vibration

VIBRATION

Vibration occurs when an object moves up and down or back and forth from its normal stationery position. A
complete vibration occurs when the object moves from one extreme position to the other extreme position and
back again. This creates a cycle. The amplitude of that cycle is the maximum distance on either side of its
stationery line. This movement and amplitude is created by vertical displacement.

A vibrating machine transfers maximum amount of energy when it vibrates at the object’s resonant frequency.
Studies regarding the physiological effects of vibration on the human body have been around for decades. We
found that most vibration experts have invested a great deal of time in the study of high frequency vibration (e.g.
hand and arm injury associated with high-speed hand-held power equipment). However, the difficult arena of
low frequency had received little focus and even less study.

HOW VIBRATION AFFECTS HUMANS
To understand why human beings are more sensitive to some frequencies than to others, it is helpful to begin
with resonance frequency.

Every object, including the human body, has its own resonance frequency range. That is to say, everything
naturally resonates within a specific Hz range. When objects are introduced to vibration in the same range,
vibration is amplified and can cause damage to that object. For example, when shipping a product by truck,
packaging experts must calculate the resonance frequency of the product to ensure it does not match the over-
road vibration it will be exposed to as it travels to its destination. Action must be taken to attenuate or move the
vibration out of the over-road range to avoid amplification and product damage.

"The vast majority of the over 130 field studies conducted during the last 30 years on the health effects of
occupational exposure to Whole Body Vibration agree that vibration is definitely related to and associated
with an increased risk for: lower back pain & sciatic pain; herniated discs; and degenerative changes in the
spinal system."

RESONANCE FREQUENCY

We know that human beings are injured during exposure to vibration. Consider the human body as
having sub-systems with their own independent resonance frequency bands. The interactions
between sub-systems are influenced by the body’s position, for example, standing or sitting.
Each part of the human body has its own resonance frequency; therefore, it reacts differently to
different frequencies. Responses to vibration on the human body depend on which part of the body
is affected.

THE WOES OF SEATED POSTURE
In seated posture, such as driving a mower, skid steer, or fork lift truck, one of the most vulnerable
parts of the human body system (with respect to vibration and shock) is the spinal pathway where
resonance occurs in the 3 to 12 Hz range. Other areas of concern include: the chest wall, the
abdominal region and the shoulder girdle.

“Vibrations close to the resonant frequency of the spine (3 to 12 Hz), such as those typically
encountered on a terrain vehicle, are amplified in the spinal pathway. This causes damage
through a cumulative process of micro-fractures to the endplates of the vertebral bodies.”

VERTICAL DISPLACEMENT

Terrain JockeyTM was designed as a “vibration isolation device” that damps vibration by means
of material rather than depending upon vertical displacement.

WHY VERTICAL DISPLACEMENT IS A CONCERN
In the typical seat suspension, vertical displacement has become a concern. When vibration or
impact occurs, the vibration is attenuated by movement in the vertical plane. This creates a
series of problems called “topping-out” and “bottoming-out,” allowing the seated driver to travel
too great a distance in both the upward and downward directions. Large amounts of this type of
vertical displacement have an adverse effect on the endplates within the spine. It “tops out” and
keeps the dynamic moving forward and then “bottoms out,” impacting the spine. This chain
reaction travels throughout the spine and damage can occur.

Topping and bottoming of seat suspensions are observed in cases where high-level inputs are
reached through irregular ground, potholes, small obstacles or dock plates. To avoid metal to
metal contact and reduce the severity of end stop impacts, many of these seats are mounted
with rubber end stop buffers located at the base of the seat which create their very own impact
and repetitive trauma.

"ISO standards for 2006 limit the vertical displacement range of seat suspensions to only 50
mm, or approximately 2 inches. The Terrain Jockey SuspensionTM moves a mere 6 to 7 mm
(1/4") with optimal weight loads."

RESONANT AMPLIFICATION

The resonant amplification of vibration energy within the human body results in various physio-
logical effects on humans. It can manifest itself as increased pulse rate or respiratory rate or more
seriously as ailments of spinal muscle or gastro-intestinal systems.

We know that low frequency (between 3 to 6 Hz) and cyclic motions (such as those found in
industrial driving) can put the seated posture body into dangerous resonance bands. The
magnitude of vibration transmitted to the human spine through the seat is most damaging at
resonant frequencies from 4 to 5.5 Hz (the natural resonant frequency of the spinal pathway).

SEATS: THE LAST LINE OF DEFENSE
The operators of mowers, skid steers and fork lift trucks are often exposed to a low frequency
vibration environment. This is mainly caused by movement of the vehicles over uneven surfaces.
Vibration increases pressure within the discs and muscles are fatigued by vibration exposure.
Our research addresses the adverse effects of irregular ground, inclines, or dock plates and the
low frequency vibration in the range of 4 Hz. The seat constitutes the last stage of suspension for
the driver—in fact, in many cases, the only suspension.

“Just ONE HOUR of seated vibration exposure can cause muscle fatigue, weaken the soft
tissues, and make a driver more susceptible to back injury!”