Note: Descriptions are shown in the official language in which they were submitted.
1057163
The invention relates to an arrangement for
influencing the operating quantity of a servo-motor that is
loadable by an external force, comprising a pressure responsive
reducing valve in the return conduit, of which the setting
element is loaded in the first direction by a desired value
spring and in the other direction by a pilo~ pressure existing
in a first pressure chamber.
In a known arrangement, the two connecting
conduits for the servo-motor are connected to the pump and the
tank by a four-way switching valve which, apart from the
neutral position, has one position for right-hand operation
and one position for left-hand operation. In both motor
connecting conduits there is a braking valve which functions
only on return flow operation because it is bridged by a
check valve that is open towards the servo-motor. The pressure
in the respective other motor connecting conduit, i.e. the supply
conduit, serves as pilot pressure for t~ie braking valve. The
braking valve is loaded in the closing direction by the
desired value spring and opens under the influence of the
pressure in the supply conduit. If, now, an external load
acts on the servo-motor, e.g. the weight of a load in the
case of lifting tackle, the force of the flow of water in the
case of a control rudder, or the like, which tends to move the
servo-motor more rapidly than the speed corresponding to
the arriving pressure fluid, the pressure in the supply conduit
drops and the braking valve is moved in the closing direction.
However, this pressure in the supply conduit does not give a
correct signal, particularly in the case of loading the servo-
motor by an external force (negative load)O In addition,
the throttling that is effected thereby affects the said
pressure only with a large time constant. Furthe~, the speed
of the servo-motor cannot be controlled in this manner.
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Still further, the load cannot be decreased when there is
no pump pressure.
- To control a servo-motor, it is known to include
an adjustable throttle element in the supply conduit of
a servo-motor. A defined throughflow quantity can be
associated with the setting path of the throttle element
if the throttle element is preceded by a regulating valve
which keeps the pressure drop at the throttle element constant.
~ The invention is based on the problem of providing
an arrangement of the aforementioned kind with which the,
throughflow quantity of a servo-motor can be adjusted at
will independently of the load of an external force (negative
load).
' This problem is solved according to the invention
in that an adjustable first throttle element is disposed
in the return conduit behind the reducing valve and that the
pilot pressure is derived from between the reducing valve and
the first throttle element.
~ The reducing valve seeks to keep the pressure
constant at a value predetermined by the desired value spring.
The quantity flowing through the throttle element is therefore
a clear function of the orifice cross-section or the setting
path of the first throttle element. In this way one can, for
example, control the lowering speed of a fork life truck,
lift or the like independently of the actual loading. In
this connection it is immaterial whether the servo-motor has
a single-acting or double-acting cylinder.
The operating conditions are altered only very
little if, in addition to the desired value spring, still other
forces act on the setting element of the reducing valve~
particularly if they are only small or likewise constant.
In an arrangement for a servo-motor having a supply
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and an outlet, this fact is advantageously utilized in that
in the supply conduit that is fed from the pump there is an
adjustable second throttle element couplied to the first throttle
element, and that the setting element of the reducing valve is
additionally, in the same sense as the desired value spring,
loaded by the pressure existing in a second pressure chamber
between the second throttle element and the servo-motor.
During normal operation, the reducing valve is thereby brought
to the open position because the pressure in the supply conduit
is considerably higher than that between the reducing valve
and the first throttle element. If, however, an external
load acts on the servo-motor in the operating direction, and
the pressure in the supply conduit thereby drops, the
reducing valve goes over to a throttling position which
prevents excessively rapid outflow of the return flow quantity.
If dangerously low pressures that could give rise to
cavitation occur on the supply side, the desired value
spring practically functions along, so that the above-described
conditions arise in the return conduit. During normal operation,
therefore, the control of the operating quantity is effected
by the second throttle element and with a negative load by the
first throttle element, switching over taking place
automatically.
It is of particular advantage if a regulating valve
is connected in series with the second throttle element for
holding the pressure drop at the second throttle element
constant. In this way the orifice cross-section or the
setting path is likewise a clear indication for the throughflow
quantity in the case of the second throttle element.
Desirably, the reducing valve is designed so that
the pressure drop influenced by it at the first throttle
element is at most equal to the pressure drop at the second
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throttle element as determined by the regulating valve. With
such dimensioning, it is ensured that the pressure between
the first throttle element and the servo-motor will never
drop below the tank pressure, i.e. cavitation phenomena are
avoided in every case.
It is also favourable if a check valve openable
towards the servo-motor is in parallel with the reducing
valve. In this way, the reducing valve will be inoperative
when the motor connecting conduit is used as a supply conduit
instead of a return conduit.
A similar effect is achieved if the conduit with
the reducing valve is usable as a return conduit in a first
operative position of a switch element and as a supply conduit
in a second operative position of the switch element and if
in at least the second operative position the second pressure
chamber is fed with the pressure between the second throttle
element and the servo-motor. In this way the same pressure
will exist during supply operation in both pressure chambers
and the reducing valve will be open under the influence of the
desired value spring.
If a negative load can act in both directions of
movemeht of the motor, e.g. in the case of a control rudder,
a reducing valve may also be provided in each of the two
motor connecting conduits.
Both throttle elements preferably always assume
substantially the same orifice cross-section. This results
in clear conditions and also permits an exchange
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of the first and second throttle elements during reverse operation.
In particular, the ~irst and second throttle elements
can be combined as a four-way valve with intermediate throttling
positions. With such a valve, synchronism of the orifice cross-
sections is ensured in a simple manner.
Further, the four-way valve may serve as first switch
element and possess further switching paths forming the second
switch element.
Further, it is advisable to provide the motor connection
on the return flow side with a safety valve. This safety valve
lowers pressure peaks and therefore not only protects the conduit
system but also the series circuit of the reducing valve and
the first throttle element.
In its broadest form, therefore and in summary of
the above, the present invention may be defined as a fluid
pressure supply and exhaust system for controlling a two port
servomotor wherein negative loads are anticipated for one side
of the servomotor, the system comprising, conduit means with
switchable parallel supply and exhaust branches, pump and tank
means connected to the servomotor by the conduit means, switch
means connecting the supply and exhaust branches to the one and
the other side of the servomotor respectively, pressure regulating
valve means in the supply branch and reducing valve means in the
exhaust branch, throttle valve means in the supply branch
downstream from the pressure regulating valve, desired valve
spring means for the pressure regulating and reducing valve
means exerting opening biasing forces, means for supplying the
pressure at a point on the downstream side of the throttle valve
means in the supply branch to the reducing valve means so that a
sharp negative load on the one side of the servomotor tends to
simultaneously reduce the pressure at the point while increasing
the pressure on the downstream side of the reducing valve means
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which simultaneously reduces the supply pressure to the
servomotor and increases the throttling of the return flow.
The invention will now be described in more detail
with reference to the examples shown in the drawings, wherein: -
Fig. 1 is a simplified circuit for a single-acting
servo-motor;
Fig. 2 is a more complete circuit for a single-acting
servo-motor;
Fig~ 3 is a circuit for a double-acting servo-motor in
which the negative load can have an influence only in one direct-
ion, and
Fig. 4, appearing on the same sheet as Fig. 1, shows
a modified part of Fig. 3 with a double-acting servo-motor in
which the negative load can have an influence in both directions.
Fig. 1 illustrates a single-acting servo-motor 1 with
cylinder 2 and telescopic piston 3 of which the connecting
conduit 4 can, with the aid of a three-way
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switching valve 5, be selectively closed (rest position a),
connected to a supply conduit 7 fed by a pump 6 (operative
position b) or connected to a return conduit 8 (operative
position c). In the return conduit 8 there is a reducing
valve 9 and behind that an adjustable throttle element 10
from which fluid flows directly to the tank 11.
The reducing valve possesses a setting element 12
of which the piston 13 controls a valve orifice 14. A first
pressure chamber 15 is connected to a point 16 between the
reducing valve 9 and the throttle element 10. A second
pressure chamber 17 is connected to the tank 11 and contains
a preferably adjustable desired value spring 18.
When the switching valve 5 moves to its operative
position b, the piston 3 of the servo-motor 1 is lifted
against a load L by the pressure fluid delivered by the pump 6.
This displacement can, for example, be controlled by means of
a regulatable pump drive. The load L can assume different
values, e.g. in a fork lift truck that has to transport
dîfferent goods. When the switching valve 5 is brought to
the other operative position c, pressure fluid flows in
from the servo-motor under the influence of the load L through
the return conduit 8 to the tank 11. The reducing valve 9 that
was fully open in the rest condition under the influence of the
desired value spring 18 is now subjected to the pressure Ps at
the point 16 and displaced in the closing direction until a
pressure as determined by the desired value spring 18 obtains
at the point 16. Since the tank pressure Pt is constant,
a constant pressure drop Ps ~ Pt also occurs at the throttle
element. Since these conditions are also substantially
maintained on a change in the throttle setting and independently
of a change in the load L, the return flow quantity is
practically exclusively governed by the set orifice cross-
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section of the throttle element 10. By adjusting the
throttle element, the lowering movement can therefore be
con-trolled.
In the embodiment according to Fig. 2, the same
reference numerals as in Fig. 1 are used for similar
components. The motor connecting conduit 4 is connected by
way of the reducing valve 9 to a connection l9 of a
switching valve 20 and can be selectively connected to the pump
. 6 by way of a pressure regulating valve 21 and a throttle path
22 or to a tank conduit 23 by way of a throttle path serving
as first throttle element 10. The pressure chamber 17 is
connected to a conduit 24 which, in the illustrated rest
position a and in the operative position c associated with return
flow, is likewise connected to the tank conduit 23 by way of
passages 25 but, in the operative position b associated with
forward flow, is connected to a point 27 behind the adjustable
supply throttle by way of a switching path 26. The pressure in
the conduit 24 is in addition supplied to the pressure regulating
valve 21 in the same sense as the force of a spring 28 whereas
in the opposite direction a pressure Pr acts between the
regulating valve 21 and the switching valve 20. Further, a
safety valve 29 which opens in the usual manner when a predeter-
mined pump pressure is exceeded is connected to the output of the
pump 6. A similar safety valve 30 leads from the motor connecting
conduit 4 to the tank conduit 23.
The following manner of operation applies to this
arrangement. When the switching valve 20 is moved by means of
the handle 31 through a predetermined setting path x into the
operative position b, fluid flows from the pump 6 through the
regulating valve 21, the throttling path 22 and the reducing
valve 9 to the servo-motor lo In the conduit 24, and therefore
also in the pressure chamber 17, there is a pressure Pd which also
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exists in the pressure chamber 15c The reducing valve 9 therefore
opens completely under the influence of the desired value
spring 18. The regulating valve 21 is under the influence
of the pressure drop at the switching valve 20 and holds this
constant. Consequently, the supply quantity and therefore the
displacement speed of the servo-motor 1 is determined solely by
the orifice cross-section of the throttling path 22, irrespective
of the size of the load L. Desirably, the regulating valve 21
is designed so that it leads excessive pressure fluid directly
to the tank.
If the switching valve 20 is displaced by means of the
handle 31 by a predetermined setting path y to the operative
position c, then pressure fluid flows from the servo-motor 1
through the re~ucing valve 9 and the first thro~tle element 10 to
the tank 11. The tank pressure ~t exists in the pressure chamber
17 and the pressure Ps exists in the pressure chamber 15. The
same conditions therefore obtain as those described in connection
with Fig. 1.
In the embodiment according to Fig. 3, a servo-motor
32 is provided which comprises a cylinder 33 and a piston 34
that can be loaded by an external load L. Accordingly, two
motor connecting conduits 35 and 36 are also provided which
communicate with the tank conduit 23 through a safety valve
37 and 38. For the purpose of replenishment, the connecting
conduit 36 is connected to the tank conduit 23 by a check valve
3~. The connecting conduit 35 communicates with a connection 40
through the reducing valve 9 and the connecting conduit 36
communicates directly with a connection 41 of a four-way switching
valve 42 which is adjustable by a handle 43O For each operating
direction there is a first throttle element 10 or 10' for the
return flow and a second throttie element 44 or 44' for the
forward flow. No throttle elements are adjustable in unison.
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For the operative position b (setting path x) and the operative
position c (setting path y) they have the same orifice
cross-section in pairs. In other particuIars, the circuit
corresponds to Fig. 2.
The manner of operation is in this case as follows.
When the switching valve 42 is displaced through a setting
path x into the operative position b, pressure. fluid flows from
the pump 6 through the regulating valve 21, the second throttle
element 44' and the reducing valve 9 to the left-hand side
of the servo-motor 32. At the same time, pressure medium flows
through the first throttle element 10' to the tank. The pressure
Pd is applied to the conduit 17 through the switching path 26.
Accordingly, the reducing valve 9 is opened fully by the desired
value spring 18. The regulating valve 21 holds the pressure drop
at the switching valve 42 constant. The arriving quantity is
therefore determined by thesetting path x and the corresponding
throttle orifice in the throttle element 44'.
If the switching valve 42 is displaced to the
operative position c through a setting path y, arriving pressure
fluid flows through the throttle element 44 direct to the right-
hand side of the servo-motor 32. Simultaneously, the pressure
chamber 17 of the reducing valve 9 is again supplied with the
pressure Pd, whilst the pressure Ps at the point 16 is regulated
by the reducing valve 9. ~he returning quantity of pressure fluid
flows through the now regulated reducing valve 9 and then through
the settable throttle element lOr When no negative load L is
present, the pressure Pd in the pressure chamber 17 is considerably
higher than the pressure Ps in the pressure chamber 15, so that
the reducing valve 9 is completely open and the control of the
operating quantity is effected solely by the throttle element 44.
If, however, the negative load L becomes larger, the pressure Pd
drops until the pressure Ps finally becomes so larg.e that the
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reducing valve 9 is moved in the closing direction. This
results in throttling of the quantity of return flow, so that the
control operation for the supply conduit can still be maintained.
If, however, the pressure in the connecting conduit 36 becomes
so low by reason of the negative load L that control on the supply
side is no longer possible, the pressure in the pressure chamber 17
is likewise so low that the pressure Ps can be kept constant by
the reducing valve 9 and the control of the throughflow quantity
is now effected by the throttle element 10.
The following numerical example will make the manner
of operation still clearer. It is assumed that the pressure
drop over the throttle element 44 or 44' is kept constant at
4 bar by means of the regulating valve 21, that the load
L = 200 bar, that the cross-section of the cylinder on the left-
hand side is twice that on the right-hand side, that the desired
value spring 18 exerts a pressure of 3 bar, and that the throttle
elements 10, 44 or 10', 44' have the same orifice in pairs.
Further, the setting is to be such that S litres flow through the
throttle element 44'.
a) If the switching valve 42 is displaced to the
operative position b, the pressure drop at the throttle element
10 can be calculated to be 1 bar. When the tank pressure is O
bar, the pressure Ps is 201 bar and the pump pressure 204 bar.
b) If the switching valve 42 is displaced to the
operative position c, the pressure drop at the throttle element
10 can be calculated to be 16 bar. Since this pressure is equal
to the pressure Pd in the supply conduit plus a constant pressure
emanating from the spring 18, this pressure Pd amounts to 13 bar
end consequently the pump pressure pp amounts to 17 bar.
In the embodiment according to Fig. 4, only the part
of the circuit disposed beyond the switching valve 42 is
illustrated. This time both connecting conduits 35 and 36 are
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connected to the associated connections 40 and 41 by way of a
respective reducing valve 9 or 9'. Both are bridged by a check
valve 45~ 45' that opens towards the motor 32. The pressure
chamber 15 communicates with the point 16 and the pressure
chamber 15' with the point 16'. The pressure chamber 17
communicates with a point 46 at the connection 41 or at
the connecting conduit 36 and the pressure chamber 17'
communicates with a point 46 at the connection 40 or at the
connecting conduit 35.
When the connection 41 transmits supply pressure,
the servo-motor 32 is fed by way of the check valve 45'. The
reducing valve 9 goes over the open position so that the
pressure fluid can flow off without hindrance. Only when the
load L is too large and the pressure in the connection 41 drops
will the reducing valve have a throttling effect in the
described m~nner. The same also applies to adjustment of the
motor 32 in the reverse direction with a correspondingly
reversely acting external load.
Instead of the check valves 45, 45' one can also
use reducing valves 9, 9' controlled in the nature of the
reducing valve 9 of Fig. 3 by way of the switching valve 42.
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