KR200206902Y1 - A device for inspecting and assembling the burner assembly - Google Patents

Abstract

The present invention relates to an inspection and assembling apparatus of an overhead heating ceiling burner, and examines the shape and size of the arc-shaped inner surface of the refractory block of the overhead heating burner, and a plurality of anchor bolts combining the body portion and the refractory block are the same. It allows the clamping force to be fastened and the nozzle can be mounted to have an appropriate height.

Configuration for this, the surface plate 10 can be adjusted horizontally; A block inspection table (20) mounted to the surface plate (10) to form a curved surface (22) and having a plurality of limit switches (24); A cylinder 40 mounted to the surface plate 10 and having a rod 42; A nozzle detector 48 mounted to a limit switch 24a and fixed to an end of the rod 42; An impact trench (60) slidably mounted on the sliding guide (50) of the surface plate (10); A sliding body (80) mounted to the sliding guide (50) so as to be slidable and mounted with a pressing plate (70) at a lower portion thereof; A tightening nut 85 and a pressurizer 90 fastened by screwing to an upper end of the upper fixing stand 45 which is detachably mounted on the surface plate 10; And a lamp display unit 30 having a lamp 32 electrically connected to the limit switches 24 and 24 '.

Description

A device for inspecting and assembling the burner assembly}

The present invention relates to a device for inspecting and assembling an upper heated ceiling burner, and more particularly, to examine the shape and size of an arc-shaped inner surface of a refractory block of an upper heated ceiling burner, and to combine a plurality of bodies and a refractory block. The present invention relates to an inspection and assembling apparatus for an overhead heating ceiling burner that enables anchor bolts to be fastened with the same fastening force and to mount nozzles to have an appropriate height.

In general, the heating furnace of the walking beam hearth type (Walking beam hearth type), the burner is installed only in the ceiling of the furnace, a typical structure for such an overhead heating ceiling burner 100 is shown in FIG.

It has a body portion 110 formed of a hollow hollow cylindrical shape to have an upper inner shell 112 to have the same inner diameter, and the lower inner shell 114 is integrally fixed to the lower end of the upper iron shell 112 and the inner diameter is enlarged. The inner surface of the lower shell 114 is a refractory 115 is constructed so that the inner diameter is the same as the inner diameter of the upper shell 112, the lower end of the lower shell 114 is a flange 114a bent outwardly Has

The refractory block 120 is fixed to the lower end of the body portion 110 via the anchor bolt 104, the refractory block 120 is formed in the vertical direction through the center of the body portion 110 It has a flame discharge hole 122 having the same inner diameter as the inner diameter of the upper shell 112 and the refractory 115, the appropriate length of the lower end portion of the flame discharge hole 122 is an arc-shaped inner surface of which the inner diameter increases toward the lower side; It is formed to have 102a.

In addition, a hollow disk-shaped upper plate 116 having an inner diameter smaller than the inner diameter of the upper steel bar 112 and having an outer diameter larger than the outer diameter of the upper steel bar 112 is formed integrally on the upper end of the body part 110. The burner cap 130 is fixed to the upper portion of the upper plate 116 by a bolt member or the like.

The burner cap 130 has a hollow cylindrical cylindrical body 132 having an outer diameter that is the same size as the central through hole 116a of the upper plate 116, and is fixed in a flange shape outside the center of the cap body 132. Flange 134.

The nozzle assembly 140 is mounted inside the lower end of the cap body 132, which is fixed to the lower end of the nozzle tube 142 and the nozzle tube 142 screwed to the lower end of the cap body 132. The gap 145 is formed to have a fixed nozzle 144.

In order to mount the upper heated ceiling burner 100 to the ceiling of the heating furnace, first, the process of assembling the body 110, the refractory block 120, the burner cap 130 and the nozzle assembly 140, An important factor in this assembly process is to check whether the arc-shaped inner surface 122a of the refractory block 120 has a set curvature, and the sealing wool in which the body portion 110 and the refractory block 120 are sandwiched therebetween. 151, 152 and when combined with each other when the tightening force of the plurality of anchor bolts 104 to match each other, and the height of the nozzle 144 is appropriately coupled to the nozzle 144, etc. In spite of the fact that these factors directly affect the combustion efficiency as well as the deterioration of the equipment, the shape and size of the main components of the ceiling burner 100 are examined. And the assembly can be done normally To the auxiliary device that had not been disclosed at all.

The present invention was devised to solve such a conventional problem, and its purpose is to examine the shape and size of the arc-shaped inner surface of the refractory block of the top heated ceiling burner, and to connect the body and the refractory block. The present invention provides an apparatus for inspecting and assembling an overhead heating ceiling burner, which enables fastening with the same fastening force and enables mounting of a nozzle to have a proper height.

1 is a cross-sectional view showing a typical top heated ceiling burner;

Figure 2 is an external perspective view showing an inspection and assembly apparatus of the top heated ceiling burner according to the present invention;

Figure 3 is a cross-sectional view showing an inspection and assembly apparatus of the top heated ceiling burner according to the present invention;

Figure 4 is a cross-sectional detail view showing in detail the pressurizer configuration for the device of the present invention;

5a and 5b are explanatory views showing the inspection of the refractory block of the ceiling burner by the device of the present invention;

Figure 6 is a cross-sectional view showing a state of fastening the anchor bolt for assembling the body portion and the refractory block of the ceiling burner by the device of the present invention;

7a and 7b are explanatory views showing a state of inspecting the position of the nozzle assembly of the ceiling burner by the device of the present invention;

Explanation of symbols on the main parts of the drawings

10: surface plate 12: support stand

14: adjusting nut 17: seating plate

18: level gauge 20: block inspection system

22: curved surface 24, 24 ': limit switch

24a, 24a ': detection part 30: lamp display part

32: lamp 40: cylinder

42: rod 44: lower fixture

45: upper fixing stand 48: nozzle detection zone

49: reference plate 50: sliding guide

60: impact trench 62: extension

63: handle 70: pressing plate

80: sliding body 82: connecting rod

84: sleeve 90: pressurizer

94: piston

The present invention as a technical configuration for achieving the above object, the surface plate capable of horizontal adjustment; A block inspection table provided with a plurality of limit switches, each having a sensing unit protruding outward from the curved surface by forming a curved surface and fixed to the center of the surface plate; A cylinder mounted to the surface plate and having a rod operating vertically upward in the center; A nozzle detecting unit fixed to an end of the rod by mounting a limit switch having a sensing unit protruding upward; An impact trench slidably mounted on a sliding guide fixed to a plurality of supports vertically fixed to the surface plate; A sliding body slidably mounted to the sliding guide and mounted on a lower portion of the pressing plate; A tightening nut and a pressurizer fastened by screwing to an upper end of the upper fixing stand detachably mounted on an upper portion of the lower fixing stand vertically fixed to the center of the surface plate; And a lamp display unit having a lamp electrically connected to the limit switch, wherein the upper heating ceiling burner is provided with an inspection and assembly device.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 2 is a perspective view showing an inspection and assembly apparatus of an overhead heating ceiling burner according to the present invention as a whole, and FIG. 3 is a cross-sectional view, wherein the apparatus 1 has a square plate 10 having a predetermined area.

The surface plate 10 should have a stiffness that is not deformed in the state where the upper surface is smooth and the other components described below and the ceiling burner 100 are mounted, and the corners of the four sides penetrate upwards and downwards ( 19 is formed so that the support 12 vertically penetrates vertically through the hole 19, and the support 12 is fixed to the lower surface of the surface plate 10 and the outer surface of the support 12. The support member 11 is firmly fixed in a state perpendicular to the surface plate 10.

The base 16 is mounted on the lower end of each of the supports 12 through the adjustment nut 14, and the lower end of the base 16 has a large area and forms a seating plate 17 having a flat lower surface. do. The adjusting nut 14 has female threads 14a and 14b in different directions at upper and lower portions of the inner surface thereof, respectively, and the male threads formed at the outer surface of the lower end of the support 12 on the upper female thread 14a. 12a) is coupled to the lower female thread (14b) is coupled to the male screw thread (16a) formed on the outer surface of the upper end of the base 16 by rotating the adjustment nut 14, the surface plate 10 can move in the vertical direction Make sure Further, at least one rod fitting hole 14c may be formed in the outer surface portion of the adjusting nut 14 so that the adjusting nut 14 may be rotated using the rod jig by allowing the rod jig to be fitted. Do.

In addition, it is possible to determine the horizontal state of the surface plate 10 by installing a level gauge 18 in the upper proper position of the surface plate 10, the horizontal state of the surface plate 10 is to the rotation of the adjustment nut 14 Is possible by Of course, the horizontal system 18 is preferably installed in one direction and the direction perpendicular to the one direction, it is preferable to be able to grasp the horizontal level of the surface plate 10 more accurate.

A block inspection table 20 is mounted at the center of the upper surface of the surface plate 10, and the surface plate 10 is provided by bolts 21 inserted from the bottom through the bolt holes 10a penetrating in the vertical direction of the surface plate 10. It is firmly fixed to the block inspection table 20 is formed of a curved surface 22 having an outer surface shaped to match the arcuate inner surface 122a of the refractory block 120 shown in FIG. The curved surface 22 provides a standard of specification for the shape and size of the arcuate inner surface 122a of the refractory block 120. In addition, the center of the block inspection table 20 penetrates up and down to form a hollow portion 21.

The block inspection table 20 has a plurality of limit switches 24, the limit switch 24 is located in a state where the sensing section 24a of the tip is slightly protruded to the surface of the curved surface 22 Bar 24 is mounted so as to generate a signal when it is pushed inward, each of the limit switch 24 is installed in the limit switch mounting groove 20a radially formed in the convex test table 20, these limit switches Each of the 24 has a wire receiving groove 20b formed in the block inspection table 20, a wire receiving groove 10b formed to penetrate in the up and down direction of the surface plate 10 so as to correspond to the wire mounting groove 20b. It is connected to one or more lamp display unit 30 mounted on one side of the upper surface of the surface plate 10 through the wire receiving tube 26 connected to the lower end of the wire receiving groove 10b. In addition, the limit switch mounting groove 20a and the limit switch 24 mounted thereon are preferably arranged in a plurality of rows in the circumferential direction of the block inspection table 20 at the same time. Do.

The lamp display unit 30 has a plurality of lamps 32, the lamps 32 protrude outward to determine whether the lamp 32 is lit from the outside, each of the lamps 32 The limit switch 24 is connected one-to-one. Then, a predetermined mark is displayed on the lamp display unit 30 so that an inspector can know which lamp 32 is connected to which limit switch 24, and the lamp display unit 30 is A power supply means (not shown) of batteries or the like necessary for lighting should be provided.

In the center of the surface plate 10 is formed a hole 10c penetrated up and down, the lower end of the hollow pipe-shaped lower fixing base 44 having a predetermined length is fitted and fixed to the hole 10c, 44, the lower end of the hollow pipe-shaped upper fixing stand 45 having a predetermined length is fixed by screwing, and if necessary, a separate connection considering the thickness of the lower fixing stand 44 and the upper fixing stand 45. It is preferred to use screws 46. In the case of using such a connection screw 46, the lower end of the connection screw 46 is fixed to the inside of the upper end of the lower fixing base 44, and a thread is formed on the outer surface of the upper end of the connecting screw 46, the upper fixing stand The upper end of the connection screw 46 can be coupled to the thread formed on the inner surface of the lower end of the 45, or, conversely, the upper end of the connection screw 45 is inserted into the lower end of the upper fixing rod 45 and the outer surface of the lower end of the connection screw 46 is fixed. A screw thread may be formed in the upper fixing part 44 so as to be coupled to the screw thread formed on the inner surface of the upper end of the lower stand 44.

In addition, the support bracket 41 is mounted to the center lower portion of the cylinder 40 by using a bolt or the like, and the cylinder 40 is vertically fixed to the support bracket 41 so that the rod 42 is positioned at the top thereof. Therefore, the rod 42 is located inside the lower end of the lower fixing stand 44. Of course, the cylinder 40 is supplied with hydraulic pressure or pneumatic through the hydraulic or pneumatic line not shown to enable the rod 42 to move forward and backward. Further, the forward position with respect to the rod 42 of the cylinder 40 is set to correspond to the lower surface position of the nozzle 144 shown in FIG.

The nozzle position detecting unit 48 is mounted on the top of the rod 42, and the nozzle position detecting unit 48 is formed in the shape of a hollow pipe having a predetermined length so that the lower end thereof is integrated with the upper end of the rod 42. Is fixed. In addition, a reference plate 49 having a smooth upper surface is integrally formed at an upper end of the nozzle position detecting unit 48, and the limit switch mounting groove 49a vertically penetrated in the center of the reference plate 49 is provided. The limit switch mounting groove 49a is provided with a limit switch 24 'connected to the lamp display unit 30 via a wire, and the sensing unit 24a' of the limit switch 24 'is mounted on the limit switch mounting groove 49a. The lamp 32 provided in the lamp display unit 30 is operated when the sensing unit 24a ′ is pressed downward by setting it to maintain the state protruding slightly upward from the upper surface of the reference plate 49. Of course, this lamp 32 is connected to the limit switch 24 '.

The upper end of each of the support 12, the lower end of the sliding guide 50 having a hollow pipe shape is mounted by screwing the sliding guide 50 and the support 12 is disposed on the same center line, Each of the sliding guide (50) is mounted so as to slide the extension 62 integrally fixed to the impact trench (60), the handle having a predetermined length on the outer surface of the extension (62) ( 63 is integrally fixed so that the impact trench 60 can be rotated and lifted up and down on the slide guide 50 by using the handle 63, and the impact trench 60 has a nut fitting provided therein. 60a is fixed to the extension 62 so as to face downward, and the nut fitting hole 60a is fitted to the head of the anchor bolt 104 shown in FIG. 1 and is rotated by the impact trench 60. It is provided to rotate the anchor bolt 104. Of course, the impact trench 60 is connected to a pneumatic line (not shown) and operated by the pneumatic pressure supplied therefrom.

In addition, the upper fixing plate 45 is equipped with a pressing plate 70 having a predetermined thickness and a lower surface thereof, and the pressing plate 70 forms a hole 70a penetrating up and down in the center thereof. The upper fixing stand 45 is fitted to the 70a to be able to move up and down along the upper fixing stand 45, and the contact plate 72 made of plastic is integrally fixed to the lower surface of the pressing plate 70. do. Of course, the hole 72a having the same size as the hole 70a of the pressing plate 70 is formed in the center of the contact plate 72.

In addition, a lower end of the sliding body 80 is integrally fixed to the upper surface of the pressing plate 70, and a hole having the same size as that of the hole 70a of the pressing plate 70 in the center of the sliding body 80. 80a). The connecting table 82 is radially fixed to all four outer surfaces of the sliding member 80, and the outer end of the connecting table 82 is fixed to the sleeve 84 fitted to be slidable to the sliding guide 50, respectively. do. In addition, a ring-shaped ring 83 is fixed to each center upper portion of the connecting table 82 to be connected to a hook (not shown) of the ceiling crane.

In addition, the tightening nut 85 is mounted on the upper end of the sliding member 80, and the tightening nut 85 forms a female thread 85a penetrating through the center to the outer surface of the upper end of the upper fixing stand 45. A predetermined length is fitted to the male screw thread 45a. In addition, a plurality of fitting grooves 85b are formed on the outer surface of the tightening nut 85 so that the tightening nut 85 may be rotated using a rod-shaped jig not shown.

The male screw thread 45a of the upper fixing stand 45 is fitted with a female screw thread 90a formed to penetrate up and down in the center of the presser 90, and the presser 90 includes a plurality of piston chambers (up and down). 92 is formed therein, and the piston 94 is mounted inside the piston chamber 92 so as to operate in an up and down direction, and a pressure rod 96 is provided at each lower end of the piston 94. It is integrally formed to protrude further downward than the lower end of the pressurizer 90. In addition, the piston 94 is provided with an elastic force upward by the elastic spring 98, the working fluid inlet hole 99 to which hydraulic or pneumatic pressure can be transferred to the upper portion of the piston 94 relative to the piston chamber 92 ) Is connected to a hydraulic or pneumatic line, not shown. Of course, the pressurizer 90 is located above the tightening nut 85.

By using the device of the present invention having such a configuration, it is possible to inspect the shape and size of the main part of the top heated ceiling burner and to assemble it in an optimal state, and the shape of the components of the ceiling burner prior to the description thereof. And what is the size is what is important, and what assembling state will be described in the optimum state.

The upper heated ceiling burner 100 as shown in FIG. 1 should have an equilibrium flame in the form of a flame discharged therethrough, and the equilibrium flame is an arc-shaped inner surface 122a formed on the inner surface of the lower end of the refractory block 120. ) Will be most affected by the size and shape. That is, the arc-shaped inner surface 122a of the refractory block 120 is formed such that its inner diameter is gradually increased toward the lower end, so that the flame spreads thinly to form an equilibrium flame shaped as a dish. It is very important to check whether the arcuate inner surface 122a has a size and shape that conforms to a specified specification since the size and shape depend on the arcuate inner surface 122a and directly affect combustion efficiency. Do.

And, the shape and length of the flame is changed by the position of the nozzle 144 provided at the lower end of the nozzle assembly 140, the flame is shortened if the position of the nozzle 144 is above the set position. At the same time, the end of the flame causes the phenomenon of splitting, lowering the combustion efficiency, as well as deteriorating the upper steel shell 112 and the lower steel shell 114 forming the outside of the body portion 110 to shorten its life. On the contrary, when the position of the nozzle 144 is lower than the set position, the flame becomes longer and at the same time, the equilibrium flame is prevented from being formed so that it directly hits the material of the heating furnace, causing excessive generation of scale or causing temperature deviation of the material. It is. That is, it is very important to keep the position of the nozzle 144 in the set position.

And, when the body portion 110 and the refractory block 120 is coupled to the sealing wool 151, 152 to be inserted therebetween, the wool 151, 152 is rock wool (Ceramic wool) It is made to seal at the interface between the body portion 110 and the refractory block 120 under the influence of the tightening input by the anchor bolt 104, but when tightening the anchor bolt 104 with a different size of force The spacing between the body 110 and the refractory block 120 may be different for each position, which may be changed to a position with respect to other components, and the flame may be emitted to a portion having a relatively large spacing. Deterioration of the upper steel shell 112 and the lower steel shell 114 forming the outside of the body portion 110 is to shorten its life. That is, it is very important to tighten the anchor bolt 104 with the same force so that the spacing between the body portion 110 and the refractory block 120 is the same in all positions.

Hereinafter, a description will be given of how to inspect the shape and size of the main part for the top heated ceiling burner and assembling in an optimal state using the device of the present invention.

In order to maintain the precision of the apparatus 1 of the present invention, it is necessary to check in advance whether the table 10 is in a horizontal state and whether the limit switches 24, 24 'and the lamp 32 operate properly.

It is possible to observe whether the surface plate 10 is in a horizontal state by observing the level gauge 18, and if the surface plate 10 determines that the surface plate 10 is not in the horizontal state, the adjustment nut 14 is rotated. The surface 10 can be leveled by adjusting the inclination of the surface 10.

Whether the limit switches 24, 24 'and the lamp 32 operate properly can be determined by the user pressing each of the limit switches 24 and 24' to check whether the corresponding lamp 32 is lit. .

Of course, it is preferable to check whether the impact trench 60 and the pressurizer 90 are also operating normally.

After checking the operation state of the components of the apparatus as described above, the sliding body 80 and the pressurizer 90 are removed from the apparatus by using the ceiling crane or the like, and the refractory block 120 of the block inspection table 20 Put on the top. This is to check whether the arcuate inner surface 122a of the refractory block 120 satisfies a predetermined standard, and when the arcuate inner surface 122a does not satisfy a predetermined standard, as shown in FIG. 5A. One or more of the limit switches 24 or two or more detection parts 24a are not pushed into the inside because they are not in close contact with the curved surface 22 of the block inspection table 20, which is connected to the limit switch 24. 32) does not light up, so that the inspector can determine that the arc-shaped inner surface 122a of the refractory block 120 does not satisfy a predetermined standard. On the other hand, when the arc-shaped inner surface 122a satisfies a predetermined standard, as shown in FIG. 5B, the entire surface is in close contact with the curved surface 22 of the block inspection table 20, and thus the limit switch 24 is applied to all of the limit switches 24. The detection unit 24a is pushed into the interior, which causes all lamps 32 connected to the limit switch 24 to be turned on, thereby allowing the inspector to designate the arc-shaped inner surface 122a of the refractory block 120 to a predetermined size. You will find that you are satisfied.

Next, the sealing wool 151, 152 is placed on the upper surface of the refractory block 120, and then the body 110 is mounted, and the sliding body 80 and the pressurizer 90 are mounted in the state shown in FIG. Attach to (1). In this state, when the working fluid of a predetermined pressure is supplied through the working fluid inlet hole 99, the piston 94 descends by the pressure, and the pressure rod 96 connected thereto presses the sliding body 80 downward, and continues. The force is transmitted to the sealing wool 151, 152 through the pressing plate 70 and the body portion 110. That is, the sealing wool 151, 152 is uniformly compressed by a predetermined set pressure. At this time, the tightening nut 85 is rotated using a jig to closely adhere to the upper surface of the sliding body 70. After that, the impact trench 60 is inserted into the head of the anchor bolt 104 by using the handle 63, and then the impact trench 60 is operated so that the body portion 110 and the refractory block by the anchor bolt 104. Assemble 120. This allows all of the anchor bolts 104 to be tightened by the same force.

Next, the sliding body 80 and the pressurizer 90 are again removed from the apparatus 1, and the upper fixing stand 45 is removed, and then the burner cap 130 and the burner cap at the upper end of the body portion 110 are removed. Put the nozzle assembly 140 in the assembled state (130). In this state, the cylinder 40 is operated to raise the rod 42 by a predetermined length, which raises the nozzle sensing table 48 integrally connected to the rod 42.

At this time, when the nozzle 144 of the nozzle assembly 140 is located lower than the preset position, as shown in FIG. 7A, the nozzle 144 is moved by the upper reference plate 49 of the nozzle detector 48. Lifted upwards, which pushes the nozzle assembly 140 and burner cap 130 upward, and is also defined between the flange 134 of the burner cap 130 and the top plate 116 of the body portion 110. A gap is generated, whereby the inspector can determine that the nozzle 144 is located below the set position, and thus the inspector 140 is screwed to the cap body 132 of the burner cap 130. The position of the nozzle 144 may be moved upward by rotating the nozzle tube 142 of the bottom side.

And, when the nozzle 144 of the nozzle assembly 140 is located above the predetermined position, as shown in Figure 7b, the limit switch 24 mounted on the upper reference plate 49 of the nozzle detector 48 '(24') does not come into contact with the nozzle 144, so that the lamp 32 connected to the limit switch 24 'does not light up and the inspector is positioned above the set position. In this case, the inspector may rotate the nozzle tube 142 of the nozzle assembly 140 screwed to the cap body 132 of the burner cap 130 to move the position of the nozzle 144 downward. Can be.

Of course, when the nozzle 144 is accurately positioned at a preset position, the lamp 32 connected to the limit switch 24 'is turned on without the burner cap 130 being lifted upward. By placing the 144 in a predetermined position and then fixing the burner cap 130 to the upper plate 116 of the body 110 using a bolt, the inspection and assembly of the upper heated ceiling burner 100 may be completed. will be.

As described above, according to the inspection and assembly apparatus of the upper heating ceiling burner according to the present invention, it is possible to perform the standard inspection of the arc-shaped inner surface of the refractory block very simply and accurately by using the block inspection table.

In addition, by using a pressurizer and an impact trench, the entire surface of the sealing wool sandwiched between the refractory block and the body may be assembled in a uniformly compressed state, thereby improving airtightness and improving assembly precision.

In addition, by using the cylinder has the effect that can be inspected and adjusted so that the nozzle which is the flame discharge portion of the nozzle assembly is in a predetermined position.

Claims (1)

Platen 10 that can be adjusted horizontally; Block inspection table 20 in which a plurality of limit switches 24 are formed having a curved surface 22 and having a sensing unit 24a protruding outward from the curved surface 22 and fixed to the center of the surface plate 10. ; A cylinder 40 mounted to the surface plate 10 and having a rod 42 operating vertically upward in the center; A nozzle detector (48) fixed to an end of the rod (42) by mounting a limit switch (24a) having a sensing portion (24a ') protruding upward; An impact trench (60) slidably mounted on a sliding guide (50) fixed to a plurality of supports (12) fixed perpendicularly to the surface plate (10); A sliding body (80) mounted to the sliding guide (50) so as to be slidable and mounted with a pressing plate (70) at a lower portion thereof; A tightening nut 85 and a pressurizer 90 fastened by screwing to an upper end of the upper fixing stand 45 detachably mounted on an upper portion of the lower fixing stand 44 vertically fixed to the center of the surface plate 10; And, And a lamp display unit (30) having a lamp (32) electrically connected to the limit switch (24) (24 ').