Mechanism Development - Report Example

Name Professor`s name Subject Date Introduction Band saws are very crucial instruments in the workshop for cutting up metal pieces into required sizes for various uses. Most of the band saws are powered by electricity from the grid or solar. However, grid power is not accessible everywhere. Such places include forest parts. There is therefore need for a band saw that is powered by human legs. Convectional band saws use power that is more than 200W depending on the motor that is used to power it. In this case we are limited to a power of 200W which a man can comfortably provide for a considerable amount of time. This is too means that the system must be extremely simple in design with very high efficiency to ensure that most of the power produced by leg is used to do useful work. Efficiency is hardly 100% because of the weight of the moving parts and friction. Also, in case the system was not geometrically precise, a lot of energy may be wasted. Analysis and calculations Gears, shafts and belts are all used to transmit power and motion from one section to another and from one mechanism to the next. In this case the best mode of power transmission will be adopted to minimize on cost and energy and still ensuring highest efficiency possible. For easy use, velocity ratio must be very high. For easier calculation, the force that is applied on the final load has to be assumed. Band saws are used for a variety of applications. Some are used to cut metals, wood, meat and other softer materials. In this case it is assumed that the final load required to do the cutting is 50kg (490.5N) The mechanical advantage (MA) = Equation 1 = Equation 2 = 2.4525 The mechanical advantage is therefore 2.4525 To calculate speed of the wheels the equation below are employed Force * speed. =Power Equation 3 200W = 200N* speed Equation 4 1m/s = Speed Equation 5 Converting that to angular velocity the equation below is used V=ω*r Equation 6 Where ω is the angular velocity and r is the radius of the wheel rotated by the foot. The power supplied by the foot is equivalent to 21kg if the efficiency is to be 100% 1m/s=ω*r Equation 7 ω= Equation 8 For higher mechanical advantage, the wheel rotated by the foot must be greater than the wheel carrying the cutting blade. The wheel rotated by the foot can be taken as 40cm in diameter. The radius is 20cm The angular velocity becomes ω= Equation 9 = 5 rad/s The torque supplied by the foot can therefore be calculated as follows; Torque =f*r Equation 10 Where f is the force supplied by the foot R is the radius of the wheel rotated by the foot Torque supplied = 0.2 * 200N Equation 11 Torque supplied =40NM Working with the assumption that the efficiency is 100%, the torque supplied is equal to the torque that reaches the load. This way the radius of the pulleys carrying the blade can be easily calculated. Force required to cut the load = 490.5N Torque =f*r 40 = 490.5N*r3 Equation 12 R3= R3=0.081549 = 8.155cm Velocity ratio (VR)= Equation 13 VR= = 2.4525 Since the efficiency is never 100% we take the VR to be 2.7 The radius of the blade pulleys now becomes =20/2.7 = 7.5cm The efficiency = = x 100 = 90.83333% efficient Of the 200N it is only 180N that are effectively used. The 20N is used to rotate the pulleys and to overcome friction. It is required that the blade to move with a speed of 8m/s This then can be converted into angular velocity as shown below; From equation 6 V=ω*r ω= ω= ω=106.6667rad/s Design and interaction of systems and subsystems The system is extremely simple in design and also on operation. The system is made up of 4 pulleys which are connected directly or via a belt. The v belt is the best to minimise the slipping when it is operating. Below is the diagram of the system; Figure 1.the front view of the band saw When the foot steps on the peddle, the power is transmitted by the connecting to the first pulley (pulley 1). When the first pulley is rotated, it rotates the pulley 2 via a v belt. Belts are applied here because of their simplicity. Pulley 2 is coupled directly to the blade pulley such that their speeds are equal. When blade pulley 2 is made to rotate, it rotates with the blade and so does the pulley blade 2. The materials used to make the pulleys are from the mild steel so that they would not wear off easily. The distance between the two power pulleys can be varied depending on the length of the connecting rod. Peddle is fixed to the ground on one end so that the person operating will use it effectively without any occurrence of injuries. Material and part selection In every design situation, durability is a factor that has to consider. The materials selected for the use in any part must be durable to reduce the cost incurred during repair and replacement. While designing the system above, mild steel materials are selected. Mild steel is a relatively cheap material yet it is capable of handling the tensile and compressive stresses are well as the thermo stresses likely to be experienced here. The base is made of wrought iron which is much cheaper that mild steel. The base must be heavier than most of the parts to ensure that the centre of gravity is as low as possible. The base is also wide. The pulleys are made from mild steel because of tear and wear experienced is a bit small. The v belts are better than every other type of belts since they have little or no slipping at all. However, changing the v belts is a bit harder compared to flat belts. Part list number part description quantity 1 blade The standard 1 2 pulleys 1. Diameter 40 cm 2. Diameter 15 cm 3. Diameter 30 cm 1. 1 2. 1 3. 1 3 belt V belt 1 4 Connecting rod 30 cm long 1 5 peddle standard 1 The geometry of the system and subsystem For the system to operate effectively without wobbling or vibrations, the geometry of the system must be accurate. The two power pulleys must be vertically straight. Other than being vertically straight, they must be perfectly in line. The power pulley 2 and the blade pulley 2 are coupled together and are horizontal. If they are at an angle they would bring about vibrations which would destroy the machine and the floor where the machine is place would start to crack. The blade pulley 1 must be vertically above the power pulley 1. Sustainability of the system As stated earlier on, almost all parts of the system are recyclable and as a matter of fact they can be reused after they are worn out. This ensures that there is minimal effect on the environment. System reliability and serviceability Mild steel is a very strong metal with very high tensile strength and equally high ultimate yield strength. It is therefore anticipated that the forces in operation can be comfortably contained by the mild steel frame and parts. The v belt is relatively strong component in that it can transmit about 1000W of power with speeds as high as 2000 rpm. Therefore the system will operate effectively with little failures. The life span of the machine is dictated by the components and the interactions of the subsystem as well as time of operation. The lifespan of the system is relatively high. Mild steel can be in useful state for as long as fifty years. Repair and maintenance of the system is equally simple. A lay man can easily change the worn out parts, belts and pulleys as well as the blade. If the system is kept away from water, it can stay for a very long time with no repairs or replacement. Environmental issues Iron is generally affected by water and air because of rusting. Rusting wears the metal away. The system has to be enclosed to ensure that the water does not come into contact with the moving parts. Due to health and safety issues, the moving parts have to be enclosed perfectly well. These way, accidents are minimised. The materials used here can be recycled. Mild steel can be recycled again and again without any effect. Some parts can also be reused before recycling. Belts on the other hard may be hard to recycle unless they are made from rubber materials. However the rubber used for the belt has been treated to increase the strength. However, it can be used for other application. Works cited Burton, Kenneth S. Band Saw Tips & Tricks. Cincinnati: Popular Woodworking Books, 2004. Print. Duginske, Mark. Band Saw Handbook. New York: Sterling Pub. Co, 1989. Print. Picciuto, David. The New Bandsaw Box Book: Techniques and Patterns for the Modern Woodworker. Nashville, TN: Spring House Press, 2015. Print. Read More