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Magnetic Particle Inspection of Bull Gears

The bull gear plays a pivotal role in the functioning of an automobile’s differential system. Positioned within the differential housing, it connects with the pinion gear and facilitates the distribution of torque from the driveshaft to the wheels, enabling smooth turns while accommodating varying wheel speeds. This allows the outer wheel to rotate faster than the inner one during turns, enhancing stability and control.

Magnetic Particle Testing (MPT) is of paramount importance for the bull gear’s integrity. As a critical component subjected to substantial stress and load, it is susceptible to surface cracks, defects, and imperfections that might compromise its structural integrity. MPT involves applying a magnetic field and iron particles to its surface, effectively revealing hidden flaws. By identifying such issues early on, MPT enhances safety, prevents potential gear failures, and ensures optimal performance. Regular MPT as part of maintenance and quality assurance procedures is essential for preventing accidents, prolonging the bull gear’s lifespan, and maintaining the overall reliability of the vehicle’s drivetrain.

The geometry of bull gear poses a unique challenge regarding the magnetization method to be followed. The easiest method would be to clamp the bull gear between the headstock & tailstock across its diameter & pass current through it. As the surface area of the tooth in contact is small and the current is very high, there is a high chance of localized heating & arcing on the tooth. This heating may change the microstructure of the tooth and result in failure of the bull gear while in service. Also this method would require each surface to be magnetized at least twice, thereby increasing the inspection time and also may result in skipping some indications in the overlapping area.

So we need a method where we can magnetize the entire surface of bull gear in one go without damaging its teeth.

To accomplish this goal, we developed an equipment in which a copper conductor is used to magnetize the bull gear in circular direction. This conductor passes through the bore of the bull gear. A high current as per the following formula is passed through the conductor.

Head Shot Current            =        20 X Diameter of bull gear (mm)

To create a perpendicular field, an encircling coil is also used.The current requirement for the coil must be established by using QQI shims as standard formulas are not applicable for the geometry of the bull gear.


Since the bull gear is very heavy and it’s difficult to hold it in a vertical position, a vertical clamping arrangement is used where the gear is placed in horizontal position. A turntable mechanism is provided to rotate the bull gear during inspection.

The arrangement is shown in the picture below.

Conventional Technique

A two-vector PLC-based bench-type magnetic crack detector machine is used to perform testing on the crank shafts

Bull Gear Testing machine

Testing Procedure

  • Clean up & dry the bull gear’s surface before inspecting.
  • Place the bull gear on the turntable.
  • Press the ‘Cycle Start’ button.
  • The turntable moves and places itself underneath the coil.
  • The coil assembly along with the central conductor comes down on the bull gear.
  • Now apply magnetic solution bath to the bull gear.
  • Stop the flow of bath.
  • Press Mag Button
  • Now, the current passes through the central conductor & the coil.
  • It shows up on the digital metering unit.
  • The coil and central conductor assembly goes up.
  • The turntable moves out.
  • Inspect the bull gear under UV light for all defects. Rotate if required.
  • Now turnover the bull gear & repeat all the above steps.
  • Demagnetize

suitable products

Standard MPI Machines

Engineered for precision and efficiency, the Standard Bench Type Magnetic Particle Testing Machine is suitable for entry level testing.

PLC Controlled MPI Bench

With PLC Controlled bench type Magnetic Particle Inspection Machine, you can accurately control the process parameters for reilable results.

Multidirectional MPI Bench

The ultimate solution for robust non-destructive testing, this machine offers multi-directional magnetization for inspection in all directions in one shot..


Demagnetizers stand as a key tool in maintaining excellence in production, with the flexibility to handle different materials and magnetism levels, coupled with energy-efficient and user-friendly design.



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