FAILURE IN ROLLER SHAFTS IN SUGAR

MILLS

AMAN TAKSHAKU-73892535

GRADUATE STUDENT,MECHANICAL ENGINEERING

WHAT ARE ROLLER SHAFTS?• Low carbon steel shaft used to crush raw material sugarcane to smaller

volumes of consumable sugar

• Driven by an A.C. powered motor of calibrated power.

• In Sugar Mills, roller shafts separate sucrose-containing juice from the cane extraction of juice. Hazards of sucrose are-

Eye: Dust may cause mechanical irritation. Skin: May cause skin irritation. Low hazard for usual industrial handling. Inhalation: Excessive inhalation may cause minor respiratory irritation. Chronic: Chronic inhalation of fine dusts may cause lung damage

• The power delivered is via tangential force and resultant torque or twisting moment setup within shaft.

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CONDITIONS LEADING UPTO SHAFT FAILURES

• The low carbon steel shaft was first installed replacing a shaft in which cracks were observed near the ends.

• A crack was observed in the center of the roll.

• Since no replacements were available at the time, welding was used to repair the crack using 17-4PH steel.

• Nine days later, the shaft broke on the key-way side while the machine was being set up at low speed. The roll usually operates at 630 RPM.

• https://www.youtube.com/watch?v=Y-ylOY4v6As.

FAILURE HOW?

Photograph showing fracture initiation sites, benchmarks from fracture propagation Photograph of shaft surface

indicating weld overlay flaw.

MICROSCOPIC ANALYSIS OF THE SHAFT

• Micrograph of core microstructure composed of ferrite and pearlite.• Micrograph of weld overlay microstructure composed of a pearlite matrix

with the presence of acicular ferrite.• Micrograph representing average inclusion content of the low carbon steel

core.• Micrograph showing two inclusions found in the weld overlay• Micrograph showing the fracture surface initiation site

FAILURE CRITERIONMODES OF FAILURE

DUCTILE Cup and ConeDimples

Dull SurfaceInclusion at the bottom of the dimple

BRITTLE INTERGRANULAR ShinyGrain Boundary cracking

BRITTLE TRANSGRANULAR ShinyCleavage fractures

Flat

FATIGUE Striations (SEM)Initiation sites

Propagation areaZone of final fracture

ITS FATIGUE

POSSIBLE REASONS FOR FAILURE• Endurance limit of the metal reached a maximum point• Non-reliable power source leading to irregular stress amplitudes• Faulty design• Incorrect welding procedure• Avoid bending of shafts that have been surface hardened or had weld

overlay applied due to the high possibility of inducing surface cracks.• Mechanical damage to the surface such as scratches because they can

act as crack initiation sites.

EMPIRICAL CALCULATION • Endurance limit is maximum amplitude of completely reversed stress

that a specimen can withstand• Fatigue life is number of stress cycles specimen can endure before the

first crack appears.• Considerations of different moderating factors also known as De-rating

factors Surface finish factor K(a) Size factor K(b) Reliability K(c) Stress concentration K(d)S(e)=K(a)* K(b)* K(c)* K(d)*S(e)’S(e)’= endurance limit stress (theoretical)

CALCULATIONS FOR EXISTING LOW CARBON STEELDIAMETER FOR SHAFT=760 MM

DENSITY=7.87 g/c3

STRESS CONCENTRATION FACTOR= 1.6

REVOLUTION PER MINUITE (N)= 4 RPM

POWER REQUIRED TO OPERATE ROLLER SHAFT= 650 TonsTORQUE= HORSEPOWER*4500/2*3.14*N= 119963 kg-m

Endurance limit comparison• Endurance limit for 4340 steel= 450-460 M Pa

• Endurance limit for 1080= 270-280 M Pa

Loads appliedBoundary condition: - As roller is simply supported so all degrees of freedom of roller are fixed at the bearing position. Loading detail: - The vertical component of loads due to crushing are applied on roller shell as uniformly distributed load. Total hydraulic load of shell is 760 ton. The Hydraulic load is applied at the bearing position which is 380 ton each side.

ADDRESSING FAILURE IN SHAFT• COMPOSITION IMPROVEMENTI. Strengthening mechanisms at microscopic levels in case of sudden

failure better than weldingII. Discovering some new alloying options to increase fatigue strength

• DESIGN IMPROVEMENTSI. Decrease scope of failure initiation sitesII. Providing proper fillet conditions

ADDRESSING FAULTY DESIGNINTRODUCING TEST PROCEDURESRadiography testing• Measures differences in radiation absorption.• Inclusions, Porosity, Cracks

Ultrasonic testing• Uses high frequency sonar to find surface and subsurface defects.• Inclusions, porosity, thickness of material, position of defects.

COMPOSITION IMPROVEMENT• At microscopic level, Precipitation hardening can lead to phasing out

of the excess pearlite.• Adding metals such as Boron and Cobalt as alloys increases the

machinability of 4340 steel making it less prone to excess weld• Aluminum acts as a perfect de-oxidizer and cuts off oxygen supply so

as to prevent weld overlay • Silicon content of more than 1% can facilitate pearlite formation in

the microstructure leading to failure

DESIGN IMPROVEMNTS EXISTING PRESENT DESIGN LOW CARBON STEEL THERMAL ANALYSIS OF LOW CARBON STEEL ALLOY

IMPROVED DESIGN OF RECCOMENDED DESIGN THERMAL ANALYSIS OF 4340 STEEL

CONCLUSION• Avoid mechanical damage to the surface, such as scratches and dents,

because they can act as crack initiation sites.• Use of another steel with better endurance limit and higher stress

cycles• The failure was caused by high cycle low stress fatigue, which was

initiated at inclusions in the weld overlay.• This would have increased local stress concentrations and the

possibility of crack initiation. These inclusions probably originated from the weld being applied too quickly. • Use of a better welding technique or a less time consuming and lucid

strengthening mechanism.

Q&A