Canadian Excavator Shows One-Side Travel Weakness Despite Normal Upper-Structure Hydraulics as Center-Swivel Internal Cross-Port Leakage Is Investigated

Case Background

An excavator operating in Canada developed reduced travel performance on one side while boom, arm and swing functions remained normal.

The upper hydraulic system produced normal working pressure, and the suspect travel motor did not immediately show a clear internal failure.

Because the problem affected functions located on the lower carriage while upper-structure hydraulics remained stable, technicians focused on the component that transfers hydraulic oil across the rotating joint between the upper and lower structures: the center swivel.

The Center Swivel Must Keep Multiple Hydraulic Circuits Separate

An excavator can rotate its upper structure continuously while still supplying hydraulic oil to the travel motors below.

The flow path is:

Main control valve → Center swivel → Lower-carriage hydraulic lines → Travel motors

Inside the swivel are several separate hydraulic passages and sealing elements.

These passages must remain isolated even while the upper structure turns relative to the undercarriage.

Internal Leakage Can Create Cross-Talk Between Circuits

If internal seals become worn or damaged, oil may move from one passage into another.

The result can be:

Pressure sent to one travel circuit → Part of flow leaks internally → Another port receives unintended pressure or return flow → Travel force falls

No external oil leak is required.

The swivel can appear dry while losing hydraulic separation internally.

Why Upper-Structure Functions Stayed Normal

Boom, arm and swing circuits may remain largely within the upper structure.

Travel flow, however, has to cross the center swivel.

This created a strong diagnostic boundary:

Upper Functions

Normal hydraulic response.

Travel Functions

One side weak, with pressure behavior inconsistent with the motor alone.

The location of the symptom therefore supported investigation of the rotary interface.

Canadian Excavator Travel Provided Useful Comparison

Technicians compared:

• left and right travel speed;
• travel-motor pressures;
• pressure before and after the center swivel;
• case-drain behavior;
• lower-carriage hoses;
• and swivel leakage patterns.

Testing with the upper structure positioned at different rotation angles can also provide useful information on some designs, although procedures must follow machine-specific safety requirements.

Why a Travel-Motor Replacement Could Fail

A healthy replacement motor still depends on correct flow reaching it.

If the center swivel allows oil to leak between ports, the motor may continue receiving reduced pressure or flow.

The important distinction is:

motor cannot convert hydraulic energy

versus

hydraulic energy is being lost before reaching the motor.

Difference From Final-Drive Mechanical Resistance

A final-drive fault creates mechanical drag after the hydraulic motor.

Center-swivel leakage reduces hydraulic energy before the motor.

Both can cause one-side travel weakness, but pressure and temperature patterns may differ.

Technical Lesson

Excavator diagnosis benefits from tracing the system according to machine architecture.

When upper-structure functions remain normal but travel behavior is abnormal, the rotating hydraulic interface between the two structures becomes an important diagnostic boundary.

FAQ

Can a center swivel fail internally without leaking externally?

Yes. Internal seals can allow cross-port leakage while the outside remains dry.

Can center-swivel leakage affect only one travel side?

Yes. A fault involving one specific internal passage can affect one circuit more than others.