Electromagnetic clutch

An electromagnetic clutch having a bearing supported pulley provided with a compact rotor, coil and coil housing arrangement having precisely controlled and maximized air gap areas with the compactness characterized by allowing the inner diameter of the pulley to approach the outer diameter of the coil and the precise controlling and maximizing of the air gap areas characterized by the formation of a close tolerance radial air gap at substantially the outer diameter of the bearing.

This invention relates to an electromagnetic clutch and more particularly 
to an improvement in the rotor, coil and coil housing thereof for 
compacting the electromagnetic clutch and precisely controlling and 
maximizing air gap areas thereof. 
The present invention is in an electromagnetic clutch adapted for use in a 
confined compartment on a device having a rotatable shaft surrounded by a 
bearing support, a bearing surrounding and supported by the bearing 
support in a fixed location, a rotor including a pulley rotatably mounted 
on the bearing and located thereby and provided with a clutch element of 
magnetic material, a cooperating clutch element of magnetic material 
operably connected to the shaft, and electromagnetic means including a 
coil associated with the clutch element on the rotor for attracting the 
clutch element on the shaft and a coil housing cooperating with the rotor 
for containing the coil and forming a magnetic flux path. The present 
invention is directed to improvement in the rotor, coil and coil housing 
for compacting the electromagnetic clutch and precisely controlling and 
maximizing air gap areas thereof. This is accomplished in the preferred 
embodiment by the rotor and coil housing comprising opposed annular 
members surrounding the bearing support and defined by radially inner and 
outer walls which telescope in air gap defining relationship with each 
other to form an axial elongated cavity therebetween. The coil is defined 
as a cylinder contiguous with the radially outer wall of the coil housing 
but spaced from the radially inner wall thereof and axially longer than 
the coil housing to project therefrom into the cavity unsupported at the 
pulley so that the inner diameter of the pulley can be reduced to compact 
the clutch by approaching the outer diameter of the coil. The radially 
inner wall of the rotor is made axially longer than the bearing to project 
from the bearing at substantially the outer diameter thereof into the 
space formed between the coil and the radially inner wall of the coil 
housing to precisely control air gaps between both said inner and outer 
walls where they telescope with each other and to form a radial air gap of 
maximum area with the radially inner wall of the coil housing by reason of 
the circumference of the inner rotor wall being substantially the same as 
the outer diameter of the bearing. Moreover, an axial air gap is formed 
between a radial wall of the coil housing and the radially inner wall of 
the rotor to add to the magnetic flux path between the rotor and the coil 
housing adjacent its inner diameter.

Referring to the drawing, there is shown an electromagnetic clutch 10 
including a pulley 12 which is adapted to be clutched to the drive shaft 
14 of a refrigerant compressor 16 (only a portion of which is shown). The 
compressor is of a conventional type used in vehicles with the shaft 14 
rotatably supported therein and adapted to be driven by the vehicle's 
engine (not shown) on engagement of the clutch 10 with belt drive from the 
engine to the pulley 12. The shaft 14 extends through a tubular extension 
18 formed on one of the compressor heads 19 for connection with the clutch 
and is sealed within the tubular extension by a rotary seal arrangement 
20. 
The electromagnetic clutch 10 is especially adapted for use in a confined 
compartment or space such as a vehicle engine compartment and on a device 
such as the refrigerant compressor 16 whose tubular extension 18 is 
adaptive to serve as a bearing support for the clutch. The clutch bearing 
in this case is a double-row ball bearing 22 which surrounds and is 
supported by the bearing support 18 in a fixed location. The pulley 12 
forms part of the clutch rotor 13 which is rotatably mounted on the 
bearing and is located thereby and is provided with a clutch element 24 of 
magnetic material. A cooperating clutch element 26 also of magnetic 
material is operably connected to the shaft 14 and is engaged with the 
rotor's clutch element 24 by electromagnetic means including a coil 28 
which is associated with the clutch element 24 on the rotor for attracting 
the clutch element 26 on the shaft 14 and a coil housing 30 cooperating 
with the rotor for containing the coil and forming a magnetic flux path. 
Describing now the various details of the electromagnetic clutch and 
particularly the improvement in the rotor, coil and coil housing for 
compacting the clutch and precisely controlling and maximizing the air gap 
areas thereof, the clutch element 26 is an annular plate having two 
radially spaced annular rows of slots 31 and 32 which form three distinct 
concentric annular pole rings; namely a radially outer pole ring 34, a 
radially inner pole ring 36 and an intermediate pole ring 38 (see FIGS. 1 
and 2). The clutch element 26 is riveted to one end of three leaf springs 
40 which are riveted at their other or opposite end to a drive plate 42 of 
generally triangular shape, the leaf springs being chordally arranged and 
equally annularly spaced about the drive shaft axis. The drive plate 42 is 
welded at its inner diameter to a hub 44 which is fixed by a cooperating 
key 46 and slot 48 and a nut 50 to the projecting end of the compressor 
drive shaft 14. 
The other clutch element 24 which is a part of the rotor 13 is welded at 
its outer perimeter to the inner diameter of a cylindrical flange 52 which 
is formed integral with the pulley 12 and extends rightwardly therefrom as 
shown in FIG. 1. The clutch element 24 has three radially spaced annular 
rows of arcuate slots 54, 56 and 58 similar to those in the clutch element 
26 but at different or offset radial locations so as to provide with the 
edge of flange 52 four bridging concentric annular pole rings; namely a 
radially outer pole ring 60, a radially inner pole ring 62 and two 
intermediate pole rings 64, 66. The pole rings on the clutch elements 24 
and 26 are thus located relative to each other in a bridging relationship 
as shown in FIG. 1 so as to provide a 6-pole flux path and resultantly 
high clutch torque capacity on their magnetic engagement. 
The rotor's clutch element 24 is additionally formed with a cylindrical 
inner wall 70 having a stepped bore 72 in which the outer race 74 of the 
bearing 22 is press-fitted against the step 76 of the bore and further 
retained therein by stakings 78 in the bore at the outer edge of the race. 
The inner race 80 of the bearing, on the other hand, is press-fitted on a 
stepped outer cylindrical section 82 of the tubular extension 18 and is 
retained against the step 84 thereon by a snap ring 86. The rotor 13, in 
addition, includes a cylindrical ring 88 which is welded at its right-hand 
end 90 as shown in FIG. 1 to the pulley 12 near the latter's inner 
diameter, the cylindrical ring 88 together with the pulley 12 and the 
clutch element 24 including the cylindrical section 70 thus defining 
annular rotor walls which are generally U-shaped in section and include 
radially outer and inner annular walls, i.e. 88 and 70. 
The coil housing 30 similarly has radially outer and inner annular walls 94 
and 96 which in this case are joined by an integral radial wall 98. The 
inner coil housing wall 96 is press-fitted at its inner diameter on a 
stepped cylindrical section 100 of the extension or bearing support 18 
radially outward of and adjacent the concentric smaller diameter section 
82. Moreover, the coil housing 30 is located and retained against a step 
102 on the bearing support by stakings 104. 
The rotor and coil housing are thus formed with opposed annular members 88, 
94 and 70, 96 which surround the bearing support 18 and which telescope in 
air gap defining relationship with each other to form an axially elongated 
cavity 106 therebetween and inside the pulley 12. 
The coil 28 has a cylindrical configuration with an external terminal 105 
and is potted in the coil housing 30 contiguous with the radially outer 
wall 94 thereof but spaced from the radially inner wall 96 and is axially 
longer than the coil housing to project therefrom into the cavity 106 
unsupported at the pulley. As a result, the inner diameter of the pulley 
can be reduced as shown to compact the clutch by approaching the outer 
diameter of the coil 28. The radially inner wall 70 of the rotor is 
axially longer than the bearing 74 at its outer race so as to project from 
its mounting on the bearing at substantially the outer diameter thereof 
into the space formed between the coil 28 and the radially inner wall 96 
of the coil housing to precisely control the inner and outer radial air 
gaps 108 and 110 between both the inner and outer walls 70, 96 and 88, 94 
where they telescope with each other and to form the inner radial air gap 
108 of maximum area with the radially inner wall 96 of the coil housing by 
reason of the circumference of the inner rotor wall 70 being substantially 
the same as the outer diameter of the bearing. Moreover, with the 
projection of the inner wall 70 of the rotor, there is formed an axial air 
gap 112 of substantial area between the radial wall 98 of the coil housing 
and the end of the radially inner wall 70 of the rotor and thus added flux 
path area between the coil housing and rotor adjacent the bearing. 
The rotor 13, coil 28 and coil housing 30 arrangement thus compacts the 
electromagnetic clutch and in addition, precisely controls and maximizes 
the air gap areas thereof. For example, in an actual construction of the 
electromagnetic clutch shown and described above, a highly reliable 
compact unit with precisely controlled and maximized air gap area was 
obtained with the inner diameter A of the pulley 12 reduced to 3.541" 
resulting in a reduced mean belt diameter B at the pulley of 4.930" while 
the outer diameter C of the bearing 22 was maintained at 2.440". To this 
end, the inner diameter D of the radially inner rotor wall 70 at the inner 
radial air gap 108 was made of only slightly lesser diameter than the 
outer bearing diameter and measured between 2.3307" and 2.3281" to thus 
provide a 0.052" average bearing stop radial heighth at the step 76 on the 
rotor with the radial rotor thickness over the bearing (i.e. that of the 
inner rotor wall 70) then sized to carry the required magnetic flux 
without high resistance. By keeping the rotor stop for use as a bearing 
stop to the minimum, this allows the largest circumference possible at the 
inner radial air gap 108. This large circumference times the length E of 
the inner rotor wall 70 behind the bearing gives a large area for the 
magnetic flux to cross over the inner radial air gap 108 from the rotating 
rotor to the stationary coil housing 30 at the inner coil housing wall 96. 
This air gap whose dimension is F has close tolerance because there are 
only two diameters that control it and thus close control on the length of 
path that the flux must jump from the rotating rotor to the stationary 
coil housing and was set at 0.0073" to 0.0110" so that it carries the 
majority of the magnetic flux. On the other hand, the dimension G 
determines the bearing stop to coil housing stop distance while the 
dimension H denotes the thickness of the coil housing's radial wall 98 
leaving the dimension E which is the length of the rotor's inner radial 
wall past the bearing to determine the dimension I of the axial air gap 
112. The remainder of the magnetic flux was made to jump from the rear of 
the rotor to the inside of the coil housing across the axial air gap 112 
by then varying its dimension I from 0.038" to 0.005". The outer radial 
air gap 110 on the other hand, is at such a large diameter as compared to 
both the radially inwardly located air gaps 108 and 112 that sufficient 
flux path area is obtained across the axial length of rotor ring 88 at 
approximately the inner diameter of the pulley and with a relatively large 
magnetic flux jump of 0.010" to 0.015". 
The above described preferred embodiment is illustrative of the invention 
which may be modified within the scope of the appended claims.