Hydraulic control device

In a control device (S) for a hydraulic motor (V) which is adapted to be ed upon at both sides, there are provided working conduits (4, 5) which are alternately connectable via a directional control valve (C) to a pressure source (P) and a tank (T), a load holding valve (H) which is hydraulically openable in a controlled way and is arranged in at least one working conduit (4), as well as a control pressure conduit (13) which is connected to the opening side (16) of said load holding valve (H), with pressure variations arising during the controlled opening of the load holding valve, and the amplitudes of the pressure variations being adapted to be dampened in the control pressure conduit (13) at least via a damping throttle (D). To dampen and eliminate the undesired effect of changes in the viscosity of the pressure medium and/or of a damping throttle which is too tightly set, the damping throttle (D) can be bypassed in both directions by a respective check valve (R1, R2, R1', R2,), a great biasing force which biases the one check valve being adjusted to a value which lies between the pressure values of the pressure extremes that act on said check valve and pertain to at least the first amplitude and the next amplitude of the pressure variations.

DESCRIPTION 
This invention relates to a hydraulic control device according to the 
preamble of patent claim 1. 
In a control device of this type, as is known from publication D 7100, June 
1986, edited by Heilmeier & Weinlein, the damping throttle is set in the 
control pressure conduit in such a way that, when the pressure medium is 
operatively warm and the load holding valve opened in a controlled way, it 
effects a decay of the amplitudes of the pressure variations. For 
preventing any delay in the controlled closing movement of the load 
holding valve and any after-running of the hydraulic motor under load, the 
damping throttle may be bypassed by a check valve which ensures a swift 
pressure reduction during closing. Although pressure variations in the 
pressure control conduit and load movements caused thereby are gradually 
dampened by means of the damping throttle, some of them can clearly be 
felt because the load holding valve is subject to play and the hydraulic 
motor reacts unevenly. It has been found in practice that the dampening 
effect of the damping throttle is above all unsatisfactory in load systems 
that have a strong tendency to vibrate. When the load holding valve is 
rapidly opened in a controlled way, this produces pressure variations with 
a relatively harmonious vibration curve. In such a case, at least the 
first amplitude of the pressure variations has a high maximum and a low 
minimum while the subsequent amplitudes decrease gradually. The pressure 
values of the extremes of the first amplitude(s) are known. The pressure 
variations during rapid opening of the load holding valve result not only 
from a rapid pressure build-up, but also from conditions of the load which 
moves (downwards) after the controlled opening of the load holding valve 
and vibrates and thus acts on the pressure medium column within the 
hydraulic circuit of the control device. Pressure variations are 
unavoidable, but it is desirable to dampen them as fast as possible. 
Furthermore, a damping throttle which is tightly set for achieving a 
strong dampening action may delay the controlled opening movement. This 
disadvantageous effect is above all observed with a cold and tenacious 
pressure medium, as the damping throttle is responsive to viscosity. 
In accordance with the invention, this object is attained through the 
features outlined in the characterizing part of claim 1. 
When the pressure rises, the respective check valve which is strongly 
biased opens at least during the first amplitude as soon as the biasing 
force has been overcome. Depending on whether the one or the other check 
valve is strongly biased, the peak of the amplitude is eliminated down to 
the minimum of the pressure value of the pressure variation which could 
not be dampened by the damping throttle. A rapid decay of the first and 
the subsequent amplitudes is effected at the opening side. This is 
especially advantageous with a cold pressure medium and/or with a tightly 
set damping throttle because the check valves do not only support the 
dampening action of the damping throttle, but compensate for the damping 
throttle effect which is not desired under specific operating conditions. 
The amplitudes of the pressure variations which become effective at the 
opening side of the load holding valve are dampened as rapidly as 
possible, so that the load holding valve immediately enables the hydraulic 
motor to move uniformly under load, i.e. virtually from the beginning of 
the movement. The rapid dampening of the pressure variations at the 
opening side of the load holding valve has a dampening effect on pressure 
variations in the whole system. 
According to one solution, the pressure which does not pass through the 
damping throttle opens the strongly biased check valve towards the opening 
side of the load holding valve as soon as it has overcome the biasing 
force. The upper part of the peak of the at least first amplitude of the 
pressure variations is no longer effective at the opening side of the load 
holding valve. A more easy decay of the other amplitudes is thus made 
possible. Along the falling part of the first amplitude or the first 
amplitudes, the second check valve which is almost unbiased permits a 
rapid pressure reduction, too. The two check valves cooperate with the 
damping throttle during dampening operations; they take over those parts 
of the pressure variations the damping throttle cannot cope with. Not only 
the amplitudes of the pressure variations decay rapidly, but there is the 
additional important advantage that any viscosity-dependent delay in the 
controlled opening and closing of the load holding valve through the 
damping throttle is suppressed, as well as a delay that might be caused by 
a tight setting of the damping throttle for damping reasons. 
In another embodiment the strongly biased first check valve responds not 
only to the first amplitude, but to several initial amplitudes of the 
pressure variations to effect, together with the damping throttle, a very 
rapid decay of the pressure variations that are effective at the opening 
side. This is especially expedient for a cold and thus viscous pressure 
medium because in such a case the damping throttle works in an 
unsatisfactory way on account of its viscosity dependence. 
In yet another embodiment the first amplitude or amplitudes of the pressure 
variations reach the opening side of the load holding valve through the 
only slightly biased first check valve, with the damping throttle being 
bypassed, so that the controlled opening movement of the valve takes place 
immediately. However, the lower portion of the first amplitude or 
amplitudes is diminished through the second check valve which is biased in 
the opposite direction, which promotes the rapid decay of the amplitudes. 
The second check valve is expediently biased to only such an extent that 
it is responsive to the pressure reduction established for the controlled 
closing of the load holding valve, and bypasses the damping throttle so as 
to avoid any delay in the controlled closing movement, even in the case of 
a cold and viscous pressure medium. 
According to another embodiment plurality of initial amplitudes are made to 
decay rapidly due to the response of the second, strongly biased check 
valve at the opening side. 
Another embodiment is simple from a constructional point of view, for 
spring-biased check valves are simple, reliable and inexpensive hydraulic 
members. The biasing force can be exactly adapted by means of the 
adjusting device to the operating conditions so as to effect an optimum 
damping action. 
According to another embodiment since the damping throttle can also be 
adapted to the operating conditions. The two check valves that cooperate 
with the damping throttle have the additional advantage that the damping 
throttle can be adjusted substantially independently of the course and 
extent of the pressure variations with a view to optimum damping. Hence, 
there is no longer any compromise adjustment of the damping throttle as 
has so far been practiced in conventional control systems, where the 
capacity of the damping throttle has not been exploited fully. 
To effect a rapid decay of the pressure variations also at the side of the 
damping throttle which faces away from the opening side of the load 
holding valve, the pressure medium according to another embodiment flows 
constantly off via the bypass conduit. The course of the amplitudes of the 
pressure variations is so effectively disturbed by the throttle passage in 
the control pressure conduit and the disturbance throttle passage in the 
bypass conduit, i.e. also in front of the damping throttle, that they 
decay rapidly. The pressure variations are dampened through the joint 
action of three measures, namely, damping throttle, check valves and 
bypass duct, and the control device is especially suited for 
vibration-prone or strongly vibratory systems. Since the two check valves 
take part in the damping action, the disturbance throttle passage need 
only be slightly greater than the throttle passage in the control pressure 
conduit, so that only an infinitely small amount of pressure medium flows 
off via the bypass conduit. 
Of course, since the dampening effect via the bypass duct can only take 
place if a pressure medium volume is actually moved, the bypass conduit 
can connected to the working conduit containing the load holding valve or 
directly to the tank. These features are incorporated in other embodiments 
of the invention. In the last-mentioned case, a directional control valve 
with supply controllers and a blocked central position may be used. Such a 
valve is per se critical in vibration-prone or greatly vibratory systems 
of this type because of its long transient response. In any case, the 
strong dampening effect which can be achieved by taking the 
above-mentioned measures permits the use of directional control valves 
equipped with supply controllers, which is of advantage to the control 
accuracy and the response characteristics of the control device during 
movement of the hydraulic motor in any direction.

A hydraulic control device S according to FIG. 1 serves to control the 
movement of a consumer V with which a load F is moved. Consumer V is, 
e.g., the lifting or bent cylinder of a crane with which load F is moved. 
In the hydraulic motor V, a piston 1 divides a cylinder into two chambers 2 
and 3. Each of chambers 2 and 3 is alternately connectable to a pressure 
source P and a tank T via a working conduit 4, 5 and a directional control 
valve C. At least working conduit 4 has disposed therein a load holding 
valve H which contains a valve 6 with a valve member 7 which is brought by 
a spring 8 into the illustrated closing position in which conduit 4 is 
blocked. The pressure prevailing in a precontrol conduit 10 acts in the 
same direction, whilst the pressure prevailing in a precontrol conduit 9 
acts in the opening direction. A conduit loop 11 bypasses valve 6 in 
working conduit 4 and contains a check valve 12 opening towards hydraulic 
motor V. 
A control pressure conduit 13 branches from the other working conduit 5 to 
the opening side 16 of valve 6. Control pressure conduit 13 contains a 
preferably adjustable damping throttle D. Two conduit loops 14 and 15 
bypass damping throttle D. Conduit loop 14 contains a first check valve R1 
including a valve member 17 and a biasing spring 18, which opens towards 
the opening side 16. The biasing force of spring 18 can be adjusted with 
the aid of the outlined adjusting device E. Conduit loop 15 contains a 
second check valve R2 which opens towards the second working conduit 5 and 
contains a valve member 19 and, optionally, a weak biasing spring 20. 
The two check valves R1, R2 are differently biased. The bias of the second 
check valve R2 may even become zero. In practice, a weak spring is used 
for positioning valve member 20 in the shut-off position in the 
inoperative state. By contrast, the bias of the first check valve R1 is 
great. The force with which valve member 17 is biased by spring 18 has a 
value which is smaller than the pressure value of the pressure maximum 
that acts on valve member 17 and pertains at least to the first amplitude 
(FIG. 4) of the pressure variations of pressure Pl, and is slightly 
greater than the pressure value of the pressure maximum of the subsequent 
amplitudes. 
FIG. 4 illustrates the pressure curve over time which follows from pressure 
variations in control pressure conduit 13 (pressure Pl between damping 
throttle D and working conduit 5), which pressure variations are typical 
of the rapid establishment of a lowering movement of the load. 
For the movement of hydraulic motor V under load, load holding valve H is 
opened in a controlled way, e.g., via directional control valve C, by 
exerting pressure on working conduit 5 until load holding valve H opens 
the passage of working conduit 4. Pressure Pl follows, e.g., the curve 
shown in full line. The pressure variations would have subsequent and very 
slowly decreasing amplitudes with a respective pressure maximum and a 
pressure minimum. If the pressure variations constantly acted on the 
opening side 16 of the load holding valve, the movement of hydraulic motor 
V would not be uniform. A decay of the pressure variations which is as 
rapid as possible is therefore necessary, at least at the opening side 16 
(pressure P2 in FIG. 1, curve shown in broken line in FIG. 4). The biasing 
force of spring 18 in FIG. 1 is set to the value shown in broken line, 
which is below the pressure maximum of the first amplitude and just above 
the pressure maximum of the second and subsequent amplitudes. As a result 
of the action of damping throttle D and the first check valve R1, the 
pressure increase in the first amplitude becomes effective at the opening 
side 16 with a phase shift. When the biasing force of the first check 
valve is reached, the latter opens, so that the peak of the first 
amplitude is cut off before pressure P2 approximately follows the pressure 
drop at the rear slope of the first amplitude. Damping throttle D is here 
bypassed. At the beginning of the next amplitude, damping throttle D 
becomes effective, so that the increase in pressure at the opening side 16 
is already less steep and the second amplitude is dampened. Likewise, the 
damping throttle effects a rapid decay of the other amplitudes at the 
opening side. As a result, the lowering movement of hydraulic motor V 
takes place without any jerks and in a uniform way immediately after the 
beginning of the movement, namely, at the speed set on the directional 
control valve. 
In a modification of the embodiment illustrated in FIG. 1, it is also 
possible to feed control pressure conduit 13 from an extra control 
pressure reservoir. In this case, however, pressure variations, e.g., 
according to FIG. 4 also arise when the opening pressure is rapidly 
established for stopping hydraulic motor V, as can often be observed in 
practice. 
The embodiment of FIG. 2 differs from that of FIG. 1 by the exchange of the 
bias of the two check valves used for bypassing damping throttle D. The 
first check valve R1' which opens towards the opening side 16 is biased 
with a biasing force that may even become zero, i.e. with a very small 
biasing force, whereas the second check valve R2' that opens in the 
opposite direction is biased with a great biasing force. When there are 
pressure variations (FIG. 5), this results in a dampening effect at the 
opening side 16. The first amplitude of the pressure variations of 
pressure P1 follows the first amplitude of the pressure variation of 
pressure P2 at the opening side with the phase shift effected by damping 
throttle D. The maximum of the pressure value of the first amplitude of 
pressure Pl is not reached by pressure P2 because of damping throttle D, 
but pressure P2 follows the falling slope of the first amplitude of 
pressure P1. The biasing force of the second check valve R2 has a value 
(broken horizontal line in FIG. 5) which is higher than the minimum of the 
pressure value of the first amplitude of pressure P1, but lower than the 
minimum of the pressure values of the subsequent amplitudes of pressure 
P1. Thus, when the value of the biasing force is not reached, the second 
check valve R2 opens before the first amplitude reaches its minimum 
pressure value. The bottom between the first and second amplitudes of the 
pressure variations is cut off, pressure P2 first remains at the level of 
the biasing pressure of the second check valve R2' before damping throttle 
D becomes active during renewed rise in the second amplitude and causes 
pressure P2 to rise more gently. A rapid decay of the pressure variations 
at the opening side 16 is thus achieved. 
The bias on the second check valve R2' is expediently adjusted such that 
the second check valve R2' opens when the pressure in the pressure control 
conduit is relieved for the controlled closing of the load holding valve. 
This prevents a delay in the closing movement via the damping throttle. 
In the two above-described embodiments, it is also possible to set the 
biasing force on the more strongly biased check valve such that the tops 
or bottoms of several initial amplitudes are cut off and the damping 
throttle only dampens subsequent amplitudes. 
The embodiment of FIG. 3 differs from the two above-described embodiments 
by an additional damping means in the control circuit of the load holding 
valve. This damping device consists of a bypass duct 23 which branches 
from control pressure conduit 13 at a junction 22 and which leads either 
to a connection point 24 in working conduit 4 or directly to tank T (as 
outlined by the broken line at 25). A throttle passage D1 is provided 
between working conduit 5 and junction 22. Bypass conduit 23 contains a 
disturbance throttle passage D2 which is slightly greater than throttle 
passage D1. The damping means helps to dampen the vibration amplitudes in 
that a pressure medium flows off constantly via the two throttle passages 
and disturbs the propagation of the vibration amplitudes, so that the 
latter will decay very rapidly. The damping means ensures the damping of 
pressure variations also during movement of hydraulic engine V in the load 
lifting direction and also during pressure variations when the load is 
stopped. The two check valves R1 and R2 are arranged and biased in the way 
shown for the embodiment illustrated in FIG. 1. 
However, it is also possible to use the reverse arrangement and bias of 
FIG. 2 in the embodiment of FIG. 3. The effect is similar in the two 
cases. 
In all embodiments, damping throttle D can also be set tightly for 
achieving optimum damping. Nevertheless, any delay in the controlled 
closing and opening movements of the load holding valve is prevented in a 
cold and thus viscous pressure medium.