Tractor Unit (Power Unit)

The tractor unit provides moving power for the paver wheels or tracks and for all powered machinery on the paver. The tractor unit includes the receiving hopper, feed conveyor, flow control gates, distributing augers (or spreading screws), power plant (engine), transmissions, dual controls, and operator's seat.

The tractor unit power plant (engine) propels the paver, pulls the screed (leveling) unit, and provides power to the other components through transmissions. Hot mix is deposited in the hopper and is then carried by the feed conveyor through the flow control gates to the distributing augers (spreading screws). The augers distribute the mix evenly across the full width of the paver, thus providing uniform placement of the mix onto _{the roadway surface.} These operations are controlled by the paver operator by means of dual controls within easy reach of the operator's seat. Figure 16-40 shows the operating controls of one type of finisher used in the NCF.

To ensure the paver functions properly, you should inspect the paver before commencement of paving. Below are some of the compments you should check.

PAVER TIRES OR TRACKS.- When the paver is equipped with pneumatic tires, tire condition and air pressure must be checked. It is particularly important for the pressure to be the same in the tires on both sides of the paver. When the paver moves on tracks (crawlers), the tracks should be checked to be certain they are snug but not tight, and the drive sprockets should be checked for excessive wear. Low tire pressure or loose tracks can cause unnecessary movement of the paver, which is transmitted to the screed unit, resulting in an uneven pavement surface. There should be no buildup of material on the tires or on the tracks.

GOVERNORS.- The governor on the engine must also be checked to be sure that there is no periodic surge in the engine rpm. When the governor is not working properly, there can be a lag in power when the engine is loaded (strained). Such a lag causes temporary failure of the vibrators or tamping bars in the screed unit, resulting in a stretch of pavement that is less dense or

Figure 16-40.-Operating controls, Barber-Greene model SA-35 finisher.

contains slightly less material than the immediately _{adjacent area.} After rolling, you can see that an area shows up as a transverse ripple in the pavement. A power lag can also interfere with the smooth and consistent operation of the electronic screed controls.

HOPPERS, FLOW GATES, AND AUGERS.- The hopper, the slats on the feed conveyor, the flow gates, and the augers should be checked for excessive wear and observed to be certain they are operating properly. Necessary adjustments should be made to ensure that these components are functioning as designed and are able to deliver a smooth flow of mixture from the hopper to the roadway. This includes adjustments to any automatic feed controls.

The speed of the conveyor and the opening of the control gates at the back of the hopper should be adjusted, so just enough mixture is being delivered to

Figure 16-41.-Tamping bar.

the augers to keep the augers operating about 85 percent of the time.'This allows a uniform quantity of mix to be maintained in front of the screed.When additional mix is required to allow an increase in the thickness being placed, the flow control gates should be adjusted. Augers should be kept about three-quarters full of mixture during paver operations.