ELECTROMAGNETIC CLUTCHES

Abstract: An electromagnetic clutch (l) comprises a rotor (2), an armature plate (l0) and an electromagnet (5). The armature plate (l0) is disposed to face the rotor (2) with an axial gap. In one embodiment, the rotor (2) and the armature plate (l0) are provided with frictional surfaces (2a,l0a) respectively each of which is formed with spirally curved depressions and projections (l4) with a certain pitch. At least one of the frictional surfaces, such as (2a), has a layer of metal plating (l3) with lower hardness than the magnetic materials of the rotor (2) and armature plate (l0). In another embodiment, either of the frictional surfaces (2a,l0a) is provided with said spirally curved depressions and projections, and said metal plating (l3), the other of the frictional surfaces (2a,l0a) being provided with a plurality of irregularly formed small depressions and projections, such as those formed by grinding. High torque transfer is obtained.

Description: This invention relates to electromagnetic clutches, and more particularly, relates to frictional surfaces for rotatable members of the electromagnetic clutches. Electromagnetic clutches are well known in the prior art and are often used for controlling the transfer of power from an automobile engine to the refrigerant compressor of an automobile air conditioning system. With reference to Figure l, an electromagnetic clutch of the type in the prior art will be described, Figure l being a cross-sectional view showing an electro­ magnetic clutch l mounted on a refrigerant compressor housing 3. The electromagnetic clutch l has a ring-shaped rotor 2 rotatably supported on tubular projecting portion 3a of the compressor housing 3 through radial bearing 4. An electromagnet 5 is stationarily disposed in hollow portion 6 which is formed in rotor 2 and fixed on the compressor housing 3 by means of bolts. Electromagnet 5 comprises annular U-shaped housing 5a and electro­ magnetic coil 5b contained in housing 5a. Drive shaft 7 is rotatably supported in compressor housing 3 through a radial bearing (not shown) which is mounted in tubular projecting portion 3a. Hub 8 is secured on the outer end of drive shaft 7 which extends from tubular projecting portion 3a and connected to ring-shaped armature plate l0 through a plurality of leaf springs 9. Armature plate l0 is thus supported by leaf springs 9 around hub 8 with a radial gap and faces annular pole faces 2a of rotor 2 with an axial gap. Armature plate l0 may be disposed between rotor 2 and a flange portion on hub 8. Also armature plate l0 is provided with concentric annular magnetic pole faces l0a. In this construction of electromagnetic clutch, when coil 5b of electromagnet 5 is energised, magnetic flux is generated and passed through annular housing 5a, rotor 2, armature plate l0, rotor 2 and housing 5a. Armature plate l0 is thus attracted to pole faces 2a of rotor 2. Accordingly, the rotational force of rotor 2 is transmitted to drive shaft 7 through armature plate l0. On the other hand, when coil 5b of electromagnet 5 is de-energised, the magnetic flux disappears, and armature plate l0 is disconnected from pole faces 2a of rotor 2 by recoil strength of leaf springs 9. Therefore, the rotational force of rotor 2 is not transmitted to drive shaft 7. In the above-mentioned electromagnetic clutch, a frictional surface defined by pole faces 2a of rotor 2 and a frictional surface defined by pole faces l0a of armature plate l0 are formed by turning, and then plated and thereafter treated by grinding. These frictional surfaces are thus provided with irregular depressions and projections as shown in Figure 2. The initial contact area between the opposed rotor 2 and armature plate l0, when their frictional surfaces are produced by grinding, is only below twenty percent of their potential area of contact. Referring to Figure 3, when the frictional surfaces of rotor 2 and armature plate l0 are initially engaged with one another, wedge-shaped projection 2b of rotor 2 and wedge-shaped projection l0b of armature plate l0 scratch each other. Therefore, a large frictional force is produced therebetween. However, shears are generated on the top of the projections by engagement of rotor 2 and armature plate l0. Accordingly, after projections 2b and l0b have been engaged two or three times, the tops of the projections 2b and l0b are shaved away, and the torque transfer is decreased. Thereafter, the area of contact between rotor 2 and armature plate l0 is gradually increased by abrasion caused by continuous engagement, and thus the torque transfer of electromagnetic clutch l is increased. Finally, the torque transfer of electromagnetic clutch l becomes three or four times its required (initial) value. However, when the number of engagement time is less than a certain number, e.g. fifty times, the torque transfer of electromagnetic clutch l is not increased. Also, in general, the materials of the frictional surfaces have the same hardness. Therefore, if there is only partial contact between the frictional surfaces in the initial stage of engagement in an electromagnetic clutch, a weld deposit with high hardness is formed between the frictional surfaces. The frictional surfaces become rough. Possible solutions to obtain large torque transfer in the initial stage of engagement, such as below fifty times, are to enlarge the area of the frictional surfaces by increasing dimension D in Figure l, or to increase the magnetic force by enlarging the sectional area of coil 5b, but these solutions increase the size and weight of the electromagnetic clutch. A construction for improvement of torque transfer without change of dimension of an electromagnetic clutch is shown in Japanese patent application publication no: 54-l47,345. The rotor or armature plate has a thin layer made of soft metal, which is softer and has a higher frictional factor than magnetic material forming both frictional surfaces. An electromagnetic clutch, according to the present invention, comprises a first rotatable member rotatably supported on a bearing, said first rotatable member having an axial end magnetic frictional surface, a second rotatable member rotatably supported on a second bearing, an annular magnetic armature member supported on said second rotatable member so as to be capable of limited axial movement, and having an axial end magnetic frictional surface which faces said axial end magnetic frictional surface of said first rotatable member with an axial gap therebetween, and electromagnetic means for attracting the facing frictional surfaces into contact with one another, characterised in that at least one of said facing frictional surfaces of said first rotatable member and said armature member is provided with spirally curved depressions and projections with a predetermined pitch and further provided with a layer of metal plating with lower hardness than the magnetic materials of said rotatable member and said armature member. The electromagnetic clutch of this invention can have a high level of torque transfer, can be compact and light in weight, and can at the same time be low in cost and high in effectiveness. Said layer of metal plating can be galvanising, or said layer of metal plating can be tin and zinc plating. In one embodiment of the present invention, both of said facing frictional surfaces are provided with said spirally curved depressions and projections, but only one of said facing frictional surfaces is further provided with said layer of metal plating. In another embodiment of the present invention, either one of said facing frictional surfaces is provided with said spirally curved depressions and projections and further provided with said layer of metal plating, the other of said facing frictional surfaces being provided with a plurality of irregularly formed small depressions and projections. Said irregularly formed small depressions and projections may be formed by grinding or pressing. Electromagnetic clutches, according to the present invention, will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure l is a vertical cross sectional view of an electromagnetic clutch known in the prior art; Figure 2 is an explanatory rough sketch illustrating a frictional surface of an armature plate or a rotor of the electromagnetic clutch of Figure l; Figure 3 is an explanatory cross sectional view illustrating frictional surfaces of an armature plate and a rotor when those are fitted in initial stage in the electromagnetic clutch of Figure l; Figure 4 is a vertical cross sectional view of an electromagnetic clutch embodying either embodiment of the present invention; Figure 5 is an explanatory front view illustrating a frictional surface of an armature plate or a rotor of a first embodiment of the present invention; Figure 6 is an explanatory enlarged cross sectional view illustrating the frictional surfaces of an armature plate and a rotor of the first embodiment of the present invention; Figure 7 is an explanatory enlarged cross sectional view illustrating the initial stage of fitting between the frictional surfaces of the armature plate and the rotor of Figure 6; Figure 8 is an explanatory enlarged cross sectional view illustrating the result of contact between the frictional surfaces of the armature plate and the rotor of Figure 6; Figure 9 is a graph which illustrates change of torque transfer corresponding to the engagement times of the rotor and the armature plate in electromagnetic clutches of the prior art and the first embodiment of the present invention; Figure l0 is an explanatory enlarged cross sectional view illustrating a frictional surface of a rotor of a second embodiment of the present invention; Figure ll is an explanatory enlarged cross sectional view illustrating the initial stage of fitting between the frictional surfaces of the armature plate and the rotor of Figure l0; Figure l2 is an explanatory enlarged cross sectional view illustrating the result of contact between the frictional surfaces of the armature plate and the rotor of Figure l0; and Figure l3 is a graph which illustrates change of torque transfer corresponding to the engagement times of the rotor and the armature plate in electromagnetic clutches of the prior art and the second embodiment of the present invention. With reference to Figures 4 and 5, there is shown an electromagnetic clutch of the present invention which is similar to the electromagnetic clutch shown in the Figure l, except for the construction of the rotor and armature plate. Therefore, similar parts are represented by the same reference numbers as shown in Figure l and a detailed description of these parts is omitted. In this embodiment of the present invention rotor 2 is formed of an integrated body of magnetic material, such as steel, and comprises inner cylindrical portion 2l, outer cylindrical portion 22 and end plate portion 23 which is connected at an axial end of the rotor 2 between both cylindrical portions 2l and 22. End plate portion 23 is provided with a plurality of concentric slits 23a,23b. These slits form between them concentric annular magnetic pole faces 2a which define a frictional surface. The frictional surface of pole faces 2a of rotor 2 is finished by machining and is then plated. The front end view of the frictional surface of a pole face 2a after machining is shown in Figure 5, i.e. small spirally curved depressions and projections are formed on the frictional surface of pole faces 2a of rotor 2 which face armature plate l0. Rotor 2 is then disposed in a plating tank to form a layer of plating. Thus, layer l3 of plating is formed on frictional surface of pole faces 2a, as shown in Figure 6. Layer l3 of plating may be treated on the surface by electric zinc galvanising, and has forty five degree as Brinell hardness. Armature plate l0 is also formed of magnetic material, such as steel plates, and has sixty two degree as Brinell hardness. Similar spirally curved depressions and projections are formed on the frictional surface of the pole faces l0a of armature plate l0. The spirally curved depressions and projections formed on rotor 2 and armature plate l0 have a certain interval, or predetermined pitch, e.g. from 0.3 mm to 0.5 mm, and extend in the same spiral direction as each other. The depressions and projections are easily formed by machining. When magnetic coil 5b is energised, armature plate l0 which is connected with drive shaft 7 is attracted to axial end plate 23 and is engaged with the frictional surface of pole faces 2a of rotor 2 by magnetic flux which is generated by coil 5b of electro­ magnet 5. Thus, driving force is transmitted to drive shaft 7 from rotor 2 through armature plate l0. In this operation, when armature plate l0 and rotor 2 initially engage, as shown in Figure 7, projections l4 formed on frictional surface of pole faces l0a of armature plate l0 gnaw into layer l3 of plating due to attractive force, whereas the projections of layer l3 of plating are deformed and the contact between both frictional surfaces is thus securely close. Furthermore, when rotor 2 and armature plate l0 engage each other, layer l3 of plating is partly sloughed off, as shown in Figure 8, and portions ll adhere on the frictional surface of armature plate l0. Adherent portions ll on the armature plate l0 grow due to repeated engagement of armature plate l0 and rotor 2. Adhesion means that small projections of the frictional surface of armature plate l0 turn layer l3 of plating on the frictional surface of rotor 2 thin, the sloughed off metal being locally gathered, pressed, combined and thus coagulated. At the same time, shaved off portions l2 are formed on layer l3 of plating of the frictional surface of rotor 2. Adherent portion ll and shaved off portion l2 are formed on the same circumference at several locations which face each other, and also are formed arc-shaped. Adherent portion ll and shaved off portion l2 are formed on initial engagement. Adherent portion ll and shaved off portion l2 produce sufficient frictional force to prevent the decreasing of transmission torque even though attractive force to armature plate l0 is lessened by decreasing the magnetic force. Therefore, after armature plate l0 has been attracted to rotor 2, the volume of electric current supplied to coil 5b of electromagnet 5 may be decreased. The reason for increase of transmission torque is that adherent portion ll and shaved off portion l2 are made by forcedly tearing the same body off, the engagement between adherent portion ll and shaved off portion l2 maintaining close contact. When adherent portion ll and shaved off portion l2 are engaged with one another, resistance force against this tearing is added to the frictional force between adherent portion ll and shaved off portion l2, and thus the transmission torque is made large. The transmission torque of electromagnetic clutch is not affected by the co-efficient of friction of the plating metal. Therefore, a plating metal which has lower hardness and is more inexpensive than magnetic materials should be utilised, e.g. zinc, or tin and zinc alloy. Referring to Figure 9, change of transmission torque within five thousand times of engagement is shown. In the embodiment of this invention, layer l3 of plating on rotor 2 is shaved off by projections l4 of the frictional surface of armature plate l0, and eventually disappears. Therefore, transmission torque of this invention is equal to that of a conventional embodiment. Also, in a conventional embodiment, a deposit is produced on the frictional surfaces of rotor 2 and armature plate l0 within fifty times of engagement, and hard facing and failure by heating may be produced. On the other hand, in the electromagnetic clutch of this invention, adherent portions of the plating metal are dispersedly produced between the frictional surfaces of rotor 2 and armature plate l0. Transmission torque is thus surely increased by the adherent portions. The direction of the spirally curved depressions and projections of rotor 2 may be the same as the direction of rotation of rotor 2 or be opposite to the direction of rotation of rotor 2. Also, the direction of the spirally curved depressions and projections of rotor 2 and armature plate l0 may be opposite each other. A second embodiment of the present invention will now be described with reference to Figures l0 to l3. In fact, the above recited description of the manner of operation of the first embodiment applies equally to the second embodiment, even though in the second embodiment only the rotor 2 has been formed with the above recited spirally curved depressions and projections. The frictional surface of the pole faces l0a of armature plate l0 in the second embodiment is finished by grinding as in the known prior art. Therefore, a plurality of irregularly formed small depressions and projections l4' are formed on the pole faces l0a. The projections l4' gnaw into the layer l3 of metal plating on the rotor 2 in the same manner as outlined hereinbefore. It should be noted, however, that Figures ll and l2 are of an even greater magnification than Figures 6 and 7. The electromagnetic clutches of the present invention are not restricted to the embodiments described hereinabove, for example the electromagnet could be re-located to be associated with the armature plate rather than the rotor.

Claims: 1. An electromagnetic clutch (l) comprising a first rotatable member (2) rotatably supported on a bearing (4), said first rotatable member having an axial end magnetic frictional surface (2a), a second rotatable member (7) rotatably supported on a second bearing, an annular magnetic armature member (l0) supported on said second rotatable member (7) so as to be capable of limited axial movement, and having an axial end magnetic frictional surface (l0a) which faces said axial end magnetic frictional surface (2a) of said first rotatable member (2) with an axial gap therebetween, and electro­ magnetic means (5) for attracting the facing frictional surfaces (2a,l0a) into contact with one another, characterised in that at least one of said facing frictional surfaces (2a,l0a) of said first rotatable member (2) and said armature member (l0) is provided with spirally curved depressions and projections with a predetermined pitch and further provided with a layer of metal plating (l3) with lower hardness than the magnetic materials of said rotatable member (2) and said armature member (l0). 2. An electromagnetic clutch according to claim l, characterised in that said layer of metal plating (l3) is galvanising. 3. An electromagnetic clutch according to claim l, characterised in that said layer of metal plating (l3) is tin and zinc plating. 4. An electromagnetic clutch according to any one of claims l to 3, characterised in that both of said facing frictional surfaces (2a,l0a) are provided with said spirally curved depressions and projections (l4), but only one of said facing frictional surfaces (2a) is further provided with said layer of metal plating (l3). 5. An electromagnetic clutch according to any one of claims l to 3, characterised in that either one of said facing frictional surfaces (2a) is provided with said spirally curved depressions and projections and further provided with said layer of metal plating (l3), the other of said facing frictional surfaces (l0a) being provided with a plurality of irregularly formed small depressions and projections (l4'). 6. An electromagnetic clutch according to claim 5, characterised in that said irregularly formed small depressions and projections (l4') are formed by grinding or pressing.

Inventors: KOITABASHI, TAKATOSHI
Applicants: SANDEN CORPORATION
Application year: 1986
Main classification: F16D27
IPC codes: F16D27