What To Do When A Kiln Tire Stops Migrating One of our kiln tires has s topped topped mig rating rating on the s hell. hell. What What should we do?
An answer to this frequently asked question invites further questions and discussion on wh y tires should migrate (creep) in the first place. There are two different designs of roller-supported rotating equipment that must be considered. First there are Type I units, those that are refractory lined, typified by kilns, calciners and reactors, etc. Secondly there are Type II units, those that are not refractory lined typified by dryers, coolers and granulators etc. Although migrating tires are very typical of Type I vessels, they are less common for Type II vessels, as we shall see. Since the issue is usually much more critical with type I units, we'll discuss them first.
Type I units (Kilns, calciners, reactors etc.) To fit the rotating shell with a migrating tire is to accommodate differential thermal expansion between the shell and the tire. The heat of the process will heat up the shell more than the tire. Consequently the shell will expand its diameter more than the tire. For this reason some clearance, when cold, is desirable between them so that the tire does not pinch the shell when the unit reaches operating temperature. Otherwise a pinching tire will frequently and often permanently distort the shell and cause refractories to be crushed. Costly damage, if it needs to be repaired. Since the temperature difference between the shell and tire vary due to natural variations of process conditions, purposely having a slightly larger gap is desirable. This allows some temperature excursions without risking damage. As a result this difference in diameters makes one revolution of the shell occur slightly ahead of the time it takes for the tire to make one revolution. This difference is called tire migration or creep. Its magnitude is usually relative to the difference in diameters. When the tire stops turning, the safety margin is gone. The alarm now sounded suggests the need for understanding the reason for this condition. Quantifying the magnitude of the problem will give us the options of what to do.
Two possible reasons. The tire may have stopped rotating for two reasons. First and most usually the shell has expanded faster than the tire, the tire-to-shell gap has disappeared and the tire locks onto the shell. Often this is caused by too rapid a kiln start-up or a rapid temperature rise of the shell associated with partial coating or refractory loss. A second reason is that the tire i s simply jammed on the shell. I t may not be free to rotat e due to the introduction of a foreign object or a broken piece of one of its support elements into the gap. A wedged piece of steel like this will slowly rip its way along between the bore of the tire and the shell, stopping normal tire migration for the time being. This then is not temperature related but entirely mechanical in nature. Careful visual inspection may give evidence of which situation is at hand. But nothing is better than knowing what the usual range of tire-to-shell temperature difference is. Most kilns now have shell temperature monitors, like the "T-Scanner", from which a history of temperature differences can provide immediate assessment of the present condition. Foresight suggests that the preparation of a table of temperature differences vs. creep values will give a warning of incipient problems. The mechanical condition is of less immediate concern. Ample and appropriate lubrication is the immediate "fix". When a shut down permits, the mechanical deficiencies can be addressed.
Loss of clearance, on the other hand, may need immediate intervention. If due to a rapid warm-up schedule, slow it down. If due to partial loss of coating, changing the location burning zone by moving it back or forward, may be possible. Loss of coating, along with partial refractory loss, may be creating a "hot spot" underneath the tire which is reason enough for an immediate kiln shut-down.
A possible third and most difficult problem. Sometimes insufficient clearance for normal operation exists. This often happens on an older kiln with a section replacement or filler bar (chair pads, or shell pads) repair, when the work has resulted in a fit that's too tight. Such a repair is expensive to reverse. It may be possible to cope with this for awhile, maybe even long enough for natural wear to create a passable situation. These thermally-caused tire lock-ups may be addressed by the following:
pointing shell cooling fans to the area immediately to each side of the shell adjacent to the tire placing "rosebud" torches to heat up the tire building an enclosure, similar to a gear guard, to contain heat and reduce the normal tire-to-shell temperature difference any other methods that reduce the shell-to-tire temperature difference
A snug tire, as long as it does not pinch the shell, has its benefits. A m igrating tire unavoidably causes a lot of wear. A tire which doesn’ t migrate would eliminate this wear, extending the service life of the parts considerably. Doing this by simple thermal expansion is risky, as explained, but if temperatures are monitored and controlled, it is no reason for immediate panic. New tire mounting designs using splined tires are available. They may be costly, but they certainly eliminate the problems surrounding this entire issue. They should be considered when shell section replacements involving tires are required.
Type II units. (Coolers, dryers and granulators etc.) Unless the process vessel is a fired unit, a migrating tire design should be questioned. A fixed tire has no wear between it and the shell; a god send for any maintenance department. Unless differential thermal expansion is a real problem (without refractory it becomes a much reduced concern) there is little reason for using a migrating tire. It is not unusual however, to find designs like this. Without the need to consider refractory, there seems little to worry about if a migrating tire stops moving. Excessive temperature, so much so that it permanently distorts the shell, is difficult to conceive. If such process temperatures were attainable, wouldn't refractory be part of the design? Tire-locking at a lower temperature may still slightly distort the shell. But since the shape of the shell isn't critical in terms of maintaining a refractory arch for example, it becomes a moot point.
Roller and Tire Lubrication
Why s hould the contact s urfaces of the rollers and tires not be lubricated with oil or g reas e? There is little chance that the meeting of the mating surfaces of the tire and the rollers through the pinch points is one of pure rolling action. There are several reasons for this. The most obvious is that the rolling axis of the roller is not parallel to that of the tire. It would be exceptional for this to prevail for any significant period of time given the nature of the equipment both in its design, installation and operation.
Additionally there is always axial movement of the tire on the rollers caused by either or both thermal expansion and mechanical clearance between tire and shell and tire and thrust rollers. Then there is also tire wobble which is always present to some degree. Tire wobble at the pinch points is axial movement that cycles back and forth with each rotation. All of the above detracts from the surfaces meeting in pure rolling by introducing some amount of face dragging or skidding. Lubrication therefore is mandated to prevent surface galling as would be required for any similar materials sliding face to face. Reducing these extends the life of the components and can have a significant effect on the drive power requirements. In the past oil and grease have been used as lubricants but for a number of years, rotary vessel designers and suppliers and indeed end users have turned to graphite blocks as an alternative. The old grease/oil based lubricant can permeate the grain structure of the metal. As the rolling surfaces meet, tremendous hydraulic pressures are generated within the actual grain structure of the tire and roller. Liquids such as water and oil are incompressible and in compression, when going through the pinch point, are actually much stronger than steel. Therefore as the liquid lubricant tries to escape it fractures the metal grain causing cracking and spalling of the running surfaces. Added to this is the affinity of liquid lubricants to attract dirt and debris this can result in impregnation damage to the rolling surfaces as well as the formation of a "grinding paste" which can quickly erode the contact surfaces and even support shafts if the "paste" is allowed to build up between the roller and bearing. Continuous application of liquid lubricants is also a problem both drip and pressure systems are prone to malfunction and a uniform film of lubricant is difficult to achieve which in turn makes control (float) of a vessel even more difficult. We would be the first to agree that proper lubrication is essential for maintenance of any mechanical device, tires and support rollers are no exception. Rotating vessel designers now almost universally agree that rolling surfaces should be lubricated with dry lubricants, such as graphite blocks. Properly selected graphite blocks not only provide excellent lubrication but also act as scrapers to prevent most dirt from rolling through the pinch point. They should be one-piece and at least as wide as the support roller. Use of one piece blocks will prevent product or dirt accumulation where multiple blocks join which can cause circumferential bands of pits and grooves on the rolling surfaces. Graphite blocks do wear but are easily monitored and replaced, the graphite debris is fine and dry and is easily cleared away, this is certainly not the case with grease or oil lubricants. We cannot readily recall any vessel suppliers who now advocate the use of oil, grease or even water as a lubricant on support roller faces. We trust the foregoing goes some way as to explain why we chose to supply graphite lubrication for your vessel, but if you have any queries or comments please do not hesitate to contact us.
Using dial indicators for axial gear measurements
When "dialing-in" the gear, axially, and the kiln has been adjusted to run continuously against the thrust roller, can it be done using only one dial indicator? No. Axial run-out measurement on a kiln gear can never be done satisfactorily using only one dial indicator. The use of two dial indicators for measuring axial run-out is a means to eliminate the effects of axial "endfloat" that is inherent in all rotating, roller supported equipment. The kiln shell, due to design features that accommodate thermal expansion and other factors such as thrust control and simply economics of construction has the ability to move in the axial direction. This movement, measured at the gear, should normally not exceed the difference between gear face and pinion face dimensions, nominally about ± ½" [12mm]. The thrust tire, the one which is constrained from axial movement by thrust rollers, is normally the closest tire to the gear. This is the fixed point on the shell from which thermal expansion causes the shell to grow in either direction away from it. The gear, therefore, being as close as possible to this point, has its position closely fixed to line up with the pinion, almost independently of thermal expansion conditions of the shell. If then, with the thrust tire running continuously on its thrust roller, is the axial "end-float" of the shell not eliminated permitting the use of only one dial indicator? This presumes that the thrust tire is running true to the shell axis of rotation and that the shell axis is perfectly straight. In other words the tire has no wobble. This is virtually never the case. All tires, thrust tires included, wobble. The only thing we can discuss is how much it wobbles. Axial run-out requirements are fairly tight, in the neighborhood of 0.030 0.060 inches total [0.8 - 1.5 mm]. Each gear will have its specific allowable maximum. We can see therefore that the slightest amount of shell movement or the slightest surface imperfections on the thrust faces can introduce sufficient axial "end-play" to produce erroneous axial run-out readings on the gear. Any wobble of the thrust tir e would be seen at the gear as axial r un-out of the gear when only one dial indicator is used. There is no way to tell the difference. Ironically, the gear will then unwittingly be set with an axial run-out to exactly (or as close as the practitioner can manage) oppose the run-out of the tire. The single indicator will then show acceptable axial run-out. At best all that has been accomplished is to make the gear run-out mirror the tire run-out. Its magnitude will be the same as the tire while its direction will be
opposite. Irregularities in the surfaces of the thrust faces may introduce sufficient "noise" into the readings making an otherwise straight forward task very confusing. The real problem is, when only one dial indicator has been used, that the axial run-out is still unknown. It simply has still not been measured. There is so little extra effort involved in making these measurements correctly, by using two indicators, that nothing else should be acceptable. The use of only one indicator either shows a lack of understanding of kiln mechanics or a blatant disregard for them. In this illustration the shell has been partially removed from view to show the dial indicator set-up positions required for axial run-out measurement.
Which is better - fixed or floating tires?
Which i s better - fixed or floating tires ?
If the process is such that the temperature difference (delta T) between the shell and the tire at normal operating conditions is less than 40degC(~100degF)then a fixed tire is best. A fixed tire by shrink fit (3mm on the diameter) gives superior service compared to any other method or combination of methods of fixing such as welding, keying, pinning, wedging, etc. When the delta T is higher then a floating or migrating tire is suggested particularly when the process requires a refractory lining inside the tire supported shell. As a rule of thumb when shells are refractory lined migrating tires are used. When no refractory is necessary fixed tires should be used. Migrating or floating tires come with a severe penalty in terms of the comparitively high maintenance that ensues. So unless the process dictates that protection of the refractory is mandated by leaving room for shell expansion within the always less expanding tire, a fixed tire arrangement should be the preference. The ultimate solution for high delta T applications is a splined tire. This configuration leaves room for expansion but also locks the tire from rotating on the shell,
hence mechanical maintenance and wear are greatly reduced. The illustration is from a Polysius patent of 1969.
How to find the bottom dead center of a kiln tyre? How to find the bottom dead center of a kiln tyre?
Kiln Doctor answer: Is the kiln running or shut down? If shut down simply find the center of the tire axially with a tape measure and scribe a line for about 12' circumference. Then use a level up against the bottom of the tire. When level, mark the tangent point on the scribed line. This is BDC. Use a precision level for better accuracy. For still more accuracy use an optical autolevel. Fix the zero end of a machinist's scale or optical tooling scale to a magnet and place the magnet end on the tire. The scale needs to be pointing out radially from the tire. With someone looking at the scale through the autolevel run the magnet/scale along the scribed circumferential line through bottom dead center. The observer through the autolevel will identify BDC when the scale reading reaches a maximum. If the kiln is running: Place the autolevel near the centerline of the kiln and look downhill to the bottom edge of the tire. The autolevel will have to be fitted on a vertical rack to allow vertical adjustment. Run the autolevel up/down so that the horizontal line of the cross hair is tangent to the edge of the tire. By carefully sighting the crosshairs at the bottom tangent point the center of the tire can be optically identified. Swing the scope and center punch a column. That will give you at least the elevation of BDC, downhill edge. Spray the bottom of the stationary tire with light oil. The oild will slowly drip off at the center of the tire.
Shell and Tire Stress
K indly explain the following Hertz P ress ure and its s ig nificance. Tangential bending s tres s in live ring s Tang ential bending s tres s in ki ln shell under live ring s . Long itudinal bending s tres s in the clear s pan of the ki ln s hell Hertz pressure is the contact pressure between the roller and the tire. It is a function of their diameters but mostly the face width of the tire. As the "line of contact" increases i.e. we make the tire and roller wider so the Hertz pressure decreases. High Hertz pressure can lead to surface spalling. High Hertz pressure can be caused by overloading the drum or limited face contact due to non cylindrical rolling surfaces or bad alignment. Suggested Allowable Hertz surface pressure < 58,000 - 62,000 psi = 400 428 MPa depending on application ( speed, etc. ) Tire bending stress (tangential bending stress) is calculated by taking the tire as a uniformly loaded and simply supported curved beam. The bending stress is a function of the tire face width, thickness, radius and the reactions at each roller. Suggested
limits: Allowable Bending stress < 10,000 psi = 69 MPa ( forging ) Allowable Bending str ess < 8,000 psi = 55 MPa ( cast steel ). The tangential bending stress of the shell under the tire is not calculated. It would be a very low number since it is almost continuously supported by the tire for most of the circumference around the lower 270 degrees. In that area the bending stress would only be induced by changes of curvature of the tire which are insignificant with respect to tangential shell stress. In the upper quadrant, where the shell is not in contact with the tire (applies to non fixed tire designs only) the critical shell parameter is deflection rather than stress. This is commonly referred to as shell ovality or shell flexing. As the tire itself continuously changes shape during rotation, its ovality must be calculated and limited. At the design stage the proportions of the tire may then be altered to satisfy maximum allowable ovality as calculated by Nies. Since the shell is not in contact with the tire in the upper quadrant its "ovality" will be approximately double that of the tire. Shell ovality is also not calculated but simply measured. Shell ovality increases as the tire bore and the shell chairs wear out over the years. High ovality is one common cause of premature refractory failures. For kilns up to 5 meters in diameter the shell ovality should not exceed 10% of the shell diameter in meters. E.g. a 5 meter kiln shell would have a maximum allowable ovality around 0.5%. Some would also say that 0.5% is the maximum allowable ovality for kilns larger than 5 meters. (0.5% of 5000mm = 25mm, which is the change in shell diameter during one complete rotation.) Suggested limits: Shell ovality (%) less than (kiln I.D. / 10) (m) with 2 mm gap Example: Kiln I.D. is 3.6 m => shell ovality < 0.36 % Tire ovality (Nies) is then about 0.12 †¦ 0.20 % and should not exceed 0.35 % in any case If no refractories inside, shell ovality may be higher, but after 0.7 †¦ 1.0 % we may see fatigue cracking in shell because of high deformations. Longitudinal shell bending stress usually peaks at the center of the tires and not at mid span as one might think. Between spans there are points where the longitudinal bending stress is typically zero (see attached shell stress diagram, which shows 4 such points between spans). The shell bending stress is calculated by using the shell as a simply supported beam with the cross section of a thin walled cylinder. When the shell has more than two supports it is statically indeterminate and the support reactions are iteratively calculated. Because the maximum stress is usually at the supports the shell plate thickness then also increases as we approach each tire and is maximum thickness directly under the tire. There are cases (somewhat unusual because they present other support related problems)where a long shell with a small diameter may have only two supports, each located at the extreme ends of the shell. In such cases the maximum longitudinal bending stress is at mid span and so we find the thickest shell plate extent in the center of the shell. It is also clear from the diagram that the circumferential welds joining the different plate thickness on each side of the tire are usually the highest stressed points on the shell. For that reason any circumferential cracks developing in or near these seams are potentially catastrophic. See Photo. Suggested allowable limits are: Heavy plate bending stress < 1,450 psi = 10 MPa Heavy plate shear stress < 400 psi = 2.76 MPa Flanking plate bending stress < 2,175 psi = 15 MPa Balance shell ( next to flanking plate ) bending stress < 2,900 psi = 20 MPa Balance shell ( at mid span ) bending stress < 2,175 psi = 15 MPa Shell max. deflection 0.25" = 6 mm Shell max. slope ( from deflection ) 0.004 "/ft = 0.0191 deg. = 0.0333 %
Increasing the kiln speed? We wis h to modify the kiln s peed from 4.3 to 6.1 rpm. Wh at mechanical problems may be appear in (g irth g ear, tyers, bearing s and thrus t) etc.? Apart from the obvious, needing a lar ger drive motor and gearbox, is t he gear large enough (suff icient face width) to deliver an added 50% power? Please inspect the support roller oiling system. If it is a standard Ferris wheel with buckets, the bucket angle will have to change in the very least. Calculations may have to be made on the pour rate to make sure that such an oiling system can even work at such high speeds. Applying a forced lubrication system may be necessary. Drive vibrations often develop with speed. This will be related to torsional harmonics in the drive train. A torsional vibration analysis will have to be done on the complete drive train, motor shaft, high speed coupling, reducer shafts, slow speed coupling and pinion shaft, to see if such a potential problem exists. The OEM made such an analysis when the kiln was first designed. They should now review this in consideration of a 50% speed increase.
Other factors namely alignment and ovality will become critical. Basically long term mechanical service life will be reduced and fatigue failures can be expected much sooner. Fatigue life is inversely proportional to speed. Increasing speed by 50% reduces fatigue life by 50% - unless the kiln was originally built to run at the higher speed.
Axial and Radial Run-Out
Is the maxi mum allowable axi al & radial run-out for g irth g ears the thesame ?
The graphs are in imperial measure and can be converted to metric. The pitch diameter is given in feet. As a rule of thumb the allowable axial runout is 0.001"/ft of pitch diameter. Radial run out has a 50% higher allowable tolerance of 0.0015"/ft of pitch diameter. The highest curve on each graph reflects the added tolerance allowed for kilns due to thermal bending of the shell usually exhibited in those kilns that have variable shell temperature profiles. Transient shell bending near the gear will cause transient gear runout. When this causes occasional vibration for part of the shell rotation the usual remedy is to separate the mesh somewhat by adjusting the pinion side rollers in closer to the kiln. This has its limits of course.
Basic Nomenclature
To weld or not to weld the face of kiln rollers? Is it poss ible to weld repair the face of a kiln roller?
The repair involves partial or complete coverage of the rolling face to build up material to replace worn off material by welding. This type of repair is frequently attempted but does not give satisfactory results in most instances. There are many factors involved which determine the success or failure of the repair. In order to understand where it may succeed and where it may fail an understanding of the mechanics of the rolling surfaces is necessary. Hertz pressure is the contact pressure between the rolling surfaces of the tire and the roller. For any given load that a roller must support, the maximum allowable Hertz pressure must not be exceeded. The diameter of the tire is mostly dictated by the diameter of the vessel it must circumvent and the thickness it must have to provide sufficient beam strength and stiffness to support the weight of the shell and its contents. The tire must therefore have an adequate section modulus. But both the thickness and the width of the tire determine the section modulus. The diameter of the roller is a determining factor for Hertz pressure. The larger the diameter of the roller the more the Hertz pressure is reduced. But the length of the line of contact between the tire and the roller (tire face width) has a more direct effect on Hertz pressure than does roller diameter. Therefore the wider the tire the lower the Hertz
pressure. The line of contact between the tire and the roller, the pinch point, when viewed closely enough, is actually an area rather than a line. This is necessarily so because even steel has elasticity. Like the tire on a car, where it flattens slightly as it meets the surface of the road, so do both tire and roller surfaces flatten when they pass through the pinch point. See fig. 1. The line of contact between the tire and the roller, the pinch point, when viewed closely enough, is actually an area rather than a line. This is necessarily so because even steel has elasticity. Like the tire on a car, where it flattens slightly as it meets the surface of the road, so do both tire and roller surfaces flatten when they pass through the pinch point. See fig. 1. The force of contact begins as the curved surfaces meet. The force of contact rises to a maximum at the center of the width of contact. This would be the point on the surface intersecting the line joining the center of the tir e to the center of the roll er. See fig. 2. Then there is the phenomenon defined as Poisson's Ratio, which states that if material is compressed in one direction it grows in the other. See fig. 3. When the surface of the roller is welded, the new layer of metal will have different mechanical properties than that of the base material. No matter how carefully the metallurgy of the welding rod was matched to the base metal, no matter how thoroughly the work was preheated, post heated etc. there will be a difference. Often none of these precautions are taken which makes matters worse. See fig. 4. As the surface passes through the pinch point the metal compresses as shown. The depth of compression varies with geometry and total load. But the upper layer compresses differently from the underlying material because of the differences in their mechanical properties. Poisson's ratio is the measurement of this difference. Shear stresses in the transition zone can therefore easily act to separate the materials. Microscopic cracking develops and soon the weld material spalls, peels or otherwise separates itself from the base metal no matter how well the welding was done. If the roller is large enough to make repairs worth while, machining away the remaining portion of the rim is the first step to lasting repair. A new sleeve, usually a newly forged rim is then shrunk fit onto the old machined roller. This assembly is then finish machined. This is a superior type of repair. The final assembly will provide good serviceability.
Wave patterns and thrust roller failure.
We have recently failed our uphi ll thrus t roller s haft. It is my opinion that the failure of the thrus t roller s haft is ultimately due to extended periods of uphill thrus t of the unit. The root caus e(s ) for that need to be evaluated. The cy clic fatig ue followed by brittle fracture evident on the beari ng s tation ary s haft is lik ely due to s teel " waves " forming on the south thrus t face (varyi ng @ roug hly 1.5 - 3" intervals) compounded by extended time under continuous thrus t. A lternatively, any c rack
that had propag ated throug h the s haft may have caused roller deflection and caus ed the waves (now I am definitely s peculating ). In any cas e, I feel that cons tant thrus t alone would not have failed the roller in the same way; lik ely the bearing would have failed vi a hi g h temp mechani s ms . Thes e waves have formed relatively recently (5 months ) and it is not s omething that many people have seen before. If the past roller did con tribute or dir ectly caus e the waves, we may have eliminated further propagation by i ns talling a new one (unlikely - washboard effect on g ravel road s top sig ns ? ), however I thi nk i t may be more than that. Note the period between waves s eems to have s ome common speci fic i nterval, but do vary in places and may overlap. The cycles r equir ed for thi s type of failure is easi ly unders tandable gi ven the period of the wave and the drum rpm (pretty darn quick. ) A ny i nput on the wave formation and how to move forward would be much appreci ated. Cliff Dear Cliff, I think your analysis is pretty much correct. Let me fill in a few more details. Wave patterns, which are not uncommon, have two potential sources; 1) gear and pinion - uneven power transfer from tooth to tooth due either to badly worn teeth or misalignment or 2) high thrust loads due to excessive support roller skew. The difference in symptoms between case 1 and case 2 is the spacing of the "waves". If they are spaced like the gear teeth we have case 1 if the spacing is greater and not necessarily uniform we have case 2. I think we are looking at a case two scenario here as you have indicated. When there is a strong thrust due to excessive roller skew the system loads up like spring. When the stored energy is high enough it breaks the friction between the support rollers and the tires and the drum "jumps back" and the process repeats, causing the wave patterns to develop. The thrust roller support pedestal with its lack of stiffening flanges acts like a tree in the wind and is itself a spring that stores energy and contributes in this process. In due course fatigue sets in and breaks the roller shaft (and/or the bearings) as you have correctly concluded. This is not a theory. I have personally seen this in action. Anecdote: On a much larger unit I have seen a 40" diameter thrust roller repeatedly bend more than 1" after every 5 shell rotations. Enough energy was stored and then instantly released to "jump" the unit (weighing approximately a thousand tons). Needless to say that was nothing short of an earthquake and could not be run in that condition but required immediate support roller adjustment. In your situation the unit is lighter and is running much faster and so produces a very short cycle time, hence the wave patterns. Your solution is just as simple. Adjust the support rollers correctly. You can also consider a more substantial thrust roller assembly. Larger diameter thrust roller, larger diameter shaft and a support that has some attitude (radial support gussets between the plate at the bottom and the barrel) so it does not act like a pole bending in the breeze.
Shell Run-Out
What is maxi mum allowance redial run-out for k iln s hell.
It all depends where on the kiln you
are measuring the run out and what damage it is causing. In some areas like mid-span 100 to 150mm is often seen without much problem. This would likely be a big problem if it is at the ends of the kiln. More than 12mm run-out at any tire would likely be a problem. No matter how much run-out is present if it can be tied to any of: -short refractory life -cyclical roller loading causing bearing or shaft failures -uneven drive loading -excessive ovality -shell cracking -drive vibrations -other problems ---then you have too much run-out.
Shell to Tire Clearance
What is the clearance between chairs and the inner diameter of tire ri ng ?
The clearance between the chairs and the tire is specific for each tire and is specified in the OEM documents (kiln assembly drawings) as cold clearance. The cold clearance will be different for each tire as stated because it is based on the designer's expected shell temperatures at each tire. During operation, when the shell is at "normal" operating temperature the clearance at each tire will have diminished due to differential shell to tire expansion, but now it is reasonable to expect that they all be approximately the same. To determine what this hot clearance is, measure the tire creep and divide by PI(). Typically for any tires and shell chairs that are in good condition the tire creep will be in the range of 6 to 12mm meaning that the theoretical clearance will be 2 to 4 mm if the tire and shell were truly circular. But if you were to instantly stop the kiln and measure the gap at the top it would be about 50% larger because of shell ovality. (The circular relationship of dividing creep by PI() does not prevail since the shell is not round but more like an oval with a pronounced flattening at the top.)
New Kiln Start Up!
What to do/look for or be careful of when s tarting a kiln up for the firs t time?
One cannot comment directly with an unfamiliar installation but based on general experience with starting up new kilns some general comments can be offered. These are restrict ed to mechanical issues only and not process related considerations. 1)Inspect all points were lubricant is supposed to be, including steel flex couplings for example. Verify gear reducers, open gears and bearings. 2)Also make sure that the tire bores (assuming they are migrating tires) are properly cleaned out (compressed air) and lubricated. Otherwise new tires have a nasty tendency to throw walnut sized nuggets for a while in some cases. 3)Verify cooling systems like water-jacketed bearing housings for temperature and flow. 4)On Ferris wheel type oiling systems on support bearings make sure the buckets have not been installed backwards to rotation. 5)On Ferris wheel type oiling systems on support bearings make sure that a liter of oil is poured on each oil tray a few minutes before the first turn. 6)With commencement of continuous rotation verify that oil is delivered in sufficient quantity in each housing. 7)With first rotation watch both inlet and outlet seals for any mechanical problems. 8)Make sure the open gear lub system is functioning. 9)Make sure the pinion bearings are greased. 10)For the first 24 hours of continuous rotation put a full time watch on bearing temperatures. Record the temperatures at each bearing each hour. Make hourly rounds to check on anything that moves. Use a checklist to make sure nothing is overlooked. 11)The inspection crew must be suspicious of and report unusual noises. 12)Observe the thrust rollers for high loads. Do not expect the kiln to be properly aligned particularly in terms of roller skew. 13)Have a well prepared crew with an experienced supervisor at the ready to make some roller adjustments. 14)Organize and equip this latter team to make roller skewing adjustments to fine tune the rollers over a period of several days. A typical 4 pier kiln will take about 2 weeks to tune out. This is based on working 2 to 4 hours each day leaving the interim for observation. 15)Within the first three months of steady operation, do a full, third party, hot kiln alignment with ovality measurements. These documents should be part of the kilns formal Mechanical Certification and Acceptance for the owner. This may highlight existing minor deficiencies that are not otherwise discernable and most importantly provide a benchmark for analyzing future problems as they undoubtedly will arise. Most of the above can be further detailed as needed. This list may not be comprehensive and is only offered as a general guide.
Shell Flexing often referred to as Ovality I have been as ked the ques tion how the deformation value in mm relates to the ovality %. For example, I have a deformation (mm) 0.54 and an ovality % 0.41 for pos ition A on an uphill tyre meas ured on a 5.6m dia. kiln. C an you als o tell me how the Delta Temp is work ed out. Thank s %Shell Flexing (Ovality) is taken as a % movement of the nominal diameter of the shell. Then mm deformation is the "delta H" in the Rosenblad formula and where t is the shell plate thickness at the point of measurement e.g. 25mm: {4/3 x (KilnDiameter+2xt)^2 x deltaH}/KilnDiameter*100%= %ovality. In your case: {4/3 x (5.6+.050)^2}xdeltaH/5.6*100%=0.41%. This calculation is valid for an ovality instrument of 1 meter length. The amount of shell flexing per rotation is therefore: 0.43/100*5600mm= 24mm diametric change. Delta T is simply the average shell temperature around the tire less the average tire temperature.
Extra Gap?
In y our es timate of cold g ap, you introduce a factor of 6.35/3.14mm (2mm) as an " extra g ap" . i would like to know the orig in of this factor. This is simply a "safety factor". The residual gap, when the kiln is in full operation, is always dependent on the temperature difference between the shell and the tyre. This temperature difference varies with the process, fuel and feed chemistry and other factors such as remaining refractory thickness, seasonal changes and so on. The "extra gap" is there to manage these variations without pinching off the shell, which could otherwise result in crushing refractory.
Slim or Wide Filler Bars? What is the effect of us ing s lim filler bars about 100mm wide as pads for rotary k iln tyres , could they pos s ibly introduce load peaks on the s hell suc h that could caus e cracks ? What is their g eneral dis advantage over muc h wider pads of s ay 400mm widenes s in relation to a kiln of 4.6m internal diameter. As long as the narrow bars (100mm) are sufficient in number (aim for at least 65 to 70% coverage around the circumference) then no shell stress issues will arise. Narrow bars are often used as an economic choice since they will likely be flat bars and not rolled like the wider bars need to be. The bars should fit intimately with the shell. Fit specifications usually require that any space between the bar and the shell be no more than .3mm. This will vary according to bar style particularly if they are a welded style arrangement. This is less critical for floating bars. The narrow bars have two disadvantages. First it is difficult to get a high percentage of circumferential coverage since the space between the bars, needed for the stop blocks, is likely significant compared to the width of the bar; the wider the bar the better the coverage; more coverage produces less wear. When the same space between them exists, the narrow bars will wear faster than the wider ones. Secondly the narrow bars will have a tendency to bend in the middle due to tire drag. This will get to look like the bars are smiling on the upturning side of the kiln and frowning on the down turning side of the kiln. Putting stops in the middle of the bar underneath the tire to prevent this is an added cost that negates the economic advantage of using narrow bars. Wider bars are always recommended for good service life.
Shell fatigue In cons idering the fatig ue life of our K ilns it has been s ug g ested that both the circumferential and long itudinal welds on the ki ln s hell are the weakes t points and are expected to fail after a pre determined time. From my information [theOE M] has not s upported this. Do you have examples or witness ed any k ilns where these welds have failed as a result of reaching their des ig n fatig ue life and not due to other contributing factors s uch as refractory failure, mechanical damag e or other catastrophic failures ? That circumferential welds are the weakest points on the kiln in terms of bending strength is a fact demonstrable per the stress analysis performed during each kiln's design stage used to determine shell plate thickness and the points at which thicknesses change. The idea that these points are expected to fail after a pre determined time is not something we've heard stated before. We do talk about this bending stress undergoing 100% reversal with each turn of the shell and fatigue taking its toll in the long run. For that reason the OEM's have drawn from experience and limit the shell bending stress to 1,500psi in the hot end and 2000psi elsewhere (typical values; some manufacturers vary from this but not significantly). Given that mild steel has yield strength of about 36,000psi this gives a factor of safety ranging from 24:1 to 18:1. That is their way of managing long term fatigue; i.e. high safety factors. Many kilns never fail or even come close to failing over the very long haul. We think it is safe to opine that most likely it takes
more than simple fatigue; things like manufacturing/installation defects, unusual loading conditions, changing to higher speeds, higher than expected shell temperatures, bad alignment and excessive shell flexing etc. etc. to cause failures. There are plenty of examples to draw from here as well. Given the many failures we do see it is difficult if not impossible to say which of these many factors including fatigue, was the chief culprit. But obviously we see more problems with older kilns rather than new ones so we must say the older the kiln the higher the influence of fatigue contributing to failure. If failure is to happen then it is natural to say that the weakest point on the shell will be the likely place for it to happen. This in no ways means it is expected or will happen at a predetermined time.
Floating the kiln A t the pelletis ing plant and i'm having a problem on the K iln, es pecially when the load is incr eas ed. My question i s what is the required s peed if the tonnag es are increas ed, this happen in a weeks time that caus ed the ki ln to dri ft. The were damag es inc urr ed on the s eal of the ki ln and this created a down time on our production. Axial travel of the kiln is ca used by roller skew. Usually, for a f ixed positive skew of the rollers, the kiln has a tendency to travel uphill with an increase of either or both kiln loading and speed of rotation. The roller skewing should be set to float the kiln at maximum speed and feed rate. Floating means moving gently between the thrust rollers. This is usually a very temporary, transient condition. As far as the speed and loading relationship is concerned I cannot be specific about that since I am not familiar with your process. One common practice, as in cement kilns for example, is to tie speed and feed rates together; as the feed is increased so is the speed. This results in a constant bed depth no matter what feed rate prevails. Other processes follow other conventions and so end up with different bed depths as the feed and or speed is changed. Because of the kiln slope, the kiln during running has the tendency to slide towards kiln h ood at the discharge end due to gravitational force.so to avoid this and to keep it at its position, thruster is used,which pushes it u pwards after a set downward position is reached as indicated by the sensor.And when a set upward po sition is reached the thruster releases its push and then kiln again starts slowly sliding downwards,so this cycle continues.It is required for all modern kilns of different capacities.
The position of Kiln Tires with respect to Rollers is different in Cold Condition & Hot Condition. Therefore, Roller width is kept higher as compared to that of tire.In normal working conditions when the Kiln is hot, if the kiln continues to run in one position there may b e a groove formation on the roller. To avoid this kiln is floated up and down so that there is uniform wear on the surface of the roller.
