Linear motion device with lubricating film monitoring and insulated raceway insert

Abstract

Linear motion device (10; 10') with a guide element (12) which is elongated parallel to a longitudinal axis (11) and with a movement element (20), wherein the movement element (20) has at least one raceway (23) which extends parallel to the longitudinal axis (11), wherein it is in rolling contact with an associated row of rolling elements (14), wherein said row of rolling elements (14) is in rolling contact with the guide element (12), wherein resistance detection means (50) are provided with which the complex electrical resistance between a first and a second measuring point (61;62), wherein a lubrication state determination device (60) is provided, with which a lubrication state in the region of the rolling elements (14) can be determined using the said complex electrical resistance, characterized in that the movement element (20) comprises a separate base body (24) and a separate first raceway insert (21), wherein at least one raceway (23) is arranged on the first raceway insert (21), wherein an electrical insulation (40) is arranged between the first raceway insert (21) and the base body (24) such that the ohmic resistance between the base body (24) and the first raceway insert (21) is more than 1 MΩ, wherein the first measuring point (61) is electrically connected to the first raceway insert (21);

Description

The invention relates to a linear motion device according to the preamble of claim 1 and a method for operating the same.WO 2008 / 093652 A1 discloses a linear movement device. The guide element in the form of a guide rail extends parallel to a longitudinal axis. The movement element in the form of a guide carriage is mounted on the guide carriage such that it can be moved in the direction of the longitudinal axis via rolling bodies. The lubricating film in the region of the rolling bodies is monitored by an electrical resistance measurement between guide element and movement element. For this purpose, both the guide element and the movement element are electrically insulated from the environment.JP 2009-63 397 A discloses a radial groove ball bearing in which an electrical capacitance between an inner ring and an outer ring is measured.JP 2012-159 127 A discloses a linear rolling bearing in which a tension between a guide carriage and a guide rail is measured.JP 2005-54 828 A discloses a linear rolling bearing in which a voltage between a guide carriage and a guide rail is measured, this voltage being generated indirectly by a battery.An advantage of the present invention is that the required electrical insulation of the rolling body contact from the environment can already be carried out during the production of the movement element. The user of the linear motion device does not need to make any related efforts. Furthermore, the electrical insulation can be produced particularly simply and cost-effectively. Moreover, the reliability of monitoring the lubrication film is improved.According to claim 1, it is proposed that the movement element comprises a separate base body and a separate first raceway insert, wherein at least one raceway is arranged on the first raceway insert, wherein an electrical insulation is arranged between the first raceway insert and the base body in such a way that the ohmic resistance between the base body and the first raceway insert is more than 1 MΩ, wherein the first measuring point is electrically connected to the first raceway insert. The electrical insulation is thus located in the interior of the guide element, so that it can be produced simultaneously without problems during its production. The user of the linear motion device does not contact the electrical insulation under normal circumstances.The linear movement device can be a rail guide, wherein the guide element is the guide rail, wherein the movement element is the guide carriage. However, it can also be a ball bush guide in which the guide element is a circular cylindrical shaft, wherein the movement element is a ball bush. The complex electrical resistor preferably has an ohmic component (real part of the complex resistor) and / or a capacitive component (negative imaginary part of the complex resistor), wherein it is at least conceivable that it also has an inductive component. The guide element is preferably made of steel, which is hardened at least in the region of the rolling contact. The first and / or the second raceway insert preferably consist of hardened steel. The base body is preferably made of uncured steel.The dependent claims specify advantageous refinements and improvements of the invention.It can be provided that the movement element has at least two raceways, wherein a separate second raceway insert is provided on which at least one raceway is arranged, wherein an electrical insulation is provided between the second raceway insert and the base body in such a way that the electrical resistance between the base body, the first raceway insert and the second raceway insert is in each case more than 1 MΩ in pairs, wherein the second measuring point is electrically connected to the second raceway insert at least indirectly via a ground. In the case of a guide element that is not grounded, this makes it possible to monitor the lubrication state. The first and second raceway inserts including the electrical insulation are preferably designed mirror-symmetrically or identically to one another.It can be provided that the second measuring point is electrically connected to the guide element, at least indirectly via a ground. In the case of a grounded guide element, this makes it possible to monitor the lubrication state.It can be provided that the electrical insulation is formed in the form of a coating of the first and / or the second raceway insert and / or the base body with an electrically insulating layer. Such a layer can be produced particularly simply and cost-effectively. The thickness of the layer is preferably less than 10 μm so as not to substantially affect the bias ratios of the linear motion device. The electrically insulating layer is preferably connected in a materially integral manner to the first or second raceway insert or the base body. The raceway inserts are preferably coated, since they are small relative to the base body. The coating can thus be carried out particularly cost-effectively.It can be provided that the electrically insulating layer is an aluminum oxide layer, a phosphate layer, a diamond-like carbon layer or an adhesive layer. These layers have a good insulating effect and can be produced cost-effectively. In the case of an adhesive layer, the corresponding adhesive is preferably an electrically insulating polymer. The adhesive preferably comprises an electrically insulating filler, which is most preferably designed in the form of balls made of PEEK and / or glass. In this way, a metallic contact of the raceway insert and the base body can be reliably prevented.It can be provided that the electrically insulating layer is formed by a separate foil which is connected in a materially integral manner to the first and / or the second raceway insert and / or the base body. The film preferably consists of a polymer and / or a ceramic.It can be provided that the electrically insulating layer extends at least over the region at which the first or the second raceway insert abuts the base body. This ensures the desired insulation between the raceway insert and the base body. Preferably, the raceway is not covered with the electrically insulating layer so as not to interfere with the monitoring of the lubrication film. In this case, it is conceivable for the first and / or the second raceway insert to be provided first with the electrically insulating layer on its entire surface, wherein the electrically insulating layer is subsequently removed again in the region of the at least one raceway. This can be done, on the one hand, by grinding the relevant raceway after the coating. However, it is also conceivable for the coating in the region of the at least one raceway to be removed again during operation by the hard rolling bodies.It can be provided that the base body or the guide element, viewed in cross section, is U-shaped with a base and a first and a second leg, wherein the first leg is assigned the first raceway insert, wherein the second leg is assigned a or the second raceway insert. In the case of a U-shaped base body, the respective raceway insert preferably abuts the relevant leg via the electrical insulation. In the case of a U-shaped guide element, the relevant rolling bodies are preferably arranged between the raceway insert and the relevant leg.It can be provided that the resistance detection means comprise a current source and a voltage measuring device. The voltage measuring device can be an analog-to-digital converter. The current source can be a digital-to-analog converter, followed by an amplifier.Within the scope of a method for operating a linear movement device according to the invention, a constant or a time-variable measurement current is impressed at the first measurement point, wherein a measurement voltage is measured between the first and the second measurement point, wherein the complex electrical resistance is determined from the measurement voltage and the measurement current. The above method is preferably performed by the resistance detecting means, which are most preferably arranged according to this method. The time-variable measurement current is preferably a sinusoidal measurement current. The amount of the measurement current is preferably selected such that a current density of 0.1 A / mm 2 is not exceeded in the region of an individual rolling contact in order to avoid damage to the raceway and / or the rolling bodies.It can be provided that the injection of the measurement current and the measurement of the measurement voltage are carried out while the movement element moves relative to the guide element, preferably at a constant speed. In the standstill state, the rolling bodies bear at least pointwise directly against the associated raceways, so that there is a direct electrical contact. Only by the rolling movement of the rolling bodies is a lubricating film formed, which results in a complex resistance that is clearly different from zero. Since the lubrication film is formed only when sufficient lubricant is present, sufficient lubrication can be inferred by measuring the complex resistance.The electrical connection between the resistance detection means and the first or the second raceway insert can be effected, for example, by means of a fixed or non-detachable connection of the corresponding electrical lines, in particular by adhesive bonding, welding or soldering. However, it is also conceivable to use a releasable electrical connection which comprises in particular a spring, for example a flat spring or a spiral spring. As a result, the corresponding electrical line or an associated contact can be pressed against the associated first or second raceway insert. In order to improve the electrical contact, a conductive paste may be used.It is conceivable to utilize the current-induced Barkhausen effect according to EP 918 999 B1. For this purpose, a time-varying electric current is impressed into the relevant electrically insulated raceway insert. This changes the magnetization of the ferromagnetic raceway insert. The magnetization of the raceway insert is preferably measured with a Hall sensor in order to determine the Barkhausen noise. Consequently, for example, material fatigue on the relevant raceway can be detected.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention.The invention is explained in more detail below with reference to the attached drawings. It shows: FIG. 1 is a linear motion device according to a first embodiment of the invention; FIG. 2 is a linear motion device according to a second embodiment of the invention; and FIG. 3 is a partial cross-section in the region of the first raceway insert.FIG. 1 shows a linear motion device 10 according to a first embodiment of the invention. The linear movement device 10 comprises a guide element 12 and a movement element 20, which are embodied, with the exception of the electrical insulation (no. 40 in FIG. 3 ), for example according to EP 2 952 761 A1. The entire contents of EP 2 952 761 A1 are referred to and made part of the content of the present application. The guide element 12 is in the present case designed in the form of a guide rail which extends with the constant cross-sectional shape shown in FIG. 1 parallel to a longitudinal axis 11 which is oriented perpendicular to the plane of the drawing of FIG. 1. The guide element 12 is made of steel and is hardened in the region of the counter-raceways (no. 13 in FIG. 3 ). The guide element 12 is thus electrically conductive. The movement element 20 is designed in the present case in the form of a guide carriage which is mounted on the guide element 12 such that it can move in the direction of the longitudinal axis 11 over a total of four rows of continuously revolving rolling bodies 14. The movement element 20 has a U-shaped base body 24 with a base 25 and a first and a second limb 26; 27.A first raceway insert 21 abuts on the inside of the first limb 26, wherein a second raceway insert 22 abuts on the inside of the second limb 27. The two raceway inserts 21; 22 are designed mirror-symmetrically to one another, wherein two raceways (no. 23 in FIG. 3 ) are arranged on each of the two raceway inserts 21; 22, which raceways each extend parallel to the longitudinal axis 11. Each row of rolling bodies 14 rolls on an associated raceway and an associated counter raceway. The rolling bodies are balls in the present case. The two raceway inserts 21; 22 and the rolling bodies 14 consist of hardened steel, so that they are electrically conductive. The base body 24 is made of uncured steel, so that it is also electrically conductive. Between the first or the second raceway insert 21; 22 and the base body 24, an electrical insulation (no. 40 in FIG. 3 ) is provided, which is explained in more detail with reference to FIG. 3. The electrical insulation has the effect that the raceway inserts 21; 22 are substantially completely insulated with respect to the base body 24.Resistance detection means 50 are also provided, which have a first and a second measurement point 61; 62. The resistance detection means 50 comprise a current source 51 and a voltage measuring device 52, which are connected in parallel between the first and the second measuring point 61; 62. The first measuring point 61 is electrically connected to the first raceway insert 21 via a connecting line. The second measuring point 62 is indirectly connected to the second raceway insert 22 via a ground 53, wherein it can also be connected to the second raceway insert 22 via a connecting line. The first embodiment is intended for applications in which the guide element 12 is electrically insulated from the ground 53. Here, for example, a guide rail is intended which is mounted on a machine bed made of electrically insulating polymer concrete.If a measurement current 54 is now impressed by means of the current source 51, it can flow substantially exclusively toward the first raceway insert 21, since the voltage measurement device 52 has a very high internal resistance. The measuring current flows from there further via the rolling elements 14 assigned to the first raceway insert 21 to the guide element 12, further via the rolling elements 14 assigned to the second raceway insert 22 to the second raceway insert 22, The measuring current 54 flows from the second raceway insert 22 via the ground 53 back to the current source 51, so that the circuit is closed. Ground 53 provides a 0 volt potential.An appreciable voltage drop occurs in the above-explained current path solely in the rolling element contacts, since there may be an electrically nonconductive lubricating film which separates the rolling elements 14 from the associated raceway or counter raceway. In the case of an alternating current, the capacitive effect of the lubricating film additionally allows a phase shift between the measurement current and the measurement voltage to be observed. If the movement element 20 does not move with respect to the guide element 12, the rolling bodies 14 are pressed directly against the raceway and the counter raceway by the typically present prestress, any lubricant present there being displaced. The voltage drop is accordingly small. Only when the rolling bodies 14 roll off sufficiently quickly does a supporting lubricating film form, which causes a voltage drop, which is measured with the voltage measuring device 52. It is understood that this lubricating film is formed only when sufficient lubricant is present. Thus, the above-mentioned voltage drop or the measurement voltage 55 is an indicator for sufficient lubrication of the linear movement device 10.The power source 51 and the voltage measuring device 52 are connected to a lubrication state determining device 60. The lubrication state determination device 60 preferably comprises a programmable digital computer. The voltage measuring device preferably comprises an analog-to-digital converter. The digital computer and the analog-to-digital converter can be combined in a microcontroller. The lubrication state determination device 60 preferably turns on the power source 51 at a time when the linear motion device 10 moves at a constant speed. The corresponding measurement current is preferably selected such that the current density in all rolling element contacts does not exceed 0.1 A / mm 2 in order to avoid damage to the rolling element contacts. The measurement current 54 can be a direct current and / or an alternating current. If the measurement current 54 is a sinusoidal alternating current, the complex electrical resistance is the quotient of the complex amplitude of the measurement voltage 55 and the complex amplitude of the measurement current 54.Depending on the viscosity of the lubricant and the travel speed of the linear movement device 10, a more or less bearing lubricating film is formed. The more pronounced this is, the greater the electrical resistance also becomes. This is reflected in a correspondingly higher measured measurement voltage 55 or in a higher reactance (this corresponds to a smaller electrical capacitance).Since voltage flashovers can occur in a formed lubricating film due to the applied measurement voltage 55, it is expedient to carry out both the voltage measurement as an effective value measurement and to carry out the entire measurement not permanently, but only occasionally. Ideally, the measurement is performed only when the linear motion device 10 moves at a nearly constant speed. This measurement is preferably carried out at one or more defined reference speeds.Depending on the measurement results, i.e. upon detection of a lubrication film non-order signal for the lubrication film quality or a significant deterioration of the lubrication quality, a manual or automatic re-lubrication can take place.It should also be noted that the base body 24 of the movement element 20 is frequently electrically connected to the ground 53. The electrical insulation according to the invention (no. 40 in FIG. 3 ) is required especially in these cases, since a current path via the base body 24, further via the ground 53, back to the resistance detection means 50 is blocked as a result. This would have the effect that the measurement voltage 55 is always 0 volts, essentially independently of the lubrication state.Fig. 2 shows a linear motion device 10' according to a second embodiment of the invention. The second embodiment is identical to the first embodiment except for the differences described below, so that reference is made in this respect to the explanations relating to FIG. 1. In FIGS. 1 and 2, identical or corresponding parts are provided with the same reference numerals.The second embodiment is intended for applications in which the guide element 12 is electrically connected to the ground 53. Here, for example, a guide rail is intended which is mounted on a machine bed made of electrically conductive cast iron, wherein the machine bed is grounded. In contrast to the first embodiment, the path of the measurement current 54 runs solely over the first raceway insert 21, so that solely the lubrication state there can be determined by means of the lubrication state determination device 60. The accuracy of the lubrication state determination is higher than the first embodiment. In particular, cases can be detected in which deficient lubrication is present only at one raceway insert 21; 22. Preferably, a changeover device (not shown) is provided, by means of which the first or the second raceway insert 21 can be selectively connected to the resistance detection means 50, so that a lubricating film monitoring can be carried out on both raceway inserts 21; 22.FIG. 3 shows a partial cross section in the region of the first raceway insert 21, which detail is present identically in both the first and the second embodiment.It can be seen in FIG. 3 how the spherical rolling bodies 14 each bear on an adapted raceway 23, on the first raceway insert 21 and on an opposing raceway 13 on the guide element 12. The lubricating film is formed there, which is to be monitored with the present invention. It can also be seen how the first raceway insert 21 rests on the base body 24 on the side facing away from the raceways 23. In the present case, a V-shaped groove 29 is provided in the base body 24, in which the first raceway insert 21 is accommodated. In order to prevent an electrical contact between the first raceway insert 21 and the base body 24 there, an electrical insulation 40 is provided.The electrical insulation 40 is embodied in the present case in the form of an electrically insulating layer 41 on the first raceway insert 21. This is preferably only a few micrometers thick, so that it substantially does not interfere with the bias conditions in the linear motion device. Due to the small thickness, it may be that no complete electrical insulation results, but rather a high contact resistance, which is preferably greater than 1 MΩ.Since the electrically insulating layer 41 would not be visible in FIG. 3 on account of its small thickness when shown to scale, its area of extent is indicated by a dashed line 41. Accordingly, only the rear side of the first raceway insert 21 facing away from the raceways 23 is provided with the electrically insulating layer. The opposite front side with the raceways 23 is preferably not coated, so that the contact resistance there depends mainly on the thickness of the corresponding lubricating film, it being very small if no separating lubricating film is present. The electrically insulating layer 41 is preferably an aluminum oxide layer, a phosphate layer or an adhesive layer.The second raceway insert (no. 22 in FIGS. 1 ; 2 ) is preferably designed mirror-symmetrically with respect to the first raceway insert 21, so that the above explanations apply analogously to the second raceway insert.The electrically insulating layer 41 can also be attached solely to the base body 24, wherein it is also conceivable for both the raceway insert 21; 22 and the base body 24 to be coated.It should also be noted that the rolling body retaining parts denoted by the reference numeral 28 in FIG. 3 are typically made of electrically insulating plastic, so that they have substantially no influence on the determination of the lubrication state.Reference numerals denote reference numerals10 Linear movement device (first embodiment) 10' Linear movement device (second embodiment) 11 Longitudinal axis 12 Guide member 13 Counter raceway 14 Rolling elements 20 Movement member 21 First raceway insert 22 Second raceway insert 23 Raceway 24 Base body 25 Base 26 First leg 27 Second leg 28 Rolling element holding part 29 V-shaped groove 40 Electrical insulation 41 Electrically insulating layer 50 Resistance detection means 51 Current source 52 Voltage measurement device 53 Grounding 54 Measurement current 55 Measurement voltage 60 Lubrication state determination device 61 First measurement point 62 Second measurement point

Claims

Linear movement device (10; 10') having a guide element (12) which is elongate parallel to a longitudinal axis (11), and a movement element (20), wherein the movement element (20) has at least one raceway (23) which extends parallel to the longitudinal axis (11), wherein it is in rolling contact with an associated row of rolling bodies (14), wherein said row of rolling bodies (14) is in rolling contact with the guide element (12), wherein resistance detection means (50) are provided with which the complex electrical resistance between a first and a second measurement point (61; 62), wherein a lubrication state determination device (60) is provided, with which a lubrication state in the region of the rolling bodies (14) can be determined using said complex electrical resistance, characterized in that the movement element (20) comprises a separate base body (24) and a separate first raceway insert (21), wherein at least one raceway (23) is arranged on the first raceway insert (21), wherein an electrical insulation (40) is arranged between the first raceway insert (21) and the base body (24) in such a way that the ohmic resistance between the base body (24) and the first raceway insert (21) is more than 1 MΩ, wherein the first measuring point (61) is electrically connected to the first raceway insert (21).Linear movement device (10) according to Claim 1, wherein the movement element (20) has at least two raceways (23), wherein a separate second raceway insert (22) is provided on which at least one raceway (23) is arranged, wherein an electrical insulation (40) is provided between the second raceway insert (22) and the base body (24) in such a way that the electrical resistance between the base body (24), the first raceway insert (21) and the second raceway insert (22) is in each case more than 1 MΩ in pairs, wherein the second measurement point (62) is electrically connected to the second raceway insert (22) at least indirectly via a ground (53).Linear movement device (10') according to Claim 1, wherein the second measurement point (62) is electrically connected to the guide element (12) at least indirectly via a grounding (53).Linear movement device (10; 10') according to one of the preceding claims, wherein the electrical insulation (40) is formed in the form of a coating of the first and / or the second raceway insert (21; 22) and / or the base body (24) with an electrically insulating layer (41).The linear motion device (10; 10') according to claim 4, wherein the electrically insulating layer (41) is an alumina layer, a phosphate layer, a diamond-like carbon layer, or an adhesive layer.Linear movement device according to Claim 4, wherein the electrically insulating layer (41) is formed by a separate film which is connected in a materially integral manner to the first and / or the second raceway insert (21; 22) and / or the base body (24).Linear movement device (10:10') according to one of Claims 4 to 6, wherein the electrically insulating layer (41) extends at least over the region at which the first or the second raceway insert (21; 22) bears against the base body (24).Linear movement device (10; 10') according to one of the preceding claims, wherein the base body (24) or the guide element (12), as viewed in cross section, is formed in a U-shape with a base (25) and a first and a second limb (26; 27), wherein the first limb (26) is assigned the first raceway insert (21), wherein the second limb (27) is assigned one or the second raceway insert (22).A linear motion device according to any preceding claim, wherein the resistance detecting means (50) comprises a current source (51) and a voltage measuring device (52).Method for operating a linear movement device (10; 10') according to one of the preceding claims, wherein a constant or a time-variable measurement current (54) is impressed at the first measurement point (61), wherein a measurement voltage (55) is measured between the first and the second measurement point (61; 62), wherein the complex electrical resistance is determined from the measurement voltage and the measurement current.Method according to claim 9, wherein the injection of the measurement current (54) and the measurement of the measurement voltage (55) are carried out while the movement element (20) moves relative to the guide element (12), preferably at a constant speed.

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