All you need to know about GEAR FAILURE AND HEAT TREATMENT METHODS

 

Gear is a simple rotating machine element that meshes with the other gear to transmit power over a certain distance. We will see some types of gear failures and heat treatment methods that prevent the wear and tear of gear and increases the fatigue life.

     

    Types of gear failures

·       SCORING FAILURE:

Scoring is a type of gear failure which occurs if the oil film fails due to overheating of the meshing parts and the metal to metal contact produces tearing of the parts which are in relative motion and removes the metal rapidly from the surface.

The criteria for selecting the best lubricating oil include choosing appropriate viscosity grade, additives, and base oil type also it should have anti-foam properties. For determining viscosity the ANSI(American National Standards Institute) or AGMA(American Gear Manufacturers Association) standard are the most common methods employed. After selecting the viscosity grade, the basic types of lubricants such as R&O (rust and oxidation inhibitors), Anti scuff (For high pressure) & compounded gear lubricants are chosen.

WEAR FAILURE:

Wear is the removal of layers of metal from the surface. This gear tooth failure occurs by metal to metal contact due to lack of oil film, abrasive particles, and chemical wear due to the mixing of oil and additives. Wear is further classified as:

Adhesive wear: Adhesive wear is caused by the fusing of parts in contact. This type of wear is hard to detect and is noticeable after millions of cycles. 

Abrasive wear: Abrasive wear is the scratching of the gear tooth surface which further leads to the creation of debris inside the system. This debris contaminates the oil and progressively damages the gear tooth surface.

PITTING FAILURE:

Any component which undergoes sliding/rolling contact under heavy loads leads to pitting failure. Pitting occurs due to repeated loading when the contact stress exceeds the fatigue strength of the material. Here also lubricating oil plays an important role.

Pitting leads to the removal of material after millions of cycles of running. At a certain amount of time the surface crack increases and causes stress concentration at that point & further leads to failure of gear. Foreign particles also cause the pitting failure of gear.

ABRASIVE WEAR:

        When abrasive wear has taken place, contacting surfaces show signs of a lapped finish, radial scratch marks or grooves, or another unmistakable indication that contact has taken place.
Cause: Foreign material within the lubrication system ordinarily causes abrasive wear. The particles may
be metallic debris from the gear and bearing system, weld spatter, scale, rust, sand, etc. Abrasive wear is usually noted soon after startup of a replacement installation, before the filter has had an opportunity to wash the system.

CORROSIVE WEAR:

        This is a deterioration of the surface thanks to chemical process. It is often caused by active ingredients within the grease , like acid, moisture, and extreme-pressure additives.
Cause: The oil breaks down in order that corrosive chemicals present within the oil attack contacting sur faces. Often this action affects the grain boundaries, causing fine pitting more or less uniformly over the tooth surfaces. At high temperatures, extreme pressure additives sometimes form very active corrosive agents. Lubricants can also become contaminated from absorption of foreign material from external sources.

SPALLING:

        Spalling is analogous to destructive pitting except that the pits are usually larger in diameter and quite shallow. Often the spalled area doesn't have a consistent diameter.
Spalling often occurs in medium-hard material, also as in highly loaded fully hardened material. Spalling of this type shouldn't be confused with "case crushing" which is related to case hardened gear material.
Cause: Spalling is typically caused by excessively high contact stresses. Usually, large pits are formed; because stress levels are high, the sides of the initial pits break free rapidly and enormous irregular voids are formed. Often these voids join together.

FROSTING:

        Frosting occurs in the early stages of scoring. Usually the dedendum section of the driving gear is the first to show signs of surface distress, although frosting can first show up on the addendum section.
As the name implies, the wear pattern appears frosted. The normal polish of the surface has an etch-like finish. Under magnification, the surface appears to be a field of very fine micro-pits less than 0.0001 in. deep. The frosted pattern will sometimes follow the marginally higher ridges caused from cutter marks or other surface undulations.
Cause: Frosting is caused by heat within the mesh, which ends up in just marginal lubrication. The heat of the mesh and therefore the bulk temperature of the rotating gears combine to interrupt down the lubrication film.

HEAT TREATMENT METHODS 

1) CASE HARDENING PROCESSES

a) PLASMA NITRIDING

Plasma nitriding is a thermo-chemical surface treatment also known as ion nitriding. It works on the phenomenon of glow discharge and increases the case hardness of gears. Nitriding is a heat-treating process that takes place at elevated temperatures and diffuses nitrogen into the surface of a metal to create a case-hardened surface.

This process is carried out in a furnace which consists of an anode and a cathode and an electrical voltage is passed through the gear and furnace. Here the furnace acts as an anode while the gear act as a cathode. Firstly the vessel is drained and ammonia gas is passed into the vessel. For the glowing phenomenon to take place a very low pressure near to vacuum is maintained.

As soon as the required pressure is set up, the electric current is passed and the glowing phenomenon takes place. The ammonia gas hits the cathode(gear) with high kinetic energy and splits into nitrogen and hydrogen, this leads to friction and thus produces heat. The nitrogen reacts with the alloy steel and forms alloy nitride which increases the wear and corrosion resistance of the material.

This process takes place around 400°C to 600 °C in a furnace. The hardness of the nitrided layer formed can be up to 1400 HV(Vickers hardness). During nitriding, the volume of the component increases by 3% of the layer thickness. This process is mostly preferred over carburizing because it requires a comparatively lower temperature than carburizing.

 

b) CARBURIZING

Carburizing is a case hardening process that takes place at elevated temperatures in a closed container. It not only makes the surface wear resistance but also increases its toughness and strength. It is similar to nitriding as in the case of nitriding gas containing nitrogen is released onto the surface of gears while in carburizing the parts to be hardened are kept inside a closed vessel. The diffusion of carbon onto the surface of the component makes it hard & wear-resistant.

          Gas carburizing is the most common method. This method is carried out in a gaseous atmosphere containing carbon. During the process, both the temperature and atmosphere are maintained. The temperature range for this process varies from 850°C to 950°C. This is mostly preferred for mass production.

          Pack carburizing is also a carburizing method in which the sample to be hardened is kept inside a closed container. This container comprises granules of charcoal or other materials which has more amount of carbon content. This process usually takes 12 to 72 hours and at a temperature of 900°C. Due to high temperature, the carbon diffuses inside the sample. This sample is then slowly cooled and brought back to ambient temperature.  

2)   SURFACE HARDENING PROCESS

The main difference between case hardening and surface hardening is case hardening leads to the hardening of the surface by forming a thick layer above the sample surface while in surface hardening the surface becomes harder and the core remains soft.

a) INDUCTION HARDENING

Induction hardening is a surface hardening method. As the name suggests, it makes use of the electric current for heating. The sample to be hardened must have 0.4 to 0.5% of carbon content or some alloying element such as nickel, chromium. The sample is held between the helical coils and a water sprayer system is employed around the gear sample.

When an AC is passed through the induction coil, according to Faraday's law of Electromagnetic induction, alternating magnetic flux is generated across the sample, due to this eddy current is formed and leads to the generation of heat and increases the temperature of the sample. Hence the outer surface of the gear sample gets heated and becomes harder while the core remains softer.

According to the requirement, the water spray system is held across the sample. Water quenching may sometimes lead to cracking. To prevent the sample from cracking poly-alkaline glycol mixture is used. Hence in case hardening, quenching is not required immediately while in surface hardening quenching is immediately required after induction.

b) FLAME HARDENING

Flame hardening is similar to induction hardening only the difference is the electric current is replaced by the oxy-gas flame. It makes use of direct heat for the hardening of the surface. The heating is carried until the temperature reaches its austenization form so that a more open structure of the austenite is then able to absorb carbon from the iron-carbides in carbon steel.

This causes the surface to become tougher and wear resistance while the core remains soft. The hardness obtained depends upon the duration of heating, carbon content, and temperature of the quenching process. The gas to be used is selected based on how fast and effective heating is obtained.

CONCLUSION

So we saw how the failure of gears occurs and their reasons for failure. The heat treatment methods such as nitriding, carburizing, etc discussed above are one of the common methods used worldwide. From all the above discussion we can say that lubrication plays a very significant role and prevents the gears from failure. So selecting appropriate lubricating oil is very essential. 

    So what are your thoughts? Do you have any questions or queries? Let me know in the comments below and I'll be happy to help. 

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SY ME B_ Batch - 3_Group – 4

81 - Chinde Utkarsha

82 - Kudale Tanay 

83 - Patil Anudipsing

84 - Patil Tanmay

85 - Solankar Rutuja

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Dr. S. P. Chippa