Machine element or mechanism – Mechanical movements – Rotary to or from reciprocating or oscillating
Patent
1998-04-02
1999-12-07
Graysay, Tamara L.
Machine element or mechanism
Mechanical movements
Rotary to or from reciprocating or oscillating
74 89, F16H 2116
Patent
active
059964316
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to twist action friction drives.
PRIOR ART
From the prior art is known a twist action friction drive in which a roller means, driven in rotation about-: an axis inclined to that of a driven member, comprises three rollers all hydrostatically mounted about axes inclined relative to that of the driven member. The arrangement is complex, and requires that all the rollers are mounted at precisely the same inclination or helix angle if conflicting drive forces, leading to an unpredictable advance of the driven member, are to be avoided. This and other prior art arrangements are also disadvantageous in that the rollers rotate around the outside of the driven member.
THE INVENTION
According to the invention, there is provided a twist roller friction drive comprising a rotating drive bar which drives in rotation roller means the axis of rotation of which is inclined relative to the axis of a rotationally fixed driven member with which the roller means engages, wherein the inclined roller means comprises a single annular roller urged from the inside into driving contact with the driven member by one or more hydrostatic pads.
Preferably the driven member is a tube and the skewed annular roller is in frictional engagement with the bore of the tube. The tube is preferably a steel tube, with a hardened and ground bore. In a typical use, the tube is fixed to the carriage of a machine tool, aligned with the machine axis.
oil for the hydrostatic pads acting on the roller is preferably supplied through the drive bar along the axis thereof. Such drive bar preferably comprises a cylindrical tube.
At its trailing end, the drive bar rotationally drives a skewed roller assembly in which the annular roller is incorporated, the remote forward end of the drive bar being driven in rotation, as by an electric motor. The rotatlionally driven, forward end of the drive bar is preferably located, albeit indirectly, by hydrostatic bearings. The oil supply for these bearings therefore constitutes a convenient source from which oil may be supplied along the central axis of the drive bar to the skewed roller assembly, via suitable restictors.
At the skewed roller assembly, drillings are preferably provided to supply oil to the bore and end faces of the roller, so that it is fully floating hydrostatically. Further drillings may supply oil to four equianbyularly spaced hydrostatic pads which urge the skewed roller assembly radially so that the roller is in frictional driving engagement with the driven tube. Oil seeping from the roller assembly is preferably returned to a sump at the driven end of the drive bar, conveniently via the interior of an extensible tube (which may be corrugated for this purpose or of a telescoping design), which is secured between the forward end of the driven tube and a housing at the forward end of the drive bar wherein drillings are provided to supply oil to the hydrostatic bearings which locate a rotating assembly carrying the forward end of the drive bar.
In the arrangement according to the invention, the axial movement of the driven member is principally determined by the angle of skew of the roller. Thus, if this angle is made very small, small precise axial movements of say 1 nm (nanometre) or less of the driven member can readily be achieved per revolution of the drive bar, as compared, for example, to axial movements of the order of 150 mm per revolution with a capstan drive, and 10 to 15 mm in the case of a hydrostatic nut drive. It follows that a relatively high speed drive motor can be employed, which is advantageous for velocity feedback control.
DESCRIPTION OF EMBODIMENT
A practical example of twist action friction drive is now described by way of example with reference to the accompanying drawings, in which:
FIG. 1 shows the complete drive in axial cross section;
FIGS. 2 and 3 are axial cross sections, in different planes, through the skewed roller assembly; and
FIG. 4 is a transverse cross section through the skewed roller assembly.
The twist ac
REFERENCES:
patent: 4203328 (1980-05-01), De Boynton
patent: 4760864 (1988-08-01), Leclerc
patent: 4921207 (1990-05-01), Baker
patent: 5363711 (1994-11-01), Seto
Marks' Standard Handbook For Mechanical Engineers, 10th Ed, pp. 8-130 to 8-131.
Fenstermacher David
Graysay Tamara L.
Unova U.K. Limited
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