Magnetic Particle Inspection of Bull Gears

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.

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