Know Your Air Brakes
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Part 23—Three-line Braking: the Tractive Unit Basic System
I HAVE already described brake controls I and wheel chambers on the three-line system, and having examined the working of each valve and unit in earlier articles, it should be easier to understand the sequence of events. Let us take the basic system first and follow the path of the air as the foot treadle is operated.
Air flowing from the compresser passes through the unloader valve to the reservoir, being prevented from leaking back to the cornpresser by the non-return valve in the outlet from the unloader valve. From the reservoir, air has free passage to both the treadle valve and the governor valve, as with the system fully charged, full pressure is always at these points.
As the treadle is operated, pressure flows through the treadle valve via the pipe lines to the wheel chambers front and rear at equal pressure. In the line to the tractor rear brakes, a T piece is fitted which conveys air to the relay valve at the pressure determined by the treadle position, the air then passing through the open supply valve in the base of the relay valve.
This is brought about by the reservoir pressure having already opened the governor valve, in turn moving the diaphragm in the relay valve and allowing the foot-treadle pressure to pass through a double check valve to the trailer stand pipe connection. From this point the air passes to the relay valve on the semi-trailer to operate the trailer brakes.
Although 70 p.s.i. is required to open the governor valve, a lower pressure will operate the diaphragm in the relay valve and open the supply valve; thus when the reservoir is charged, the supply valve will remain open until the auxiliary reservoir is nearly empty.
Since the governor valve closes below 70 p.s.i. two things apply: (a) a safe supply is always available in the auxiliary system in the event of failure in the service line; (b)
should the trailer break away and drop the pressure to zero in the auxiliary section beyond the governor, the diaphragm will close the supply valve, sealing the service line from the break in the standpipe line. Thus the tractor can be brought to a standstill by the residual pressure of .70 p.s.i. in the main reservoir, although at a slightly lower level of efficiency. This then is the basic footbrake system on the modern tractor. Next week I will describe the auxiliary system.