Magnetic devices

July 30, 1968 r. f. searle 3,394,784

MAGNETIC DEVICES

Filed July 20. 1966

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F I G. 2

INVENTOR.

ROBERT F. SEARLE

BY / ATTORNEY

United States Patent Office

3,394,784

Patented July 30, 1968

3,394,784 MAGNETIC DEVICES Robert F. Searle, Amherst, N.H., assignor to Vibrac Corporation, Chelmsford, Mass., a corporation of Massachusetts 5

Filed July 20,1966, Ser. No. 566,590

Claims. (Cl. 192—21.5)

ABSTRACT OF THE DISCLOSURE

A magnetic particle torque transmitting device comprising means for magnetically constraining the magnetic particles so that they are removed from the immediate area of the shaft seals during operation.

This invention relates to magnetic torque transmitting devices and more particularly to those devices employing magnetic particles for coupling together two relatively rotatable members. 20

Magnetic particle devices of various types are known to the art. Most commonly they are constructed to function as quick-acting electrically actuated clutches or brakes, but they also can be designed to impart drag to a rotatable member as, for example, where it is desired to ²⁵maintain tension. The magnetic particles may be in loose form or may be suspended in a liquid of selected viscosity, such as oil. Such devices offer many advantages, including fast response, adjustability of output torque over a relatively wide range, smooth chatter-free opera- ³θ tion and ability to operate in the slip condition. The present invention is concerned with an improvement that greatly improves the reliability and service life of magnetic particle torque transmitting devices. The reliability and operating life of magnetic particle devices depends ³⁵on the ability to retain the magnetic particles in the area where they are effective to transmit torque. Typically devices of the type to which the invention relates consist of an armature assembly comprising two spaced pole pieces or armature members, a shaft disposed within the armature assembly and carrying a disc that extends into the space between the two pole pieces, bearing means supporting the shaft for rotation relative to the armature assembly, magnetic particles disposed between the disc and the pole pieces, and means for providing a magnetic field axially across the armature assembly and the disc so that the magnetic particles will lock into torque-transmitting chains coupling the disc and the pole pieces. Where the device is designed to function as a clutch, the armature assembly constitutes part of a rotor assembly that is mounted for rotation relative to a surrounding housing and functions as an input or driving member while the rotatable shaft on which the disc is mounted functions as an output or driven member. Where the _ggdevice is designed to operate as a brake, the armature assembly is locked against movement relative to the housing and the applied magnetic field operates to oppose rotation of the disc relative to the armature assembly. In both cases shaft seals are required between the θθ shaft and the two pole pieces to confine the magnetic particles to the spaces between the disc and the pole pieces. Extensive research and development has yielded a variety of high performance shaft seals. However, it has been determined that the magnetic particles have a _ggtendency to migrate into and become entrapped in the seals, and once entrapped they act as an abrasive to accelerate seal wear.

Accordingly the primary object of this invention is to extend the useful life of magnetic devices of the type de- ₇θ scribed above by an improvement that minimizes shaft seal wear.

A further object is to provide improved magnetic torque transmitting devices of the character described wherein the shaft seals have a service life equivalent to bearing life.

I have discovered that shaft seal wear can be greatly reduced if the magnetic particles are constrained from engaging or migrating to the shaft seals and are confined to an area where they are effective to transmit torque between the armature assembly and the disc.

Therefore, a more specific object of the invention is to provide in a magnetic particle torque transmitting device of the character described means for magnetically constraining the magnetic particles so that they are removed from the immediate area of the shaft seals during operation.

The foregoing and other objects are attained by providing a disc comprising an inner ring made of a nonmagnetic material and an outer ring made of magnetic material. The inner ring is coextensive radially with the shaft seals. When the magnetic field is applied its flux path is confined so as to pass through the outer ring, whereupon magnetic particles in proximity to the shaft and the shaft seals are attracted radially outward into alignment and coupling relation with the outer ring and the pole pieces. In an alternative embodiment the pole pieces are made in two concentric sections, with the inner section consisting of non-magnetic material and the outer section of magnetic material. The modified pole piece construction also functions to confine the magnetic field so that it is effective to draw the magnetic particles away from the shaft seals.

Other objects and many of the attendant advantages of this invention are believed to be apparent from the following specification which is to be considered together with the accompanying drawings, wherein:

FIG. 1 is a longitudinal sectional view of a magnetic particle clutch constituting a preferred embodiment of my invention;

FIG. 2 is an enlargement of a portion of FIG. 1; and

FIG. 3 is a fragmentary longitudinal sectional view of an alternative embodiment of my invention.

Turning now to FIG. 1 there is illustrated a magnetic particle clutch comprising a stationary field coil and rotatable input and output rotor assemblies mounted in a housing consisting of a cylindrical shell 2 fitted with annular end members 4 and 6. The field coil, secured between shell 2 and its two end members, is an annular coil assembly 8 of known construction. The opposite ends of the coil are connected to a pair of suitable terminals mounted in the shell 2, one of such terminals being shown at 10. These terminals are used to couple the coil to an external power source (not shown) whereby it is energized. The shell and its end members 4 and 6 are made of magnetic material, with the inner transversely extending faces 14 and 16 of end members 4 and 6 respectively functioning as poles to develop a magnetic field through adjacent portions of the input rotor assembly which is identified generally by numeral 18.

The input rotor assembly comprises two driving members 20 and 22 that are formed of magnetic material so as to function as armatures. These members are connected to each other by a sleeve 24 made of non-magnetic material. Sleeve 24 may be press-fitted onto the driving members 20 and 22 or may be secured thereto by other means, as for example, by means of screws 26. The two driving members 20 and 22 are rotatably supported in the end members 4 and 6 by means of bearings 28 and 30, the positions of which are determined by shoulders 32 and 34 formed in the two driving members and by retainer rings 36 and 38 that snap into grooves formed in the end members 4 and 6.

3,394,784 . . ³

The two driving members 20 and 22 function as a bearing support for an output rotor assembly identified generally by numeral ,40. The output rotor assembly comprises a shaft 42 constructed of non-magnetic material and a disc 44, the construction of which is described below. The shaft 42 is rotatably supported by the input rotor assembly by means of bearings 46 and 48, the positions of which are defined by shoulders 50 and 52 on the shaft and by retainer rings 56 and 58 that snap into grooves formed in driving members 20 and 22 respectively. Disc 44 is disposed between the adjacent driving members 20 and 22 and the adjacent ends of the latter confronting disc 44 are provided with circular grooves 60 in which are secured shaft seals 62 and 64 of conventional design. Preferably these seals are of the type comprising rings of U-shaped cross-section made of non-magnetic material such as brass and filled with a resilient sealing material that engages shaft 42. The two shaft seals 62 and 64 together with sleeve 24 function to close off the annular space between disc 44 and the two driving members 20 and 22. This space is filled with dry magnetic particles 66 (see FIG. 2).

As seen best in FIG. 2, the disc 44 is made of two concentric rings that are inductively soldered together. The inner ring 68 is made of non-magnetic material, preferably type 303 stainless steel. The outer ring 70 is made of a magnetic material, preferably #2 relay steel. The outer diameter of the inner ring 68 is substantially the same as the outer diameter of the seals 62 and 64. The outer diameter of the outer ring 70 is slightly less than the inner diameter of sleeve 24. The disc 44 may be attached to shaft 40 in a variety of ways, as, by press fitting, but preferably by inductive soldering.

Operation of the above-described clutch will now be described with reference to both FIGS. 1 and 2. When the coil 8 is energized a magnetic field is established across the particles 66 through the highly permeable end members 4 and 6, the driving armature members 20 and 22 and the driven disc 44. With such flux linkage across the magnetic particles, the latter will lock in chains between driving members 20 and 22 and disc 44, thereby coupling the input rotor assembly 18 with the output rotor assembly 4. The transmitted torque is controllable by varying the strength of the magnetic field. The clutch may be operated at any desired torque level or between any two or more torque levels by selection of appropriate energizing currents. Generally the energized clutch is operated such that the input rotor assembly 18 rotates faster than the output rotor assembly 40. This is known as operating in the slip condition and involves shearing of the magnetic bonds within the powder. The degree of slip varies inversely with the degree of magnetizing force and is also a function of the load on the output rotor assembly. Such a clutch is characterized by extremely fast response with the response time being substantially independent of the direction of rotation of the input rotor assembly.

The significance of the two-piece construction of disc 44 will now be described. After the clutch has been assembled but before it is operated, the particles 66 are more or less uniformly distributed on both sides of disc 44. When the clutch coil is energized the particles will be attracted by the resulting magnetic field. Since the inner ring of disc 44 is non-magnetic, the flux through the disc will tend to be confined to the outer magnetic ring 70. In other words the flux density between disc 44 and driving members 20 and 22 will be relatively high in the region of ring 70 and quite small in the region of ring 68. Hence the magnetic field will urge particles 66 outwardly away from shaft 40 so that the concentration of particles will be greatest between driving members 20 and 22 and the outer ring 70 of disc 44. Since ring 70 is larger in diameter than seals 62 and 64, the effect of concentrating the particles in the region of ring 70 is to diminish the concentration of particles in contact with and in the immediate vicinity of shaft 42 and seals 62 and’ 64. Thus during operation of the clutch the likelihood of particles migrating in between shaft 42 and its seals 62 and 64 is greatly reduced, with a consequent improvement in seal and clutch life. While the benefits of this invention are particularly apparent in installations where the clutch is continuously energized or is operated for long periods in the slip condition, they also are present where the clutch is operated intermittently. In this connection it is to be noted that when a clutch is deenergized, the driving members 20 and 22 will have a small residual magnetism that tends to maintain the particles in alignment with the outer ring of disc 44. The same improvement in useful life occurs if the two piece disc is used in a magnetic particle brake, even though in a brake the seals and the two armature members confronting the disc are locked against rotation relative to the housing.

The significant improvement in useful life resulting from use of the two piece disc 44 have been confirmed by comparison life tests. By way of example, clutches constructed as above described except that the disc 44 is one piece have been found to exhibit substantial seal wear due to abrasion by the magnetic particles after approximately 10 million cycles operating continuously at 75 percentage slip. Percentage slip is defined as

r.p.m. input rotor less r.p.m. output rotor r.p.m. input rotor

On the other hand, clutches of identical construction but including two piece discs as provided by the present invention have been found to have a useful life of at least 100 million cycles without any substantial seal wear and without any perceptible loss of magnetic power.

FIG. 3 shows a modification of the invention. In this modification, the same numerals are used to identify elements identical with those illustrated in FIG. 1 and 2. In this modification the output rotor assembly comprising shaft 42 and the two piece disc 44 are rotatably supported in an input rotor assembly comprising two members identified generally by the numerals 80 and 82. These two members are formed in two parts.

The member 80 consists of a rotor in the form of a sleeve 84 made of a non-magnetic alloy steel and a ring 86. The outer end of sleeve 84 (not shown) is adapted to receive bearings like bearings 46 of FIG. 1 for rotatably supporting the shaft 42. The inner end face of sleeve 84 is provided with a groove 88 in which is secured a seal 90 corresponding to seal 62 of FIG. 1. Ring 86 is made of a suitable magnetic material such as #2 relay steel and is affixed to the inner end of sleeve 84. The other member 82 of the input rotor assembly also comprises a rotor in the form of a sleeve 92. The latter also is made of a non-magnetic alloy steel and its outer end (not shown) is adapted to receive bearings like bearings 48 of FIG. 1 for rotatably supporting shaft 42. The inner end face of sleeve 92 also is provided with a groove 94 in which is secured a shaft seal 96 like seal 64 shown in FIG. 1. A ring of magnetic material 98 corresponding in size to ring 86 is affixed to the inner end of sleeve 92. The two members 80 and 82 of the input rotor assembly are secured to each other by the sleeve 24 which may be secured in any appropriate way, as for example, by screws 26. The two members 80 and 82 of the input rotor assembly are rotatably supported in the end members 4 and 6 (not shown) of the housing by means of bearings (also not shown) corresponding to bearings 28 and 30 of FIG. 1.

The two annular magnetic rings 86 and 98 have an inner diameter corresponding to the inner diameter of the ring 70 of disc 44. Rings 86 and 98 also are in line with the end members 4 and 6 and, therefore, can function as pole pieces or armatures. With this modified construction, the magnetic field is confined outwardly of the seals by coaction of the outer ring 70 of disc 44 and also by the rings 86 and 98.

3,394,784

In essence the magnetic flux of the applied magnetic field follows a circuit consisting of shell 2, end members 4 and 6, the annular rings 86 and 98, the outer ring 70 of disc 44 and the magnetic particles 66 that are interposed between the disc and the two members 80 and 82 of the input rotor assembly. The influence of this magnetic field on the magnetic particles is substantially as previously described in connection with the embodiment of FIGS. 1 and 2. Since the magnetic field is confined to the annular rings 86 and 98 and the outer ring 70 of disc 44, the particles 66 are attracted away from shaft 42 and the seals 90 and 96 while the coil is energized. Accordingly when the clutch is operated, i.e., when the coil is energized and the input and output rotor assemblies are rotating, the particles are held away from the immediate vicinity of the two shaft seals. The net result is little or no migration of magnetic particle into the two shaft seals and an improvement in seal life. It is believed to be apparent that in this modification the magnetic field is confined beyond the seals to a greater degree than is possible with the embodiment of FIGS. 1 and 2. However, this alternative modification is more costly due to the need of providing the annular rings 86 and 98 and attaching them to the sleeves 84 and 92 respectively. Preferably the annular rings are secured to the sleeves 84 and 92 by a shrink fit. However, other means of securing the rings to the sleeves are known to persons skilled in the art and may be used with equally good results.

It is to be appreciated that the present invention provides a marked improvement in shaft seal life not only in magnetic particle devices intended to function as clutches but also in magnetic particle brakes. In this connection it is to be noted that the clutch construction illustrated in the drawings may be converted to a brake by locking the armature members against rotation relative to the housing, in which case the magnetic field produced by energizing coil 8 will operate to brake rotation of shaft 40. However, in the usual brake construction the bearings 28 and 30 are omitted and the armature members 20 and 22 are made integral with end members 4 and 6.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts specifically described or illustrated, and that within the scope of the appended claims, it may be practiced otherwise than as specifically described or illustrated.