Interlock system for the locking mechanism of a cargo container lifting spreader

In a cargo container lifting crane employing a lifting spreader for lockingly suspending a cargo container by means of four corner twist locks, an improved electrical interlock system for interlocking the twist lock manual control switch located in the cab of the lifting crane, said interlock system including normally closed, parallel connected proximity switches which are disposed adjacent the twist locks of the lifting spreader and which are connected to a relay whereby the relay is de-energized to close a relay circuit which supplies power to the twist lock manual control switch when all of the four corner proximity switches indicate full engagement of all twist locks into the corresponding locking parts of the cargo container. An adjustable time delay is provided between full lifting spreader engagement and application of control switch power, and a by-pass switch is provided for overriding the interlock system. Also, indicating and alarm lights and horn are provided to alert crane maintenance and supervisory personnel, as well as the crane operator, of certain operating and alarm conditions in respect to the interlock system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is related to cargo container lifting apparatus, and 
more particularly to the operation of the twist locks of a cargo container 
lifting spreader, and still more particularly to means for interlocking 
the twist lock control switch to prevent improper operation thereof. 
2. Description of the Prior Art 
Cargo containers, which are standardized in size for ease in handling, have 
in recent years come into widespread use in the shipping industry. Such 
containers are conventionally handled by lifting cranes which pick up 
containers with a so-called lifting spreader which is a load spreading 
lifting device conforming in shape to the top of the container. The 
spreader locks onto the container by means of corner twist locks which 
must be aligned and dropped into twist lock receptacle castings built into 
each of the container's top corners. To move a container, the crane 
operator first positions the spreader over the container to be lifted such 
that the twist locks are lined up over the container's corner castings; he 
then lowers the spreader onto the top of the container whereupon a manual 
control switch is thrown to rotate the twist lock into a locked position. 
Similarly, to disengage a spreader from a container which has already been 
moved, the operator first throws the same control switch to unlock the 
twist locks and then lifts the spreader away from the container. 
Problems arise, however, when the lifting spreader is not properly engaged 
with the container before the operator turns the manual control switch to 
close the twist locks. If this happens, which it often does, the twist 
locks may be rotated against a structural part of the container resulting 
in damage to the twist lock or the container or both. This situation often 
occurs because of operator haste which itself is often motivated by a pay 
formula based on the number of containers an operator handles. 
To prevent premature rotation of the twist lock and the resulting equipment 
damage, interlock systems have been devised to permit rotation of the 
twist lock only upon proper engagement of the spreader. Conventionally, 
such interlocks are comprised of a mechanical pressure cam disposed next 
to each twist lock such that the cam will be depressed to release its 
associated twist lock when the lifting spreader contacts the top of the 
container. Such mechanical interlocks, however, have reliability problems 
and are not suitably adapted to interface with other electrical circuitry 
of the crane control system. To provide for electrical interlocking of the 
twist locks, an electrical interlock system has been devised utilizing 
proximity sensors at the twist locks for sensing the proximity between the 
lifting spreader and the container. This known system uses, in place of 
the mechanical cams, proximity switches which are series connected, 
normally open switches which provide a relay energizing current when all 
the proximity switches are closed. Upon closing of all the proximity 
switches of the system, power is supplied to the operator's twist lock 
manual control switch. It has been found, however, that this electrical 
interlock system operates irratically, in that, quite often the last in 
the series of sensors would not operate properly, thereby preventing 
proper operation of the overall system. It also does not provide the 
important visual and audio operation controls as will be described in the 
present invention. 
The present invention is an improvement on the above-described electrical 
interlock system, in that, it overcomes the problem of reliability in 
respect to sensor operation. It also provides a solution to the 
over-eagerness problem which occurs with many container crane operators as 
they proceed through the container lifting operations, namely, engaging a 
container with a lifting spreader, moving the container to its new 
location, setting the container back down, and finally disengaging from 
the container. 
SUMMARY OF THE INVENTION 
The present invention involves an improved electrical interlock system for 
the locking mechanisms of a lifting spreader used by a container lifting 
crane. The locking mechanisms which are controlled would conventionally be 
twist locks but might include other types of locking mechanisms. The 
invention is comprised of proximity switches associated with each of the 
locking mechanisms of the spreader wherein the proximity switches are of 
the normally closed type. The proximity switches are connected in parallel 
to provide an energizing current to a relay switching means, the relay 
circuit of which operatively controls the availability of power to the 
twist lock manual control switch which is under the control of the crane 
operator. The normally closed, parallel arrangement of the proximity 
switches is such that power is supplied to the twist lock manual control 
switch when the energizing circuit of the relay switching means is 
de-energized rather than energized. Using the interlock system of the 
present invention, the reliability problems associated with the operation 
of the invention's proximity switches is substantially eliminated. 
Other aspects of the present invention include means for delaying the time 
between full lifting spreader to cargo container engagement and power 
availability to the twist lock manual control switch; they further include 
means for alerting the crane operator that power has indeed been supplied 
to the control switch after the time delay. In still other aspects of the 
present invention means are provided for discouraging the crane operator 
from prematurely throwing the twist lock control switch to its unlock 
position when the suspended load is being lowered into position. Also, 
means are provided to permit the interlock system to be by-passed in the 
event of a system malfunction and to simultaneously alert nearby crane 
maintenance personnel of the by-pass so that the malfunction can be 
corrected at the earliest possible time. 
It is therefore seen that the primary object of the present invention is to 
provide an improved electrical interlock system which reliably operates in 
field conditions and which directs and encourages the proper operation of 
the twist locks by the crane operator. Other objects of the invention will 
be evident from the following specification and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, the operator of a container lifting crane 
conventionally sits in a cab 11 situated near the top of the crane such 
that the crane operator will have a complete view of his working area 
through the cab windows, such as denoted by numeral 13. The operator will 
normally sit in an operator's seat 15 next to which, and within easy 
reach, there is provided a control panel 17 containing all the necessary 
controls for operating the crane. Also in the cab 11, and within easy view 
of the operator seated on the cab seat 15, is a control box 19 containing 
suitable indicator lights 21, 23, 25, and a interlock system by-pass 
switch 27, the operation of which will hereinafter be described. The 
arrangement of the indicator lights and by-pass switch of the control box 
are most clearly indicated in FIG. 2. 
It is noted that located underneath the operator's cab 11 are additional 
warning lights 29, 31 and an audio horn 33. These alarm mechanisms, which 
will be more fully described below, are located such that they are easily 
viewed and heard by crane maintenance and supervisory personnel who are on 
the ground in the vicinity of the container crane; they particularly 
provide a highly reliable means of communicating with these personnel with 
regard to certain aspects of the crane's operation. As will be described, 
the cab light 29, preferably a red light, acts together with the sounding 
of the horn 33 to indicate that the operator is operating the crane in a 
by-pass mode; the yellow and preferably rotating warning light 31, also 
together with the horn, is used to alert all persons, including the crane 
operator, that the crane is being operated improperly. 
Referring to FIGS. 3 and 4, there is shown a conventional lifting spreader 
35 which attaches at the end of the crane's suspension ropes (not shown) 
as a means for lockingly engaging the cargo container to be lifted. As can 
be seen, the spreader is rectangular in shape corresponding to the outer 
dimensions of the top surface of a cargo container, and has locking 
mechanisms, conventionally twist locks 37, which depend from the four 
lifting corners 39 of the lifting spreader 35 so that they are capable of 
engaging corresponding locking parts on the cargo container (not shown). 
The container locking parts are conventionally corner receptacle castings 
located in the four top corners of the cargo container at standardized 
spacings. It is noted that in the industry there are two standardized 
container lengths and therefore the spreader is designed to be adjustable 
to these lengths. Also, an additional offset pair of twist locks 37a are 
retractably mounted to the spreader so that the spreader can be reversed 
to permit certain close-in lifting operations. 
Details of the spreader lifting corners 39 with the twist locks mounted 
therein are shown in FIG. 5 of the drawings. The twist lock 37 has an 
extended shaft 40 rotatably mounted within the corner structure 41 of the 
lifting spreader. The twist lock can be rotated through an angle of 
90.degree. by means of a remotely rotatable control arm 43 secured to the 
top of the twist lock shaft 40. In one rotation the twist lock, which has 
extended tabs 38, is oriented such that it can be lowered into a corner 
receptacle casting of the cargo container which has a twist lock receiving 
opening substantially in the shape of the opening 45 shown in FIG. 6; in 
its opposite rotation the twist lock will be oriented cross-wise with 
respect to the receptacle opening to firmly lock within the receptacle. 
When the lifting spreader 35 engages the top of a cargo container the 
bottom plate 49 of the spreader's lifting corners 39 contacts the top of 
the cargo container, and particularly the metal top plate of the cargo 
container's corner receptacles (not shown). Conventionally, a mechanical 
cam protrudes through the opening 51 in this bottom plate 49 such that 
contact with the cam depresses the cam upwardly to interlock the twist 
lock. In the present invention, proximity switching means are instead 
provided, preferably by placing a proximity switch 53 in the existing 
opening 51 in the spreader corner bottom plate 49 of each spreader lifting 
corner 39. In this position the proximity switches are capable of sensing, 
by non-mechanical means, the proximity of the container receptacle 
castings. 
The distance between the sensing end surface 55 of the proximity switch and 
the bottom surface 57 of the container corner's bottom plate 49 is crucial 
to the operation of the proximity switches and can be set by locking nut 
59 through field tests. The switching distance of the particular proximity 
switch or sensor used should closely correspond to the distance between 
these two surfaces, 55, 57 whereby switching occurs substantially at the 
point of contact between the spreader corners 39 and the container corner 
castings. A TURCK proximity switch, model Berz-G10 having a rated 
switching distance of 0.393 inches, has been used, with the sensing 
surface 55 to bottom plate surface 57 spacing being nominially set to 0.39 
inches. The proximity switches 53 can be wired through their lead wires 61 
to the interlock circuit relays (described below) which are preferably 
located in the control box 19. 
The preferred interlock circuit of the invention is shown in schematic form 
in FIG. 7 of the drawings. As shown therein, the circuit includes 
proximity switches 53 and 53a, with the proximity switches 53a being 
associated with the offset twist locks 37a. It is seen that in this 
preferred embodiment the circuit is powered through a protection fuse 66 
by a 120 volt ac power source 65. The interlock circuitry acts to control 
the availability of this power to the twist lock manual control switch 67, 
which is located on the control panel 17 in the crane's cab 11. The manual 
control switch, which can be conveniently switched to either a lock 
position 69 or an unlock position 71, permits the crane operator to 
selectively supply power to the twist locks by means of locking and 
unlocking control circuits, such as, respectively, solenoid switches 73 
and 75. Power to the twist lock manual control switch 67 is interlocked by 
a relay switching means preferably comprised of the relay R1 connected in 
tandem with a time delay relay R2. As shown, relay R1 has its energizing 
circuit 77 connected in series with the proximity sensors 53 and its relay 
circuit 78 connected in series with the energizing circuit 79 of time 
delay relay R2; the relay circuit 80 of relay R2 controls the power to 
manual control switch 67. Thus, the R2 relay circuit 80 is in effect 
switched in response to a change in the flow of current through the 
energizing circuit 77 of relay R1, and then only after a delay time which 
can be preadjusted by adjusting the time delay of relay R2. Therefore, it 
can be seen that power will not be supplied to the control switch 
immediately upon the proper engagement of the lifting spreader with the 
cargo container, but rather after a preset delay. This will ensure that 
the crane operator does not hastily throw the twist lock control switch 
before the lifting spreader is fully settled. 
The proximity switches 53 are preferably of a normally closed type which 
are wired in parallel as shown in FIG. 7. Such an arrangement of normally 
closed proximity switches means that energizing current will flow through 
the energizing circuit 77 of relay R1 until all of the proximity sensors 
used in the circuit sense contact with its associated lifting spreader 
corner, thus indicating that all four twist locks are properly engaged in 
the container's corner receptacle castings. When four corner engagement is 
achieved the energizing current through energizing circuit 77 is prevented 
from flowing thereby de-energizing relay R1. The relay logic is such that 
relay R1 is normally closed and time delay relay R2 is normally open in 
respect to its own energizing circuit 79, but normally closed in respect 
to the energizing circuit 77 of relay R1. In other words, de-energizing 
the energizing circuit 77 will cause power to be supplied to manual 
control switch 67. It is noted that the offset proximity switches 53a can 
replace the top two proximity switches 53 in the interlock circuit by 
means of a manual offset relay switch 83, which through energizing circuit 
84 simultaneously actuates both the normally closed and normally open 
relay circuits denoted 85 and 87. 
It is further noted here that preferably a shunt resister 90 is placed 
across the energizing circuit 77 of relay R1. It has been found that this 
resistor, by reducing the bleed current which flows through this 
energizing circuit, will substantially eliminate any relay chatter or 
anomalies associated with the bleed current. 
A means for indicating the four corner contact of the lifting spreader with 
the cargo container is provided by the four corner light 23 which is 
switched on by means of the relay circuit 80 of relay R2. This light, 
which is preferably located on the control box 19 mounted in the operator 
cab 11, signals the operator, after a delayed time, that all of the twist 
locks of the lifting spreader are so engaged with the corner casting of 
the cargo container as to permit the unobstructed rotation of the twist 
locks. 
Additional unlock condition alarm means are provided to provide a 
continuing alarm if the operator throws the twist lock manual control 
switch 67 to its unlock position while a cargo container is suspended by 
the lifting crane. During suspension, the twist locks, due to the weight 
of the container, cannot rotate from a locked to an unlocked position. 
Because of this, crane operators, to speed up operations, commonly throw 
the control switch 67 before the container is set down, a practice which 
raises the possibility that a twist lock will prematurely release, such as 
when a corner of the spreader knocks against a structural part in the hold 
of a ship. The result is damage to the twist lock and/or the container. 
To deter this practice an alarm means, preferably consisting of rotating 
yellow light 31 and horn 33, are powered through the unlock position 71 of 
the twist lock manual control switch 67 by means of a normally closed 
relay R4, the energizing circuit 91 of which, like relay R1, is placed in 
series with the proximity switches 53. If the proximity switches indicate 
four corner contact of the spreader with the cargo container and if the 
control switch 67 is rotated to the unlock position, the relay circuit 93 
of relay R4 will energize both the alarm horn 33 and the alarm light 31 
located underneath the operator cab 11. The horn is preferably chosen to 
have a high decibel level to give a clear warning and deterance to the 
operator concerning the practice of prematurely throwing the control 
switch 67 to an unlock position before it is time to properly do so, and 
to incidently alert the supervising personnel that this practice is going 
on. It is noted that when the load is properly set down such that no 
weight appears on the twist locks, throwing the manual control switch 67 
to its unlock position will cause all the twist locks to unlock, thereby 
releasing the cargo container from the sensing range of proximity sensor 
53 before the alarm light 31 and horn 33 have an opportunity to be 
activated. 
In the event there is a malfunction of any part of the interlock circuit, 
such as in one of the proximity switches 53, the by-pass switch 27 can be 
turned to by-pass the relay circuit of relay R2. This directly supplies 
power to the manual control switch 67. The shown circuit additionally 
provides that the by-pass switch will energize a by-pass light 25, 
preferably located on the control box 19, as well as the alarm horn 33 and 
alarm light 29. Thus, when this by-pass switch is used, the operator, as 
well as maintenance and supervisory personnel in the vicinity of the 
crane, will be alerted to the fact that the crane twist locks are being 
operated in a by-pass condition and that the interlock malfunction which 
created the need for the by-pass should be attended to immediately. This 
by-pass alarm scheme will prevent the crane operator from intentionally 
avoiding the protections provided by the interlock circuits. 
As to the operation of the circuit of FIG. 7, it is seen that a crane 
operator maneuvers and lowers the lifting spreader 35 onto the top of a 
cargo container until all of the twist locks 37 of the spreader lifting 
corners 39 are fully engaged into the container's twist lock receptacle 
castings whereupon normally closed proximity sensors 53 simultaneously 
de-energize both the energizing circuit 77 of relay R1 and the energizing 
circuit 91 of relay R4. The normally closed relay circuit 78 of relay R1 
thereupon closes to energize the energizing circuit 79 of the time delay 
relay R2. After its pre-set time delay, the normally open relay circuit of 
relay R2 closes to supply power to the twist lock control switch 67, 
simultaneously lighting the four corner indication light 23 located inside 
the crane's operator cab 11. The crane operator can then throw the manual 
control switch 67 to its lock position 69 to energize solenoid 73 for 
rotating the twist locks to a closed position. The manual control switch 
can also now be rotated to the unlock position for supplying power to 
solenoid 75. The result is an unlocking force to the twist locks, however, 
if this force is supplied when the container is suspended and thus before 
all the twist locks are free to rotate, the alarm 33 and alarm light 31 
will be continuously activated through the closed relay circuit 93 of 
relay R4. If the interlock system malfunctions preventing power to the 
manual control switch 67, then in order to operate the crane the operator 
simply throws the by-pass switch 27 at the same time alerting maintenance 
personnel as to the malfunction condition. 
Therefore, it can be seen that the present invention provides an improved 
electrical twist lock system which provides for the reliable operation of 
the interlock circuit and which provides suitable alarm and light 
indication means for promoting the optimum operation of the container 
lifting crane. 
Although the present invention has been described in considerable detail in 
the foregoing specification, it is not intended that the invention be 
limited to such detail, except as necessitated by the appended claims.