Hydrostatic Drive Testing for Early Bobcat 975 Models

[MikePhua]

Introduction
Bobcat 975 skid steer loaders, particularly those from the early production years, feature a hydrostatic drive system that requires specialized testing and maintenance. Hydrostatic systems are commonly found in various heavy equipment for their ability to provide smooth, variable-speed control without the need for a manual transmission. However, diagnosing and troubleshooting issues with the hydrostatic drive system requires a methodical approach, as failure to pinpoint the right cause can lead to costly repairs or inefficiencies in operation.
This article will explore how to test the hydrostatic drive in the early Bobcat 975 models, discussing the components involved, common issues that can arise, and step-by-step procedures for troubleshooting the system effectively.
Understanding the Hydrostatic Drive System
The hydrostatic drive system in the Bobcat 975 is designed to provide infinite speed control, allowing the operator to vary the machine's speed smoothly without shifting gears. This system consists of several key components:

• Hydraulic Pump: The hydraulic pump is powered by the engine and provides the flow of hydraulic fluid that drives the motor.
  
• Hydraulic Motor: This motor is connected to the wheels or tracks and converts hydraulic pressure into rotational motion, propelling the machine forward or backward.
  
• Transmission Control Valve: The valve regulates the flow of hydraulic fluid to the motor, controlling speed and direction.
  
• Drive Axles: These are connected to the hydraulic motor, transferring the rotational energy to the wheels or tracks of the skid steer.
  

When functioning correctly, the hydrostatic drive system allows for seamless operation. However, issues such as poor acceleration, erratic movements, or even complete loss of drive power can indicate problems within the system that require testing.
Common Issues in the Hydrostatic Drive System
Before diving into the testing procedure, it's important to recognize the common issues that could affect the hydrostatic drive:

1. Loss of Power: If the Bobcat 975 is struggling to move, the issue may lie with the hydraulic fluid, the hydraulic pump, or the motor.
   
2. Erratic Movement or Stalling: Irregular movement can be a sign of air trapped in the system, a clogged filter, or faulty control valves.
   
3. Poor Steering Control: The hydrostatic steering system might be malfunctioning, leading to poor directional control or uneven steering.
   
4. Overheating: Overheating of the system can occur due to low fluid levels, old fluid, or a clogged cooler.
   
5. Inconsistent Speeds: Sudden speed variations can be a result of a malfunctioning valve or issues with the pump.
   

Step-by-Step Guide for Testing the Hydrostatic Drive System
To properly diagnose issues with the hydrostatic drive system on the Bobcat 975, you should follow a systematic approach. The testing procedure involves checking the key components of the system and making adjustments or replacements as necessary.
1. Check the Hydraulic Fluid Levels
The first step in diagnosing any issues with the hydrostatic drive system is to inspect the hydraulic fluid levels. Low or dirty fluid can lead to poor pump performance, leading to erratic machine behavior.

• Fluid Inspection: Check the fluid level using the dipstick or the fill sight glass. Ensure the fluid is at the proper level and is clean.
  
• Fluid Quality: Look at the condition of the hydraulic fluid. If it is dark or has a burnt smell, it is likely that the fluid has degraded and needs to be replaced.
  
• Fluid Type: Ensure the correct type of hydraulic fluid is being used. Using the wrong fluid can cause damage to the system.
  

2. Test the Hydraulic Pump
The hydraulic pump is the heart of the hydrostatic drive system, and any issues with it can directly affect the performance of the machine.

• Pressure Test: Using a pressure gauge, test the pressure coming from the hydraulic pump. The pressure should match the specifications listed in the Bobcat 975 service manual.
  
• Flow Test: A flow test checks if the pump is supplying the proper amount of hydraulic fluid. This test requires the use of a flow meter and is typically done at various engine speeds.
  
• Check for Leaks: Inspect the pump for any signs of leaks, which could indicate internal damage or seal failure.
  

3. Check the Hydraulic Motor
If the pump is functioning properly, the next step is to check the hydraulic motor. A faulty motor can cause the machine to lose power or fail to move.

• Test for Proper Operation: With the engine running, check if the motor is rotating properly. If it is sluggish or making strange noises, it could indicate internal wear or failure.
  
• Check for Fluid Flow: Ensure that the hydraulic fluid is reaching the motor. If the motor is not receiving sufficient fluid, it could be due to a clogged hose or valve.
  
• Test for Leaks: Inspect the motor for any signs of external leakage.
  

4. Inspect the Transmission Control Valve
The transmission control valve is responsible for directing hydraulic fluid to the motor. A malfunctioning valve can cause speed or directional control issues.

• Check Valve Operation: Test the valve to ensure it is switching smoothly between forward, reverse, and neutral positions.
  
• Check for Obstructions: Ensure that the valve is not clogged with debris or dirt, which can interfere with its function.
  
• Test for Leaks: Inspect the valve housing and seals for any signs of hydraulic fluid leakage.
  

5. Check the Drive Axles and Wheel Motors
Finally, inspect the drive axles and wheel motors for wear or damage. While these components are generally robust, issues can arise from prolonged use or lack of maintenance.

• Check for Wear: Inspect the drive axles for signs of excessive wear, such as scoring or uneven surfaces. If the axles are worn, they may need to be replaced.
  
• Inspect Wheel Motors: Ensure that the wheel motors are turning smoothly and not making grinding or whining noises. If the motors are defective, the machine may lose power or fail to move altogether.
  

6. Inspect for Air in the System
Air in the hydraulic system can lead to erratic behavior or a loss of power. To remove air, follow these steps:

• Bleed the System: Use the bleeder valves located on the hydraulic lines or fittings to remove any trapped air.
  
• Check for Bubbles: After bleeding the system, observe the fluid in the reservoir for any bubbles, which can indicate air is still present.
  
• Test Again: After purging the air, test the system again to ensure smooth operation.
  

7. Perform Steering and Functionality Tests
Once the drive system has been inspected, it is important to test the steering and operational functions of the skid steer:

• Steering Control: Check the steering to ensure smooth and responsive operation.
  
• Speed Control: Test the speed control by gradually increasing the throttle and noting any irregularities in speed or acceleration.
  

Conclusion
Testing the hydrostatic drive system of an early Bobcat 975 model involves a combination of fluid checks, pressure tests, and mechanical inspections. By following a structured diagnostic approach, operators can identify the root cause of performance issues and take corrective actions to restore the skid steer’s functionality. Regular maintenance, including fluid changes and system inspections, will ensure optimal performance and extend the lifespan of the machine. Understanding how to diagnose and address hydrostatic drive problems is an essential skill for operators and maintenance technicians alike.