Rotary vibration isolator bushing

In a rotary fan (10) having a tubular drive shaft (14) and a lower cylindrical hub (16), an isolator bushing (20) interconnects the drive shaft (14) and hub (16) so as to substantially reduce vibrations. The bushing (20) is elastomeric, tapered and mounted within the hub (16) in an uncompressed state to maintain vibration-reducing properties. The drive shaft (14) extends into and terminates within a central bore (42) of the bushing (20), and the end portion of the shaft (14) is secured at a lower portion of the bushing (20). The hub (16) is secured to an upper end surface of the bushing (20) and the length of the bushing (20) provides dampening of vibrations caused by rotary motion. The tapering of the bushing (20), while maintained in an uncompressed state, provides dampening of vibrations caused by resonant frequency characteristics.

DESCRIPTION 
1. Technical Field 
The invention relates to a vibration isolator bushing mounted between the 
drive shaft of a motor and the hub of a rotary device such as a fan. 
2. Background Art 
In rotary fans it is common to mount a rubber or similar elastomeric 
bushing between the drive shaft of the fan motor and the hub on which the 
fan blades are supported. In such a conventional fan the bushing serves to 
damp vibrations generated during rotation of the drive shaft so as to 
reduce noise during operation of the fan. The rubber bushing also can 
connect the drive shaft to the rotary hub which supports the fan blades. 
This connection may be in addition to or in place of a set screw or other 
clamping means which connects the drive shaft to the hub. The bushing 
dampens the transmission of vibrations from the motor to the blades as 
well as prevents oscillation of the motor shaft which could cause annoying 
noise if the shaft were to hit another portion of the hub assembly. 
One such example of a mounting for a rotary hub which supports fan blades 
is disclosed in U.S. Pat. No. 2,558,589, to Skofield issued June 26, 1951. 
In the Skofield U.S. Pat. No. 2,558,589 a cylindrical, elastomeric 
bushing, preferably made of rubber, is mounted between the drive shaft of 
a motor and the rotary hub of the fan. The hub includes a bore which 
receives the bushing in a compression fit. In this regard, the inside 
diameter of the bore through the hub is less than or equal to the outside 
diameter of the bushing. In this way, the bushing is compressed when 
mounted in the hub. The bushing includes a bead which is received in a 
groove in the drive shaft so as to lock the bushing and drive shaft 
together. The bushing prevents the transmission and amplification of 
vibrations between the motor and fan so as to reduce noise. 
The conventional bushing mount is formed of an elastomeric material such as 
rubber which is mounted in a compressed state. When compressed, the rubber 
loses a portion of its elasticity and does not have the resiliency 
necessary to damp vibrations of the drive shaft and prevent the 
transmission of such vibrations from the shaft to the hub. Further, a 
rubber or similar elastomeric bushing does not provide the rigidity needed 
to connect the drive shaft of the motor to the hub. A conventional rubber 
bushing which is mounted in the compressed state also allows for angular 
movement or "wobble" of the drive shaft which causes wear and tear on the 
motor and hub as well as generates noise. Further, rubber is a less stable 
material as compared to modern synthetic elastomeric materials and 
deteriorates in the presence of oils or the like. The lack of resiliency 
of the rubber when compressed reduces its capability to damp vibrations of 
the shaft and hub and prevent transmission of the vibrations. Further, the 
need to mount the bushing in a compressed state requires special tools 
needed to compress the bushing between the drive shaft and the fan hub. 
DISCLOSURE OF INVENTION 
In accordance with the invention it has been found that vibrations in the 
shaft and hub of a rotary device having a cylindrical hub and a rotary 
drive shaft coaxially mounted within the hub can be dampened by mounting a 
bushing between the hub and shaft in a substantially uncompressed state, 
which bushing also serves to operatively connect the drive shaft and hub. 
The bushing has first and second end surfaces having central apertures 
therein and a central bore extending therethrough which receives the drive 
shaft in a close-fitting relationship. The end of the drive shaft is 
spaced from the second end surface of the bushing so that the drive shaft 
does not extend all the way through the bushing. The bushing is fastened 
between the shaft and the hub and has selected elastomeric properties 
which provide for dampening of vibrations in the shaft and hub. It has 
been found that the use of such a bushing provides effective dampening of 
vibrations generated during rotation of the hub and shaft as well as 
provides a connection between the shaft and hub for rotation together. 
The bushing is secured to the shaft by a set screw and is further secured 
to the hub by suitable fasteners. In order to accomplish this attachment, 
the bushing is provided with an annular plate which receives the 
fasteners. The bushing also includes an annular collar which is embedded 
within an enlarged annulus in the central bore of the bushing so that the 
wall of the bore and the inner wall of the collar are flush. The collar 
includes a threaded bore which receives the set screw so as to connect the 
set screw to the collar and the drive shaft. 
In practice, the bushing has a slightly conical configuration having a wall 
thickness which tapers from the second end surface to the first end 
surface, which variation of wall thickness provides the desired dampening 
characteristics. Further, the shaft is preferably a hollow member and the 
rotary device includes a stationary axis which extends therethrough so 
that the shaft and attached hub rotate about the axis. In this 
configuration the bushing includes a bearing member which supports the 
stationary axis against angular movement. The bearing is preferably an 
annular disc which is embedded in the second end surface of the bushing. 
The bushing is preferably formed from a 70-durometer polyurethane while the 
bearing member embedded therein is harder than the remainder of the 
bushing and is preferably formed from a 75-durometer polyurethane and 
impregnated with a highly lubricitive material such as molybdenum 
disulfide. The durometer of the polyurethane is selected so as to provide 
the desired dampening of vibrations in the shaft and hub as well as 
provide a connection between the drive shaft and hub.

BEST MODE FOR CARRYING OUT THE INVENTION 
The hub portion of a ceiling fan is shown in FIG. 1. The remainder of the 
fan is not shown but it is understood that it is conventional in nature 
and includes a mounting plate for securing the fan to, for example, a 
ceiling, a decorative shroud or casing disposed over the fan motor and fan 
blades. 
The fan 10 includes a conventional electric motor 12 having a tubular drive 
shaft 14 extending therefrom. The drive shaft 14 is preferably a hollow 
tube. The drive shaft 14 is connected to a fan hub 16 which supports a 
plurality of fan blades 18, one of which is illustrated. The fan blades 18 
are secured to the hub 16 in a suitable fashion such as by riveting or the 
like. 
Disposed between the hub 16 and the drive shaft 14 is a vibration isolator 
bushing 20 in accordance with the invention. The isolator bushing 20 
serves as a means to connect the drive shaft 14 to the hub 16 so as to 
rotate the fan blades when the motor is operated as well as a means to 
damp vibrations and eliminate noise during operation of the fan 10. 
The fan 10 includes a switch housing 22 which contains appropriate 
electrical circuitry and switches for turning the fan on and off and 
operating the fan at varying speeds. The switch housing 22 and its 
circuitry are conventional in nature and do not form a part of this 
invention. Extending through the hub 16 is a hollow center tube 24 having 
one end secured in the switch housing 22 by a retainer nut 26 and the 
other end (not shown) fixed in the fan motor 12 or mounting plate. The 
center tube 24 provides an axis about which the drive shaft 14 and hub 
rotate 16. The center tube 24 is fixed against rotation and supports the 
switch housing 22. 
The hub 16 has a cylindrical wall 30 and an end wall 32. The end wall 32 
includes a central aperture 34 having a diameter equal to the diameter of 
the drive shaft 14. Spaced from the central aperture 34 are througholes 36 
which receive fastening means for securing the isolator bushing 20 within 
the hub 16, as will be described below. The inner dimension and 
configuration of the cylindrical wall 30 of the hub 16 corresponds to the 
size and shape of the isolator bushing 20. The wall 30 includes a cut-away 
portion 38 which allows for the insertion of a fastening means through the 
wall 30 and into the isolator bushing 20, as will be described below. 
There is no direct connection between the hub 16 and drive shaft 14 of the 
motor 12 but rather the hub and shaft 14 are connected by the isolator 
bushing 20. 
With reference to FIGS. 2-4, the isolator bushing 20 is a slightly-tapering 
conical member having an upper section 40 which is made of an elastomeric 
material. The upper section 40 includes a central bore 42 therethrough 
which receives the drive shaft 14. The lower portion of the central bore 
42 includes an inner annulus 44 which receives a metal collar 46, 
preferably formed from steel. The metal collar 46 is preferably bonded to 
the bushing 16 during forming thereof. A through-hole 48 perpendicular to 
the central bore 42 extends through the lower portion of the upper section 
40 of the bushing 16 and through the collar 46. The portion of the 
through-hole 48 in the collar 46 includes a thread 50 for receiving a 
fastening member for securing the isolator bushing 20 to the drive shaft 
14, as will be described below. 
The lower portion of section 40 of the isolator bushing 20 carries a flat 
bearing member 52 which is embedded within the isolator bushing 20 during 
forming thereof. The bearing member 52 includes a central aperture 54 
having a diameter slightly larger than the diameter of the center tube 24 
which extends through the drive shaft 14. It can be seen that the bearing 
52 supports the center tube 24 so as to prevent angular movement thereof. 
The top portion of the isolator bushing 20 carries a flat annular plate 56 
preferably made of steel. The plate 56 is embedded in the isolator bushing 
20 during forming thereof and includes two apertures 58 and 60 which 
receive fastening means for securing the isolator bushing 20 within the 
hub 16. The plate 56 includes a central aperture 62 through which the 
drive shaft 14 is inserted. 
The isolator bushing 20 is secured within the interior of the hub 16 by 
means of rivets 62 and 64. The rivets are inserted through the througholes 
36 in the end wall 32 of the hub 16 and are received in the corresponding 
apertures 58 and 60 in the plate 56. In this way, the isolator bushing 20 
is fixed against movement within the hub 16. 
The drive shaft 14 is connected to the hub 16 through the isolator bushing 
20. In this regard, a set screw 66 is inserted through the through-hole 48 
which extends through the bushing 20 and the collar 46. The set screw 66 
is threaded into the threaded portion 50 of the through-hole 48 and is 
received in the lower portion of the drive shaft 14. In this way a 
connection between the drive shaft 14 and the hub 16 is made. Access to 
the through-hole 48 is provided by the cut-away portion 38 in the wall 30 
of the hub 16. 
The drive shaft 14 extends into the central bore 42 of the isolator 
bushings 20 so that its end is spaced from the bearing member 52. It can 
be seen that the drive shaft 14 and the center tube 24 are concentric and 
radially spaced apart. 
The isolator bushing 20 is not compressed between the drive shaft 14 and 
the wall 30 of the hub 16 so that the elasticity and compressibility of 
the isolator bushing 20 are not reduced when it is mounted in place. There 
is a frictional engagement between the drive shaft 14 and the isolator 
bushing 20 in addition to the set screw connection both of which serve to 
connect the drive shaft 14 to the bushing 20 and thus to the hub 16. 
The bushing 20 is preferably made of polyurethane and is substantially 
unstressed and uncompressed in its mounted condition. Preferably, the 
upper section 40 of the isolator bushing 20 has a durometer of 70 on the 
"A" scale. The cylindrical bearing member 52 is also preferably made of a 
polyurethane having a durometer of 75 on the "D" scale. The bearing member 
52 is more rigid than the remaining portion of the isolator bushing 20 and 
does not provide any dampening properties nor serve to connect the drive 
shaft with the hub. The durometer of the polyurethane is selected to 
provide the desired dampening characteristics. In this regard, a material 
which is too hard would not provide enough dampening and would not absorb 
the vibrations generated during rotation of the drive shaft. A material 
which is too soft would allow for too much flexibility of the drive shaft 
so that it would oscillate and wobble during rotation due to the torsional 
vibration thereof. it is contemplated that the particular durometer of the 
polyurethane is selected so as to provide the desired dampening 
characteristics for particular motors and hub and fan blade assemblies. 
The conical shape of the isolator bushing provides for a close-fitting 
mounting of the isolator bushing within the hub. Further, the variation of 
the thickness of the wall 30 of the isolator bushing due to the taper 
provides different dampening characteristics which will absorb different 
frequencies of vibration. The slight taper also allows for ease in 
manufacture of the bushing by providing a draft which permits the bushing, 
which is a molded part, to be easily removed from the mold. 
The isolator bushing has enough rigidity or integrity so that it serves as 
a means to connect the drive shaft to the hub. In this regard, the bushing 
is not compressed between the drive shaft and the hub as is conventional 
in many bushed connections. Instead, the isolator bushing is mounted in an 
substantially unstressed and uncompressed state. The frictional engagement 
between the drive shaft and the bushing serves to connect the hub and 
drive shaft along with the set screw connection. 
The isolator bushing dampens torsional vibrations generated during rotation 
of the drive shaft. The torsional vibration generates annoying motor hum 
and noise which is undesirable. The bushing dampens the vibration of the 
drive shaft so as to reduce or substantially eliminate any such vibration 
and resulting noise. Further, vibrations are undesirable in that they tend 
to cause wear on the motor and the connection between the drive shaft and 
hub. Dampening of these vibrations reduces the wear problem caused by 
gyroscopic movement of the shaft since these vibrations would not be 
transmitted from the shaft to the hub. 
The dampening of the vibrations also serves to prevent the center tube 24 
from striking the drive shaft through which it is inserted. In this 
regard, the center shaft is spaced from the drive shaft so as to provide 
an axis about which the hub and drive shaft rotate. In the past, 
conventional bushings would not provide enough dampening and would allow 
the center tube to strike the drive shaft and generate noise. The 
dampening of the vibrations provided by the bushing in combination with 
the integral bearing which supports the lower end of the center tube 
greatly reduces the contacting of the drive shaft and center tube which 
generates annoying noise and causes wear and tear on the tube and its 
bearing. The isolator bushing provides sufficient dampening so that the 
center tube can vibrate without striking the drive shaft. The polyurethane 
used to form the bearing 52 is preferably impregnated with a highly 
lubricitive material such as molybdeum disulfide which adds lubricity to 
the polyurethane. In this way, friction between the center tube 24 and the 
bearing 52 during rotation of the hub 16 is reduced. 
The above-described isolator bushing provides desired dampening of 
torsional vibrations of the drive shaft which produce annoying noises and 
cause wear and tear on the shaft and hub. The isolator bushing serves to 
reduce this undesirable vibration which generates annoying noise. It has 
been found that a remarkable reduction of the noise is accomplished by use 
of such an isolator bushing. Further, the isolator bushing is easily 
installed within the hub since it is not compressed between the drive 
shaft and wall of the hub as in conventional elastomeric bushings. 
Reasonable variation and modification are possible within the scope of the 
foregoing disclosure and drawings without departing from the spirit of the 
invention which is defined in the appended claims.