Overload protection systems - Essay Example

? PAPER TOPIC SYSTEM By Overload Protection Systems In industrial applications, overload occurs when machines are loaded beyond their capacity leading to breakdown of the machine. For this reasons, machines must be protected against mechanical overloading using overload protection systems. Depending on the type of overload, the appropriate protection systems are chosen and employed is design of machines. For instance, relays, overload release and circuit breakers are used to protect systems against mechanical overload (Bhalja & Maheshwari, 2011). The most common overload protection systems are mechanical and hydraulic overload protection systems. Mechanical overload protection systems are used in mechanical presses; the working principle is based on a delicate shear plate that requires approximately 130% capacity to be broken. When the shear plate is broken, a slide in position has some space of free distance that allows the press to go through a complete stroke. The shear plate is replaced after each case of overload. Hydraulic overload protection systems are more common because the force of the press can be restricted by controlling the hydraulic pressure, and the press can be restarted after an incidence of overload without having to change the plates (Altan & Tekkaya, 2012). Overload protection systems (a) mechanical (b) hydraulic (Altan & Tekkaya, 2012). The system consists of an oil pad placed between the slide and the connecting rod to which pressure is applied when force is developing. The pressure from the oil is used to move another piston, which acts to intensify the pressurized air. In case the oil pressure (air pressure) exceeds a threshold value, the oil is sapped back into a tank. Therefore, the force applied is restricted and some space is created for free movement (Altan & Tekkaya, 2012). a) Torque guard Torque guards are overload protection devices used in power transmission applications. A spring with a load cam follower placed in a core and detent causes the core and load to rotate jointly. Thus the load always resets in one position and stays in phase when tripped. The system automatically resets when it goes back into the set position after tripping, normally by moving the machine after the overload is cleared. Torque guards assist to protect mechanical systems from damage that may result from extreme torque produced by jamming and overloads. The automatic reset and instant reaction capacity offers consummate guard and reduces down-time. The system disengages at the stipulated torque limit (Grote & Antonsson, 2009). b) Torque limiter A Torque Limiter is a torque overload system consisting of a spring loaded friction style. The load connected to the friction pads is regulated in order to transmit the process torque is transmitted. An overload torque that exceeds the set torque makes the system to slip, preventing overloads from passing through the system. The torque limiter stops slipping when the load drops below the set level, transmitting the torque. In a torque limiter, the phase between input and output is not kept constant. This system can be used in a chain drive with a machined sprocket placed between the friction pads to act as the slip interface so that shutdown and restart is not necessary (Grote & Antonsson, 2009). Torque limiters are regularly used in applications such as conveyor systems, machine tools, and off-highway mobile equipment. c) Axial guard Axial guards are used in systems where the load acts in the linear rather than revolving direction. This overload protection device provides greatly accurate and repeatable trip point. The point of load set is simply adjusted by rotating a screw. The Axial Guard instantly trips when overload occurs and the tie between the load side and drive side is disengaged. The Axial Guard is reset by a slight push or pull in the reverse direction, allowing production to be revived swiftly and downtime to be reduced (Grote & Antonsson, 2009). The mechanical overload system usually operates either by lateral displacement of the interior gearing or free rotation of the primary / intermediate gearbox. Lateral displacement overload systems are frequently used on clarifier drives composed a chain and sprocket set, primary reducer, intermediate reducer, and a spur gear set. The overload system is found on the intermediate gear reducer, usually a worm and worm gear set designed as an internal part of the clarifier drive (Wittbrodt, 2013). During transmission of torque back via the drive, the input worm pinion is made to move sideways with opposition being generated by a spring mechanism. Each torque load on the clarifier is associated with the worm pinion movement. An independent torque monitor device contains a range of limit switches designed to trigger an "alarm" followed by a "shut-off" by closing contacts. Some systems may have a mechanical "shear-pin" placed on the driven sprocket to act as an extra safety device (Altan & Tekkaya, 2012). This pin is made to shear, when a specific torque value is reached, in case the electrical switches do not engage. These overload protection systems provide enough protection to the drive and clarifier, given that they are is correctly set and regularly maintained. The "rotational reducer" overload system was created because many gearbox units in market were not equipped with basic overload devices. In this system, the primary (and sometimes intermediate) gearbox is allowed to rotate under load and the system is made to monitor torque loads on the clarifier. This system operates on a similar design principle as the lateral displacement system; torque is transmitted back via the drive train and the freely rotating reducer as it increases. While the reducer rotates, a plunger is depressed and engages switches situated inside the torque monitor that work on the same principle as those in a lateral displacement system (Vani?c?Ek & Vani?c?Ek, 2008). Rotational overload systems are associated with the common problem of a direct result of unusual loads on the support gearing and bearings which leads to early wear of the intermediate bearings and gears. This leads to incorrect gauge readings and possible failure of the entire overload system. In motor systems, overload occurs when devices are overloaded mechanically past their horsepower rating, causing the motors to draw more current that their ratings. Overload protection is offered at the ends of motor circuit conductors using a motor starter overload device designed to imitate the thermal characteristics of the motors. The condition of overload may not destroy the circuit immediately, but if left to persist for a longer period of time, the excessive current caused by overload conditions may damage the motor and circuits. Electrical connections between overload protection systems and contacts (Senty, 2013). 2. i. A machine is defined as an arrangement of rigid or resistant parts, formed and in a uniform manner such that they move with defined relative movements transmitting force from the source of power to the resistance to be overcome. The two functions of a machine are transmission of force and transmission of definite relative motion. These functions involve rigidity and strength to convey the forces (Vanoff, 2006). ii. When the force applied to a machine can raise a heavier load, the machine saves effort. Mechanical Advantage (MA) refers to the ratio of load to effort in a machine. For a pulley Block with n pulleys, the upper block is fixed to a support while the lower one is movable and the load attached to it. The rope is attached to the lower or upper block, and then passed through a fixed and movable pulley in turn and finally over a fixed pulley. The effort is applied at the free end. The pulleys are assumed to be frictionless and tension round the rope is equally distributed and is equal to the effort. The number of position of the rope in the lower block is r, the total upward force on it is rP and must be equivalent to the load L. Therefore;             rP = L + l, when l, is the weight of the lower block. M.A. = L/P = r- L/p If M.A.>1, the load is greater than effort applied; this implies that the effort can move a heavier load. The greater the mechanical advantage, the less the effort applied to the machine. However, in practical situations, the weight of the machine and friction also affect the load that can be lifted and the mechanical advantage.  Velocity Ratio (VR) of a machine is the ratio of the distance moved by effort to the distance moved by load. For this system, VR = n, (n = number of ropes between the sets of pulleys). The design of machines determines their velocity ration, depending on the friction and the weight of the machines. A velocity ration Read More