Feeding into smoothness

Delivery from the hopper to the screed key to success

Asphalt Article January 24, 2003
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The way the feeder system is set up and operated has a major
impact on a wide range of mat quality issues. Regardless of the mix type, the
paving width or the paving depth, the feeder system must deliver material from
the hopper to the auger chamber and across the screed in an uninterrupted,
uniform manner. Consistency and doing the fundamental things right are the keys
to good feeder system operation.

Watch your head

The correct head of material is achieved when the mix in the
auger chamber covers one-half of the augers. This level should be maintained
from the center of the auger shaft out to the ends of the auger shaft. The head
of material must be maintained at the correct level; variations will cause the
screed to rise or fall and also can cause mat texture problems.

When the head of material is too low, the forces acting on
the screed are reduced and the screed will fall. As mat thickness decreases, a
depression is created and smoothness will suffer.

This condition often occurs during truck exchanges when the
operator continues to pave and consumes all the mix in the hopper. As the
conveyors run empty, the head of material drops and the screed settles.

When compacted, the area behind the left screed extension
may show variations in density. Smoothness will be affected, and the crew will
probably have a hard time trying to match curbs or longitudinal mat joints.

When the head of material is too high, the forces acting
against the face of the screed will increase. As there is increased resistance,
the screed will rise with resultant increase in mat thickness.

The crew will sometimes experience this situation during the
truck exchange sequence. In reaction to a low head of material, the crew will
use feeder system manual overrides to fill the auger chamber. As they try to
fill the area in front of the extensions, too much mix is delivered. When the
auger chamber is grossly overfilled, a large hump will be created in the mat.
It may be necessary to shovel out some of the mix to reduce the level and
minimize the mat defect.

A too large head of material also can be caused by a feeder
sensor being improperly positioned or improperly calibrated.

The perfect height

Many pavers are equipped with adjustable auger height. The
height of the augers affects not only the head of material, it also affects mat
texture.

A good starting point for the augers is 2 in. above the
level of the mat prior to compaction. This is the distance from the bottom of
the screed plate to the bottom of the auger segments. Be sure to check the
auger height at the start of each shift because the augers were probably raised
when the paver was parked at the end of the last shift or when it was loaded
onto a truck for transport.

When the auger height is too low, it is common to see two
texture stripes in the mat directly behind the right and left augers. This is
particularly true when paving with a base course mix that usually has large
aggregates. As the augers are turning, large aggregates may be left near the
surface of the mat if the augers are too low.

To tighten up the mat surface, raise the augers in small
increments until the surface of the mat is tight and uniform in appearance.

When the auger height is too high, the head of material also
will be too high and the screed will ride up. To counter the forces that lift
the screed, the angle of attack is usually decreased so the screed runs nose
down or flat. As a result, the texture of the mat will generally be open across
the entire width of the mat. The screed will be harder to control in this
condition and mat smoothness will suffer. You also may see large temperature
differentials across the mat.

Lower the augers and correct the screed angle of attack
until the mat texture is tight and uniform.

Watch your speed

The speed at which the augers turn affects mat texture, the
occurrence of segregation and the uniformity of mat temperature. When auger
speed is consistent and in the correct speed range, the mat should be uniform
in appearance with no striping. Mat temperature should be consistent from edge
to edge with no more than about -10°C (15°F) variation.

The augers should run continuously in the range of 30-40 rpm
while the paver is at a consistent paving speed. Control the auger speed by
adjusting the amount of material coming off the position of the feeder sensors
and by the sensitivity of the feeder sensors. Remember, any time the paving
speed is changed, there should be a corresponding adjustment in feeder controls
to make the augers run uniformly in the correct speed range.

If the augers are running on/off or are running at very low
speeds, mix will not move across the face of the screed in a uniform manner.
Larger aggregates in the mix will tend to segregate in the "dead"
areas under the chain case and outer bearing supports. The head of material in
front of the extensions is another area where segregation stripes can occur
when the auger speed is too low.

To increase auger speed, decrease the amount of mix coming
off the conveyors. Adjust flow gates or ratio control dials depending on the
type of paver being used.

When the auger speed is too high, you may get stripes in
several places, especially if the mix has large aggregates. It is common to see
segregation stripes at the outer edges of the mat as the high-speed augers tend
to cast the large stones to the area in front of the extensions. You also may
see a centerline stripe.

To reduce auger speed, increase the flow of material coming
off the conveyor.

Erratic auger speed will cause intermittent striping and the
head of material to fluctuate. Erratic auger speeds are caused by frequent
changes in paving speed or by feeder sensors improperly positioned. If the
feeder sensors are targeting the wave of material coming right off the end of
the augers, the feeder system will operate on/off. This is a very common cause
of erratic auger speed.

To correct erratic auger speeds, be sure paddle or sonic
feeder sensors are targeting the active pile of mix that is about 18 in.
outside of the last auger segment. And, be sure that paving speed is
consistent.

About the author: 
Information for this article provided by Caterpillar Paving Products Inc., Minneapolis.
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