Preventing Bridge Skew & Misalignment

January 24, 2024 Maintenance 5 min read

Bridge skew, often called "crabbing," occurs when one end of the crane bridge travels faster than the other. This causes the crane to move diagonally down the runway, leading to severe wheel flange wear, rail damage, and structural stress. Here is how to prevent and fix it.

1. Check Drive Synchronization

On dual-drive cranes (motors on both sides), the motors must run at the exact same speed. If one motor is slightly faster, the crane will skew.

VFD Control: Modern cranes use VFDs with "shaft synchronization" or closed-loop feedback to ensure both motors turn at the same rate.
Slip Ring Motors: Ensure the rotor resistance is balanced on both sides.

2. Brake Adjustment

Uneven braking is a common culprit. If the brake on the left side grabs harder or faster than the brake on the right, the crane will pivot around the left wheel every time you stop.

Action: Regularly check brake torque settings and air gaps. Both brakes must release and set simultaneously.

3. Wheel Diameter Matching

If you replace a wheel on one side, you MUST replace the corresponding wheel on the other side. Why?

A new wheel has a larger diameter than a worn wheel. Since they rotate at the same RPM, the larger wheel will cover more distance per revolution, causing that side of the crane to race ahead.

Rule: Always change drive wheels in pairs.

4. Squaring the Crane

Sometimes the crane structure itself is out of square. This can happen after a collision or severe shock load.

How to check: Measure the diagonal distance between the wheels. The measurement from Front-Left to Rear-Right should match Front-Right to Rear-Left. If the diagonals are not equal, the frame is twisted.

5. Rail Survey

If the crane is perfect but the runway is bad, skewing is inevitable. Perform a rail survey to check:

Span: Is the distance between rails constant?
Straightness: Are the rails straight?
Elevation: Are the rails level?

Conclusion

Skewing is not just a nuisance; it's a machine killer. By ensuring synchronized drives, balanced brakes, and matched wheel diameters, you can keep your crane running straight and true.