Soft-strip-free induction quenching equipment

Abstract

The invention discloses induction quenching equipment without a soft belt. The induction quenching equipment comprises a mounting platform used for mounting an annular workpiece and a first driver used for driving the mounting platform to rotate around the axis of the mounting platform; the clamping mechanism is used for circumferentially positioning the annular workpiece on the mounting platform, the clamping mechanism is provided with a plurality of clamping pieces which are annularly arranged, and the clamping pieces can move in the radial direction of the annular workpiece; the driving mechanisms are annularly arranged around the mounting platform, induction heaters used for quenching the annular workpiece are arranged on the driving mechanisms, and a heating area used for heating the whole annular workpiece is defined by the induction heaters; the driving mechanism can be used for driving the induction heater to move in the axial direction and the radial direction of the annular workpiece. The induction quenching device has the beneficial effects that induction quenching can be carried out on annular workpieces with different diameters and heights, and the problem that the annular workpieces are prone to generating soft belts during quenching is solved.

Description

Technical Field

[0001] This invention relates to an induction hardening device without a soft strip, belonging to the field of heat treatment technology for ring-shaped workpieces. Background Technology

[0002] Induction hardening is a heat treatment method that uses the principle of electromagnetic induction to rapidly heat and cool the surface of a workpiece. It can effectively improve the wear resistance, fatigue strength and service life of parts, and is mainly used for processing and machining ring-shaped workpieces such as bearings.

[0003] Currently, the main method for induction hardening is scanning induction heating with relative movement between the inductor and the workpiece. This involves mounting the induction heater on a gantry, and quenching the workpiece by moving the gantry relative to it. However, this method is limited by the gantry's size, making it difficult to meet the quenching requirements of small-diameter workpieces. Furthermore, in continuous quenching of ring-shaped workpieces using gantry movement, the scanning endpoint and starting point of the induction heater can easily overlap. This causes the already hardened area to be reheated to the tempering temperature, resulting in a decrease in hardness, uneven hardness of the hardened layer, and the formation of closed soft bands, all of which negatively impact workpiece quality. Summary of the Invention

[0004] The purpose of this invention is to provide a soft-strip induction hardening device that can perform induction hardening on ring-shaped workpieces of different diameters and heights, and solve the problem that ring-shaped workpieces are prone to producing soft strips during hardening.

[0005] The present invention is achieved through the following technical solution.

[0006] A soft-strip induction hardening device includes: A mounting platform for mounting workpieces and a first driver for driving the mounting platform to rotate about its own axis; A clamping mechanism is used to circumferentially position the annular workpiece on the mounting platform. The clamping mechanism has at least a plurality of clamping members arranged in a ring, and the clamping members are movable radially along the annular workpiece. Multiple drive mechanisms are arranged in a ring around the mounting platform. Each drive mechanism is equipped with an induction heater for quenching the ring-shaped workpiece. The multiple induction heaters enclose a heating area for heating the entire ring-shaped workpiece. The drive mechanisms can be used to drive the induction heaters to move along the axial and radial directions of the ring-shaped workpiece.

[0007] As a further improvement of the present invention, the clamping mechanism includes a plurality of clamping components corresponding one-to-one with the clamping members, the clamping components including: A second driver and a push rod, wherein the second driver is used to drive the push rod to move radially along the annular workpiece, and the movement of the push rod pushes the clamping member to move; An elastic element is provided to the clamping member with a radial elastic force toward the annular workpiece, and the elastic deformation of the elastic element allows the radial expansion of the annular workpiece when heated to drive the clamping member radially away from the annular workpiece; A limiting block is disposed on the side of the clamping member away from the annular workpiece and is used to abut against the clamping member; in the initial state, the limiting block and the clamping member have a preset distance along the radial direction of the annular workpiece, and the preset distance is used to limit the maximum radial expansion deformation of the annular workpiece when it is heated.

[0008] As a further improvement of the present invention, the push rod is connected to the clamping member through a guide limiting member, the guide limiting member including at least a guide rod and a limiting part, the clamping member is slidably sleeved outside the guide rod along the radial direction of the annular workpiece, the limiting part is used to abut against the clamping member to prevent the clamping member from disengaging from the push rod; and the elastic force provided by the elastic member to the clamping member keeps the clamping member in abutting state with the limiting part.

[0009] As a further improvement of the present invention, the push rod is connected to the limiting block, one end of the guide limiting member is connected to the limiting block, and the limiting block is provided with a connecting hole for fixing the guide limiting member; the connecting hole extends radially along the annular workpiece, and the position of the guide limiting member relative to the connecting hole is adjustable radially along the annular workpiece.

[0010] As a further improvement of the present invention, the limiting block and the clamping member are provided with an inwardly recessed mounting groove for each elastic element, and the two corresponding mounting grooves are combined to limit the position of the elastic element.

[0011] As a further improvement of the present invention, the clamping member has a clamping surface extending circumferentially along the annular workpiece at one end near the annular workpiece, and the clamping surface is in surface contact with the outer peripheral surface of the annular workpiece.

[0012] As a further improvement of the present invention, the bottom end of the clamping member extends outward to form a support portion, the support portion having a support plane for supporting the annular workpiece; multiple support planes belonging to different support portions form a support structure for supporting the annular workpiece.

[0013] As a further improvement of the present invention, the driving mechanism includes a first support base, a second support base slidably connected to the first support base radially, a mounting bracket for mounting the induction heater slidably connected to the second support base axially, a third driver for driving the second support base to move radially, and a fourth driver for driving the mounting bracket to move vertically.

[0014] As a further improvement of the present invention, the first support base is provided with at least a first slide rail extending radially, and the second support base is provided with at least one first sliding member corresponding to each of the first slide rails, and the first guide rail is slidably connected to the first sliding member.

[0015] As a further improvement of the present invention, the first support base is provided with a second slide rail extending along the axial direction, and the mounting bracket is provided with at least one second sliding member corresponding to each second slide rail, and the second slide rail and the second sliding member are slidably connected.

[0016] The beneficial effects of this invention are: 1. During induction hardening, the heating zone formed by multiple induction heaters can heat the entire ring-shaped workpiece. The matching mounting platform drives the axial rotation of the ring-shaped workpiece, achieving uniform heating of the entire ring-shaped workpiece. Compared with the traditional continuous hardening method, it can shorten the heating time of the ring-shaped workpiece, improve heating efficiency, and reduce the risk of deformation of the ring-shaped workpiece caused by long-term heating. On the other hand, it can avoid the "soft band" problem during hardening and improve the consistency of the hardened layer.

[0017] 2. Multiple clamping components are evenly arranged circumferentially along the annular workpiece and are radially movable, enabling the clamping and fixing of annular workpieces of different diameters. Furthermore, the uniform distribution of the multiple clamping components ensures a uniform distribution of clamping force on the annular workpiece, maintaining its positional stability and preventing displacement during high-speed rotation or heating. Simultaneously, the drive mechanism can drive the induction heater to move along the axial and radial directions of the workpiece. On one hand, this allows adjustment of the axial and radial positions of the induction heater relative to the annular workpiece when its axial and radial dimensions change, adapting to different sizes of annular workpieces. On the other hand, the depth of the hardened layer can be controlled by adjusting the individual axial position of the induction heater.

[0018] 3. The design of the elastic element allows the clamping component to retract radially when the annular workpiece expands due to heat. On the one hand, this avoids deformation or cracking caused by the mechanical rigidity of the clamping component on the annular workpiece. On the other hand, the elastic element can continuously provide elastic force to the clamping component when it retracts radially, ensuring that the clamping component can always maintain close contact with the surface of the annular workpiece and guaranteeing the stability of the annular workpiece's position during quenching. At the same time, the initial distance between the limiting block and the clamping component ensures that the annular workpiece has a certain expansion space when heated, while preventing excessive displacement and excessive radial expansion of the annular workpiece, thus improving the fatigue strength and reliability of the annular workpiece after quenching. Attached Figure Description

[0019] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings to aid in understanding the objectives and advantages of the present invention, wherein: Figure 1This is a schematic diagram of the structure of an induction hardening device without a soft strip according to the present invention; Figure 2 A schematic diagram showing the structure of the mounting platform, clamping mechanism, and workpiece in conjunction. Figure 3 for Figure 2 Enlarged view of section A; Figure 4 A structural diagram showing the assembly of the push rod, limit block, guide limit component, and clamping component; Figure 5 This is a schematic diagram of the clamping component. Figure 6 This is a schematic diagram of the limiting block. Figure 7 This is a schematic diagram of the structure in which the drive mechanism and the induction heater work together; Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0021] The directional terms such as up, down, left, right, front, back, front, back, top, and bottom mentioned or possibly used in this specification are defined relative to the construction shown in the accompanying drawings. The terms "inner" and "outer" refer to directions toward or away from the geometric center of a specific component, respectively. These are relative concepts and may therefore vary depending on their location and usage. Therefore, these or other directional terms should not be interpreted as restrictive.

[0022] This embodiment provides a non-soft strip induction hardening device for induction hardening of ring-shaped workpieces m, such as bearing parts, as described above. Figure 1-7 The system includes a mounting platform 11 for mounting a ring-shaped workpiece m, a first driver 12 for driving the mounting platform 11 to rotate about its own axis, a clamping mechanism, and multiple driving mechanisms 4. The clamping mechanism positions the ring-shaped workpiece m on the mounting platform 11 and has multiple clamping members 21 arranged in a ring. The clamping members 21 can move repeatedly radially along the ring-shaped workpiece m, and the multiple clamping members 21 fix the ring-shaped workpiece m by contacting its outer circumferential surface. The system also includes multiple driving mechanisms 4 arranged in a ring around the mounting platform 11. Each driving mechanism 4 is equipped with an induction heater 3 for quenching the ring-shaped workpiece m. The multiple induction heaters 3 are arranged in a ring and enclose a heating area along the circumference for heating the entire ring-shaped workpiece m. The driving mechanisms 4 can drive the induction heaters 3 to move axially and radially along the ring-shaped workpiece m.

[0023] During induction hardening, the heating zone formed by multiple induction heaters 3 can heat the entire annular workpiece. The matching mounting platform 11 drives the axial rotation of the annular workpiece m, achieving uniform heating of the entire annular workpiece m. Compared with the traditional continuous hardening method, on the one hand, it can shorten the heating time of the annular workpiece m, improve heating efficiency, and reduce the risk of excessive deformation of the annular workpiece m caused by long-term heating. On the other hand, it can avoid the "soft band" problem during hardening and improve the consistency of the hardened layer.

[0024] Multiple clamping components 21 are evenly arranged along the circumference of the annular workpiece m and are radially movable, which can achieve clamping and fixing of annular workpieces of different diameters; and the uniform distribution of multiple clamping components 21 makes the clamping force on the annular workpiece m uniformly distributed, which can keep the position of the annular workpiece m stable and prevent it from shifting its position when rotating at high speed or heated.

[0025] Meanwhile, the drive mechanism 4 can drive the induction heater to move along the axial and radial directions of the workpiece. On the one hand, when the axial and radial dimensions of the annular workpiece m change, the axial and radial positions of the induction heater 3 relative to the annular workpiece can be adjusted to adapt to annular workpieces of different sizes. On the other hand, the depth of the hardened layer can be controlled by adjusting the axial position of the induction heater alone.

[0026] It should be mentioned that during induction hardening, when the induction heater 3 heats the annular workpiece m, the surface temperature of the annular workpiece m rises rapidly. At this time, the surface of the annular workpiece m will undergo radial expansion due to the heat. However, excessive radial expansion can easily lead to excessive deformation of the annular workpiece m, affecting the assembly accuracy. Insufficient radial expansion can easily prevent the structural stress from being fully released, causing the annular workpiece m to crack during use. Therefore, controlling the radial expansion of the annular workpiece m during induction hardening is crucial.

[0027] In this embodiment, refer to Figure 2 , Figure 3 and Figure 4The clamping mechanism includes multiple clamping assemblies corresponding one-to-one with the clamping members 21. Each clamping assembly includes a second driver (not shown in the figure), a push rod 22, an elastic element 23, and a limiting block 24. The second driver drives the push rod 22 to move radially along the annular workpiece m. The push rod 22 is connected to the clamping member 21. The second driver can be a pneumatic cylinder, a hydraulic cylinder, or an electric push rod, and is fixedly connected to the mounting platform 11. When the second driver is working, the movement of the push rod 22 causes the clamping member 21 to move radially. When multiple radially arranged clamping members 21 simultaneously contact the surface of the annular workpiece m, the position of the annular workpiece m is clamped. The holding mechanism is fixed, and the elastic element 23 is used to provide radial elastic force to the clamping member 21 toward the annular workpiece m. The elastic element 23 is made of elastic material, which can deform and provide elastic force when subjected to external force, and return to its initial shape after the external force disappears. Thus, the elastic deformation of the elastic element 23 allows the radial expansion of the annular workpiece m when heated to drive the clamping member 21 away from the annular workpiece m radially. The limiting block 24 is set on the push rod 22 and is used to abut against the clamping member 21. In the initial state, there is a preset distance 25 between the limiting block 24 and the clamping member 21 along the radial direction of the annular workpiece m to limit the maximum radial expansion of the annular workpiece m.

[0028] It should be noted that the initial state mentioned above refers to the state before induction hardening, when multiple clamping parts 21 simultaneously contact and fix the annular workpiece m.

[0029] The design of the elastic element 23 allows the clamping element 21 to retract radially when the annular workpiece m expands due to heat. On the one hand, this avoids deformation or cracking caused by the mechanical rigid constraint of the clamping element 21 on the annular workpiece m. On the other hand, the elastic element 23 can continuously provide elastic force to the clamping element 21 when it retracts radially, so that the clamping element 21 can always be in close contact with the surface of the annular workpiece m, ensuring the stability of the position of the annular workpiece m during quenching. At the same time, the initial distance between the limiting block 24 and the clamping element 21 ensures that the annular workpiece m has a certain expansion space when heated, while preventing excessive displacement and excessive unidirectional expansion of the annular workpiece m, thus improving the fatigue strength and reliability of the annular workpiece m after quenching.

[0030] Meanwhile, the design of the limiting block 24 can accurately control the radial deformation of the ring workpiece m during quenching. On the one hand, it can ensure the quenching accuracy and improve the strength of the ring workpiece after quenching. On the other hand, it can avoid the subsequent additional processing caused by uneven deformation of the ring workpiece m.

[0031] In this embodiment, refer to Figure 4The push rod 22 is connected to the clamping member 21 through the guide limiting member 26. The guide limiting member 26 includes a guide rod 261 extending radially and a limiting part 262. The clamping member 21 has a through hole that allows the guide rod 261 to pass through. The clamping member 21 is slidably sleeved on the guide rod 261. The limiting part 262 is formed at one end of the guide rod 261 near the annular workpiece m. The limiting part 262 is used to abut against the clamping member 21 to restrict the clamping member 21 from disengaging from the push rod 22. The elastic force provided by the elastic member 23 to the clamping member 21 keeps the clamping member 21 in abutting state with the limiting part 262. Therefore, the clamping member 21 is fixed in the position of abutting with the limiting part 262 under the action of the elastic force. When the clamping member 21 is subjected to the radial expansion force of the annular workpiece m, the clamping member 21 moves away from the annular workpiece m. During this process, the guide limiting member 26 is used to limit the movement direction of the clamping member 21 to ensure the accuracy of the movement direction of the clamping member 21 when subjected to the expansion force of the annular workpiece m.

[0032] Meanwhile, to facilitate the installation of the elastic element 23 and the guide limiting element 26, one end of the push rod 22 is connected to the limiting block 24, and one end of the guide limiting element 26 is connected to the limiting block 24. The limiting block 24 is provided with a connecting hole 27 for installing the guide limiting element 26, and the connecting hole 27 extends radially along the annular workpiece m. The guide limiting element 26 is radially adjustable relative to the connecting hole 27 along the annular workpiece m, thereby adjusting the size of the preset distance 25 between the limiting block 24 and the clamping element 21. Therefore, for annular workpieces m of different materials or different sizes, it is possible to control them to have different maximum radial expansion amounts.

[0033] In this embodiment, refer to Figure 4 , Figure 5 and Figure 6 The guide limiting member 26 and the connecting hole 27 can be threaded together, meaning that the outer surface of the guide limiting member 26 has external threads, and the interior of the connecting hole 27 has corresponding internal threads. The position of the guide limiting member 26 relative to the limiting block 24 can be adjusted by rotating the guide limiting member 26, thereby adjusting the size of the preset distance 25 between the clamping member 21 and the limiting block 24. It should be noted that the guide limiting member 26 and the connecting hole 27 can also be snap-fitted together, meaning that the fixing and relative position adjustment of the guide limiting member 26 and the connecting hole 27 are achieved through the engagement of a slot and teeth.

[0034] Furthermore, the limiting block 24 and the clamping member 21 each have an inwardly recessed mounting groove 28 corresponding to each elastic member 23. The two corresponding mounting grooves 28 are combined to limit the position of the elastic member 23, that is, the elastic member 23 abuts against the bottom wall of the two mounting grooves 28 respectively. And when the clamping member 21 abuts against the limiting block 24, the elastic member 23 is completely within the two mounting grooves 28.

[0035] In this embodiment, the elastic element 23 is preferably a compression spring. The two ends of the compression spring abut against the clamping member 21 and the limiting block 24, respectively. In the initial state, the compression spring is in a compressed state, ensuring that it always provides a spring force to the clamping member 21 towards the annular workpiece m. It should be noted that in this embodiment, the number of elastic elements 23 corresponding to each clamping member 21 can be set to one or more to adjust the spring force applied to the clamping member 21.

[0036] In this embodiment, refer to Figure 5 The clamping member 21 has a clamping surface 211 extending circumferentially along the ring workpiece m at one end near the ring workpiece m. When the clamping member 21 contacts the ring workpiece m, the clamping surface 211 can fit tightly against the outer circumferential surface of the ring workpiece m, that is, the two are in surface contact. In this structure, the clamping member 21 and the ring workpiece m have a large contact area, which helps to improve the positioning stability of the ring workpiece m. The bottom end of the clamping member 21 extends outward to form a support part 212. The support part 212 has a support plane for supporting the ring workpiece m. Multiple support planes belonging to different support parts 212 form a support structure for supporting the ring workpiece m, which facilitates the installation and positioning of the ring workpiece m.

[0037] In this embodiment, refer to Figure 7 The drive mechanism 4 includes a first support base 41, a second support base 42 that is radially slidably disposed on the first support base 41, a bracket for mounting the induction heater 3 that is axially slidably disposed on the second support base 42, a third driver 44 for driving the second support base 42 to move radially, and a fourth driver 45 for driving the mounting bracket 43 to move axially.

[0038] The first support base 41 has at least one first slide rail 51 extending radially along the annular workpiece m. The second support base 42 has at least one first sliding member 52 corresponding to each first slide rail 51. The first slide rail 51 and the first sliding member 52 are slidably connected. Here, the first sliding member 52 and the first slide rail 51 can be in a sliding engagement manner to ensure the stability and accuracy of the second support base 42. Correspondingly, the first support base 41 has a second slide rail 61 extending axially. The mounting bracket 43 has at least one second sliding member 62 corresponding to each second slide rail 61. The second slide rail 61 and the second sliding member 62 are slidably connected. In this structure, the first support base 41 and the second support base 42 are slidably engaged radially along the annular workpiece m, and the mounting bracket 43 and the second support base 42 are slidably engaged axially along the annular workpiece m, ensuring the stability and accuracy of the movement of the induction heater 3.

[0039] It should be noted that the first driver in this embodiment can be a servo motor, that is, a servo motor is used to drive the mounting platform 11 to rotate. The third driver 44 and the fourth driver 45 can be one of an electric telescopic rod structure, a cylinder mechanism, or a ball screw structure.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A soft-strip induction hardening device, characterized in that, include: A mounting platform (11) for mounting a ring-shaped workpiece (m) and a first driver (12) for driving the mounting platform (11) to rotate about its own axis. A clamping mechanism is used to circumferentially position the annular workpiece (m) on the mounting platform (11). The clamping mechanism has a plurality of clamping members (21) arranged in a ring, and the clamping members (21) can move radially along the annular workpiece (m). Multiple drive mechanisms (4) are arranged in a ring around the mounting platform (11). Each drive mechanism (4) is equipped with an induction heater (3) for quenching the ring workpiece (m). The multiple induction heaters (3) enclose a heating area for heating the entire ring workpiece (m). The drive mechanism (4) can be used to drive the corresponding induction heater (3) to move along the axial and radial directions of the ring workpiece (m).

2. The induction hardening equipment without soft strip according to claim 1, characterized in that, The clamping mechanism includes a plurality of clamping components that correspond one-to-one with the clamping member (21), and the clamping components include: A second driver and a push rod (22), the second driver being used to drive the push rod (22) to move radially along the annular workpiece (m), the movement of the push rod (22) pushing the clamping member (21) to move; An elastic element (23) is provided to the clamping member (21) with a radial elastic force toward the annular workpiece (m), and the elastic deformation of the elastic element (23) allows the radial expansion of the annular workpiece (m) when heated to drive the clamping member (21) radially away from the annular workpiece (m). A limiting block (24) is disposed on the side of the clamping member (21) away from the annular workpiece (m) and is used to abut against the clamping member (21); in the initial state, the limiting block (24) and the clamping member (21) have a preset distance (25) along the radial direction of the annular workpiece (m), and the preset distance (25) is used to limit the maximum radial expansion deformation of the annular workpiece (m) when heated.

3. The induction hardening equipment without soft strip according to claim 2, characterized in that, The push rod (22) is connected to the clamping member (21) via a guide limiting member (26). The guide limiting member (26) includes at least a guide rod (261) and a limiting part (262). The clamping member (21) is slidably sleeved outside the guide rod (261) along the radial direction of the annular workpiece (m). The limiting part (262) is used to abut against the clamping member (21) to prevent the clamping member (21) from disengaging from the push rod (22). The elastic force provided by the elastic member (23) to the clamping member (21) keeps the clamping member (21) in abutment with the limiting part (262).

4. The induction hardening equipment without soft strip according to claim 3, characterized in that, The push rod (22) is connected to the limiting block (24), and one end of the guide limiting member (26) is connected to the limiting block (24). The limiting block (24) is provided with a connecting hole (27) for fixing the guide limiting member (26). The connecting hole (27) extends radially along the annular workpiece (m), and the position of the guide limiting member (26) relative to the connecting hole (27) is adjustable radially along the annular workpiece (m).

5. The induction hardening equipment without soft strip according to claim 4, characterized in that, The limiting block (24) and the clamping member (21) are provided with an inwardly recessed mounting groove (28) for each elastic member (23), and the two corresponding mounting grooves (28) are combined to limit the position of the elastic member (23).

6. The induction hardening equipment without soft strip according to claim 2, characterized in that, The clamping member (21) has a clamping surface (211) extending circumferentially along the annular workpiece (m) at one end near the annular workpiece (m), and the clamping surface (211) is in surface contact with the outer peripheral surface of the annular workpiece (m).

7. The induction hardening equipment without soft strip according to claim 6, characterized in that, The bottom end of the clamping member (21) extends outward to form a support portion (212), the support portion (212) having a support plane for supporting the annular workpiece (m); multiple support planes belonging to different support portions (212) form a support structure for supporting the annular workpiece (m).

8. A non-soft-strip induction hardening device according to any one of claims 1 to 7, characterized in that, The drive mechanism (4) includes a first support base (41), a second support base (42) that is radially slidably connected to the first support base (41), a mounting bracket (43) that is axially slidably connected to the second support base (42) and used to mount the induction heater (3), a third driver (44) for driving the second support base (42) to move radially, and a fourth driver (45) for driving the mounting bracket (43) to move axially.

9. The induction hardening equipment without soft strip according to claim 8, characterized in that, The first support (41) is provided with at least one first slide rail (51) extending radially, and the second support (42) is provided with at least one first sliding member (52) corresponding to each of the first slide rails (51), and the first slide rail (51) is slidably connected to the first sliding member (52).

10. The induction hardening equipment without soft strip according to claim 8, characterized in that, The first support base (41) is provided with a second slide rail (61) extending along the axial direction, and the mounting bracket (43) is provided with at least one second sliding member (62) corresponding to each second slide rail (61), and the second slide rail (61) and the second sliding member (62) are slidably connected.

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