Material Selection Process for Bevel Gear

Selecting an engineering material for any engineered product is a regular process in any engineering design task. Material selection has a special importance in engineering design tasks as it will define the performance of the product designed. Material selection process is not so simple; it usually consists of many steps which mostly depend on the importance or value of the produce being designed.

Following are some of the steps of material selection process which are usually followed in regular designing tasks

1.      Establishing Selection Criteria

2.      Short listing material by literature review

3.      Evaluate material on selection criteria

4.      Selecting best material

4.1 Establishing selection criteria

In material selection process for any engineering design task, establishing selection criteria is first step in which a selection criteria for material selection is establish and usually it consist of properties required to full fill design requirements. Following are the some of the criteria on which all materials will be selected.

·         Mechanical Properties

·         Manufacturing Properties

·         Environmental Properties

·         Economical Aspects

·         Availability Issue

Mechanical properties have first position in selection criterial as they will define mass, strength and performance of the product. After mechanical properties manufacturing properties of material are test as they will define the proper manufacturing process and the cost of the manufacturing process of product and thus will affect the final cost of the product. Environmental properties are third in selection criteria as they will define the environmental effect of material from which the produce will be manufacturing. It is very important to select a material that effect the environmental lest during its life span and death period. Economical aspects material will define the cost of raw material and will have great impact of the final cost of the product. Material cost should be kept as low as possible in order to keep the final product low and make the product cost effective. Availability issue is also very important is material selection the selected material should be easily available in local market. This will reduce the transportation cost and will save a lot of time required for material transportation.

4.2 Short listing materials

There are slot of materials that have the required potential to be used as base material for a particular product production. It is very difficult to study each and every material available for product. So make this process easy, usually three to four materials are short listed form all available materials and this is done by studying similar products.

4.3 Material Evaluation

All short list materials are evaluated on the base of selection criteria establish for a particular product. Marks or points are given to each material based on its performance in each and every field of selection criteria. Material are arranged on descending sequence based on their overall marks

4.4 Selection

In most of cases first material is selected for the production of product but in some case first two material or one which easy available is used for production.

5.0 Material selection process

Material selection process will be conducted for gears based on the instruction provided above.

5.1 Selection criteria

1.1     5.1.1 Mechanical Properties

Mechanical properties have first position in selection criterial as they will define mass, strength and performance of the product. There are about 22 mechanical properties that any engineering material can have and from all of these properties following the few which will affect the performance of the gear.

·         Density

·         Strength

·         Ductility

·         Wear and Tear resistance

1.2     5.1.2 Manufacturing properties

Manufacturing properties are very important for cost effective manufacturing of product. In selection criteria they have considerable weightage. Machinability, cast ability and malleability are some of the manufacturing properties which material should have for cost effective manufacturing of gears

1.3     5.1.3 Environmental properties

With increase awareness in environmental protection selecting material which did not affect or have least affect from the available material on environment is becoming more important. Following are some of environmental properties for which any material should have minimum value.

·         Embodied energy

·         CO2 footprints

Embodied energy is energy used for any material during its extraction, refining, manufacturing, transportation and decomposition. More the embodied energy any material will have more it will affect the environment. CO2 foot prints means the amount of carbon dioxide any material will release during its life time. A good material should have minimum CO2 foot prints.

1.4     5.1.4 Economical aspects

In order to make a cost effective product, material selected for that product should be cost effective. So the selected material should have lowest cost from all the available materials

1.5     5.5.5 Availability

Material selected for the manufacturing of any product should be easily available in local market in order to minimize the transportation cost associated material.

5.2 Material short listing

There are a lot of materials that can be used for manufacturing gears but out of all those material following three are the most common materials used for gear manufacturing.

·         Alloy Steel

·         Bronze

·         Aluminum Alloy

5.3 Material Evaluation

1.6     5.3.1 Mechanical properties

Table 6 Mechanical properties

Materials	Density	Yield Strength	Ductility	Wear Resistance
Alloy Steel	7700	620400000	Good	High
Bronze	7700	210000000	Good	Good
Aluminum Alloy	24980	240000000	High	Satisfactory

1.7     5.3.2 Manufacturing properties

As gears are mainly manufactured by machining process called hobbing, so only machine-ability will be consider.

Table 7 Manufacturing properties

Materials	Machine ability
Alloy Steel	Excellent
Bronze	Good
Aluminum Alloy	Good

1.8     5.3.3 Environmental properties

Table 8 environmental properties

Materials	Embodied Energy	CO2 Foot Prints
Alloy Steel	3110	1.93
Bronze	22700	3.43
Aluminum Alloy	21500	12.2

1.9     5.3.4 Economical Aspects

Table 9 economical aspects

Materials	Cost $/lb
Alloy Steel	0.272
Bronze	2.78
Aluminum Alloy	1.07

1.10       5.3.5 Availability

Table 10 availability

Materials	Availability
Alloy Steel	Good
Bronze	Good
Aluminum Alloy	Good

5.4 Selection

Based on the evaluation shown above Alloy Steel is the best material for this project as it has highest yield strength, high wear resistance, lowest embodied energy, lowest CO2 foot prints and lowest cost. So Alloy Steel is the material select for manufacturing of gears.

Gear Box Material Selection Process

Material selection is one of the main and most important step involve in designing of any mechanical product. Mechanical product may consist of many components and it is very important to select the best material for each and every component of the product. Gear box designed in this project has many parts like lever, body, gears, shafts and bearings. From all these components lever and body are not the major load bearing components but gears, shafts and bearing are the load bearing components and their material will define the life and performance of the product. From these three component bearing that will be used will be pre-manufactured so only two component shaft and gear remains for which material has to be selected in this selection process.

5.1 Selection strategy

Selection of a best material for any component required the evaluation of all the available material for that component. Evaluation of the materials is usually made on number of factors which involve mechanical properties, manufacturing properties, environmental aspects and economic considerations. Each of these factors has many sub divisions and all of those factors and their sub division are explained below.

1.1     5.1.1 Mechanical Properties

Mechanical properties are the first and most important factor of material selection process as they properties will define the strength, mass and life of the components. There are almost twenty two mechanical properties related to a component but not all of them are required at a time, so from these twenty two properties few has to be selected in order to select the material with best mechanical properties. Following are some of the mechanical properties required for the selection of material that can be used in shaft and gears for the gear box.

·         Yield Strength

·         Young Modulus

·         Shear Modulus

·         Density

Yield strength

Yield strength of the material will define the limit of the stresses or the limit of the forces which can be applied on the component. This very important that material should have high value of yield strength as in gear box gears teethes will be under the bending load and shaft will be under bending and shearing load during operation. So the material of shaft and gear should have high value of yield strength on order to get safe and reliable service.

Young Modulus

Young modulus or the modulus of elasticity is the property of the material that will define the amount of deflection of the certain part of the component which is under load. It is very important that selected material should not deflect too much because large deflection of component will cause decrease in efficient and inaccuracy in performance. Deflection in shaft cause by the bending stresses will affect the gear meshing and deflection in gear teeth will affect the proper working of gear train of the selected material should not show large deflection

Shear Modulus

Just like young modulus, shear modulus is also a material property which shows the deflection of material but under the shear stresses. These stresses will only be in shaft as it will have torque because it is main power transfer in gear box. Like the young modulus, shear modulus of the material should also not allow component to deflect to much because this can affect the operation of gear box.

Density

Density of the material is an important mechanical property as it will define the final weight of the components. Density of gear box will define its efficiency and performance because more the weight of the shafts and gears more will be the extra load on the system other the load for which the gear box is designed. So for best gear box, materials for its shafts and gears should have low density as much as possible.

1.2     5.1.2 Manufacturing Properties

 Machine-ability of the material is a property which will define the ease with which that material can be machined to manufacture. This property is more than just defining the machining ability of the material; it will also define the cost of the manufacturing of the product because material with good machinability will be cost effective in manufacturing than the one which has bad machinability.

1.3     Environmental Aspects

Environmental aspects of the material are the property which has huge impact on the material selection with the increase in awareness of environment. Environmental aspects of the material include embodied energy which is the energy used during the extraction, production, transportation and using process, CO2 foot prints which defines the amount of CO2 emitted during the life of the material and recyclability which is the ability of the material to be reused after its main life and use.

1.4    Economic Condition

Economic condition of the material which needed to be used for the component production should be good. Economic condition of material included its availability and cost in the open market. Material with the good availability but high cost will not be good choice as it will increase the cost of the product and like that material with low cost and low availability of will also not going to be the best choice so material used for the shafts and gears should have good availability and low cost.

5.2 Materials Selection

Gear and shafts in gear box will under the high stresses during the operation of gear box, So the material used for the gear and shaft should be good enough for satisfying the need of high stresses. There are many materials that can be used for the manufacturing of gear and shafts but following are the three materials that can be used for the gears manufacturing.

Table 2 Material properties

Material	Aluminum Alloy	Alloy Steel	Cast Iron
Yield Strength ksi	72.5	104	98.6
Density lb/ft^3	181	0.278 lb/in^3	453
Elastic Modulus psi	11.9*10^6	30.4*10^6	26.1*10^6
Shear Modulus psi	3900	11.45*10^6	5.9*10^6
Machinability	Good	Good	Good
Recyclability	Yes	Yes	Yes
Cost USD/lb	1.17	0.299	0.349

Three material that were short listed from all the other available material are been presented above. From these three material alloys steel is the best choice because it has the highest value of yield strength, elastic modulus, shears modulus and has lowest cost. Along with this it has alloy steel has high machinability and good recyclability. So alloy steel is the material for the shafts and gears for this gear box.

Material Selection of Helical Spur Gear

It is a common practice to select a material for a product but in the case of the gear box it has special attention as it will decide the life, cost, weight and over all dimensions of the product. In gear box special attention is required for the gear material as they are main component the will be manufactured and they are the one under high load due to which they will face high wear and tear. Material selection process of any product consists of simply evaluating the material available for that product based on the mechanical properties, manufacturing properties, environmental aspect and economic conditions.

Following are the four materials short listed for the gears

·         Ductile Cast Iron

·         Alloy Steel

·         Medium Carbon Steel

·         Aluminum Alloy

Mechanical Properties Evaluation

There are approximately twenty two mechanical properties on which any material can be evaluated but on this section only few selected will be used for evaluation of material for gears. Following are some of the mechanical properties that will be used for the evaluation

·         Elastic limit

·         Density

·         Hardness

·         Fatigue strength

·         Fracture toughness

Table 3 Material mechanical properties

Properties	Elastic Limit	Density	Hardness	Fatigue Strength	Fracture Toughness
Ductile Cast Iron	23.9	440	115	26.1	20
Alloy Steel	85	487	140	36	12.7
Medium carbon steel	44.2	487	120	33.2	10.9
Aluminum Alloy	43.5	156	12	3.13	20

In this section elastic limit defines the ability of the material to take load means how much load or stresses a material can take before it plastically deform. In the case of gears plastic deformation of gear will make gear use less as gear teeth profile in very important for proper power transfer. In order to get the best gear box Elastic limit of the selected material should be as high as possible.  Elastic limit of all four materials in mention above according to that alloy steel is on first place due to highest value of Elastic limit followed by Medium carbon steel, aluminum alloy and at last the ductile cast iron.

Density is very important mechanical property in this selection process as it defines the total weight of the product. In order to get the best gear box density of the selected material should be as low as possible. Density of all four materials in mention above according to that aluminum alloy is at the first position with the lowest density followed by the ductile cast iron at second and medium cast steel and alloy steel at last position. 

Hardness is the property which enables material to resist the change in its shape in the case of any external force or stress applied in it. It is very important mechanical property in the case of gear designing as it will define the ability of gear to resist the external loads to main train its proper shape. In order to get the best gear box hardness of the selected material should be as high as possible. Hardness of all four materials in mention above according to that alloy steel is the at the first position due to highest value of hardness followed by Medium carbon steel, ductile cast iron and at last the aluminum alloy.

Fatigue strength of the material is its ability to resist against the fatigue loading when a fluctuating load is applied. In the case of gear designing t is very important property because gear will be under fatigue loading due to the central lathe operation procedure and gear box usual loading process. . In order to get the best gear box fatigue resistance of the selected material should be as high as possible. Fatigue resistance of all four materials in mention above according to that alloy steel is the at the first position due to highest value of fatigue resistance followed by Medium carbon steel, ductile cast iron and at last the aluminum alloy.

Fracture toughness on any material will define its ability to resist the propagation of cracks in the material in the case if there is any defect in the material or final product produce by the manufacturing process. . In order to get the best gear box Fracture toughness of the selected material should be as high as possible. Fracture toughness of all four materials in mention above according to that aluminum alloy and Ductile Cast Iron is the at the first position due to highest value of fatigue resistance followed by Alloy Steel and Medium carbon steel.

Manufacturing Properties

Manufacturing properties of any material are those properties which influence the manufacturing process of that material. Those manufacturing properties are machinability and cast-ability; machinability will define how easy it is to machine a certain material and cast-ability will define how easy it is to cast a certain material. For the low manufacturing cost selected material should have good machinability and cast-ability. All the four selected for material has good cast-ability but machinability is inversely proportional to the strength of the material. From the four selected materials ductile cast iron will have the best machinability followed by the aluminum alloy, medium carbon steel and at last alloy steel.

Environmental Aspects

 Environmental aspects of any material are those aspects which shown the effect made by the use of that material on the environment. Embodied energy, CO2 foot print and recyclability are the factor which decides the environmental effect associated with material.

Table 4 Material environmental aspects

Material & Properties	Cost	Embodied energy	CO2 foot print	Recyclability
Ductile Cast Iron	0.318	1.95e^3	1.7	Yes
Alloy Steel	0.272	3.11e^3	1.93	Yes
Medium Carbon Steel	0.254	2.72e^3	1.72	Yes
Aluminum Alloy	1.07	2.15e^4	12.2	Yes

Embodied energy is the energy associated with the extraction, manufacturing, transportation and decomposition of the material. In order to get the best gear material, embodied energy of the selected material should be as low as possible.  Embodied energy of all four materials in mention above according to that ductile cast iron is on first place due to lowest value of embodied energy followed by Medium carbon steel, alloy steel and at last the aluminum alloy.

CO2 foot prints show the amount of CO2 any material will release during its life type. In order to get the best gear material, CO2 foot prints of the selected material should be as low as possible.  CO2 foot prints of all four materials in mention above according to that ductile cast iron is on first place due to lowest value of CO2 foot prints energy followed by Medium carbon steel, alloy steel and at last the aluminum alloy. Recyclability is the property which defines weather a material can be recycled or not.

Economic aspects

Economic aspect of material involves the cost of the material which will influence the cost of the final product. In order to get the best gear material, cost of the selected material should be as low as possible.  Cost of all four materials in mention above according to that alloy steel is on first place due to lowest value of cost followed by Medium carbon steel, aluminum alloy and at last the ductile cast iron.

Conclusion

From all the discussion made above it is clear that alloy steel is the best material for the gears because it has highest value of elastic limit, fracture toughness, hardness and fatigue loading. Along with this it has moderate values of environmental aspects and lowest cost of material.