Tire and Tire Elements

Kiln tires serve two basic functions. The first is obvious, the tires are the main bearing rings on which the kiln turns. In this regard, the tire is similar in function to the race of a spherical roller bearing. The second function is less obvious but equally important. The tires are also the structural member that support the kiln shell. For this reason, the way the tires are mounted to the shell is critical for long term operation. Since, by function, the kiln involves thermal processing, the shell, the tires and the tire mounting system all undergo changes from thermal expansion and contraction. It is impossible to restrain the effects of thermal expansion. Because the expansion rate of the components is usually unequal to that of the shell, the problem of managing these differences becomes an important focus for maintenance personnel. Any time that inspection of a rotary unit is carried out, the tires and tire elements should be carefully examined. Once a small problem develops with these components, larger problems are sure to follow.

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Riding rings can be manufactured from cast iron, cast steel, or from a one piece seamless forging. Configurations include cast and hollow, forged and rolled, fabricated, multi-piece or segmented, and a drive type, in which the girth gear is directly bolted. FORGED A typical riding ring may be manufactured from 1045 normalized material or equivalent, and hardness falls between 180 BHN and 220 BHN. CAST Some cast riding rings use a ASTM, A-551-81 Class C locomotive steel with carbon content of .7 to .85. This is non-weldable and can reach hardness up to 400 BHN to 540 BHN. These tires require the use of special flame-hardened rollers. If a tire needs to be repaired it is essential to identify the material exactly so that correct welding procedures can be performed. After repairs have been made it is imperative that the ring be resurfaced to establish its true contact surface, and to remove any out-ofroundness that may have developed from the welding process. The specific way a tire is mounted on the shell determines what problems may arise.

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Tire & Tire Elements

TIRES Riding rings/tires provide substantial strength to the shell by maintaining shell roundness. Much of the shell’s integrity is directly related to the thickness, width and mounting style of the riding ring. Riding rings can be manufactured from cast iron and cast steel. They can be cast and hollow, forged and rolled, or fabricated, multi-pieced and segmented. A typical kiln riding ring is manufactured from 1045 normalized material or its equivalent, and hardness typically falls between 180 BHN and 220 BHN. In the broadest sense tires can be put into two classes; fixed tires and loose, migrating or floating tires. Loose tires are normally required to accommodate differential thermal expansion between the tire and the shell, especially on those shells whose surface temperature is high. Fixed tires are usually found on unfired vessels and equipment whose shell temperature is below 200°F or 100°C. FIXED -There are various designs that have been used to mount and fix the tires on the shell. Some tires are welded directly to the shell. Others may be shrink fitted, wedged, pinned, keyed, splined, or otherwise mechanically fixed. If a tire’s mounting system begins to require frequent repair, an evaluation of the method of mounting should be made. MIGRATING/LOOSE - Migrating tires are free to rotate on the shell. The mounting is designed to allow for a different rate of thermal expansion between the shell and the tire. Migration, even when sufficient and controlled, and even when appropriate lubrication is present, will lead to wear of the mating parts (filler bars, stop blocks, wedges, etc.). A migrating tire also becomes a problem when it becomes too loose to properly support the shell.

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Side Wall Wear - This problem occurs when a tire is migrating on the shell, whether it is designed to or not. Visual signs of undercutting on the side wall are a clear indication that a problem exists. Conditions that cause or contribute to this wear problem usually exist in some combination of varying degrees with one another and will be; warped shell, improperly skewed or out of slope rollers. Surface Deterioration - Surface craters on the surface of the tire, commonly referred to as spalling, is a sign of metal failure. There is always some metal distortion present in the pinch point of the roller and tire. If a tire starts to wobble or if for some other reason the pressure at the pinch point becomes excessive, the steel will distort beyond the elastic limit. The flexing of the workhardened layer against the softer underlying layer will create small cracks to develop. Once cracking starts it continues until chunks of metal break free. The two major factors that cause surface spalling are reduced contact area because of abnormal wear patterns and excessive thrust due to improperly adjusted rollers. Excessive Gap/Flex - The looser the tire, the less support it gives to the shell. When support of the shell is reduced the shell flexes more. Flexing in the extreme is detrimental to the shell and can cause fatigue cracks to develop in the area around the tire mounting elements. Excessive shell flex must be controlled. Excessive Creep - Wear accelerates as creep increases. Shell flex increases causing shell fatigue cracks to develop in the shell. Tires can wobble and get cocked on the shell. Stop blocks and retainers can wear and undercut the tires. In all cases it is critical to minimize excessive axial and radial run-outs of the riding ring. Excessive run-outs may affect the gear train, thrust rollers, feed and discharge seals and the crucial rolling surfaces of the carrying rollers.

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Tire & Tire Elements

Tire Wobble – Excessive axial run out of the tire is referred to as “wobble”. The contact between a wobbling tire and its’ rollers will vary throughout each revolution of the kiln. Through part of the revolution the tire will be contacting predominately on one side of the rollers. Then, throughout the balance of the revolution the tire will contact on the opposite side of the rollers. This results in reduced contact area between the tire and the roller and uneven wear patterns will develop. As a result of the reduced contact area, the metal is overstressed and in extreme conditions, surface spalling can occur. Tire wobble can also contribute to tire side wall wear and tire retainer wear or failure. Proper roller adjustments can be extremely difficult to make with a wobbling tire. The thrust reaction of tire and the rollers becomes erratic as a result of the changing contact. Tire wobble is generally caused by some type of bend or bow in the shell’s rotational axis that is created by distorted shell plate resulting from localized overheating during a refractory failure. It can also be a temporary condition caused from uneven coating or some process condition that causes an unbalanced thermal profile in the kiln.

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Side Wall Wear – Aggressive wear or undercutting of the side face of the tire can occur when a tire is migrating on the shell under high load conditions. Any signs of excessive wear to the tire retaining blocks or excessive scuffing on the side wall of the tire are a clear indication that a problem exists. Visual observation of the space between the tire and the retainer blocks gives a good idea of what is going on. For a new assembly this space is about 1/16” – 1/8”. Over the years, as a result of wear, this space may increase to about 1/2” to 3/4” . Any more than this and the tires may run off the edge of their rollers and the gear may overhang the pinion Heavy side loading of the tire against the retainers can also cause the retainers to break off. If the tire retainers are welded, cracks may appear. If they are bolted on, the bolts may shear. Fixing cracks and worn parts, or replacing missing pieces may not resolve the problem. It is important to carefully analyze the condition, since redesigning the way the tire is retained may be required Conditions that cause or contribute to this wear problem are; a collapsed shell under the tire, a warped shell that is creating a bow or bend to the shell’s axis, out of slope rollers, or improperly skewed rollers. They usually exist in some combination with one another and to varying degrees.

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The presence of surface craters on the rolling face of the tire, commonly referred to as spalling, is a sign of metal failure. Since steel is an elastic material, there is always some metal distortion present in the pinch point between the roller and the tire. On a microscopic level this can be seen as an area flattening out, similar to that of a rubber tire on a car where it contacts the road. If for any reason the pressure in a localized area of the pinch point becomes excessive, the distortion of the steel will go beyond the elastic limit. The flexing of the work-hardened layer against the underlying softer mass will cause fatigue cracks to develop. Once this fatigue cracking starts, it can propagate until chunks of metal break free.

The two major factors that cause surface spalling are reduced contact area because of abnormal wear patterns and excessive thrust due to improperly adjusted rollers. The example shown is caused by an excessive thrust condition. This is a direct result of improperly adjusted support rollers. If any roller is skewed excessively with respect to the rotational axis of the tire, a sliding or scuffing action occurs that creates a high friction load that can lead to spalling of the surfaces. This will be discussed in greater detail in the section on support rollers skewing. Spalling on the contact surfaces of the tire and rollers can also be caused by the use of oil to lubricate these surfaces. As the rolling contact surfaces meet, since oil is a noncompressible fluid, hydraulic pressure forces the oil down into any small fatigue cracks. Over a period of time this hydraulic pressure causes small pieces of material to fracture loose.

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Any area of the tire that has been welded can be prone to cracking. This happens for two reasons. First, the material used for the tire is typically a medium carbon steel which requires very specific weld procedures for a successful weld. This typically includes a localized preheating prior to welding and some type of post-weld stress relieving as a minimum. If proper weld procedures are not followed cracks will most certainly develop. Second, since the tire is subject to cyclical loads, it is subject to fatigue failure which can occur at stress levels as much as 40% below the ultimate strength of the tire material. Any stress risers exacerbate this problem. Any tire mounting elements that are welded to the tire, such as anti-rotation keys or mounting chairs, create a stress riser in the tire.

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FIXED TIRE MOUNTING STYLES Shrink Fit - For low temperature applications this is by far the best way to fix a tire. It is mechanically simple because it has no tire mounting elements. There is a requirement to accurately machine the shell however, which is an added expense. A shrink fit tire grips the shell for its full bore, circumference and width. No other method does this. Additional welding is not recommended because the flexing with each rotation soon fatigues the heat effected zone causing eventual cracking. Direct weld - This method should also only be used on low temperature units since it does not allow for any variation in the thermal expansion of the shell. When troubleshooting this type of mounting, always look for fractures in the weldment. Any weld fractures can propagate as cracks into the shell and will cause a circumferential split. Wedge & Shoe - In this design a guide shoe is welded to the shell surface and a tapered wedge is driven in to force the riding ring tight to the shell body. The wedge is then welded to the shoe. Caution must be used when tightening the wedges so as not to collapse the shell by driving the wedges to tight. Also, if proper procedures are not followed when wedging the ring, we can actually reposition or re-center the shell within the bore of the tire. This can affect the gear run out and the run out on the ends of the drum shell. Anti-rotation keys - The keys are welded into the riding ring and prevent the ring from migrating around the shell during rotation. Because most drums flex at the 12 o’clock position the keys are constantly fretting at the point where they contact the tire supports. Listen for a banging noise which indicates worn keys. It is not uncommon for a key to become pinched and fracture the tire.

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Spring Chair Supports This design is a fixed tire arrangement that is intended to allow for some difference in the thermal expansion of the tire and the shell. It consists of bars that act as a springs, welded to the riding ring and bolted to the shell. The concept of the sprung chair support is to provide a suspension system for the riding ring. As the shell temperature changes at a different rate than the tire the, shell expands and the springs flex to allow this to happen without damaging the shell. As the tire wears down over time it becomes thinner and begins to flex more. The areas where the spring plates are welded to the tire can become prone to cracking. Opposing wedges - Two wedges are driven from either side of the riding ring to tighten the shell. Stop blocks are welded to the shell to limit the axial movement of the tire. It is common for cracks to appear at weldments and for them to propagate to the shell. It is important that the wedges are not installed too tightly or the shell can actually “neck down” under supporting assemblies.

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Tire & Tire Elements

Irregular wear patterns develop between the tires and their rollers as a result of misadjusted and out of slope rollers, drive train problems and wobbling tires. Regular inspections can be made of these surfaces by laying a precision straight edge across the face and making a visual comparison. Also, circumferential measurements can me made using a tachometer counting device or by physically taping the tire and roller circumferences. Since many of the wear pattern that develop on the ring are a result of problems with the rollers, the causes and symptoms of the various types of wear patterns will be covered in more detail in the roller troubleshooting section.

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MIGRATING TIRE MOUNTING SYSTEMS Units requiring floating tires demand the most service performance from the tire mounting components. Filler bar style tire mounting systems - Filler bars can be designed to allow for different rates of thermal expansion between the tire and shell and yet provide support to the shell within the bore of the riding ring. The style of filler bar and method of installation can be selected for each particular rotary unit as unique conditions dictate. Over the years the “full floating filler bar” design has evolved as providing one of the best compromises of the conflicting service demands of the migrating tire mount.

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Refractory-lined shells that require loose tires demand the most service performance from the tire mounting components. Over the years various tire mounting systems have been used. Design has evolved in an attempt to provide the best compromise to meet the severe service demands of the migrating tire arrangement. We will review this evolution with an eye to the maintenance problems associated with each design.

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Welding did not become accepted in industry until after the second world war (1945) up until then shells were riveted and filler bars usually bolted. Often tires and filler bars with integral stop blocks were cast and bolted to the shell. Even today some manufacturers still like cast and bolt arrangements. This design is prone to bolts shearing off due to thermal expansion. The shell under the filler bar gets hotter than the filler bar. This means there will be some shear force present. More significant is the force of the tire dragging over the filler bar which acts to shear off the bolts. Some improvements have been made to this design in recent years by adding special elastic inserts to cushion the bolts from the shear forces. These have only been partially successful. In most installations involving bolted filler bars and or stop blocks it does not take too much concentration to spot missing bolts, nuts and cracked filler bars. The real disadvantage to the bolted filler bar is that bolt replacement involves going inside the shell and removing refractory. This makes maintenance expensive. Having to drill the shell in the first place to accept the bolts makes this an all around expensive arrangement. PKS does not recommend it.

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Tire & Tire Elements

Bolted filler bar design – Some manufacturers use a bolted filler bar system. The bolts are prone to breaking as a result of the shearing forces created as the tire was across the filler bars and from the differential thermal expansion between the filler bars and the shell. Replacing the bolts is a difficult task which requires a kiln shutdown and some refractory removal. Many installations with a bolted bar arrangement have been converted to a free floating bar system. The holes in the shell are plugged and welded closed and a new pad system is installed. Since the filler bars are generally larger in a free floating system, there will be fewer pads with a different spacing. This requires any weld that is in the area under the tire to be gouged off and ground smooth.

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THE EVOLUTION OF FILLER BAR MOUNTING SYSTEMS Fully welded filler bar design - The expansion and contraction forces and the dragging force from the tire as it slides across the filler bars are transferred to the welds. The difference in shell expansion acts to crack the end welds in tension. The welds along both sides of the bar run parallel to the lines of stress resulting from circumferential shell flexing. Additionally the filler bar acts like a stiffener, trying to resist the shell flexing. This means that the area between the stiffeners will flex more. These stress lines cross at the corners producing a high stress concentration. Chances are this is where the cracks will appear first. This type of arrangement is prone to developing cracks in the weld, which when left unattended eventually run into and through the shell plate. Cracks running from one side of the tire to the other are not unusual. When this happens no amount of weld will hold. A better design is needed.

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Semi trapped filler bar system - To solve the problem of expansion stresses it would seem obvious to not attach both, but just one end of the bar to the shell. Then, by simply trapping the free end of the bar in the circumferential direction to take the tire drag load the assembly would function correctly and not be prone to developing cracks from tensile stresses. Since the filler bar is now only welded to the shell at one end it does not act so much as a stiffener and circumferential stresses due to shell flexing are not as concentrated. In fact the line of axial stress concentration is now broken up to some degree. We also see in this illustration that the stop blocks are staggered. For the semi trapped arrangement with its addition of trap blocks it allows the filler bars to be put closer together. Getting the maximum circumferential shell coverage is always desirable. If stop blocks were mounted at both ends of the filler bars the stagger coupled with the shell expanding more than the filler bars, half the blocks would loose contact with the side of the tire. Theoretically at least you could argue that blocks are only needed at the fixed end of the bars. The clincher of course is that this design is cheaper because of the reduction of the number of stop blocks. This design started to address key issues but more was yet to come.

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Full floating filler bar system - The advantages of the semi-trapped design quickly led to a “full floating” arrangement, which had even more advantages. By eliminating all welds between the filler bars and the shell and adding keepers at the stop block end a fully trapped design was achieved. The stresses associated with the bar acting as a stiffener plate were now completely eliminated. There remain welds on the shell from the smaller keeper blocks but since they are small and short the weld material can easily take the differential expansion without exceeding the material yield point. To further reduce stress concentration every attempt is made to have welds run diagonally across the shell flexing (axial) stress lines. This is why the filler bar keepers are somewhat triangular in shape. The side keepers should also have large radius corners. An additional advantage this arrangement provides is the ability to remove the filler bar for replacement or re-shimming without having to do any gouging or welding on the shell. Only the stop block needs to be removed from the top of the filler bar and it can be pulled out from beneath the tire.

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Tire & Tire Elements

An improvement to handle tire side face wear - One of the major remaining problems with migrating tires is stop block wear against the side of the tires. Lack of lubrication and unusual loading conditions will increase the wear rate. Staggered stop blocks may not provide enough bearing surface to take the rubbing pressure of the tire. One solution, space permitting, is to introduce keyed rings. These rings are installed with a cold clearance the same as the tire but are prevented from migrating with the tire by keys. There should be a minimum of six keys for each ring. Too few keys and they will tear the filler bar away. The bearing area is now between the rings and the side face of the tire. This area is now larger and continuous, which greatly reduces the unit stress and subsequently wear. The contacting surface between rings and stop blocks has no relative movement in the circumferential direction which all but eliminates wear on the stop block. This makes the rings the sacrificial elements, which are easier to replace than are the welded stop blocks. In all cases it is critical to minimize the axial movement of the riding ring. Excessive axial tire movement may affect the gear train, thrust rollers, feed and discharge seals and the crucial rolling surfaces of the carrying rollers.

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This variation has an improvement on the full floating ring in that the segments are easier to manufacture transport and fit to the pads. The danger of having ring keys shear off the filler bar pad retainers is also eliminated. Although the segmented rings have a slightly smaller bearing area than complete rings and require more weld to be removed in case of a need to replace them, they do provide equal service life. Barring a customer’s express desire for another arrangement this would be the PKS recommendation. This is a more manageable installation, which makes for quality results. With very little sacrifice of service life the segmented style with stop block backup to keep the rings from rolling over is the best arrangement provided space exists to fit in.

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Tire & Tire Elements

Sometimes the side faces of the tires have been seriously worn. Elevating the block increases the bearing area and reaches up to unused areas of the tire face. This method is likely not an option on tires equipped with thrust rollers due to the height limitation.

We have only shown some of the typical kinds of filler bar designs. These cover the basic variations although many different styles with other combinations of features exist.

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SPLINED DESIGN - In this design the load of the rotary kiln is transmitted, via the X-shaped retaining blocks welded to the shell, to the floating tire shoes, to the wedges, then to the tire splines. The shell is supported by the tire at 3 o’clock and 9 o’clock. Consequently there is almost as much gap at the bottom as there is on the top although ovality still makes the top gap larger. The ovality for this type of shell support is considerably less than conventional designs. The clearance between the tire splines and the shell pads is taken up by the slow wedges. As this clearance varies with tire to shell temperature differences, the wedges migrate in and out. This results in a tight assembly circumferentially at all temperatures yet leaves room for the the shell to expand/contract radially within the tire. The wedge and spring assemblies should be lubricated to insure that they can move freely. The welds between the X-shaped blocks should be checked regularly for cracks. Since the X blocks are welded solid to the shell, they can act as a stiffener trying to resist any flexing of the shell and can be prone to cracking.

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Tire & Tire Elements

“CREEP” MEASUREMENT ON MIGRATING TIRES The riding rings provide substantial strength to the shell by maintaining shell roundness. Because the shell naturally flattens out at the 12 o’clock position like a flexible bag of water, the riding ring system must maintain shell integrity by minimizing flex. To accommodate any difference in the expansion rate of the shell and the tire, there is a difference in the size of the shell’s outside diameter (OD) and the tire’s inside diameter (ID), the tire having a larger diameter. Because of this difference the tire naturally wants to creep, or migrate at a slower rotational speed than the shell. The shell is actually rotating at one speed and the tire is lagging behind at a slightly slower speed. By making a mark with a soapstone from the side face of the tire to the surface of a filler bar, or along a stop block, it is possible to witness the marks slowly separate during each rotation. This separation is a direct measurement of the fit between the shell OD and the riding ring ID. Worn filler bars, or supports, allow excess gap at the shell’s 12 o’clock position, thus allowing excessive flexing of the shell plate as the drum rotates. This reduces the shell support provided by the riding ring, accelerates and compounds support pad wear, and leads to fatigue cracks in the welds of the mounting system and can eventually lead to shell cracks.

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When we go to the top of the kiln and measure the actual gap we find that it is larger than the difference in circumferences (Tire bore circumference less shell filler bars circumference) divided by . The reason for this is ovality, meaning the shell sags across the top. Another way to think of this is that the shell and tire are not perfect eccentric circles. If they where then the gap would be equal to the creep/ . Even this assumes that the creep is the result of true rolling action with no slip or hang-up. The Obourg device shows us the complete story. It shows us the relationship between slip, gap and the effects of ovality. The amplitude “s” of the resulting plot is the actual gap. The period “ U” is the prevailing creep. U/s but something more like 2 to 2.5. This ovality ratio varies from kiln to kiln and tire to tire. This may seem like a very academic issue but it has great significance when it comes to calculating the expected filler bar thickness when reducing the gap to correct ovality is necessary. Although this is an excellent diagnostic tool its use is often limited by the presence of thrust rollers and high speed kilns.

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Tire & Tire Elements

Measuring diagram after 4 revolutions S

U U = Relative Motion (creep) S = Actual Gap U < Pi S

2 - 2.5

Typical Ovality Ratios

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PROBLEMS CAUSED BY EXCESSIVE TIRE CREEP 1. Wear accelerates as creep increases. 2. Shell flexing increases which can lead to: Brick failure due to crushing Shell fatigue leading to cracking in shell plate and filler bar welds. 3. Stop blocks and retainers wear out then undercut tires. Undercut tires lead to the continuous problem of matching up stop blocks. There is a direct relationship between tire creep and the gap between the tire and the shell. The more tire creep the more gap. The more gap the looser the tire. The looser the tire the less support it gives to the shell. Worn filler bars, or supports, create excessive gap thus allowing excessive flattening or flexing of the shell at the 12 o’clock position. This accelerates and compounds support pad wear, and leads to shell cracks. Cracks are a sign of complete failure and will eventually adversely affect the total operation of the kiln.

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Tire & Tire Elements

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If the tire has cracked, is severely undercut on the side face or if the wear is so severe that the tire no longer has sufficient strength to support the kiln, replacement is required. If the contact surfaces have developed irregular wear patterns or are pitted or spalled, retruing by grinding the surfaces smooth is a prudent step to returning the equipment back to a dependable condition (See “Resurfacing and Grinding” section). If there is excessive tire migration and high shell ovality flexing, a solution may be to replace the worn filler bars and stop blocks. Prior to this step the shell under the tire should be carefully inspected. If the shell has collapsed under the tire, a section may have to be replaced. If the bore of the tire is severely damaged from the excessive migration, the tire may need to be taken off and sent to a machine shop for true-up during the filler bar or section change out. Tires and Tire Elements

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If indications are for filler bar replacement, be aware that in some of the old filler bar designs the bars are welded to the shell in the area under the riding ring. This is important to know because when it comes time to replace the bars, the riding ring must be pushed off the bars to facilitate removal of the weld and grinding these areas smooth. This takes extra time and equipment which must be planned into the shutdown for the job. The replacement filler bars are generally ordered in a stock thickness. A full set of 1/16, 1/32, and 1/8 shims are supplied so that the bars can be shimmed as required for the proper clearance based on the operating temperatures for the shell and tire. In addition, the shims are used to help fit the out of round shell to the bore of the tire.

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PREPARING FOR A SHUTDOWN Always make a map of the position of the kiln components based on hot and normal conditions, prior to the shutdown. Once the outage starts, the kiln is cool and it is difficult to determine what, if any, adjustments should be made to correct problem conditions. With a written record for reference, corrections or replacements can be more accurately determined. Check the position of the tires with respect to their support rollers and the rollers with respect to their housings. If the tire is not centered over the rollers, note how much and in which direction the tire should move. Note how much space is between the edge of each tire and its’ retainers. Make a note if repositioning is necessary. Also look to see if the gear is centered over the pinion and where the kiln is at between the thrust rollers. Is there excessive clearance between the thrust rollers that allows too much axial movement of the kiln? Measure and document the shell temperature on either side of the tire and the temperature of the tire. This information will be used to help determine the proper amount of “cold clearance” when reshimming a tire.

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Preparation and Protecting the Equipment •

Measure the position of the tire as it relates to the transition.

•

Check the gear and pinion axial alignment.

•

Review inspection reports and discuss with the customer if the tire has full contact with the carrying rollers during operation. Review all inspection data for positions and conditions of all relevant tire(s) with relationship to carrying rollers, thrust rollers, stops, etc.

•

Does the above information require the tire to be repositioned?

•

Cover the carrying rollers. (Nothing works better than a sheet of ½” plywood.) Blow out the contact area of the rollers and tire with high-pressure air before each kiln turn.

•

Protect areas below the pier. (Make it a regular event to barricade ribbon off the area below for personnel people protection.)

•

Grounding – always ground directly to the kiln shell when welding on the shell or filler bars. Do not ground through carrying roller frames or bearings.

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CALCULATING COLD GAP Kiln Shell Diameter x Temperature Difference between Tire and Shell x Expansion Coefficient = Reduction of the Filler Bar to Tire ID Gap. This is the differential expansion. EXTRA GAP is a safety factor that we apply since we can only work with average temperature values and temperatures fluctuate during normal operation. Since these temperature differences cannot be strictly controlled and a locked tire is something to be avoided, leaving a residual or extra gap is the accepted norm. The final step would be to take the sum of the differential EXPANSION and the Cold Kiln shell diameter subtracted from the cold bore diameter of the tire, divided by two to give the theoretical filler bar thickness. This information can then be used to specify machining diameters for new sections or replacement filler bar base thickness and shim packs in case of repairs.

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THE QUESTION OF LUBRICATION The question of lubrication between the bore of a migrating tire and the shell has always been controversial. Although lubricating the tire bore may seem to be a natural requirement of good kiln operation, some experts advise against it. One argument is that “greasy” lubricants may attract dust and debris which then act as grinding compounds and accelerate wear. The second argument says that lubrication will promote slippage and creep, again, hastening wear. 1. A dry form of lubrication should be used so as not to attract air born contamination and debris. 2. Slippage will occur with or without lubrication. True rolling action can never be assured and is at best, a transient condition. Lubrication, therefore, is not applied to induce slippage, but to prevent any local areas from galling and hanging up to the point where metal failure occurs. A greater amount of creep may be seen with the use of lubricant than without, which, if it acts to polish the surfaces, is infinitely more desirable than creep which is limited. Inhibited creep will eventually tear the metal.

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Galling occurs when dry steel slides by dry steel and the surfaces attach themselves on a microscopic level. The steel balls up and forms slugs or spitzers. These are created at the sides of the tire where they contact the retaining blocks. Lubricating here is essential to prevent undercutting the tire and consuming the stop blocks. Lubricating the entire area is completely appropriate. Graphite is most frequently used for this application. Other lubricants specifically formulated for this are colloids containing molybdenum, aluminum etc. These are solid lubricating materials in a carrier that is designed to evaporate at low temperatures. In no way should this carrier be confused with “grease”. The carrier quickly dissipates leaving solids as a nonsticky residue, which closely adheres to the surface of the steel components. Lubrication does not correct misalignment but lubrication in the areas discussed is an important step toward good equipment maintenance.

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