INTRODUCTION Vibration has become an important consideration in engineering. Development of industries and vehicles make the subject even more important. In today’s world every family has a car. There for it becomes essential to study the phenomenon of vehicle vibration and its effects on humans. Human vibration is defined as the effect of mechanical vibration on the human body. During our normal daily lives we are exposed to vibrations of one or other sort e.g. in buses, trains and cars. Many people are also exposed to other vibrations during their working day, for example vibrations produced by hand-tools, machinery, or heavy vehicles.
Man, as a mechanical system, is extremely complex and his mechanical properties readily undergo change. The vibrations generated in vehicle produce Mechanical Damage, Physiological Response, & Subjective Responses to humans. The short time exposure to vibration causes small physiological effects such as increase in heart rate, increase in muscle tension long term exposure to vibration causes effects such as disk to spine & effects on digestive system.
Types of human vibration There are mainly two types of human vibrations- Whole-body vibration Hand-arm vibration.
Whole body vibration Whole-body vibration is transmitted to the body as a whole, generally through the supporting surface (that is, feet, buttocks, back, etc.). A person driving a vehicle, for example, is subjected to whole-body vibration through the buttocks. Effects of exposure to whole body vibration--Exposure to whole-body vibration can either cause permanent physicaldamage, or disturb the nervous system.-Exposure to whole-body vibration can disturb the central nervous system.Symptoms of this disturbance usually appear during, or shortly afterexposure in the form of fatigue, insomnia, headache and "shakiness". Manypeople have experienced these nervous symptoms after they have completeda long car trip or boat trip.
Hand arm vibration Hand-arm vibration is transmitted to the hands and arms. It is mainly experienced by operators of hand-held power tools.Effect of Exposure to hand-armvibrations-Daily exposure to hand-arm vibration over anumber of years can cause permanent physicaldamage usually resulting in what is commonlyknown as "white-finger syndrome", In the earlystages of "white finger syndrome" the symptoms aretingling, numbness, and loss of feeling and controlin those fingers which are affected.Damage to the wrist or elbow joints is often causedby long-term exposure to the vibrations produced bylow blow rate percussive tools (e.g. asphalthammers and rock drills). This damage causes painin the joints and muscles of the forearm.
Cause of vibrations in vehicles Unbalancing of engines. Whirling of shafts. Road roughness.
Frequency response of the human body Mechanical vibration of a machine is caused by the moving components of the machine. Every moving component has a certain frequency associated with its movement. To understand why human beings are more sensitive to some frequencies than to others it is useful to consider the human body as a mechanical system. This illustration shows a greatly simplified mechanical mod el of the body, where each section is represented by a mass, spring and damper unit. The human body is a strongly damped system and therefore, when a part of it is excited at its natural frequency, it will resonate over a range of frequencies instead of at a single frequency
ISO Evaluation of human exposure to whole-body vibrationThere are four important physical factors to consider when assessing the effectof a vibrational environment on the human-body, namely- The equivalent acceleration value (aeq) of the vibration The various frequencies which make up the vibration The direction of excitation of the vibration The time of exposure to the vibration.The ISO Standard 2631 for whole-body vibration distinguishes three mainhuman criteria which can be used to assess vibrations in different situations: The preservation of working efficiency (the fatigue decreased proficiency boundary); The preservation of health or safety (exposure limit), and The preservation of comfort (reduced comfort boundary).
Vibration control Inclusions of good suspension system like hydraulic shock absorbers in vehicles which drive over rough terrain. Decreased exposure time of driver from vibrations. Increasing man’s mechanical resistance to mechanical forces by properly selecting the body position with respect to the direction of forces. By mounting the driver on elastic cushion so that the vibration is not easily transmitted to driver’s body.