US3517830A

US3517830A - Cranes

Info

Publication number: US3517830A
Application number: US674154A
Authority: US
Inventors: Vilkko Antero Virkkala
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-10-10
Filing date: 1967-10-10
Publication date: 1970-06-30
Anticipated expiration: 1987-06-30

Description

June 30, 1 970 v. 4; VIRKKALA ORANES Filed Oct. 10. 1967 2 Sh'eets-ESheet 1 A .I F 1 4.. T7. I V o All! June 30, 1970 v. x; VIRKKALAa I 3,517,330

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Filed Oct. 10, 1967 2 Sheets-Sheet 2 MOTOR United States Patent 3,517,830 CRANES Vilkko Antero Virkkala, Purjetuulenkuja 11, Helsinki, Finland Filed Oct. 10, 1967, Ser. No. 674,154 Int. Cl. B66c 19/00 U.S. C]. 212132 Claims ABSTRACT OF THE DISCLOSURE An arrangement for reducing the oscillations of a load pendulating from a crane, which load is moved horizontally. A moving mechanism is provided with a synchronizing device, automatically functioning so that each change of acceleration as controlled by the driver is automatically succeeded by another equally great and similarly directed change of acceleration after a time, which is half the length of the oscillation period of the load.

By the term load as used herein is meant the load which is suspended by members of suspension, i.e. cables or chains, and the loading device, which may consist merely of a crane hook for suspension, but may also be composed of a more complicated device.

When a load which pendulates at the end of a cable is moved horizontally by a crane, the load generally gets into more or less heavy oscillation, which often is quite a drawback to the work. These oscillations can be damped either in such a manner that the load is manoeuvered with the hands, which is not always possible, or by means of suitably synchronized driving movements. To do this correctly, however, requires great skill by the driver, wherefore automatic arrangements for reducing such oscillations have been invented. However, the devices so far known are so expensive in use that they have been employed only for cranes with an exceedingly high capacity.

The arrangement, according to this invention, is chiefly intended for use with slow cranes with a fairly small capacity. As cranes of this kind do not often have their own driver, they are manoeuvered by more or less casual workers, who are not as a rule trained to use synchronized driving movements in order to damp the oscillations. The active principle of this arrangement, is however, applicable for cranes with comparatively rapid movements, too.

In accordance with one embodiment of this invention the oscillations of the load may be prevented in that the driving mechanism, which moves the load horizontally is provided with a synchronizing device, which automatically causes each change of acceleration to be succeeded by another equally large and similarly directed change of acceleration over a period, which is half the oscillation time of the load.

The invention is described in greater detail in the following with reference to the appended drawings, in which:

FIG. 1 shows a bridge crane, with a load suspended from a trolley which is moving on a bridge;

FIG. 2 shows graphically the movements of the trolley and the load;

FIG. 3 shows graphically difierently arranged movements of the trolley;

FIG. 4 shows an electrical circuit for the case illustrated in FIG. 2; and

FIG. 5 shows an electrical circuit for the case, illustrated in FIG. 3.

The principle of this arrangement appears in FIGS. 1

and 2. FIG. 1 shows a bridge crane and a bridge 5 along with a lifting trolley 6, moving on the bridge. An embodiment of the invention is shown in connection with the horizontal movement of the trolley, though the trolley, in practice, may be placed anywhere in any equipment having horizontal movements.

The driving mechanism of the trolley consists of an electric motor and electrically operated brake as well as a suitable transmission device, which is not shown in FIG. 1. A two speed squirrel-cage motor is employed for the simplest embodiment of this invention and the motor is so wound that its speeds are proportional to one another approximately in a ratio of 1:2. The trolley besides, is furnished with a lifting device with the attached load 7 pendulating from a cable or a chain. The center of gravity of the load is considered to be at point 8. When the trolley is moving, the load oscillates in relation to the trolley so that the time of oscillation is about T =21r seconds in 'which IJ=the distance from the center of gravity of the load to the fulcrum (m) and g=the acceleration of the gravitation (m./s. The load may be at rest in relation to the trolley only when the trolley is at rest, when it is in even or in an evenly accelerating motion. Should the speed of the trolley not be great, the changes of acceleration are in general so rapid as to appear .almost momentous in comparison to the oscillation speed of the load. Each change of acceleration of this type releases a series of oscillations, with an oscillation time T and the natural damping of the oscillation is then as a rule exceedingly slight in practice. Provided the horizontal movements of the trolley were now adjusted so that each change of acceleration should be after a period of /2 T succeeded by an equally great and similarly directed change, then the effects of these changes would oppose each other in such a manner that the oscillations of the load are but slight when the trolley stops. This is illustrated in FIG. 2.

The speed v of trolley as the function of time t, is presented in the upper part of FIG. 2. The trolley 6 in the first place has been accelerated to half its speed by means of a comparatively great constant acceleration and after the time /2 T to its full speed with the same acceleration. The time T can be measured from the onset of the first part-acceleration up to the onset of the second part-acceleration. It has been presupposed that the second partacceleration should be released by an automatic device, which later on will be described in greater detail.

Each change of acceleration of trolley 6 sets the load 7 in an oscillating movement, whose time period is T. These diverging oscillations are illustrated by the

curves

1, 2, 3 and 4 in FIG. 2, and the proportional speed of the load in relation to the trolley is shown as function of the time. It is, thus, obvious that the sum total of the partcurves always =0 after a complete acceleration, or in other Words the load is at rest in relation to the trolley. The stopping of the load has been, as indicated in FIG. 2, brought about in the same way as the acceleration, viz by two synchronized steps, these part-oscillations which were released by the different changes of acceleration opposing each other.

With a view to the fact that the load, according to the principles embodied in FIG. 2, cannot be moved very short distances, another arrangement is required for such cases.

For short distance transport, the oscillation can be damped as described in FIG. 3. The crane driver now for the time t only presses the button. The pressure time is so brief, that the automatic acceleration adjusted to function after A: T does not have time to start functioning, but the trolley starts stopping immediately after the moment t In any event, the automatic devices have anyway recorded the length of the pressure time and after /2 T again start the trolley so that it makes another similar short movement in the same direction. When both the accelerations, the retardations and the driving times are uniform, the aforementioned principle is fulfilled which infers that the oscillation caused by each change of acceleration is opposed by another similar change of acceleration, synchronized to be released within the period /2 T.

The function which is released in accordance with the chief principles mentioned above, can be embodied in many different ways. The essential part, however, always is the synchronization device, which gives a suitable signal after the time /2 T, counted from the onset of the previous part-acceleration. As the period T depends upon the pendulum length L, in conformity with the aforementioned equation this must be produced with sufficient exactitude by the device.

Apparatus suited for this purpose is illustrated in FIG. 4. The transmission device connected with the cable drum of the crane winch 9 sets the potentiometer R in motion. A condenser C is charged from a voltage source +U to a voltage which depends on the height of the lifting hook. When the measuring of the half of the oscillation time begins, the change-over switch 10, connected with the condenser, is operated and the condenser begins to discharge through the resistor R The resistor R is, further, provided with a relay A equipped with a transistor amplifier, which starts functioning when the condenser voltage decays to the value U In case the voltage of the condenser was U at the onset of the measuring, this means that the time relative to the function complies with the law It is presumed that the internal resistance of relay A is very great compared with the resistance R The adjustable resistors R and R may be adjusted so that the initial voltage of the condenser obeys the law in which L=the distance in meters from the axis of the cable drum of the winch to the center of gravity of a typical load. When, in addition, C and R have been selected so that the time of the discharging circuit R C=2.13 seconds, the following equation is arrived at for the function time in seconds of relay A Adjusted in this manner the function time of relay A, within wide limits, is practically exactly the same as half the desired oscillation time If the distance from the hook 11 to the center of gravity of the load 8 shows perceptible variations, this will make the exactitude of the device suffer with regard to its function, but its damping effect is still comparatively strong, though the true oscillation length should deviate from the assumed by 3040 percent. Similar or almost similar loads are anyway always dealt with in a great many important cases.

Another important embodiment, in accordance with the principles of this invention, is a device, made to record the length of given short driving impulses and to repeat them by order of the abovementioned time relay. Such a device can be constructed quite advantageously, provided the lengths of the impulses are stored in analog form as a charge of a condenser.

' FIG. 5 illustrates an embodiment of the above. The contact A is closed during the time the crane driver presses the control button for the horizontal movement. The condenser C is then charged to a voltage which is proportional to the time the contact A is closed. The charging current passes through diode D and transistor Tr The latter is so connected that it limits the charging current to a given value, which remains practically constant until the voltage of the condenser has almost reached the voltage of the source of the current. However, the circuit can be easily arranged so that the condenser is not fully charged during the full time /2 T, which is the longest impulse necessary to be recorded. The condenser is then charged with a voltage, which is directly proportional to the length of the ordered impulse. When the aforesaid time relay gives a signal indicating a repetition of the impulse, the contact B is closed and the condenser starts discharging. The discharge current passes through diode D and transistor Tr which also functions as a constant current device. The main part of the dis charging current further passes via transistor Tr and brings its collector voltage to almost zero. When the condenser is fully discharged, Tr ceases to receive base current and its collector voltage rises to that of the source of current. This may be simply used as a signal for the driving movement to stop. As the condenser is both charged and discharged by the same constant current device, these phases are practically the same length, and no change of the components caused by aging or temperature fluctuations evokes any functional errors.

In some cases, where the load is great in comparison to the weight of the trolley, errors may be caused by the load braking the trolley during the first impulse while tending to pull it forward during the time of the later impulse. The accelerations and retardations, thus, are dissimilar in this instance. The error hereby caused may be reduced, if the distances which the crane travels can be made equally long during the time of both the impulses. To effect this a switch may be placed within the current circuit of the condenser, which device then permits the current to run during a constant length of time, when the trolley has travelled the fixed short distance. Such a device may be easily constructed by making good use of the known technique, if for example the axis of the driving motor of the trolley is supplied with a suitable magnetic toothed wheel. The signal for the repeated impulse to end is now that the collector voltage of the transistor Tr no longer drops to almost zero during the time of the pulse.

What I claim is:

1. Apparatus comprising suspension means for suspending a load and providing for the latter a pivot about which the load will oscillate, acceleration and moving means for moving the suspension means and thereby the load in a horizontal direction whereby the load, as a result, will tend to oscillate according to an oscillation period resulting from the acceleration and movement and distance from said pivot to the center of gravity of the load, and synchronizing means controlling the acceleration and moving means so that each change of acceleration is automatically succeeded by an equal and similar change of ac celeration after one-half of the resulting oscillation period, the synchronizing means including means for controlling the acceleration and moving means so that the suspension means moves in short impulses the length of which is half the oscillation period and so that such movement is succeeded by another similar movement the start of which is delayed by half an oscillation period.

2. Apparatus as claimed in claim 1, wherein the acceleration and moving means includes a two-speed motor the speed ratio of which is about 1:2.

3. Apparatus as claimed in claim 2, wherein the acceleration and moving means further includes an electrically operated brake and the synchronizing means regulates the time of acceleration between the speeds of said motor.

4. Apparatus as claimed in claim 2 wherein the synchronizing means includes a condenser, means to charge and discharge the condenser, and a time-delay relay operated by discharge of said condenser and controlling said motor.

5. Apparatus as claimed in claim 2, wherein the synchronizing means includes a condenser, means to charge 10 and discharge the condenser, and a time-delay relay sistor circuit coupled to the condenser and controlling said motor.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 1/ 1966 Germany.

HARVEY C. HORNSBY, Primary Examiner US. Cl. X.R. 21210, 21