Premature Damage to Actuator Bearings?

Are you hearing unusual noises or seeing blackened grease leaking out of your pulley housings resulting in unplanned machine downtime? If you are, you’re not alone. Your actuator may be degrading due to a phenomenon known as electrical discharge (ED).

Premature actuator damage due to ED?Stray electrical voltages are traveling from motor shafts through couplings, pulleys, bearings, and housings in a search for ground. Since ball bearings are rolling contacts with lubricant between surfaces, the voltage arcs the gap and causes material erosion, lubricant failure, heat, and ultimately premature bearing failure.

This damage can reduce the life of bearings that should last 6 to 10 years to as little as 2 to 6 months. Your root cause investigation should focus on the motor grounding system as voltage discharges as low as 3.2 volts will cause current discharges across a ball bearing and raceway lubricant gap.

 

How it works:

Electrical Discharge Machining, sometimes called “spark machining” is a common manufacturing process where material removal is done by triggering a rapidly recurring current discharge between two electrodes separated by a dielectric liquid and subjected to an electrical voltage. One of the electrodes is the part being machined and the other is the “tool” electrode, typically a wire or carbon shape.

The gap between each electrode is precisely controlled and the dielectric liquid fills this gap. As voltage is applied and increased, the intensity of the electrical field in the gap rises and becomes stronger than the dielectric liquid, allowing current to flow or arc across the gap. As a result, material is removed from each electrode and carried away by the dielectric fluid. Performing this arcing at a high frequency results in an efficient erosion machining process that is very good for hardened materials.

Whereas EDM is a practical manufacturing process, it is not desirable in Motion Control systems and applications.

How do you recognize Electrical Discharge?

Stage One:

At first, the continuous arcing will start pitting the bearing raceways and reduce the ball diameter. Ball retainers will also start eroding and break apart. The heat builds due to arcing and friction causing the grease lubrication to breakdown and become contaminated with metal particulate. Blackened grease may leak out of the bearing and be visible on the pulley housing and pulley shaft.

Stage Two:

As the bearing degrades and becomes unlubricated, heat and friction build causing metal to metal contact noise. If the actuator drive shaft is decoupled from the motor, you may feel a roughness or bumpiness when rotating the drive shaft. The electrical discharge continues and works its way to all the bearings in the actuator as these are the points of arcing.

Stage Three:

As the bearings degrade (balls, retainer cage and polymer seals disappear) and become non-functional the actuator will begin running erratic and make significant noise. In a fully catastrophic case there are no bearings remaining and the pulley is unsupported, the actuator timing will be compromised and other parts of the machine system will be affected.

Here is a Stage 2 ED-damaged drive pulley housing that has been disassembled.

The shaft-side bearing (left) shows no seal, blackened lubricant and damaged ball cage. The pulley housing and bearing (right) show no visible damage but the bearing rotates with a rough feel indicating Stage 1 damage.

Determining if your actuator has been damaged by ED:

  1.  Measure actuator pulley housings and shafts for a voltage using an Oscilloscope or an Electrical Discharge Pen TKED1 made by SKF. Presence of voltage is a good indicator that ED is contributing to the failure.
  2.  Check all drive system ground connections and cables for proper ground bonding and shielding. If inconsistencies are found, correct and recheck for voltages.
  3. Measure temperatures of the motor, adaptor and pulley housings for excess heat generation. Hot spots may indicate failure point.
  4. Replace noisy drive or idler pulley assemblies. Record change dates and monitor the performance over time. It is quite rare for a pulley assembly to fail within 6 months of installation

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