Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation |
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Rotary dryers and kilns require a lot of care. One aspect of care that is usually overlooked because it doesn't require attention on a regular basis is something called "Ovality." If you don't know what that is, then you will find out what it is in about 40 million revolutions.
The rotary dryer shown to the left is 8 1/2 ft. in diameter. After about 40 million revolutions it started cracking underneath the tire. The retaining rings were cracking and the stiffening ring was cracking. The repair of the stiffening ring is showcased in the article "Machine Design - Case Study No. 111: Welding Procedure Specification Key Parameters." The repair is visible in this picture.As the tire turns, so does the dryer. The dryer will actually turn within the tire itself because it is not locked to the tire. Over time, this motion will wear down the filler blocks or the inner diameter of the tire. As the clearance opens up, the shell will begin to flex. The more it can flex, the higher the hoop stresses. At some point the hoop stresses will be great enough to break anything that resists. The retaining ring is not supposed to handle any hoop stresses. It is there to simply keep the tire from moving about. However, when the shell begins to flex, then the retaining ring will try to resist the flexing, and the ring will eventually fail (picture lower left). In the picture to the lower left, the retaining ring cracked and the crack followed the path of least resistance (yellow dotted line). You can get a closer view of the crack between the retaining ring and the filler block by clicking on that region. An inspection of the clearance between the tire and the filler block just above this crack was 3/8". The total clearance was much greater at the top of the shell. This clearance is known in the industry as "ovality" and it is the measurement of the eccentricity of the shell from its normal round shape. As the clearance opens up, the shell becomes oval in shape. Most manufacturers want to keep the ovality to within a certain limit, after which time one must install new filler blocks, or shim the existing filler blocks. |
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