Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
2000-04-06
2003-05-27
Vidovich, Gregory (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C029S894361, C403S356000, C403S370000, C403S374300
Reexamination Certificate
active
06568063
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for mechanical power transmission and position or timing control by means of a belt or chain driven by an arrangement of a shaft and sprocket or pulley in which the shaft is coupled to the sprocket or pulley by means of a tapered bushing so as to ensure substantially concentric motion of the components and to allow phasing of the shaft-mounted components.
BACKGROUND OF THE INVENTION
Typical torques employed in both mechanical power transmission and motion control applications are often such as to cause significant problems with torque transmission between a shaft and any gears, pulleys or sprockets that are intended to be driven by the shaft. Various methods that are employed to achieve the requisite coupling of torque include set screws, pins, keys, flattened shafts, flanged bushings and clamping couplings of various sorts.
Additionally, in power transmission applications employing sufficiently large shafts, tapered bushing assemblies are known for coupling rotating shafts to sprockets so that rotary motion of the shaft may be transmitted to the sprocket. In the power transmission art, the coupling of substantial torque between a shaft and a hub by a bushing gives rise to requirements such as keys, flanges coupled to the exterior face of the sprocket, and/or large surface areas of interface between the bushing and the shaft and hub respectively. The use of large surface areas of interface is based on the proportionality of static friction, for transmission of shear forces, to the contact area between the surfaces. A typical prior art hub and bushing structure is now described with reference to
FIGS. 1 and 2
.
Numeral
10
designates generally a sprocket or pulley for driving a chain or belt (not shown), the sprocket having an outer surface
12
and a hub
14
. The subject of the present discussion is the coupling of torque between a rotatable shaft
20
of constant diameter and hub
14
by means of a bushing
22
. In the power transmission art, the coupling of substantial torque between shaft
20
and hub
14
via bushing
22
is commonly accomplished by using keyways machined into interior bore
46
of the bushing and outside diameter
44
of the shaft, with a solid square key
48
inserted into the machined spaces. The use of square keys and keyways for obtaining maximum torque is described in ANSI Standard B17.1, Keys and Keyseats, which is incorporated herein by reference. In this way, the torque that may be coupled by means of shear forces exerted on the keyways increases as the size and strength of the key and keyways increase. Alternatively, torque may be transmitted by means of one or more connectors or pins inserted through an annular flange of the bushing and a side face of the hub. When an annular flange is employed, the maximum diameter of the bushing exceeds the inside diameter of the hub.
Referring now to
FIG. 2
, a cross-sectional view is shown of the prior art sprocket
10
and bushing
22
, with the section taken on line
2
—
2
of FIG.
1
. Mating tapered surfaces of the outside
24
of bushing
22
and the inside
26
of hub
14
are shown, while internal surface
27
of bushing
22
is shown to be straight and parallel with the surface of shaft
20
. Bushing
22
is continuous throughout with the exception of a radial slot
28
of sufficient width to permit the bushing to contract during installation and to grip the shaft firmly. Tapered surfaces
24
and
26
of the bushing and hub, respectively, are adapted to slide relative to one another as the sprocket is assembled onto the shaft and the bushing is secured in place within hub
14
In a power transmission application, the scaling relationships that are known to hold require that the surface area in contact between bushing
22
and hub
14
must increase at least as fast as the torque required to be transmitted.
In order to insert bushing
22
into hub
14
of sprocket
10
, a force is required to overcome the sliding friction developed across the entire contact surface area between the bushing and the hub. A similar sliding friction must be overcome to disengage the bushing from the hub. The sliding friction that must be overcome is proportional to the surface area and thus, as discussed above, to the torque capacity of the bushing.
Since the objective of power transmission applications is typically the transmission of a maximal amount of torque for given geometrical and material constraints, it has been deemed desirable, in the prior art, to maximize the contact surface area (~T) between the bushing and hub. This makes it difficult, however, to overcome the frictional hurdles both for insertion and disengagement of the bushing. Therefore, the prior art teaches that a taper with an included angle of at least 8° (equivalent to a taper angle &agr;=4°) is placed on external surface
24
of bushing
22
and internal surface
26
of hub
14
in order to reduce the difficulty in releasing the bushing. Typically, a threaded hole
76
(shown in
FIG. 1
) is provided for insertion of a jack-screw so that force may be applied for release of the bushing.
Engagement of the bushing, moreover, requires that both the sliding friction across the contact surface as well as the lateral component due to the taper must be overcome. In the prior art, this has been accomplished by use of at least two screws
30
and
32
, along with matching tapped threading on the hub, for drawing the bushing into the hub. Consequently, clearance must be provided in the placement of the components for tightening the bushing and for removing the bushing using a jackscrew. An additional concern is the square entry of the bushing upon insertion; since the contact surface area is large (scaling~T), the bushing is prone to cocking upon insertion if care is not taken in driving the symmetrically placed screws.
Additionally, the scaling relations of the prior art power transmission applications dictate that the annular width w (shown in
FIG. 1
) of bushing
22
must scale substantially as the bushing face thickness t since t~T
½
. The annular width, however, is half the difference between the outer diameter and inner bore of the bushing. Therefore, w~d
p
−d
s
~&ggr;T
½
−&zgr;T
⅓
, where &ggr; and &zgr; are coefficients of proportionality. The annular width dimension, w, typically scales as T
½
in the limit of large torque, which is the limit of interest in prior art power transmission applications. While the foregoing scaling relations are not intended to be definitive or descriptive of all bushings available in the prior art, they are intended to illustrate the nature of design considerations governed primarily by maximizing torque capacity as taught in the power transmission art.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, there is provided a method for applying a bushing to improve the concentricity of a coupling between a shaft and a hub having a central bore. The bushing has an interior bore for surrounding the shaft and exactly one exterior slot in a direction parallel to the interior bore for matching a corresponding slot in the hub. Each matched part of the slot defines an opening for receiving a set screw. The method includes driving the set screw into the exterior slot in such a manner as to draw the bushing into the bore of the hub, thereby coupling the shaft to the hub.
In accordance with alternate embodiments of the present invention, there is provided a method for improving the concentricity of a coupling between a shaft and a hub of the type where the shaft has a diameter less than 0.5 inch. The method has the steps of providing a split tapered bushing, where the bushing has an inner bore of less than 0.5-inch diameter and an outer diameter smaller than the inner diameter of the hub, and using the bushing to couple the shaft to the hub.
In accordance with a further aspect of the present invention, there is provided a device for coupling a shaft to a hub, where t
Bennett Robert
Sweeney James
Bromberg & Sunstein LLP
Custom Machine & Tool Co., Inc.
Omgba Essama
Vidovich Gregory
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