JPS61221601A - Method and instrument for measuring clearance of ball screw - Google Patents

Abstract

PURPOSE:To measure the quantity of abrasion easily with high precision by fixing a nut which is engaged threadably with a screw shaft to the nearly center part of a leaf spring, and putting the clearance between the nut and screw shaft close in one direction and returning the leaf spring to a neutral point. CONSTITUTION:An instrument is equipped with a screw shaft supporting device 2, a screw shaft rotary driving mechanism 3, a screw shaft rotational angle detector 4, a base 5, etc. Then, the nut 34 is engaged threadably with the screw shaft 9 and then fixed to the nearly center part 6a of the leaf spring 6 whose both ends are fixed to the base 5. The shaft 9 is rotated in one direction to move the nut 34 in one direction and thus the spring 6 is inflected in one direction to make a pressing force operate on the shaft 9 from the nut 34; and the shaft 9 is rotated in the opposite direction to move the nut 34 in the other direction and the spring 6 is returned to the neutral state and then inflected in the other direction. Simultaneously, the angle of rotation of the shaft 9 and the extent of movement of the nut 34 are detected and recorded. The axial clearance of a ball screw 43 is measured from the length of a blind sector wherein the nut 34 stops moving against the rotation of the shaft 9 while the spring 6 is in the neutral state.

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a method and device for measuring the clearance of a ball screw, and particularly to a method and apparatus for measuring the clearance in the axial direction between the screw shaft and the nut when the nut is screwed onto the screw shaft. The present invention also relates to a method and apparatus that enable highly accurate measurement and clarify the wear characteristics of a ball screw.

It is said that the wear that occurs in conventional ball screws is extremely small under normal lubrication conditions. However, in the environment where ball screws are actually used, there is a risk that cutting dust, cutting oil, foreign matter in the atmosphere, etc. may get into the screws, and the wipers and seals currently used in ball screws are There are cases where a considerable amount of wear occurs on the balls and thread grooves due to reasons such as insufficient dustproofing effects.

The occurrence of such wear not only causes the loss of preload in the fixed position preload screw and reduces the axial rigidity of the ball screw, but also causes a worsening of positioning accuracy, which is a major problem when using the ball screw. It is a point.

Therefore, there has been a need to clarify the actual state of wear in ball screws, but among the wear that occurs in various parts of ball screws, it is easy to measure the wear that occurs in the balls with relatively high accuracy. However, it has been extremely difficult to directly measure the amount of wear in thread grooves. In other words, conventional methods have been used to determine the wear amount of a screw shaft from the results of measuring the effective diameter using a three-needle or steel ball, but with this method, the wear profile of the screw groove is This technique has the disadvantage that highly accurate measurement cannot be expected because it depends on the ball contour, which is undergoing wear.

Furthermore, due to structural problems, even the above method could not be applied to Natsu. In addition, there is no method or device available for measuring the amount of wear that occurs on the entire ball screw with the balls and nut assembled in the screw shaft, and it is extremely difficult to clarify the actual state of wear on the ball screw. Purpose The present invention has been made in order to eliminate the drawbacks of the prior art described above, and its purpose is to provide a NAND screwed onto the screw shaft of a ball screw with both ends fixed. The leaf spring is fixed approximately at the center of the temporary spring and the leaf spring is bent in the axial direction of the screw shaft, thereby closing the gap between the nut and the screw shaft in one direction and returning the leaf spring to the neutral point. The screw shaft is rotated in one direction so that the leaf spring passes through the neutral point and is deflected in the other direction, and during this time the screw shaft rotates. By simultaneously detecting and recording the angle and the amount of nut movement, and measuring the axial clearance of the ball screw by the length of the dead zone where the nut stops moving with respect to the rotation of the screw shaft when the leaf spring is in the neutral state, The objective is to make it possible to measure easily and with extremely high accuracy the axial clearance that occurs in the entire ball screw with balls and nuts assembled in the shaft, that is, the amount of wear. To make it possible to measure the wear amount under various conditions and to clarify the wear characteristics of a ball screw under various usage conditions.

Structure: In short, the method of the present invention involves screwing a nut onto a screw shaft, fixing the nut to a substantially central portion of a leaf spring whose both ends are fixed to a base, and rotating the screw shaft in one direction. A nut is moved in one direction, the leaf spring is bent in one direction, and a pressing force is applied from the nut to the screw shaft to bring it to an initial state, and from the initial state, the screw shaft is rotated in the other direction to After the nut is moved in the other direction and the leaf spring is returned to the neutral state, the leaf spring is bent in the other direction, and during this period, the rotation angle of the screw shaft and the amount of movement of the nut are simultaneously detected and recorded. , the axial clearance of the ball screw is measured by the length of a dead zone in which movement of the nut stops with respect to rotation of the screw shaft when the leaf spring is in a neutral state.

The device of the present invention also includes a screw shaft support device that rotatably supports both ends of a screw shaft, a screw shaft rotation drive mechanism that rotates the screw shaft at a predetermined rotational speed, and a screw shaft rotation drive mechanism that detects the rotation angle of the screw shaft. A screw shaft rotation angle detection device that inputs detection results into a recording device, a base that is held stationary, and a nut that is fixed at both ends to the base and that is screwed to the screw shaft approximately at the center. A leaf spring configured to bend in the axial direction of the screw shaft to apply a pressing force from the nut to the screw shaft, and a recording device that detects the amount of movement of the nut relative to the screw shaft. The present invention is characterized by comprising a Nando movement amount detection device for inputting detection results.

The present invention will be explained below based on embodiments shown in the drawings. The ball screw clearance measuring device l according to the present invention, as shown in FIGS. 1 to 3, includes a screw shaft support device 2, a screw shaft rotation drive mechanism 3, a screw shaft rotation angle detection device 4, and a base 5. , a leaf spring 6 , and a nut movement amount detection device 8 .

The screw shaft support device 2 is provided at both ends 9a of the screw shaft 9.

9b is rotatably supported, and is provided with a pair of vertically rotatable centers 10 and 11,
The upper center 10 is fixed to a chuck 16 in which a tapered part 16a is fitted into a support part 14 fixed to an upper support plate 13 fixed to the upper end 12a of, for example, four columns 12 fixed to a base 5. The support portion 14 is screwed onto a sleeve 19 fixed to the upper support plate 13 with bolts 18. The lower center 11 is rotatably supported by a sleeve 21 fixed to the base 5 by bolts 20 via a thrust bearing 22.
The lower end 11a of the center 11 projects downward from the lower surface 23a of the lower support plate 23 of the base 5. and center IO
and II so that both ends 9a and 9b of the screw shaft 9 can be rotatably supported from above and below as shown in the drawings, and the support portion 14 is screwed onto the sleeve 19. By freely rotating the knob 14a, the upper center 10 can be moved up and down to apply an appropriate pressing force to the screw shaft 9.

As shown in FIGS. 1 and 3, the screw shaft rotation drive mechanism 3 includes a gear 25 attached to one end 9b of the screw shaft 9 and restrained in the rotational direction by a key 24, and a gear 26 meshing with the gear. , a reducer 28 with the gear 26 fixed to an output shaft 28a, a chain 29 that rotates the input shaft 28b of the reducer, a motor 30 that drives the chain, and a support base 31 that supports the motor. The reducer 28 is slidably attached to the lower support plate 23 so that the distance between the centers of the pair of gears 25 and 26 can be changed from a meshed state to a non-meshed state. and screw shaft 90
When screwing the gear 26 into the device 1 of the present invention, the gear 26 is connected to the gear 2.
5 and when measuring the clearance, these gears 2
5 and 26 are engaged with each other so that the screw shaft 9 can be rotated at a constant speed by the motor 30.

The screw shaft rotation angle detection device 4 detects the rotation angle of the screw shaft 9 and inputs the detection result to an x-y recorder 25, which is an example of a recording device. It is attached to the lower end 11a of the amplifier 29 and is connected to an amplifier 29 by a conductor 28. The rotation angle of the screw shaft 9 can be detected with an accuracy of about 0.1 degree on the minimum scale.

The base 5 is integrated with a lower support plate 23 and bolts 30. A horizontal plate 31 is fixed to the upper part of the base 5, and a pair of leaf springs are attached to the horizontal plate by bolts 32. A stand 33 is fixed. A concave portion 33a is formed in the center of the horizontal plate 31, so that rotation of the screw shaft 9 is not hindered.

As shown in FIGS. 1 to 4, the leaf spring 6 is configured such that both ends thereof are fixed to the base 5, and a nut 34 that is screwed onto the screw shaft 9 is fixed approximately at the center portion 6a. , screw shaft 9
The nut applies a pressing force to the screw shaft by bending in the axial direction of the nut, and its plate thickness is set to have an appropriate spring constant. For example, three bolts 35 are inserted into the holes 6b formed at both ends of the leaf spring 6, and the leaf spring 6 is attached to the horizontal plate 31 through a block 36 (not shown in FIG. 3). It is designed to be removably fixed. Further, a flange portion 34a of a nut 34 is detachably attached to the substantially central portion 6a with a bolt 38. The reason why the leaf spring 6 is used to apply the pressing force from the nut 34 to the screw shaft 9 is to make the unsteady zone region of the displacement curve recorded by the X-Y recorder 25 more clear, and to This is to provide sufficient rigidity against the rotational moment acting on 34.

The nut movement amount detection device 8 detects the amount of movement of the nut 34 with respect to the screw shaft 9, and inputs the detection result to an XY recorder 25, which is an example of a recording device.
For example, an electric micrometer 41 having a pickup 4o in contact with the upper end 34a of the nut 34 can be used. In this case, the electric micrometer 41 is connected to an amplifier 29 by a conducting wire (not shown), and its output is amplified and input to the X-Y recorder 25. The X-Y recorder 25 is capable of automatically recording the relationship between the rotation angle of the screw shaft 9 on the horizontal axis and the movement amount of the nut 34 on the vertical axis in the form of a displacement curve.

As shown in FIG. 3, a plurality of balls 42 are provided between the napht 34 and the threaded shaft 9 in the threaded groove 9C of the threaded shaft 9 and the shaft 3.
Needless to say, it is incorporated so that it can be moved along the thread groove 34C of No. 4.

In the method for measuring the clearance of a ball screw 43 according to the present invention, the nut 34 is screwed onto the screw shaft 9 via the balls 4' 2L, and both ends 9a and 9b of the nut are fixed to the base 5. The plate spring 6 is fixed to approximately the center part 6a of the temporary spring 6, and the screw shaft 9 is rotated in one direction to move the nut 34 in one direction (for example, downward), and the plate spring 6 is bent in one direction (for example, downward). Pressure is applied upward from the nut 34 to the screw shaft 9 to bring it into an initial state, and from the initial state the screw shaft 9
is rotated in the other direction to move the nut 34 in the other direction (for example, upward), and after returning the leaf spring 6 to the neutral state, the temporary spring is bent in the other direction (for example, upward), and during this time, the screw shaft 9 is The rotation angle and the amount of movement of the nut 34 are simultaneously detected and recorded, and the axial clearance of the ball screw 43 is calculated based on the length of the dead zone where the nut 34 stops moving with respect to the rotation of the screw shaft 9 when the temporary spring 6 is in the neutral state. It was designed to measure.

Function The present invention is constructed as described above, and its function will be explained below. To measure the axial clearance of the ball screw 43, as shown in FIG.
Fix it to a. And as shown in Figures 1 to 3,
Rotate the knob 14a of the support part 14 of the screw shaft support device 2 to raise the support part 14 relative to the sleeve 19, move the upper center 10 upward, and move the one end 9b of the screw shaft 9 to the lower center. 11, and the one end 9b is rotatably supported by the center. Then, the center 10 is lowered by rotating the knob 14a of the support part 14 in the opposite direction to the above, and one end 9a' of the screw shaft 9 is rotatably supported by the center, and both ends of the leaf spring 6 are removed from the center. Attachment is hole 6
The plate springs are attached to the horizontal plate 31 fixed to the base 5 by b to the base 33 using a plurality of bolts 35.

Further, since the gear 25 is fixed to one end 9b of the screw shaft 9, in this state, the reducer 28 of the screw shaft rotation drive mechanism 3 is moved so that the gear 26 approaches the gear 25,
The distance between the centers of these gears is determined accurately to the extent that bank crash does not occur, and the reducer 28 is fixed to the lower support plate 23. In this case, the movement of the reducer 28 is absorbed by the chain 29 and does not interfere with power transmission from the motor 3o.

After the ball screw 43 has been attached to the ball screw clearance measuring device 1 of the present invention in this way, the electric micrometer 41 of the nut movement amount detection device 8 is fixed with the stand 33, for example, when attaching a leaf spring. The pickup 4o is set so as to come into contact with the upper end 34a of the nut 34. As a result, the amount of movement of the nut 34 in the axial direction with respect to the screw shaft 9 can be electrically detected. Further, since a low rotation encoder 26, which is an example of the screw shaft rotation angle detecting device 4, is attached to the lower end 11a of the lower center 11, the friction force between the screw shaft 9 and the center 11 causes the center 11 to By rotating completely together with the screw shaft 9, the rotation angle of the screw shaft 9 is detected by the rotary encoder 26, and the detection result is once input to the amplifier 29 and then input to the X-Y recorder 25. .

Next, when such preparations are completed, the motor 3° is rotated to drive the reducer 28 by the chain 29, and the gear 26
First, rotate the gear 25 by rotating the screw shaft 9 in one direction, for example, if the screw shaft 9 has a right-hand thread, the screw shaft is rotated counterclockwise when viewed from above, that is, in the direction of arrow A in FIG. 3. As a result, the nut 34 is slightly lowered, and as a result, the leaf spring 6 is deflected from the neutral point shown by the imaginary line to the state shown by the solid line.
The leaf spring 6 presses the nut 34 upward. As a result, the ball 42 is removed from the bottom surface of the threaded groove 34c of the nut 34.
A force is applied to push up the ball 42, and a force is also applied to press the top surface of the screw groove 9c of the screw shaft 9 upward from the top surface of the ball 42.

As a result, an axial clearance between the nut 34 and the screw shaft 9 is created between the ball 42 and the lower surface of the thread groove 9c of the screw shaft 9, and a clearance is created between the upper surface of the thread groove 9C and the ball 42. It disappears. In this initial state, that is, the nut 34 and the screw shaft 9
When a preload is applied between the nut 34 and the nut 34, the nut 34 starts to rise when the motor 3° is reversely rotated and the gear 25 is rotated clockwise when viewed from above, that is, in the direction of arrow B in FIG.

While the nut 34 is rising to the neutral point of the spring 6, the state shown in FIG. When it returns, the diaphragm spring does not bend in either the up or down direction, so no force acts on the screw shaft 9 from the nut 34. Therefore, since the screw shaft 9 is held in a free state, if the screw shaft continues to rotate in the direction of arrow B, the nut 34 will be rotated by the clearance at this neutral point. Even though the screw shaft 9 rotates, it does not move. That is, with respect to the rotation of the screw shaft 9, the nut 34
A dead zone occurs in which the motor does not move in its axial direction. After passing through this unfavorable zone, the screw shaft 9 further rotates in the direction of arrow B, so that the nut 34 eventually starts to rise again, and the leaf spring 6 moves in the opposite direction to that shown in FIG. 3, that is, upward. It begins to bend. By bending the leaf spring 6 upward in this way, a downward force is applied from the nut 34 to the screw shaft 9. As a result, a gap occurs in the opposite direction to the case shown in FIG.
2 in contact with the lower surface of the threaded groove 9C of the screw shaft 9.

During this time, the rotation angle of the screw shaft 9 and the amount of movement of the nut 34 are detected by the rotary encoder 26 and the electric micrometer 41, respectively, and at the same time, these detection results are recorded by the X-Y recorder 25, and are shown as solid lines in FIG. A displacement curve 44 of the nut 34 with respect to the rotation angle of the screw shaft 9 is drawn as shown in FIG. When the leaf spring 6 is bent downward, the nut 9 is moved proportionally with the rotation of the screw shaft 9, and an upward curve PQ is drawn, and the leaf spring 6 is imaginary as shown in FIG. When the nut 34 is returned to the neutral point as shown by the line, the displacement curve 44 becomes horizontal and a curve QR is drawn, and when the nut 34 passes through this neutral point, it starts to rise again and a curve R3 is drawn. The distance between the curve PQ and the curve R3 represents the axial clearance between the nut 34 and the screw shaft 9, and when measured in FIG. 5, it is found to be, for example, 20.6 μm. By measuring the length of the dead zone in this way, the ball screw 43
The axial clearance between the nut 34 and the screw shaft 9 can be accurately measured. The displacement curve 45 shown by the broken line in FIG. 5 is a displacement curve obtained by calculation assuming that all parts of the ball screw clearance measuring device 1 according to the present invention are a spring system, and this calculation result also corresponds to the actual situation of the device of the present invention. For example, from this calculation result, the axial clearance of the ball screw 43 in this case is 20.6.
This shows the state calculated as μm.

Next, FIG. 6 shows two cases where the difference in diameter of the ball 42 is 3.5 μm.
This shows the measurement results when different types of balls 42 are assembled into the same ball screw 43.The axial clearance was determined using the same method as shown in Fig. 5, and the difference between the two was converted into the diameter of the balls 42. However, a dimensional difference of 3.3 μm was obtained, and it was confirmed that the measurement accuracy was also good. That is, the smaller ball 4
The axial clearance when using ball 2 is 30 μm, while the clearance when using the larger ball 42 is 20.6 μm.
When converted to the diameter of the ball 42, the difference in diameter is 3.3 μm, which is a slight error of 0.2 μm compared to the actual difference of 3.5 μm. This means that it is not too much.

Next, the results of the wear test of the ball screw 43 will be explained with reference to FIGS.
The diameter of 2 is 3.175, the similarity coefficient is 0.566, the contact angle is 45 degrees, the material is SCM420, the surface hardening treatment is carburizing and quenching, and the hardness is Rockwell hardness H * c) 6'O or more. Here, for the purpose of investigating the wear characteristics of the ball screw 43 with a single nut 34, a disc spring (not shown) was used to keep the load value constant during the entire test period.
A constant pressure preload was applied using the .

Further, the test conditions were that the thrust load was 200 kg=f, the rotational speed of the screw shaft 9 was 168 rpm, the stroke of the nut 34□ was 36 strokes, and rolling bearing grease No. 2 was used as the lubricant. In order to investigate the influence of foreign matter contamination, 20% by weight of the grease was mixed into 100 to 140 mesh FCC cutting powder grease. The wear test was conducted using a fatigue life tester (not shown), and when operated under the above test conditions, the temperature rise of the ball screw 43 was 25 degrees Celsius relative to room temperature.

The changes in the axial clearance measured before the start of the test and every 4 million rotations of the screw shaft 9 were as shown in FIG. All of these measurements were performed in a constant temperature room at a room temperature of 20 degrees Celsius±1 degrees Celsius, and sufficient attention was paid to the influence of temperature.

In FIG. 7, there are cases where the unsteady zone of the leaf spring 6 does not clearly appear, and this tendency is particularly noticeable when the axial clearance is small. This works on the ball screw 43.

It is presumed that this is due to the uneven contact between the ball 42 and the thread grooves 9c, 34c that occurs due to the thread precision when the load is small, and the friction within the threaded shaft 9. However, according to the measurement method using the device 1 of the present invention, it can be said that the measurement accuracy of the axial clearance is not significantly affected. That is, the displacement curve 46 in FIG. 7 shows the axial clearance in the case of 0 rotations before the test, and in this case, it was 2.8 μm. Next, a displacement curve 47 shows the case where the number of rotations of the screw shaft 9 is 4 million revolutions, and the axial clearance has increased to 6 μm. Also, the displacement curve 48
is the case where the rotation speed of the screw shaft 9 is 8 million revolutions, the axial clearance increases to 9.6 μm, and the displacement curve 49
When the rotation speed is 12 million revolutions, the axial clearance is 13.
Similarly, the displacement curve 50 increases to 9 μm at a rotation speed of 1600
In the case of 10,000 rotations, the axial clearance increased by 16.8 μm.

As described above, according to the ball screw clearance measuring method and device of the present invention, the axial clearance of the ball screw 43 can be easily and accurately measured both during the durability test and in actual use. can do.

Next, FIG. 8 shows the wear amount of the ball 42 and thread grooves 9c, 34C obtained from the results shown in FIG. 7 and the measurement results regarding the diameter of the ball 42. 9
and the wear on the nut 34, which represents the sum of the depths. Ball 42 like this 1.
When I looked at it, I found that the preload had completely lost and there was dust.
Although the amount of wear occurring in the screw grooves 9c and 34G is approximately several micrometers, it has become clear that the influence of dust contamination is significant in the case of fixed position preload screws.

As described above, according to the method and apparatus according to the present invention, the axial clearance of the ball screw 43 can be easily and highly accurately measured, so that the axial clearance of the ball screw 43 can be measured easily and with high precision. It is possible to clarify the wear characteristics of

Effects Since the present invention is constructed and operates as described above, a nut screwed onto the screw shaft of a ball screw is fixed to approximately the center of a leaf spring whose both ends are fixed, and the diaphragm spring is screwed. By bending in the axial direction of the shaft, the gap between the nut and the screw shaft is closed in one direction, and by returning the diaphragm spring to the neutral point, the screw shaft is freed. By rotating the temporary spring in one direction, it passes through the neutral point and is deflected in the other direction. During this time, the rotation angle of the screw shaft and the amount of movement of the nut are simultaneously detected and recorded. Since the axial clearance of the ball screw is measured by the length of the dead zone where the nut stops moving with respect to the rotation of the screw shaft, the entire ball screw can be measured with the ball and nut still installed in the screw shaft. This provides the revolutionary effect of being able to measure the axial clearance, that is, the amount of wear, that occurs in the axial direction easily and with extremely high accuracy.

Further, as a result, it is possible to measure the amount of wear in the ball screw under actual usage conditions, and it is possible to clarify the wear characteristics of the ball screw under various usage conditions.

[Brief explanation of the drawing]

The drawings relate to embodiments of the present invention, and FIG. 1 is an overall perspective view of a ball screw clearance measuring device, FIG. 2 is a vertical cross-sectional front view of the principal part of the ball and screw clearance measuring device, and FIG. 3 is a ball Fig. 4 is a schematic vertical cross-sectional view of the main parts showing the operating principle of the screw clearance measuring device, Fig. 4 is a perspective view showing a state in which a leaf spring is fixed to the nut of a ball screw and a gear is attached to one end of the screw shaft, Fig. 5 is a A diagram showing the amount of movement of the nut relative to the rotation angle of the screw shaft,
Figure 6 is a diagram similar to Figure 5 when measuring the axial clearance with different ball diameters, Figure 7 is a diagram showing changes in each axial clearance in a ball screw wear test, and Figure 8
The figure is a diagram showing the relationship between the amount of wear on the balls and the thread grooves with respect to the rotational speed of the threaded shaft. 1 is a ball screw clearance measuring device, 2 is a screw shaft support device, 3 is a screw shaft rotation drive mechanism, 4 is a screw shaft rotation angle detection device, 5 is a base, 6 is a leaf spring, 6a is a central portion, 8 is a Nut movement amount detection device, 9 is a screw shaft, 9a, 9b are both ends, 25
4 is an X-Y recorder which is an example of a recording device, and 43 is a ball screw. Patent applicant Kuroda Seiko Co., Ltd. Minoru Izawa

Claims (1)

[Scope of Claims] 1. A nut is screwed onto a screw shaft, the nut is fixed to a substantially central portion of a leaf spring whose both ends are fixed to a base, and the screw shaft is rotated in one direction. A nut is moved in one direction, the leaf spring is bent in one direction, a pressing force is applied from the nut to the screw shaft to bring it to an initial state, and the screw shaft is rotated in the other direction from the initial state to After moving the nut in the other direction and returning the leaf spring to a neutral state, the leaf spring is deflected in the other direction, and during this period, the rotation angle of the screw shaft and the amount of movement of the nut are simultaneously detected and recorded. A method for measuring a clearance in a ball screw, characterized in that the axial clearance of the ball screw is measured by the length of a dead zone in which movement of the nut stops with respect to rotation of the screw shaft when the leaf spring is in a neutral state. 2. A screw shaft support device that rotatably supports both ends of the screw shaft, a screw shaft rotation drive mechanism that rotates the screw shaft at a predetermined rotational speed, and detects the rotation angle of the screw shaft and records the detection results on a recording device. A screw shaft rotation angle detection device that inputs the rotation angle of a screw shaft, a base that is held stationary, and a nut that is fixed at both ends to the base and that is screwed to the screw shaft approximately in the center. A leaf spring that bends in the axial direction of the screw shaft to apply a pressing force from the nut to the screw shaft, and a nut that detects the amount of movement of the nut with respect to the screw shaft and inputs the detection result into a recording device. A ball screw clearance measuring device characterized by comprising a movement amount detection device.