluid-tight fit against a portion of sidewall 138 in piston chamber 86, which divides piston chamber 86 into a first portion 142A
(shown here having a narrower diameter) and second portion 142B (shown here having a wider diameter). Piston cylinder 90A is moved to the extended position by hydraulically pressurizing portion 142A of piston chamber 86 (i.e., the portion of piston chamber 86 between second end 104 of the piston cylinder and the endwall 124 of the piston chamber). The hydraulic fluid acts upon second end 104, urging piston cylinder 90A towards end surface 84. Piston cylinder 90A is returned to the retracted position by hydraulically pressurizing portion 142B of piston chamber 86. The hydraulic fluid acts upon a return surface 144, urging piston cylinder 90A
towards endwall 124. For the purposes of clarity, the ports, valves and lines for the hydraulics system have been omitted from the illustration.

Referring now to Fig. 5, an injection piston 85B is shown in greater detail. A
second piston cylinder, namely piston cylinder 90B (i.e., configured for single action) has an outer diameter sized smaller than sidewall 138, but rather, is sized for a fluid-tight fit against wear collar 130. Thus, piston chamber 86 is not subdivided into two portions. An annular cylinder insert 144 is located within wider sidewall 140, reducing the volume of piston chamber 86. Piston cylinder 90B is moved to the extended position by hydraulically pressurizing piston chamber 86 (which is undivided). The hydraulic fluid acts upon second end 104, urging piston cylinder 90B towards end surface 84.
Injection piston 85B is returned to the retracted position by the plasticizing of the resin against screw 46 (Fig. 1), urging piston cylinder 90B towards endwall 124. Again, for the purposes of clarity, the ports, valves and lines for the hydraulics system have been omitted from the illustration.
Thus, the present invention provides for a modular configuration in which piston cylinders can be exchanged while using the same piston housings.

Referring now to Fig. 9 and additionally to Fig. 6, drive assembly 28 will be described in greater detail. The inventors have determined some deficiencies in prior art drive units, such as the drive assembly described in the U.S. 2005/0048162 application. The aforementioned drive assembly is relatively expensive because it requires a unique hollow motor design that incorporates the motor into the injection unit itself. In contrast, in the drive assembly described herein obviates some of these deficiencies.

Drive assembly 28 includes an outboard-mounted, motor housing 146, a stator 148. a rotor 150 and an end cap 152. Motor housing 146 is mounted to an end wall 154 of piston housing 44 via fasteners 156. A stator 148 is concentrically located within motor housing 146. A control box 162 is attached to an exterior surface of motor housing 146.

The hollow rotor 150 is coaxially mounted within a void defined by stator 148 and rotates freely therein. Rotor 150 defines a axial void 166. A spline sleeve 164 is located within the axial void 166, and securely mounted at a first end to rotor 150 by fasteners 157, and is supported (but not mounted) at a second end to end cap 152. A concentric gap 153 is provided between rotor 150 and spline sleeve 164 between the first and second ends. Spline sleeve 164 defines a plurality of inward-facing splines 168. The axial void 166 is partially filled with lubricating oil and is sealed by seals 170. Alternatively, the plurality of inward-facing splines 168 could be defined directly on an inner surface of rotor 150.

As is described earlier, piston shaft 112 extends through the shaft aperture 120 in piston housing 44 into drive assembly 28. Bearings 172 are provided in piston housing 44 adjacent to shaft aperture 120 to facilitate the movement of piston shaft 112. An end portion 174 of piston shaft 112 defines an integral spline 176. End portion 174 can be an integral portion of piston shaft 112, or alternatively, could be an extension that is separately mounted to piston shaft 112.

A spline insert 178 is located over end portion 174 by integral spline 176.
The spline insert 178 includes an inner-facing spline 180 which meshes with integral spline 176 on the piston shaft 112, and a plurality of outward-facing splines 182 which meshes with the plurality of inward-facing splines 168 on spline sleeve 164. The spline insert 178 is secured to the end portion 174 by means of a hex bolt and lock washer assembly 184. Apertures 179 are provided in a spline insert 178 permit the lubricating oil to freely circulate within the axial void 166.

Through the intermeshed splines, the rotational movement of rotor 150 is kinematically coupled to screw 46 through piston cylinder 90. When injection unit 20 is in operation, the rotor 150 rotates spline sleeve 164 while molding material is being fed into the screw 46. The rotational movement of spline sleeve 164 causes spline insert 178 to rotate. Spline insert 178 transmits rotational motion to the integral spline 176 on piston shaft 112, thereby causing screw 46 to rotate.

Furthermore, as piston 85 translates between the retracted position and the extended position (either by hydraulic pressure or the flow injection material in screw 46), the spline insert 178 slides within spline sleeve 164. It will thus be appreciated that spline sleeve 164 should be sized to be at least as long as the distance of screw travel between the retracted and extended positions. As such, the present invention provides a compact and robust drive assembly 28 for injection unit 20. The combination of a short spline on the spline insert 178 and a long spline on the spline sleeve 164 provides a very compact drive assembly 28 that provides good support for all moving parts so that machine alignment is maintained.

Alternatively, the spline insert 178 could be made long and the spline sleeve 164 shortened. This approach has the benefit of easier manufacture since it is much easier to manufacture an element with an external spline than it is to manufacture an element with an internal spline. However, if the spline insert 178 was made longer it would be necessary to increase the distance between the end cap 152 and the end wall 154 on piston housing 44 to accommodate the extra length of the spline insert 178. The increase in the distance would be directly related to the amount of the increase in the length of the spline insert 178.

The drive assembly 28 is readily configurable or replaceable. By removing fasteners 156, the entire drive assembly can be removed from injection unit 20 without disassembly of the other components.
Servicing of drive assembly 28 and its components can easily be more easily accomplished. In addition, field upgrading of components can be readily done. For example, if the entire drive assembly is to be replaced, a more powerful (and typically larger) drive assembly 28 could be mounted to the back of piston housing 44. Alternatively, if a larger or smaller piston is to be used, the spline sleeve 164 and/or spline insert 178 could be exchanged, while still using the rest of the original drive assembly 28, such as the and use the original motor housing 146, stator 148 and rotor 150.

A position sensor 188 is mounted to end cap 152, and is operable to determine the current position of the piston between its fully retracted position and fully extended position. Preferably, the position sensor 188 uses a temposonic sensor, but other types of linear positioning sensors could be used. As can be seen in Fig. 9, the end cap 152 defines a concavity 192 within the hollow of rotor 150. A
sensor housing 190 for position sensor 188 (containing the sensor head and electronics) is bolted to the exterior facing surface of concavity 192, and preferably, is fully recessed within motor housing 146. Position sensor 188 further includes a sensor tube 194 which extends through the centre of axial void 166 into a bore 196 defined coaxially within the end portion 174 of piston shaft 112. A
position magnet assembly 198 mounted to the distal end of end portion 174, concentrically around bore 196.As sensor tube 194 does not move, a greater or less portion of sensor tube 194 will be located within bore 196, depending on the current position of the piston shaft 112 (between its fully retracted and fully extended position). The movement of position magnet assembly 198 creates a magnetic strain pulse which travels along sensor tube until it reaches the sensor head within sensor housing 190, thereby indicating the current position of screw 46.

It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention.

The description of the non-limiting embodiments provides non-limiting examples of the present invention; these non-limiting examples do not limit the scope of the claims of the present invention.
The non-limiting embodiments described are within the scope of the claims of the present invention.
The non-limiting embodiments described above may be: (i) adapted, modified and/or enhanced, as may be expected by persons skilled in the art, for specific conditions and/or functions, without departing from the scope of the claims herein, and/or (ii) further extended to a variety of other applications without departing from the scope of the claims herein. It is to be understood that the non-limiting embodiments illustrate the aspects of the present invention.
Reference herein to details and description of the non-limiting embodiments is not intended to limit the scope of the claims of the present invention. Other non-limiting embodiments, which may not have been described above, may be within the scope of the appended claims. It is understood that: (i) the scope of the present invention is limited by the claims, (ii) the claims themselves recite those features regarded as essential to the present invention, and (iii) preferable embodiments of the present invention are the subject of dependent claims. Therefore, what is to be protected by way of letters patent are limited only by the scope of the following claims:

Claims (20)

1. A piston assembly for injection unit of a molding machine, comprising:
a piston housing, defining a piston chamber and a first aperture and a second aperture on opposing ends of the piston chamber, the piston housing being selectively pressurizable;
an injection piston, slidably located at least partially within the piston chamber and extending through both the first aperture and the second aperture, the injection piston being movable between a retracted position and an extended position by selective pressurization of the piston chamber; and support structures for the injection piston located within the piston housing proximate the first aperture and the second aperture, the support structures operable to maintain pressure in the piston chamber.

2. The piston assembly of claim 1, wherein the injection piston includes a piston cylinder and a piston rod.

3. The piston assembly of claim 2, wherein an end on the piston cylinder includes a coupler for a screw.

4. The piston assembly of claim 2, wherein the piston cylinder extends through the first aperture.

5. The piston assembly of claim 2, wherein the piston rod extends through the second aperture.

6. The piston assembly of claim 1, wherein a wear collar is located in the piston housing adjacent the first aperture.

7. The piston assembly of claim 1, wherein seals are provided proximate the second aperture.

8. The piston assembly of claim 1, wherein seals are provided proximate the first aperture.

9. The piston assembly of claim 7, wherein seals are located within niches defined in the piston housing.

10. The piston assembly of claim 8, wherein seals are located within niches defined in the piston housing.

11 11. An injection unit of a molding machine having a piston assembly, the piston assembly comprising:

a piston housing, defining a piston chamber and a first aperture and a second aperture on opposing ends of the piston chamber, the piston housing being selectively pressurizable;
an injection piston, slidably located at least partially within the piston chamber and extending through both the first aperture and the second aperture, the injection piston being movable between a retracted position and an extended position by selective pressurization of the piston chamber; and support structures for the injection piston located within the piston housing proximate the first aperture and the second aperture, the support structures operable to maintain pressure in the piston chamber.

12. The piston assembly of claim 11, wherein the injection piston includes a piston cylinder and a piston rod.

13. The piston assembly of claim 12, wherein an end on the piston cylinder includes a coupler for a screw.

14. The piston assembly of claim 12, wherein the piston cylinder extends through the first aperture.

15. The piston assembly of claim 12, wherein the piston rod extends through the second aperture.

16. The piston assembly of claim 11, wherein a wear collar is located in the piston housing adjacent the first aperture.

17. The piston assembly of claim 11, wherein seals are provided proximate the second aperture.

18. The piston assembly of claim 11, wherein seals are provided proximate the first aperture.

19. The piston assembly of claim 17, wherein seals are located within niches defined in the piston housing.

20. The piston assembly of claim 18, wherein seals are located within niches defined in the piston housing.