Helical Gears and Vibration In Terms Of PeakVue - Research Paper Example

Helical Gears and Vibration In Terms Of PeakVue Introduction The extraction of vibration signals emitted by rotating gears in machineries tends to reveal vast and vital information pertaining to the operating conditions of various machineries. In order to obtain concrete information, these vibrations signals usually processed with special regard at a specific location of the whole machinery component. This paper analyzes helical gears and the probable reasons for failure such gears. Moreover, the paper also highlights the how vibrations work in monitoring and subsequent diagnosis of faults in rotating gears. Vibrations According to Ebrahimi (2012) the application of vibration analysis has been quite successful in a number of areas. In rotary machinery, vibration analysis is quite effective in the detection of primary characteristics of machines and consequent extraction of faulty characteristics. Vibration analysis for faults in gear systems is largely employed in various industries, such as transport and aviation, where train drives as well as helicopter engines among other rotor systems are usually fitted with vibration sensors for monitoring their health condition. Each machine even in their best operating condition, tends to have some form of vibration, which will have a certain level that may be deemed natural or normal (Sheffer and Girdhar 2004). Nevertheless, when a machinery vibration increases, such a change can be attributed to a mechanical problem. In helical gears, just as is the case with other types of gears, vibrations can be associated with gear tooth contacts. There are different techniques employed in the detection of faults by vibration analysis. According to Shreve (1994), these include: the time-domain technique, frequency domain technique and the time-frequency domain technique. The time domain technique normally entails analyzing certain statistical aspects of vibration signals like standard deviation, the peak level as well as the skewness of a vibration signal. On the other hand, the frequency domain technique is a confirmatory approach that normally provides additional information based on the analysis of the time-frequency details of a vibration signal. This is achieved by the use of power spectra and vibration amplitude (Shreve 1994). Adams (2001) explains that vibration analysis is quite efficient due to the fact that virtually all rotor machineries have some vibration at least once in a single revolution frequency component. This is due to the fact that it is almost impossible to have a perfectly balanced rotor. He further expounds that there are certain quantified safe upper limits for acceptable levels of vibration on nearly all types of rotating gears. PeakVue Vibration frequency signals can be analyzed through a special technique known as PeakVue. PeakVue is a technique that normally serves to capture the peak value of stress waves produced by gear vibrations and consequently obtain the repletion frequency of the impacts through a spectral analysis. Based on the Emerson Process Management report (2011), the technique can be quite useful in detection of stress waves as a result of contact at an early stage of impact-related failures such as gear loss due to wear or contamination. By measuring the true peak acceleration at high frequencies, the technique provides an indication of an imminent failure (Emerson Process Management. 2011). Vibration sensors normally detect analog signals from a gear vibration, which is then digitally processed and converted into digital signal that is expresses in g or velocity units. The signal is subsequently channeled through a high order low pass filter in order to rid off signals that are within the acceptable range and therefore greater than the Nyquist frequency rate (Narayana 2006). Narayana (2006) further explains that PeakVue is quite instrumental in vibration analysis, especially since it separates low energy faults that occur in gears, thereby enhancing the signal causing the faults to stand above the spectral noise level. This makes the faults easier to identify. PeakVue first separates the stress waves from the vibration waveform using a high pass filter. It is conditioned to enhance amplitude and pulse width of the gear shift vibrations making it Fast Fourier Transform (FFT)-friendly (Emerson Process Management. 2011). PeakVue is therefore quite crucial in revealing some faults that may have gone undetected in their earlier stages, perhaps because they may have been buried in the noise floor of the vibration spectrum. Helical gears A helical gear is a type of gear that is characterized by teeth that are formed on cylinder, which are inclined at an angle such that each tooth wraps around the gear in the shape of a shallow helix (Narayana 2006). They tend to have spiral teeth. Helical gears serves to transmit motion from one machine compartment to another. Meshing of two complimentary gears will normally result in the rotation of the other gear when one of the gears is turned. Helical gears are identified by having teeth that are inclined to the axis of rotation at a helix angle. When using helical gears, their shaft bearings tend to be subjected to thrust loads that can be resisted by using a double helical gear, which is almost equal to two helical gears of opposite hand mounted in the same shaft. In a helical gearing, one gear usually has a right hand helix while the other, a left helix. Such gearing is nearly noiseless because of their gradual engagement of the teeth during meshing (Narayana 2006). They are regarded as one of the most complex types of gears and also among the most expensive in manufacturing. They offer the advantage of producing less noise and vibration when used in high-speed machinery. According to Wicker and Lewis (2006), in a typical helical gearset, tooth-to-tooth contact normally starts at one edge of a tooth and proceeds gradually across its width, thereby smoothing out teeth engagement and disengagement. Narayana (2006) further confirms that helical gears are also widely preferred due to their capability to carry greater torque and power compared to other related types of gears, such as the spur gear. This is attributed to the fact that helical gears’ tooth-to-tooth forces are spread over more surface area and the contact stresses are reduced. Depending on the diameter of the two gears, there is usually a variation of torque as well as speed. Helical gears are mainly used to convey motion between two shafts that do not have an intersection and can be at any angle to one another. There are two main ways in which two helical gears can be meshed: crossed and parallel. In crossed meshing of a pair of helical gears, the shafts of the gears are oriented at the sum of the helix angle, whereas parallel meshing entails adjusting the gear shafts at the difference of the 45 degree helical angles of the two complimentary gears. In crossed meshing, point contact between tooth surfaces is the most commonly achieved configuration, implying that the helices lack tangential contact. On the contrary, in parallel configuration the helices of the meshing teeth converge with each other at a tangent thus, the contact between the two gears therefore forms a curve that extends across the face widths of the two gears. Parallel meshing is commonly preferred and deemed mechanically firm (Narayana 2006). Faults There are a number of defects that tend to occur on gears. For instance, a local defect such as a crack on a helical gear tooth can generate a short duration impulsive signal. This will in turn lead to the production of a series of amplitude and modulation effects to meshing components of the gears. These modulations will then be spread over various frequencies by the action of several sidebands of the tooth meshing component frequency (Sheffer and Girdhar 2004). Just like any other gear, a helical gear also has a Gear Mesh Frequency (GMF) which is the product of the running speed of a gear and the total number of teeth on the gear shaft. As such, a gearbox will tend to have different peaks due to the shaft variation. All peaks usually have low amplitudes which can only be excited by the occurrence of a fault. There are a numerous gearing defects that can interfere with the proper functioning of a gear system. These are usually found in gearboxes. Defects in gearboxes can lead to a low-frequency rhythm in the vibration spectrum or high activity in the high-frequency spectrum depending on the effect of the defect on the gear teeth impact. Sheffer and Girdhar (2004) posit that the amplitudes of vibrations produced by helical gears tend to have a directional proportionality with the amount of dynamic forces generated, such that an increase in the force will automatically result in an increase in the vibrations. According to Eftekharnejad (2010), one reason for the development of a fault in rotor machineries is tooth wear and backlashes of the gears, which have the capability to excite the natural frequencies of gears, sidebands as well as their GMFs that can be analyzed by the PeakVue analysis technique. These frequencies tend to be spaced within the running speed of the faulty gear. Another possible fault that is likely to affect the proper functioning of a gear, which can be detected by vibration signals, as explained by Sheffer and Girdhar (2004) is the increase in load of the gear tooth. An increase in the load on a gearbox will concurrently lead to an increase in the gear system GMF. A high GMF may increase sideband amplitude indicating the presence of wear. Considerably high amplitude of sidebands around the GMF is often an indication of backlash or eccentricity of gear shafts. Such faults usually imply that the rotation of one gear may lead to the modulation of the gear vibration amplitude at the running speed of another gear. This is normally detected by the incorporation of the time-domain vibration analysis technique. A backlash often excites both the natural frequency as well as the GMF of a gear. The faulty gear can be determined by the analysis of the various spacing of the sideband frequencies (Sheffer and Girdhar 2004). Sheffer and Girdhar (2004) further posit that gear failure may also arise as a result of a broken or a cracked tooth of a gear. Such cases normally result in the increase of the amplitude that consequently excites the natural frequency of a gear. Detection of such anomaly can be achieved through the time-frequency domain, which will indicate the sharp rise on each instance when the faulty tooth meshes with a good tooth of a complementary gear. Additionally, a gear fault can arise due to misalignment, which in most instances, may cause some excitement to the higher harmonics of the GMF. This is detected by the presence of sidebands in the running speed. In particular, there will be higher amplitudes at higher GMFs and lower amplitudes at lower GMF (Sheffer and Girdhar 2004). Furthermore, they also present that some manufacturing processes or gear mishandling may lead to hunting tooth problems. These problems have very high vibrations but with low frequencies such that they may pass unnoticed. Gears with such problems can also produce certain beat frequencies that are audible to the human ear. They tend to emit repetitive growling vibration sounds that can be established by counting with a stop watch as explained by Yung (2011). It is important to note that helical and spur gears normally experience both rolling and sliding on one flank of the pitch point as well as pure rolling at the pitch point. In spite of this, during gear mesh, the pitch point is usually not passed simultaneously along the gear width because of the helical gear geometry. This therefore implies that the pitch point contact for meshing of helical gears is somehow progressive in contrast to spur gear meshing. A number of researchers who have applied Acoustic Emission (AE) under both rolling and sliding conditions have observed that sharp contact plays a role in the initiation of AE. Lubricants are regarded as one of the most important elements in the maintenance of the mechanical integrity of gears. In their operation, gears normally operate at a certain film thickness designated λ to denote the ratio of thickness of oil film to the combined surface roughness of a gear. The value of λ is usually not easy to ascertain in the course of an operation. Nevertheless, elastohydrodynamic lubrications (EHL) studies have indicated that there are several factors that tend to influence the oil film thickness. They include: load, temperature, the speed as well as surface roughness of the meshing gears. Increase in speed generally tends to increase the background noise levels of the acoustic emissions from other components of a gearbox such as gears and bearings. Serrato et al. point out that speed increase of rolling elements can result in an increase in vibration damping that commonly occurs between the rolling components and the race way because of an increase in the thickness of the oil film. In summary, helical gears can be distinguished from other types of gears by their unique 450 inclination of their teeth. They are also distinct from other types of gears due to the fact that they are almost noiseless because of their gradual engagement of teeth during meshing with other complimentary gears. Since like any other machinery gears also produce vibrations, analysis of vibration is very important in constant monitoring and diagnosis of faults of such gears. The most appropriate and modern technique of vibration analysis is the PeakVue technique which tends to separate stress waves from vibration waveform while enhancing amplitude and pulse thereby detecting minute vibrations that may have been buried in the noise floor of vibration spectrum. The most probable gear faults that are normally responsible for the abnormal vibrations include tooth misalignment, tooth wear, increase load of tooth gear and cracked or broken tooth. References Adams, Maurice L, 2001. Rotating Machinery Vibration From Analysis to Troubleshooting. New York: Marcel Dekker, Inc Ebrahimi, Ebrahim, 2012. "Fault diagnosis of Spur gear using vibration analysis ." Journal of American Science, vol. 8, no. 1, pp.133-138. Eftekharnejad, Babak, July 2010."Condition monitoring of gearboxes using Acoustic Emission ." Phd Thesis. Cranfield University Emerson Process Management, 2011. "PeakVue Analysis of Antifriction Bearing Fault Detection”, Emerson Process Management, viewed 29 March 2012, http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Fwww2.emersonprocess.com%2Fsiteadmincenter%2FPM%2520Asset%2520Optimization%2520Documents%2FProductCaseStudies%2F2130_cs_CEMEX.pdf&ei=wghxT6bINeO80QXepvQH&usg=AFQj (accessed March 27, 2012). Ganeriwala, Suri. "Review of Techniques for Bearings & Gearbox Diagnostics." IMAC Conference. Jacksonville, FL: SpectraQuest, Inc., 2010. 1-37. Narayana, K L., 2006. Machine Drawing. New Delhi: New Age International Publishers Sheffer, Cornelius, and Paresh Girdhar, 2004.Practical Machinery Vibration Analysis and Predictive Maintenance. Burlington, MA: IDC Technologies Shreve, Dennis H., November 1994. Introduction to Vibration Technology , viewed 29 March 2012, http://www.irdbalancing.com/downloads/VT1_2.pdf Wickert, Jonathan, and Kemper E Lewis, 2006. An Introduction to Mechanical Engineering. Stamford, CT: Cengage Learning Yung, Chuck, 2011."Vibration analysis: what does it mean?" Plants service, viewed 29 March 2012, http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=0CFkQFjAH&url=http%3A%2F%2Fwww.plantservices.com%2Fknowledge_centers%2Fvibralign%2Fassets%2Fra_vibration_analysis.pdf&ei=4OJwT7S_Fs3trQfx5uS-DQ&usg=AFQjCNFucOxcz6Bp5R4o4FlU3y3k9Sgr5g&sig2= (accessed March 27, 2012). Read More