Electro-magnetic Braking Device

Abstract

The electro-magnetic braking device includes two coils concentrically disposed about a magnetizable core having a high residual magnetism capable of holding a brake armature in non-braking position against a spring bias even when the current is removed form the coils. When it is desired to have the brake armature spring biased into braking position when the current is removed from the coils a demagnetization circuit is connected to one of said coils to demagnetize the core.

Description

The present invention relates to an electrically controlled two position brake, which is particularly useful for fitting to a tower crane to allow the crane to behave as a weather vane when it is out of service. It is to be understood however that the brake is not limited in its field of application to cranes.

Known cranes comprise a fixed part (generally the mast) surmounted by revolving part (jib and counter-jib). In certain circumstances, it is required that the revolving part can turn on the fixed part; in other circumstances, it is required that this revolving part shall be immobilised. The cases usually met with in practice are as follows:

A. When the crane is out of service, it is generally allowed to behave as a weather vane to ensure its stability in a wind above 80 kilometres per hour; for this purpose the brakes of the directional mechanism must be free in order that the jib can itself take up the direction of the wind;

B. When the crane out of service is in addition not under voltage the brake must be held in the locked position;

C. If the crane is in ordinary service, the brake must be locked when the operator decides to stop the directional movement of the revolving part in the chosen position;

D. Finally, when the electric current is cut off whilst the crane is in service, the brake must automatically be locked to avoid the jib continuing its rotary movement under the force of the wind or of its own inertia.

The systems known up to the present to ensure that the crane is allowed to behave as a weather vane are mechanically controlled. In certain cases the crane operator has to activate the directional mechanism directly which releases the brake through a system of screws and nuts (the operator is then obliged to go into the revolving part, that is to say on the top of the crane). In other cases, the crane operator can actuate the directional mechanism from the base of the crane, by means of cables and levers.

None of the known systems offers sufficient reliability, especially if the directional mechanism is positioned on the revolving part of the crane.

The present invention has the aim of avoiding these disadvantages by providing an electrically controlled brake which possesses the characteristic of being two position, that is to say that both in the rest position and after the current is cut off, it can remain as required either locked or freed.

An electrically controlled two position brake according to the invention, intended for fitting between a stationary part and a moving part comprises a moving armature subject to the action of at least one electro-magnet and a component in a material of high residual magnetism interposed in the magnetic circuit so that the electric control of the electro-magnet permits the component of high residual magnetism to be either magnetised or de-magnetised.

According to a preferred feature of the invention, the component with high residual magnetism is constituted by a stationary heavy ring against which the armature can be attracted by overcoming a resilient load, such as that of a spring, when this remanent component is magnetised, the brake being then released; whilst on the other hand when the remanent component is de-magnetised, the action of the resilient loading repels the armature against a braking disc and the brake is locked.

In one construction a single winding is used arranged around the highly remanent ring. This winding can be fed by direct current with a given polarity when it is required to magnetise the ring and with current of the opposite polarity when it is desired to de-magnetise the ring.

In another construction two separate windings are arranged around the remanent ring, namely one winding used for magnetisation, and another used only for de-magnetisation.

If the brake is used on a crane, it will be understood that if the crane is taken off current when the remanent component is magnetised, the residual magnetism will hold the armature attracted to the remanent component the brake will remain stably in the free position. If on the other hand the crane is taken off current when the remanent component is demagnetised the brake will remain locked under the action of the resilient loading.

In a preferred arrangement, three separate contacts are used, namely a first contact in series with the magnetisation winding, a second contact in series with the de-magnetisation winding, and a third contact controlling the charge of a condenser which is arranged in parallel on the de-magnetising winding. Owing to this arrangement, when the direction movement of the crane is stopped, opening of the third contact isolates the condenser which, because the second contact is closed, discharges through the de-magnetisation winding where it produces a sufficient impulse to de-magnetise the remanent ring so that the brake remains locked from then on. This system has the advantage of remaining unchanged in the case of a break in the current supply.

One construction of a brake and its arrangement in accordance with the invention, will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is an axial section through a brake in the locked (i.e. brake applied) position,

FIG. 2 is a section similar to FIG. 1, but showing the parts in the unlocked position,

FIG. 3 shows diagrammatically the electrical connection of three control contacts in relation to the general three phase circuit of a crane, and

FIG. 4 is a diagram of the direct current electric circuit controlled by the three contacts and illustrating both a magnetisation winding and a de-magnetisation winding.

The brake shown in FIGS. 1 and 2 comprises a wound yoke formed by a circular component 1 made in mild steel, and a central core 2 in the form of a ring, manufactured from some strongly remanent material, identical to that which is used for example for the manufacture of permanent magnets, and two circular electro-magnetic windings 3 and 4, both concentric with and placed around the central core 2.

This wound yoke acts in co-operation with a moving armature 5 made of mild steel and fitted with an annular friction pad 6, on its underside. The armature 5 possesses two lugs 7 which project radially and which can slide respectively in two longitudinal slots 8 in a fixed support 9 of the brake assembly. The support is made of a substantially non-magnetic material. Thus the armature 5 can be displaced freely in the axial direction (as indicated by the double arrow 10) but it is immobilised against rotation.

Below the friction pad 6 there is placed a brake disc 11 fixed to a shaft 12 which it is desired to brake.

A compression spring 13, intended to apply the lining 6 of the armature 5 on to the braking disc 11, is mounted in the centre of the heavy ring 2, and acts between the armature 5 and a support screw 14 located in a screw-threaded hole in the top of the frame 9. The regulation of this screw allows the braking effort which will be applied to the lining 6 to be the adjusted.

The operation of the brake is as follows:

For the first stable position, that is to say when the brake is locked (FIG. 1), the force of the spring 13 applies the lining 6 to the brake disc 11, and this locks the disc 11 and the shaft 12 against rotation. To pass the second stable position (brake feed), it is necessary to supply an electric current to the winding 3, which is called the magnetisation winding. When the magnetisation winding is energised, there is an attraction of the mobile aramture 5 towards the ring 2 of the yoke and this frees the brake disc 11. The current can then be cut off, but the central core 2 remains magnetised due to its residual magnetism, and holds the mobile armature 5 firmly against the wound yoke (FIG. 2).

To return the brake to the first stable position (the brake locked), it is only necessary to feed into the winding 4, which is called the de-magnetised winding, an electric current of short duration this current flowing in such a way that it nullifies the remanent magnetisation of the central core 2. The spring 13 then repels the armature 5 and applies the direction lining 6 against the brake disc 11.

The control of these various operations can be carried out from a relay (FIG. 3) operating simultaneously a first (normally open) contact K1, a second (normally closed) K2 and a third (normally open) contact K3. This relay is connected in the three phase feed line of the directional motor M of the crane.

The contacts K1 and K3 close automatically as soon as the control for the setting in rotation of the revolving part of the crane is operated. They open as soon as the control for the cessation of movement has been operated. They likewise open in the case of a breakdown in the electrical current supply. On the other hand, the second contact K2 being normally closed, opens as soon as the directional movement is begun and closes as soon as this movement ceases. It likewise closes in the case of a breakdown in current.

These three contacts are incorporated in a direct current brake control (FIG. 4). The first contact K1 is arranged in series with the magnetisation winding 3, so that when the contact K1 is closed, the winding 3 is energised. The contact K2 is arranged in series with the demagnetisation winding 4. A condenser C is mounted in parallel with the contact K2 and the winding 4. These two parallel connections are fed through the third contact K3.

Finally there is connected in parallel with the first contact K1, a button B which is positioned at the control position of the crane, and which allows the brake to be unlocked to allow the crane to behave as a weather vane.

In particular it will be seen that when the contact K3 is open and the contact K2 is closed, if the condenser C has previously been charged, it can discharge through the de-magnetisation winding 4 which allows the freeing of the armature 5 which passes from the position shwon in FIG. 2 to that in FIG. 1 under the thrust of the spring 13.

When the operator starts the crane in directional movement, the following operations take place:

i. the contact K1 feeds the magnetisation winding 3 which releases the brake;

ii. the contact K3 charges the condenser C, and

iii. the contact K2 isolates the de-magnetisation winding 4.

To cause the movement to stop;

iv. the contact K1 opens the circuit of the magnetisation winding 3, and

v. the contact K2 connects the condenser C which discharges through the de-magnetisation winding. The brake is then locked.

It will be noticed that the process is carried out in exactly the same way in the case of a breakdown in the supply of current.

To cause the crane to act as a weather vane, it is sufficient to press the button B which puts the magnetisation winding 3 under voltage. The brake is then released. When the button B is released the brake remains unlocked because of the effect of the residual magnetiisation of the central core 2 (the de-magnetisation winding 4 is inoperative, since the condenser C has not been charged during this operation).

Naturally, a signal lamp may also be included in the electrical circuit to allow the position (locked or unlocked) of the brake to be observed.

Finally, the brake may be provided with a single winding replacing the windings 3 and 4. Magnetisation would then be obtained by passing the current in one direction, demagnetising being produced when the current passes in the opposite direction in the same winding, its itensity being limited to a predetermined value by the insertion of a resistance in the circuit.

Claims

I claim:

1. An electro-magnetically controlled brake comprising a rotatable disc, a movable armature, support means guiding said armature non-rotatably toward and away from said disc, coil means mounted on said support means, a stationary component having high residual magnetism disposed in the magnetic circuit of said armature and said coil means, resilient means normally biasing said armature into engagement with said disc to prevent rotation of said disc and control circuit means including said coil means and relay means for magnetizing said component upon energization of said relay means to overcome the force of said resilient means and for automatically de-magnetizing said component upon deenergization of said relay means.

2. An electro-magnetically controlled brake as set forth in claim 1 wherein said resilient means is comprised of a compression spring and means on said support means for adjusting the force of said spring.

3. An electro-magnetically controlled brake as set forth in claim 1 wherein said coil means is comprised of two separate windings concentrically arranged around said component, one of said windings being arranged to magnetize said component and the other of said windings being arranged to de-magnetize said component.

4. An electro-magnetically controlled brake as set forth in claim 3 wherein said relay means includes a first normally open switch connected in series with said one of said windings, a second normally open switch and a third normally closed switch connected in series with said other of said windings.

5. An electro-magnetically controlled brake as set forth in claim 4 further comprising capacitor means connected in parallel with said third normally closed switch and said other of said windings and connected in series with said second normally open switch whereby upon energization of said relay said second switch will close to charge said capacitor means while said third switch opens to prevent energization of said other of said windings and upon de-energization of said relay said second switch will open and said third switch will close to allow discharge of said capacitor through said other of said windings to de-magnetize said component.

6. An electro-magnetically controlled brake as set forth in claim 4 further comprising a manually operated switch connected in toto with said first normally open switch to selectively energize said one of said windings to magnetize said component.