Chemical reinforcing holders for glass substrate

Abstract

The present invention provides a holder that prevents a glass substrate from deforming during a chemical reinforcing process. The holder includes supporting members, each of which has a plurality of supporting concave portions. The supporting members are formed from a flat plate having a constant thickness so as not to have a portion having high thermal capacity. When the glass substrate is cooled, a thermal stress is not applied to the glass substrate.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a holder for supporting a glass substrate for an information recording medium, such as a magnetic disc, a magneto-optical disc or an optical disc. Particularly, the invention relates to a holder for a chemical reinforcing process for immersing a glass substrate in a chemical reinforcing liquid.

[0002] Japanese Laid-Open Patent Publication No. Hei 7-176045 discloses a conventional holder for immersing a glass substrate in a chemical reinforcement tank in order to chemically reinforce the glass substrate. The conventional holder is formed by three rods, which are parallel with one another, and two side plates for fixing the ends of the rods. Annular beads are inserted into the respective rods and are fixed to the rods with predetermined intervals. A flat recess is partitioned between adjacent beads. The flat recesses of the three rods receive three outer edge portions of a glass substrate, respectively. As a result, the glass substrate is supported by the holder.

[0003] Chemical reinforcement of a glass substrate will be described. A holder which holds glass substrates is put into a tank which stores a chemical reinforcing liquid containing a potassium nitrate (KNO3) liquid. After predetermined time, the holder is taken out from the tank, and the glass substrates and the holder are cooled to a room temperature.

[0004] In the conventional holder, since beads, each of which has a relatively large thermal capacity, are fixed to the rods, a cooling speed of the glass substrate at portions that contact with the rods is lower than that of the other portions in the glass substrate. For this reason, during the cooling, contraction of the portions contacting with the rods is small, but contraction of the other parts are large. Therefore, the conventional holder has a problem that a glass substrate is distorted.

SUMMARY OF THE INVENTION

[0005] An objective of the present invention is to provide a holder for glass substrates without distorting the glass substrate in chemical reinforcing process.

[0006] In order to achieve the above objective, the present invention provides a holder for holding glass substrates for an information recording medium and for immersing the glass substrates in a heated chemical reinforcing liquid. The holder includes a frame, and a plurality of supporting members mounted on the frame. Each of the supporting members has a plurality of supporting concave portions for receiving outer edges of the glass substrates, respectively, and thickness of the supporting members is constant.

[0007] A further perspective of the present invention is a holder for immersing a glass substrate in a chemical reinforcing liquid. The holder includes a frame, first, second and third supporting plates mounted on the frame. The first supporting plate has a first serrate portion that defines a first concave portion for receiving a first portion of the glass substrate. The second supporting plate has a second serrate portion that defines a second concave portion for receiving a second portion of the glass substrate. The third supporting plate has a third serrate portion that defines a third concave portion for receiving a bottom portion of the glass substrate.

[0008] A further perspective of the present invention is a holder for immersing a glass substrate in a chemical reinforcing liquid. The holder includes a frame, first, second and third supporting wires mounted on the frame. The first supporting wire has a first serrate portion that defines a first concave portion for receiving a first portion of the glass substrate. The second supporting wire has a second serrate portion that defines a second concave portion for receiving a second portion of the glass substrate. The third supporting wire has a third serrate portion that defines a third concave portion for receiving a bottom portion of the glass substrate.

[0009] Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

[0011] FIG. 1 is a partially broken perspective view of a holder according to one embodiment of the present invention.

[0012] FIG. 2 is a perspective view of a supporting member.

[0013] FIG. 3A is a plan view of the supporting member.

[0014] FIG. 3B is a cross-sectional view taken along the line 3B-3B of FIG. 3A.

[0015] FIGS. 4 and 5 are plan view and side view of the supporting member which supports a glass substrate.

[0016] FIGS. 6A, 6B and 6C show modified examples of the supporting member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A holder for a chemical reinforcing process according to one embodiment of the present invention will be described below.

[0018] As shown in FIG. 1, a holder 11 has frames 12 and 13, first, second and third supporting members 15a, 15b and 15c fixed to the frames. The frames 12 and 13 include two side plates 12 and four elongate connecting plates 13 which are mounted in the four corners of the side plates 12, respectively, and connect both the side plates 12. It is preferred that the side plates 12 are made of metal such as nickel alloy, stainless steel, pure titanium or titanium alloy.

[0019] Near the center of three sides of each side plate 12 are formed with three supporting holes or slits 14, respectively. The ends of the supporting members 15a, 15b and 15c are inserted into the two slits 14 opposed to each other, respectively. Each one end of the supporting members 15a, 15b and 15c is welded with one side plate 12, and each other end of the supporting members 15a, 15b and 15c is not welded with each slit 14 of the other side plate 12 to be supported thereto. In such a manner, the supporting members 15a, 15b and 15c are fixed between both the side plates 12.

[0020] Each of the supporting members 15a, 15b and 15c is made of a flat thin plate. Each of the supporting members 15a, 15b and 15c is formed of a metal material such as nickel alloy, stainless steel, pure titanium or titanium alloy. Of these metal materials, nickel alloy having good corrosion resistance, mechanical property and processability, or a titanium alloy with high corrosion resistance, small specific gravity and small thermal capacity is preferred. Pure titanium is more preferable.

[0021] As shown in FIG. 1, one side of each of the supporting members 15a, 15b and 15c is formed with jagged serrate portion 16a, 16b and 16c, respectively. Namely, the serrate portions 16a, 16b and 16c are flush with the relating supporting members 15a, 15b and 15c. The serrate portions 16a, 16b and 16c include a plurality of supporting concave portions 18a, 18b and 18c, respectively. The serrate portion 16a of the first or left side supporting member 15a is opposed to the serrate portion 16b of the second or right side supporting member 15b. The serrate portion 16c of the third or lower supporting member 15c faces upward.

[0022] FIG. 2 is an enlarged diagram of the left side supporting member 15a. The right side and the lower supporting members 15b and 15c have the same structure shown in FIG. 2. The plural supporting concave portions 18a, 18b and 18c are defined by ridgelines 16r of the serrate portions 16a, 16b and 16c. The ridgeline 16r is chamfered by a wet barrel polishing process, so that a burr is removed. Therefore, outer surfaces of the serrate portions 16a, 16b and 16c are smooth. As shown in FIG. 3B, it is preferred that the ridgeline 16r is of a curved surface type.

[0023] As shown in FIG. 5, a glass substrate 17 is a disc having a circular hole at its center which is used as a substrate for an information recording medium such as a magnetic disc, a magneto-optical disc or an optical disc. A glass material forming the glass substrate 17 includes sodalime glass mainly containing silicon dioxide (SiO2), sodium oxide (Na2O) and calcium oxide (CaO); aluminosilicate glass mainly containing SiO2, aluminum oxide (Al2O3) and R2O (R indicates potassium (K), sodium (Na), lithium (Li)); borosilicate glass; lithium oxide (Li2O)—SiO2 glass; Li2O—Al2O3—SiO2 glass; and R′O—Al2O3—SiO2 glass (R′ indicates magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba)). Moreover, the glass material also includes a glass for chemical reinforcing process where zirconium oxide (ZrO2) or titanium oxide (TiO2) is added to these glass materials. As shown in FIG. 4, an outer peripheral edge of the glass substrate 17 is chamfered.

[0024] As shown in FIG. 4, the three supporting concave portions 18a, 18b and 18c are arranged on the same plane so that the outer edge of the glass substrate 17 is received by the three supporting concave portions 18a, 18b and 18c. As shown in FIG. 1, the holder 11 can hold a plurality of glass substrates 17 in a vertically standing state.

[0025] One glass substrate 17 is inserted among the three supporting members 15a, 15b and 15c. As shown in FIGS. 4 and 5, a left edge portion of the glass substrate 17 is supported by the left supporting member 15a, and the bottom portion of the glass substrate 17 is supported by the supporting member 15c. A right outer peripheral surface of the glass substrate 17 is separated by a gap L from the supporting concave portion 18b of the right supporting member 15b. Preferably the gap L is 0.1 to 0.5 mm at normal temperature. In the case where the gap L is less than 0.1 mm, when the glass substrate 17 and the holder 11 are immersed in a chemical reinforcing liquid, there is a fear that the glass substrate 17 is pressed strongly between the supporting members 15a, 15b and 15c due to thermal expansion. In the case where the gap L is larger than 0.5 mm, the glass substrate 17 is not stably supported, and when it is oscillated, there is a fear that the glass substrate 17 may collide strongly with the supporting members 15a, 15b and 15c and be damaged.

[0026] As shown in FIG. 5, the glass substrate 17 comes in contact with the left supporting member 15a in a first position (supporting point) 19. It is preferred that an angle (lower angle) &thgr; between a plane which connects the supporting point 19 with a center 17c of the glass substrate 17 and a plane which connects the left supporting member 15a with the center 17c of the glass substrate 17 is 160 to 180°. In other words, it is preferred that the left supporting member 15a and the right supporting member 15b are separated from the lower supporting member 15c by 80 to 90° in a circumferential direction of the glass substrate 17 with respect to the center 17c of the glass substrate 17. For example, the angle &thgr; between the left and right supporting members 15a and 15b is 180° in a position indicated by a solid line in FIG. 5. In a position indicated by alternate long and two short dashes line, the angle &thgr; between the left and right supporting members 15a and 15b is 160°.

[0027] In the case where the lower angle &thgr; is less than 160°, there is a fear that the glass substrate 17 is oscillated relatively greatly due to vibration and is damaged. As the lower angle &thgr; is smaller, the supporting of the glass substrate 17 becomes more unstable. Meanwhile, when the lower angle &thgr; exceeds 180°, it is difficult to insert the glass substrate 17 into the holder 11.

[0028] As shown in FIG. 4, the bottom portions of the supporting concave portions 18a, 18b and 18c have slightly larger width than a thickness of the glass substrate 17. As shown in FIG. 5, the lower supporting member 15c is thicker than the left and right supporting members 15a and 15b.

[0029] Next, the chemical reinforcing process will be described.

[0030] A heated chemical reinforcing liquid is stored in a chemical reinforcement tank (not shown). Chemical reinforcement salt is, for example, KNO3 or sodium nitrate (NaNO3). The chemical reinforcing liquid contains sodium ions (Na+) or potassium ions (K+) which are monovalent metal ions having a relatively large ion radius. The holder 11 holding the glass substrate 17 is arranged in a cage, not shown, and the cage is put into the chemical reinforcement tank. As a result, the glass substrate 17 and the holder 11 are immersed in the heated chemical reinforcing liquid.

[0031] Metal ions having relatively small radius such as lithium ions (Li+) and Na+exist on the surface layer of the glass substrate 17. When the glass substrate 17 is immersed in the chemical reinforcing liquid, metal ions with relatively small surface layer are exchanged with relatively large metal ions in the chemical reinforcing liquid. With this ion exchange, a compression stress is applied to the surface layer of the glass substrate 17. As a result, the glass substrate 17 is chemically reinforced.

[0032] In the case where the glass substrate 17 is subjected to chemical reinforcing process, firstly a plurality of glass substrates 17 are inserted between the supporting concave portions 18a and 18b of the left and right supporting members 15a and 15b from above the holder 11, so that the bottom portions of the glass substrates 17 are brought into contact with the supporting concave portions 18c of the lower supporting member 15c. As a result, the plural glass substrates 17 are held at the holder 11 in the vertically standing state. A plurality of holders 11 are housed in the cage, not shown. The cage is put into the chemical reinforcement tank where the chemical reinforcing liquid is stored. As a result, the plural glass substrates 17 and holders 11 are immersed in the chemical reinforcing liquid.

[0033] In one embodiment, the chemical reinforcing liquid is obtained by heating KNO3 and NaNO3. A temperature of the chemical reinforcing liquid is preferably lower than a distortion point of a glass material by about 50 to 150° C. More preferably, the temperature of the chemical reinforcing liquid is about 350 to 400° C.

[0034] The glass substrates 17 are immersed in the chemical reinforcing liquid for predetermined time. During the immersing, Li+on the surface layer of the glass substrate 17 is ion-exchanged with Na+or K+, and Na+on the surface layer of the glass substrate 17 is ion-exchanged with K+. The strength of the glass substrate 17 is improved by the ion exchange. As a result, when the glass substrate 17 is used for an information recording medium, its breakage due to high-speed rotation can be prevented.

[0035] In addition, during the immersing, the supporting members 15a, 15b and 15c and the glass substrate 17 are thermally expanded (alternate long and two short dashes line in FIG. 5). The glass substrate 17 is separated by a gap L from the bottom portion of the right supporting concave portion 18b at normal temperature. Even if the glass substrate 17 and the supporting members 15a, 15b and 15c are expanded, the gap L is set so that the glass substrate 17 does not come in contact with both the left and right supporting members 15a and 15b. For this reason, a deformation of the glass substrate 17 is prevented.

[0036] After the immersing for predetermined time, the cage is taken out from the chemical reinforcement tank, the glass substrates 17 and the holders 11 are cooled to normal temperature. As a result, the chemical reinforcement of the glass substrates 17 is completed.

[0037] According to one embodiment, the following advantages are obtained.

[0038] The supporting members 15a, 15b and 15c are relatively thin plate materials. For this reason, the supporting members 15a, 15b and 15c do not have portions with large thermal capacity unlike conventional annular beads. Therefore, while the glass substrates 17 and the holders 11 are being cooled, a thermal stress which is generated on the glass substrate 17 is reduced. A distortion is prevented by the chemical reinforcing process from occurring on the glass substrate 17. As a result, the flat and chemically reinforced glass substrate 17 is obtained.

[0039] The serrate portions 16a, 16b and 16c which define the plural supporting concave portions 18a, 18b and 18c are formed on each one side of the supporting members 15a, 15b and 15c. As a result, the holder 11 can hold a plurality of glass substrates 17. Moreover, insertion and taking-out of the glass substrate 17 can be easily carried out.

[0040] Ridgelines 16r of the serrate portions 16a, 16b and 16c undergo the wet barrel polishing process. For this reason, a burr on the surfaces of the serrate portions 16a, 16b and 16c is removed, and the ridgeline 16r is formed into a smooth curved surface. Therefore, a damage such as a break or a crack does not occur on the glass substrate 17, and the glass substrate 17 can be easily inserted between the left and right supporting members 15a and 15b.

[0041] The left and the right supporting members 15a and 15b are separated by the lower angle &thgr;, which is set within the range of 160 to 180°, against the center 17c of the glass substrate 17. For this reason, the glass substrates 17 can be inserted into the holder 11 easily, and the glass substrates 17 can be supported by the holder 11 stably. Moreover, even if the holder 11 is vibrated, the glass substrate 17 is prevented from being oscillated and damaged.

[0042] When two places of the glass substrate 17 (first position (19) and bottom portion) come in contact with the supporting members 15a and 15c, the glass substrate 17 is separated by a gap L from the supporting member 15b. The gap L is set at 0.1 to 0.5 mm at normal temperature. For this reason, the holder 11 and the glass substrate 17 are heated, and even if the supporting members 15a, 15b and 15c and the glass substrate 17 are thermally expanded, the glass substrate 17 is not strongly pressed by the supporting members 15a, 15b and 15c. Further, even if the holder 11 is vibrated, the glass substrate 17 is prevented from being oscillated to be damaged.

[0043] The supporting members 15a, 15b and 15c are welded with one side plate 12 at each end of their longitudinal direction, and the other ends of the supporting members 15a, 15 and 15c are not welded to the other side plate 12 but are only inserted into the slits 14. For this reason, even if the holder 11 is heated, the supporting members 15a, 15b and 15c are not distorted. In other words, the frames 12 and 13 are positioned so as to allow thermal expansion of the supporting members 15a, 15b and 15c.

[0044] Next, the present invention will be described in more detail by way of Examples.

EXAMPLES 1 to 7

[0045] In Examples 1 to 3, a holder 11 having supporting members 15a, 15b and 15c where ridgelines 16r of serrate portions 16a, 16b and 16c were not polished was used. Fifty glass substrates 17 were inserted into and taken out of the holder 11, and this process repeated five times.

[0046] In Examples 4 to 7, a holder 11 having a supporting member 15 where ridgelines 16r of serrate portions 16a, 16b and 16c were polished by a wet barrel polishing process was used. One hundred glass substrates 17 were inserted into/taken out of the holder 11 five times.

[0047] A damage incidence on the glass substrate 17 was measured. The results are shown in Table 1. In Examples 1 to 3, the results are an average value of the fifty glass substrates 17, and in Examples 4 to 7, the results are an average value of the one hundred glass substrates 17. 1 TABLE 1 Example 1 2 3 4 5 6 7 Damage 10.67 27.33 5.33 0.33 0.67 0.00 1.00 incidence (%)

[0048] Damage incidence in Examples 1 to 3 was relatively low. Damage incidence in Examples 4 to 7 was lower than that in Examples 1 to 3. This is because the ridgelines 16r of the serrate portions 16a, 16b and 16c are polished by the wet barrel polishing process.

[0049] It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms.

[0050] Three supporting members 15a, 15b and 15c form one set and two or more sets of them may be mounted on the side plates 12. In this case, the holder 11 can hold two or more rows of plural glass substrates 17. The number and size of the connecting plates 13 and mounting positions for the side plates 12 are changed suitably.

[0051] The supporting members 15a, 15b and 15c may be fixed to one side plate 12 by a lock mechanism or the like other than the welding method.

[0052] It is preferred that the bottom portions of the supporting concave portions 18a, 18b and 18c are polished by a method such as the wet barrel polishing process, but need not be polished. In this case, it is preferable that side portions which are contiguous with the bottom portions of the supporting concave portions 18a, 18b and 18c are polished.

[0053] As shown in FIG. 4, in one embodiment, bottom portions of the supporting concave portions 18a, 18b and 18c of the left and right supporting members 15a and 15b are flat, but approximately semicircular protrusions may be provided on the bottom portions of the supporting concave portions 18a, 18b and 18c, respectively. In this case, since the protrusions come in contact with the outer peripheral surface at one point, occurrence of distortion on the glass substrate 17 can be further suppressed. Therefore, flatness of the glass substrate 17 can be maintained more satisfactorily.

[0054] The supporting members 15a, 15b and 15c may be made of a line material such as a wire as shown in FIGS. 6A, 6B and 6C. FIGS. 6A, 6B and 6C show modified examples of one supporting member 15a, but the remaining supporting members 15b and 15c may have the same structure.

[0055] In the supporting member 15a (15b, 15c) shown in FIG. 6A, a line material is bent with constant intervals in one plane so that a plurality of supporting concave portions 18a (18b, 18c) are formed. The width of the bottom portion of the supporting concave portion 18a (18b, 18c) adjacent to the outer edge of the glass substrate 17 is slightly larger than the thickness of the glass substrate 17. Two side portions which are contiguous with the bottom portion of the supporting concave portion 18a (18b, 18c) approach closer to each other as they are closer to the bottom portion. Namely, the supporting concave portions 18a (18b, 18c) are formed into a trapezoid shape.

[0056] In the supporting member 15a (15b, 15c) shown in FIG. 6B, a line material is bent so that a plurality of wave-shaped supporting concave portions 18a (18b, 18c) are formed.

[0057] In the supporting member 15a (15b, 15c) shown in FIG. 6C, a line material is bent with constant intervals on one plane with its plane having an approximately semi-elliptic shape, so that a plurality of supporting concave portions 18a (18b, 18c) are formed. In this case, the bottom portions of the supporting concave portions 18a (18b, 18c) have linear shape, and two side portions which are contiguous with the bottom portion gently approach each other as they are closer to the bottom portions.

[0058] A surface of the supporting member 15a (15b, 15c) made of a linear line material is notched with constant intervals so that a plurality of supporting concave portions 18a (18b, 18c) may be formed. In this case, it is preferred that the width of the bottom portion of the supporting concave portion 18a (18b, 18c) is slightly larger than the thickness of the glass substrate 17 and two side portions which are contiguous with the bottom portion approach closer to each other as they are closer to the bottom portions.

[0059] The present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.

Claims

1. A holder for holding glass substrates for an information recording medium and for immersing the glass substrates in a heated chemical reinforcing liquid, the holder comprising:

a frame; and

a plurality of supporting members mounted on the frame, wherein each of the supporting members has a plurality of supporting concave portions for receiving outer edges of the glass substrates, respectively, and wherein thickness of the supporting members is constant.

2. The holder according to claim 1, wherein the supporting members are plates having one edge formed with serrate portions including the plurality of supporting concave portions.

3. The holder according to claim 1, wherein each of the supporting members is a line member which is bent so that the supporting concave portions are formed.

4. The holder according to claim 1, wherein each of the supporting members has a ridgeline which defines the supporting concave portions, and wherein the ridgeline is chamfered.

5. The holder according to claim 4, wherein the ridgeline is chamfered by barrel polishing.

6. The holder according to claim 1, wherein the glass substrate is held vertically and the supporting members include:

a first supporting member which contacts with a first position of the outer edge of each glass substrate;

a second supporting member which is provided so as to be separated by a predetermined distance from the outer edge of the glass substrate in a second position separated from the first position of the glass substrate by 160 to 180° in a circumferential direction of the glass substrate with respect to the center of the glass substrate; and

a third supporting member for supporting the lower end of each glass substrate.

7. The holder according to claim 6, wherein the predetermined distance is set at 0.1 to 0.5 mm at normal temperature.

8. The holder according to claim 1, wherein the frame includes two side plates, each of which has a plurality of slits, and a connecting member for connecting the two side plates, and wherein both ends of the supporting members are inserted into the plurality of slits of the two side plates, respectively.

9. The holder according to claim 8, wherein one end of each of the supporting members is fixed to one of the two side plates, and the other end of each of the supporting members is mounted on the other side plate movably.

10. The holder according to claim 1, wherein each of the plural supporting members is formed so that the supporting concave portions are arranged in a plane.

11. The holder according to claim 1, wherein the chemical reinforcing liquid contains second ions which have an ion radius larger than an ion radius of first ions existing in a surface layer of each of the glass substrates, and the holder is used to exchange the first ions with the second ions and chemically reinforce the glass substrates.

12. A holder for immersing a glass substrate in a chemical reinforcing liquid, the holder comprising:

a frame;

a first supporting plate mounted on the frame, the first supporting plate having a first serrate portion that defines a first concave portion for receiving a first portion of the glass substrate;

a second supporting plate mounted on the frame, the second supporting plate having a second serrate portion that defines a second concave portion for receiving a second portion of the glass substrate; and

a third supporting plate mounted on the frame, the third supporting plate having a third serrate portion that defines a third concave portion for receiving a bottom portion of the glass substrate.

13. The holder according to claim 12, wherein the third supporting plate is thicker than the first and second supporting plates.

14. The holder according to claim 13, wherein the first, second and third supporting plates have ridgelines that defines the relating concave portions, respectively, and the ridgelines are chamfered.

15. The holder according to claim 12, wherein the first and second supporting members are separated by 160 to 180° in a circumferential direction with respect to the center of the glass substrate.

16. The holder according to claim 12, wherein the third supporting member is separated from the first and second supporting members by 80 to 90° in a circumferential direction with respect the center of the glass substrate.

17. The holder according to claim 12, wherein the frame is connected with the first, second and third supporting members so as to allow thermal expansion of the first, second and third supporting members.

18. A holder for immersing a glass substrate in a chemical reinforcing liquid, the holder comprising:

a frame:

a first supporting wire mounted on the frame, the first supporting wire having a first serrate portion that defines a first concave portion for receiving a first portion of the glass substrate;

a second supporting wire mounted on the frame, the second supporting wire having a second serrate portion that defines a second concave portion for receiving a second portion of the glass substrate; and

a third supporting wire mounted on the frame, the third supporting wire having a third serrate portion that defines a third concave portion for receiving a bottom portion of the glass substrate.

19. The holder according to claim 18, wherein the first, second and third serrate portions are formed by bending the first, second and third supporting wires.

20. The holder according to claim 19, wherein the third supporting wire is thicker than the first and second supporting wires.