1. Mechanical restriction at the control levers
    • The first reason an excavator pulls to one side could be down to a physical restriction that doesn’t allow the control spool to open fully and let the drive get the full flow of oil needed. This could be due to a stretched or unbalanced link cable between the control lever in the cab and spool, or simply a build up of debris around the control lever not allowing it to open fully.
  2. Worn undercarriage
    • Check the condition of the track, rollers and idlers. A seized or collapsed bearing or tight track links can cause an enormous amount of friction. This may appear to be an excavator weak track but it is one track motor working harder than the other giving a reduction in speed.
  3. Worn main hydraulic pump
    • In smaller excavators there is one main pump that supplies the oil to both track motors. Although there is one common pump, the valve plate is effectively ‘split’ so it is perfectly possible for one side of the valve plate to wear, causing excessive case drain for one half of the circuit, meaning one side of the machine will be getting more pressure and flow than the other side. This causes excavator tracking to one side. Larger excavators use dual pumps, a separate pump for either side of the machine. A difference in tracking speed could be down to one of the pumps being damaged, causing excessive case drain and lack of pressure/flow.
  4. Worn servo pump
    • The servo pump provides the pressure and flow to release the brake (if externally controlled) and operate the two speed tracking. It is possible for a weak servo pump to not have enough power to allow one brake to release properly or only have enough power to engage one drive into two speed.
  5. One drive stuck in high speed
    • The most common style of excavator track motor is the Axial Piston Motor with swash plate to give the function of two speed tracking. If the two speed tracking circuit gets damaged, it is possible for the two speed spool or piston in one track motor to get stuck in the high speed position.
  6. Worn motor
    • Over time it is inevitable that the track motors will wear. Most commonly it is down to contaminated oil. Inside the hydraulic motor there are metal components that have to rotate against each other. To stop them making contact and binding against each other there has to be a certain amount of controlled oil leakage (case drain). If the oil is contaminated, the debris will scratch the components in the motor, causing excessive case drain. The oil now being passed straight to the case drain line isn’t contributing to the power of the motor, causing a reduction in speed and excavator tracking problems. An easy check to see if the track motor is generating too much case drain is to remove the case drain line, measure the amount generated over one minute and check against manufacturers specifications.
  7. Worn gearbox
    • A worn gearbox will add extra load to the hydraulic motor and cause a reduction in speed. Check the condition of the oil and condition of main bearings etc.
  8. Leaking turning/swivel Joint
    • Between the upper structure and the undercarriage there is a turning joint. This joint has multiple chambers that allows the oil to flow from the upper structure to the undercarriage. There are different pressures in each chamber (flow/return are high pressure, 2 speed and brake are servo pressure and case drain is low pressure). The turning joint can leak internally, allowing the wrong pressures to be in the wrong chambers. This can cause a deviation in track speed and even catastrophic motor failure if not fixed.

It is important to remember that contamination causes 90% of all hydraulic failures and the majority of excavator tracking problems. Track motors suffer the most because they are at the very bottom of the circuit where all the sludge collects. Track motors also put the greatest demand on the hydraulic pump compared to any other function on the machine and people confuse the fact that a slow final drive might be a symptom and not the cause of another problem somewhere further up the circuit.

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