[0001] The invention relates specifically but not exclusively to a device for blocking manoeuvring
of a crane when a load on said arm reaches a load limit level.
[0002] The invention especially relates to a device comprising: a) a balance valve, arranged
on a hydraulic transmission line of a double-acting cylinder provided with a first
chamber on the cylinder base side associated to the raising of a crane arm and a second
chamber, on the stem side of the cylinder, associated to the lowering of the arm;
b) a first pilot conduit, communicating with the first chamber of the cylinder; c)
a second pilot conduit, communicating with the second chamber of the cylinder; d)
a cursor, slidingly mobile within a seating and provided on opposite sides with a
first end and a second end on which the fluids of the first and second pilot conduits
act reciprocally contrastingly, the cursor being pushed into a block position whereat
it activates a cylinder block actuator by effect of the action of the fluid of the
first pilot conduit; e) a spring, acting on the cursor contrastingly to the action
of the fluid in the first pilot conduit.
[0003] The cursor moves into the block position when the force exerted by the pressurized
fluid on the first end overcomes the spring force added to the force exerted by the
fluid on the second end. In a correct functioning of the device the difference between
the force exerted by the pressurized fluid on the first end and the force exerted
by said fluid on the second end must be proportional to the load bearing on the arm.
If this is true, it is sufficient to calibrate the spring to a value which is proportional
to the limit load on the arm, and when such load limit is reached the cursor automatically
moves into a block position.
[0004] In such devices the above-mentioned proportional rapport is conserved also during
the arm lowering phase; that is, when, due to the necessity of piloting the balance
valve, the pressure inside the second chamber of the cylinder increases and, consequently,
so does the pressure in the first chamber correspondingly. The increase in the apparent
pressure (that is, pressure increase not due to the load on the crane) in the first
chamber is compensated by causing not only the pressure signal coming from the first
chamber to act on the cursor, but also the counterpressure signal coming from the
second chamber through the second pilot conduit to follow suit. Without this consideration,
during a lowering of the arm the crane commands would block, and not due to the load
limit having been reached.
[0005] Various limiters of this type have been disclosed, in which however the rapport among
the various areas of the cylinder piston surface area on which the fluid acts on the
stem side and the base side must be exactly equal to the rapport among the surface
areas of the two ends of the cursor, on which the fluid acts. If this is not so there
cannot be a perfectly proportional rapport between, o nthe one hand, the difference
between the two forces exerted by the pressurized fluid on each end of the cursor
and, on the other hand, the effective load bearing on the arm.
[0006] The device is thus constructionally complicated and expensive, as both the cursor
and the seat must be made to extreme levels of precision and further are only suitable
for a cylinder having a predetermined stem-piston size rapport.
[0007] One solution to this problem is described in patent DE 1926236, in which the cursor
is in fact the piston of a second, small cylinder, and in which a pressure regulator
is provided on the second pilot conduit and has the function of limiting the fluid
pressure downstream of the regulator to a maximum predetermined value, or of reducing
the pressure downstream by a constant quantity with respect to the upstream pressure,
independently of the changes in the latter. This pressure regulator serves to correct
the pressure in the second pilot conduit, as the stem/piston size rapport is greater
in the case of the activating cylinder of the arm than in that of the small activating
cylinder for the crane arm block.
[0008] This solution does not however completely solve the problem, since the correction
effected by the pressure regulator, due exactly to the way in which it acts, is effective
only within a restricted pressure range in the second chamber.
[0009] As it gradually leaves that range, the difference in the forces due to the pressurized
fluid and acting on the two opposite sides of the small piston is ever less proportional
to the load bearing on the arm.
[0010] The main aim of the present invention is to offer a solution to the above-described
problem by providing a device which is constructionally simple and effective, in which
the above-mentioned force difference is always proportional to the load bearing on
the arm in all possible load conditions, and for whatever value of the second chamber
pressure; and which is easily adaptable for cylinders having any stem-piston size
rapport.
[0011] One advantage of the invention is that it can be practically applied without any
special modifications to cranes at present in use.
[0012] These aims and advantages and others besides are all achieved by the device of the
invention, as it is characterized in the accompanying claims, which comprises a drainage
conduit, communicating with a discharge and with the second end of the cursor, and
wherein a first and a second orifice, each of which exhibit a predetermined diameter
section, are provided on the second pilot conduit and on the drainage conduit.
[0013] Further characteristics and advantages of the present invention will better emerge
from the detailed description that follows, of an embodiment of the invention, illustrated
in the form of a non-limiting example in the accompanying drawing, in which figure
1 schematically shows the device applied to a crane.
[0014] With reference to the figure, the crane 1 is provided with an arm 2.
[0015] A double-acting cylinder 3 is provided with a first chamber 3a on the cylinder base
side, which first chamber 3a is associated to the raising of the arm 2 and is further
provided with a second chamber 3b on the stem side, which second chamber 3b is associated
to the lowering of the arm 2.
[0016] A balance valve 14 is arranged on a hydraulic drive line 15 of the double-acting
cylinder 3, which hydraulic drive line 15 comprises a pump 16 and a distributor 17.
[0017] The device comprises a first pilot conduit 4 communicating with the first chamber
3a and a second pilot conduit 5 communicating with the second chamber 3b.
[0018] The device further comprises a cursor 6 which is slidingly mobile in a constant-section
seating 8 and which is provided with a first end 6a and a second end 6b, on which
the fluids of respectively the first and the second pilot conduits 4 and 5 act in
reciprocal contrast. The surface areas of the ends 6a and 6b which the fluids act
upon are identical.
[0019] The cursor 6 is forced by effect of the action of the fluid from the first pilot
conduit 4 towards a block position whereat it activates a block actuator 7. In the
illustrated embodiment the cursor 6 regulates the distribution of the flow among three
pipes emerging from the seating 8, wherein a first pipe T is connected to a discharge
13, a second pipe S is connected to the hydraulic block actuator 7 and a third pipe
P is connected to the first chamber 3a of the cylinder. In the cursor 6 blocked position,
the second pipe S is placed in communication with the third pipe P, allowing for the
activation of the block actuator 7.
[0020] A spring 9 acts on the cursor 6 in contrast to the action of the fluid of the first
pilot conduit 4.
[0021] The device, according to the invention, comprises a drainage conduit 10, which communicates
with a discharge 13 and with the second end 6b of the cursor 6.
[0022] A first and a second orifice 11 and 12 are provided, each having a predetermined
diameter section, which are arranged respectively on the second pilot conduit 5 and
on the drainage conduit 10. Thanks to the two orifices 11 and 12 the fluid pressure
acting on the second end 6b is comprised between the external pressure of the discharge
13 and the pressure in the second chamber 3b. The value of this intermediate presure
depends on the orifice sections. The orifices give rise to bleeding, by means of which
a pressure acting on the second end 6b is always proportional to the pressure in the
second chamber 3b.
[0023] According to the invention the passage sections of the orifices 11 and 12 are such
that to an increase in pressure in the first chamber 3a of the cylinder, caused by
a proportional increase in the second chamber 3b due to the need to pilot the balance
valve 14 during an arm lowering operation, corresponds an equal increase in the pressure
of the fluid acting on the second end 6b. In other words, the orifices 1 and 12 are
conformed such that the pressure acting on the second end 6b is porportional to the
pressure in the second chamber 3b in accordance with the rapport between the area
of the cylinder piston on the stem side and the area of the same piston on the base
side.
[0024] With the invention it is easy to ensure that the difference in the opposed forces
on the cursor 6 due to the fluids acting on the ends 6a and 6b is constantly proportional
to the load bearing on the arm in all possible load conditions, and for whatever value
of the pressure in the second chamber. On reaching a limit load on the arm 2, the
cursor 6 automatically moves into a block position.
[0025] An advantage of the invention is that it uses a simple concept, that of a cursor
and a seating in which said cursor runs.
[0026] A further advantage of the invention is that the cursor runs in a constant-section
seating with almost negligible friction.
[0027] The device can be advantageously applied on existing-type cranes, whatever the cylinder
stem/piston size rapport might be. When the rapport changes it is sufficient to modify
the rapport between the passage sections of the orifices 11 and 12, leaving the cursor
6 unvaried.
1. A device for detecting when a limit load is bearing on a crane arm, comprising:
a balance valve (14), arranged on a hydraulic drive line (15) of a double-acting cylinder
(3) provided with a first chamber (3a) on a base side of the double-acting cylinder
(3) for raising a crane arm and a second chamber (3b), on a stem side of the double-acting
cylinder (3), for lowering the crane arm;
a first pilot conduit (4), communicating with the first chamber (3a) of the double-acting
cylinder (3);
a second pilot conduit (5), communicating with the second chamber (3b) of the double-acting
cylinder (3);
a cursor (6), which is slidingly mobile in a seating (8) and which is provided oppositely
with a first end (6a) and a second end (6b) on which first end (6a) and second end
(6b) fluids of the first pilot conduit (4) and the second pilot conduit (5) act reciprocally
contrastingly; the cursor (6) being pushed into a blocked position wherein it activates
a cylinder block actuator (7) by effect of an action of the fluid of the first pilot
conduit (4);
a spring (9), acting on the cursor (6) contrastingly to the action of the fluid in
the first pilot conduit (4);
characterised in that:
it further comprises a drainage conduit (10) in communication with a discharge (13)
and with the second end (6b) of the cursor (6);
a first orifice (11) and a second orifice (12) are afforded respectively on the second
pilot conduit (5) and the drainage conduit (10), each of which first orifice (11)
and second orifice (12) has a predetermined diameter section.
2. A device as in claim 1, characterised in that:
the first end (6a) and the second end (6b) on which the fluids act have identical
surface areas; the first orifice (11) and the second orifice (12) have passage sections
made in such a way that to an increase in pressure in the first chamber (3a) of the
cylinder, caused by a proportional increase in the second chamber (3b) for piloting
the balance valve (14) during a lowering phase of the crane arm (2), corresponds an
equal increase in pressure of the fluid acting on the second end (6b).
3. A device as in claim 1 or 2, characterised in that:
the cursor (6) regulates a distribution of a fluid flow rate among three pipes emerging
from the seating (8), wherein a first pipe (T) of the three pipes is connected to
the discharge (13), a second pipe (S) of the three pipes is connected to the hydraulic
block actuator (7) and a third pipe (P) of the three pipes is connected to the first
chamber (3a) of the cylinder;
when the cursor (6) is in the blocked position, said second pipe (3) is set in communication
with said third pipe (P).