What Is Fretting?
Fretting is a type of wear process that occurs at the asperities between the contact of two materials. What causes this wear process is an oscillatory motion between two surfaces in contact. Generally, the amplitude of the oscillatory motion is between 1 µm – 300 µm although, it can be as low as a few nanometres.
What Causes Fretting?
The common cause of the oscillatory motion between the two surfaces is vibration. A good example is found in gearboxes. The exterior of the outer race is commonly held in place via an interference fit. As a gearbox experience large amounts of vibration there is a relative motion between the outer race and bore containing the bearing resulting in fretting.
How Does It Differ To Other Wear Mechanisms
The main difference between fretting and sliding wear is the debris produced by fretting is prone to remain in the contact area which influences the progression of wear. The wear debris produced in fretting is predominantly oxide, which takes up a larger volume than the metal. In steady-state fretting conditions for steels, the wear debris consists of very fine plastic-like particles, largely alpha-Fe2O3 which has an appearance of a red-brown colour, which is redder than the usual corrosion product.
The wear rate produced by this mechanism is proportional to the normal load. Other influencing factors are temperature, sliding amplitude, number of cycles and relative humidity. These factors do not just influence fretting; they are common factors among the majority of wear processes.
Examples of Fretting
There are many different environments and conditions in which the wear process occurs. Find common examples below:
- Splines on driveshafts
- Bolt faces
- Cylinder head gaskets which are exposed to different thermal expansion coefficients
- Pitch bearings of turbines
How Can We Prevent It?
The primary way to stop this type of wear process is at the designing stage. It is to design the system/interaction of components such that no relative movement occurs.
Like other wear mechanisms, fretting can also be prevented by using a lubricant, although depending on the system, it may have unwanted effects.
The hardness ratio plays an important role within the wear mechanism. Having two materials of similar hardnesses will reduce the wear rate and increase the life of components.