What Is a Friction Burn?

Friction transmission is the transmission of motion and power by the friction pair through the friction between the contact surfaces. According to the different lubrication conditions, the transmission can be divided into two types: one is that the working surface is not lubricated, and the friction pair is composed of metal and non-metal (such as leather, rubber, wood, mixed fabric, etc.) materials, although there is a high friction factor However, the allowable contact stress is low, and the power transmitted is small; the other is that the friction pair is a hardened metal material, and the friction motion can be divided into an elastic hydrodynamic lubrication state and a mixed lubrication state.

The friction transmission mechanism consists of two

Friction pairs can be divided into high and low pairs. Friction wheels (four-column wheels, ring cone wheels, discs, rings, four balls, arc cone wheels) transmission, metal belt transmission. The key transmission is a high-pair transmission, and the service belt transmission is a low-pair transmission.

Because of its simple structure, easy manufacture, smooth operation, low noise, overload protection, and continuous and smooth adjustment of its transmission ratio, friction transmission has a wide range of applications and has become the main component of continuously variable transmission. However, due to friction transmission, there are slips in the movement (elastic sliding, geometric sliding and slipping). Affect the transmission accuracy, low transmission efficiency, large structure size, large load on the shaft and bearings, and it is mostly used for small and medium power transmission.

Friction wheel transmission is the transmission of motion and power by using the friction generated by the two wheels pressing each other. The figure below shows the simplest friction transmission, which consists of two cylindrical friction wheels that are pressed against each other.

Transmission ratio friction wheel transmission includes cylindrical friction wheel transmission, cylindrical groove friction wheel transmission and conical friction wheel transmission, etc., which are respectively used for the transmission of parallel and orthogonal axes. The failure forms of friction transmission are as follows:

1. Overload, change of compression force and reduction of friction factor, resulting in slipping and local friction and burn on the wheel surface.

2. Higher alternating contact stress causes fatigue pitting and surface crushing of the working surface.

3. High-speed running under high pressure and tight pressure causes the instantaneous temperature rise on the friction surface and the wheel surface to become glued.

When the two wheels are made of metal, they are usually calculated according to the surface fatigue strength. When one of the wheels is made of non-metallic materials, the conditional calculation is usually based on the pressure per unit contact length. ^[3]

Sliding has a great impact on the performance of friction transmission. There are three types of sliding: elastic sliding, geometric sliding and sliding.

1. Elastic sliding

The sliding caused by the elastic deformation of the material when the friction pair works is called elastic sliding. The elastic sliding area is located at the exit end of the contact area. There is no sliding at the human end of the contact area. That is, the entire contact is divided into a static area and a sliding area. There is a "slip" between the driving wheel leading and the driven wheel falling behind in the sliding zone. The sum of the micro-friction torques in the sliding area is balanced with the applied torque, so the larger the load, the larger the sliding area, and the larger the slip.

The magnitude of elastic sliding is not only related to the load, but also to the elastic modulus of the material. The larger the elastic modulus, the smaller the elastic sliding. Elastic sliding is inevitable.

2.Geometric sliding

When the friction pair works, the slip caused by the geometry is called geometric slip. For example, a cylinder rolls around the center of the disc at the end of the disc. The velocity distribution on the contact line is "vortex", and only one point is purely rolled. The size of the geometric sliding is only related to the shape and relative position of the friction pair element. There is also geometrical sliding of the friction contact of point contact. Cylindrical friction pairs or co-topped conical friction pairs have no geometric slip. Geometric sliding is not a common feature of friction pairs.

3.Slip

When the load is so large that the entire contact area slips, the friction drive will slip. Slip is an overload phenomenon. When there is geometric sliding, the effects of elastic sliding and geometric sliding must be considered at the same time.

Slip is a form of friction drive failure. Not only will the transmission efficiency be reduced, the work will be unreliable, it will even cause the wear of the work surface, and serious gluing will occur. Design with appropriate safety factors. It cannot be designed with slip as an overload protection measure.

The factors that affect slippage are: the friction coefficient or traction coefficient is too small, the normal pressure is too small, the elastic modulus of the friction pair is too small, and the geometry and relative position design is not reasonable. During oil film traction, the traction coefficient is related to the slip rate. To ensure sufficient traction coefficient, there must be a certain slip rate, which is not slippage at this time. Slipping not only refers to the loss of rotational speed of the driven shaft, but also is in a state of unstable movement. ^[3]