Electromagnetic clutch

An electromagnetic clutch 11 including a rotor assembly 14 with an attraction surface 14d, a field coil assembly with an electromagnetic coil 15 and a coil support yoke 16, and an armature assembly 19. The armature assembly has an armature hub 20 which can be connected to a driven shaft 18, a stopper plate 21 connected to the hub 20, a ring shaped armature 24 and a damper which connects the armature to the stopper plate. The damper has a rubber damper 27 which is held in a damper cover 30a, 30b or 125b. The damper cover includes an open side with bevelled edges. The bevelled edges have an arc-shaped area 30e, 30f or 125c that prevents stress contractions in the end of the rubber damper 27 or 126 to prevent cracking of the vulcanized rubber. A rivet 25 or 124 may also be in the rubber damper. A free end of the rivet has a bevelled arc-shaped area 25a or 124a that limits stress concentrations to prevent cracking of the rubber damper 27 or 126. The bevelled areas prevent cracking of the rubber damper without increasing the axial length of the electromagnetic clutch 11.

TECHNICAL FIELD 
This invention pertains to an electromagnetic coupling device which 
connects and disconnects a rotating torque transmission device and a 
driven device by magnetizing and demagnetizing an electromagnetic coil. 
BACKGROUND OF THE INVENTION 
Electromagnetic clutches and brakes are used extensively in a variety of 
machines. They are used in stationary machines and on vehicles. Use on 
vehicles includes drives for air conditioning system compressors, air 
compressors, and mechanical superchargers. These clutches can take many 
forms. Most of the clutches include a field coil assembly, a rotor 
assembly and an armature assembly. 
The field coil assembly is often mounted in a fixed position. A rotor 
assembly is rotatably mounted adjacent to the field coil in a position to 
form a portion of a magnetic flux path. The rotor assembly includes a 
drive engaging means such as a v-belt or power band engaging surface, a 
sprocket for a chain drive or a gear for gear drive. The rotor assembly 
also has an attraction surface. The armature assembly includes an armature 
hub, a stopper plate, an armature and a damper. The armature hub is 
adapted for mounting on a driven shaft. The stopper plate is attached to 
the armature hub and has generally radially extending arms which support 
the armature. There are usually three arms that support the armature in 
three places. The number of arms can vary. The damper includes rubber 
members which connect the armature to the arms of the stopper plate. The 
rubber members allow the armature to move into contact with the attraction 
surface on the rotor assembly when the field coil assembly is energized. 
The rubber members in the damper pull the armature away from the 
attraction surface when power to the field coil assembly is cut off. The 
damper assembly also deforms and absorbs the sudden increases in torque 
when the field coil assembly is energized. In addition to the high peak 
torques that are absorbed by the damper assembly when the field coil is 
energized, the damper assembly absorbs torsional vibrations from the power 
source and from the driven machine. 
The rubber members included in the damper tend to crack, around the outside 
edges where the rubber contacts a metal retainer and in the center where 
the rubber contacts a central support post. The cracks in the rubber 
members lead to deterioration and a shortened useful life for the dampers 
and armature assemblies. 
The life of the armature assemblies and the dampers, which are an integral 
part of the armature assemblies, can be extended by increasing the length 
of the metal parts to provide space for rubber filets that adhere to the 
metal parts when the rubber is vulcanized. The increase in the length of 
metal surfaces increases the axial length of the armature assembly. The 
increased axial length of the armature assembly is unacceptable in many 
vehicle installations due to the limited space available in vehicle engine 
compartments. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an electromagnetic clutch with a 
short axial length. 
Another object of the invention is to provide an electromagnetic clutch 
with an armature assembly that includes a damper with a long useful life. 
The electromagnetic coupling device includes a field coil assembly, a rotor 
assembly and an armature assembly. The field coil assembly is mounted in a 
fixed position on or adjacent to a driven unit such as a compressor. The 
rotor assembly is rotatably journalled on a support member adjacent to the 
field coil assembly. The armature assembly includes an armature hub 
mounted on a driven shaft, a stopper plate secured to the armature hub, an 
armature and a damper which connects the armature to the stopper plate and 
supports the armature adjacent to an attraction surface on the rotor 
assembly. The damper allows the armature to move axially into contact with 
the attraction surface on the rotor assembly when the field coil assembly 
is energized and pulls the armature out of contact with the attraction 
surface when the field coil assembly is not energized. The damper also 
absorbs torque changes and torsional vibrations. Attraction noise that can 
occur when the armature is pulled into contact with the attraction surface 
on the rotor assembly is essentially eliminated by the damper. 
The armature assembly includes a damper with a holding device with an open 
end that is bevelled outwardly in an arc-shape. The outwardly bevelled 
surface of the holding device contacts the rubber portion of the damper. A 
post member that is embedded in the center portion of the rubber element 
has an arc-shaped bevelled edge on its unsupported end. The arc-shaped 
bevelled end reduces the diameter of the end of the post member. 
The holding devices for the rubber element can be part of the stopper plate 
or part of the armature. Mounting arrangements for the holding devices are 
provided which have minimal axial thickness without decreasing the axial 
thickness of the rubber element. 
The foregoing and other objects, features and advantages of the present 
invention will become apparent in light of the following detailed 
description of exemplary embodiments thereof as illustrated in the 
accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
The electromagnetic clutch 11 as shown in FIG. 1 includes a rotor assembly 
14 rotatably mounted on a cylindrical member 12a, that protrudes from a 
compressor 12, by a bearing 13. A plurality of v-grooves 14a are provided 
on the outside of the rotor assembly 14. A pulley with v-grooves that 
match v-grooves 14a is provided on and driven by an engine. A power band 
belt is trained around the rotor assembly 14 and the engine driven pulley 
to drive the rotor assembly 14. The engine, and the power band belt are 
not shown in the drawing. 
A field coil assembly including a toroidal electromagnetic coil 15 is 
mounted inside a yoke 16. The yoke 16 is a toroidal shaped member with a 
U-shaped cross-section. An attachment plate 17 is secured to the yoke 16 
and bolted to the compressor 12. The attachment plate 17 supports the yoke 
16 and the toroidal electromagnetic coil 15 inside a ring groove 14b in 
the rotor assembly 14. The rotor assembly 14 rotates about the axis of a 
driven shaft 18 and the toroidal yoke 16 is concentric with the 
cylindrical member 12a so that the electromagnetic coil 15 remains 
stationary and the rotor assembly 14 is free to rotate. 
The armature assembly 19 includes an armature hub 20 that is attached to a 
driven shaft 18 of a compressor by a bolt 23. The armature hub 20 and the 
driven shaft 18 have splines 22 that prevent relative rotation between the 
armature hub and the driven shaft. A stopper plate 21 is rigidly secured 
to the armature hub 20 by rivets or other attaching means. The stopper 
plate 21 can have a triangle shape, a disc shape or other appropriate 
shape. An armature 24, which is shaped into a disk with a cylindrical hole 
24a that is larger in diameter than a flange 20a of the armature hub 20, 
is held by the stopper plate 21 and positioned between the stopper plate 
and the attraction surface 14d on the rotor 14. The armature 24 is 
connected to stopper plate 21 by a damper. The damper includes three 
damper elements. Each damper element includes a circular cup-shaped damper 
cover 26, a rivet 25 and a rubber damper 27. The rubber damper 27 is 
vulcanized inside the damper cover 26 and with the rivet 25 in the center 
of the rubber damper. The damper cover 26 is attached to the stopper plate 
21 with the rivet 25 extending through a hole 21a in the stopper plate 21. 
The hole 21a is substantially larger in diameter than the rivet 25. The 
rivet 25 is rigidly secured to the armature by swaging. An arc-shaped 
bevelled area 25a is provided on the end of the rivet 25 that is in the 
rubber damper 27. The rivet 25 is set so that the end with the bevelled 
area 25a and the rubber damper 27 cooperate to make a flat surface. Holes 
14c and 24b in the attraction surface 14d of the rotor assembly 14 and the 
armature 24 are magnetism stopping holes which detour the magnetic flux. 
During operation of the electromagnetic coupling device shown in FIG. 1, 
electricity is supplied to the electromagnetic coil 15. Magnetic flux is 
generated through the yoke 16, the rotor assembly 14, the armature 24 and 
the yoke 16. The armature 24 will be pulled toward the attraction surface 
14d, the rubber dampers 27 will deform and the armature 24 will 
magnetically adhere to the attraction surface 14d of the rotor assembly 
14. Torque, supplied to the rotor assembly 14 by an engine, will be 
transferred from the attraction surface 14d to the armature 24, to the 
rivet 25, to the rubber damper 27, to the damper cover 26, to the stopper 
plate 21, to the armature hub 20 and to the driven shaft 18. If sufficient 
torque is applied to the rotor assembly 14, the rotor assembly will rotate 
and the driven shaft 18 will rotate. The rubber damper 27 will dampen 
torsional vibrations from the drive for the rotor assembly 14 or from the 
driven shaft 18. 
Another electromagnetic clutch, which employs the invention is shown in 
FIG. 2. The same reference numbers are used in FIG. 2 that were used in 
FIG. 1 for parts that are similar. An abbreviated explanation is given for 
members in FIG. 2 which have nearly the same structure and function as 
members shown in FIG. 1. A yoke 16 is used to secure an electromagnetic 
coil 15 on the compressor 12 with the electromagnetic coil positioned 
inside a ring-shaped groove 14b in the rotor assembly 14. The rotor 
assembly 14 is rotatably supported by a cylindrical member 12 on the 
compressor 12. 
The armature assembly includes an armature hub 20 attached to the driven 
shaft 18 by a fastener 23. The stopper plate 30 includes an inner member 
30a that is attached to a flange 20a on the armature hub 20 by rivets. The 
stopper plate 30 also includes a plate-shaped outer member 30b which is 
larger in diameter than the inner member 30a. An armature 24 is riveted to 
the plate-shaped outer member 30b of the stopper plate 30. The outer 
member 30b and the inner member 30a have axially extending spaced apart 
members which define a ring-shaped empty space which receives a 
ring-shaped rubber damper 31. Bevelled arc-shaped edges 30e and 30f are 
provided on the edge portions of the inner member 30a and the outer 
members 30b that define the ring-shaped space that receives the rubber 
damper 31. The rubber damper 31 is shaped to cooperate with the bevelled 
edges 30e and 30f to form a flat surface on the end face 31a. The inner 
member 30a also includes a stop surface 30g which contacts the armature 
24 to limit movement of the armature away from the attraction surface 14d. 
The operation of the armature assembly shown in FIG. 2 is substantially the 
same as operation of the armature assembly shown in FIG. 1. The bevelled 
surfaces at 30e and 30f and the portions of the rubber damper 31 which are 
in contact with the bevelled surfaces avoid stress concentrations which 
cause cracking of the rubber damper. 
The electromagnetic clutch shown in FIG. 3 employs another version of the 
clutch. This version is substantially identical to FIG. 1 except for the 
armature assembly 19. The armature assembly 19 includes an armature hub 22 
and a stopper plate 21 that is fastened to a flange 20a by rivets. Three 
cup-shaped areas 125b are formed in the armature 25 by a plasticizing 
technique. The cup-shaped areas 125b have bevelled edges 125c with 
arc-shapes at the open edge that face the attraction surface 14d. A rubber 
damper 126 is vulcanized inside each of the cup-shaped areas 125b. A rivet 
124 with a bevelled arc-shaped end 124a is also positioned in the center 
of the rubber damper 126 when the damper is vulcanized. The rivet 124 
extends through the center of a large aperture in the armature 125 and is 
held to the stopper plate 21 by an enlarged end at 124b formed by swaging. 
Apertures 14c and 125d in the attraction surface and in the armature 125 
are magnetic flux stopping holes which change the magnetic flux path. 
FIG. 4 shows a prior art armature assembly 40 which is similar to the 
armature assembly shown in FIG. 2. The stopper plate 42 with an inner 
member 42a and an outer member 42b. The inner member 42a is riveted to an 
armature hub 20. An armature 24 is attached to the outer member 42b by 
rivets. The inner and outer members 42a and 42b of the stopper plate 42 
have concentric, axially extending and spaced apart walls which define an 
annular space. A rubber damper 45 is vulcanized in the annular space and 
connects the outer member 42b of the stopper plate 42 to the inner member 
42a. The forward surface 45a of the rubber damper 45 is recessed inward 
from the forward edges of the stopper plate 42. The recess provides space 
for the bevelled arcuate filets 45b and 45c. The filets 45b and 45c reduce 
stress concentrations and eliminate or reduce cracking of the rubber 
damper 45. However, the construction increases the axial length of the 
armature assembly 40 by a distance L shown in the drawing. This increase 
in length renders the armature assembly 40 unusable in some installations. 
The armature assembly 19 disclosed in FIG. 2 incorporating the invention 
is shorter than the unit shown in FIG. 4 by the distance L. The armature 
assemblies in FIGS. 1 and 3 are shortened by the same distance L. 
Preferred embodiments of the invention, which have been described in 
detail, are examples only and the invention is not restricted thereto. It 
will be easily understood by those skilled in the art that modifications 
and variations can easily be made within the scope of the invention: