FIELD OF THE INVENTION

The present invention relates to the field of rails for motion systems, especially using a combination of materials to provide thermal stability in combination with low weight and high wearablity.

BACKGROUND OF THE INVENTION

In motion systems in which a traveling carriage moves along a precision profile, often known as a rail, there is often a trade-off between the various properties which the rail provides to the system. Ideally, a hard, mechanically stable and wear resistant material should be used for the rail, so that its contact surfaces, along which the ball bearings, cylindrical rollers or slide surfaces of the movable carriage roll as the carriage travels along the rail, is sufficiently hard to ensure minimal friction and long life with minimal wear. Steel alloys provide one such a material. However a rail system made of steel has a number of disadvantages. Firstly, compared with softer materials such as aluminium or magnesium alloys, it would be expensive to machine and/or grind to shape. Secondly, it would be heavy. Thirdly, it would be prone to thermal distortions when mounted on the base of a system constructed of a light alloy such as aluminium, which is the type of material of choice for air-borne applications, since the thermal expansion coefficient of an aluminium alloy base unit is substantially larger than that of a steel rail mounted thereon. As a result, thermally induced mechanical distortions will be generated if a steel rail is mounted on such an aluminium alloy baseplate. This is particularly so in systems which may be used over wide temperature ranges, such as the common military specification which requires coverage of the -54°C to + 85°C range. Therefore, the use of rails made of materials such as aluminium alloys, which do not have such thermal distortion problems when bolted to bases of aluminium alloy, may have advantages, especially for motion systems which are used in airborne applications. However, aluminium alloy rails are subject to significantly faster mechanical wear and hence loss of accuracy with time, than steel rails, and hence cannot be used in high precision, long lifetime applications.

There exist in the prior art composite rails in which the body of the rail profile is made of an aluminium alloy in order to provide low cost and light weight, and the contact surfaces on which the carriage slides, and which therefore define the position of the carriage, are made of thin stainless steel strips bonded along their length to the aluminium alloy profile. However, such a composite rail system is still prone to mechanical distortion with change in temperature, because of the large differences in thermal expansion between the aluminium alloy profile and the steel surfaces firmly affixed thereto. Such rails are therefore limited to applications where high precision is not required. Thus, for instance, they would not be generally suitable for use in high precision optical applications, such as in the motion systems of zoom lenses, where the lateral and longitudinal stability and positional accuracy of the moving carriage may be required to be of the order of micrometers in high precision lenses.

There therefore exists a need for a rail system for mounting on a lightweight platform such as one constructed of aluminium alloy, but which has high accuracy and hard wearing contact surfaces and withstands temperature changes with minimal mechanical distortion, especially for use in precision optical systems, and which would therefore overcome at least some of the disadvantages of prior art systems and methods.

The disclosures of any publications mentioned in this section and in other sections of the specification are hereby incorporated by reference, each in its entirety.

SUMMARY

The present disclosure describes new exemplary motion systems which utilize a novel rail comprising a light alloy profile with hard metal strips at the contact surface of the motion carriage with the profile, generally on opposite sides of the profile. The light alloy profile generally has a relatively high thermal expansion coefficient, while the hard metal strips, generally have a relatively low thermal expansion coefficient, The rails described in the present disclosure differ from those known in the prior art in that the hard metal contact strips are not firmly fixed along their length to the rail base profile, but are free to expand and contract independently of the profile, such that essentially no thermally induced mechanical distortion of the rail assembly is generated. The metal contact strips are held in place by the sliding surfaces of the carriage itself, which, in high precision designs, are generally ball bearings or cylindrical rollers, which, because of the rolling action, provide minimal frictional forces. The walls of the carriage which incorporate the bearings enclose the rail and its associated strips, which are therefore constrained in position between those walls. The strips may advantageously be provided with V-grooves on their outer surfaces facing the walls of the carriage, such that the ball bearings of the carriage sit within those grooves, thereby defining the position of the carriage relative to the strips in the direction perpendicular to the plane on which the rail is mounted. The carriage thus has a clearly defined position both in the direction between the planes of the ball bearings and in the direction perpendicular to the mounting plane, while its motion position along the rail is defined by the usual methods of position encoders or lead screws, or any other methods in common use.

The width of the rail profile together with its contact strips is predefined relative to the internal distance between opposing ball bearings of the carriage such that when the profile with its loosely mounted contacts strips is contained beneath the carriage, sufficient pressure is exerted on the contact strips that they are held in place under the carriage in firm contact with the rail base profile. Other parts of the contact strip, in regions outside of the position of the carriage, are unattached to the rail base profile, and are free to expand longitudinally along the length of the rail. As the carriage moves along the rail, it releases the region of the contact strip which was previously held gripped to the base profile, and instead presses a new region of the contact strip into good contact with the base profile, that new region being the section of the strips now contained between the bearings within the carriage walls. Thus the contact strips are always held firmly in contact with the rail base profile disposed underneath the position of the moving carriage, while the parts of the contact strips not underneath the position of the moving carriage are free to expand or contract independently of the expansion or contraction of the base profile. Such a construction thereby enables a rail having light weight and low-cost, which can be affixed to a light alloy base structure without engendering any thermally induced mechanical distortions, and yet which has hard wearing and stable motion contact surfaces to provide high precision with low wear characteristics.

In order to ensure that the contact strips are always positioned at the correct height relative to the height of the base profile, they may advantageously be mounted within grooves preformed in the base profile, such that their height position is pre-determined. The grooves may advantageously be either V-grooves or rectilinear grooves. The fit of the strips within the grooves should be such as to enable the presence of the carriage to firmly clamp them in position, but without generating a press fit which may be sufficient to essentially bond the contact strips to the groove along their whole length. In order to prevent the contact strips from having undue freedom to vibrate or move away from the base profile in regions where the contact strips are not clamped to the base profile, it is possible to use clips at the ends of the strips, which loosely hold the strips in contact with the profile but without limiting their ability to move longitudinally.

As an alternative to the above described methods of having the contact strips totally unattached to the base profile, it is feasible to utilize what may be termed a semi-flexible method of attaching the contact strips to the base profile, such as by the use of regions of flexible adhesive material, or screw attachments which do not clamp the contact strips to the base profile but maintain them in their correct height and lateral positions relative to the rail base profile, but allow them to move longitudinally relative to the base profile. In any of these implementations, it is important to ensure that there is no intermediary layer of adhesive material between the contact strips and the base profile, such that when the carriage presses the contact strips against the base profile, its lateral and height position is accurately defined.

The novel composition material rails are described in this disclosure as comprising a combination of a lightweight alloy base profile and hard wearing metal contact strips. Though not intending to limit the invention in any manner, commonly used materials for these component parts are aluminium or magnesium alloy for the rail base profile, and high tensile machine steel or stainless steel for the contact strips. There thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a rail system for use with a motion carriage, comprising:

(i) a base profile part constructed of a base material, the profile having longitudinal grooves at opposite sides along its length, and

(ii) strips made of a material having higher wear resistance than the base profile material, a separate strip being disposed within each of the grooves, such that the presence of the motion carriage clamps the strips to the base profile part in the region of the motion carriage,

wherein the strips are not otherwise firmly attached to the base profile such that they are free to move longitudinally relative to the base profile part.

The freedom to move longitudinally relative to the base profile part enables the strips to expand and contract with change of temperature independently of expansion and contraction of the base profile.

In such a rail system, the base material may be a light alloy material, and this base material may have thermal expansion characteristics similar to those of a base structure on which the rail system is to be attached. Additionally, the strip material may be a steel.

According to additional exemplary implementations, each of the strips may be attached to the base profile at points along its length by means of at least one fixing component which provides flexibility of attachment sufficient to allow the strips to move longitudinally relative to the base profile part, such that the strips can expand and contract with change of temperature independently of expansion and contraction of the base profile.

Furthermore, in any of the previously described rail systems, the strips may comprise V- grooves on those of their surfaces not facing the base profile, such that ball bearings located within the motion carriage can roll within the V-grooves. Alternatively, the strips may comprise V-grooves on those of their surfaces not facing said base profile, such that protuberances located within the motion carriage can slide along the strip V- grooves. According to yet a further optional implementation, the motion carriage may incorporate cylindrical roller bearings which roll along the strips. Although the use of the systems described in this disclosure has been directed at the specific problem of linear rail profile systems with motion carriages moving along the rail profiles, it is to be understood that this is only one use of the invention described in this application, and the invention is not intended to be limited to that application. More generally, what is further described in this disclosure is a method of ensuring stress and distortion free motion, whether longitudinal or angular, between two mutually moving elements, over a wide temperature range. Either or both lower friction and higher wear resistance are provided by use of an intermediary material selected to have a high wear resistance and/or low friction surface, used as the contact interface between the two moving elements. The thermal expansion coefficient of such an intermediary material may generally be different from that of either of the two elements. The method is characterized in that the intermediary material is not firmly bonded to either element but is held in position only by the pressure of the contact between the two mutually moving elements in the region of contact between them. Therefore, changes in temperature enable the intermediary material to expand or contract differently from the two mutually moving elements. This is especially important for expansion or contraction along the direction of mutual motion of the two mutually moving elements, which is generally the longest dimension in the system. This method thus enables the provision of precision motion systems having high accuracy and long lasting wear characteristics, even for operation over wide temperature ranges, where prior art methods of construction may result in thermal distortion causing reduced precision in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig. l illustrates schematically an exploded isometric view of an exemplary motion system constructed using a light alloy profile with steel strip inserts;

Fig. 2 illustrates schematically the motion system of Fig. 1 fully assembled with the motion carriage in place on the rail;

Fig. 3 illustrates schematically an end cross-sectional view of the motion carriage mounted on the rail assembly; and Figs. 4A and 4B illustrate respectively schematic side elevation and plan views of a completely assembled motion system of the type described in this disclosure.

DETAILED DESCRIPTION

Reference is now made to Fig. 1, which illustrates schematically an exploded isometric view of an exemplary motion system constructed using a light alloy profile 10 with steel strips 12 for mounting in grooves 14 on either side of the aluminium rail profile 10. In the example shown in Fig. 1, the profile grooves 14 are rectangular, this being the most convenient shape from which to manufacture the grooves in the profile and the steel strips themselves for fitting into the grooves, though it is to be understood that they could be of any other suitable shape, such as V-grooves. The motion carriage 13 has sidewalls 17 which straddle the completely assembled rail, and the sliding elements 16 on the inside surfaces of these walls may be either fixed protuberances or more usually, ball or roller bearings. The steel strips 12 shown in the implementation of Fig. 1 are equipped with V-grooves 15 on their outer faces so that the matching protuberances or ball bearings 16 on the carriage 13 fit into those V-grooves 15, thereby defining the position of the motion carriage in the plane perpendicular to the mounting surface of the rail.

In its basic form, the novel rails described in this disclosure do not need any form of fixturing to attach the steel strips 12 to the aluminium profile 10, and they are primarily maintained in position in the region of the motion carriage by means of the pressure exerted laterally and inwardly by the motion carriage 13. However in order to prevent undue vibration of those parts of the steel strips not gripped by the motion carriage, such as may well occur in airborne or other such applications, it is possible to utilize some light form of fixturing in order to hold the steel strips in place, without them being firmly attached by that fixturing. The term firmly is defined to be such that the fixturing is sufficiently flexible that it does not prevent the steel strips from moving longitudinally relative to the aluminium profile, such as under the effect of changes in ambient temperature, and does not impart forces to the aluminium profile that may cause deformation that could interfere with the accuracy of the motion system. One such form of optional fixturing is shown Fig. 1 in the form of a bent clip 19, which can be made of metal sheet or plastic, and which is clipped over the end of the assembled rail once the steel strips 12 have been pushed into their grooves 14 in the aluminium profile 10 and the motion carriage 13 has been mounted on the rail assembly. The tightness of the clip must be arranged to be at a level such that it holds the steel strips in place yet does not exert sufficient lateral pressure on them to prevent them from moving freely in the longitudinal direction. Alternative fixturing could include adhesive material (not shown in Fig. 1) applied to the steel strips after insertion, the adhesive having sufficient flexibility and thickness to enable the steel strips to move longitudinally relative to the aluminium rail profile sufficiently to enable expansion over the temperature range envisaged, and yet which holds the steel strips in position within their grooves in the aluminium rail profile. As another possible alternative, the steel strips could be press fitted into their grooves, the grooved height and rail heights being designed to provide a press fit at a level such that the steel strips are held firmly within their grooves, but not so firmly that they are unable to move longitudinally along the length of the grooves during thermal expansion, without transferring stresses to the aluminium rail profile.

Reference is now made to Fig. 2, which illustrates schematically the motion system of Fig. 1 fully assembled with the motion carriage 13 in place on the rail 10, with the steel strips 12 in position within their grooves 14 in the aluminium rail 10. The novelty of the current system, though not visible in the small-scale drawing of Fig. 2, is that the steel strips 12 are not attached rigidly to the aluminium rail profile 10, but are simply inserted into place within the rail groove 14, and are firmly held in position only in the region beneath the motion carriage 13. As previously mentioned, the dimensions of the carriage and its associated protuberances or ball bearings 16 is such that the carriage grips the steel strips 12 laterally such that they are clamped firmly against the aluminium rail profile 10 by contact of the motion carriage sliding or rolling surfaces with the steel rails. The position of this clamping action moves along the rail assembly as the carriage moves along the rail assembly.

Reference is now made to Fig. 3, which illustrates schematically an end cross-sectional view of the motion carriage 13 mounted on the rail assembly 10, 12, showing how the sliding or rolling elements 16 on the sidewalls 17 of the motion carriage fit into the V- grooves of the steel strips 12, forcing the steel strips into their designated grooves in the aluminium rail 10.

Reference is now made to Figs. 4A and 4B, which illustrate schematically side elevation and plan views respectively of a completely assembled motion system of the type described in this disclosure, showing the carriage 13 mounted on the rail assembly. Fig. 4A shows how the clamped position of the steel strips defines the height of the carriage above the mounting plane on which the rail system is mounted, while Fig. 4B illustrates the thread inserts 18 by which the motion system rail assembly is attached to the mounting surface on which it operates.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.