Skew, more than any other mechanical adjustment, is the least understood, the most misused, and is often the most troubl esome mechani cal issue with a kiln. DEFINITION: Skew is the position o f the roller axis with res pect to turning axi s of the shell. If they are parallel then the roller is said to have zero skew or be neutral. Zero skew means no ax ial thrust is create d. If the roller is not parall el then it is said to be skewed or “cut” and does create an axial thrust that pushes the kiln either uphill or downhill. Because kiln shell s are not truly straight its rotating axis at the rollers is not constant. Zero skew cannot be set with rollers that have a fixed base. This is only possible if the roller support base is allowed to articulate to follow the shell/tire wobble. Skew is created with a very small (0.004 to 0.04 0 inches, 0.1 to 1.0 mm) pivoting adjustment and only changes the parallel relationship of the roller to the longitudinal axis of the rotating shell. It does not affect (to any significant degre e) the position o f the shell either in p lan or eleva tion views. In other words the roller i s pivoted bu t the shell i s not significantly raised or moved laterally. This simple, but important concept must be understoo d completely before correct roller adjustments can be made. Thrust control by skewing may be the single most important adjustment which influences the optimum mechanical operation of the unit. Roller Adjustment and Skew Page 1
Skew, more than any other mechanical adjustment, is the least understood, the mostmisused, and is often the most troublesome mechanical issue with a kiln.
DEFINITION: Skew is the position of the roller axis with respect to turning axis of theshell. If they are parallel then the roller is said to have zero skew or be neutral. Zeroskew means no axial thrust is created. If the roller is not parallel then it is said to beskewed or “cut” and does create an axial thrust that pushes the kiln either uphill ordownhill. Because kiln shells are not truly straight its rotating axis at the rollers is notconstant. Zero skew cannot be set with rollers that have a fixed base. This is onlypossible if the roller support base is allowed to articulate to follow the shell/tire wobble.
Skew is created with a very small (0.004 to 0.040 inches, 0.1 to 1.0 mm) pivotingadjustment and only changes the parallel relationship of the roller to the longitudinal axisof the rotating shell. It does not affect (to any significant degree) the position of the shelleither in plan or elevation views. In other words the roller is pivoted but the shell is notsignificantly raised or moved laterally.
This simple, but important concept must be understood completely before correct rolleradjustments can be made. Thrust control by skewing may be the single most importantadjustment which influences the optimum mechanical operation of the unit.
Skew is a description of the position of the roller axis with respect to the rotating drumaxis. If these axis are parallel the roller is neutral or has zero skew. If they are notparallel then the roller is said to be “cut” or skewed either correctly by pushing the drumuphill or incorrectly when pushing the drum downhill. The amount of skew is typically0.005 to 0.040 inches (0.1 to 1 mm) for rollers in good condition of any size.
Why Skew?
Since the drum is set on a slope, gravity pulls it downhill. Therefore something mustcontrol the axial drum position. Typically this is the job of the thrust rollers. But theskew of the carrying rollers can also counteract this gravitational pull. Often rotatingequipment is economically built with light thrust rollers that need help from the carryingroller’s skew to keep the drum from pushing downhill too hard.
Why is Proper Thrust Important?
Any amount of skew acts to deteriorate and tear up the rolling surface. If the operationof the unit requires some carrying roller skewing to limit the load on the thrust rollersthen the skewing must be set to the minimum in order to save wear and tear as much aspossible. Skewing is really a compromise whereby some of the long term running life issacrificed to save the capital cost of more expensive thrust rollers.
Hydraulic thrust rollers move the entire kiln axially about ±1 inch (± 25mm). They run onrails and are powered by hydraulic rams. They are designed to carry 100% of the thrustload of the kiln allowing the carrying rollers to be adjusted to neutral skew. Thisminimizes roller-tire wear considerably and to distribute the remaining wear across therunning faces the mechanism is adjusted to cycle one stroke every 12 to 24 hours. This“adjustment” naturally also involves setting the rollers. For actual times please refer tothe OEM’s recommendation.
A “full thrust kiln” is simply one fitted with static thrust rollers that are large enough tofully support the kiln with the carrying rollers neutral. Sometimes with larger and longerkilns there are thrust roller assemblies on two or even more piers. There are kilns thathave been built with as many as twelve (12) sets of support rollers.
Understanding which way to position/reposition a roller and understanding thesubsequent action and reaction of an adjustment is essential in gaining control of themechanical operation.
Stand on the down- turning side of the drum.
Simply steer the roller in the direction the tire should be moved.
If the roller is steered to move the tire to the right the reaction is for the roller to moveleft.
If the roller is steered to move the tire left the reaction is that roller moves right.
It’s a case of simple action – reaction. Newton once postulated “For every action thereis an equal and opposite reaction”. Pushing the tire one way causes the reaction of the
roller to drive itself the other way.The same holds true for the roller on the opposite side. Essentially the rollers should bealways be kept parallel.
There is no logical sense or purpose to have the rollers “toed in”. Toed in rollers createunnecessary wear for no benefit whatsoever.
The animation provided on the CD which accompanies this book shows how the tire canbe directed either right or left.
Assuming both rollers are skewed equally (only to simplify the calculation) the change inelevation is:
∆E A - (B- ) A B
A Radius of tire + radius of roller
B = sin(Angle) A
2 2= − −
=
×
skew2 2
For a typical kiln: Angle = 30 degrees
R - radius of tire may be 2000mm (79” or 6.5 feet)r - radius of roller may be 500mm (20”)
and the skew should not be more than 0.25mm (0.010”)
DE is then calculated to be 0.14mm (0.006”). The ratio is about 2:1 for easyreference. This ratio will not change enough to make a difference for any size kiln,
cooler or dryer etc.
Although the skew is significant at 0.25mm, the change in alignment elevation, DE isnot.
2) The line of contact between the roller and the tire changes. The line of contact is not really a line. It
is an area defined by
i) the length of contact between the roller and the tire in the axial direction.
ii) the width of contact which varies according to:
a)roller diameter
b)tire diameter
c)hardness of the material
d)roller slope matching the tire slope
e)amount of skew
It is most desirable to have the area of contact as rectangular as possible. e.g... view (a). When skewingis required, which is the case for many units by design, then clearly the minimum amount of skew to just
balance the down thrust of the shell, should be sought. The skewing should be shared equally by all
the rollers. For illustration purposes diagram (b) shows excessive skewing, so much so that only half theroller face is in contact. Since the load this roller carries has not changed, the stresses in this reduced
area must necessarily be higher. Visually the stress volume of the yellow shape at “a” must equal thatof “b”.
We can see therefore that excessive skewing decreases the contact area and increases the unit load,and stress, in that area. The contact area behaves similarly to a car tire in contact with the road. The
contact area actually flattens out and the material in the flat area deforms. When this deformationexceeds the elastic limits of the material, it fails.
Skewing causes edge loading as seen in “b”. This can be catastrophic if the skewing isexcessive. The symptoms would include mushrooming, edge cracks in the rim and
ultimately large pieces coming out of the loaded edge of the roller. Since some skewingis required in most cases, changing the roller slope by shimming is beneficial. Onlybearing housings that have self aligning bearing sleeves or spherical roller bearings areeasily adjusted in this way. Bearing housings with fixed sleeve bearings can beshimmed using tapered shims but this is a more complex procedure. Shimming as acompensation for skew is often restricted to larger units. When it is required it will bestated in the kiln manufacturer’s documentation.
When the roller slope is adjusted for skew the load carried by the roller is distributed asshown in “c”. The peak stress is moved back to the center of the roller, the stressreduces towards the edges and is symmetrically distributed. This is a much better
distribution pattern and makes the effort to do this worth while.
On the upturning side of the kiln shell the downhill bearing is shimmed and on thedownturning side the uphill bearing is shimmed. The shim thickness is about 0.6 timesthe amount of skew.
With the highest load centered on the roller we see why convex/concave wear is anatural result.
As long as the the tire and rollers are free to shift they do so until the roller shaft reaches and
seats on a thrust bearing. Similarly the tire will shift until it bumps up against a thrust roller.When neither the ring or the roller can shift, the thrust load is relieved by slippage. Therefore,with skewed rollers we no longer have pure rolling action. Slippage is another effect that causes
problems. It tears the rolling surfaces. An overly skewed support roller can generate more thrustthan the thrust bearing can handle. The oil film in the bearings becomes too thin, metal to metalcontact occurs, the surfaces heat up which reduces the oil viscosity further, and the bearing fails.
Once the thrust bearing fails the heat generated is usually enough to fail the support bearing aswell. When support rollers are fitted with spherical roller bearings the situation is more criticalsince the thrust load and the support load both act on the one bearing simultaneously. These
will tend to fail more frequently than journal bearings with thrust rings or thrust buttons.
Since a skewed roller no longer runs against the tire with a pure rolling action, but induces some
slippage, lubrication of the outside diameter with dry graphite is highly desirable, and helpspreserve the surfaces. Oil lubrication on the rolling surface should be avoided as it can promote
spalling.
Once again we have good cause to avoid skewing when possible and to limit it to a minimumwhen it is required.
Until you get used to visualizing the actual motions of the roller and tire, this “hand rule”may help sort things out.
The palms: Stand and face the tire as it moves in front of you. If the tire surface ismoving up – hold your hands out, palms up. If the tire is moving down – hold out yourhands, palms down.
Fingers: Curl the fingers into your palm. They point into the direction the top of the rolleris moving. When palms are up the fingers curl up towards you which is the way the topof the roller is moving. When palms are down they curl down and away from you, againthe way the top of the roller is moving.
Index finger: Points to the direction which the bearing is to be moved.Thumb: Points to the direction the shell will move as a result. For (a) pushing the leftroller in will cause the shell to move to the right, and so on.
Remember, the thumb points into the direction the shell will move. The roller reacts byshifting itself in the opposite direction of the shell.
By looking at a full roller support station assembly we should easily be able to predict thereaction of the shell to a skewing adjustment of the rollers.
Given the information shown in this illustration, which way is this kiln turning, up ordown?
Fixed Plain Sleeve Bearing with Thrust Buttons on the End Caps.
CHECKING AND DOCUMENTING THRUST
Checking the thrust on a housing that has the thrust buttons in the end caps is prettysimple. Using a 3 or 4 LB. hammer, and lightly striking the end cap on or near thecenter, will produce one of two different tones. One is a hollow “bong”, or empty sound,which indicates that this end cap has no load on it. The other sound is a very solid, high“ping” like striking an anvil, indicating that the roller is loading up against this end cap.This style of roller is considered a “pusher”. When thrusted, the shaft will load up against
one end cap and push the kiln in the opposite direction. For example, if the uphill endcap sounds hollow, and the downhill end cap sounds solid, the roller is positioneddownhill and is pushing the unit uphill.
Remember to sound both end caps, even though the first one you strike may produceone of the distinct sounds mentioned above. If the roller is midway in the bearing this willcause both ends to sound hollow.
DETERMINING THRUST DIRECTION BY ROLLER POSITION (Type I Housing)
Both uphill and downhill bearing housings are keyed into the bases such that the spacebetween the thrust buttons is ¼ - ½” or 6 - 12 mm larger than the length of the shaft.This allows the roller to have that much axial float. When the roller is skewed to drive theshell slightly uphill, its reaction is to slide downhill. The normal and expected position forall the rollers is to be in contact with the downhill thrust button.
Sleeve Bearing, Self-Aligning with Thrust Collars on the Shaft
The thrust collars are located on the ends of the shaft or on the shoulder of the shaftnear the roller. Visual inspection through the inspection ports of the housing allows us tolocate the gap. This is the gap between the thrust collars and the thrust bearing.
With the thrust arrangement as shown above, the normal expectation is to have the gapon the downhill end of the shaft. This indicates the roller is positioned downhill and ispushing the shell uphill.
DETERMINING THRUST DIRECTION BY ROLLER POSITION (Type II Housing)
The normal and expected position of all the rollers, if slightly skewed to push the shelluphill, is to position itself downhill. With Type II style bearings we then expect to see nogap on the uphill side and a ¼ - ½” or 6 - 12 mm gap on the downhill side. Tapping theend covers on this style of bearing housing does not tell us anything.
Determining thrust on Type II style housings is a matter of removing the inspection ports
and examining the position of the roller. When the ports are removed you will see whereone thrust washer is tight by noticing that oil has been wiped clean from its surface. Thiscan only be seen on the roller on the down turning side of the shell. The other shouldshow a gap in which the oil runs freely over the thrust washer. This type of roller isconsidered a “puller”. This means that the shaft will move until it seats against the thrustcollar.
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OPEN
Notice the gap between the thrust collar and washer.
TIGHT
Notice how the thrust washer and collar are tight.
Spherical Roller Bearings (No separate thrust bearings)
This is the most difficult type of bearing to deal with for setting skew. The previous styleof bearings are specifically designed to utilize the “action-reaction” phenomenon of skewby allowing room for a small amount of axial shift. That ¼ - ½” or 6 - 12 mm float isessential for setting skew correctly. With spherical roller bearings there is noaccommodating float to show us skew direction. Spherical roller bearings are mostlyinstalled on smaller faster-turning units. Faster turning means a proportionately higherthrust for any given skew. Unfortunately these bearings have a low tolerance for thrust
load. Consequently we see a much higher failure rate with spherical roller bearings ascompared to Type I and Type II bearings.
There are however a significant number of full sized kilns that also use spherical rollerbearings as shown here. Making fine roller adjustments for a close determination ofskew must first be done by very careful measurement. After that, long term observationof the surface condition is the final guide.
The biggest draw back in using spherical roller bearings for support rollers is that we
cannot take advantage of literally seeing the roller move axially when it is adjustedthrough the neutral position. The sleeve bearing design commonly used in large kilnspurposely allow about ¼ to ½ “ axial float so that the neutral position can be establishedby simple adjustment.
With spherical roller bearings careful measurement will get us very close, often within 10to 20 thousandths of an inch with respect to neutral. After that we have to use visualand tactile observations to establish weather further fine adjustment is needed.
The safest method is to measure to the shafts using a framing square fitted with atorpedo level to a piano wire as described in the 2-pier alignment procedure. Betterwould be to make these measurements optically. That simply replaces the wire.
Laser equipped levels are now available with a magnetic base. These are quite handyto measure the roller position with respect to the tire using a small scale as shown.Point the laser in two directions as shown and repeat with three or four different kilnpositions and at each side of the roller. The more measurements are made the moreaccurate the results will become.
All these procedures can only be safely done with the unit shut down.
Spherical Roller Bearings (No separate thrust bearings)
This is the most difficult type of bearing to deal with for setting skew. In contrast, sleevebearings are specifically designed to utilize the “action-reaction” phenomenon of skew byallowing room for a small amount of axial shift. This feature is not available withspherical roller bearings. These bearings also have a low tolerance for thrust load.Consequently proper and limited thrust adjustment is required.
By fixing a dial indicator as shown, thrust load may still be detected since there usually issome axial play in the bearing assembly. Any thrust load will tend to load the bearingslightly. Since the tire will never run true axially this wobble is seen with each turn of thedrum by a constantly swinging indicator reading. The greater the thrust load, the greaterthe amount of movement. By making small roller adjustments, if that’s possible with theunit running, it is usually possible to detect whether the roller is favoring a load uphill ordownhill. The roller should never be adjusted to push the drum downhill. A properlyadjusted roller will thrust the unit lightly uphill necessitating that the roller will be pushedslightly downhill.
Small adjustments, a few thousandths of an inch, if they make the roller axis cross theneutral point, will show up on the dial indicator when positioned as illustrated.
Phillips Kiln Services has solved the problem of fine tuning roller skew whenever
spherical roller bearings are used with the introduction of the Thrust Monitor.A sensor is attached to one of the two bearings supporting the roller shaft. This sensorwill send signals indicating the exact amount and direction of any thrust developed as aresult of rotation. The Thrust Monitor does require considerable preparation to functionproperly. The tires should be freshly ground flat and be free of any type of lubricant.The rollers should also be new or freshly ground and be free of lubricant as well. A fullalignment must then be done in the conventional manner with particular emphasis onattaining the correct roller slopes by careful shimming if necessary. The calibrationprocedure of the Thrust Monitor is then most conveniently done as part of thealignment/shimming work since lifting the drum free of the roller is a necessary step to
define the condition of zero skew.
The sensors must be mounted on the bearing with the fixing ring, which is usuallymounted in the downhill bearing.
As there are at least 4 rollers there will be at least 4 sensors, one for each roller. These
are cabled to a central control unit. A computer is connected to the control unit and thethrust levels can then be observed and monitored on the computer screen numerically orgraphically.
For permanent installations the sensor signals are carried via Ethernet to a web port andso the data can be monitored remotely.
The Thrust Monitor is particularly well suited to rotating drums supported by rollersfitted with spherical roller bearings because it is the only way that thrust can be
measured on such assemblies.However axial thrust forces are developed by a skewed roller no matter which type ofbearing is employed so the Thrust Monitor can be employed in all cases where thrustmeasurement is of interest.
There is a great variety of process equipment who’s mechanical integrity can besignificantly enhanced by maintaining minimum and balanced thrust on the supporting
rollers.In almost all cases where heavy metal flow on the rolling surfaces occurs or wherebearings fail or where thrust rollers fail, improperly skewed or heavily skewed rollers arethe cause.
Improper skew very often results from roller adjustments made to improve face contactwhen the rollers are not on the correct slope. Anytime a roller is off its required slope,adjustment for improved face contact creates axial thrust. Stated another way; anytime a roller is off slope, a neutrally set roller can never have full face contact.
This condition is by far the most common cause of mechanical problems with rollers.
Use of the thrust monitor will immediately identify this condition.
When the unit is said to be “floating” it means that the the natural pull of gravity on theunit down the slope is exactly balanced by the combine force of the skewed rollerspushing the shell up. The forces from each of the rollers should be equal. This floatingcondition is of course transient since the thrust from skewing varies according to shellspeed and the load in it. This means that occasionally the upper thrust roller will berotating but more often we expect the lower one to be rotating. Looking at the relativeshininess of the upper and lower thrust rollers tells us much of what is going on.
Look at the six units above. Unit #3 has a rusted lower thrust roller. Obviously the rollers
are skewed too aggressively. At the other extreme is two, five, and eight where theupper thrust rollers never seem to see contact.
Attitudes and the level of respect given the equipment is largely dictated by generalhousekeeping. The biggest problem is usually oil or water mixed in with product thatcontaminates (sometimes buries) part of the base assemblies. If pier tops are coveredby spent oil or buried in spilled product the expectation of good maintenance is largelyundermined.
Clean up as required, prepare and assemble all the necessary tools to do the work.
A very important step is to assure there is a meaningful way to measure bearing oiltemperature. If a problem should arise chances are that the first indication will be arapid rise in bearing oil temperature. Monitoring all the sump temperatures through out
If there is a history of hot bearings or if problems are anticipated for whatever reasonsbe prepared to deal with the situation of hot bearings. For units with sleeve bearings If a
problem arises as a result of roller adjustments hot bearings are usually the first in thelist. Be prepared.
The problem is either excessive thrust where the thrust bearing heats up or more usualthere are grooves in the bearing shaft and brass sleeve which prevent smooth axial float.
If there is excessive thrust graphite powder can be applied liberally to the roller face.This will relieve all thrust and may allow the trunnion to cool. The graphite must becontinuously applied until counteracted moves can be made, and the trunnion can be putinto a position where it will run cool.
Do not blow air into the housing through the inspection port. This may cause anexplosion.
It is not recommended that water be run directly on the trunnion face. Make sure water isflowing freely through the cooling jackets.
Usually the best method of cooling is to use a water to oil heat exchanger or oil cooler.
These are readily available commercial units. The suction side of the pump isconnected to the oil drain on the trunnion. The oil is then dispensed onto the top of thetrunnion shaft through the inspection port. It is also recommended to add an oil filter.Caution must be used to keep the filter as free-flowing as possible.
If a bearing is known to be a problem synthetic oil should be used before any moves areattempted. Synthetic oil retains its viscosity to 450°F [230 °C]. If a petroleum oil is beingused be prepared for the possibility of having to change oil “on the fly” to a synthetic tosustain a high rise in temperature. Some synthetic oils are not compatible withpetroleum oils. The changeover must be total without cross contamination. Continueflushing with synthetic until the change is complete.
If none of these methods bring the temperature under control, bearing failure isimminent. Prepare to slow and stop the kiln. A slowed kiln may allow the problembearing to “seat-in”. However an overheated bearing is damaged and cannot repairitself by continued operation. Even if the temperature is brought under control thesituation can redevelop at any time. The sleeves and roller need to be change at firstopportunity.
Moves can be properly recorded using a dial indicator, one for each bearing assembly,positioned at the backside of the bearing as shown. Often the magnetic bases for the
dial indicators are inadequate to hold the indicator reliably over the course of anadjustment campaign. Weld brackets to the base and use clamps to hold the indicatorsfor 100% reliability.
Adjustments using the “flats” of the adjustment screw is good enough for “ball park”adjustment but must never be relied on for recording the actual moves made. Thebearing housing may take some time to seat in. Leave indicators in place for as long as24 hours after the last adjustment, before recording the final bearing position.
Only one person should be given authority to have moves made. He should providewritten instructions as to which bearing should be moved and in what direction.Personnel making the moves should record, date and sign a record of their work.Inspection sheet as shown here for example is an efficient way to do this. These sheetsshould then be kept in a log book. A running history of roller adjustments is necessary tomaintain control of the mechanical condition of the kiln.
If multiple moves on one roller are anticipated this sheet needs to be accompanied by atable on which moves, times and temperatures can be listed.
Once preparations are complete the first task is to record the temperatures on allbearing at all piers. An adjustment on one roller changes the overall thrust on the kilnand can cause it to shift with the potential of causing a hot bearing on another pier.Recording all bearing temperatures frequently throughout this process is required.
The first move would be a small one of about 0.5mm (0.020”) in one bearing. Thebearing first moved would be the one which would bring the roller closer to neutral. Waitabout 20 minutes until the roller has had a chance to shift. This is also enough time torecord all temperatures again. Trouble can be identified by a temperature rise anywhere,not just in the bearing being moved. Assuming that the shaft journals and bearings are innormal condition and no temperature rise was encountered, these steps would berepeated as necessary until the roller shifts position. The roller’s shifting positionindicates that the neutral point has been crossed. This is the most critical aspect of thewhole procedure: to get the roller to shift position without any significant temperature risein any of the bearings.
Once it is seen that the roller shifts easily without a temperature rise, then the size of themoves can be increased to say 2mm (0.80”) per bearing. The sequence of the movesshould alternate from one bearing to the other with the shaft sliding across with eachmove. Waiting 20 minutes between moves is also unnecessary as long as the shaftshifts easily with each move. The work can continue smartly providing there are no othermitigating circumstances like a bowed shell, or a rise of oil temperature anywhere etc.This continues until the average of the moves for both bearings reaches the desiredtotal, 15 mm for this example. The final moves should be very small ones to leave theminimum amount of skew on the roller.
Even the largest rollers, and there are some as large as 10 feet (3050 mm) in diameter,will respond quickly to a 0.10mm (0.004”) skew adjustment.
Naturally all the work must be monitored with dial indicators and must be done with theunit in operation.
This procedure is used with units that have sleeved bearings. See “Two-Pier Alignment”
for the procedure using spherical roller bearings and pillow blocks. The principle of rollerreaction is always valid even though thrust direction is not seen by axial roller shift.Secondary techniques need to be used.
Toed-in rollers can balance each other, one pushing the tire down as hard as the other ispushing it up. If situation exists the shell may be “floating”. Floating means that theshell is balanced between thrust rollers. But it is not the desired situation. Leftuncorrected there is unnecessary wear and tear on the whole support. Left for longperiods of time the tires and rollers will wear into a cone shape.
How do you quickly identify if a roller is skewed?
Look at the surface. A roller with little or no skew will polish up to a mirror finish. If thesurface is dull and gray and appears rough, then skew is present and probablyexcessive.
Rollers should be parallel to each other, set in the same direction on all piers. Oftenmeasuring between bearings or shafts, as “a” above, may reveal that they are paralleland not toed-in as in our previous example. Unless these measurements are tied into acommon reference line, the above situation will not be identified.
Once again this situation could be present with the shell “floating”. Assuming from thatobservation alone that all is well, will lead to excessive wear and tear of all the supportcomponents. Careful inspection using the simple techniques already described will showthis immediately.
Unfortunately many designs require that support rollers be skewed. The thrustmechanisms of these designs are inadequate to support the entire downward thrust ofthe shell. This is especially true of large long rotary kilns. Since most of this type ofrotary trunnion-supported equipment is installed on a slope, there is a natural componentof force acting in the axial direction of the shell. If this force cannot be completelymanaged by the thrust mechanism(s) it is the skewing of the support rollers that musthelp out.
Skewing is a compromise. Skewing accelerates the wear and tear of the supportmechanisms but then allows smaller, less costly thrust mechanisms to operatesuccessfully. If skewing is insufficient the thrust mechanisms will fail prematurely. Ifskewing is excessive additional wear and tear of the support components takes placeand the thrust mechanism can still fail. If rollers are skewed against each other, wearand tear takes place but the advantage supposedly gained by skewing is lost.
The maximum performance life of rotary equipment that requires skewing, can only beachieved by skewing correctly and keeping it to a minimum.
The amount of skew shown in the illustration may be sufficient for most installations
• Distributing the unit’s thrust load evenly, so one trunnion is notworking any harder than another, reducing the wear rate.
• Distributing the load across the trunnion face equally among all of the rollers sothat tire and trunnion wear is reduced.
• Reduction in stresses on the tire and its support components.
• Possible reduction in the unit’s main motor load. This will reduceelectricity consumption and save money.
For a fixed amount of skew the resultant thrust force varies with:
• Load. The heavier the shell, the harder it bears down on the rollers the more frictionforce develops. Lightly loaded the shell tends to sit downhill. The heavier its loaded
the more it tends to run up hill.• Speed. The amount of thrust developed is directly proportional to speed. A slow
running shell will tend to stay downhill. The more it is sped up the more it will tend toclimb uphill.
• Surface lubrication, temperature and ambient conditions, anything that will influence
