Vibration Analysis and Design - Case Study Example

Vibration Analysis and Design Vibration Analysis and Design Introduction Safety and ergonomics are significant issues in the work place over the past years. Technology has enabled additional safety in equipment and products. On the other hand, technology has developed new risks. However, safety standards have been developed by various institutions to ensure employee safety. The European Standards serve as a guideline that employers can use to ensure safe working environment. As WBV and HV become common in the work places, European Standards among others are found fit for a healthy working environment. Scope of BS EN 14253:2003+A1 Vibration at the work place poses great danger to employees. The sources of vibration vary as well as its intensity. This depends on the nature of work and equipment used serving as the source of vibration. This implies that vibration should be measured and evaluated according to the general requirements outlined in ISO 2631-1. This is a European Standard that provides practical guidelines to protect employees from vibration exposure that may lead to illness. According to the standards, precautions ought to be taken in the work place are outlined. This entails measuring and determining daily exposure per operation. This facilitates calculation of the daily exposure value that is referenced to the 8 hour period. The BS EN 14253:2003+A1:2007 outlines what establishes the important operations ought to be considered to determine vibration exposure. According to this standard, people exposed to whole-body vibration, as well as other forms of vibration must be monitored. However, the standard is limited to whole-body vibrations quantities evaluated and considers frequency range between 0.5Hz to 80Hz (Finucane, 2006). This standard underestimates harsh vibrations such as shocks and the harshness of the vibration. Referenced normatives The following norms are referenced in the BS EN 14253:2003+A1:2007: EN ISO 8041 EN 30326-1 ISO 2631-1 ISO 5347 ISO 5348 ISO 16063 Evaluation quantities for an operation exposed of vibration There are two guiding principles for evaluation for exposure to vibration; Frequency-weighted acceleration and Total duration per day. The former principle is measured in m/s², and expressed as r.m.s. (root mean square) values. This is taken at the supporting surface per axis. The latter is expressed as Ti, implying vibration exposure for a particular operation i. According to the Physical Agents Directive 2002/44/EC, an employee is allowed an 8-h VDV (vibration dose value). Exposure Profile The purpose of exposure profile is to identify operations that produce the overall value A(8). It also defines the state at which the measurements are made. The sources of vibrations must be measured since they contribute to the daily vibration exposure. As each operation is identified, the following issues must be known: Equipment used by the employee The conditions under which the employee operates The ground of operation The duration the operation takes How many times the operation is done The nature of the operation and The condition of the seat (Ignacio & et al, 2006). The case study discussed is about annual exposure to whole body vibration on family farms using mixed production profile. Organizing Measurements Farmers are exposed to mechanical vibration which is a physical hazard in the working environment. The sources of these vibrations are; agricultural vehicles such as tractors, and power as well as hand propelled machines. It is necessary to evaluate the annual vibration exposure in farmers. Organizing the measurements Select 25 family farms in the area of interest. The framers must be engaged in similar farm activities. Check on the time-schedule for the agricultural activities by the farmers independently Measure the frequency weighted vibration acceleration in m/s² and express in Root Mean Square values (RMS) (Manfield, 2004). Duration and number of vibration measurement Since the study is done on an annual basis, the statistical data for the monthly dose in the individual month is combined. According to the study, the farmers get exposed to high levels of mechanical vibration in the months of April, August, and September. The vibration doses for the above months are 2.51m/s².h, 2.83m/s².h and 2.03m/s².h respectively. This is due to intensive farming activities. The farmers experienced low dosage in the months of january, february, march, july and september (0.62, 0.63, 0.98, 0.97, and 0.97m/s².h respectively). In order to evaluate the degree at which the farmers get exposed to mechanical vibration, the values obtained on monthly basis are recalculated into vibration acceleration values and weighted for the 8-hour exposure. According to the months of study, the dosage remained within the standard range of 0.28-0.60m/s². the standard value (A8=0.8m/s²). this implies that the monthly mena daily exposure to mechanical vibration was below the allowable levels. Choice of equipment There are four vibration parameters related to vibration; vibration frequency unit, displacement unit, velocity unit, and acceleration unit. Vibration frequency unit symbolised as f is measured in Hertz and refers to the number of times the vibrating equipment vibrates per second. The intensity of the vibration frequency is the period T. The displacement unit vibration parameter is measured in m, mm or µmm and is denoted with the symbol D. It is defined as the distance the vibrating equipment displaces from the point of reference. The velocity parameter is denoted by the unit m/s or mm/s with the symbol V and refers to the optimum speed the vibrating object achieves during the vibration cycle. Finally, the fourth parameter known as acceleration uses m/s² for its unit and is denoted by the symbol A. It refers to the rate at which the velocity of the vibrating equipment changes per unit time. Accelerators are the best equipment used to detect vibration on surfaces. The accelerometer must be well calibrated and have well built in frequency weightings and integrating modes. The choice of accelerator used depends on the frequency range in question, the physical characteristics of the surface in question, as well as the environment. The dynamic rage accelerometer suits best for this study. Since this equipment is sensitive to temperature, humidity and other environmental factors, it is best fitted on the under surface of the equipment and currently monitored (Finucane, 2006). Other farm machines such as tractors are mobile. It is essential that the equipment is secured with belts or straps. When the farmer use a machine while standing, the accelerometer is placed on the surface supporting the farmer’s feet. Checking and verification of the measurement equipment It is important to verify the measurement system. This is a regular practice that can be done annually or after two years. The purpose of verification checks is to ensure that the system functions within the defined tolerance according to EN ISO 8041. The transducers of the system must be checked according to ISO 16063 and also verified after any rough handling. Estimating uncertainty in the measurements The accelerometer has an increased sensitivity at its high frequency ends due to resonance. This implies that its output will not be practical and give the true representation of vibration measured. To avoid this error, it is important to recognize the high frequency peak and ignore. If the wideband reading is taken, the reading is termed inaccurate. This is replicated in the vibration measured. This problem is also overcome by selecting an accelerometer that has a wide frequency range and use of a low-pass filter. The low-pass filter is placed in the vibration meters and preamplifiers for the purpose of cutting away wrong signals from accelerometer resonance. Mechanical filters are used to remove confined low frequencies and are made of resilient medium typical rubber that is bonded on the mounting disks and surface. They reduce the upper frequency limit resulting from resonance by 0.5-5kHz. Other sources of error are cable connector problems, electromagnetic interference, triboelectric effect, high frequency vibration, and contact loss. Uncertainty may occur when measuring the vibration transmitted. This could be due to faulty instrument, calibration faults, interference from electrical errors, positioning of the accelerometer, alterations from normal operations of equipment, and changes influenced by the machine operator. Other sources of uncertainty are exposure duration uncertainty, daily vibration exposure uncertainty Advantages of the BS EN 14253:2003, +A1:2007 As the world became industrialized, employees got exposed to various hazards at the work place. Vibrating equipment has posed great danger to the lives of many employees. It is important that employees work in safe environment with well maintained vibrating machines. Exposure to vibration should be monitored so that employees are not exposed to harsh radiation that cause illness such as aching backbones. This document outlining European Standards for vibration exposure helps in calculating and reporting vibration levels employees are exposed to. This helps in determining vibration dosage value the employees are exposed to. Once these values are established, employees are advised on how to control vibration exposure. Since the vibration equipment is also monitored, machines with harmful effects can be serviced or not used. The document also highlights the sources of errors that would arise when measuring vibration levels. This enables one to get accurate reliable readings for evaluation. Conclusion Since technological advancements have developed machines, exposure to vibration remains inevitable. This means that employers need to implement the European Standards such as the BS EN 14253:2003, +A1:2007 so that the working environment remains safe to the employees. This will minimize exposure to vibrations as well as illness derived from vibration. The European Standards will serve useful to the employers in monitoring their sources of vibration, while the employees enjoy safety. Bibliography Finucane, E. W., 2006. definitions, Conversions, and Calculations for Occupational Safety and Health Professionals. 3rd ed. London: CRC Press. Ignacio, J. E. & et al, 2006. A Strategy for Assessing and Managing Occupational Exposure. New York: AIHA. Manfield, N. J., 2004. Human Response to Vibration. London: CRC Press. Read More