Flywheel is a mechanical device used to store the rotational kinetic energy. The main application of the flywheel is to provide the kinetic energy whenever the load torque is greater the drive torque. Amount of energy store in a flywheel depends on three things geometry of flywheel, angular velocity of flywheel and density of material used. Whereas the flywheel ability to handle the stresses and range of angular velocity at which it can rotate safely, depends on the material of the flywheel. Therefore the material selection is one of most important aspect of flywheel manufacturing.
Flywheel which is discussed in this report is for an electric motor, manufactured form aluminum casting alloy using sand casting.
Current material selection and issues related with it. Currently aluminum casting alloy was selected by manufacturer to manufacture flywheel for given job because of following four reasons.
1. Manufacturing properties required
According to the requirements listed by the customer following are the properties required by any material to perform the job perfectly and safely. (Mikell P. Groover, 2010, Chapter 10)
• Good Fluidity
• Pouring Temperature
• Good machinability
• Shrinkage during solidification
Cast-ability means how much easy it is to manufacture a quality finished product without the machine processes like surface finish and property enhancing process like heat treatment. Material fluidity means molten material ability to flow into the mold cavity. More the fluidity less will be the damage done to cavity walls and better will be material distribution into the cavity. Low pouring temperature will help us to work at low temperature. As metal melting temperature is also its freezing temperature so low melting temperature means material will solidify at lower temperature and also solidify quickly.
Machinability of a material means the ease with which machine operation like drilling and surface finishing can be done. Material with good machinability usually required fewer forces which directly increase your tool life and decrease the time of machining. Every material has three phase liquid contraction, phase changing contraction and thermal contraction. Material having greater value of linear shrinkage need larger pattern than material having lower value of linear shrinkage. Shrinkage of material causes defects in material like void and shrinkage cavity formation.
2. Mechanical properties required
According to the requirements listed by the customer following are the properties required by any material to perform the job perfectly and safely. (Mikell P. Groover, 2010, Chapter 3)
• High ductility
• Low density
• High yield strength
• Corrosion resistance
• High material index
A material having high ductility have a clear change in its shape before it breaks or burst (flywheel burst like pressurize cylinder upon failure).
This property will help us to replace the part before it causes any serious damage to machine but high ductility affect the working of flywheel in such a way that at very high rpm it start to lose its shape, affecting the working where flywheel is enclose inside a machine. Low density of a material allow you to have minimum mass under a given geometry, making your flywheel light weight, easy to carry and easy to assemble and disassemble but low less mass result is less kinetic energy of flywheel at given rpm which directly affect your output.
High yield strength of a material enables him to restore its original shape after elastic deformation. It will help flywheel of resist the high rpm without being permanent deformed. Giving desire shape to a material required machining and for this good machinability is required. For long life material should be stable and should not react with surrounding atmosphere to form oxides. For this flywheel material should have good corrosion resistance. Material index is the most important property which shows material strength to density ratio enables us to select the best material.
3. Local Market Condition
Local market condition greatly affects the selection of material. Material you select should be easily available, cheap and should have continuous supply for nonstop manufacturing and reducing the throughput and inventory cost of material.
4. Environment affects
Material you select should not affect the environment that is melting of it should not emit harmful gases and waste material can be recycled.
5. Company ability to process material
Selection of material depends on the manufacturing facilities available and technical skills of labor working there.
From available material for flywheel, aluminum casting alloy is the best candidate but still there are some problem associated with its casting. First is to make the mixture of materials, which need complete technical knowledge of all the materials involve. Second is the melting of mixture, melting the mixture at higher temperature to melt the material having high melting temperature will do the work but too much high temperature will boil the low melting materials resulting the metal oxides of that material because its vapor reacts with oxygen. (Tomasz Stuczyñski, 1997)
Current manufacturing process selection
and issues related with it
After the selection of material the process of selecting the manufacturing process starts and like material selection this selection also has some important factors govern the manufacturing method selection. Some of them are as follow (K. G. Swift and J. D. Booker, 2013, Chapter 2)
1. Production volume
It is the number of pieces to be produce and it decide whether to use expandable mold or permanent mold and this factor also decide whether the organization should go for automation of the process or do it manually. If the volume is high the organization should go for automation to reduce the unit production time and cost but if the volume is small and design variation is high the organization should think of the integration of designing and manufacturing to have the fastest response to change in design of product.
2. Product Size
Dimensions of the product is really important for determine of the manufacturing process because of the factors associates with the size like mold size, amount of molten material, pouring rate, cooling rate and total solidification time.
3. Product shape features
Product shape features defines the complexity involves in the manufacturing of the product for example irregular external shape and core for internal cavity can resist the flow of molten material.
4. Product Value
Product value means the sensitivity of the function which it is going to perform. Product associated with human life security need more careful manufacturing.
Sand casting which is most widely used manufacturing process was selected by manufacturer because of low production volume, small size, simple shape and low product value. This can produce near net shape and also can provide you desire mechanical properties by the use of chillers or by careful design of mold V/A ratio. Sand use in this process can be reused again and again and waste material because of molding can be recycled making this process highly economical. Automation of sand casting can also be done to reduce the unit production time. Sand casting is a simple process but still there are some problems associated with it. (Mikell P. Groover, 2010, Chapter 11)
One of the most important is human health security issue because the molten metal is usually pored manually by the workers and any accident here can seriously damage human life also the heat and gases coming from the molten metal are harmful for human health. To overcome this, a manual operated mechanical system can be established so that worker does not come in direct contact with molten metal. (Mikell P. Groover, 2010, Chapter 11)
Another issue which is associated with sand casting of aluminum alloy is the creations of defects due the reason that alloy solidify at a range of temperature rather than at single temperature. Defects usually occur near hub of the flywheel because this is the place which solidifies in last. To overcome this, externals chills can be used near hub so that cooling process can be speed up there. (Mikell P. Groover, 2010, Chapter 10)
As already mention that these flywheels are being manufactured for an electric motor therefore an assembling process should be there to mount flywheel on electric motor shaft. This was done by making a key way on shaft of motor and also in the hub of flywheel and then after placing the flywheel was hammered onto the shaft of motor. This need experience because little carelessness can damage the shaft and bearing of the motor thus making the whole electric motor useless.
Quality control was a big problem in the organization because only destructive testing method was available. For testing the flywheel one from every batch was rotated at rpm which increase slowly until the flywheel have permanent deformation. The rpm at which the flywheel fails is then compared with rpm at which it was design to rotate. ( K.Gopinath and M.M.Mayuram, n.d, pg 8)
There was no recurring and non-recurring coat associated with this project but to manage the inventory cost delivery system was made on weekly bases and to handle the material cost recycling of the waste material was done.