Feeding into smoothness

Jan. 24, 2003

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.

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.

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