Abstract:
Disclosed is an electromagnetic coupling designed to strengthen the magnetic force exerted from an electromagnetic coil in the direction in which a pilot clutch is operated. In the electromagnetic coupling with the pilot clutch for activating a main clutch wherein clutch plates are thrust with a retaining ring moved by magnetic force produced by an electromagnetic coil in the pilot clutch, the electromagnetic coil in the pilot clutch is surrounded with ferromagnetic material over at least two of its three facial sides other than its magnetic force exerting facial side, namely over at least its rear and outer facial sides and the ferromagnetic material is covered with nonmagnetic material.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an electromagnetic coupling for operating a main clutch into ON (connecting) and OFF (disconnecting) states by means of a pilot clutch operated by electromagnetic force.  
         [0003]     2. Description of the Prior Art  
         [0004]     A conventional electromagnetic coupling of this type is made up, for example, as shown in  FIG. 1 . Referring to the Figure, the electromagnetic coupling includes an input shaft  1  that is connected to an engine (not shown) side, an output shaft  2  for transmitting power to rear wheels, and a main clutch  3  for connecting and disconnecting the input shaft  1  to and from the output shaft  2 . The main clutch  3  comprises inner plates  3   a  whose inner diameter sides are coupled in spline engagement with the outer periphery of the input shaft  1  and outer plates  3   b  whose outer diameter sides are coupled in spline engagement with the inner periphery of a casing  2   a  united to the output shaft  2  in a manner to be described later, the inner and outer plates  3   a  and  3   b  being alternately positioned. A fixed plate  4  is fastened to an end of the casing  21  and a thrust means  5  has a push plate  6  such that pushing the inner and outer plates  3   a  and  3   b  against the fixed plate  4  with the push plate  6  under a pressure engages the input shaft  1  with the output shaft  2  and removing the pushing pressure or thrust disengages the output shaft  2  from the input shaft  1 . See, e. g., JP 2002-266904 A (pages 5 to 6 and  FIG. 1 ).  
         [0005]     The thrust means  5  comprises the abovementioned push plate  6 , a backing plate  7  disposed behind the push plate  6 , and a thrust generating mechanism  8  for actuating the push plate  6  to push with respect to the backing plate  7  via balls disposed between them.  
         [0006]     The inner diameter side of the push plate  6  is coupled in spline engagement with the input shaft  1 . And, the backing plate  7  with its end face supported by a thrust bearing  16  is supported rotatably relative to a boss section  2   b  of the output shaft  2  and has its inner diameter side fitted with the outer periphery of the input shaft  1  so that it can be rotated while in sliding contact with the latter. Further, this backing plate  7  is coupled via a pilot clutch  9  lying outwards thereof to the casing  2   a  united with the output shaft  2  so that it can engage with and disengage from the casing  2   a  in a direction in which it is rotated.  
         [0007]     In the pilot clutch  9 , a retaining ring  10  is axially inserted into an annular space formed between the outer periphery of the backing plate  7  and an inner periphery of the casing  2   a  so that it can be axially moved in sliding contact with these peripheries in that space to an extent that it comes into contact with a stop ring  17 . And, between the retaining ring  10  and a rotor section  2   a ′ of the casing  2   a  there are positioned alternately clutch plates  9   a  of a first group whose inner diameter sides are coupled in spline engagement with the outer periphery of the backing plate  7  and clutch plates  9   b  of a second group whose outer diameter sides are coupled in spline engagement with the inner periphery of the casing  2   a . And, axially rearwards (at the left hand side as shown) of the rotor section  2   a ′ of the casing  2   a  there is provided a ring shaped electromagnetic coil  11  for electromagnetically attracting the retaining ring  10  towards the rotor section  2   a ′ of the casing  2   a  to bring the clutch plates  9   a ,  9   b  into pressure contact with one another, thereby turning the pilot clutch  9  into its ON state.  
         [0008]     The electromagnetic coil  11  is disposed in a ring shaped chamber  12  formed by a stepped recess formed in the casing  2   a  and an outer periphery of the boss section  2   b  of the output shaft  2  and arranged therein so that it is juxtaposed with the pilot clutch  9  across the rotor section  2   a ′. And, this electromagnetic coil  11  is supported from a cover member  13  which is securely connected to a frame (not shown) and which is fitted over and rotatably coupled to the output shaft  2  via a bearing  18 .  
         [0009]     Opposed to the electromagnetic coil  11 , the rotor section  2   a ′ of the casing  2   a  and the flange  2   b ′ of the boss section  2   b , which are positioned diametrically outwards and inwards, respectively, as separate members, are united together with a ring of nonmagnetic material  14  that is composed of stainless steel as a nonmagnetic material. Also, the clutch plates  9   a ,  9   b  of the pilot clutch  9  are provided at respective diametrical positions thereof flush with the inner and outer peripheral surfaces of the ring of nonmagnetic material  14  with cutouts  19  and bridges (not indicated) interconnecting them for preventing a magnetic force on a magnetic loop  15  of the electromagnetic coil  11  from short-circuiting.  
         [0010]     The electromagnetic coupling constructed as mentioned above operates as stated below.  
         [0011]     Turning current conduction to the electromagnetic coil  110 N generates the magnetic loop  15  in which the retaining ring  10  is attracted magnetically by the electromagnetic coil  11 . This causes the clutch plates  9   a  and  9   b  in the pilot clutch  9  to be pushed against the rotor section  2   a ′ of the casing  2   a  whereby the pilot clutch  9  is turned into its ON state and the backing plate  7  in the thrust means  5  is then brought into engagement with the casing  2   a , namely with the output shaft  2 . In a 4WD coupling, its 4 WD state is taken by this state.  
         [0012]     If in this state a difference in speed of rotation between the input and output shafts  1  and  2  is produced due to a slip of the rear wheels or the like, then a corresponding difference in speed of rotation is produced between the backing plate  7  in engagement with the output shaft  2  and the push plate  6  in engagement with the input shaft  1 . Then, the thrust generating mechanism  8  is brought into operation by such a difference in speed of rotation to push the push plate  6  with respect to the backing plate  7 , thereby turning the main clutch  3  into its ON state. Thus, the input and output shafts  1  and  2  are interconnected and rotated in one united body.  
         [0013]     The abovementioned thrust generating mechanism  8  comprises balls  8   a  each of which as shown in  FIG. 2A  is rollably interposed between a pair of opposed reentrant cam surfaces  6   a  and  7   a  of the push and backing plates  6  and  7 . When a difference in speed of rotation is produced between the push and backing plates  6  and  7 , the ball  8   a  tends to run on the reentrant cam surfaces  6   a  and  7   a  as shown in  FIG. 2B  with the result that the push plate  6  is moved away from the backing plate  7  in the direction of the arrow shown. Consequently, the plates  3   a ,  3   b  of the main clutch  3  is moved and pushed to the fixed plate  4 .  
         [0014]     The pilot clutch in a conventional electromagnetic coupling as mentioned above must have its capacity increased to meet with the need for a greater torque transmission of the main clutch. If a large torque is to be transmitted in the main clutch, an insufficient capacity of the pilot clutch creates troubles such as that the clutch slips. Also, in strengthening the magnetic force, measures have hitherto be taken of devising the configurations and thicknesses of inner and outer diametrical parts of the housing in which the electromagnetic coil is accommodated. As a result, a considerable number of man-hour and its attendant time period have so far been required in manufacturing an electromagnetic coupling.  
         [0015]     On the other hand, while increasing the magnitude of electric current to the electromagnetic coil may strengthening the magnetic field it produces to increase the electromagnetic force, the magnetic saturation of a magnetic material imposes a limitation on increasing the electromagnetic force by increasing the current magnitude.  
       BRIEF SUMMARY OF THE INVENTION  
       [0016]     The present inventors in their zealous research to solve the problems mentioned above have ascertained through several simulations that surrounding an electromagnetic coil with a ferromagnetic material increases the force of attraction of the electromagnetic coil to a pilot clutch.  
         [0017]     Accordingly, there is provided in accordance with the present invention an electromagnetic coupling with a pilot clutch for activating a main clutch wherein clutch plates are thrust with a retaining ring moved by magnetic force produced by an electromagnetic coil in the pilot clutch, characterized in that the electromagnetic coil in the pilot clutch is surrounded with ferromagnetic material over at least two of its three facial sides other than its magnetic force exerting facial side, namely over at least its rear and outer facial sides and further that the magnetic material is covered with nonmagnetic material.  
         [0018]     In the electromagnetic coupling mentioned above, at least one of the retaining ring and the clutch plate in the pilot clutch may be made of ferromagnetic material; and the ferromagnetic material mentioned above is preferably one of materials selected from the class which consists of a spherical graphite cast iron having a chemical composition containing by weight 2.7 to 3.9% of C, 3.3 to 4.8% of Si, 0.3 to 1.2% of Mn, P≦0.1%, S≦0.1%, 0.01 to 0.1% of Mg, 0.01 to 0.1% of at least one of Ce and La and the balance Fe and having a carbon equivalent (C-E)≧4.3 and a silicon steel containing by weight 2.8 to 3.3% of Si, not greater than 1.0% of Al, 0.1 to 0.2% of Mn, not greater than 0.002% of C and the balance Fe. Further, the spherical graphite cast iron may have 0.1 to 1.5% by weight of Mo added thereto  
         [0019]     According to the present invention, the magnetic force exerted from an electromagnetic coil in the direction in which a pilot clutch is operated can be strengthened. Also, the force exerted by the electromagnetic coil to attract the retaining ring and clutch plates can be increased. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     These and other objects, features and advantages of the present invention as well as other manners of its implementation will become more readily apparent, and the invention itself will also be better understood, from the following detailed description when taken with reference to the drawings attached hereto showing certain illustrative forms of implementation of the present invention. In the drawings:  
         [0021]      FIG. 1  is a cross sectional view illustrating a typical electromagnetic coupling in the prior art;  
         [0022]      FIGS. 2A and 2B  are each a cross sectional view illustrating the operation of a thrust generating mechanism in the conventional electromagnetic coupling shown in  FIG. 1 ;  
         [0023]      FIG. 3  is a cross sectional view illustrating a form of implementation of the electromagnetic coupling according to the present invention;  
         [0024]      FIG. 4  is a cross sectional view illustrating another form of implementation of the electromagnetic coupling according to the present invention; and  
         [0025]      FIGS. 5, 6 ,  7  and  8  are graphs comparing a preferred and a conventional composition of ferromagnetic material in respect of magnetic characteristic, magnetic saturation, magnetic permeability and magnetic hysteresis, respectively. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Suitable forms of implementation of the present invention are described below with reference to  FIG. 3  ff. In the description to follow, it should be noted that the same components as in the prior art shown in and described in connection with  FIG. 1  are indicated by the same reference characters and their repeated descriptions are omitted.  
         [0027]     Referring to  FIG. 3 , there is shown a casing  21  in which acting parts for the main and pilot clutches  3  and  9  are accommodated including the retaining ring  10 , the casing  21  being united with the output shaft  2 . Coupled in spline engagement with inner peripheries of the casing  21  are the outer diametrical ends of the outer plates  3   b  in the main clutch  3  and the outer diametrical ends of the clutch plates of one group  9   a  in the pilot clutch  9 . And, between the rotor section  21   a  of the casing  21  and the flange  2   b  of the boss section  2   b  of the output shaft  2  there lies, here also, the ring of nonmagnetic material  14 , made of stainless steel and fitted to unite them together. Further, the retaining ring  10  and the clutch plates  9   a ,  9   b  of the pilot clutch  9  are each composed of a ferromagnetic material, of which use is preferably made of a high-silicon spherical graphite cast iron excelling in electromagnetic properties, by having a chemical composition containing by weight 2.7 to 3.9% of C, 3.3 to 4.8% of Si, 0.3 to 1.2% of Mn, P≦0.1%, S≦0.1%, 0.1 to 1.5% of Mo, 0.01 to 0.1% of Mg, 0.01 to 0.1% of at least one of Ce and La and the balance Fe and having a carbon equivalent (C-E)≧4.3 as disclosed in JP 2001-33470 A by the present applicant. The spherical graphite cast iron so composed is also a material that is low in remanence and magnetic hysteresis.  
         [0028]     A cover member  22  with which the output shaft  2  is covered and coupled rotatably via the bearing  18  is securely connected to the frame (not shown) and made of a nonmagnetic material such as Al, or synthetic resin which in the form of implementation illustrated is nylon (registered trade mark). Within the cover member  22 , an electromagnetic coil  23  is disposed so that its magnetic force exerting facial side (at its front) is opposed to the clutch plates  9   a ,  9   b  of the pilot clutch  9  and that other three sides thereof, namely its rear side, and inner and outer peripheral sides, are surrounded with a ferromagnetic material  24  disposed in the cover member  22 . And, to be so disposed, the electromagnetic coil  23  and the ferromagnetic material  24  are molded together into the cover member  22  made of resin. Electrical leads connected to the electromagnetic coil  23  are not shown.  
         [0029]     The ferromagnetic material  24  with which the electromagnetic coil  23  is to be surrounded may be composed of the high silicon spherical graphite cast iron of which the retaining ring  10  is also preferably composed.  
         [0030]     According to this configuration, looped lines of magnetic force generated around the inner and outer peripheries of the magnetic coil by energizing the magnetic coil  23  with electric current are confined within the cover member  22  made of nonmagnetic material while being emanated through the magnetic material from the rear side of the electromagnetic coil along those inner and outer peripheries towards the clutch plates  9   a ,  9   b  of the pilot clutch  9 .  
         [0031]     Then, with the electromagnetic coil  23  surrounded over its three facial sides with the magnetic material  24 , the lines of magnetic force from the electromagnetic coil are exerted efficiently. As a result, the electromagnetic coil for emanating lines of magnetic force of a desired strength can be made smaller in size and, in other words, an electromagnetic coil made smaller in size can be manufactured which produces a force of attraction that is sufficiently large.  
         [0032]     While in the form of implementation in  FIG. 3  the electromagnetic coil  23  is shown surrounded over three facial sides thereof with the ferromagnetic material  24 , it may be surrounded over two facial sides thereof, e.g., its rear and outer peripheral sides, with the ferromagnetic material  24  in another form of implementation shown in  FIG. 4  so that its inner peripheral side is opposed to the outer periphery of the output shaft  2  united with the casing  21 . The output shaft  2  is here composed of a carbon for machine construction, e.g., S25C.  
         [0033]     A high silicon spherical graphite cast iron mentioned above is not only a ferromagnetic material but also a material low in magnetic hysteresis. It is compared with a carbon steel for machine construction as the conventional material so far commonly used, with respect to magnetic property (BH curve), magnetic saturation (Bs), magnetic permeability (μ) and hysteresis as shown in  FIGS. 5, 6 ,  7  and  8 , respectively. In this comparison, the carbon steel for machine construction is S25C while the high silicon spherical graphite cast iron has a composition containing by weight 3.33% of C, 4.23% of Si, 0.25% of Mn, 0.021% of P, 0.008% of S, 0.036% of Mg, 0.011% of Ce and the balance Fe.  
         [0034]     In  FIG. 5  which shows the magnetic property taken for magnetic flux density (B) with respect to magnetic field (H), curve a represents the BH curve of the conventional material and curve b represents that of high silicon spherical graphite cast iron, from which it is seen that the latter is superior to the former in the BH property.  
         [0035]     In  FIG. 6  which shows magnetic saturation (B), curve c represents that of the conventional material and curve d represents that of high silicon spherical graphite cast iron, from which it is seen that the latter is greater than the former in magnetization.  
         [0036]     In  FIG. 7  which shows magnetic permeability (μ), curve e represents that of the conventional material and curve f represents that of high silicon spherical graphite cast iron, from which it is seen that the latter produces greater magnetization than the former with a small current when the coil current is relatively small.  
         [0037]     In  FIG. 8  which shows magnetic hysteresis, curve g represents that of the conventional material and curve h represents that of high silicon spherical graphite cast iron, from which it is seen that the latter is smaller than the former in both coercive force (Hc) and residual flux density (Br).  
         [0038]     The retaining ring  10  and the clutch plates  9   a ,  9   b  of the pilot clutch  9  and the ferromagnetic material  24  may alternative to high silicon spherical graphite cast iron be composed of a silicon steel. The silicon steel should preferably has a chemical composition containing by weight 2.8 to 3.3% of Si, not greater than 1.0% of Al, 0.1 to 0.2% of Mn, not greater than 0.002% of C and the balance Fe. Such silicon steel has an elongation of 20% and can well withstand a press forging. This material is not as good as the high silicon spherical graphite cast iron with respect to electromagnetic characteristics, but is high in productivity and inexpensive.  
         [0039]     The addition of Mo is not indispensable as far as a part or component of the high silicon spherical graphite cast iron composition is allowed to work at the room temperature and its electromagnetic characteristics are also unaffected by no addition of Mo. In an environment, however, where its temperature rises to 300° C. or more, the part or component may be embrittled and broken. In order to preclude this possibility, the addition of Mo in an amount of 0.1 to 1.5% by weight has been found effective. Thus, the tendency of high silicon spherical graphite cast iron to embrittle can be eliminated by adding thereto Mo in an amount of, say, 1.0% by weight. Also, while in the form of implementation illustrated above both the retaining ring  9  and the clutch plates  9   a ,  9   b  are shown to be composed of ferromagnetic material, one of them may be composed of a carbon steel for machine construction such as S25C.  
         [0040]     Although the present invention has hereinbefore been set forth with respect to certain illustrative embodiments thereof, it will readily be appreciated to be obvious to those skilled in the art that many alterations thereof, omissions therefrom and additions thereto can be made without departing from the essences and scope of the present invention. Accordingly, it should be understood that the invention is not intended to be limited to the specific embodiments thereof set forth above, but to include all possible embodiments that can be made within the scope with respect to the features specifically set forth in the appended claims and to encompass all the equivalents thereof.