GEAR FAILURE

In the process of designing a gear, maintenance needs to be taken in order to prevent the failure of the gear. Failure of gear may take place in various types or modes.

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The failure of the gear takes place in five types they are

  • Wear
  • Scoring
  • Plastic flow
  • Pitting
  • Tooth fracture
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Wear:

Wear happens in the tooth which means damage is observed in the tooth. The damage occurs in the form of metal removal in the tooth or less consistently from the surface. This is none other than removal of metal from the surface. Therefore the tooth of the gear is weakened. Wear in the gear tooth is caused by the lack of oil film, there must be a metal to metal contact in this case, abrasive particles present is allowed to mix with the oil; due to oil composition there happens to be a chemical wear.

wear in gear

Wear must be classified in to three types they are

  • Corrosive wear
  • Abrasive wear
  • Adhesive wear

Corrosive wear:

Corrosive wear]

Due to the chemical action of the additives and the lubricating oil the corrosive wear occurs. Due to the wear the tooth is roughened.

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Abrasive wear:

The abrasive wear is the main reason for the mis-carriage of the open and closed gearing of the operating machines are polluted by the abrasive materials. The wear must be changes and it depends upon the shape, size and machinery transportation.

Abrasive wear

Abrasive wear must be divided in to two types they are:

  • Server abrasive
  • Mild abrasive

Server abrasive

The server abrasive wear occurs due to the entry of the large abrasive particles into the lubricating system. And also the wear must be due to the higher particles concentration. In the direction of sliding, on the gear tooth the particles are provided to make the grooves on the gear surface. Due to the high wear it reduces the thickness of the tooth. So the weakness of the gear leads to the failure

Mild abrasive

When the dust particles enter into the lubricating oil, the particles that have entered are abrasive in nature then it leads to the failure of the gear. If the abrasive particles are very fine in size then the metal removal rate is slow. So it takes long time to notice the wear. It looks like a polished surface.

We can see the mild abrasive wear in the ore grinding mills and cement mills. Due to the entry of the dust particles into the lubricating system we can see the abrasions. Marks present on the surface must be disappears and the surface looks like highly polished. The wear occurs over for a long time. Sticking development and minor pressurization in the gear box with air reduce the entry of the dust particles and wear is reduced.

Adhesive wear:

It is difficult to identify the defect. In this wear occurs from since the beginning. The wear is low. It takes more time to notice the wear. Earlier to the full load diffusion, the gears run in various sections of full load, for a number of cycles. The surface points are suppressed over a stretched period of running and the surface gives a polished look. This is known as polishing wear or mild wear.

Abrasive wear

The adhesive wear must be divided into three types they are

  • Polishing wear
  • Moderate wear
  • Severe wear

Adhesive wear is also known as the polishing wear

Moderate wear:

Moderate wear

When the speed and load operations are higher than the mild wear conditions the moderate wear takes place with the high rate. Damaged sections look shiny and brighter. However it occurs over a long period of time.

Severe wear

The Severe wear occurs due to the metal to metal contact

Scoring

The amalgamation of two separate activities is known as scoring. The first failure is due to the lubrication in the contact area. The second failure is due to the contact between the metals. Tearing and the welding action results in the removal of the contact metal and continuous speed, oil temperature and load remain in the same level.

The scoring must be divided in to three types they are

  • Initial scoring
  • Moderate scoring
  • Destructive scoring

Initial scoring

The initial scoring happens at the high spots left by the previous machining process. The lubrication fails at the spots leads to friction and initial scoring. If the spots are removed, then the stress comes down as load is circulated over a large area. If the load, speed and oil system remains reduced or unchanged then the scoring is stopped. The initial scoring is unprogressive and has helpful action connected with it.  This is known as the initial scoring.

Moderate scoring:

After the initial scoring if the speed, oil temperature and load increases then the scoring is extended over to a large area. The scoring improves at tolerable rates. So this is known as moderate scoring.

Destructive scoring:

After the completion of the initial scoring, if the oil temperature, speed and load increase significantly. Then the severe scoring sets with high metal torn areas to spread quickly. Generally scoring is major over the pitch line area for lubrication in the Elasto hydrodynamic is the least at that section.

Plastic flow:

Plastic flow

Plastic flow in the tooth results in the high contact stress under sliding action and rolling. Due to the subsurface material or yielding of surface deformation takes place. Generally they occur in the softer gear material. In case of heavy loads it result the hardened gears. In case of bevel gears the tooth tip can be seen clearly in the cold flow materials. This is also known as the cold flow.

Plastic flow must be divided in to three types they are

  • Cold flow
  • Ridging
  • Rippling

The basic plastic flow must be known as the cold flow

Ridging

Ridging is known as the formation of deep ridges by plastic flow of surface and subsurface sliding. In the direction of sliding it shows the definite peaks and ridges or valleys across the surface of the tooth. This ridging is caused due to the high relative sliding and also high compressive stresses. On gear drives and hypoid pinions often present heavy loaded worn and worm wheel drives. This is to reduce the ridging by reducing the stresses during the contact time and also by increases the material hardness. During the process high viscous lubricating oil is used with extreme pressure additives.

Rippling

At the right angle a periodic wave is formed to the direction of the sliding, which is known as rippling. We can see this type of rippling in the hardened gear surface and it appears like a fish scale. It is a kind of plastic deformation or wear in a micro method with very thin oil film.

Pitting of gear:

Pitting of gear

Pitting is known as the failure of surface fatigue in the gear tooth. Mainly it occurs due to the frequent loading of the tooth surface and the contact stress is above the surface fatigue strength of the material. In the fatigue region material is removed and there is a formation of pit in the material. Due to the presence of the pit, which causes the stress concentration plus the pitting is spread over to the adjacent region. The pitting covers the whole surface.

Due to the high loads it resulting causes fracture to already weakened tooth. The failure takes place during the cycle of running.

The pitting is divided into two types they are

  • Initial pitting
  • Progressive pitting

Initial pitting:

The initial pitting is also known as incipient pitting. Mainly the initial pitting occurs during the running oversized peaks on the surface where it gets removed. Later, a small pit having 25 to 50 μm deep are formed below the pitch line region. On the large surface region the load gets distributed and the stress comes down, which may stop the growth of pitting.

For helical gears the pitting started with excess load due the small misalignment developed through the tooth in the dedendum share to mid face. Then the pitting must be stopped and surface began to polish and clean over. We can observe this in the medium hard gear which is a common phenomenon. Here, over the gears of the materials which are driven off the pits, wells etc. may get cased in the later stages after running in. This in reality has no influence on the presentation of the drive. This is due to the reason that, the well or the pit that are moulded slowly grow smoother by the rolling performance. The initial pitting is unprogressive.

Progressive pitting:

Progressive pitting is also known as destructive pitting. At the time of initial pitting the loads are great and it unable to destroy the pitting progress due to the high corrective action of initial pitting. So due the above reason destructive pitting takes place. All over the tooth distance pitting is spread. In the un-pitted surface high pressure may lead to pitting. Where pressing the lubricants into pits and lastly to gripping of the surface. On the tooth flanks the pitting begin and it is near to the line along the tooth passing. The tooth must be passing through the pitch where there are great friction forces due to low sliding velocity. Then the pitting must be spreads to the entire surface of the flank. In rare cases the tooth faces are subjected to pitting. So the destructive pitting spreads through entire tooth and it results in weakened tooth and it gets fractured at the tip leading to failure.

Spalling:

Spalling

The spalling is also known as flaking. In case of surface hardened gears the stress lies under the layer and this may lead to the surface fatigue. So it results in the spalling of the material from the surface.

Pitting – frosting

The frosting normally occurs in the dedendum portion of the driving gear first and then later on the addendum. The wear shape does not have regular metal polish but it has etched finish. Below magnification the surface discloses to a very fine micro pits of nearly 2.5μm. These shapes follow the greater ridges affected by cutter marks. Frosting effects from very high oil film and some sharp contact.

Tooth fracture

Tooth fracture

Tooth fracture is the dangerous kind of failure in the gear and it leads to the deactivation of the drive where it gets damaged frequently by the components like bearings, shafts by pieces of the broken teeth. Due to high over loads one can observe the tooth breakage, overload resulting in low cycle fatigue.

Tooth breakage

Tooth breakage

 

 

Over a long period of time the bending fatigue may take place. At the weakest points the cracks are seen initially in some cases, as they start at the teeth roots or the fillet where there is high stress developed with the high tensile strength from bending. The cracks are spread over to 80% to 90% of the life. The cracks are spread over in the tooth very fast and results to fracture. So the fractured surface exhibits the marks in the slow crack region and brittle fracture behavior in the sudden fracture region. So the time taken for the failure is low and it is known as the high cycle fatigue.

Tooth breakage: Bending fatigue

Tooth breakage Bending fatigue

Overload breakage or low cycle fatigue causes tough appearance in broken ductile material. In harder materials break are crystalline or silly exteriors.

Tooth breakage: high cycle fatigue

Tooth breakage high cycle fatigue

Breakage is often due to load attention along the tooth length, as a result of errors changes are observed at the machining and assembly or large elastic deformation of the shaft. The tooth wear results in the weakening of the teeth and it also results in dynamic loads. Shifting of sliding gear takes place without stooping the rotation of the shaft. Generally cracks are observed occurred at the root of the teeth on the sides of the stretched fiber where the highest tensile stresses occur together with local stresses due to the shape of the teeth. Mainly the fabrication occurs at the cross sections through the root of the teeth.

In case of the failure fatigue, fracture occurs at the concave form in the body of the gear, it looks like a convex shape due to the overload failure which can take place. The teeth of the wide face helical gears or herringbone gears usually break-off with the slanting cross section. To stop the tooth breakage, the gear teeth beam strength needs to be calculated. Fatigue pitting of the surface layers of the gear teeth is the most serious and wide spread kind of tooth damage. The gear damage may occurs when they are enclosed or protected against the dirt and also well lubricated

Gear noise:

Due to the several reasons the gear noise rises. Error occur at the contact point due to the presence of the gear profile, rolling and sliding, impact of tooth, churning of the lubricant, bearings and wind age. The method of the combating noise is: changing over to helical gearing, improving the finishing operation of the tooth, contact ratio must be increased, modifying the profile by flanking, equalizing the load along the face width of the tooth rim, improving the design of the housing and  covers, using crowned gears.

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