You might not know it but gearboxes are found on almost every vehicle. They are used in trucks, cars, boats, and even on some chainsaws. These gearboxes are used to change the speed of the vehicle or equipment and they have a number of different uses. If the gearbox fails, the vehicle or equipment will be rendered useless. If you are a gearbox mechanic, it’s important to be able to identify some of the most common failures and how you can repair them.
Gearbox failures can be caused by many reasons, but one of the most common causes of gearbox failure is due to a lubricant issue. A correct failure mode diagnosis is the first step in correcting this issue and preventing additional failures. Preventing gearbox failures can be done by addressing five key failure modes:
1. Micropitting
Micropitting, a common form of wear in gearboxes, can affect both gears and bearings. This defect occurs when a thin lubricant film is not able to protect the surfaces from high amounts of sliding action. Micropitting results in a frosted or matte surface that can be seen in the above image. Loss of lubrication may be prevented by changing the type of lubricant or reducing surface roughness.
2. Macropitting
Macropitting can also affect gears and bearings. Macropitting occurs when contact stress exceeds the materials fatigue strength.The design life of gears and bearings is typically 20 years, so macropitting before the end of this design life is an indication that one or more design assumptions were not met.
Macropitting can happen to your gear and bearing rings, and it is a sign of fatigue. There may be beach marks where the bearings and gears rub together with corrosion and lubricant in the cracks. Macropit fails can be prevented by reducing load on these parts, improving the profile of the gears and bearings to reduce stress, using cleaner steel, or increasing material strength. You can add alloy selection or heat treatment to these methods.
3. Axial Crack
Axial cracking is a problem with bearings and it’s especially common in through-hardened bearings. Axial cracks occur in the axial direction, perpendicular to the direction of rolling. This phenomenon can be prevented by using case-carburized bearings, ensuring that there is enough retained austenite present, applying a black oxide coating, and ensuring the correct interference fit.
4. Bending Fatigue
Gear teeth are complicated. Gear teeth break, or fail, for many reasons. One of these is bending fatigue. This happens when the stress at the root of the tooth exceeds the strength of the tooth material. This can be due to high loads or improper heat treatment. Other features that can identify bending fatigue include ratchet marks and multiple crack origins. To prevent it, one should decrease load, increase tooth material’s strength, or optimise the root fillet geometry.
5. Fretting Corrosion
When gears and bearings don’t have a protective lubricant on them, fretting corrosion can happen. This is when there are small oscillating motions and no lubricant between two touching surfaces. It often occurs in gearboxes due to transport or long periods of non-movement. Fretting corrosion can be identified by the presence of ruts along the lines of contact and reddish-brown or black debris.
Final Words
It’s important to understand what problems you might encounter with your gearbox, so you can make a smart buying decision. Avoiding common gearbox problems with gearbox repair can save you lots of headache, money, and time.
