FIELD

The invention relates to percussion devices and to rock drills incorporating percussion devices. The invention is described herein in the context of rock drills by way of example only. Various embodiments of the invention may suit other applications.

BACKGROUND

In the applicant's own international patent application WO 2012/031311 (Figure 7a of which is reproduced as Figure 1 a herein), a rock drill is disclosed. The rock drill incorporates a percussion device in which a member 92, 93, 94 is driven by fluid to which back pressure from a rock face is applied so as to vary the stroke length of an impact piston 66. This drill is a significant advance over earlier drills in which changes in the characteristics of the rock being drilled could cause destructive over-stroking and required periodic manual resetting of the drill. In contrast, this drill automatically optimises its stroke length to suit changing rock characteristics, thereby improving the drilling rate, reducing down time and reducing the likelihood of damage to the drill related to over-stroking.

Field testing has shown that the automatic stroke length adjustment mechanism of this drill can deteriorate over time, such that after a period of time the stroke length is not optimal. The invention aims to address this deterioration.

It is not admitted that any of the information in this patent specification is common general knowledge, or that the person skilled in the art could be reasonably expected to ascertain or understand it, regard it as relevant or combine it in any way at the priority date. SUMMARY

One aspect of the invention provides a percussion device including an impact piston; a system for driving the impact piston; a seal; a lubrication path; and a member; the member being mounted to slide through the seal; and having at least one portion, on one side of the seal, arranged to adjust the system to vary a stroke length of the impact piston as the member slides through the seal; the lubrication path being arranged to convey fluid to lubricate, from the other side of the seal, an interface between the seal and the member.

In a preferred form of the invention, the system is a fluid system, in which case the lubrication path may be arranged to convey fluid from the fluid system to so lubricate.

Preferably the fluid system is configured to drive the fluid along the impact piston from one or more high pressure zones to one or more low pressure zones to drive the impact piston, and the lubrication path is arranged to convey the fluid from at least one of the low pressure zones to so lubricate. The at least one low pressure zone may be between two of the high pressure zones, in which case the fluid system is preferably configured to alternately pressurise the two high pressure zones to reciprocally drive the impact piston.

The device may include an integral body encircling the impact piston and the seal, in which case the lubrication path may at least predominantly consist of a conduit (e.g. a drilling) formed in the integral body.

Preferably the fluid system includes conduits respectively connected to the impact piston and the at least one portion of the member selectively closes the respectively connected conduits, to vary the stroke length of the impact piston, as the member slides through the seal. Another aspect of the invention provides a percussion device including an impact piston; a fluid system for driving the impact piston; a seal; and a stroke length adjustment mechanism configured to optimise a stroke length of the impact piston; the stroke length adjustment mechanism including a fluid path; and a member; the member being mounted to slide through the seal; and having on one side of the seal at least one portion exposed to fluid of the fluid system and arranged to adjust the fluid system to vary a stroke length of the impact piston as the member slides through the seal; and an end; and the fluid path fluidly connecting the at least one portion of the member to the end of the member.

The device preferably includes, on the one side of the seal, a non-sealing guide to guide the member and through which the member slides. Optionally the fluid system is configured to drive the fluid along the impact piston from one or more high pressure zones to one or more low pressure zones to drive the impact piston. In this case, a conduit may communicate at least one of the high pressure zones to the at least one portion of the member such that the at least one portion of the member and the end of the member are exposed to high pressure lubricant. The device may be a rock drill percussion device, and another aspect of the invention provides a rock drill including the device. The rock drill preferably includes structure defining a chamber such that a back pressure from a rock face is applied to fluid in the chamber, and an arrangement by which the fluid, to which back pressure is applied, drives the member to slide through the seal to vary the stroke length in response to the back pressure.

BRIEF DESCRIPTION OF DRAWINGS

Various arrangements will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1a is a schematic cross-section view of a prior art percussion device; Figure 1 b is a perspective view of a rock drill according to an embodiment of the invention, mounted upon a cradle movable along a beam;

Figure 2 is a front elevation of the embodiment of Figure 1 b;

Figure 3 is a cross-sectional view of the embodiment of Figure 2, taken along the line A-A on Figure 2;

Figure 4 is a cross-sectional view of the embodiment of Figure 2, taken along the line B-B on Figure 2;

Figure 5 is a detail view of part of Figure 4;

Figure 6 is an exploded view of the embodiment of Figure 1 b;

Figure 7a is a sectional and schematic view of the impact piston and associated hydraulic driving circuit, in which the stroke adjustor is shown in a minimum stroke position;

Figure 7b is a sectional and schematic view of the impact piston and associated hydraulic driving circuit, in which the stroke adjustor is shown in an intermediate stroke position;

Figure 7c is a sectional and schematic view of the impact piston and associated hydraulic driving circuit, in which the stroke adjustor is shown in a maximum stroke position;

Figure 8 is a schematic of a hydraulic circuit according to an embodiment of the invention;

Figure 9 is a block diagram of a method according to an embodiment of the invention;

Figure 10a is a perspective view of a housing;

Figure 10b is a cut-away view of the housing of Figure 10a;

Figure 1 1 a is a cut-away view of a percussion device; and

Figure 11 b is a close-up view of a seal assembly shown in Figure 1 1 a.

DESCRIPTION OF A PREFERRED EMBODIMENT

Figures 1 b and 2 show a rock drill 2 in drill operation configuration, the drill 2 having a front head 20, cover plate 30, housing of a rotation generation mechanism or gear box 40, a percussive or back pressure damping mechanism for damping a drill rod (also known as back pressure damping body 50), stroke adjustor 90 for adjusting the stroke length of an impact piston 66 and percussion device 60. They are supported on cradle assembly 110, which in operation moves longitudinally along a beam 120. A drill rod or shank adaptor 10 associated with the rock drill is also shown in the operational position. In use, drill rod 10 is connected to a drill string (not shown) for transmission of rotary motion and percussive force to a drill bit (not shown) working at a rock face. The rock drill 2 advances along the beam 120 to keep the tip of the drill bit (not shown) under pressure at the rock face. As rock is fluidised, the rock drill continues to move forward, drilling a hole in the rock. When the drill 2 has advanced to the forward end of the beam 120 it is repositioned at the rear end, and the beam and drill string reconfigured to recommence operations.

As shown in Figure 3, the drill rod or shank adaptor 10 is held by a drill chuck 42. The drill chuck 42 is rotatable about the central drill axis 4, and can also move through a limited range longitudinally along the drill axis 4. During operation, as the rock drill 2 advances along the beam 120, the tip of the drill bit (not shown) works at the rock face. The continued advancement of the rock drill 2 creates a back pressure along the drill string and drill rod 10 from the drill bit at the rock face to the drill chuck 42. The drill chuck 42 moves in a limited range forwardly or rearwardly relative to the back pressure damping body 50 along the longitudinal axis 4, in response to the back pressure, i.e. the pressure at the working rock face. The back pressure, and vibrational forces or percussive shock imparted to and reflecting from the working face, are as far as possible isolated or damped from the rest of the drill by back pressure damping body 50. These vibrational forces may also be known as reflex waves. A rotary drive shaft 46 drives gearbox 40 having cover plate 30, gears 48 and tapered thrust bearings 47, 49. This in turn drives chuck 42 and drill rod 10. The front head 20 has a flushing seal carrier 150 via which flushing medium is supplied to the hollow centre 12 of the drill rod 10. Back pressure damping body 50 has or is associated with a back pressure damping fluid chamber 52, and a damping piston 54. The damping piston 54 is longitudinally aligned with the drill chuck 42 and is driven by the drill chuck 42. As the drill chuck 42 moves rearwardly along the longitudinal axis 4 due to the back pressure transmitted through the drill rod 10, the damping piston 54 also moves rearwardly and compresses the cushioning hydraulic fluid in the back pressure damping fluid chamber 52. The greater the compression, the greater the pressure in the damping fluid chamber and the greater the resistance to further movement in a rearward direction. Where the back pressure is low or none, the damping piston 54 is driven to a forward position by the hydraulic fluid. The longitudinal movement of the drill chuck 42 thus changes the position of the drill rod 10 relative to the impact body 60 and impact piston 66. It is noted that the back pressure damping fluid chamber 52 is a generally annular chamber extending around the impact piston 66, and that the damping piston 54 also extends in an annular manner around the impact piston 66. The damping body 50 and its associated back pressure damping chamber 52 are not in fluid communication with any hydraulic circuits associated with the motor for driving the gear box 40.

Turning to Figures 4 and 5, the impact piston 66 is driven by fluid in a fluid circuit 70 which in this embodiment is a hydraulic circuit receiving pressure supply from a 'central' hydraulic system (not shown). The terms "driving fluid circuit" 70 and "first fluid circuit" 70 are used herein to distinguish it from the "damping fluid system" 56 (also referred to as "second fluid system" 56) associated with the back pressure damping chamber 52. The "damping fluid circuit" 56 is isolated from the driving fluid circuit and does not receive pressure supply from the 'central' hydraulic system, but rather pressure changes are due to movement of the damping piston 54, responding to the drill chuck holding drill rod 10. Use of the term "driving fluid circuit" 70 should not be taken to require a particular fluid path of the paths within that circuit to drive an impact piston.

Turning to Figures 7a to 7c which are in a mirror image orientation compared to Figures 4 and 5, the hydraulic driving fluid circuit 70 has an associated control valve, being first spool valve 80 which is not visible in Figures 4 and 5. The impact piston 66 is reciprocally driven forward and rearward within impact piston housing 60 at around 60Hz. The piston 66 is a solid, predominantly cylindrical body. It includes a pair of spools 66a, 66b spaced along its length. Each of the spools 66a, 66b includes four

circumferential ribs radially projecting a short distance beyond the piston's cylindrical exterior.

The housing 60 defines a predominantly cylindrical bore 6066 (Figure 10b) to slidingly receive the piston 66 and in particular its spools 66a, 77b.

Fluid is supplied through the first control spool valve 80 into the impact piston housing 60 alternately via ports 210 or 214 into respective annular spaces surrounding the piston 66 fore and aft of the spools 66a, 66b. These annular spaces constitute high pressure zones 210', 214' (Figure 10b). When one of these zones is pressurised, fluid passes along piston 66, and the bore of the housing 60, past the adjacent one of the spools 66a, 66b into an annular zone 21 1' bracketed by the spools 66a, 66b. In so passing the spool, pressure energy is lost to driving the piston 66 whereby the annular zone bracketed by the spools 66a, 66b is a low pressure zone. A port 211 opens to the low pressure zone 211'. Fluid is returned from the impact piston 66 to the valve 80 via ports 210, 211 and/or 214. Alternatively the port 21 1 may communicate directly with drain R.

The valve 80 is a 6-port, 2-position spool valve. The position of the spool determines the path through which fluid is supplied and thus the direction of travel of the impact piston 66. The control spool valve 80 has pressure supply port 202, and return or drain ports 201 and 203. It also has working side ports 204, 205 and 206, through which fluid is supplied to and returned from the impact piston 66. The spool valve 80 is a directional valve in which a position change is triggered via fluid flow to end port 207 or 208.

A change in direction of the impact piston 66 is triggered by hydraulic feedback from the impact piston 66 caused by opening and closing of a forward and a rearward port such as forward ports 215, 216, 217, 218 and rearward ports 212, 213 due to movement of the impact piston 66. The feedback to a respective end 207, 208 of the spool valve 80 causes the spool to move to another position.

Each of the ports 215, 216, 217, 218 is fluidly connected to the impact piston 66 and when open constitutes a trigger point at which the direction in which the piston is driven ^' may be reversed. By providing at least two forward (or rearward) hydraulic feedback ports a change in the stroke length of the impact piston 66 can be obtained depending on which of the ports is connected to an end of the spool valve. A regulating pin can be used to block or open a fluid path to reconfigure the circuit 70 and thus the impact piston stroke length.

A regulating pin may be used to manually reconfigure the driving circuit to a particular stroke length was of use to manually set the drill 2 to work at a known rock face hardness level. However, where for example hard rock is suddenly changed to a soft rock section, the drill 2 still suffers from over-stroking of the impact piston. The over- stroking can result in the impact piston cycling at high frequency and is most

undesirable. While it is possible to change the regulating pin manually, this does not prevent over-stroking incidents, but rather in the event that the drill was not damaged, simply enables the drill to be put back into service with a different regulating pin giving a new stroke length setting for the apparent new conditions. The regulating pin is manually removed from the drill housing with an associated risk that dirt and contaminants enter the hydraulic fluid circuit driving the impact piston.

In the embodiment of the invention shown the Figures, the stroke adjustor 90 includes a stroke length control mechanism 91 , in this embodiment a hydraulic control valve also referred to herein as stroke length control valve 9 , second control valve 91 or switch 91. The stroke length control valve 91 includes a stroke adjustor pin 92, residing in adjustor housing 98, an actuator or stroke adjustor piston 94 for driving the pin 92, a coaxial cylindrical extension 93 of the pin 92, and a return spring 95. The pin 92, extension 93 and piston 94 together constitute a member. Alternatively, the member 92, 93, 94 may be integrally formed. Extension 93 of member 92, 93, 94 slides through a seal in the form of rear seal assembly 98b.

The stroke length control mechanism 91 is operable to reconfigure the driving fluid circuit 70. Movement of the member 92, 93, 94 causes a portion of the member (in this case pin 92) to open and/or close the first fluid circuit feedback paths 72. The position of the hydraulically driven stroke adjustor pin 92 controls or switches the forward feedback fluid path through which hydraulic fluid will travel in the driving fluid circuit 70. This in turn changes the point in the impact piston's travel or stroke at which the first control spool valve 80 triggers a direction change in the movement of the impact piston 66 within impact piston housing 60. The four forward feedback ports 215, 216, 217, and 218 in impact housing 60 are spaced at a distance from the two rearward feedback ports 212, 213 so that, in conjunction with the configuration of the impact piston 66, they provide an adjustment in the impact piston stroke length of 0mm, 9mm, 17.5mm, 31 mm

respectively. Thus in the embodiment shown the impact piston 66 may have any of four stroke lengths corresponding to the four forward fluid ports 220, 221 , 222 and 223 in impact housing 98.

Each of the four forward feedback ports 215, 216, 217, and 218 has a respective fluid path connecting to respective ports 220, 221 , 222 and 223 in adjustor pin housing 98. The adjustor pin position determines which of ports 220, 221 , 222 and 223 are open to connect to directional feedback take-off 224 and thus to fluid path 224-207 to supply pressure to the first control valve 80 at the forward end of the spool to trigger a spool position change and thus an impact piston direction change.

Thus as shown in Figure 7a, the impact piston 66 is triggered in a direction change when the fluid path connecting ports 215-220 is open at both ends, and thus has a shorter stroke length than in Figure 7b where a direction change is not triggered until the impact piston 66 has travelled far enough to open fluid path 216-221 , as in Figure 7b fluid path 215-220 is blocked by the stroke adjustor pin 92. Similarly, a maximum stroke length is obtained in Figure 7c as the impact piston 66 moves until it opens fluid path 218-224, as the adjustor pin 92 blocks fluid paths 215-220, 216-221 , and 217-222.

Importantly as shown in Figures 7a to 7c, the stroke length control mechanism 91 is hydraulically operated, driven by the pressure in the damping fluid chamber 52 and damping fluid circuit 56 to reconfigure the driving fluid circuit 70. The stroke adjustor pin 92 is driven by an actuator or stroke adjustor piston 94, located in housing 96 (see Figure 5). The stroke adjustor piston 94 has an associated return spring 95. Where the pressure in the damping fluid chamber 52 is increased, fluid from the damping fluid chamber 52 will flow through damping fluid path 56 into piston housing 96 at an end zone 97 _x via port 225, hydraulically driving the piston 94 against the return spring 95. This also drives the adjustor pin 92. Movement of adjustor pin 92 results in switching of the forward feedback fluid paths as described above in response to the pressure in damping fluid chamber 52.

The stroke adjustor piston 94 has a working face which together with an end wall of the piston housing 96 defines an end zone 97, into which damping fluid path 56 enables passage of damping fluid from the damping chamber 52. The working face of the piston 94 is prevented from "sticking" to the end wall of the housing by the left end of stroke adjustor pin 92 reaching its maximum travel within adjustor housing 98, preventing the piston 94 from contacting the end wall. The stroke adjustor piston 94 has a return spring 95 which seats against piston shoulder 94a. The piston 94 also has a balancing port 94b, being a hollow centre channel that functions as a passage through the piston 94, from the end zone 97 side of the piston 94 to the return spring 95 side of the piston 94. The balancing port 94b allows equalisation of fluid pressure to either side of the piston 94, which can significantly reduce the force acting on the return spring 95. The fatigue life of the return spring 95 can thus be significantly increased. The stroke adjustor piston housing 96 has seals 99 between it and the adjustor pin housing 98, which prevent damping fluid from exiting the housing 96 while allowing the piston 94 to extend into the adjustor pin housing 98 and drive the adjustor pin 92

As best viewed in Figure 7c the adjustor pin housing 98 has a small stepped shoulder 98a which defines a first end zone 93a.

The adjustor pin 92 also has a piston shoulder 92a which in the maximum stroke position shown in Figure 7c is seated against the end wall of the housing 98. When not in the maximum stroke position the shoulder 92a and housing 98 define a second end zone 93b, as viewed in Figures 7a and 7b. The stroke adjustor pin 92 also has a balancing port 92b as shown in Figures 7a, 7b and 7c, being a hollow centre channel that functions as a passage through the pin 92 from the first end zone 93a side to the second end zone side 93b.

The balancing port 92b allows equalisation of fluid pressure to either side of the adjustor pin 92, which alleviates the effect of high percussion pressure in the driving or first fluid circuit acting on stroke adjustor pin 92 as the impact piston 66 is operated. Thus the stroke adjustor mechanism is primarily controlled or affected by pressure in the damping or second fluid circuit i.e. back pressure.

Thus the feedback fluid ports and paths, and adjustor pin 92 have been arranged (in conjunction with return spring 95 and the sizing of the faces of piston 94 etc) to result in the stroke length of the impact pis. ton 66 being automatically matched to the back pressure from the rock face (as back pressure determines the pressure in damping fluid chamber 52).

It should be noted for clarity that the damping fluid system including damping fluid chamber 52 and damping fluid path 56 is a separate fluid system from the driving fluid circuit 70 - the stroke adjustor piston 94 and stroke adjustor pin 92 have associated seals preventing leaks from one system to the other. Thus the hydraulically actuated stroke length control mechanism or switch 91 (including stroke adjustor pin 92), is directly responsive to changes in pressure in the damping fluid chamber, enabling the stroke length of the impact piston to be automatically and hydraulically adjusted during operations. This can avoid the over-stroking feedback loop which causes problems in the prior art, since stroke length is continually adjusted according to back pressure. The hydraulic operation avoids the need for expensive and delicate electronic instrumentation and controls to shut down the drill if it over-strokes.

The embodiment disclosed has four fluid paths defining four stroke length settings.

However, greater or fewer fluid paths could be provided to change the number of stroke length settings available, and the difference in stroke length enabled by each path may be selected to suit particular requirements.

The arrangement of driving fluid circuit 70, stroke length control mechanism 91 , adjustor pin 92, impact piston 66 and spool valve 80 can be made in a variety of configurations. For example the size and location of cavities defined between the impact piston and its housing, or between the adjustor pin and its housing, type and configuration of the directional valve and the location of fluid ports, may be varied in a number of ways to achieve the same or similar result. The above is thus a description of one embodiment of such a driving fluid circuit.

The arrangement of the damping fluid circuit 56 can also be made in a variety of configurations. Depending on mechanical configuration, the stroke length control mechanism 91 could be located directly adjacent the damping fluid chamber 52 and remove the need for a conduit, fluid channel or path between chamber 52 and piston 94. Nonetheless, in such a case the damping chamber 52 is considered to comprise the second fluid circuit 56. Pneumatic, rather than hydraulic, operation may also be feasible in some applications.

Figure 9 shows a block diagram of a method of adjusting the driveable stroke length of an impact piston. In step 1 , fluid pressure in the damping chamber 52 alters in response to movement of the drill rod due to rock backpressure. In step 2, the fluid pressure change hydraulically actuates the stroke length control mechanism 92 to a new position via the damping fluid circuit 56. In step 3, movement of the adjustor pin 92 in the mechanism 92 opens and/or closes forward feedback paths 72 connected to ports 215, 216, 217 and 218 in the driving fluid circuit 70. This adjusts the paths and ports through which fluid is fed back to the control valve 80 that triggers a direction change to the impact piston, and thus the position of the impact piston when a direction change is triggered. Thus the driveable stroke length of the impact piston is hydraulically adjusted in response to rock face back pressure in the damping body. As best shown in Figure 10b, the housing 60 defines a pair of predominantly cylindrical bores 6066, 6092. The bores 6066, 6092 are parallel and lie in a common horizontal plane. Figure 10b is a cut-away view through that horizontal plane. The bores are dimensioned to slidingly receive the piston 66 and the slidable member 92, 93, 94 respectively, such that the piston and member are encircled by the housing. The housing is an integral body of steel.

The bore 6066 includes annular formations spaced along its length defining the high pressure zone 210' into which the port 210 opens, the low pressure 21 1' into which the port 21 1 opens, and the high pressure zone 214' into which the port 214 opens.

Five parallel drillings bisect the bore 6092 and open into the bore 6066 to form the conduits 215, 216, 217, 218 and a conduit 300. In this example, these drillings are perpendicular to, and lie in the same plane as, the axes of the bores 6066, 6092. The outer free ends of these drillings are countersunk and closed by suitable threaded fasteners to isolate the bore 6092 from the outside world. Port 224 (by which the conduits 215, 216, 217, 218 are communicated with the valve 80) is also evident in Figure 10b. A rearward end of the bore 6092 internally carries the seal assembly 98b and is capped by a cup-shaped member defining a cylindrical interior 92' to receive the extension 93 when the member 92, 93, 94 moves rearwardly.

The seal assembly 98b is an annular arrangement including an outer bushing 98b' which has a stepped exterior complementary to an interior of the bore 6092 and internally carries a seal 98b". The outer bushing 98b' is preferably formed of bronze.

The conduit 300 fluidly communicates the low pressure zone 211' with the interior 92' to convey fluid from the fluid system 70 to the interior 92' so as to lubricate the seal 98b" from its rearward side. The present inventors have recognised that the deterioration in the performance of the stroke length adjustment mechanism of the drill of WO 2012/03131 1 is associated with the accumulation of lubricant in the interior 92'. This lubricant acts on the member 92, 93, 94, changing its position so as to have an unplanned effect on the stroke length of the drill. Counter-intuitively, in the disclosed example of the drill, the solution to the problem associated with this accumulation of lubricant in the interior 92' is a path for supplying lubricant to the interior 92'.

The inventors have determined that the problematic accumulation of lubricant is associated with lubricant from the high pressure zone 210' leaking past the seal 98b" (via the conduit 72) and in the turn that this leaking is associated with the seal wearing with use.

The conduit 300 serves to supply lubricant, in the form of oil, to the seal 98b" so as to reduce the rate at which it wears.

The lubricant conveyed by the conduit 300 is only a fraction of the pressure of the lubricant in the high pressure zone 210' and so does not adversely interfere with the movement of the member 92, 93, 94. As illustrated in Figure 1 b, the cup-shaped member defining the interior 92', when fully screwed into position, partially blocks the conduit 300. However, the member and the surrounding structure are co-operably configured to leave a gap and there are no seals in this vicinity, such that the lubricant travels into the interior 92' and inevitably reaches the seal 98b" via this route.

While the above description refers to one embodiment of a rock drill, it will be

appreciated that other embodiments can be adopted by way of different combinations of features. By way of example, high pressure lubricant might be supplied to interior 92' and the member 92, 93, 94 and spring 95 adjusted to suit. Such embodiments fall within the spirit and scope of this invention.

In yet another example of the invention, the seal 98b" is eliminated altogether. The bushing 98b' forms a loose sliding fit through which the member 92, 93, 94 slides. This loose sliding fit is not a sealing engagement whereby the high pressure lubricant (to which the portions 92, 93 are exposed) flows through the annular space between the member bushing 98b' and the surrounded portion of the member 93.

This flow of lubricant serves to ensure that the free end of the member 92, 93, 94, and indeed all portions of that member on that side of the seal 99, are bathed in a

continuous body of lubricant. This continuous bathing is another approach to controlling the pressure forces on the member 92, 93, 94 and avoiding problematic changes in those pressure forces due to wearing seals.

In this other example of the invention the annular space connects the operative central portion of the adjustment pin 92, 93, 94 to the end of the pin when the drill is

commissioned (i.e. first operated) and stroke length adjustment mechanism is configured to optimise the stroke length whilst this fluid path (including the annular space) is open. The mechanism may be so calibrated by varying one or more of the dimensions of the pin 92, 93, 94, the length and/or stiffness of the spring 95, the position of the stop against which the spring 95 acts, and the locations of the ports 72. The term "comprises" and its grammatical variants have a meaning that is determined by the context in which they appear. Accordingly, the term should not be interpreted restrictively unless the context dictates so.