FIELD OF THE INVENTION AND PRIOR ART
[0001] The present invention relates to a hydraulic crane, preferably a lorry crane, a method
for calculation of the fatigue stress of such a crane and a method for registration
of the lifting up and putting down of a load in such a crane.
[0002] A hydraulic crane is, like all other mechanical constructions, subjected to an accumulating
fatigue stress during use. When the fatigue stress has reached a certain value large
risks of crane breakdowns ensue, which can result in serious personal injuries or
expensive damages to the crane itself or objects in the vicinity of the crane. In
order to make possible a safe and trouble-free operation of a crane it is therefor
a great need of calculating the accumulated fatigue stres's of the crane. Based on
such a calculated fatigue stress, it is possible to estimate the condition of the
crane and its need of maintenance.
[0003] A method today used for calculation of the accumulated fatigue stress of hydraulic
cranes is based on that the operating time of the crane is registered by means of
a time counter. In this connection it is normally registered the time during which
the pump included in the hydraulic system is switched in. However, this constitutes
a relatively bad measure of the accumulated fatigue stress of the crane, since one
and the same crane often is used for many types of working operations having a very
varying intensity.
OBJECT OF THE INVENTION
[0004] The object of the present invention is to achieve a hydraulic crane in which it is
possible to calculate the accumulated fatigue stress of the crane in a simple and
efficient manner.
SUMMERY OF THE INVENTION
[0005] According to the present invention, this object is achieved in that the crane comprises
means connected to a pressure sensor in the lifting cylinder of the crane, which means
is adapted to register the lifting up and the putting down of a load by detecting
the velocity of the pressure variations in said cylinder, the means being adapted
to register a lifting up and a putting down, respectively, of a load when the velocity
of a detected pressure variation exceeds a predetermined value.
[0006] In this way liftings up and puttings down of load performed by the crane can be registered
in a very simple manner and with simple and cheap means. By means of the liftings
up and puttings down of load registered by the means it will be possible to determine
the number of lifting cycles performed by the crane, which constitutes a measure well
related to the accumulated fatigue stress of the crane. It has appeared that the number
of performed lifting cycles constitutes a considerably better measure of the fatigue
stress of the crane than the time during which the crane has been in operation. Each
lifting cycle is namely contributing in increasing the fatigue stress of the crane
and the duration of a lifting cycle can vary from for instance 30 seconds, when the
crane is used for excavation with a hydraulic bucket, up to several hours, when the
crane is used in assembly work in order to for instance lift and position a transformer
and to keep this in place until it has been fixed on the intended place. In the first
mentioned case, the total fatigue stress of the crane will be considerably higher
than in the last mentioned case during one and the same time period.
[0007] According to a preferred embodiment of the invention, the means is connected to one
or several sensors arranged to deliver information to the means concerning the control
and/or movements of the crane, said means being adapted to determine by means of this
information whether or not a lowering movement of the crane is taking place. In this
connection the means is adapted not to register a lifting up of load when a rapid
pressure variation in the cylinder is detected in connection with a determined lowering
movement. In this way the means is prevented from being "fooled" by the rapid pressure
variation caused by the induced pressure on the piston side of the cylinder that can
ensue during lowering movements due to the fact that a certain pressure is required
on the piston stem side in order to open the load holding valve of the hydraulic system.
[0008] The invention also relates to a method for registration of the lifting up and putting
down of a load in a hydraulic crane according to claim 5 and a method for calculation
of the fatigue stress of a hydraulic crane according to claim 7.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] With reference to the enclosed drawings, a more specific description of embodiment
examples of the invention will follow hereinbelow. It is shown in:
- Fig 1
- a sectional view of a hydraulic crane provided with a bucket,
- Fig 2
- a sectional view of a hydraulic crane provided with a jib,
- Fig 3
- a schematic view of an embodiment of the invention, and
- Fig 4
- a schematic view of a control unit with a number of control devices for control of
different crane functions.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0010] In this description the expression operating means is used to designate the hydraulic
force members which execute the crane movements ordered by the operator of the crane.
The expression operating means consequently embraces the hydraulic cylinders 8, 9,
10, 14, 17 and 19 mentioned hereinbelow. The expression control device refers to the
devices, for instance operating levers, by means of which the operator regulates the
valve members included in the control system which control the flow of hydraulic fluid
to the respective operating means. In the described embodiment, said valve members
consist of so-called directional-control-valve sections.
[0011] In fig 1 a hydraulic crane 1 attached to a frame 2 is shown, which frame for instance
can be connected to a lorry chassis. The frame is provided with adjustable support
legs 3 for supporting the crane 1. The crane comprises a column 4, which is rotatable
in relation to the frame 2 around an essentially vertical axis. The crane further
comprises an inner boom 5 articulately fastened to the column 4, an outer boom 6 articulately
fastened to the inner boom 5 and an extension boom 7 displaceably fastened to the
outer boom 6. The inner boom 5 is operated by means of a hydraulic lifting cylinder
8, the outer boom 6 by means of a hydraulic outer boom cylinder 9 and the extension
boom 7 by means of a hydraulic extension boom cylinder 10. In the shown example a
rotator 11 is articulately fastened at the outer and of the extension boom, which
rotator in its turn carries a hydraulic grab tool in the form of a bucket 12. Two
bucket parts 13 included in the bucket 12 can be operated in relation to each other
by means of a hydraulic grab cylinder 14 for opening and closing of the bucket 12.
The rotator 11 is rotatable in relation to the extension boom 7 by means of not shown
hydraulic operating means.
[0012] In the example shown in fig 1, the crane 1 is equipped for performing excavations.
When the crane 1 is to be used for proper lifting operations the rotator 11 and the
bucket 12 can be removed and replaced by a lifting hook. In order to perform lifting
operations requiring a great range, the rotator 11 and the bucket 12 are replaced
by a jib 15, see fig 2. The jib 15 comprises a jib boom 16, which is articulately
fastened in relation to the extension boom 7 and operated by means of a hydraulic
jib boom cylinder 17. The jib can further comprise an extension boom 18 which can
be operated by means of a hydraulic extension boom cylinder 19. One and the same crane
can consequently be used for a large number of different working operations, the intensity
(lifting frequency) of which varies considerably.
[0013] Besides the crane elements shown in fig 1 and 2, the crane 1 can also be provided
With a hydraulically controllable winch, which can be used in combination with a lifting
hook either with or without jib 15. The crane 1 can also be provided with other types
of hydraulic grab tools than a bucket, for instance grab tools for handling scrap
or pallets with building material such as stone or building plates.
[0014] The control system for controlling the different crane functions, i.e. lifting/lowering
by means of the lifting cylinder 8, tilting by means of the outer boom cylinder 9,
extension/retraction by means of the extension boom cylinder 10 etc, comprises a pump
20 which pumps hydraulic fluid from a reservoir 21 to a directional-control-valve
block 22. The directional-control-valve block 22 comprises a directional-control-valve
section 23 for each of the hydraulic operating means 8, 9, 10, 14, 17, 19, to which
hydraulic fluid is supplied in a conventional manner depending on the position of
the slide member in the respective valve section 23. The position of the slide members
in the directional-control-valve sections 23 is controlled either via a number of
control devices in the form of control levers 24, each of which being connected to
its own slide member, or by remote control via a control unit 25, see fig 4, comprising
a control lever for the respective slide member. In case of remote control, the control
signals are transmitted via cable or a wireless connection from the control unit 25
to a microprocessor, which in its turn controls the position of the slide members
in the valve sections 23 of the directional-control-valve block 22 depending on the
magnitude of the respective control signal from the control unit 25.
[0015] Each separate directional-control-valve section 23 consequently controls the size
and the direction of the flow of hydraulic fluid to a specific operating means and
thereby controls a specific crane function. For the sake of clarity, only the directional-control-valve
section 23 for the lifting cylinder 8 is illustrated in fig 3.
[0016] The directional-control-valve block 22 further comprises a shunt valve 26 pumping
excessive hydraulic fluid back to the reservoir 21, and an electrically controlled
dump valve 27 which can be caused to return the entire hydraulic flow from the pump
directly to the reservoir 21.
[0017] In the shown embodiment, the directional-control-valve block 22 is of load-sensing
and pressure-compensating type, which implies that the hydraulic flow supplied to
an operating means is at all times proportional to the position of the slide member
in the corresponding directional-control-valve section 23, i.e. proportional to the
position of the lever 24. The directional-control-valve section 23 comprises a pressure-limiting
device 28, a pressure-compensating device 29 and the directional-control-valve 30
proper. Directional-control-valve blocks and directional-control-valve sections of
this type are well-known and available on market. However, also other types of directional-control-valves
than the one described here can be used.
[0018] A load holding valve 31 is arranged between the respective operating means and the
associated directional-control-valve section 23, which load holding valve makes sure
that the load will remain hanging when the hydraulic system runs out of pressure owing
to the dump valve 27 being caused to return the entire hydraulic flow from the pump
20 directly to the reservoir 21.
[0019] A sensor 32 is arranged on each of the directional-control-valve sections 23 in order
to detect the movements of the valve slide member in the respective directional-control-valve
section 23. These sensors 32 are connected to a processing unit 33 suitably constituted
by a microprocessor. By means of these sensors 32 the processing unit 33 can obtain
information that a certain valve slide member is influenced and thereby how the crane
is controlled. In case the valve slide members are regulated via a remote control
unit 25, the processing unit 33 can instead be adapted to obtain information about
how the crane is controlled by reading the control signals transmitted from the control
unit 25.
[0020] The crane further comprises a load sensing means in the form of a pressure sensor
34 adapted to measure the hydraulic pressure in the lifting cylinder 8. The pressure
sensor 34 is, just as the sensors 32 in the valve sections 23, connected to the processing
unit 33.
[0021] The crane 1 further comprises a means 36 adapted to register when the crane lifts
up and puts down, respectively, a load. The means 36, which henceforth is denominated
"lifting counter", registers this by detecting the velocity of the pressure variations
in the lifting cylinder 8 of the crane, which pressure variations are measured by
the pressure sensor 34 associated with the lifting cylinder 8. During lifting up of
a load, the pressure in the lifting cylinder 8 very rapidly increases just at the
moment when the load is lifted up from the underlay and becomes free hanging. The
same rapid pressure variation occurs when the load is put down and no more is carried
by the crane. These pressure variations are much more rapid than the pressure variations
caused by the normal natural oscillations which at all times are present in the steel
structure of the crane, and hereby the lifting counter 36 can separate "liftings up"
and "oscillations". A lifting up and a putting down, respectively, of a load is consequently
registered when the velocity of the pressure variation in the lifting cylinder 8 exceeds
a certain predetermined value.
[0022] When it comes to loads which are very small for the crane (approximately smaller
than 10 % of the maximum capacity of the crane) it might be problematic to register
a lifting up and a putting down of a load in the above described way. However, these
small loads bring about a so small contribution to the accumulated fatigue stress
that they can be neglected in this connection. However, a more serious complication
for the lifting counter 36 is the induced pressure on the piston side of the lifting
cylinder that can ensue during lowering movements due to the fact that a certain pressure
is required on the piston stem side in order to open the load holding valve 31 associated
with the lifting cylinder 8. Practical tests have shown that this can give such a
rapid pressure variation that it "fools" the lifting counter. However, this problem
can be solved in that the lifting counter 36, via the sensors which register the movements
of the slide members in the directional-control-valve sections 23, obtains information
whether or not a lowering movement of the crane is taking place or not. In this connection,
the lifting counter 36 is adapted not to register a lifting up of a load when a rapid
pressure variation in the lifting cylinder 8 takes place in connection with a simultaneous
registration of a lowering movement. The crane can also be provided with other types
of sensors than the ones here described in order to give the lifting counter 36 information
whether or not the crane performs a lowering movement by detection of the control
and/or the movements of the crane.
[0023] The lifting counter 36 is suitably constituted by a microprocessor and is preferably
connected to the processing unit 33, to which it transmits information concerning
registered liftings up and puttings down of a load. In fig 3 the lifting counter 36
is shown as a unit separated from the processing unit 33, but it can with advantage
be integrated with this.
[0024] The crane 1 further comprises means 35, preferably integrated with the processing
unit 33, for calculation of the accumulated fatigue stress of the crane, this means
35 being adapted to calculate said fatigue stress based on the number of registered
liftings up and puttings down of a load by the lifting counter 36. It is here realised
that a lifting up and a subsequent putting down of a load correspond to one performed
lifting cycle. The calculated fatigue stress is presented to the operator and/or maintenance
personal of the crane by means of suitable display means. It is also possible to let
the processing unit 33 emit a signal or an alarm when the accumulated fatigue stress
has attained a certain predetermined value.
[0025] The information from a lifting counter 36 of the type here described can also be
used for other functions in a crane than for calculation of accumulated fatigue stress.
The information can for instance be used in a system for controlling the maximum allowed
lifting force of a hydraulic crane, as more closely described in a patent application
filed simultaneously with the present patent application.
[0026] The invention is of course not limited to the embodiments described above, a number
of modifications thereof are on the contrary possible within the scope of the subsequent
claims.
1. A hydraulic crane (1) comprising an inner boom (5), a hydraulic cylinder (8) for operating
the inner boom (5), and a pressure sensor (34) arranged to measure the pressure in
the hydraulic cylinder (8), characterized in that the crane (1) further comprises a means (36) connected to the pressure sensor (34),
which means is adapted to register the lifting up and the putting down of a load by
detecting the velocity of the pressure variations in said cylinder (8), said means
(36) being adapted to register a lifting up and a putting down, respectively, of a
load when the velocity of a detected pressure variation exceeds a predetermined value.
2. A crane according to claim 1, characterized in that the means (36) consists of a microprocessor.
3. A crane according to any of the preceding claims, characterized in that the means (36) is connected to one or several sensors (32) which are adapted to deliver
information to the means (36) concerning the control and/or the movements of the crane,
said means (36) being adapted to determine by means of this information whether or
not a lowering movement of the crane (1) is taking place, and that the means (36)
is adapted not to register a lifting up of a load when a rapid pressure variation
in said cylinder (8) is detected in connection with a determined lowering movement.
4. A crane according to any of the preceding claims, characterized in that the crane (1) further comprises means (35) for calculating the fatigue stress of
the crane (1), said calculating means (35) being adapted to calculate the fatigue
stress based on the number of liftings up and puttings down of load registered by
the registration means (36).
5. A method for registration of lifting up and putting down of a load in a hydraulic
crane (1) comprising an inner boom (5), a hydraulic cylinder (8) for operating the
inner boom (5), and a pressure sensor (34) for measuring the pressure in the hydraulic
cylinder (8), characterized in that the lifting up and the putting down, respectively, of a load is registered by detection
of the velocity of the pressure variations in said cylinder (8), a lifting up and
a putting down, respectively, of a load being registered when the velocity of a detected
pressure variation exceeds a predetermined value.
6. A method according to claim 5, characterized in that it is determined by means of information from a sensor (32) concerning the controls
and/or the movements of the crane whether or not a lowering movement of the crane
(1) is taking place, and that a lifting up of a load is not registered when a rapid
pressure variation in said cylinder (8) is detected in connection with a determined
lowering movement.
7. A method for calculation of the fatigue stress of a hydraulic crane (1) comprising
an inner boom (5), a hydraulic cylinder (8) for operating the inner boom (5), and
a pressure sensor (34) for measuring the pressure in the hydraulic cylinder (8), characterized in that the lifting up and the putting down, respectively, of a load is registered by detection
of the velocity of the pressure variations in said cylinder (8), a lifting up and
a putting down, respectively, of a load being registered when the velocity of a detected
pressure variation exceeds a predetermined value, and that the fatigue stress is calculated
based on the number of registered liftings up and puttings down of load.
8. A method according to claim 7, characterized in that it is determined by means of information from sensors (32) concerning the control
and/or the movements of the crane whether or not a lowering movement of the crane
(1) is taking place, and that a lifting up of a load is not registered when a rapid
pressure variation in said cylinder (8) is detected in connection with a determined
lowering movement.