This document is a service manual for the Yanmar YM240 Diesel Tractor, specifically focusing on its hydraulic device. It provides detailed information on the hydraulic system's piping, nomenclature, operation, and troubleshooting.
The primary function of the hydraulic device is to provide power for various operations of the tractor, particularly for lifting and lowering implements. It utilizes hydraulic fluid, pressurized by a pump, to actuate a cylinder, which in turn moves the lift arm. The system is designed to handle different operational scenarios, including normal work, sudden shocks, and the use of various implements.
Hydraulic Piping System:
The manual illustrates the main hydraulic piping system, showing the flow of hydraulic fluid from the engine-driven pump through a relief valve, stop valve, and control valve to the hydraulic cylinder. A reservoir and oil strainer are also part of this circuit. The system includes a slow-return valve to control the return flow of fluid to the reservoir.
Nomenclature of the Hydraulic System:
Key components of the hydraulic system are identified and described:
- Stop valve: Prevents high-pressure fluid from flowing backward from the cylinder.
- Relief valve: Operates when the pressure in the cylinder exceeds a specified limit, protecting the system from overpressure.
- Slow-return valve: Controls the volume of hydraulic fluid flowing from the cylinder back to the reservoir, influencing the speed of implement descent.
- Hydraulic pump: Generates the pressure needed to operate the hydraulic system.
Usage Features:
The manual details the operation of the hydraulic system for various tasks:
- Raising the lift arm: When the control lever is pulled backward, hydraulic fluid flows into the cylinder, pushing the piston and causing the lift arm to rise. A feed-back rod and lever system, along with a spring, return the control valve to a neutral position, stopping the piston movement once the desired height is reached.
- Lowering the lift arm: Pushing the control lever forward allows the weight of the implement to push the piston, driving fluid out of the cylinder through the slow-return valve and back to the reservoir. The speed of descent is controlled by the slow-return valve's opening.
- Control valve in the neutral position:
- Lift arm fully raised: In this state, the control lever is in the rear position, and the control valve is neutral. Hydraulic fluid from the pump returns directly to the reservoir, and the cylinder circuit is closed, preventing oil movement.
- Lift arm fully lowered: Similar to the fully raised neutral position, the control lever is in the rear, and the control valve is neutral. Fluid from the pump returns to the reservoir, and the cylinder circuit is shut off.
- Relief valve operation:
- Under normal conditions: If the fluid pressure exceeds the specified value (e.g., when the stop valve is closed and the control lever is pushed backward), the relief valve opens to maintain normal pressure and protect the pipes. This operation is accompanied by a noise.
- Under a sudden shock: When the lift arm experiences a sudden shock (e.g., driving over rough roads), the resulting overload pressure can exceed the allowable limit. The relief valve opens to prevent damage to the hydraulic device and pipes. For this protection to be effective, the stop valve should always be left open when driving the tractor with an implement mounted.
- Use of implements:
- For outlet B: This section describes how to find the neutral position of the control lever and how to operate an implement connected to outlet B. The control lever's movement for raising and lowering implements is within a narrow range (about 10 degrees forward and backward from neutral). If the implement cannot be raised sufficiently, the neutral position needs to be reset.
- For outlet A & B: For implements connected to outlet A, a "five ports and three actions" type control valve is required. It's crucial to connect the tank port directly to the reservoir. A safety note advises against driving the tractor fast with both front and rear implements attached to protect the hydraulic circuit.
Maintenance Features (Troubleshooting):
The manual includes a comprehensive troubleshooting section to address common issues:
- Free descent of the implement (too fast or unstable neutral position):
- Causes: Broken O-ring in the stop valve or slow return valve, weak return spring, faulty seats or worn spool, broken O-ring or back-up ring in the piston head.
- Remedies: Replace the faulty components (O-ring, spring, spool, control valve body, piston head rings).
- Implement does not descend:
- Causes: Faulty seat in the stop valve, piston not moving smoothly, foreign matter in the spool and body, broken return spring, stuck slow return valve ("closed"), piston stuck to cylinder.
- Remedies: Replace stop valve seat, replace piston, clean or replace spool and control valve body, replace return spring, open the slow return valve, replace piston.
- Implement cannot be lifted or lifting is too slow:
- Causes: Air sucked through the oil seal of the pump shaft, worn pump gears, insufficient pump movement, faulty O-ring in the stop valve, weak return spring, broken crank arm, broken O-ring or back-up ring in the relief valve seat, low relief valve pressure, broken O-ring or back-up ring in the piston head, clogged strainer, insufficient hydraulic fluid, or high fluid viscosity.
- Remedies: Exchange oil seal, replace pump, replace stop valve O-ring, replace return spring, replace crank arm, replace relief valve seat O-rings, adjust relief valve pressure to 155 kg/sq. cm, replace piston head O-rings, clean strainer, supplement hydraulic fluid, or replace fluid.
- Abnormal noises from the pump or control valve:
- Causes: Worn moving portion of the pump shaft, bent shaft, insufficient rod distance in the feed-back arm, clogged strainer, high fluid viscosity, or water mixed with hydraulic fluid.
- Remedies: Replace pump shaft, readjust feed-back arm, clean strainer, replace fluid, or check for water leakage into the hydraulic line.
General Notes on Hydraulic Fluid:
The manual emphasizes the importance of using clean hydraulic fluid and the correct type ("John Deere Type 303" or equivalent) due to the high precision of the hydraulic device parts. It also notes that the hydraulic fluid is shared between the transmission and the hydraulic device, and the fluid level should always be checked and refilled to the dipstick mark before operation. When assembling or disassembling, the service manual procedures must be followed.
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