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