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OCCUPANT RESPONSE TO VEHICULAR VIBRATION
Vibration & Mechanical ShockVibration is a time varying disturbance of
a mechanical /biological system from an equilibrium condition for which the long-term average of the motion tend to zero, and on which may be superimposed either by translations/ rotations/both.
The mechanical forces may be distributed /concentrated over a small area of the body and may be applied at an angle to the surface (eg. Tangential/normal)
Vibration may contain random/deterministic components or both they may also vary with time (i.e; nonstationary)
Deterministic vibration may contain harmonically related components or pure tones (with sinusoidal time dependence) and may form “SHOCKS”
A mechanical shock is a nonperiodic disturbance characterized by suddenness and severity with for the human body, the max. forces being reached within a few tenths of a second and a total duration of upto about a second.
IMPACTAn impact occurs when body/body part
collides with an objectHuman response to vibration , shock and
impact depends on the frequency content of the stimulus as well as the magnitude
Vibration ExposureHealth disturbances and injuries are related
to the magnitude of the stimulus, its frequency content and its duration
Human Response to Vibration, Mechanical Shock and ImpactMechanical damage can occur at large vibration
magnitudes at exposure to shocks eg. bone fracture, brain injury, organ hemorrhage and tearing /crushing of soft tissues
At moderate magnitudes, there can be physiological effects leading to chronic injury such as to the spine and disorders affecting the hands
At all magnitudes above the threshold of perception there can be behavioral responses ranging from discomfort to interference with tasks involving visual/manual activities
Injury from VibrationWhole Body Vibration – WBVHand transmitted vibration
WBVSmall animals (eg. Mice, dog) may even be
killed by intense vibration lasting only a few minutes
Internal injuries may be caused commonly to heart , lungs also gastro intestinal bleeding are consistent with the organs beating against each other and the rib cage due to the resonance motion of the heart and lungs on their suspensions.
WBVIn human these organ suspension resonances
are at frequencies b/w 3 & 8 HzChronic exposure of WBV results in an
increased risk of low back pain, sciatic pain and prolapsed / herniated lumbar disks (compared to that not exposed to vibration)
These injuries occur predominantly in crane operators, tractor drivers and drivers in the transportation industry
HTVChronic injuries may be produced when the
hand is exposed to vibration. Symptoms of numbness or parasthesia in the fingers / hands are common
Reduced grip strength and muscle weakness may also be experienced and episodic finger blanching often called colloquially “ White finger”/ “White hand” / “Dead hand” may occur in occupational groups (eg. Operators of pneumatic drills, grinders, chipping hammers ,riveting guns and chain saws)
Contd..Repeated flexing of the wrist can injure the
tendons, tendon sheaths, muscles, ligaments, joints and nerves of the hand & fore arm
This repetative strain injury commonly occur in occupations involving repeated hand wrist deviations (eg. Keyboard and computer operators)
Carpal tunnel syndrome – frequently involve nerve compressions at the wrist
HAVS – Hand arm vibration syndrome is blood vessel, nerve and muscle disorder associated with the regular use of hand held power tools
Injury from Shock and ImpactVertical shock – exposure to single shock applied to a
seated person directed from seat pan towards the head may result in spinal injury & vertebral fractures
Horizontal shock – Accident statistics indicate that serious injuries to the occupants of motor vehicles involved in frontal collisions are most commonly to the head, neck and torso including the abdomen
Injuries to the head usually involve diffuse / focal brain lesions either with/ without skull fracture.
Most common neck injury is caused by rearward flexion and forward extension resulting in dislocation/fracture of cervical vertebrae and compression of spinal cord
Vehicle vibrationVibration occurring on the seats of vehicles
may cause driver & passenger discomfort and may also interfere with their activities.
Epidemiological studies results suggest that long term exposure to WBV may additionally contribute to problems such as low back pain, early degeneration of the spine & herniated discs
Low back pain can be caused by the strain imposed on the lumbar spine from differential motion occurring b/w the seat back and seat surface
Contd..Experimental studies of the vibrations
transmitted to and tolerated by drivers indicate that the drivers are subjected to extremely uncomfortable levels of vertical and pitch vibrations in the frequency range of 1.5 – 11 Hz
Short term and long term consequences of WBV adversely affects occupational health and safety
Some of these effects can lead to permanent disabilities especially to musculoskeletal , spinal and gastro intestinal disorders
Contd..One of the most common complaints from
personnel exposed to high levels of WBV is lower back pain, an ailment difficult to diagnose
Otherwise reduced ride comfort can lead to fatigue & reduced alertness of drivers which ultimately leads to safety risks when they make poor decisions or fail to react proactively when hazardous situations develop
The reason of tiredness caused by vibrations seems to be the muscular activity caused by instant effort of the human body to compensate the vibrations, to stabilize the human body
Protection against VibrationThe level of vertical vibration on the seat top of an
occupied seat will be influenced by several factors – type of road (condition), vehicle suspension, speed of travel, physical size of driver, type of vehicle and seat’s dynamic characteristics
The vibration experienced by a driver / passenger can be greatly modified by the dynamic response of a vehicle seat
Decoupling seat and operator from vehicle motion by employing seat suspension – only effective if it is well matched to the motion characteristics of the vehicle