Types of Belts and Pulleys - Research Paper Example

Types of Belts and Pulleys Belts: Belts are used for power transmission from one shaft to another quietly, smoothly and economically (Jadon & Verma, 2010). They are made of continuous construction from materials like leather strands, fabric, rubberized cord or reinforced rayon, nylon, steel or glass fiber. Belts are used on sheaves and shaft mounted pulleys. There are various types of belts designed for different applications. These include the V-belt, flat belts, and positive drive belts. V belts: These are the most commonly used belts, having a variety of operating conditions and applications. These belts operate efficiently at speeds of 1500-6500 feet per minute and temperatures of between -30 to 185 degrees Fahrenheit. These belts are categorized into industrial (narrow, conventional, light-duty and double V cross section), automotive and agricultural. V belts have reinforcing cords in the belt that provide load-carrying capacity. This reinforcement is made of nylon, steel, rayon or glass fibers embedded in a cushion section (soft rubber material). The cushion section is externally covered with tough rubber and the interior is covered with a material that is resistant to abrasion. As the belt bends around the pulley, the length of its pitch line remains the same. The pitch diameter if the pulley is determined using the pitch line (Jadon & Verma, 2010). Flat belts: These types of belts are normally used in applications where high speeds are required or long center distances are involved. Flat belts are more efficient at high speeds compared to low speeds since they tend to slip under load. They are also used where drives which have nonparallel shafts are necessary as they allow twisting to contain the relationship of the shafts. Positive drive belts: These belts have a notched underside that establishes contact with a pulley with similar design on the circumference. The belts have similar benefits to chain and gear drives because of their positive contact with the pulley. Positive drive belts are appropriate in operations where high efficiency, constant velocity of timing is required. They can also be used to reduce the size of the pulley and provide the same operating performance as large sized V-belt pulleys (Jadon & Verma, 2010). Pulleys Pulleys are components that are used for power transmission in machines by mounting them on shafts over which belts run. These components are generally made of iron, steel plates or by welded construction. Pulleys may be of a split type or a single piece depending on the application. The types of pulleys can be categorized based on the belts used with the pulley into: a. Flat belt pulleys: These pulleys are of varying shapes and sizes, designed to fit the functional requirements. The main types of pulleys used with flat belts include: Armed Pulley The boss, spokes/arms and rim are the main parts of a pulley. The arms of the pulley are either curved or straight while the cross section is the shape of an ellipse. The rims of these pulleys have a slight convexity to prevent the belt from axial slipping during operation. Pulley with a Web The boss and rim of a pulley are connected with a web in the form of a disk when the diameter of the pulley is very small. Holes are made in the web to make the pulley lighter. Cone Pulleys These types of pulley are mounted on the driver and driven shafts facing opposite sides to offer varying speed ratios between the two and maintain the speed of the driver shaft at a constant. The steps in the two pulleys have a diameter designed such that one belt can operate on the surface of any pair of steps. This type of pulley is used in drilling machines and lathes. Fast and Loose Pulleys In applications that involve a number of machines operated from one source of power, each machine has a fast and loose pulley configuration. Using this arrangement, any machine can be stopped or started independently while the shaft runs. Using a keyed joint, the fast pulley is mounted on the shaft while the loose pulley is lefty to run freely on the shaft. The lose pulley has a diameter which is slightly less than that of the fast pulley, therefore tensioned is reduced when the belt is shifted on the loose pulley. This configuration only transmits power when the belt is on the fast pulley (Textbook of machine design, 2007). b. V-belt Pulleys When using a V belt is used to transmit power, the rim of the pulley is modified using wedge shaped grooves to allow the belts to run on the grooves. The types of pulleys used with V-belts include: Step Cone Pulley This pulley operates on the same principle and purpose as the one for step cone pulley for flat belt. Rope Pulley When transmitting power over large distances, a rope drive is used. The ropes are made of manila, cotton or hemp and fitted into grooves on the circumference of the pulleys used to transmit power. The drive can either use single of multiple ropes. The pulley is designed similar to a V-belt but slight changes are made to the groove. It is preferred for transmitting power between shafts at varying distances and different elevations. In high power transmission and overhead cranes, steel ropes are used (Kong, & Parker, 2006). Failure Modes and Mechanisms, Materials Used In Various Belts and Pulleys Belts i. Normal Belt Wear and Failure This failure occurs the belt reaches its critical tensile cord fatigue life, after operation for a long period of running. This is considered to be an ideal type of failure. For V-belts, synchronous belt teeth may fail after a long time of service; this is deemed a non-ideal kind of belt failure. The belt teeth may look worn out and protruding fibers from the jacket may make the belt teeth appear fuzzy (Kong, & Parker, 2006). ii. Belt Crimp Failures This type of belt failure often bears a resemblance to a straight tensile failure. This straight type of break may be due to bending of the belt tensile cords around an extremely small diameter. A sharp bend might cause great compressive forces in the tensile parts forcing fibers to crimp or crimp. This reduces the general critical tensile strength of the belt. Belt crimp failures are most commonly related to inadequate belt installation tension, belt mishandling, sub-minimal sprocket diameters, or entrance of strange objects into the belt drive. Belt crimp failures due to poor handling may be due to improper packaging, inappropriate storage practices, and handling of the belt before and during installation (Kong, & Parker, 2006). When belts are operating in under-tensioned state, the belt teeth may ride out of the sprockets until an adequate level of belt tension is reached. This occurrence is referred to as “self-tensioning”, and can be clearly observed at the point of minimum dynamic belt span tension, or at the point of entry of the belt teeth into the driven sprocket grooves. As the belt undergoes self-tensioning, its teeth slide up outside the sprocket grooves until the belt teeth back are forced down into the sprocket grooves by higher span tension from the approaching fixed side tension. Belt tensile cord damage can occur at the point where the teeth are forced back downward into the grooves of the sprocket. This is because the force creates a sharp, transitory point of bending. This point where tensile cord damage occurs is called a crimp. The belt ratchets if the side tension fails to force the belt teeth back downward into the grooves of the sprocket. Ratcheting of the belt can also cause belt tooth damage and tensile cord crimp. Crimping and belt tensile cord damage can also occur if the belt is subjected to sub-nominal bend diameters. This can either be due to flat backside idlers or sprockets in sub-nominal sizes, or excessively sharp hand bending of the belt (Kong, & Parker, 2006). If foreign objects enter the space between the belt and sprocket, belt crimp failures may occur. The foreign object may shove the belt away from the sprocket at a sharp angle, resulting in a point of tensile cord crimp. In addition, crimping may be caused by tools used to force belts onto sprockets, such as bars or screwdrivers. Belts may not fail completely when subjected to inappropriate use of tools or foreign objects, but their overall lifetime will be reduced. iii. Shock Load Shock loading occurs in belt drives when cyclic or intermittent torque loads higher than normal are produced by the driven equipment. These shock loads may cause belt stresses that are higher than normal and may encourage belt failure. Whereas normal V-belt drives might experience intermittent slip when subjected to peak torque load situations, synchronous belt drives have to convey the total magnitude of the peak loads (The SAE journal, 1998). Serious shock loads may cause tensile breaks in belts with uneven and a ragged appearance. The specific belt teeth placed in the sprocket at the time of the shock load can also exhibit tooth shear or develop root cracks. The remaining belt teeth may appear normal if the shock load is cyclical and repetitious at one particular point around the belt or occurred only once. Pulleys A fixed pulley uses more effort than the load to lift the load from the ground. When connected to an unmovable object, the fixed pulley acts as a first class lever with the fulcrum being positioned at the axis, but the arrangement can be slightly changed so that the bar becomes a rope.  The fixed pulley has one advantage in that when using it, the pulley does not have to be pushed up and down. However, it is disadvantageous since more effort than load has to be applied (Rattan, 2005). A movable pulley moves with the load. It allows the effort to be less compared to the load and also acts as a second class lever. The position of the load is between the fulcrum and the effort. This pulley has the advantage of allowing less effort to be used to pull the load. The disadvantage of a movable pulley is that one does not have to push or pull the pulley up and down. It is easier to use a combined pulley because the effort required to lift the load is less than half the load. It also travels a longer distance. When a second pulley is added, the amount of effort required to lift the heavy load is much less. For instance, lifting a box of weight 150 N requires a force of 150 N to be exerted without pulleys. However, when two pulleys are used, only 50 N of force is required (Kong, & Parker, 2006). Flat belts and V-belts: These may be used to transmit power from one shaft to another where unnecessary to keep an accurate speed ratio between the two shafts. Power losses as a result of slip and creep contribute to from three to five percent for most belt drives. The following design has assumed that the shafts are parallel (Singh & Joshi, 2006). Belt Design This either involves the proper belt choice to transmit a requisite power or determining the power that might be transmitted by a V-belt or flat belt. In the first instance, the width of the belt is unknown, while in the second one, the width is known. We assume the thickness of the belt thickness for both cases. The power transmitted by a belt drive is a function of the tension in the belt and the speed of the belt. Power = (T1 –T2) v, W or = (T1 –T2) V/550, hp (Rao & Durgaiah, 2005). Where: T1 = belt tension in tight side, N T2 = belt tension in loose side, N V = belt speed, m/s The formula for finding the stress, σ2, for the flat belts is used when the width of the belt is unknown but the thickness is given: Where: = maximum allowable stress, N/m2 = stress in the slack side of the belt, N/m2 m = mass of 1.0 meter of belt 1.0 m2 in cross section, kg f = coefficient of friction between belt and pulley  = angle of wrap of belt on pulley, rad. (Narayana et al., 2009) The required cross-section area of the flat belt in this case of the width unknown can be determined using the formula: Required area = Therefore, the required flat belt width b is: B = area/thickness We may determine the value of (T1 –T2) from the horsepower requirement, P = (T1 – T2) V, W The allowable stress of the belt material determines the tension in the tight side of the belt. Normally, the allowable stress for rubber belting is between 1.0 and 1.7 MN/m2 and allowable tensile stress for leather belting is from 2.0 to 3.45 MN/m2. This will depend on the quality of the material. It is easy to obtain leather belting in different single ply thicknesses. Double and triple ply belts can also be obtained (Kong, & Parker, 2006). Using the following formula, we can determine the value of T2 for both V-belt and flat belts is used when we know the width and thickness of the belt: Where: m = b.t.p = mass of 1.0 meter of belt, kg/m b = belt width, m, t = belt thickness, m, p = belt density, kg/m3  = groove angle for the V-belt ( is 180 for a flat belt). The quantity mV2 is as a result of centrifugal force, which tends to cause the belt to leave the pulley and lower the power that may be transmitted (Kong, & Parker, 2006). Angle of wrap: We may determine the angles of wrap for an open belt by: (Meriam & Kraige, 2003). For a crossed belt drive, the angles of wrap may be determined by: When the tangential force F exceeds the frictional force between two surfaces, the surfaces start to slide relative to each other. Normally, the sliding frictional resistance is different from the static frictional resistance. We can express the coefficient of sliding friction using a formula similar to that of the static coefficient. The coefficient of sliding friction is generally less than the static coefficient of friction (Madsen, 2004). Determine the coefficient of sliding friction for rubber against wood. Diameter of large pulley:3.667 in. radius = 1.8335 Diameter of smaller pulley: 2.737 in. radius = 1.3685 Belt thickness 0.141 in Sin a = (1.8335-1.3685)/0.141 = 3.2978 a = 0.0575 References Jadon, V. K., & Verma, S. (2010). Analysis and design of machine elements. New Delhi: I.K. International Pub. House. Kong, L., & Parker, R. G. (2006). Mechanics And Sliding Friction In Belt Drives With Pulley Grooves. Journal of Mechanical Design, 128(2), 494. Madsen, D. A. (2004). Engineering drawing & design. Albany, N.Y: Delmar Thomson Learning. Meriam, J. L., & Kraige, L. G. (2003). Engineering mechanics (5th ed.). Hoboken, N.J.: Wiley. Narayana, K. L., Kannaiah, P., & Reddy, K. V. (2009). Machine drawing. New Delhi: New Age International Publishers. Rao, M. V. S., & Durgaiah, D. R. (2005). Engineering mechanics. Hyderabad: University Press (India. Rattan, S. S. (2005). Theory of machines. New Delhi: Tata McGraw-Hill. Singh, M. D., & Joshi, J. G. (2006). Mechatronics. Delhi: Prentice-Hall of India. The SAE journal. (1998). New York, N.Y: Society of Automotive Engineers. Textbook of machine design. (2007). S.l.: Laxmi Publications. Read More