Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
2001-02-02
2003-05-06
Fenstermacher, David (Department: 3682)
Machine element or mechanism
Gearing
Directly cooperating gears
C074S424830, C192S150000, C254S098000
Reexamination Certificate
active
06557431
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fastening structure having a screw with outer helical grooves, an associated nut with corresponding inner helical grooves, and bearing balls interposed in the nut and screw grooves, where the screw and nut remain engaged even after loss or removal of the bearing balls.
BACKGROUND OF THE INVENTION
A ball screw assembly is a relatively well known mechanism for converting rotary motion into axial movement, as disclosed in U.S. Pat. No. 1,831,080. As illustrated in
FIGS. 1A-1C
, a standard ball assembly has a screw
10
with a thread
20
, a nut
30
positioned around the screw and internally threaded with a thread
40
having the same pitch as the screw thread
20
, and a multitude of bearings
60
disposed in a channel
70
defined between the screw
10
and nut
30
by the screw and nut threads
20
and
40
. The channel
70
may be shaped to match closely the shape and dimensions of the bearings
60
. The ball screw assembly further includes a return tube
80
in the nut
30
for transferring the bearings
60
between the opposite ends of the nut. In operation of the ball nut assembly, counter-clockwise rotation of the screw causes the bearings
60
to leave the channel
70
at a distal end
31
of the nut
30
and travel via the return tube
80
to the proximal end
32
nut where the bearings
60
are returned into the channel
70
. In order to direct the cycling of the bearings
60
, the nut may have a structure, such as a finger (not illustrated) that extends into the channel
70
to direct the bearings
60
in to the return tube
80
.
The structure of the ball screw assembly allows rotation of the screw
10
relative to the nut
30
to cause precise axial motion of the screw
10
relative to the nut
30
. The ball screw assembly has very high mechanical efficiencies and can bear large loads, which make it possible to obtain very high yields and to shift heavy loads using a very low torque input. Because of these properties, the ball screw assembly is frequently employed in moving and manipulating heavy loads. In addition, the ball screw assembly is wear resistant and exhibit very little play. Accordingly, the ball screw assembly is also well-suited for use in precision mechanisms and machine tools employed in commercial, manufacturing, and industrial applications.
A well-known apparatus for mechanically rotating the ball screw assembly is illustrated in FIG.
2
. In that figure, a housing, generally designated
1
, is provided for the ball screw
10
which is extensible from the housing
1
. Received on the ball screw
10
, is the nut
30
, which circulates the bearings
60
in a recirculating path in the usual, above-described manner. The nut
30
may be driven in rotation by a pinion gear
3
via a worm gear
4
which is in mesh with it. The worm gear shaft
5
may be driven by a motor drive system (not shown). Provided on the nut
30
is an integrated base flange
8
which is received within and coupled to the pinion gear
3
.
In operation, rotation of the worm shaft
5
drives the nut
30
in rotation via the worm
4
and pinion
3
. With rotation of the ball nut
30
, the bearings
60
, which travel in the complemental internal threads
40
and external threads
20
of the nut
30
and ball screw
10
respectively, move the screw
10
in axial motion up and down as desired.
Unfortunately, the traditional ball screw assembly has the problem of occasional catastrophic failure. In particular, the balls
60
start to fatigue and deteriorate when used for many cycles, depositing debris in the channel
70
and the return tube
80
. This presence of the debris causes the bearings
60
to deteriorate even further. Eventually, the bearings
60
may become so worn and distorted that they are no longer able to prevent separation of the screw
10
from the nut
30
. Alternatively, the debris from the deterioration of the bearings
60
collects and blocks the return tube
80
, such that the bearings
60
leave the ball screw assembly instead of returning the beginning of the channel
70
.
In either case, catastrophic failure occurs as the screw
10
separates from the nut
30
with little resistance. As seen in
FIG. 1D
, the traditional ball screw assembly has no structural feature to prevent the screw
10
from easily separating from the nut
30
after the bearings
60
are removed from the channel
70
. The screw and nut may violently separate, causing the load supported by the ball screw assembly to be abruptly released and dropped, potentially damaging the contents of the load. This failure of the ball screw assembly is sudden and without warning, potentially occurring almost immediately upon the loss of the bearings
60
.
Accordingly, it is a goal of the present invention to provide an improved ball screw assembly that is resistant to catastrophic failure and the resulting sudden separation while preserving the ball screw's benefits of high mechanical efficiencies. A further goal of the present invention to provide a ball screw assembly that gives an indication of potential failure, thereby allowing preventive measures such as repair or replacement of the ball-screw assembly.
SUMMARY OF THE INVENTION
These and other goals are addressed through the fail-safe ball screw assembly of the present invention. The ball screw assembly has a double start screw with a first helix with a truncated, smaller diameter flat and an intertwined second helix with an extended, larger diameter flat. The assembly also has corresponding nut with a first helix with an extended, larger diameter flat and an intertwined second helix with a truncated, smaller diameter flat. When the screw is inserted with the nut, the screw's truncated first helix is paired with the nut's extended first helix. Likewise, the screw's extended second helix is paired with the nut's truncated second helix. The nut and screw helixes combine to form channels in which bearings balls travel. With this structure, the fail-safe ball screw assembly operates with the efficiency and precision of a standard ball screw, while catastrophic failure caused by the removal or loss of the bearing balls is prevented. In particular, the extended, larger diameter flats of the second screw helix and the first nut helix interact, similar to threads in standard screw
ut combinations, to prevent the unintended separation of the screw and nut. When contact occurs between the larger diameter flats of the second screw helix and the first nut helix, the resulting friction diminishes the ball screw assembly's efficiency, thereby requiring more energy to rotate the ball screw relative to the nut. To take advantage of this indication of deterioration and failure of the ball screw assembly, another implementation of the present invention connects a torque sensor to the ball screw assembly to detect any increase in force needed to turn the ball screw.
REFERENCES:
patent: 610044 (1898-08-01), Glenn
patent: 2069471 (1937-02-01), Baker
patent: 2855790 (1958-10-01), Smith
patent: 3304794 (1967-02-01), Bird
patent: 3468401 (1969-09-01), Letz
patent: 3687234 (1972-08-01), Gendreau
patent: 4962674 (1990-10-01), Payne
patent: 5295406 (1994-03-01), Alfano
patent: 6206784 (2001-03-01), Kato
Fenstermacher David
Siemens Medical Solutions USA , Inc.
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