05 March 2011

IMPROVEMENT OF BREAKER OFFCENTERING


IMPROVEMENT OF BREAKER
OFF CENTERING


By


RENJITH. P.R
 


          
STUDY OF OFFCENTREING OF BREAKERS

BREAKERS:

            These are short plies with low EPI (ends per inch) cut at an angle and are positioned centrally between tire casing and tread to strengthen carcass against impact.  They also provide cushioning effect and increase the modulus of tread area.  They are made by rubberizing dipped fabric on calendar and then cutting them by bias cutting machine. Breakers are used mainly in biased tires where as in radial tires they are replaced by steel belts.

            During rotation the carcass transmits all the forces on to the ground through the tread at contact patch.  The tread being a rubber mass undergoes considerable deformation while transmitting these forces as well as engaging on the road deformities.  This deformation of tread cause excessive stress strain concentration at tread to carcass interface and consequent premature failures.  In order to dilute this concentration, a graded structure is opted for biased carcass in which the outer most layer in contact with tread are low modulus suspended plies, or breakers with high gauge of rubber insulation (Low EP (840/2) followed by intermediate layer and real pressure bearing inner plies.

            The EPI (ends per inch), i.e. number of cords per unit area is much lower than carcass plies. This helps for rubber compound penetration between the cards which help in achieving better bond between tread and carcass as well as it provides impact resistance.

           


In general the function of breakers can be summarized as

i)                   It provides stability to the tread and helps in distributing forces uniformly throughout the tire structure.
ii)                Breakers hold the tread firmly to carcass and also help in providing impact resistance to tread region.

OFFCENTERING
Off centering is actually the displacement of breaker from the central position in a band or a tire building drum. This defect can occur mainly during two stages of tyre construction; they are band building and tire building.
Off centering of breakers in tires can lead to;
1) Tread shoulder separation
2) Reduction in tyre life due to early failures.

To study about off centering of breakers mainly we deal with 2 departments

1)                 Band building department
2)                 Tire building department

Band building
In band building department the components of carcass, i.e. the inner liner plies and ply squeegees are assembled to form bands.  Usually for a truck tire mainly 3 bands are employed.

First band – it consists of the inner liner, plies and ply squeegees.  First the inner liner is applied to drum followed by 3 plies with 3 ply squeegees in between.

Second band – It consists of 3 plies with 3 ply squeegees in between

Third band – it consists of 2 plies, 2 ply squeegees and 2 breakers.

            Band is usually constructed in a band building machine.  Band building machine consists of a metallic drum driven by a shaft, containing a canvas that is held by a tension bar beneath it. During construction of bands the plies are placed in opposite direction. This is same in case of breakers also. The placement of breakers in 3rd band should be in such a way that distance of breaker from both ends of ply should be zero or less than 5. When the difference is more than 5 which is the tolerable value the band is said to be offcentred. 

Off centering of breaker in band building machine can occur mainly because of

1.         Machine related problems like

a)                 Canvas shift
b)                 Tension bar alignment
c)                 Light setting vibration
2.         Material related problems like

a)                 Variation in width of incoming material
3.         Worker related problems like

a)                 Error in adjustment of light setting
b)                 Stretching of plies and breaker during band building
Tire Building

            In tire building process all the components of a tire are assembled to make a raw tire called green tire. The building machine consists of a metallic drum, which is mounted on a drive shaft. The ends of the drum are flanged to suit the bead configuration of the tire to be built. In all cases the overall diameter of the drum exceeds the nominal tire diameter. This difference is known as drum crown height and varies from tire type & size. Radial ply tire require complex and costlier machinery having inflatable textile reinforced diaphragms, overlying a skeletal metal drum shell, to shape the carcass plies and other components up to the diameter for belt fitting.
 
                                      In tire building machine there is located a bead carrier ring on either side of the drum. This ring is concentric with the drum and is capable of moving inwardly to provide an interference contact with the drum shoulders and there by transfer and consolidate the tire beads against the partly build casing structure. In the carrier ring frame, there are inbuilt spring steel fingers, which forms a circle turn the plies down the contact shoulder of the drum immediately before the beads make contact.  Attach to the building machine base plate and to the rear of the drum are two pneumatically operated component consolidating assemblies each comprising two pairs of shaped disc rollers. The rollers of one assembly are located on either side of the drum and pivot around the tire bead area for the purpose of turning and interlocking various components around the bead. The pair of rollers forming the second assembly traverses laterally, out word from the central line of the drum to consolidate the sidewall elements of to the carcass.  In case of ply building system servicer is placed at the rear of the machine. More common is to build bands of plies before building tire. Bands with 3 to for plies are assembled together on band building machine. No of plies in a band depends upon the service requirements. No. of bands in a tire also depends upon the service requirements. The diameter of the band is kept lower than that of the drum.
                
                                            Drum is collapsed, the two beads are placed on the carrier rings and the drum is expanded. A rubber base adhesive is applied to the drum shoulders. First band is then slipped over the drum under rotation with the help of the stick to apply pressure at the edges. The bead carrier ring assemblies then automatically turn down the ply edges around the drum and consolidate the beads.   This is followed by the other bands and the second bead, in between the ply edges are turned around the beads. Then the chafer strips are applied and down with the ply edges. After this tread is applied and consolidated followed by applying of side walls on both sides. Final consolidation is done by traversing rollers. On completion drum is collapsed and green tire is removed from the building machine.
                             In case of radial ply construction (which differs from biased ply construction in the angle of orientation of chord in the plies) different approaches are followed. Earlier building and shaping operations were carried out on different machines. But in modern industries both the operations are carried out in the same machine.

           
Offcentring of breaker in tire building can occur mainly because of 

1.         Error in light setting
        
             



REASONS FOR OFFCENTERING IN BAND BUILDING

Machine Related  problems.

1.         Canvas Shift

            In a band building machine, bands are constructed over an in extensible canvas that holds the tension bar and the drum. Depending upon the nature of bands to constructed different canvas is used. The canvas lies holding the building drum and the tension bar. During band construction first the canvas is inserted over the drum by keeping the tension bar in closed position. After insertion on the drum, the tension bar is lowered; this keeps the canvas held tightly over machine. Now plies are rolled over canvas and then made into a band.  For the proper construction of a band it is necessary that the canvas should have the same perimeter in both the side (machine side and operator side).  In case there is variation in perimeter in both side (i.e., difference between perimeter in both sides is greater than 10) then canvas shifting will occur.  I.e. canvas during construction of band will move from one side to another over the building drum.  (That is canvas moves from area having lesser perimeter to be having more perimeters).  In such cases when the builder places the breaker on to the drum, over the plies this shifting movement causes breaker to be placed in such way that distance of breaker from ply ends unequal.

            When the canvas shifts whole ply which is placed over the canvas also shifts then builder places. The breaker onto the ply according to the light setting which is incorrect then.  The result will be huge difference in distance of breaker from both ply ends.  The region of canvas with higher perimeter will have distance of breaker from ply ends greater than the other region of canvas with lower perimeter.  The ultimate result will be off centering of breaker.


b)        Tension bar alignment
Every band building machine consists of a iron bar with a rubberized coating over it placed below the building drum.  The main function of tension bar is to hold the canvas in position. The tension bar is usually placed as a cant lever.  Now the alignment of tension bar is critical for production of an okay band.  The correctly aligned tension bar will have same distance from building drum with canvas in open position that is difference in distance between tension bar and building drum should be zero.  If the difference exceeds 10mm then it will put more pressure on 10 the canvas inducing canvas shifting. This will lead to off centering.

c)         Light setting vibration

Every band building machine consists of a light setting (which consists two to three laser beams)

Three laser beams are placed with respect to the edge of band building drum

1.                  First beam for correct placement of plies.
2.                  Second and third for correct placement of I and II breakers worker places the breakers and plies to form a band based on this light setting if there is vibration in this light setting because of some kind of machine problems then it will induce the worker to make an off band or offcentred band. Since vibration causes variation in distance set on drum.



2.         Material related problems

            a)        Variation in Incoming material

                        Fort he purpose of making an ok band, it is necessary that the plies and breakers should have some specified dimensions.  When plies and breakers are concerned width forms main area of concern.  It is based on this width that light setting in done in band building machine.  For example is apply is having a width of 845mm, first breaker – 415mm and 2nd breaker 210 mm the light setting is adjusted in such a way that distance of first breaker from ply ends is 317.5mm and that of 2nd breaker is 215mm.  But this speficaition work only if width of all these components comes in the tolerable range (ie ±5m).  Whenever the width of breakers and plies exceeds more than 5mm, it becomes difficult to centre the breaker correctly on the band building drum.


[Even if builder changes to set the breaker correctly on onside because of incoming material width variation the distance of breaker may very badly on other side, leading to off centering]

            The breakers and plies are given a tolerance of ±5mm when the width exceeds 5mm or goes down below 5mm it may lead to off centering.  In coming material variation occur mainly due to problems in bias cutting.

3.         Worker related problems
            i)  Error in light setting adjustment
                        Each band building machine consists of a light setting comprising of three laser beams over the drum. Three laser beams are provided over the drum with respect to drum edges.  First beam for placement of ply, 2nd and 3rd for placement of breakers. For the purpose of building a correct band it is necessary that this light setting adjustment is done correctly by the builder.  When the builder creates some error in this light setting the result will be variation from specification, which will ultimately lead to an offcentred band. 

2)        Ply stretch during band building

            Since plies and breakers are rubberized fabric they are likely to be deformed when they are excursively stretched when plies or breakers are stretched it will lead to reduction in width of both plies and breakers.  When width reduces below a tolerable limit say 5mm, it becomes impossible to centre the breaker on the band building drum. The ultimate result will be correct reference distance on. One side of the band and incorrect distance on other side.  (That is huge difference in distance between breakers from ply ends)

REASONS FOR OFFCENTERING IN TIRE BUILDING

            In tyre building the only chance of error likely to occur is in light setting.
After bands are made they are inserted on to the tire building drum for making green tire. During insertion of band care should be taken to see that the breaker comes exactly in the centre of the drum i.e., distance of band edges from either side of building drum should be equal. In order to center the band perfectly, light setting is provided at both the ends of the building drum. Light setting consists of a laser beam fitted on to the top of a steel rod placed parallel to axis of building drum.  For the perfect centering of band on the tire building drum the edge of band inserted should coincide with the laser beam. As the distance of band edge from light setting increases off centering also increases. 
ACTIVITIES

BAND BUILDING
Studies for off centering were carried out on 4 band building machines building the 3rd band.

1)           Machine related problems
a)           Canvas shift
i)             All the machines were examined for any kind of technical defects which could affect off centering.
ii)          Perimeter of canvas was taken in both machine side and operator’s side. [Dimensions are measured after removing canvas from the band building drum]
iii)        Shifting of canvas for 3 revelations of drum are noted.  (3 revelations forward and backward)
b)           Tension bar alignment
Distance of tension bar form the band building drum is noted with canvas and without canvas.  3 check points are made i.e., perimeter of canvas in operators side, middle and machine side).  The distance between drum and tension bar is noted in closed and open position without canvas also.
c)      Light setting vibration
i)      Light setting is checked continuously after every 3 bands made.
ii)     Material related problems

-                     Breakers and plies from bias cutting departments are continuously checked for width variation.
                  For every breaker and ply both upper and lower part are examined for width variation.  Cut to cut variation is noted.

3.      Worker Related issues

a)     Light setting variation

Error in Light setting is a worker related issue so every worker is examined for any kind of variation in light setting.  So each worker is examined for correct light setting every machine is surveyed for minimum 3 workers.
b)     Stretching of plies and breaker closing band building

- The process of band building requires some kind of force to be applied on to the plies and breakers for the purpose of making a band.  Plies from conveyer are pulled on to the building drum to form the band.  The breakers are also applied in the same way. To see whether stretching has any affect on off centering first points are marked on the ply before it is made into a band and its width is noted.  The dimensions are again checked after the ply is made into a band at the same point marked. The same procedure is carried out for breakers also.
Tire building

            Light setting validation is done in tyre building section studies conducted include:

            Centering of breaker on tyre building drum.


 
RESULTS

Band building

The following conclusions were made from the study
.
1) Incoming material width variation


a) Variation in width of incoming ply material were found to be 7mm more than the specified value.

b) Variation in width of incoming 1st breaker was found to be 10 mm more than the specified value. Rare cases recorded shows deviation of 8mm lower than that of specified value.

c) Width of 2nd breaker was found to vary badly when compared to plies and first breaker. The deviation of actual values from the specified values was found to be about 10 mm.
The above mentioned variations were sufficient to cause breaker off centering.

2) Machine related problems

a) Survey was conducted on four band building machines.
Out of four machines two machines showed higher canvas shift of about 12 to 15mm which is very high.
This data pinpoints the role of canvas shift in off centering.

3) Worker related problems

a) Ply stretch during band building

Stretching of plies during band building could cause only around 2mm to 3mm reduction in width which is negligible.
Both first and second breaker width was found to reduce by 2mm to3 mm during band formation
Studies conducted indicate that ply stretching has no significant role in off centering of breakers in band building.

b) Error in light setting

Light setting variation for first and second breakers was checked for 6 operators.
For first breaker, out of 6 operators, 2 to 3 operators were found to vary reference distance by more than 5mm from spec value.
For second breaker 1to 2 operators were found to vary reference distance by about 5mm more than specified value.
The above data clearly indicates the error in light setting which shows that it has a significant role in breaker off centering.

Tire building

Light setting validation was done in tire building section.
Data collected from tire building did not show any kind of variation
From the above data it could be concluded that only source of off centering issue in tire building is off centered bands

 
CONCLUSIONS


The following conclusions were made from the studies conducted


BAND BUILDING

Ø  Incoming material variation seemed to be the main material related problem that caused off centering of breakers

Ø  Out of all machine related problems canvas shift proved to be the main reason for breaker off centering.
Ø  Other machine related issues like light setting vibration and tension bar alignment did not have any effect on off centering.

Ø  Main worker related problem that caused offcentering of breakers was the error in light setting
Ø  Other worker related issues like stretching of plies during band building did not have any significant effect on off centering.

TYRE BUILDING

Ø  Data collected from tire building did not show any kind of variation
Ø  From the above data it could be concluded that only source of off centering issue in tire building is off centered bands

Ø So the best possible option for improvement of breaker off centering is to reduce the above mentioned problems.



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