Cracking the compactor code

Making some sense of vibratory roller specifications and how they relate to output horsepower

Asphalt Article April 16, 2003
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Within the last couple of years, a number of high frequency
vibratory rollers have hit the road building market. The trend is growing so
rapidly that frequency has become the one specification in which potential
buyers seem most interested. Why?

The reason for their increased interest is quite logical.
Higher frequency rollers offer the promise of performance at a faster travel
speed. When contractors bid a job, they are expected to get the project done to
a certain quality specification and within a particular time frame. If the
contractor does not meet the quality and time requirements, they may incur a
monetary penalty.

Conversely, if the job is done well and ahead of schedule,
they may be rewarded with bonus dollars--not to mention achieving decreased
labor, equipment and maintenance expense. Thus, any piece of equipment that can
offer the potential of more profit is understandably of great interest to the
contractor.

It is reasonable to conclude that because higher vibration
frequency results in faster travel speed, productivity and performance should
increase as well--but that's not exactly accurate. There are some major pieces missing
from this formula. Frequency is an important factor in calculating compaction
performance, but it is only one factor. Although frequency capabilities
influence roller productivity and performance, it does not necessarily
translate into better compaction results.

Selecting the right roller for a particular operation does
not need to be confusing, nor does it need to involve a certified mathematician
to calculate countless performance values. It simply requires understanding
some of the basic specifications found on most manufacturers' product sheets,
such as frequency, amplitude and centrifugal force, and having a general idea
of how these all relate to one another. Additionally, there is another rarely
published specification known as "output horsepower."

Derived from all three of the previously mentioned values,
output horsepower can be easily calculated using information provided on any
complete product sheet and provides an absolute measurement value to compare
one roller's performance to another.

What's the frequency?

At its most basic level, frequency is the rotational speed
of the exciter mechanism within a roller's drum. A function of engine
horsepower, exciter design, physical weight of the drum and the size and
efficiency of the hydraulic components, frequency can be measured in
revolutions per minute (rpm)--or using the industry standard vibrations per
minute (vpm). For every revolution of the exciter mechanism there is one
vibration. Thus, in the case of a 4,000-vpm roller, the internal shaft rotates
4,000 times per minute, in turn delivering 4,000 vibrations, or impacts, into
the work surface.

With a higher frequency of vibration, the maximum production
travel speed of a roller is going to be faster while still maintaining at least
10 impacts per foot, which is the minimum requirement to achieve proper
relative smoothness for an asphalt surface. The faster one can compact, the
more time is saved--and saved time equals more money for the contractor.

For example, compare a roller offering a high frequency of
3,000 vpm with one of the new rollers capable of 4,000 vpm. Again, given that
the compactor must be set to travel at a speed allowing for at least 10 impacts
per foot, the 3,000-vpm machine can achieve a maximum speed of 3.4 miles per hour,
whereas the 4,000-vpm unit can go 4.5 miles per hour.

To the layperson, a difference of 1.1 miles per hour might
not seem like much, but in relative terms the higher frequency roller is 25%
faster. When figuring that this can mean finishing a job in 25% less time, with
a potential for 25% less investment in labor, equipment and maintenance, it's
no surprise that frequency has received so much attention recently.

On the other hand, a roller with higher frequency will not
necessarily achieve density faster than one with lower frequency. Even though
the unit is traveling faster and delivering a certain number of vibrations per
minute, the energy produced with each vibration may be greatly reduced to
accommodate the higher frequency. In fact, depending on the roller's
performance and the characteristics of the asphalt, a 4,000 vpm roller may
require two passes to do the same amount of work that a 3,000 vpm roller can do
in one pass--negating any benefit of the faster travel speed. Thus, amplitude
also plays its part.

Don't forget amplitude

In simple terms, amplitude is the physical jump, or lift, of
a roller's vibratory drum from the work surface. Influenced by the sheer
physical mass of the given roller's drum assembly and the eccentricity of the
exciter design itself, amplitude produces both positive and negative travel,
meaning the drum lifts up off the surface as much as it drops down. Measured in
thousandths of an inch, the industry standard is to present amplitude as half
of the total travel value. Thus, if a roller has a total excursion (positive
and negative travel) of .040 at a certain performance setting, the unit would
be published as having an amplitude of .020.

Amplitude that is properly proportioned to the drum mass
will result in good compaction performance. Many times this amplitude rating
will determine the lift thickness or material type that a given roller will be
successful on. To understand how this relates to the value published on a
specification sheet, it is important to realize the reciprocal relationship of
amplitude to frequency.

There is a certain amount of give and take between amplitude
and frequency. When running at higher frequencies, amplitude must be lowered,
and when at a greater amplitude setting, frequency is reduced. If this does not
occur, the mechanical stress is too great and the result is a shortened machine
life. This offset relationship illustrates the reason higher frequency does not
necessarily guarantee good performance. In order to achieve an increased
vibration value, there has to be a compromise somewhere--and that somewhere is
amplitude.

One problem with reading specification sheets to determine
roller performance is that this relationship between frequency and amplitude is
rarely explained. In fact, sometimes only the highest values are listed, even
though they cannot coexist. Potential buyers are impressed by the supposed
combination of high frequency and amplitude, only to find on closer inspection
that things are not as they seem. The same problem can be found with
centrifugal force.

Amplitude, frequency collide

A measurement of drum compaction energy, centrifugal force,
sometimes referred to as total output energy or dynamic force, is defined by
calculating the mass of the drum with frequency and amplitude. Measured in
foot-pounds, centrifugal force determines the amount of energy the drum is
releasing into the material. So, if a roller is said to have a centrifugal
force value of 40,000 lb at a certain setting, it is delivering this amount of
energy with every vibration.

Though a calculation of amplitude and frequency, centrifugal
force can- not be considered the lone value in determining a roller's
performance. Again, this is due to the influence one factor has on the other. A
machine may have a high centrifugal force capability of more than 40,000 lb,
but this is at a higher amplitude setting, which results in lower frequency.
Increase the frequency and, in turn, amplitude suffers resulting in reduced
centrifugal force.

Even with a clear understanding of all the factors that go
into compaction performance and knowing exactly the range of materials the unit
will be working on, it is still difficult to determine how one machine compares
to the other. But figuring a roller's output horsepower could help clarify the
situation.

Output horsepower

A relatively new calculation for roller performance, output
horsepower does not receive attention on specification sheets because it
currently has little meaning for the user. For years, people have compared
products using frequency, amplitude and centrifugal force values, but output
horsepower is unfamiliar. This is unfortunate because it is the one value that
factors in all major performance specifications and provides an
apples-to-apples comparison from one roller to another.

As with centrifugal force, output horsepower measures the
energy that the drum generates, but it also defines an efficiency of the drum
to produce horsepower. The higher the measurement, the greater the efficiency.
In order to calculate output horsepower, one must have access to the roller's
frequency and amplitude values, as well as the relative centrifugal force.

For example, assume a roller has a high frequency of 4,000
vpm at .015 amplitude and a resulting centrifugal force of 32,000 lb. On the other
hand, a competitive roller has a high frequency of 4,000 vpm at .017 amplitude
and a centrifugal force of 34,000 lb. To calculate output horsepower, multiply
the first roller's top vpm value (4,000) by the resulting amplitude (.015) by
two (amplitude is typically defined as only half of its actual value). This
results in 120. Multiply 120 by the centrifugal force value of 32,000 and this
yields 3,840,000. Divide by 12, converting the number into in./lb, and divide
again by 33,000, which is a constant used in every formula for horsepower. The
result is an output horsepower value of 9.69. Though the other suggested roller
is listed as only slightly higher in amplitude and centrifugal force, the
resulting output horsepower value is 11.67--a substantial difference in roller
efficiency.

As with frequency, amplitude and centrifugal force, output
horsepower has little meaning by itself, but it provides a level playing field
on which to compare rollers based on performance.

It's all about performance

With the growing options available to the user, clearly
defining true compaction performance is becoming more and more difficult. One
can read through a specification sheet, but this does not necessarily tell the
tale as far as a machine's ability to perform and achieve compaction results.

New technologies, such as 4,000-vpm rollers, have their
limitations, but overall are improving performance capabilities on certain
asphalt mixes. The trick is to know your material and understand what will work
best to compact it effectively.

Fortunately, most manufacturers are building their machines
with different performance settings to accommodate varying material demands.
Additionally, there is a reason manufacturers continue to produce rollers with
frequency ratings of 3,000 vpm and lower. Though there might be a bigger,
faster or more powerful roller available, finding one that performs for your
unique operation is what truly matters.

About the author: 
Wilson is manager, marketing services, for Compaction America, Kewanee, Ill.
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