GEAR FAILURE

Failure Modes in Gears

Gear Systems or gear trains tend to play a very vital role in all industries and also in our day to day life. Any failure to the gear system leads to the total system failure.Now let us look into the various modes by which the gears fail:
The various major areas by which the gears tend to fail are
 * Shock Loading Failures
* Fatigue Failures
* Failures due to Wear
* Failures due to Scuffing

SHOCK LOADING

1. Breakage by sudden shock loads: When the gear system is subjected to a sudden shock load more than it is designed for, it instantly breaks. Though this cannot be identified prematurely, utmost care can be taken to avoid such mishaps.

FATIGUE FAILURES

1. Bending Fatigue:

 Gears based on their rotational motion and the heavy amount of load they carry are subjected to fatigue. Due to the application of load on the gear teeth, the gear tooth is subjected to bending. When the same is repeated for a very huge number of cycles Bending Fatigue failure of the gear sets in. The fatigue in the gears induces the formation of cracks in the root of the gear tooth which propagates with each rotational cycle of the gear and ultimately leads to the failure of the gear tooth.

2. Pitting: 

This is a major cause of gear failure accounting for nearly 60% of the gear failures. Pitting is the formation of craters on the gear tooth surface. These craters are formed due to the high amount of compressive contact stresses in the gear surface occurring during transmission of the torque or in simple terms due to compressive fatigue on the gear tooth surface. 

a) Micro Pitting: 

These are basically formed due to

•   Inherent Errors in the gears
•  Presence of water in the lubricant that is lubricating the gears
•  Wrong viscosity selection of the lubricant used.

Visually, micro pitting is not so clearly visible at the first go.They appear as very small dots which one can feel when he runs his finger over the gear tooth.This sort of pitting normally tends to make the gear useless and damages the whole gear system.

FAILURES DUE TO WEAR

1. Moderate Wear:

This is a most common type of wear mode caused mainly due to lubricant contamination. The other contributors to this are limitations of lubricant viscosity, gear speed, and temperature

2. Abrasive Wear:

 This is normally characterized by scratch marks on the tooth surface when we look at it visually. This type at certain times is misjudged as Micro Pitting marks due to the similarity in the visualization. Abrasive wear is caused due to the presence of metal particles from the gears and bearings, weld spatter, scale, rust, sand and dirt in the lubricant of the gears

3. Corrosive Wear: 

As the name says this type of wear regime occurs due to corrosion action on the teeth of the gears. This type of wear regime is visually looked at as spotty dots appearing scattered on the tooth of the gear. The common causes for this type of wear are

• chemical action of active ingredients in the lubricant like acid,
•  presence of moisture, foreign materials in the lubricant
•  extreme-pressure additives added to the lubricant which react with the gear teeth

Scuffing damage occurs on gear teeth if they are operated with an inadequate lubricant film between the teeth or when the viscosity of the lubricant is very less. They can be identified by ripples on the contacting gear tooth surface. This type of failure is also characterized by a burning smell of the lubricating oil during running and also development of burn marks on the contact teeth surface. This can be prevented by proper selection of the lubricant and monitoring of the gearbox temperature at equal intervals to ensure that the temperature does not exceed the maximum running temperature of the lubricating oil.

how gears are manufactured

GEAR MANUFACTURING

Gears can be manufactured by various process that can be classified under the following  3 process 

1. Gear milling
2. Gear generating
3. Gear molding

a) Gear Milling:

•  This is one of the initial and best known and metal removal process for making gears. 
• This method requires the usage of a milling machine
•  It is also to be noted that this method can produce nearly all types of gears.
•  The method involves the use of a form cutter, which is passed through the gear blank to create the tooth gap. 
• This method is right now used only for the manufacture of gears requiring very less dimensional accuracy. 
• Surface finish  is good.

b)GEAR GENERATING:

• In the generating process ,teeth are formed in series of passes by a generating tool shaped like a mating gear.
• Either hobs or shapers can be used.

1. Hobbing:

•  Gear Hobbing is a continuous generating process in which the tooth flanks of the constantly moving work piece are formed by equally spaced cutting edges of the hob.
•  The main advantage of this process is its versatility to produce a variety of gears including Spur, Helical, Worm Wheels, Serrations, Splines, etc. 
• The main advantage of the method is the higher productivity rate of the gears.
• Hobbing closely controls tooth spacing,lead and profile.
  

     

2.SHAPING:

•  Gear shaping is a generating process.
• The cutter used is virtually a gear provided with cutting edges. 
• The tool is rotated at the required velocity ratio relative to the gear to be manufactured and any one manufactured gear tooth space is formed by one complete cutter tooth. 
• This method can be used to produce cluster gears, internal gears, racks, etc with ease, which may not have the possibility to be manufactured in gear hobbing.
• Using a shaper ,teeth may be generated with either a pinion cutter  or a rack cutter.
  

3.GEAR MOLDING:

Mass production of gears can be achieved by molding. 

• Injection molding:It produces light weight gears of thermoplastic material.
• Die casting:It is a similar process using molten metal.Zinc, brass, aluminium,and magnesium gears are made by this process.
• Sintering: It is used in small , heavy-duty gears for instruments and pumps.Iron and brass are mostly used for this process.
• Investment casting: It produces medium-duty iron and steel gears for rough applications.
  

GEAR MATERIAL

Gear Materials

• The selection of gear materials depends upon:

1. Type of service
2. Method of manufacture
3. Wear and shock resistance
4. Space and weight limitations
5. Safety and other considerations
6. Peripheral speed
7. Degree of accuracy required
8. Cost of material
9. High load ,impact loads,and longer life requirements

• The gear material should have the following properties:

1. High tensile strength to prevent failure against static loads
2. High endurance strength to withstand dynamic loads
3. Low coefficient of friction
4. Good manufacturability

•  The gears may be manufactured from metallic or non-metallic materials. The metallic gears with cut teeth are commercially obtainable in cast iron, steel and bronze. The non-metallic materials like wood, rawhide, compressed paper and synthetic resins like nylon are used for gears, especially for reducing noise
• The following  shows the some of commonly used gear materials.

1.METALLIC MATERIALS

a)STEEL:

• So far  the most widely used material in gear manufacture is steel. Almost all types of steel have been have been used for this purpose.
• To combine the property of toughness and tooth hardness ,steel gears are heat treated.
• Steels with brinell hardness number(BHN) <  350 are used  in light and medium duty drives.But steels with BHN > 350 are used in heavy duty drives and also where compactness is required.The plain carbon steels used for medium duty applications are 50 C 8 , 45 C 8 ,  50 C 4 , and 55 C  8.For heavy duty applications ,alloy steels   40 Cr 1 ,30 Ni 4 Cr 1 ,and 40 Ni 3 Cr 65 Mo are used.
• For cast steels the tensile strength ranges from 650 N/sq.mm to 1020 N/sq.mm/

b)CAST IRON

• It is used extensively  as gear material because of its low cost ,good machinability, and moderate mechanical properties.
• Generally ,large size gears  are made up of grey cast of iron of grades FG 200 , FG 260 ,or FG 350.
• Its main disadvantage  is its low tensile strength.

c)BRONZE:

• It is mainly used in worm gear drives because of their ability  to withstand heavy sliding loads.
• Bronze gears are also used where corrosion and wear  are a problem .
• Its main disadvantage is its high cost.

2.NON-METALLIC MATERIALS

• The non-metallic materials like wood ,rawhide, compressed paper and synthetic resins like nylons are used for gears .
• Its advantages are              

1. Noiseless operation;
2. Cheaper in cost;
3. Damping of shock and vibration;

• Its disadvantages are 

1. Low load carrying capacity;
2. Low heat conductivity;

It also has to be noted that the material for pinion should always be better than material of the mating gear because the teeth of pinion undergoes more number of cycles than those of gear and hence quicker wear.

classification of gears

CLASSIFICATION OF GEARS

 The gears may be classified as follows:

1.ACCORDING TO THE POSITION OF AXES OF THE SHAFT:

• parallel
• intersecting
• non-intersecting and non-parallel 

i)Parallel

  The two parallel and co-planar shafts are connected by  spur gears.They have teeth parallel to the axis of rotation and  are used to transmit motion from one shaft to another parallel shaft

If the teeth are inclined to axis then such type of gear is known as helical gear.The double helical gears are known as herringbone gear.

ii)Intersecting shaft.

INTERSECTING SHAFTS

    The two intersecting ,but co planar shafts connected by gears known as bevel gears.they have teeth formed on conical surface.

BEVEL GEAR

iii).Non intersecting and non parallel

     The two non intersecting and non-planar i.e. non-coplanar  shafts are connected by skew bevel gear or spiral gear.this type gearing has line contact ,the rotation of which about the axes generates the pitch surfaces known as hyperboloids.

2.ACCORDING TO THE PERIPHERAL VELOCITY OF THE GEARS :

• Low velocity
• Medium velocity
• High velocity

The gears having velocity less than 3 m/s are termed as low velocity.The gears having velocity between 3 and 15 m/s are known as medium velocity gears.If the velocity of gear is more than 15 m/s then these are called high speed gears.

3.ACCORDING TO THE TYPE OF GEARING:

• External gearing 
• Internal gearing
• Rack and Pinion

In external gearing .the gears of two shafts mesh externally with each other.The larger of two wheels is known as spur wheel and smaller wheel is known as pinion.In an external gearing ,the motion of the two wheels is always unlike .

In internal gearing the gears of two shafts mesh internally with each other.The larger of two wheels is known as annular wheel and smaller wheel is known as pinion.In an internal gearing ,the motion of two wheels is always   like.

INTERNAL GEARING

In some cases ,the gear of shaft meshes externally and internally with the gears  in straight line.such type of gear is known as rack and pinon.It is used to convert linear motion into rotary motion and vice versa.

4.ACCORDING TO THE POSITION OF TEETH ON GEAR SURFACE:

• Straight gears-spur gear
• Inclined gears-helical gear
• curved gears-spiral gear