Grapple Chattering When The Grapple Is Lowered At Idle Or At Low Engine Speeds Caterpillar

Problem:

There have been several complaints about grapple chattering when the grapple is lowered at idle or at low engine speeds. The cause of the chattering is that at rest the pump destrokes to standby pressure status. When the boom is lowered, gravity will over run the pump, effectively lowering the pressure in both the pump and cylinder to zero. This causes the selector and pressure control valve along with the main control spool on the grapple control valve to shift back to neutral. The pump then recharges the cylinder and valve providing the system with pressure. This cycle repeats at a high rate and causes the chattering noise.

Solution:

The chattering is a feature that is inherent to the system design. The chattering does not constitute or lead to any system failure and can be reduced or eliminated when the engine is operated at higher speeds.

Until the necessary changes are implemented in production, this issue can be addressed by adding a pressure accumulator in the pilot circuit which will provide enough pressure to lower the boom when the pump is not supplying the needed flow.

Installation Procedure:

1. Park the machine on a smooth, level surface. Lower all implements to the ground. Remove the hydraulic system pressure.

2. Install a 6V-9836 Tee between connection of 122-9034 Hose (1) and 6V-9836 Tee (2). Use two 6V-8397 O-ring seals.

3. Charge the 2G-8111 Accumulator with 690 ± 69 kPa (100 psi) of nitrogen. This will need to be done before the installation of the accumulator because there will be no room for charging after installation. Install 2G-8111 Accumulator (3). Use a fabricated bracket or a 7I-3921 Clip, a 9B-7237 Bolt, and an 8T-4896 Washer to mount the accumulator to the front wall of the compartment under the cab as shown in the above illustration. The accumulator may be installed on the rear wall of the compartment under the cab if preferred.

4. Install a 6V-8723 Elbow (4) on the end of 2G-8111 Accumulator (3). Use a 3K-0360 O-ring Seal.

5. Install a 122-9034 Hose Assembly (5) between the 6V-9836 Tee and the 6V-8723 Elbow (4). Use two 6V-8397 O-ring Seals.

NOTE: If the accumulator is mounted in the rear wall, a longer hose will need to be used. To assemble a longer hose, use 122-6869 Hose and two 124-2114 Couplings.

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