Industrial Machinery Repair Part 10 pptx

Gears and Gearboxes 299 Figure 14.24 Miter gears with spiral teeth viewed from opposite sides; changing the position of the gear cannot change the hand of the tooth’s helix angle. A pair of helical gears, as illustrated in Figure 14.27, must have the same pitch and helix angle but must be of opposite hands (one right hand and one left hand). Helical gears may also be used to connect nonparallel shafts. When used for this purpose, they are often called “spiral” gears or crossed-axis helical gears. This style of helical gearing is shown in Figure 14.28. Worm The worm and worm gear, illustrated in Figure 14.29, are used to transmit motion and power when a high-ratio speed reduction is required. They provide a steady quiet transmission of power between shafts at right angles. The worm is always the driver and the worm gear the driven member. Like helical gears, worms and worm gears have “hand.” The hand is determined by the direction of the angle of the teeth. Thus, in order for a worm and worm gear to mesh correctly, they must be the same hand. 300 Gears and Gearboxes Figure 14.25 Typical set of helical gears Helix an g le Figure 14.26 Illustrating the angle at which the teeth are cut The most commonly used worms have either one, two, three, or four sep- arate threads and are called single, double, triple, and quadruple thread worms. The number of threads in a worm is determined by counting the number of starts or entrances at the end of the worm. The thread of the Gears and Gearboxes 301 Angle Angle Hub on left side Hub on right side Figure 14.27 Helix angle of the teeth the same regardless of side from which the gear is viewed Figure 14.28 Typical set of spiral gears 302 Gears and Gearboxes Figure 14.29 Typical set of worm gears worm is an important feature in worm design, as it is a major factor in worm ratios. The ratio of a mating worm and worm gear is found by dividing the number of teeth in the worm gear by the number of threads in the worm. Herringbone To overcome the disadvantage of the high end thrust present in helical gears, the herringbone gear, illustrated in Figure 14.30, was developed. It consists simply of two sets of gear teeth, one right-hand and one left-hand, on the same gear. The gear teeth of both hands cause the thrust of one set to cancel out the thrust of the other. Thus, the advantage of helical gears is obtained, and quiet, smooth operation at higher speeds is possible. Obviously they can only be used for transmitting power between parallel shafts. Gear Dynamics and Failure Modes Many machine trains utilize gear drive assemblies to connect the driver to the primary machine. Gears and gearboxes typically have several vibration spectra associated with normal operation. Characterization of a gearbox’s Gears and Gearboxes 303 Figure 14.30 Herringbone gear vibration signature box is difficult to acquire but is an invaluable tool for diagnosing machine-train problems. The difficulty is that: (1) it is often difficult to mount the transducer close to the individual gears and (2) the number of vibration sources in a multigear drive results in a complex assort- ment of gear mesh, modulation, and running frequencies. Severe drive-train vibrations (gearbox) are usually due to resonance between a system’s nat- ural frequency and the speed of some shaft. The resonant excitation arises from, and is proportional to, gear inaccuracies that cause small periodic fluctuations in pitch-line velocity. Complex machines usually have many resonance zones within their operating speed range because each shaft can excite a system resonance. At resonance these cyclic excitations may cause large vibration amplitudes and stresses. Basically, forcing torque arising from gear inaccuracies is small. However, under resonant conditions torsional amplitude growth is restrained only by damping in that mode of vibration. In typical gearboxes this damping is often small and permits the gear-excited torque to generate large vibration amplitudes under resonant conditions. One other important fact about gear sets is that all gear-sets have a designed preload and create an induced load (thrust) in normal operation. The direc- tion, radial or axial, of the thrust load of typical gear-sets will provide some 304 Gears and Gearboxes insight into the normal preload and induced loads associated with each type of gear. To implement a predictive maintenance program, a great deal of time should be spent understanding the dynamics of gear/gearbox operation and the fre- quencies typically associated with the gearbox. As a minimum, the following should be identified. Gears generate a unique dynamic profile that can be used to evaluate gear condition. In addition, this profile can be used as a tool to evaluate the operating dynamics of the gearbox and its related process system. Gear Damage All gear sets create a frequency component, called gear mesh. The funda- mental gear mesh frequency is equal to the number of gear teeth times the running speed of the shaft. In addition, all gear sets will create a series of sidebands or modulations that will be visible on both sides of the primary gear mesh frequency. In a normal gear set, each of the sidebands will be spaced at exactly the 1X or running speed of the shaft and the profile of the entire gear mesh will be symmetrical. Normal Profile In addition, the sidebands will always occur in pairs, one below and one above the gear mesh frequency. The amplitude of each of these pairs will be identical. For example, the sideband pair indicated, as −1 and +1 in Figure 14.31, will be spaced at exactly input speed and have the same amplitude. If the gear mesh profile were split by drawing a vertical line through the actual mesh, i.e., the number of teeth times the input shaft speed, the two halves would be exactly identical. Any deviation from a symmetrical gear Frequency Gear mesh Ϫ4 ϩ4 Ϫ3 ϩ3 Ϫ2 ϩ2 Ϫ1 ϩ1 Amplitude Figure 14.31 Normal profile is symmetrical Gears and Gearboxes 305 mesh profile is indicative of a gear problem. However, care must be exer- cised to ensure that the problem is internal to the gears and induced by outside influences. External misalignment, abnormal induced loads and a variety of other outside influences will destroy the symmetry of the gear mesh profile. For example, the single reduction gearbox used to trans- mit power to the mold oscillator system on a continuous caster drives two eccentrics. The eccentric rotation of these two cams is transmitted directly into the gearbox and will create the appearance of eccentric meshing of the gears. The spacing and amplitude of the gear mesh profile will be destroyed by this abnormal induced load. Excessive Wear Figure 14.32 illustrates a typical gear profile with worn gears. Note that the spacing between the sidebands becomes erratic and is no longer spaced at the input shaft speed. The sidebands will tend to vary between the input and output speeds but will not be evenly spaced. In addition to gear tooth wear, center-to-center distance between shafts will create an erratic spacing and amplitude. If the shafts are too close together, the spacing will tend to be at input shaft speed, but the amplitude will drop drastically. Because the gears are deeply meshed, i.e., below the normal pitch line, the teeth will maintain contact through the entire mesh. This loss of clearance will result in lower amplitudes but will exaggerate any tooth profile defect that may be present. If the shafts are too far apart, the teeth will mesh above the pitch line. This type of meshing will increase the clearance between teeth and amplify the 19 Hz 16 Hz 20 Hz 1x INPUT FREQUENCY AMPLITUDE Figure 14.32 Wear or excessive clearance changes sideband spacing 306 Gears and Gearboxes FREQUENCY AMPLITUDE Figure 14.33 A broken tooth will produce an asymmetrical sideband profile energy of the actual gear mesh frequency and all of its sidebands. In addition, the load bearing characteristics of the gear teeth will be greatly reduced. Since the pressure is focused on the tip of each tooth, there is less cross- section and strength in the teeth. The potential for tooth failure is increased in direct proportion to the amount of excess clearance between shafts. Cracked or Broken Tooth Figure 14.33 illustrates the profile of a gear set with a broken tooth. As the gear rotates, the space left by the chipped or broken tooth will increase the mechanical clearance between the pinion and bullgear. The result will be a low amplitude sideband that will occur to the left of the actual gear mesh frequency. When the next, undamaged teeth mesh, the added clearance will result in a higher energy impact. The resultant sideband, to the right of the mesh frequency, will have much higher amplitude. The paired sidebands will have nonsymmetrical amplitude that represents this disproportional clearance and impact energy. If the gear set develops problems, the amplitude of the gear mesh frequency will increase, and the symmetry of the sidebands will change. The pat- tern illustrated in Figure 14.34 is typical of a defective gear set. Note the asymmetrical relationship of the sidebands. Common Characteristics You should have a clear understanding of the types of gears generally utilized in today’s machinery, how they interact, and the forces they gen- erate on a rotating shaft. There are two basic classifications of gear drives: (1) shaft centers parallel, and (2) shaft centers not parallel. Within these two classifications are several typical gear types. Gears and Gearboxes 307 LOW SPEED SHAFT ROTATIONAL FREQUENCY HIGH SPEED SHAFT ROTATIONAL FREQUENCY INTERMEDIATE FREQUENCIES SIDE BANDS TWICE GEAR MESH THREE TIMES GEAR MESH GEAR MESH FREQUENCY Figure 14.34 Typical defective gear mesh signature Shaft Centers Parallel There are four basic gear types that are typically used in this classification. All are mounted on parallel shafts and, unless an idler gear in also used, will have opposite rotation between the drive and driven gear (if the drive gear has a clockwise rotation, then the driven gear will have a counterclockwise rotation). The gear sets commonly used in machinery include the following: Spur Gears The shafts are in the same plane and parallel. The teeth are cut straight and parallel to the axis of the shaft rotation. No more than two sets of teeth are in mesh at one time, so the load is transferred from one tooth to the next tooth rapidly. Usually spur gears are used for moderate- to low-speed applications. Rotation of spur gear sets is opposite unless one or more idler gears are included in the gearbox. Typically, spur gear sets will generate a radial load (preload) opposite the mesh on their shaft support bearings and little or no axial load. 308 Gears and Gearboxes Backlash is an important factor in proper spur gear installation. A certain amount of backlash must be built into the gear drive allowing for tolerances in concentricity and tooth form. Insufficient backlash will cause early failure due to overloading. As indicated in Figure 14.11, spur gears by design have a preload opposite the mesh and generate an induced load, or tangential force, TF, in the direction of rotation. This force can be calculated as: TF = 126,000 ∗hp D p ∗ rpm In addition, a spur gear will generate a Separating Force, S TF , that can be calculated as: S TF = TF ∗tan φ Where: TF = Tangential force hp = Input horsepower to pinion or gear D p = Pitch diameter of pinion or gear rpm = Speed of pinion or gear φ = Pinion or gear tooth pressure angle Helical Gears The shafts are in the same plane and parallel but the teeth are cut at an angle to the centerline of the shafts. Helical teeth have an increased length of contact, run more quietly and have a greater strength and capacity than spur gears. Normally the angle created by a line through the center of the tooth and a line parallel to the shaft axis is 45 degrees. However, other angles may be found in machinery. Helical gears also have a preload by design; the critical force to be considered, however, is the thrust load (axial) generated in normal operation; see Figure 14.12. TF = 126,000 ∗hp D p ∗ RPM S TF = TF ∗tan φ cos λ T TF = TF ∗tan λ [...]... time in seconds for 60 cubic centimeters of a fluid to flow through the orifice of the Standard Saybolt Viscometer at a given temperature under specified conditions.) Temperatures taken are 100 ◦ F and 210 F (100 ◦ , 130◦ , 210 F—Shell Oil) For example, one sample of oil will take 60 seconds to flow through the opening, while another sample of oil of the same volume takes 600 seconds The oil taking 60 seconds... can lead to positive or negative consequences See Figure 15.2 Press Hydraulic System Hydraulic Fluid Samples Potential component failure Particle count / PPM Component failure 200 150 100 50 0 1 2 3 4 5 6 7 Month Monthly samples Figure 15.2 Hydraulic fluid samples 8 9 10 11 12 Hydraulics 323 Fluid analysis will prove the need for better filtration The addition of a 3-micron absolute return line filter to... or tubing 316 Hydraulics Best Maintenance Hydraulic Repair Practices In order to maintain your hydraulic systems, you must have preventive maintenance procedures and you must have a good understanding and knowledge of “Best Maintenance Practices” for hydraulic systems We will convey these practices to you See Table 15.1 Table 15.1 Best maintenance repair practices: hydraulics Component Component knowledge... they must be accurate and understandable by all maintenance personnel from entry level to master Preventive maintenance procedures must be a part of the PM job plan, which includes (see Figure 15.1): ● Tools or special equipment required for performing the task; ● Parts or material required for performing the procedure with store room number; ● Safety precautions for this procedure; ● Environmental concerns... properly written and followed properly will allow equipment to operate to its full potential and life cycle Preventive maintenance allows a maintenance department to control a hydraulic system rather than the system controlling the maintenance department We must control a hydraulic system by telling it when we will perform maintenance on it and how much money we will spend on the maintenance for the... the downtime (tracked daily) ● Parts and material cost? ● Labor cost? 322 Hydraulics ● Production downtime cost? ● Any other cost you may know that can be associated with a hydraulic system failure? 2 Track hydraulic system fluid analysis Track the following from the results (taking samples once a month): ● Copper content ● Silicon content ● H2 O ● Iron content ● ISO particulate count ● Fluid condition... identify specific forcing functions or root causes of gear failure 15 Hydraulics “Only Permanent Repairs Made Here” Hydraulic Knowledge People say knowledge is power This is true in hydraulic maintenance Many maintenance organizations do not know what their maintenance personnel should know We believe in an industrial maintenance organization where we should divide the hydraulic skill necessary into two... reservoir oil if needed Install a petcock valve on the front of the reservoir, which will be used for consistent oil sampling Equipment and parts needed: ● Quick disconnects that meet or exceed the flow rating of the portable filter pump ● Two gate valves with pipe nipples ● One 10- micron filter breather 326 Hydraulics WARNING: Do not weld on a hydraulic reservoir to install the quick disconnects or air filter... and their relationship to a hydraulic system) ● Calculating flow, pressure, and speed ● Calculating the system filtration necessary to achieve the system’s proper ISO particulate code Hydraulics 315 Skill: ● Trace a hydraulic circuit to 100 % proficiency ● Set the pressure on a pressure compensated pump ● Tune the voltage on an amplifier card ● Null a servo valve ● Troubleshoot a hydraulic system and utilize... maintenance organization where we should divide the hydraulic skill necessary into two groups One is the hydraulic troubleshooter; they must be your experts in maintenance, and this should be as a rule of thumb 10% or less of your maintenance workforce The other 90% plus would be your general hydraulic maintenance personnel They are the personnel that provide the preventive maintenance expertise The percentages . system filtration necessary to achieve the system’s proper ISO particulate code Hydraulics 315 Skill: ● Trace a hydraulic circuit to 100 % proficiency ● Set the pressure on a pressure compensated. Characteristics You should have a clear understanding of the types of gears generally utilized in today’s machinery, how they interact, and the forces they gen- erate on a rotating shaft. There are two. rotation, then the driven gear will have a counterclockwise rotation). The gear sets commonly used in machinery include the following: Spur Gears The shafts are in the same plane and parallel. The teeth