Loss Of Logs From Grapple When Operating Boom Or Arch Functions At Low Engine Speeds Caterpillar

THIS TIB REPLACES THE DECEMBER 17, 1997 WHEEL SKIDDER TIB, COMPONENT CODE: 5051; "LOSS OF LOGS FROM GRAPPLE WHEN OPERATING BOOM OR ARCH FUNCTIONS AT LOW ENGINE SPEEDS". DESTROY THE DECEMBER 17, 1997 TIB. IT WILL NOT APPEAR IN THE SEMI-QUARTERLY INDEX.

Problem:

Reference: Service Manual Module, SENR6686; "Electronic Transmission And Grapple Control System"; May 13, 1996.

Reference: Service Magazine; July 21, 1997; Page 18; "New Control Group Now Used with Improved Auto Grab Function."

The design of the control valve in the grapple circuit allows back flow from the grapple cylinder when operating a low pressure circuit.

When the grapple is in the auto grab mode and the boom or arch lower functions are operated, logs can be dropped. Boom and arch functions operate at lower pressure than the grapple close function when holding a load of logs.

If the auto grab signals the electronic control to increase grapple pressure, the grapple control valve will open creating a parallel connection between the grapple cylinder and the boom and arch cylinder.

At low engine speeds, pump flow is not sufficient to supply flow to the boom or arch function, and increased pressure in the grapple circuit. High pressure oil in the grapple cylinder will flow to the low pressure boom or arch cylinder, causing the grapple to open.

Solution:

A check valve can be added to the grapple control valve group.

Procedure to add check valve to grapple control valve group.

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Warning Personal injury or death can result without releasing all of the hydraulic pressure. Release all the pressure from the hydraulic system before any lines are disconnected.

1. Remove the one piece flow compensator (1) in grapple control valve. (Illustration 1)

2. Install a two piece flow compensator (2) in grapple control valve. (Illustration 2)

Illustration 1.

Illustration 2.

Parts needed:

2. One 9T-7177 Control Valve Service Kit. (Contains parts for two machines)

3. Two Spools. (Part of 9T-7177).

4. Two Poppets. (Part of 9T-7177)

5. Two 4T-4514 Springs. (Part of 9T-7177).

Existing Parts:

6. 4T-4534 Spring.

7. 9T-3729 Spring.

8. 2M-9780 O-Ring Seal.

9. 9S-8006 O-Ring Plug.

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