The main relief valve of a CAB type of power drive serves two primary purposes. First, it protects the CAB unit from excessive fluid pressure conditions. Second, it prevents cavitation of the A-end by directing supercharging fluid to the pump.

The main relief valve (also called a safety relief valve) mounts to the outside of the valve plate. (External location can be seen in figs. 4-27 and 4-28.) The valve is connected into both main fluid passages that join the A-end and B-end. In this manner it protects the CAB unit when the A-end is tilted in either direction.

The valve is classified as a compound-relief valve since a small pilot valve controls a larger main valve. The simplified schematics of the main relief valve (figs. 4-37 and 4-38) show its major component parts. The valve works on the differential-in-pressure principle. The area of the main valve plunger exposed in chamber A is equal to the area exposed in chamber B. These two areas combined are equal to that area exposed in all of chamber C. Therefore, the fluid pressure in either chamber A or chamber B must be slightly more than twice the fluid pressure in chamber C to unseat the main valve plunger.

Depending on the direction of CAB rotation, fluid pressure (from the A-end) may build up in either chamber A or chamber B. When the A-end is not pumping, supercharge fluid pressure fills these chambers. The main relief valve fumctions during power drive start, normal, and excessive fluid pressure conditions. The fluid pressures involved are supercharge, servo, and a variable high pressure from the A-end and the tank.

Start Condition Operation

To understand the operation of the main relief valve when the power drive is initially started, you must realize two facts. First, fluid pressures developed by hydraulic systems do not instantly reach their normal levels or values. It takes a few seconds for fluid pressures to buildup to a normal, or at least a minimum, level before they can be effective. Second, tilt plates "drift" or tend to come off neutral at the A-end when the power drive is secured. This means that when the power drive is restarted, the pistons of the A-end are automatically positioned to start pumping to the B-end. The B-end would rotate and move the equipment. This

Figure 4-37.-Start condition of a main relief valve. 4-28

Figure 4-38.-Normal condition of a main relief valve.

could result in damage to certain components (like a power-off brake). It is also dangerous to life and limb if you are unable to jump out of the way fast enough.

The main relief valve performs two important functions when a power drive is started. It directs supercharge fluid, which builds to its low pressure (100 psi) very quickly, to the A-end and power-off brake lines. This initially fills the system to compensate for fluid lost through leakage or which "drained from the lines while the system was secured. The main relief valve also prevents an off-neutral A-end from driving the B-end through a set brake. Refer to figure 4-37 as we see how these actions are performed.

Supercharge fluid enters the relief valve and unseats the No. 1 and No. 2 ball type of check valves, The No. 1 check valve permits fluid to fill chamber A and one transmission line (A) to the A-end. The No. 2 check valve permits fluid to fill chamber B and the other transmission line (B) to the A-end. The double-check valve is also unseated. It allows supercharge fluid to act on the pilot valve. The 100-psi pressure of supercharge fluid is not enough to shift the spring-loaded pilot valve. Therefore, supercharge fluid flows through the lands of the pilot valve plunger and fills chamber C. It is also directed to the brake-release valve. Supercharge fluid initially assists in keeping the brake set until servo fluid pressure can build up and take over. Since the supercharge fluid pressures in chambers A, B, and C are all equal and acting on the same-size areas (A + B = C), they cancel each other. The large spring of the main valve plunger is rated at about 75 pounds of force. It is the only force holding the valve downward on its seat.

If the A-end is on neutral, there will be no problems or pumping action. If the A-end is not on neutral, one of the transmission lines (line A or line B) will be pressurized. Let's say line A is the pressurized line. Line B is the suction or return line.

As fluid pressure of the A-end enters chamber A, it seats the lower ball of the double-check valve. It also seats check valve No. 1 in the supercharge fluid pressure line. Spring pressure on the pilot valve plunger holds it downward. With the main valve held on its seat by only a 75-pound spring force, any fluid pressure above 75 psi of the A-end causes the main valve to lift. The excess fluid pressure is ported back to the suction line of the A-end (through line B) and bypasses the B-end. The maximum fluid pressure buildup between the A-end and B-end is limited to about 75 psi. That is not enough to move the brake-held B-end.

When the power-off brake solenoid energizes, it positions the release power-off brake and main relief control valve. The valve is positioned so it ports servo fluid pressure to the top of the main valve plunger. This action seats the main relief valve. With servo fluid pressure on top of the main valve, the pilot valve regulates the pressure in the main relief valve.

Will the main relief valve stay in this condition? No, because, as other fluid pressures build in the system, the control assembly automatically returns the A-end to neutral. Pumping action quickly stops and the main relief valve returns to its seat.