Note: Descriptions are shown in the official language in which they were submitted.
S~6
HYDRAULIC TRIM TILT SYSTEM FOR OUTBOARD
PROPULSION UNITS USING A PRESSURE AMPLIFIER
FIELD OF THE INVENTION
This invention relates to the field of hydraulic
amplifiers and to the use of a hydraulic amplifier to
provide higher pressure and force to move objects such as
an outboard propulsion assembly in a marine drive.
BA~KGROUND OF THE XNVENTIO_
Hydraulic trim tilt systems for outboard
propulsion units are well-knwon. Further, such systems'
units employing a single cylinder to displace the
propulsion system such as rotating them through an arc are
well-known. One such example is shown in U.S. patent No.
3,799,104 which issued to L. Kurling on March 26, 1974.
U.S. patent No. 3,842,789 which issued to K. A.
Bergstedt on October 22, 1974 also shows a system for
l; using a single cylinder for power trim and tilt. Patent
No. 4,064,824 which issued to J. R. Draxler on December
27, 1977 and 4,096,820 which issued to C. B. Hall on June
27, 1978, both show systems for applying hydraulic forces
to separate trim and tilt cylinders. Other patents
showing hydraulic systems used with marine propulsion
units are 3,885,517 which issued to G. M. Borst on May 27,
1975; 3,434,450 and 3,434,448 which issued to D. F.
McCormick and W.L. Woodfill, respectively, on March 25
1969.
All of the prior systems, U.S. Patent No.
3,842,789 identified above, being an example, require a
particular valving arrangement to change the amount of
hydraulic pressure to the actuator and to rotate the
propulsion unit through the trim and tilt range. In that
patent, manual adjustment of a valve in the hydraulic
supply lines is required. Although the object of this
patent and of other patents is to provide a higher
pressure to a system when requiring greater force and less
rapid movement during trim operation and a lower pressure
and more rapid movement during tilt operation, the prior
, , ~ .
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5~6
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devices either require separate actuator cylinders to
rotate the propulsion unit through trim and tilt, or
require special hydraulic switching systems to change the
distribution area or the hydraulic fluid in the actuator.
DISCLOSURE OF THE INVENTION
In accordance with an aspect of the invention
there is provided an hydraulic amplifier for amplifying
pressure of a hydraulic fluid, said hydraulic amplifier
having a displaceable means movable by said fluid in a -
displacement path, said displaceable means having switch
means, said switch means being open to connect a first
port of said amplifier to a second port of said amplifier
and to provide a fluid path through said amplifier sub-
stantially without amplification responsive to a pressure
differential across said switch, said amplifier having
retarding means in said displacement path for retarding
the movement of said displaceable means, said switch means
being in a closed position when said displaceable means is
moved in said displacement path, movement of said
displaceable means being retarded upon reaching said
retarding means in said path and increasing said pressure
differential to provide fluid through said amplifier
substantially without amplification.
This invention is a hydraulic system for moving,
displacing or rotating a load for example, such as in a
marine propulsion unit through its trim range and through
a tilt range. In the trim range, when the boat is
underway, and the angle of propulsion unit must be
adjusted, a considerably greater force is required to
displace the propulsion unit then to tilt it out of the
water when power is removed. The trim range is limited
with respect to the range of movement for the hydraulic
propulsion unit. During trim, hydraulic fluid is provided
to the hydraulic cylinder actuator unit with greater
pressure and force and with a resultant slower movement of
the hydraulic cy:Linder and the propulsion unit.
~51S(~6
-2a-
Accordingly, this invention uses a hydraulic
amplifier which is connected in the hydraulic circuit
between the hydraulic pump and the hydraulic cylinder
actuator unit. The amplifier increases the pump pressure
during the trim operation as necessary to increase the
force to the hydraulic actuator connected to the
propulsion unit, at the end of the trim and start of the
tilt range, the amplifier is bypassed and further
displacement of the propulsion unit is then in the tilt
range where the propulsion unit is then rapidly lifted
clear of the water.
The hydraulic amplifier includes a displaceable
or movable means which may be a piston and which has a
displacement proportional to the displacement of the
hydraulic actuator cylinder and the propulsion unit in the
trim range. Movement of the piston within the amplifier
to one end of its displacement path may correspond to
movement of the propulsion unit to the end of its trim
range and to the start of its tilt range.
~lS~S06
At this point, a switch means such as a valve
within the hydraulic amplifier and which may be mounted
on the piston, bypasses the amplifier and connects the
fluid from the hydraulic pump directly through to the
actuator unit. m e valve may be made responsive to dif-
ferential pressure. The differential pressure will be
large enough to open the valve when the load placed on the
actuator cylinder is reduced or removed. In the system
this will occur when the piston reaches the end of its
10 displacement path and the load is removed from the actuator.
The flow of the hydraulic fluid into ~he actuator unit is
then at a lower pressure, but at a faster rate. As the
propulsion unit at this point is to be tilted out of the
water with power to propulsion unit being removed, less
15 force is necessary and the lower hydraulic pressure moving
at a higher velocity through to the actuator is s~fficient
to rapidly move the hydraulic actuator and the propulsion
unit through its tilt range out of the water.
When it is desired to move the propulsion unit
20 through its tilt range bacX into the water and into the
trim range, the pump is reversed, displacing the fluid in
the hydraulic system in the opposi e direction and causing
the actuator cylinder to rotate the propulsion unit down-
wards towards the water and correspondingly displacing the
25 piston within the hydraulic amplifier. As stated above,
the displaceable or movable means may be a slidable piston,
moving within a cylinder. Movement of the fluid under the
force of the pump forces the amplifier piston and actuator
piston to be displaced as the actuator forces the propul-
sion unit in the opposite direction, the movement of the
30 piston within the hydraulic amplifier corresponding to the
movement of the actuator piston and of the propulsion unit.
The hydraulic amplifier includes a limit means
- which may be removPd from an end and placed within the
path of movement of the said amplifier piston or may be,
35 but not necessarily at the oth r extreme end of its dis-
placement path~ When the amplifier piston reaches that
limit means, the switch corresponding to the valve is
actuated causing the amplifier to be bypassed such that
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iL5~
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f~ fr~m the actuator ~lows directly through the ampli-
fier, bypassing the amplifier and through to the pump.
The continued operation of the pump then displaces the
fluid from the actuator and through the hydraulic amplifier
without amplification to further displace the hydraulic
cylinder and the propulsion unit down through the balance
of its tilt range. Continued operation of the pump forces
the propulsion unit through the trim range until the
extreme of the trim range is reached where further move-
10 ment is prevented.
When during this movement of the propulsion unitin the downward direction, the pump is reversed, the hy-
draulic pressure in the system will force the amplifier
piston to move away from the limit means and in the other
15 direction along its displacement path and towards the one
end. At this point, the switch within the amplifier will
be actuated connecting the amplifier within the hydraulic
circ~it, and providing hydraulic fluid with increased
pre~sure but at a slower velocity to the hydraulic cylinder
20 actuator.
In the contemplated use of this device, this
part of the operation will take place after the system has
been operated in reverse to bring the hydraulic propuLsion
unit from its full tilt position downward and to a posi-
25 tion where it immersed in the water and where furtherreversal of the pump and fluid flow will be for the purpose
of causing a desired trim.
In a system for providing hydraulic fluid at a
first higher pressure to a hydraulic cylinder to move a
30 propulsion unit through a trim rang~, and at a lower pre-
ssure but at a faster rate to move the hydraulic cylinder
and a propulsion unit through a tilt range, a kydraulic
amplifier is provided with means for switching the ampli-
fier in and out of the hydraulic circuit ~o produce a
35 change in pressure required for the respective trim and
tilt operation.
Thi9 invention accomplishes its result ky means
o~ a novel arrangement of a switch being a valve mounted on
the piston wlthin the hydraulic ampli~ier and dispenses
.
~51506
-5-
with ex~er~al switches ~nd control s~stems to alter the
system from a trim to a tilt operation and visa-versa.
Additionally, this system uses a single hydrau-
lic cylinder for both trim ancl tilt operations. Use of
this single cylinder is made possible by the hydraulic
amplifier and its internal va].ve arrangement. When the
propulsion unit and the hydraulic cylinder has moved
through a range corresponding to the trim range, the valve
arrangement within the pressure amplifier is switched by
differential fluid pressure and fluid at a lower pressure
10 is provided directly to the actuator but at a faster rate
for tilt operation.
- The invention is described with xeference to its
use with a load such as a marine propulsion unit and parti-
cularly where the propulsion unit is tilted or rotated out
15 of the water and where the unit may also be rotated
through a trimming range when immersed in the water.
As is known, a marine propulsion unit, under-
drive, exhibits a strong force in one direction, tending
to rotate or drive the propulsion unit towards the transom
20 of the boat. Hydraulic systems are most commonly used to
displace the propulsion unit forward and backward within a
trim range when the propulsion unit is immersed in the
water. ~hen underdrive and to displace the propulsion unit
away from the transom of the boat, a considerable force
25 must be developed within a hydraulic actuator.
Where the propulsion unit is displaced through
its trim range, further rotation or movement of the hydrau-
lic propulsion unit then tilts the propulsion unit out of
the water. For tilt, the drive is removed from the marine
30 propulsion unit and less hydraulic pressure is required
for the actuator to rotate or-displace the propulsion unit
through the balance of its range of movement and particu-
larly through the tilt range.
With regard to this application and for refer-
35 ence purposes, movement of the propulsion unit from aposition where it is closest to the transom of the boat,
through its trim range, into and through its til~ range to
its other extreme position out of the water, is defined as
. . .
~si~o~
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movement in a first direction. The first direction then i.s
identified as movement of the propulsion unit through its
trim range and its tilt range rotating it in an upper
direction removing the propulsion unit from the water.
Accordingly, movement of the propulsion unit in
the reverse or second direction would be downward towards
the transom of the boat and through its tilt range and its
trim position immersing the marine propulsion unit in the
water.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. la through ld show in schematic form, the
system in various stages of displacement. FIG. la showing
the amplifier system wi~h the piston at its limit means
wherein the amplifier is bypassed. FIG. lb showing the
piston movable being midway between its limit means and
its opposite end position in its displacement path and
wherein it provides an amplified pressure to the actuator.
FIG. lc shows the amplifier piston at its end posltion and
stopped f-rom further movement and wherein the amplifier
is bypassed to provide fluid to the actuator at the pump
velocity and pump pressure, and FIG. ld shows movement of
the piston within the amplifier to its limit means
responsive to movement of the actuator from its tilt posi-
tion with the pump pressure reversed from that shown in
FIGS. la through lc.
FIG. 2 shows in cross section, the amplifier
assembly with the piston at one end at its stopped position.
FIGS. 2a, 2b and 2c show in cross-section the
surfa¢es of the piston at various positions along its axial
length.
FIG. 3 shows in cross-section, a detail of the
switch means, being a hydraulic valve within the ampli-
fier piston.
FIG.4 (appearing on the same sheet of drawings as
Fig. 2) is an end view of the amplifier assembly of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
I
FIGS. la, lb, lc and ld shows operation of the
hydraulic system in a first direction, proceeding through
. ..~ ,.
cl'`
~51506
-7-
the tri~ r~nge t~ the tilt x?nje to~a full extensiQn sf
the hydraulic actuator cylinder where the propulsion unit
is in its full tilt position.
The hydraulic unit is shown in schematic form
and wherein the same numerals indicate the same and simi-
larly operating parts.
As shown, a hydraulic pump 3 provides fluid to a
hydraulic amplifier indicate generally as 5. m e hydrau-
lic amplifier 5 has a first port 7 connected to the pump
3 and a second port 9 connected to the hydraulic actuator
cylinder shown as 11. The hydraulic amplifier 5 as is
well~known, has a displaceable means piston which may be
a piston 13, having a first corss-section area 15 shown
in FIG. 2b and a second cross-sectional area 17, shown in
FIG. 2c, area 17 being an annular area represented by the
difference between the area of face 15 of the piston 13
and the area of face 19 of piston 13, shown in FIG. 2a.
The piston 13 is displaced or moves within the
cylinder assembly 21 of the hydraulic amplifier 5 between
a first extreme position at the top of the ampli~ier cylin-
der 21 shown as position B and a second extreme position
shown as position A at the bottom of the amplifier 21.
It being understood that the terms top and
bottom are used for reference purposes only and this
invention in no way requires that the hydraulic cylinder
be aligned as shown in the drawing with respect to top and
bottom.
Seals 23 and 28 are provided for allowing move-
ment of the amplifier piston 13 within the cylinder 21
without loss of fluid and without fluid communication
between that volume bound by face 15 of piston 13 and that
volume bound by face 17 of piston 13.
~ e piston 13 is shown as having a switch means
29 which may be a valve assembly as shown having a ~alve
33 and valve seat 34, a means for closing the ~alve shown
as spring 3;2 and means for opening the valve shown as
plunser 36 positioned to a limit means shown as bottom
wall 35 of c~ylinder 21.
~le fluid may be communicated to the actuator
..
.
-8-
cylind~ 3A th~ough ~crt g of ~he Gyli..nfl~?~ ?1 c~r,d~ 3
and port 45 within the cylinder 11.
Operation in the system is described assuming
that the load 49 is a~ its fully dow~wardly ex~ended posi-
~ion with the hydraulic cylinder 11 and the load propulsionunit 49, being at ievel C at one end of the trim range.
Assuming now that the pump 3 is operated in a
first dixection to apply pre-~sure to the system, fluid under
pressure is applied to the hydraulic amplifier 5 through
10 port 7 and to that volume adjoining the sur~ace 15 of
piston 13.
Although the plunger 31 in contact with the bot-
tom of the amplifier cylinder 35 as shown in FIG. la,
forces the valve 29 to open, communicating the volume of
15 the amplifier cylinder adjoining face 15 of piston 13 to
the volume of the cylinder adjoining face 17 of the piston
13 sufficient force is developed due to the differences in
affected surface area to move the piston 13 up towards
position B from the position A shown in FIG la.
As shown in FIG. 15, movement of the piston
from position A to position A-B between extreme positions
A and B correspondingly moves the plunger 31 from the
bottom 3~ of the amplifier allowing the valve 33 to close
under the force of the valve closing means shown as spring
25 32.
During this time when the amplifier piston 13 is
moving between end position A and end position B as shown
in FIG. lb, the valve will be closed under the force or 32
and the area or volume withi~ the amplifier cylinder
30 adjoining face lS of piston 13 will be isolated from the
area adjoining ace 17 of piston 13. With the valve 33
closed, movement of the amplifiar piston 19 will force
rluid from the amplifier cylinder 21 through port 9 to the
hydraulic actuator cylinder 11 and displace the hydraul;c
35 actuator cylinder from its initial rest position to a new
position D shown as in FIG. lb.
During the time the amplifier piston is moving,
the pressure within the hydraulic cylinder 5 will be an
amplified prassure relative to the pressure of the pump.
, - -
~51S06
g
As is well-kno~.~ in hydx~ amplifie~s, t~e ~e.5sure at
port 9 will be equal to the pump pressure multiplied by
the ratio of the surfa~e area of face 15 of piston 13 to
the sur~ace area of the annu].ar surface 17 of piston 19.
During movement of the piston, the force pro-
duced by the di~ferential pressure between the volume
adjoining surface 17 and the volume adjoining surface 15
will be less than the force of the valve closing means 32
within valve assembly 29 and the valve will be maintained
closed as shown in FIG.'lb.
In FIG. lc, the amplifier piston 13 has been
moved to one-end of its displacement path at position B.
When this point is reached, the amplifier is constructed
so that further mechanical displacement of the piston 13 is
not possible in the direction from A to B. At this point,
15 and assuming power ~ the propulsion unit is removed the
differential pressure between the fluid in the volume bound
by ~a~e 15 and the volume bound by annular face 17 will
force the valve 33 to open against the force of spring 32,
permitting communication between that volume bounded by
face 15 and the volume bounded by face 17. The further
20 operation of the pump in the first direction, rorces fluid
from the pump 3 through port 7 of amplifier 5 through the
valve assembly 29 and conduit 37, through the volume of the
amplifier cylinder 21 adjoining surface 17, through port 9
of amplifier 21, and conduit 43 connecting the amplifiar to
25 the hydraulic cylinder ll at port 45.
When the piston 13 of the hydraulic amplifier is
moved to its extreme position B, and the differential pres-
sure opens valve 33, allowing communication of the fluid
through the amp,lifier 5 without amplification, the fluid is
30 then moved at a lower pressure but at the velocity of the
pump further displacing the piston 47 within the hydraulic
cylinder 11 from position D to position E as shown in FIG.
lc .
As explained above, where the propuLsion system
35 shown as load 49 is at its lowest position shown as C in
FIG. la, initial displacement, corresponding to the dis-
placement between A and B of the amplifier piston 12 will
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b9 through the trim range oosition~ the di.splacement ~eLn~.j...
C to D of the piston 47 and the propulsion unit 49.
When the amplifier piston is at position B, and
with the v21ve 33 open as explained above, the pump will be
connected directly through to the hydraulic cylinder 41
bypassing the hydraulic amplifier 5. The fluid within the
system will be moved at the pump velocity but at the lower
pump pressure, directiy to the hydraulic actuator and will
rapidly move the actuator and the propulsion unit from pos-
ition D to E representing the tilt range.
Operation of the device is now shown with refer-
ence to FIG. ld wherein the propulsion unit 49 shown as
position 1 in Phantom at the extreme end of its tilt posi-
tion, is lowered through its tilt range and its trim range,
back to its extreme from position C.
As can be seen in FIG. lc, when the pump 3 is
stopped or reversed, the differential pressure exist m g at
face 15 of the amplifier piston 13 will no longer maintain
the valve 33 in valve assembly open, valve 33 will close
connecting the hydraulic amplifier S in the hydraulic
20 circuit, so the hydraulic amplifier piston 13 is displaced
downwardly in a second direction from position B towards
position A. As the hydraulic amplifier pistcn 13 moves
from position B to A, a corresponding displacement of the
actuator piston rod 47 and the propulsion unit 49 from its
25 extreme tilted position E to an intermediate position shown
as F will take place corresponding to the movement of the
piston within the hydraulic amplifier from B to A.
Ac explained above at position A, the force of
the plunger 36 against the limit means shown as the bottom
30 35 of the cylinder 21 of the amplifier 5 will open the
valve 33 allowing communication of the actuator cylinder
11 through port 9, conduit 37, the valve 29 and port 7 of
the amplifier 5 through to the pump 3. As the valve will
be open under the force of plunger 36, the amplifier will
35 be bypassed and switched out of the hydraulic circuit, and
the piston 47 of the hydraulic actuator 11 will be moved
under the direct force of the pump nd with the velocity of
the pump through to its other extreme bottom and moving the
~L~5~5S~6
propulsion unit 49 to position CO
Operation of the pump in the reverse direction
with the unit within its tilt range at F for example, and
with piston 13 at B will cau~;e displacement of the fluid,
moving the piston 47 within t:he hydraulic actuator cylinder
11 and moving the propulsion unit.
Movement of the pump in the sècond direction then
displaces or rotates the propulsion unit downward towards
the water, and continued move~m,ent and operation of the
10 pump displaces the propulsio~ unit 49 to the trim range
be~ween positions ~ and D. When the propulsion unit is
located in the trim range, operation of the pump in the
first direction will cause movement of the hydraulic ampli-
fier piston 19 away * om the limit means shown as the bot-
15 tom wall 35, removing the force of the opening means sho~nas plunger 36 from the valve 33 closing the valve under the
force of closing means shown as spring 32 and placing the
amplifier within the hydraulic circuit. The amplifier then
provides an amplified pressure to permit displacement of
20 the load shown as a propulsion unit 49 to ef~ect proper
trimming of the propulsion unit.
Operation of the pump in the reverse direction
with the unit within its tilt range at F for example, and
with piston 13 at B will cause displacement of the fluid,
25 moving the piston 47 within the hydraulic actuator cylinder
11 and the propulsion unit toward position C.
The amplifier piston 13 will also experience cor-
responding movement until it reaches a limit means where
the valve 33 will open allowing direct communication of
30 the hydraulic actuator cylinder 11 with the pump 3 and
rapid movement of the piston 47 and the associated propul-
sion unit ~9 at the speed and pressuxe of the pump 3.
In the described operation o~ the amplifier S,
valve 33 op~ns when the piston reaches position A. However,
3~ it should be understood that position A is not necessarily
at an end or limit of the displacement path of the piston
13 within th,e amplifier.
As will be understood by one skilled in the art,
a limit means for contacting a means to open valve 33 can
5~ 5~ 6
-12-
be placed within the hydraulic amplifier ~ to open the
valve 33 before the piston l9 reaches position A. For
( example, a contact means can be placed within the ampli-
fier 5 which contacts the plungex 36 when the piston assem-
bly is somewhere between position B and A.
It should be understood that the limit means
shown as bottom wall 35 in this embodiment, may ~e placed
anywhere along the path of the piston 13 between position
A and position B and may be any suitable means which con-
tacts the actuator means shown as plunger 36 responsive to
10 movement of the piston 13 towards the limiting means, and
wherein further movement of the piston 13 in the same
direction towards the limiting means, forces the actuator
means against the valve, opening the val~e and allowing
communication between the two volumes.
Operation of the am~lifier is now shown with
reference to FIGS. 2, 3 and 4 wherein a detail of the
valve assembly 29 is shown.
m e valve is mounted in the displaceable piston
13 and includes a means shown as spring 32 forcing valve
_ 20 body 33 ag~inst valve seat 34. An actuator means for
o~ening th~ valve is shown as a plunger 36 having an end
~ 38 protruding below the surface 15 of the piston to engage
a limit means such as the bottom wall 35 of cylinder 21.
Movement of the amplifier piston 13 towards posi-
25 tion A places the plunger 36 in contact with the bottomwall 35 of the amplifier cylinder. The plunger 36 then
displacPs valve body 33 against valve spring 32 opening
the valve and permitting fluid to flow from the volume
bounded by surface 17 through conduit 37, ~alve seat 34 and
30 valve body 33, through to port 7. A small annular space 24
may be left between piston 13 and the inner wall of cylinder
21 to permit fluid to flow out conduit 37.
It should be noted that it is not necessary for
the piston to be displaced to the bottom or to the bottom
35 wall 35 of the amplifier cylinder 21 as any suitable means
for contacting the plunger 31 and forcing the plunger
against the valve body 33 and against the force of the
spring 32 opening the valve may be used. For example, a
.
~*~lS~6
-13-
suitable mechanical structure may be placed close to posi-
tion A and removed from position B to contact the plunger
extension 38 and open the valve before the valve reaches
the bottom wall 35 of the cyLinder. However, the bottom
wall is a convenien~ place for locating the limit means
for the valve and for that reason is shown as such in the
preferred embodiment.
Further, it should be known that a plunger is
not required as any suitable actuating means ~ay be used,
responsive to movement of the piston, to contact the valve,
10 and open the valve. For exa~lple, a structure stationarily
mounted in the cylinder, and located in the path of the
valve so it contacts the valve when the valve reaches a
corresponding limit position, opening the valve and
communicating the volume bounded by surface 15 to the
15 volume bounded by surface 17.
As hydraulic fluid is driven in a first direc-
tion, the differential force against the surface 15 forces
the piston to be displaced towards position B from position
A, moving the piston away from bottom wall 35 and allowing
20 the valve to close. The piston is then displaced up until
the point where the annular surface 17 contacts the stop
23 located in the cylinder 21. At this point, and assum-
ing no power is provided to the drive unit, the differen-
tial pressure on the plunger side of the valve compared to
25 the force of the spring against the valve opens the valve
and provides hydraulic fluid under the speed and pressure
of the pump to the hydraulic actuating cylinder 11 to dis-
place the cylinder through its tilt range. r~hen the pump
is stopped or reversed, the differential pressure forcing
30 the valve open is removed and spring 32 forces the valve
closed.
However, if power is supplied to the propulsion
drive unit, the force of the pump by itself will be insuf-
ficient to displace the hydraulic cylinder against the
35 force of the propulsion unit and the system will go into
hydraulic stall or fluid will be directed through a bleed
valve.
With the piston of the hydraulic amplifier located
~5~5~6
-14-
at position B, operation of the hydraulic pump in the
second direction, displaces fluid ~rom the hydraulic cylin-
der to the amplifiex, and to the volume defined by surface
17 displacing the piston from position B to position A,
S When the piston reaches the limit means, plunger 36 is
forced against the valve body 33, and opens valve and
permitting fluid to flow around the valve seat 34, fluid
under the speed and pressure of the pump is forced to the
hydraulic cylinder allowing the hydraulic cylinder to move
10 its end position where the hydraulic unit is placed to its
fully lowered position in the water corresponding to posi-
tion C in FIG. 1,
The principles of this invention have been shown
with reference to the preferred embodiment, but should not
15 be limited to the description of this embodiment as shown.
For example, the manner of opening the switch
means mounted in the movable portion of the amplifier, by
driving the movable means to one end of its path may be
changed to any other suitable arrangement~ One possible
20 arrangement would be to place a limit means in the path of
the movable means, a limit distance from the end of the
path. In this way, the amplifier could have two limit
means, neither at the end of the path, or one limit means
which may or may not be at the end of the path, with the
25 the switch being opened by the third pressure when it is
at the respective other end of th0 path, or the switch may
be operated by the fluid under pressure at both ends of the
path.
