|Title of Invention||
"MECHANICAL TRANSLATOR WITH ULTRA LOW FRICTION FERROFLUID BEARINGS
|Abstract||A mechanical translator includes at least one magnet that is disposed on a substrate to carry a load and has a magnetic axis generally transverse to the substrate, establishing a magnetic field with maximum external density adjacent the substrate. An ultra low friction interface is obtained with ferrofluid bearings, such as a light mineral oil medium mixed with isoparaffinic acid, which establish a critical angle of displacement from a horizontal static position of less than 1 degree, and preferably less than 10 minutes. A controller of magnetic material can be placed on the opposite side of the substrate to control the movement of the magnets.|
|Full Text||BACKGROUND OF THE INVENTION Field of the Invention
This invention, relates to magnetic-based mechanical
translators, and more particularly to mechanical transla tors with ferrofluid bearings.
Description of the Related Art
Ferrofluids are dispersions of finely divided mag-netic or magnetizable particles, generally ranging between about 30 and 150 Angstroms in size, dispersed in a liquid carrier. The magnetic particles are nypicaliy covered with surfactants or a dispersing agent. The surfactants assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal forces and magnetic interaction, and also provide a chemical composition on the outer layer of the covered particles which is compatible with the Iiauid carrier and the chemicals in the surrounding environment. Ferrites and ferric oxides employed as magnet particles offer a number of physical and chemical properties to the ferrofluid, including saturation magnetization, viscosity, magnetic stability and chemical stability. Several types of ferrofluids are provided by Ferrotec (USA) Corporation of Nashua, New Hampshire. A summary of patents related to the preparation of ferrfluids is provided in
Patent Mo. 6,056,839, while the use of ferrofluid bear-
ings in a moving magnet electrical generator is discussed
in copending Patent Application Serial No. 10/078,724
er.cicled "Electrical Generator With Ferrofluid Bearings",
filed on the same day as the present invention by Jeffrey T. Cheung and Hao Y.in, and also assigned to Innovative Technology Licensing, LLC, the assignee of the present invention. The contents of this copending application are hereby incorporated herein by reference.
A ferrofluid's frictional coefficient is roughly related to its viscosity (measured in centipoise (cp)), but not directly. For example, a ferrofluid with a viscosity of 300 cp has been found to have a static friction coefficient of about 0.015, the EFH1 ferrofiuid from Ferrotec (USA) Corporation has a viscosity on the order of 6 cp and a static friction coefficient of about 0.002, but a water based ferrofluid with a viscosity of 5 cp has been found to have a static friction coefficient of about 0.01. The higher friction coefficient for the somewhat lower viscosity composition has been attributed to surface tension associated with a water based solvent.
Low friction systems can have other problems. For example, depositing a thin film over a substrate by vapor deposition is performed in a vacuum chamber by rotating and translating the substrate inside the chamber to achieve uniform deposition over a large area. The mechanism that controls this motion is cumbersome, consisting of gears, chains, and both rotary and translational motion vacuum feedthroughs . A simpler less expensive mo-0 tion control for the substrates would be highly desirable .
Traditional lubricants employed to assist movement of a Load bearing mechanism on a substrate, while considerably reducing tr.e frictional forces resisting motion, still involve a considerable amount of friction. For example, whereas trie static, coefficient of friction between unlubricated hard steel surfaces is typically about 0.6, the corresponding coefficient is about 0.08-0.1 for vegetable and animal- oil lubricants, 0.14-0.2 for mineral oils, 0.12 for graphitised oils, 0.1 for molybdenum di-sulfide, 0.08 for oleic acid, 0.4 for alcohol and benzene, and 0.2 for glycerine. Even lower frictional coefficients, making it easier to translate a load, wcuici be desirable.
SUMMARY OF THE INVENTION
The present invention provides a mechanical translator system with an ultra low degree of friction, and a novel ferrofluid composition that can be used as a lubricant for the translator. The term "translation" as used herein includes both changes of position and/or rotation. In a preferred embodiment the translator includes a move-able magnet that is arranged to carry a load and is disposed for movement on a substrate. Its magnetic axis is generally transverse to the substrate, such that its magnetic field has a maximum density adjacent the substrate. A ferrofluid lubricant is preferably provided.between the magnet and substrate. A ferrofluid composition that can be used to-achieve a particularly low friction has a viscosity substantially less than 5 cp. It consists of a light mineral oil medium mixed with isoparaffinic acid, with the ratio of isoparaffinic acid to light mineral oil
preferably in the range of about 2:1 to about 4:1. The resulting coefficient of static friction has been found to be in the approximate range of 0.0008-0.0012.
A plurality of moveabie magnets, each with a mag-netic axis generally transverse to the substrate, car. be used to support a platform upon which a load can be placed, or which itself can comprise a load. The magnets are preferably connected to respective fixed locations on the platform, which can be formed from a magnetic material, or a nonmagnetic material which might include an alloy that shields magnetic fields. A ferrofluici lubricant is provided between the magnets and substrate for ultra low frictional movement. A ferrofluid with a low vapor 'pressure should be selected for environments in which other ferrofluids tend to dry out.
When applied to vapor deposition apparatus, the moveabie magnets are disposed on the floor of a vacuum chamber which includes a vapor source for depositing a thin film on a substrate carried by tne platform. A controller formed from a magnetic material outside the vacuum chamber, on the opposite side of the floor from the moveabie magnets, controls the movement of the magnets and platform to achieve uniform deposition, eliminating the need for the cumbersome control equipment ana its mechanical communication through the vacuum chamber than were previously required.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram .illustrating the magnetic field lines associated with a bar magnet;
FIG. 2 is a diagram illustrating the ultra low friction achieved with a ferrofluid Lubricant in accordance with one aspect of the invention;
FIGs. 3a and 3b are respectively frontal and side elevation views of a low friction, load bearing mechanical translator in accordance with the invention; and
FIG. 4 is a diagram of a vapor deposition chamber using one embodiment of a mechanical translator in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a bar magnet 2 and its associated magnetic field lines 4. As is well known, the field lines radiate out mostly from the magnet's north pole, and loop around to return to the magnet's south pole. The greatest magnetic field concentrations external to the magnet are at its opposite poles, and it is in these regions that a ferrofluid will tend to accumulate when the ferrofluid is presented to the magnet. The ferro-fluid concentration formed at the opposite ends of the magnet, indicated by dashed ovals 6 and 8 around the north and south magnet poles, respectively, can be used as bearings to provide a lubricated movement of the magnet along a surface. Ferrofluid bearings would also tend to form at the opposite ends of an electro-magnet as well as a permanent magnet.
Preferring r.ow to FIG. 2, a magnet 10 _-• shown sup-oorted on a substrate 11, which in turn is on a honcon-
tal support surface 14. A ferrofluid bearing 16 provides an ultra low friction interface between the magnet 10 and. substrate 12. The magnet is oriented with its magnetic axis 18 generally transverse to the substrate 12. Thus, the magnet's magnetic field has it maximum external density adjacent the substrate. Although for purposes of this invention the magnet's magnetic axis will normally be orthogonal to substrate 12, it can also be tilted at an angle up to about 45° from a perpendicular to the substrate, preferably with a corresponding chamfer at the end of the magnet adjacent the substrate.
With the use of an appropriate ferrofluid 16 between the substrate and magnet, ultra low degrees of friction can be achieved between the magnet and substrate, making the
EFHl light mineral oil ferrorluid in the range of approximately 2:1 tc 4:1, ultra low static coefficients of friction in the range of 0.0008-0.0C12 were achieved. Trie viscosity of the mixture was significantly less than 2 cp, on the order of 1 cp . While the mixture's static friction coefficient was even lower than for the EFH1 ferrofluid by itself, the EFH1 composition has a somewhat greater load bearing capability.
The static friction coefficient was measured by raising one end of the substrate 12 off the horizontal surface 14 until a mass supported by the ferrofiuid oeoan to slide along the substrate, determining the substrate's critical off-horizontal angle of displacement at which sliding movement began, returning the substrate to horizontal, lifting its other end until the mass started to slide in the opposite direction, determining the critical angle of displacement from horizontal for sliding to begin in that direction, and averaging the two angles. With the preferred EFKl/isoparaffinic acid mixture, the mass began to slide at an average angle of much less than 1 degree, and even considerably less than 10 minutes. In fact, the critical angle for displacement from a horizontal static position was found to be approximately 0.07 degree.
FIGs . 3a and 3b illustrate the invention applied to an ultra low friction translatable load bearing platform. A set of magn-ets 20 are disposed with their magnetic axes generally transverse to a substrate 22, with one end of the magnets provided with ierrofiuia bearings 24 tor ultra low friction movement over the substrate, ana -he other end of the magnets attached to a load bearing plat-
form 26. A load to be carried by the platform is indicated by dashed lines 28. The shape and size of the olatform, as well as the number of magnetic posts, -are arbitrary and can be chosen, depending upon the load 10 be placed on the platform. The platform can be formed from a magnetic material so that it diffuses the mac net! :• field, leaving the strongest field location at the opposite end of the magnet adjacent the subsrrate. This tends to concentrate the ferrofluid away from the platform and towards the bottom of the magnet, where it functions as a lubricant for magnet movement: over the substrate. The platform can also be formed from a nonmagnetic material. Another option is to form the platform from a nonmagnetic alloy that provides effective shielding of the magnetic field, thereby creating a zone above the platform which is free of magnetic flux. Such a material is available from Spang & Company Corp. under the trademark. MUMETAL. The magnet posts can be attached to the platform by mechanical devices such as clamps, bolts or adhesives.
The ferrofluid solution is applied to the bottom of the posts where, because of their magnetic property, they bond strongly to engulf the post bottoms. The platform thus rests on ferrofluid "cushions", without a direct contact between the magnets and substrate. Because of the low friction achievable with ferrofluid bearings, the platform car.' be pushed to move freely with a slight force. The movement can be controlled by either applying it directly to the platform and/or magnets, or by moving a magnetic control object (formed from either a magnet-iced or a magnetizable material on the other side of the
subs crate. The movement or tne platform can be controlled remotely by moving the external controller on the contact to the platform assembly rtseif.
An application for this type of motion control is a vapor deposition chamber, illustrated in FIG. A. The chamber consists of a vacuum enclosure 30, trie floor 32 of which comprises a substrate upon which a load bearing platform 34 moves via attached magnets 36 and ferrcfluid bearings 38. As illustrated, the magnets 36 in FIG. 4 have a magnetic polarity opposite to that illustrated in FIGs. 3a and 3b; the choice is arbitrary. A substrate 40 upon which a thin film is to be deposited is placed on the platform 34.
The chamber is evacuated a vacuum pump 42, while a vapor source generically indicated by reference number 44 provides a material to be deposited on the substrate. In practice, the vapor deposition process can take several forms, such as thermal evaporation, e-beam evaporation or different forms of sputtering. These all require that the substrate holder be able to rotate and translate inside the vacuum chamber to achieve a uniform deposition over a large substrate area. The prior need for gears, chains, and both rotary and translational motion vacuum feedthroughs are eliminated with the use of an external magnetized or magnetizable control mass 46, placed on the opposite side of substrate 32 from the platform 34 and aligned with its support magnets. The external controller 46 can be translated in an x-y plane, as indicated bv arrows 48 and 50, or rotated as ind-catsa ov circular arrow 52, imparting a correspcndina movement: to the inte-
rior magnets and the platform they support. The external' controller 46 is preferably a single mass which encompasses the area subtended by the interior magnets 33, or can equivalently be an array of separate controller-aligned with the individual magnets 36 and moved together. The platform and the substrate which it carrifs can be rotated and 'translated over a large area simply by imparting the same movements to the easily accessible external controller. If the controller is magnetized, its polarity should be oriented in the same direction as the internal magnets for mutual attraction.
APG S10 grade ferrofluid from Ferrotec (USA) Corporation was used for the vapor deposition application because of its low vapor pressure, which allows it to be used-in a vacuum or ambient atmosphere with a long operational lifetime. Some other ferrofluids have a tendency to dry out in this environment. A 7.6 cm diameter platform with four 0.95 cm diameter, 6.4 cm length, Grade 30 NdFeB magnets, cushioned by the APG SiO grade ferrofiuid, was found to be capable of supporting a 250 gram load, while maintaining a low static coefficient of friction in the range 0.01-0.02. While not as low as the other far~ rofluid compositions discussed above, this value was still considerably lower than traditional lubricants.
In addition to providing a lower level of friction, the use of a ferrofluid allows the movement of the platform to be controlled externally by a moving magnet body. With traditional lubricants, placing a magnet on the apposite side of the wall from the platform can attract the platform's magnet posts strongly enough that the lubricant is squeezed out, leaving the posts in a direct high
friction contact with the wall. However, due to the attraction between the magnet posts and the magnetic p. ancpar t icles in a ferrotluid, a cushion of ferroflui a lubricant will remain between th.e magnet posts and trie chamber wall.
While various embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.
1. A mechanical translator, comprising:
a substrate (12),
at least one magnet (10) disposed for movement on said substrate, each magnet positioned to carry a load and having a magnetic axis (18) transverse to said substrate, with a surface of each magnet facing said substrate, and
characterized in that a ferrofluid lubricant (16) is concentrated between each magnet and said substrate, said ferrofluid extending across and in direct contact with each said magnet surface to provide a low friction area support for each magnet to translate across said substrate.
2. A mechanical translator as claimed in claim 1 wherein
a surface of each magnet(l0) faces said substrate(12) and each magnet is positioned to carry a load and establishing a magnetic field (4) which has its maximum external density adjacent to said substrate(12).
3. A mechanical translator as claimed in claim 1 wherein
each magnet has a magnetic axis at an angle of at least 45° to said substrate and a critical angle of displacement (a) from a horixontal static position of less than 1 degree, with a surface of each magnet facing said substrate.
4. A mechanical translator as claimed in claims 1 and 2 wherein said at
least one magnet has a critical angle of displacement (a) from a
horizontal static position of less than 1 degree.
5. A mechanical translator as claimed in claim 2 and 3 comprising a
respective ferrofluid bearing (16, 24, 38) providing a lubricant for each
6. The mechanical translator as claimed in claim 1, 2 and 3 comprising a
controller (46) of magnetic material on the opposite side of said
substrate from said at least one magnet, said controller controlling the
movement of said at least one magnet.
7. The mechanical translator as claimed in claim 6, said substrate
comprising a portion of an enclosure, with said at least one magnet
inside and said controller outside said enclosure.
8. The mechanical translator as claimed in claim 1, 2 and 3 said at least
one magnet comprising a plurality of magnets (20) carrying a
platform (26) and connected to respective fixed locations on said
9. A mechanical translator substantially as herein described with
reference to the forgoing description and the accompanying Drawings.
|Indian Patent Application Number||2274/DELNP/2004|
|PG Journal Number||33/2013|
|Date of Filing||04-Aug-2004|
|Name of Patentee||ROCKWELL SCIENTIFIC LICENSING,LLC|
|Applicant Address||1049 CAMINO DOS RIOS, P.O. BOX 1085, MC A15, THOUSAND OAKS, CALIFORNIA 91358-0085, U.S.A.|
|PCT International Classification Number||F16C 29/02|
|PCT International Application Number||PCT/US03/05058|
|PCT International Filing date||2003-02-18|