Span gap sled runner and method
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a runner for mounting on a roll gap sled for measuring the misalignment of rolls in a machine for continuously casting a metal slab. 
     2. Prior Art 
     It is known to provide a sled-type apparatus for passing through the opposed rolls of a continuous caster in order to measure the gap between each pair of opposed rolls. Such a sled is fitted with transducers which measure the magnitude of the gap between opposed pairs of caster segment rolls. The length of the sled is less than the spacing of three rolls in the casting direction. This measurement value is assigned to the pair of rolls for which the measurement is made and is called the &#34;roll gap&#34; or simply the &#34;gap&#34; between the rolls so measured. 
     Roll gap has a direct effect on product quality and an incorrect roll gap contributes to slab casting defects such as core cracking. By measuring roll gap, it is possible to detect failed bearings and other problems in a caster segment by thus detecting a roll gap value which is larger than that called for in the caster design. 
     Traditional roll gap sleds cannot detect and measure roll segment alignment, i.e. a segment-by-segment transition of the slab passline. It also is necessary that the segment-to-segment transition, or alignment, be held within specified tolerances to allow for the production of a high quality, defect-free slab. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of the present invention to provide a novel gap sled runner which is capable of detecting and measuring misalignments in the rolls of a continuous caster. 
     This objective is fulfilled by the provision of a &#34;span gap&#34; runner, which, by spanning a segment of three rolls, can measure possible misalignment of the middle or spanned roll. For such purpose, the span gap runner has a middle portion recessed, and a position transducer mounted in the center of the recessed portion, whereby a span gap measurement is made between the surface of the spanned roll and a line or plane tangent to the surface of the two adjacent rolls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a span gap runner in accordance with the invention. 
     FIG. 2 is a side elevation of the span gap runner in measurement position adjacent a three roll segment of minimum roll spacing. 
     FIG. 3 is a similar side elevation showing the span gap runner in measurement position adjacent a three roll segment of maximum roll spacing. 
     FIG. 4 is an isometric view of a gap sled with two span gap runners in place on the sled. 
     FIG. 5A is a side elevation of a flat bottomed span gap runner in measurement position in a curved segment of a caster. 
     FIG. 5B is a side elevation of a flat bottomed span gap runner in measurement position in a straight segment of a caster and where there is no misalignment of the measured rolls. 
     FIG. 5C is a side elevation of a flat bottomed span gap runner in measurement position in a straight segment of a caster and where there is a negative span gap (spanned roll higher than the two adjacent rolls). 
     FIG. 5D is a side elevation of the span gap runner of the invention in measurement position in a straight segment of a caster and where there is a negative span gap. 
     FIG. 6 is a bar graph relating roll number, by segment, in a continuous caster, and showing segment misalignments. 
     FIG. 7 is a similar bar graph showing substantially complete freedom from segment misalignment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1 there is shown a span gap runner 1 having a position transducer 2 mounted in an aperture in the center of the runner. Runner 1 has a middle portion 4, of length 5 (representing a minimum roll spacing of rolls 8--FIG. 2), recessed from a bottom surface of the runner, e.g. about 0.100 inch. Recession of the middle portion of runner 1 leaves end portions 3 extending outwardly of the middle portion and spanning a distance 6 representing a maximum roll spacing of rolls 9, as shown in FIG. 3. The runner 1 terminates in extremites extending outwardly of end portions 3 for a distance 7 and having a rounded lower corner. 
     As shown in FIG. 4, a pair of span gap runners 11, with transducers 15, may be mounted on a conventional roll gap sled 13, having roll gap transducers 14, adjacent a set of three rolls 12. The two longer span gap runners are designed to completely span three rolls instead of only two as in the case of the conventional roll gap sled. The position transducers which are mounted in the middle of the span gap runners are positioned to measure the distance from the surface of the spanned, middle roll and a line or plane tangent to the surface of the two adjacent, upstream and downstream, rolls. In a single pass of the sled through the rolls, measurements of both roll gap, using the conventional roll gap transducers, and span gap, using the span gap runners, can be made. 
     As shown in FIGS. 2 and 3, a positive span gap, represented by the numeral 10, exists when the spanned roll is lower than the two adjacent rolls. The span gap runner is designed to work with roll spacings varying from the minimum roll spacing (FIG. 2) to the maximum roll spacing (FIG. 3) found in the casting machine. The portion 4 of the runner between the roll contact zones is recessed upwardly away from the roll surfaces so that the runner may function in the straight part of the caster. Thus, even a hypothetical flat bottomed runner would function as a span gap runner in the curved part of the caster, as shown in FIG. 5A since the curve of the roll path would result in displacement of the middle roll away from the bottom of the runner. Also a hypothetical flat bottomed runner would function as a span gap runner in the straight part of the caster, but only if the alignment of the spanned roll is zero or positive, that is, lower than the adjacent rolls, as shown in FIG. 5B. However, a hypothetical flat bottomed runner would not function as a span gap runner in the straight part of the caster if the span gap is negative, that is, higher than the two adjacent rolls. As shown in FIG. 5C, such a condition would result in a span gap measurement of zero; however, such a value is not correct, because a misalignment condition exists that has merely bottomed out the sensor and lifted one end of the runner off the adjacent roll. Such a condition is successfully addressed by the span gap runner of the invention, as shown in FIG. 5D. 
     The span gap runners can be designed to work on either the inner (higher) or the outer (lower) bow of rolls. The outer bow of rolls is preferable for making span gap measurements because they are more directly connected to the segment mount on the caster frame than the upper rolls. 
     In operation, a span gap runner makes a measurement of the span gap for each caster roll as the gap sled passes through the cast line. If the roll positions in a caster are numbered, e.g. 1, 2, 3, 4, 5, 6 etc., a span gap measurement across rolls 1-3 is assigned to roll 2; a span gap measurement across rolls 2-4 is assigned to roll 3, etc. Each span gap runner generates one distinct span gap value for each roll so measured. 
     The number of distinct span gap values generated for each caster roll is equal to the number of span gap runners employed in the gap sled. For the gap sled shown in FIG. 4, two distinct span gap values will be generated for each caster roll position. The runners function independently of one another, both mechanically and electronically. Multiple span gap values generated from the same roll position may or may not be combined to obtain a single result. 
     When the data are assembled after a run, the individual span gap values are reported by assigning to them their respective roll position numbers from which the measurement was made. The span gap runner has no ability to report its passage from one segment to another. It is known by operating personnel which roll numbers are located in which segment. It is then known from the roll numbering which span gap values are indicators of a segment to segment alignment, and which are indicators of a roll misalignment within the segment. This information is used, e.g. by a computer, to provide a recommendation for shimming the segment/machine frame connection to correct the misalignment. 
     The effectiveness of application of the invention can be seen by a comparison of the data of FIGS. 6 and 7. In FIG. 6, before adoption of the invention, there are relatively large deviations in span gap transition from one segment to another. In contrast, in FIG. 7, with use of the invention, transition from one segment to another is essentially smooth.