Fluid pressure operated piston engine assembly

A fluid pressure operated piston engine assembly, such as a pump assembly, including a fluid pressure operated piston engine and a fluid valve for coupling fluid under pressure to alternative portions of the piston chamber of the piston engine. The fluid valve includes a valve spool which is translated to effect the alternative modes of coupling pressurized fluid to the piston chamber. The translation is accomplished by a shifter assembly having a shifter rod attached to the valve spool. The shifter rod includes a pair of diametrically opposed magnets attached thereto. A fork is attached to the piston engine drive shaft at one end and is mounted for limited movement along the shifter rod between the pair of magnets of the shifter rod. The interaction between the flux generated by a magnet carried by the fork with one of the magnets of the shifter rod causes the shifter rod, and in turn the valve spool, to shift from one position to another, thereby re-directing the flow of fluid to the piston chamber to effect the reversal of direction of travel of the piston and its associated drive shaft.

This invention relates generally to a fluid pressure operated piston engine 
assembly. The invention more particularly concerns such an assembly 
including a fluid valve for coupling fluid under pressure to alternative 
portions of the piston chamber of the piston engine so that, as the drive 
shaft of the piston engine approaches each end of its stroke, fluid under 
pressure is coupled to a portion of the piston chamber to effect reversal 
of the direction of travel of the drive shaft. It also concerns a shifter 
assembly for actuating the fluid valve. 
In a fluid pressure operated piston engine, a pressurized fluid is used to 
reciprocate a piston and an attached drive shaft to perform mechanical 
work. To do this, a pressurized fluid valve is generally interposed 
between a source of pressurized fluid and the piston chamber of the piston 
engine to alternatively pressurize and exhaust each end of the piston 
chamber. As the piston approaches an end of the chamber, and hence as the 
attached drive shaft approaches an end of its stroke, the valve must be 
actuated to effect reversal of the direction of travel of the piston and 
drive shaft. 
Typically, in order to do this, some form of mechanical coupling is 
provided between the drive shaft and the pressurized fluid valve. One 
known form of fluid pressure operated piston engine, for example, is a 
pneumatically driven pump, such as may be used for pumping hot melt 
adhesive. One form of such a pump is described in U.S. Pat. No. 4,550,642 
to Langer which also describes various other prior art systems, the 
disclosure of this patent describing these systems is hereby incorporated 
herein by reference. 
One problem with the heretofore shifter assemblies is that they contain 
many moving parts, or require mechanical interaction (contact) between 
parts, or are complicated, etc. All of these, either individually or 
collectively, can lead to fatigue of the shifter and/or stalling of the 
pump assembly while also being difficult to trouble shoot. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved fluid 
pressure operated piston engine assembly which includes a pressurized 
fluid valve effectively cooperating with the piston engine to couple fluid 
under pressure to portions of the piston chamber to effect reversal of the 
direction of travel of the drive shaft as the drive shaft approaches each 
end of its stroke. 
According to one aspect of the invention, it is also an object of the 
invention to provide a means for causing the fluid valve to shift from one 
position to another when the piston reaches the end of its stroke that is 
substantially non-contact in nature. 
These and other objectives have been accomplished by providing an assembly 
comprising: a fluid valve having an inlet for coupling to a source of 
fluid under pressure, first and second discharge outlets, and a valve 
spool translatable between a first and second position such that in the 
first position the inlet is coupled to the first discharge outlet and in 
the second position the inlet is coupled to the second discharge outlet; 
and a shifter including a shifter rod coupled to the valve spool, a pair 
of diametrically opposed magnets carried by the shifter rod, and a means 
movable between said magnets for causing the shifter rod to move from 
either a first position relative to the valve spool to a second position, 
or from the second position relative to the valve rod to the first 
position, wherein coupling of the inlet to a discharge outlet of the fluid 
valve is shifted from either the first to the second outlet or from the 
second to the first outlet. 
These objectives and others have also been accomplished by a fluid pressure 
operated piston engine assembly comprising: a fluid pressure operated 
piston engine including a piston chamber, a piston reciprocable in the 
chamber, and a drive shaft attached to the piston and reciprocable 
therewith through a drive shaft stroke having a first end and a second 
end; fluid valve means for coupling fluid under pressure to alternative 
portions of the piston chamber, including a valve spool translatable to 
(a) a first position in which the valve means is operable to couple fluid 
under pressure to a first portion of the piston chamber, tending to move 
the drive shaft toward the second end of its stroke and (b) a second 
position in which the valve means is operable to couple fluid under 
pressure to a second portion of the piston chamber, tending to move the 
drive shaft toward the first end of its stroke; first means for coupling 
to the fluid valve means, mounted for reciprocal movement, and including a 
pair of diametrically opposed magnets; and second means for coupling the 
first means to the piston engine drive shaft such that as the drive shaft 
approaches the first end of a stroke, the first means is moved to a first 
position relative to the valve spool and such that as the drive shaft 
approaches the second end of its stroke the first means is moved to a 
second position relative to the valve spool, wherein, as the drive shaft 
approaches each end of its stroke, fluid under pressure is coupled to one 
of the portions of the piston chamber to effect reversal of the direction 
of travel of the drive shaft. 
Other objects and advantages of the invention, and the manner of their 
implementation, will become apparent upon reading the following detailed 
description and upon reference to the drawings.

DESCRIPTION OF THE INVENTION 
With reference now to the figures, a fluid pressure operated piston engine 
assembly, shown generally as reference numeral 10, includes a fluid 
pressure operated piston engine 12 and a fluid valve 14 for coupling fluid 
under pressure to the piston engine. The piston engine 12 includes a 
housing 16 defining a piston chamber 18 in which a piston 20 reciprocates. 
Attached to, and reciprocable with, the piston 20 is a drive shaft 22. The 
drive shaft 22 may serve as a pump shaft, for example, if the piston 
engine 12 is employed as a pump. When employed as a pump, this assembly is 
especially suited for pumping adhesives, such as for example, hot melt 
adhesive. 
The pressurized fluid valve 14, in the illustrated form, is a pneumatic 
valve for selectively coupling pressurized air from a pressurized air 
source (not shown) through an air inlet 24 to the piston chamber 18. A 
valve spool 26, which serves as a flow-directing valve element, is 
translatable within a sleeve 28, having a multi-stepped bore mounted 
within a housing 30 of the fluid valve 14. 
In the illustrated form, pressurized air communicates through the inlet 24 
into an annulus 32 forming a portion of the bore of the sleeve 28. The 
pressurized air communicates from the annulus 32 to either annulus 34 or 
36 of the bore via reduced diameter portions 38, 40, respectively, of the 
bore, depending upon the position of the valve spool 26. The outer 
diameter of the valve spool 26 varies to form stepped portions for 
directing the flow of pressurized air. 
With the valve spool 26 positioned as shown in FIGS. 2 and 4, the 
pressurized air is coupled from the air inlet 24, through the annulus 32, 
portion 40, and annulus 36 of the bore of the sleeve, and through a 
passageway 42 to the top of the piston chamber 18. When the spool 26 is in 
the position shown in FIGS. 3 and 5, the pressurized air is coupled from 
the air inlet 24, through the annulus 32, portion 38, and annulus 34 of 
the bore, and through a passageway 44 communicating with the bottom of the 
piston chamber 18. Passageways 42 and 44 are shown in a diagonal or 
crossing pattern for clarity, however, they could both extend 
substantially in the vertical direction with respect to FIG. 2. 
An exhaust annulus 46 of the bore of the sleeve 28 is couple to the annulus 
34 via a reduced diameter portion 50. In like manner, as exhaust annulus 
48 of the bore is coupled to the annulus 36 via a reduced diameter portion 
52. As before, the flow of air between each pair of annuluses 34, 46; 36, 
48 is dependent upon the position of the valve spool 26. Each exhaust 
annulus is coupled to an opening in the housing 30 of the fluid valve so 
that air may be vented from the piston chamber as the piston moves from 
one end of the chamber to the other. 
In order to reciprocate the piston 20 and the drive shaft 22, and, for 
reference, referring initially to the positions of the valve spool 26 and 
piston 20 shown in FIG. 2, the pressurized air is coupled through the 
inlet 24 and the annulus 32. The air passes through the reduce diameter 
portion 40 to annulus 36, but is prevented from passing to annulus 34 by a 
larger diameter portion 56 of the valve spool 26. From annulus 36 the air 
passes through the passageway 42 to the upper portion of the piston 
chamber 18. The pressurized air acts upon the upper face of the piston 20, 
forcing the piston and the drive shaft 22 downwardly. As the piston 20 
moves downwardly, the air in the lower portion of the chamber 18 is 
exhausted through the passageway 44 to the annulus 34, through the reduced 
diameter portion 50, annulus 46, and then the exhaust opening 60 in the 
top of the valve housing 30 were upon the air is vented out of the 
assembly. 
In a manner to be described further below, when the piston 20 nears the 
bottom of the chamber 18, the valve spool 26 is slideably moved within the 
sleeve. With reference to FIGS. 2 and 4, the valve spool 26 will be moved 
upwardly to the position shown in FIGS. 3 and 5. This causes the various 
different portions of the stepped outer diameter of the valve spool 26 to 
align differently with the stepped bore of the sleeve 28, thereby causing 
the air flow path to be redirected along a different flow path. 
Pressurized air is then coupled through the fluid valve 14 so that the 
pressurized air is coupled to the passageway 44 to act upon the lower face 
of the piston 20. The thus-applied force on the lower face of the piston 
20 drives the piston back to the top of the chamber 18, whereupon the 
fluid valve 14 is again slideably moved to its previous position of FIGS. 
2 and 4. The cycle is then repeated. During the time that the piston 20 
moves upwardly through the chamber 18, the air in the upper portion of the 
chamber is exhausted through the passageway 42, annulus 36, portion 52, 
annulus 48 and finally out of the opening 62 in the bottom of the valve 
housing 30. 
Attached to the upper end of the valve spool 26 is a stop plate 64 having a 
pair of rings 66. The stop plate 64 is held by the upper shoulder 68 of 
the spool valve and by a nut 70 threadably attached to the upper end of 
the spool valve 26. The stop plate 64 provides a detent for limiting the 
travel of the spool valve 26 as it slideably moves within the bore of the 
sleeve 28. With reference to FIG. 3, the stop plate 64 limits the upward 
travel of the spool by interacting with the cap portion 72 of the housing 
30 of the fluid valve. The rings 66 are somewhat resilient so that they 
act similar to a shock absorber. The rings may be, for example, comprised 
of an elastomer, Tetrafluoroethylene, or other material. 
To insure that the air valve is resistant to the formation of deposits 
within the air valve, it is preferred that the outer stepped portion of 
the spool, such as the larger diameter portion 56, wipes across the bore, 
of the housing, such as reduced diameter portion 38. Deposits will then be 
wiped into the larger portions, such as 34 of the bore or various pockets 
57. 
Normal valve design tolerances for a half inch diameter spool (as measured 
at the larger diameter portions, such as 56) are .+-.0.0001 inches. 
However, for pump assemblies for use in hot melt dispensing systems, it 
has been found that spools and sleeves manufactured to this tolerance had 
a tendency to gum up and stick, while those manufactured to .+-.0.001 
inches tended to leak. Therefore, the preferred tolerance lies somewhere 
there between. Good results have been obtained however, for spools and 
sleeves manufactured of a hardened stainless steel and having a .+-.0.0005 
tolerance. This is a larger clearance than is found with typical air 
valves. It has also been found for this particular combination that it is 
preferred that the air utilized by the air valve is non-lubricated air. 
In order to activate the fluid valve 14 as the piston 20 approaches the top 
or bottom of the piston chamber 18, and consequently as the drive shaft 22 
approaches the ends of its stroke, the motion of the drive shaft is 
transmitted to the valve spool 26 via a shifter assembly 80. The shifter 
assembly 80 includes a shifter rod 82 which is threadably attached to the 
valve spool 26. The shifter rod 82 extends from the fluid valve 14 and 
through an opening in the end cap 84 of the shifter assembly 80. The 
shifter rod 80 is substantially parallel to the drive shaft 22 of the 
piston engine. The shifter assembly 80 further includes a fork 86, 
attached to the piston drive shaft 22 and is mounted for limited 
translation upon the shifter rod 82. For example, a screw 88 extends from 
an end 90 of the fork to a mid portion 92 of the fork, between these two 
positions, the fork forms substantially a "C" about the piston drive shaft 
22. As the screw 88 mates with the portion 92 of the fork, the "C" of the 
fork is tightened to grip the drive shaft 22. 
The forked end 94 of the fork 86 carries a magnet 96 which is located in a 
milled pocket of the fork 86. The magnet 96 is substantially "C" shaped as 
viewed in FIG. 6, wherein the shifter rod 82 and a sleeve 98 are disposed 
between the tines of the "C". It is preferred that the shifter rod 82 and 
the sleeve 98 are able to freely slide there through as the valve spool 26 
moves in reciprocal motion and allows for the fork 86 to move along the 
sleeve 98 of the shifter rod 82 in conjunction with the reciprocal motion 
of the drive shaft 22. It is therefore preferred that the sleeve 98 is 
spaced apart from the tines of the fork and magnet 96 so that the fork and 
the magnet straddle, but do not contact the sleeve 98. It is not 
recommended that the magnet 96 be allowed to make slidable contact with 
the sleeve 98. Therefore, it is preferred that the spacing between the 
tines 94a, 94b of the fork is less than the spacing between the tines of 
the magnet. In other words, the magnet is spaced further from the sleeve 
98 than the fork, so that if the fork comes in contact with the sleeve 98, 
the magnet will not, thereby preventing wear and/or damage to the magnet. 
At the end of the shifter rod 82, farthest from fluid valve 14, is another 
magnet 100. This magnet is similar to the magnet 96, but instead of being 
"C" shaped, it is substantially ring like or circular. The magnet 100 is 
sandwiched between a pair of caps 102, 104 which help prevent physical 
damage to the magnets. The magnet 100 is secured to the shifter rod by a 
nut 106 at one end and the interaction of the cap 102 and the sleeve 98 at 
the other end. 
Between the end cap 84 and the fork 86 is a third magnet 108 which is 
similar to the magnet 100. Again, the magnet 108 is sandwiched between two 
caps 110, 112 and are secured to the shifter rod 82 by a shoulder 114 of 
the shift rod 82 at one end and the sleeve 98 of the shifter rod at the 
other. 
The magnets 96, 100, 108 are permanent magnets. If fluid pressure piston 
engine assembly is to be used to pump hot melt adhesives, it is preferred 
that the permanent magnets be of a samarian cobalt, SM.sub.2 CO.sub.17, 
magnet construction. This is because it is well known that heat can affect 
the magnetic strength of a permanent magnet. Therefore, the choice of a 
permanent magnet for the pumping of hot melt adhesives must be able to 
withstand the temperatures commonly experienced in the heating and melting 
of such hot melt adhesives. For example, in a hot melt adhesive system, it 
could be expected that the shifter could be exposed to temperatures from 
about 200.degree. F. (93.3.degree. C.) to about 350.degree. F. 
(177.degree. C.). Samarian cobalt magnets, typically operate well at 
temperatures below 450.degree. F. (232.degree. C.). Therefore, if this 
embodiment is to be used in the dispensing of hot melt adhesives, then it 
is believed that samarian cobalt magnets are preferred. 
Each permanent magnet produces its own associated field of flux. The 
interaction of these fields is important to the effectiveness of the 
shifting. In order to provide smooth shifting in either direction, it is 
preferred that the shifter magnets 100, 108 are substantially the same 
size and configuration. In like manner, the fork magnet 96 is similar to 
the shifter magnets 100, 108. The forked magnet 96 could be circular with 
the shifter rod 82 and sleeve 96 passing through its center. Such a 
configuration is more difficult to assemble and disassemble. However, by 
providing a slot in a circular configuration, the fork magnet retains 
substantially the same circular configuration while allowing the shifter 
rod 82 and sleeve 96 to be easily disengaged from the fork, thereby 
facilitating assembly and disassembly. 
In that ferro-magnetic materials can affect the field (either focusing or 
distorting it) of a magnet, shifter rod 82, its associated sleeve 98, and 
the fork 86 should be of a non-magnetic material. For example, a 
passivated stainless steel may be used, such as 300 series stainless 
steel, or other non-magnetic materials such as aluminum, brass, etc. 
Similarly, it is believed that it is preferred that the magnet caps 102, 
104, 110, and 112 associated with each respective magnet be also of a 
non-ferro-magnetic material. 
Due to the presence of the magnetic fields, it is preferred that the valve 
spool 26 and the sleeve 28 of the fluid valve are also manufactured from a 
non-magnetic material or of a material which is only somewhat magnetic, 
such as a hardened stainless steel. For example, valve spools and sleeves 
of stainless steel having a 45-55 Rockwell "C" rating work well for hot 
melt applications. This prevents the possibility that one or both of these 
parts could become magnetized, thereby preventing or hindering the sliding 
movement of the valve spool 26 within the sleeve 28, and thus interfering 
with the direction of the flow of air to and from the piston chamber 18. 
In such embodiment, the housing 30 was aluminum and there were a plurality 
of o-rings 31 to accommodate the expansion and contraction of the two 
dissimilar metals. 
On the other hand, certain elements of the assembly should be of a 
ferro-magnetic material, so as to aid in the directing of the magnetic 
field so that it can be more effectively utilized and/or contained. As 
such, it is preferred that the end caps 84, 116 of the shifter assembly be 
of a ferromagnetic material. This also provides a detent mechanism which 
will be more fully described below. 
The polarity of the magnets are arranged such that as the fork magnet 96 is 
moved toward either of the shifter rod magnets 100, 108, there will be an 
attraction therebetween. For example, if the shifter rod magnets 100, 108 
are installed such that a north pole is located in conjunction with the 
upper caps 102, 110, respectively, then the fork magnet 96 will have its 
north pole located towards the upper shifter rod magnet 108. Shifting of 
the fluid valve 14 is accomplished by bringing the magnet 96 of the fork 
86 within close proximity to one of the spool magnets. At some point the 
attraction between the magnet 96 of the fork 86 and the spool magnet will 
be great enough to cause the spool magnet, along with the shifter rod 82, 
and a valve spool 26 to move towards the magnet 96 of the fork. This 
sliding movement will cause the elements of the valve spool 26 to realign 
themselves causing the piston to move in the opposite direction. 
For example, with reference to FIG. 2, as the drive shaft 22 and the piston 
20 approach the end of its stroke, the fork 86 will be moved towards the 
shifter rod magnet 100. As the force of attraction between fork magnet 96 
and the shifter rod magnet 100 increases, it will eventually be great 
enough to pull the shifter rod magnet 100 towards the magnet 96 of the 
fork 86. This in turn cause shifter rod 82 and the valve spool 26 to be 
moved in the same direction, such as is illustrated in FIG. 3. Once 
shifted, the fluid valve 14 redirects the air flow as described above such 
that the direction of motion of the drive shaft 22 and its associated 
piston 20 is reversed. This in turn moves the fork 86 towards the other 
shifter rod magnet 100. Again, as the drive shaft and piston approach the 
end of a stroke, the attraction between the magnet 96 of the fork and the 
magnet 108 of the shifter rod 82 will cause the magnet 108 to move towards 
the fork 86. This in turn causes the fluid valve 14 to shift, thereby 
reversing the flow of air and returning the assembly to that as 
illustrated in FIG. 2. By properly positioning and sizing the various 
magnets, the shifting of fluid valve 14 may be accomplished as a 
non-contact operation. In other words, the magnets of the shifter rod may 
become adjacent to, but do not contact the magnet of the fork. A 
non-contact operation should have improved wear characteristics, and, 
therefore, improved durability over previous designs. Furthermore, by 
keeping the magnets spaced a predetermined distance apart, the force 
required to separate the magnets will be less than if they were in a 
contact position. Also, in that the force exerted on or between the 
respective magnets increases as the magnets are brought closer and closer 
together during the stroke of the piston and the drive shaft, there is 
less likelihood that the fluid valve will be prevented from shifting, 
which in turn produces a less likelihood that the pump will stall. 
In prior art air valves, it is common for various contaminants, such as 
varnish like substances, to accumulate within the air valve so that more 
and more force is required to shift the air valve completely from one 
position to the other. Some shifter assemblies exert the greatest amount 
of force at the beginning of the shift and taper off to a lesser force as 
the shifting process is completed. For example, a shifter utilizing a 
spring will work in this manner because the spring's force is typically 
the greatest at the beginning of the shift. In such shifters, there may be 
enough force to overcome the contaminants and cause the valve to begin to 
shift. However, as the force of the shifter diminishes as the air valve 
moves, it is possible that the force will diminish to a point were it will 
not be able to overcome the resistive force caused by the contaminants. 
This results in the air spool failing to completely move from one position 
to the other. This, in turn, causes the pump to stall. 
With reference to FIG. 2, the shifting force exerted by the shifter on the 
air valve increases as the shifter moves from one position to another. For 
example, as the magnet 96 carried by the force 86 moves from its lower 
position (as oriented with reference to FIG. 2) near magnet 100, the force 
of attraction continuously increases between it and the upper magnet 108. 
At some point, the force of attraction between the magnets 108 and 96 will 
become so great that the spool, shifter rod, and the magnet 108 will begin 
to move downward. Once they begin to move downward, they should continue 
to shift because the force drawing them downward continuously increases 
until the air valve has completely shifted downward and the fork has 
reached its most upward portion of the stroke. Therefore, once the air 
valve begins to move, there is a much greater probability that the shift 
will be completed because the force of attraction is increasing, thereby 
being less sensitive to the build-up of contaminants. 
The interaction between the magnets 100, 108 of the shifter rod 82 with the 
respective end cap 84, 116 provide a detent in order to prevent the 
shifter and the fluid valve from moving from one position to another as 
the fork moves between the shifter rod magnets. Therefore, whichever 
shifter rod magnet is located closest to its respective end cap, the force 
of attraction therebetween should be strong enough to prevent inadvertent 
movements of the shifter and the fluid valve, but not strong enough to 
prevent the shifter rod magnet from moving towards the magnet of the fork 
at the time of shifting. 
Alternatively, the "C" magnet may be replaced with two parallel bar 
magnets. The shifter rod would pass between the spaced apart magnets 
similar to the slot in the "C" magnet of the fork. In this alternate 
embodiment, the length of the bar magnets must have a longer dimension 
than the outer diameter of the shifter rod magnets. This embodiment 
provides a means for reducing or eliminating side loading which may be 
associated with the shifter rod. With ring magnets and "C" magnets, side 
loading of the shifter rod may occur due to misalignment between the 
shifter rod magnet and the fork magnet. This misalignment can result from 
tolerance differences which cause the physical parts to be misaligned, or 
from differences wherein the magnetic center of the magnet varies from its 
geometric center. If there is a misalignment between the "C" shaped magnet 
and a ring magnet, then they will tend to resist any force that tries to 
move them out of, or hold them out of, their true magnetic alignment. For 
example, if the fork magnet and the shifter rod magnets are held out of 
magnetic alignment by their connections to the pump piston and the air 
valve respectively, they will exert a force on these components, in the 
form of a side load, which in turn can cause increased friction and wear 
to these components. Providing a means for allowing the fork magnet and 
magnet shaft magnets to move into magnetic alignment will eliminate this 
problem. The slot formed by the bar magnets provides an adjustment which 
allows the ring magnet of the shifter rod to compensate for any 
misalignment between it and the magnet 96 of the fork. 
Alternately, with reference to FIG. 7, the one piece fork may be replaced 
with a two piece fork 86b, in which members 86c and 86d are connected 
together by a hinge 87. The connection of the fork 86b to the drive shaft 
22b is changed to allow it to pivot about the shifter rod. This allows the 
"C" shaped fork magnet 96b the freedom to swing in an arc about the 
shifter rod and position itself within the shifter rod so that it can 
magnetically align itself with the shifter rod magnet and thus eliminate 
or reduce side loading. 
Also, since the fork magnet has less cross-sectional area due to the hole 
and slot for the shifter rod, its magnetic center is not necessarily the 
circular center of the magnet. Therefore, it is believed to be preferable 
to position the shifter rod at the magnet's centroid. 
Spacing the fork and its associated magnet from the sleeve 98 and shifter 
rod 82 provides as an aid in trouble shooting the system. For example, if 
the pump was to stall, the air valve may be manually activated by pushing 
on either the end 26a of the spool 26 or the end 82a of the shifter rod 
82. If the air valve moves freely, then the stall was probably not the 
result of the air valve. In other prior art shifters, it is necessary to 
first physically disconnect the shifter from the pump drive shaft, which 
can be difficult and time consuming. 
While the invention has been described with reference to a preferred 
embodiment, it should be understood by those skilled in the art that 
various changes may be made and equivalents may be substituted for 
elements thereof without departing from the scope of the invention. For 
example, with reference to FIG. 8, there is illustrated a cross-sectional 
view of an alternate shifter assembly 80a. In this arrangement, the fork 
86a is of a ferromagnetic material and does not contain a fork magnet. The 
fork 86a is attached to the drive shaft 22a of the piston as before. The 
shifter rod magnets 100a, 108a are mounted in steel cups 118, 120 
respectively to contain the lines of flux and increase the force of 
attraction between the shifter rod magnets and the ferro-magnetic fork 
86a. As before, as the drive shaft 22a of the piston reaches the end of a 
stroke, the fork 86a approaches one of the shifter rod magnets. As the gap 
between the fork 86a and a shifter rod magnet decreases, the force of 
attraction will increase. This force of attraction will increase until the 
shifter rod magnet is pulled toward the fork 86a. Thus shifting the fluid 
valve 14 to cause the air directed in the piston chamber to be reversed, 
thereby reversing the direction of the piston and the drive shaft 22a.