Installation Of Pressure Limiting Circuit Caterpillar

Problem:

Additional heat is added to the hydraulic system when a down pressure circuit is plumbed for air seeder applications. This is due to the down pressure circuit causing the implement system to run at high pressure stand-by while the fan circuit requires less system pressure. The difference between the fan circuit pressure and high pressure stand-by, times the fan flow that can be anywhere between 45 to 68 L/min (12 to 18 gpm) is all added heat to the hydraulic system.

Solution:

The following instructions outline the principle of operation, installation procedure, and necessary parts to install a pressure limiting network for a constant down pressure circuit. This rework procedure will allow the hydraulic system to function optimally without adding any additional heat.

Principles of Operation

This circuit is designed to limit the load sense (LS) signal sent to the pump from the "lower" port of the down pressure circuit. During "raising", the LS signal is not limited. This is necessary to get adequate raise times of the openers. The manual valve is in the circuit so that the "pressure limiting" circuit can be turned off when using other implements on circuit #1. By pulling the knob out and rotating so the roll pin sits on the upper level of the valve body will turn the "pressure limiting" circuit on. By rotating, and pushing in to the lower position on the manual valve, the "pressure limiting" circuit is disabled.

The LS signal from the "lower" side of the down pressure circuit is sent to the specialized primary resolver with an orifice. The orifice limits the oil that can be bled out of the sequence valve that lowers the power loss. The LS signal is sent down the valve stack to section #2 where it is pulled out of the valve stack by the specialized secondary resolver plug that is installed. The LS signal is then branched at the tee in the resolver plug. One line from the tee goes to a resolver valve that resolves section #1 with the other sections. The other line goes to the sequence valve that bleeds off oil to maintain the desired work port pressure. The LS signal acts against the spring in the sequence valve. By turning the adjustment screw, the spring force is changed, thus varying the work port pressure. By measuring the LS pressure sent to the pump, set the pressure to approximately 8620 kPa (1,250 psi). This will ensure adequate pressure for the down pressure circuit. Oil bled off by the sequence valve is then routed through the manual on/off valve and back to the return screen on the machine.

The line that is teed into the "raise" work port of the down pressure circuit is used as a signal to be able to attain full system pressure while rising. The signal is directed to the spring end of the sequence valve. The pressure forces on the spool in the valve are now balanced, and the spring force keeps the valve closed.

Circuit #1 must be used as the Down Pressure Circuit

The following steps are required to install the pressure limiting circuit for air seeders.

1) Install vacuum line on the hydraulic oil tank to eliminate excessive oil leakage when changing parts on the machine.

2) Remove the detent housing on circuit #1. Remove the primary resolver and replace with the orificed primary resolver. Replace the detent housing on circuit #1.

3) Remove the secondary resolver body and ball on circuit #2. Replace these two components with the one secondary resolver plug and 6V-8774 Tee.

4) Connect the manual shuttle valve and the sequence valve together with the 5P-4272 Connector. The valves can be mounted to the PTO scatter shield. (If your machine does not have a PTO, mount the valves to the machine in a appropriate manner.)

You must then determine lines and fittings needed to connect the valves, as shown in this document.

Valve Adjustment

a) Open the manual directional valve so the pressure limiting circuit is engaged.

b) Attach a test line and a 0 to 24800 kPa (0 to 3,600 psi) gauge to the LS port on the machine.

c) Start the machine and cycle the disc openers up and down a few times and start and stop the motor, to get the air out of the system and check for leaks.

d) Lower the disc openers down until the desired down pressure is built.

e) Turn the sequence valve adjustment screw in or out until the LS pressure is approximately 8620 kPa (1,250 psi).

f) Raise the openers and see that the LS pressure goes above the 8620 kPa (1,250 psi) sequence valve setting. This will check for proper hook-up of the lines. NOTE: When fully lifted, the pressure should be high pressure stand-by.

g) Set the oil flow for an adequate raise/lower time interval without greatly affecting the fan speeds.

Proper plumbing is critical, the hydraulic schematic showing the resolver network is included in this document.

Resolver Network

Hydraulic Schematic

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