JP6368583B2 - Adhesive application mechanism, and solid lubricant embedding device provided with the same - Google Patents

Description

  The present invention relates to an adhesive application mechanism capable of uniformly applying an adhesive to the surface of a solid lubricant.

  A solid lubricant is used for a sliding member used in an environment where oil or grease cannot be used. FIG. 7 is a diagram illustrating an example of a general external shape of a bush type sliding member. As shown in the figure, the sliding member S is accommodated in a sliding member main body B made of a copper alloy and a plurality of holes (through holes or non-through holes) provided on the cylindrical side surface of the sliding member main body. It has a plurality of solid lubricants A that are fixed in a substantially cylindrical shape. As the solid lubricant, graphite (graphite) or the like is often used.

  The shape of the sliding member S is not limited to the bush type as shown in FIG. 7, and there are various types of plate type, block type, and washer type.

  When manufacturing such a sliding member S, the operation | work which embeds the solid lubricant A which apply | coated the adhesive agent to the hole provided in the sliding member main-body part B is needed. Conventionally, the operation of applying the adhesive to the solid lubricant and the operation of embedding the solid lubricant have been performed manually.

  However, the manual application of the adhesive to the solid lubricant has a quality control problem that the amount of application varies depending on human experience (kang and knack). Further, the solid lubricant burying work in the sliding member by hand has a problem in that the cycle time varies depending on the experience value of the worker, and when the worker is absent, the burying work itself is stopped.

  In addition, as a method of applying an adhesive to a cylindrical workpiece surface by mechanization regardless of manual work, a method using an application roller (see, for example, Patent Document 1) or a method of bumping a workpiece (for example, a patent) Reference 2) and other methods such as brush painting, spraying, and line painting with a nozzle are disclosed in the prior art.

JP 2010-075904 JP 2006-314933 JP

  However, in these conventional mechanized methods, it takes time to apply the adhesive and it is difficult to adjust the amount of adhesive applied. Furthermore, when the embedding process after the adhesive application process is performed by a robot, there is a problem that the adhesive adheres to the robot hand or the like due to dripping of the adhesive and solidifies.

  Therefore, when automating and mechanizing the solid lubricant application operation and the embedding operation, it is necessary to appropriately and uniformly control the application of the adhesive to the surface of the solid lubricant.

  The present invention has been made to solve such a conventional problem, and it is an object of the present invention to provide an adhesive application mechanism capable of uniformly applying an adhesive to the surface of a solid lubricant.

The adhesive application mechanism of the present invention is an adhesive application mechanism for applying an adhesive to the outer peripheral surface of a solid lubricant fixed in a plurality of holes of a sliding member body, wherein a predetermined solid lubricant is a predetermined solid lubricant. A hollow cylindrical shape that can be passed with a gap between the inner surface of the hollow cylinder and a concave portion that serves as an adhesive reservoir, and a solid lubricant at an opening at the rear end of the adhesive coating portion. A solid lubricant driving unit that supplies and discharges the solid lubricant from the opening at the tip of the adhesive application unit, and an adhesive supply unit that supplies the adhesive to the recess, the recess of the adhesive application unit is Kumiawasedea is, the axial length of the recess of the adhesive application section of the plurality of turns of the helical groove, the combination of a plurality circumferential of R groove helix and bilateral circumferential R grooves, or, a plurality of turns of the circumferential groove and the longitudinal groove is Ru der than 6 times 1.5 times the diameter of the solid lubricant.

Further, the adhesive application mechanism of the present invention is such that the solid lubricant fixing portion has a solid lubricant at the solid lubricant discharge position at the tip of the hollow cylindrical adhesive application portion in which the solid lubricant is closely disposed. The solid lubricant driving part pushes the solid lubricant at the solid lubricant charging position at the other end of the adhesive application part into the adhesive application part, and the solid lubricant fixation part solidifies the solid lubricant. Fixed at the lubricant discharge position, the adhesive supply unit injects a predetermined amount of adhesive into the recess provided on the inner diameter surface of the hollow cylinder of the adhesive application unit , and the solid lubricant supply unit supplies the solid lubricant to the solid lubricant. The solid lubricant supply unit transfers the solid lubricant from the solid lubricant supply position to the solid lubricant charging position, and the concave portion of the adhesive application unit includes a plurality of spiral grooves and a plurality of R grooves. Combination of spiral and both-side circumferential R-grooves or multiple circumferential grooves and longitudinal grooves And the combined axial length of the recess of the adhesive application unit is less 6 times 1.5 times the diameter of the solid lubricant.

  According to the adhesive application mechanism of the present invention, it is possible to uniformly apply the adhesive to the surface of the solid lubricant.

FIG. 1 is a diagram illustrating an overall configuration of an adhesive application mechanism according to the present embodiment. FIG. 2 is a diagram illustrating the adhesive application unit of the present embodiment. FIG. 3 is a diagram showing variations in the shape of the concave portion of the adhesive application portion of the present embodiment. FIG. 4 is a view showing the solid lubricant fixing portion of the present embodiment as viewed from the X direction of FIG. FIG. 5 is a diagram showing a detailed structure of the solid lubricant supply unit of the present embodiment. FIG. 6 is a diagram illustrating the overall configuration of the solid lubricant embedding apparatus according to the present embodiment. FIG. 7 is a diagram illustrating an example of a general external shape of a bush type sliding member.

  Hereinafter, an adhesive application mechanism 100 according to an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram illustrating an overall configuration of an adhesive application mechanism 100 according to the present embodiment.

  As shown in FIG. 1, the adhesive application mechanism 100 of this embodiment includes an adhesive application unit 110, an adhesive supply unit 120, a solid lubricant driving unit 130, a solid lubricant fixing unit 140, and solid lubrication. Agent supply unit 150. The adhesive application mechanism 100 according to the present embodiment performs an operation of inserting the solid lubricant A applied with an adhesive into a plurality of holes Bh of a sliding member main body B fixed to a handler 213 described later.

  FIG. 2 is a diagram illustrating the adhesive application unit 110 of the present embodiment. The adhesive application part 110 of this embodiment has a hollow cylindrical part 111 through which a predetermined cylindrical solid lubricant A can pass with a predetermined gap, and is bonded to the inner diameter surface 111a of the hollow cylindrical part 111. It has the recessed part 112 used as an agent pool. In this figure, for the sake of clarity, the solid lubricant A is shown one by one at the entrance / exit of the inner diameter surface 111a of the hollow cylindrical portion 111, and the others are omitted, but a plurality of solid lubricants A are applied during actual application. Are closely aligned with each other in series.

  In the adhesive application mechanism 100 of the present embodiment, the solid lubricant A is passed through the hollow cylindrical portion 111 of the adhesive application unit 110 to apply the adhesive to the surface of the cylindrical side surface of the solid lubricant A. .

  As an example of the solid lubricant A used in the present embodiment, graphite (graphite) is used. However, the solid lubricant A is not limited to this. Moreover, it is preferable to use a one-pack epoxy adhesive for the adhesive used in the present embodiment. However, the adhesive is not limited to this, and a two-component epoxy adhesive, an ultraviolet curable adhesive, or the like is also used.

  According to the adhesive application unit 110 of this embodiment, since the solid lubricant A continuously passes through the hollow cylindrical portion 111 of the adhesive application unit 110, a complicated operation such as a rotation operation for application is performed. Can be eliminated, and mechanical failure factors can be reduced. Further, the coating cycle time can be shortened as compared with conventional spin coating and the like.

  The adhesive application part 110 of the present embodiment changes the size of the gap generated due to the difference between the inner diameter D of the hollow cylindrical part 111 and the diameter d of the solid lubricant A, so that the solid lubricant A is applied to the outer peripheral surface. The application thickness of the adhesive can be adjusted. Further, by preparing the hollow cylindrical portion 111 having a different inner diameter D for each diameter of the solid lubricant A, the solid lubricant A of various sizes can be handled.

  The adhesive application unit 110 according to the present embodiment has a concave portion 112 serving as an adhesive reservoir on the inner diameter surface 111 a of the hollow cylindrical portion 111.

  When the inner diameter surface 111a of the hollow cylindrical portion 111 of the adhesive application portion 110 does not have the concave portion 112 but is merely a cylindrical surface, the inner diameter surface 111a of the adhesive application portion 110 and the outer periphery of the solid lubricant A are always provided. Since the surface is rubbed against the surface, there is a problem that the adhesive is rubbed and scraped off. Further, when the adhesive is supplied from the end face of the adhesive application unit 110, the adhesive accumulates only at the supply position, and it is difficult to uniformly apply the solid lubricant A to the outer peripheral surface.

  According to the adhesive application unit 110 of the present embodiment, the adhesive application unit 110 is formed in a hollow cylindrical shape, and a concave portion 112 serving as an adhesive reservoir is provided on the inner diameter surface 111a, and the adhesive is supplied to the concave portion 112. As a result, a substantially constant amount of adhesive is accumulated in the recess 112. As a result, a substantially constant amount of the adhesive contacts the outer peripheral surface of the solid lubricant A, and the adhesive can be uniformly applied to the outer peripheral surface of the solid lubricant A passing through the adhesive application unit 110. .

  In addition, as shown in FIG. 2, the adhesive application unit 110 of this embodiment may have an O-ring 113 at the solid lubricant A supply inlet portion at the rear end 111 b of the hollow cylindrical portion 111. Thereby, it is possible to prevent the adhesive from flowing backward from within the adhesive application unit 110.

  FIG. 3 is a diagram showing variations in the shape of the recess 112 of the adhesive application unit 110 of the present embodiment. As shown to Fig.3 (a), it is preferable that the shape of the recessed part 112 of the adhesive agent application part 110 of this embodiment is the spiral groove 112a. The shape of the recess 112 is not limited to this, and a combination of the R groove spiral 112b and the both-side circumferential R groove 112c as shown in FIG. 3B, or a circumferential groove 112d and a vertical groove 112e as shown in FIG. 3C. Or a radial hole 112f as shown in FIG. 3 (d).

  However, in order to smoothly supply adhesive to the entire groove from the inlet to the outlet of the inner diameter surface 111a of the hollow cylindrical portion 111 and accumulate a certain amount of adhesive in the recess 112, the shape of the recess 112 is changed to the spiral groove 112a. It is preferable that

  Here, as shown in FIG. 3 (a), the axial length l of the adhesive application part 110 of the present embodiment is preferably 1.5 to 6 times the diameter d of the solid lubricant A, more preferably. Is 3 to 5 times the diameter d of the solid lubricant A.

  By making the length 1 of the adhesive application part 110 of this embodiment about 3 to 5 times the diameter d of the solid lubricant A, the adhesive collected in the recess 112 and the wet state are further improved, and the solid lubricant It becomes possible to apply uniformly to the outer peripheral surface of A.

  As shown in FIGS. 1 to 3, the adhesive supply unit 120 of this embodiment is connected to the recess 112 of the adhesive application unit 110 and supplies the adhesive to the recess 112. For the adhesive supply unit 120 of the present embodiment, a known quantitative adhesive supply device (a so-called dispenser; the dispenser main body is not shown) or the like can be used.

  Here, as shown in FIGS. 2 and 3, the positions 120 a and 120 b of the concave portion 112 to which the adhesive is supplied from the adhesive supply unit 120 are the positions of the hollow cylindrical portion 111 when viewed from the tip of the adhesive application unit 110. Two or more are preferably arranged at equal angular intervals with the central axis Cl as the central point.

  Evenly adheres to the outer periphery of the solid lubricant A by supplying adhesive from a plurality of positions 120a and 120b of the recess 112 at equal angular intervals with the central axis Cl of the hollow cylindrical part 111 of the adhesive application part 110 as the center point. The pressure of the agent is applied, the solid lubricant A is held at the center of the adhesive application part 110, and the application thickness of the adhesive can be kept more uniform.

  As shown in FIGS. 1 and 2, the solid lubricant driving unit 130 of the present embodiment is coupled to the rear end 111 b side of the hollow cylindrical part 111 of the adhesive application unit 110, and drives the rod of the actuator 131. The solid lubricant A at the solid lubricant charging position P2 in the vicinity of the opening at the rear end 111b of the hollow cylindrical portion 111 of the adhesive application unit 110 shown in FIG. The solid lubricant A is discharged from the solid lubricant discharge position P1 in the vicinity of the opening 111c.

  As the actuator 131 of the solid lubricant driving unit 130 of the present embodiment, an air cylinder, a linear motion motor actuator, or the like can be used. In addition, a push block 132 for pressing the entire end surface of the solid lubricant A is provided at the rod tip of the actuator 131 of the present embodiment.

  The solid lubricant driving unit 130 according to the present embodiment has a plurality of solid lubricants A aligned in a row from the solid lubricant charging position P2 to the solid lubricant discharging position P1 with the cylindrical end surfaces in close contact with each other. It is preferable to supply to the hollow cylindrical part 111 of the agent application part 110.

  By supplying the cylindrical end surfaces of the plurality of solid lubricants A to the hollow cylindrical part 111 of the adhesive application unit 110 in close contact with each other, the adhesive agent to the cylindrical end surfaces of the solid lubricant A that does not require application of the adhesives is supplied. Adhesion is alleviated and the amount of adhesive used can be reduced. Furthermore, since it becomes possible to apply | coat an adhesive agent to the some solid lubricant A continuously, shortening of cycle time can be aimed at.

  FIG. 4 is a view showing the solid lubricant fixing portion 140 of the present embodiment as viewed from the X direction of FIG. As shown in FIG. 4, the solid lubricant fixing portion 140 of the present embodiment includes a mounting portion 142 and an actuator 141 attached to the outer periphery of the mounting portion.

  As shown in FIGS. 1 and 2, in the solid lubricant fixing portion 140 of this embodiment, the mounting portion 142 is disposed on the front end 111 c side so as to engage with the hollow cylindrical portion 111 of the adhesive application portion 110.

  The solid lubricant fixing portion 140 of the present embodiment drives the rod of the actuator 141 and presses the tip of the rod against the solid lubricant A, thereby discharging the solid lubricant A at the tip 111c of the hollow cylindrical portion 111. Fix at position P1. An air cylinder, a linear motion motor actuator, or the like can be used as the actuator 141 of the solid lubricant fixing portion 140 of the present embodiment.

  In the present embodiment, by providing the solid lubricant fixing portion 140, the solid lubricant A can counteract the supply pressure of the adhesive even when the adhesive is supplied to the recess 112, and a plurality of solid lubricants are provided. The close state between the end faces of the agent A is maintained, and adhesion of the adhesive to the end face of the solid lubricant A is mitigated. Furthermore, since the solid lubricant A is fixed to the solid lubricant discharge position P1 of the adhesive application mechanism 100 even when embedded in the sliding member main body B, it can be reliably embedded without shifting during positioning.

  As shown in FIG. 1, the solid lubricant supply unit 150 of the present embodiment uses the solid lubricant A supplied from the parts feeder (not shown) to the solid lubricant supply position P3 by the air pipe 152 and the rod of the actuator 151. Driven and supplied to the solid lubricant charging position P2. An air cylinder, a linear motion motor actuator, or the like can be used as the actuator 151 of the solid lubricant supply unit 150 of the present embodiment.

  FIG. 5 is a diagram showing a detailed structure of the solid lubricant supply unit 150 of the present embodiment. As shown in the figure, a solid lubricant transport block 153 is provided at the tip of the rod of the actuator 151. The solid lubricant transport block 153 is provided with two through holes 153a and 153b. Each of the through holes 153a and 153b can accommodate the solid lubricant A. The solid lubricant transport block 153 reciprocates up and down in the groove 154 by driving the actuator 151. Thereby, the solid lubricant A at the solid lubricant supply position P3 is transferred to the solid lubricant charging position P2.

  Next, the operation of the adhesive application mechanism 100 according to the embodiment of the present invention will be described.

  In step 1, the cylinder rod of the actuator 141 of the solid lubricant fixing part 140 is returned to loosen the solid lubricant A at the solid lubricant discharge position P1.

  In Step 2, the cylinder rod of the actuator 131 of the solid lubricant driving unit 130 is pushed, and the solid lubricant A at the solid lubricant charging position P2 is pushed out into the adhesive application unit 110.

  In step 3, the cylinder rod of the actuator 141 of the solid lubricant fixing unit 140 is pushed to fix the solid lubricant A to the solid lubricant discharge position P1.

  In step 4, a predetermined amount of adhesive is injected from the adhesive supply unit 120 into the adhesive application unit 110.

  In step 5, the injection of the adhesive from the adhesive supply unit 120 is stopped.

  In step 6, the cylinder rod of the actuator 131 of the solid lubricant driving unit 130 is returned.

  In step 7, the cylinder rod of the actuator 151 of the solid lubricant supply unit 150 is returned.

  In step 8, the solid lubricant A is supplied from the air pipe 152 of the solid lubricant supply unit 150 to the through hole 153b of the solid lubricant transport block 153 at the solid lubricant supply position P3 by air pressure feeding.

  In step 9, the air pressure feeding of the air piping 152 of the solid lubricant supply unit 150 is stopped.

  In Step 10, the cylinder rod of the actuator 151 of the solid lubricant supply unit 150 is pushed to move the solid lubricant transport block 153 upward, and the solid lubricant A stored in the through hole 153b is moved from the solid lubricant supply position P3. Transfer to the solid lubricant charging position P2.

  Next, the solid lubricant embedding device 200 according to the embodiment of the present invention will be described in detail with reference to the drawings. FIG. 6 is a diagram showing an overall configuration of the solid lubricant embedding apparatus 200 of the present embodiment.

  The solid lubricant embedding device 200 of the present embodiment aligns the adhesive application mechanism 100 described above with the hole Bh of the sliding member main body B to which the solid lubricant A is fixed and the adhesive application mechanism 100. It has a positioning mechanism 210 and a handler 213 that holds and fixes the sliding member main body B from above and below. In FIG. 6, only the sliding member main body B is shown in a cross-sectional view in order to make it easy to understand the state of clamping by the handler 213.

  As the positioning mechanism 210 of the present embodiment, a combination of a robot hand 211 that drives the adhesive application mechanism 100 and an index table 212 that indexes the sliding member main body B can be used. However, the configuration of the positioning mechanism 210 is not limited to this, and a mechanism capable of performing alignment between the hole Bh of the sliding member main body B and the adhesive application mechanism 100 may be appropriately combined. For example, instead of the robot hand 211, the adhesive application mechanism 100 may be driven using an actuator as appropriate.

  In the solid lubricant embedding device 200 of the present embodiment, first, the index table 212 is rotated, and the hole Bh of the sliding member main body B to which the solid lubricant A is fixed is moved to the front of the adhesive application mechanism 100. To do.

  Next, the robot hand 211 is driven, and the adhesive-coated solid lubricant A fixed to the tip of the adhesive application mechanism 100 is aligned with the hole Bh of the sliding member main body B, and the robot hand 211 and / or the actuator 131 of the solid lubricant driving unit 130 is driven to insert the solid lubricant A into the hole of the sliding member main body B.

  According to the solid lubricant embedding apparatus 200 of the present embodiment, by attaching the adhesive application mechanism 100 to the robot hand 211 as it is, there is no need to re-grasp the solid lubricant A with the robot hand during the embedding process after the application process. Become. As a result, the cycle time can be shortened.

  Further, when the solid lubricant A is embedded, it is not necessary to separately hold the solid lubricant A with the robot hand, so it is necessary to provide a grip allowance for gripping the robot hand on the solid lubricant A where it is difficult to manage the size of the adhesive. In addition, it is possible to prevent an embedding failure due to insufficient grip of the robot hand.

  As described above, according to the adhesive application mechanism 100 of the present embodiment, the adhesive application part 110 is formed into a hollow cylindrical shape, a concave portion is provided on the inner diameter surface thereof, and the adhesive is supplied to the concave part, thereby allowing adhesion. The adhesive can be uniformly applied to the outer peripheral surface of the solid lubricant A that passes through the agent application unit 110.

  Since it is a simple mechanism for the solid lubricant A to pass through the hollow cylinder of the adhesive application unit 110, a complicated operation such as a rotation operation is not required for the application, and a mechanical failure factor can be reduced. Further, the coating cycle time can be shortened as compared with conventional spin coating and the like.

  Moreover, by making the length of the adhesive application part 110 about 3 to 5 times the diameter of the solid lubricant A, the adhesive accumulated in the groove and the wet state are improved, and the solid lubricant A is uniformly distributed on the outer peripheral surface. It becomes possible to apply to.

  Furthermore, by supplying the solid lubricant A with the end surfaces of the solid lubricant A in close contact with each other, the adhesion of the adhesive to the end surface of the solid lubricant A that does not require application of the adhesive is reduced. The amount of adhesive used can be reduced. Furthermore, since it becomes possible to apply | coat an adhesive agent to the some solid lubricant A continuously, shortening of cycle time can be aimed at.

  By providing the solid lubricant fixing portion 140, it becomes possible to resist the pressure even when the adhesive is supplied to the recess, and the close state of the end surfaces of the plurality of solid lubricants A is maintained. Adhesion of the adhesive to the end face is alleviated. Furthermore, since the solid lubricant A is fixed to the tip of the adhesive application mechanism 100 even when it is embedded, it can be reliably embedded without being displaced during positioning.

  Furthermore, according to the adhesive application mechanism 100 of the present embodiment, by attaching the adhesive application mechanism 100 to the robot hand as it is, an operation of re-gripping the solid lubricant A with the robot hand during the embedding process after the application process is unnecessary. It becomes. As a result, the tact time can be shortened.

  Further, in the embedding process, it is not necessary to separately hold the solid lubricant A with the robot hand, so it is not necessary to provide a grip allowance for holding the robot hand in the solid lubricant A, which is difficult to manage the size of the adhesive. Improper embedding due to insufficient grip of the robot hand can be prevented.

100: Adhesive application mechanism 110: Adhesive application unit 120: Adhesive supply unit 130: Solid lubricant driving unit 140: Solid lubricant fixing unit 150: Solid lubricant supply unit 200: Solid lubricant embedding device 210: Positioning mechanism

Claims (6)

An adhesive application mechanism for applying an adhesive to the outer peripheral surface of a solid lubricant fixed in a plurality of holes of a sliding member body,
An adhesive application part having a hollow cylindrical shape through which the solid lubricant can pass with a predetermined gap, and having a recess serving as an adhesive reservoir on the inner diameter surface of the hollow cylinder;
A solid lubricant driving unit that supplies the solid lubricant to an opening at a rear end of the adhesive application unit, and discharges the solid lubricant from an opening at a front end of the adhesive application unit;
An adhesive supply section for supplying an adhesive to the recess;
Have
The recess of the adhesive application unit, a combination of a plurality circumferential spiral groove, the plurality of turns R grooves spiral and bilateral circumferential R grooves or, Ri Kumiawasedea with multiple peripheral circumferential groove and the longitudinal groove, the adhesive agent axial length of the recess of the application part, the adhesive applying mechanism, wherein 6 times or less der Rukoto 1.5 times the diameter of the solid lubricant.   The adhesive supply unit supplies the adhesive from a plurality of positions at equiangular intervals around the central axis of the hollow cylinder of the adhesive application unit when viewed from the tip of the adhesive application unit. The adhesive application mechanism according to claim 1.   The solid lubricant driving unit supplies a plurality of solid lubricants to the adhesive application unit in a line from the rear end to the front end in close contact with each other. Item 3. The adhesive application mechanism according to Item 1 or 2.   The adhesive application mechanism according to any one of claims 1 to 3, further comprising a solid lubricant fixing part capable of fixing the solid lubricant at the tip of the adhesive application part. The adhesive application mechanism according to any one of claims 1 to 4,
A positioning mechanism for aligning the adhesive-coated solid lubricant fixed to the tip of the adhesive application mechanism and the hole of the sliding member main body;
A solid lubricant embedding device characterized by comprising: The solid lubricant fixing part loosens the fixing of the solid lubricant at the solid lubricant discharge position at the tip of the hollow cylindrical adhesive application part where the solid lubricant is closely arranged inside,
The solid lubricant driving unit pushes the solid lubricant at the solid lubricant charging position at the other end of the adhesive application unit into the adhesive application unit,
The solid lubricant fixing portion fixes the solid lubricant at the solid lubricant discharge position;
The adhesive supply unit injects a predetermined amount of adhesive into the concave portion provided in the hollow cylindrical inner diameter surface of the adhesive application unit,
The solid lubricant supply unit supplies the solid lubricant to the solid lubricant supply position,
The solid lubricant supply unit, transferring the solid lubricant in the solid lubricant-on position from the solid lubricant supply position,
The concave portion of the adhesive application part is a spiral groove of a plurality of circumferences, a combination of a spiral of a plurality of R grooves and a circumferential R groove on both sides, or a combination of a plurality of circumferential grooves and a longitudinal groove. The solid lubricant embedding mechanism , wherein the axial length of the concave portion of the application portion is 1.5 to 6 times the diameter of the solid lubricant.