* Improper Installation: The most common cause. If the nut wasn't tightened to the manufacturer's specified torque using a torque wrench, it's likely to loosen over time, especially with driving vibrations. This could be due to a previous repair done incorrectly.
* Damaged Threads: Stripped or damaged threads on the axle or the nut itself will prevent the nut from seating properly and holding securely. This could be from previous over-tightening or corrosion.
* Missing or Damaged Cotter Pin/Washer: The cotter pin (or a similar retaining device like a castle nut and cotter pin) is crucial for preventing the nut from backing off. If it's missing, bent, or damaged, the nut will loosen. A missing lock washer can also contribute to this.
* Corrosion: Rust and corrosion can weaken the threads and the nut itself, making it less likely to stay tight. This is especially likely in areas with harsh winters and road salt.
* Wheel Hub Bearing Failure: A failing wheel bearing can put extra stress on the axle nut, contributing to its loosening. You might notice other symptoms like rumbling or grinding noises from the wheel.
* Impact Damage: A significant impact to the wheel or suspension could jar the nut loose.
What to do:
1. Do not drive the vehicle. A loose axle nut can lead to wheel separation and a serious accident.
2. Have the vehicle towed to a qualified mechanic. They can properly diagnose the problem and repair it safely, including verifying the wheel bearing condition. They'll likely replace the nut and cotter pin (or equivalent retaining device) and ensure the proper torque is applied.
3. Do not attempt to fix this yourself unless you have significant mechanical experience and the proper tools (including a torque wrench). This is a critical safety issue that requires professional attention.
Ignoring a loose axle nut is extremely dangerous. Get it fixed immediately.
