Friction transmission isfrictionA pair transmits motion and power through friction between contact surfaces.According to 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-metallic materials (such as leather, rubber, wood, mixed fabrics, etc.). Although it has a high friction coefficient, the allowable contact stress is low, and the power transmission rate is small;The other is that the friction pairs are all hardened metal materials, and the friction movement can be divided into elastohydrodynamic lubrication state and mixed lubrication state.
Chinese name
Friction drive
Foreign name
Friction drive
Purpose
Friction transmission ring for friction press
Composition
Friction block and connecting piece
Advantages
Wear resistance, temperature resistance, stability and reliability
The friction transmission mechanism is composed of two mutually compressedFriction wheelAnd pressing device.It transfers motion and power by friction between contact surfaces.This mechanism has the advantages of simple structure, easy manufacturing, stable operation, slipping under overload (which can prevent damage to important parts in the equipment), and stepless change of transmission ratio, so it has a wide range of applications.However, due to the shortcomings of sliding, low transmission efficiency, large structure size, and large load acting on the shaft and bearing during operation, it is only suitable for occasions with small power transmission.The commonly used friction transmission mechanisms are mainly cylindrical flat friction, cylindrical groove friction, cone friction, roller disc friction, roller cone friction, etc.[1]
Friction pair
Announce
edit
Friction pair can be divided into high pair and low pair transmission.Friction wheel (four column wheel, ring cone wheel, disc, ring, four ball, arc cone wheel) transmission, metal belt transmission.The key drive is a high pair drive, and the belt drive is a low pair drive.
Because of its simple structure, easy manufacturing, stable operation, low noise, overload protection, and the ability to continuously and smoothly adjust its transmission ratio, friction drive has a wide range of applicationsStepless speed changeMain components of transmission.However, friction transmission has sliding in motion (elastic sliding, geometric sliding and sliding).It affects the transmission accuracy. The transmission efficiency is low, the structure size is large, and the load acting on the shaft and bearing is large, which is mostly used for small and medium power transmission.
Working principle of friction transmission
Announce
edit
Friction wheel driveIt is the transmission of motion and power through the friction force generated by pressing between two wheels.The following figure "Friction Drive" shows the simplest friction drive, which is composed of two cylindrical friction wheels that are pressed against each other.
Friction drive
In normal operation, the driving wheel can drive the driven wheel to rotate by the friction force.In order to ensure that the wheel surface contact does not slip (that is, it can only be used for pure rolling) when two wheels drive, there must be enough friction at the contact of two wheels.Usually installed on friction wheelSpringOr an appropriate force application device, or lining the working surface of another friction wheel with a layer of asbestos, leather, rubber and other materials to increase friction.
According to physics, friction is equal to the product of friction coefficient and the positive pressure of two wheels.When the positive contact pressure of two wheels is constant, the greater the friction coefficient, the greater the friction obtained, and the better the transmission effect obtained.Generally, the friction coefficient of plastic and gray cast iron contact transmission, or leather and gray cast iron contact transmission can reach 0.3~0.4;The contact transmission between rubber and gray cast iron can reach 0.7~0.8.[2]
Failure mode of friction transmission
Announce
edit
Transmission ratio Friction wheel drive includes cylindrical friction wheel drive, cylindrical groove friction wheel drive and conical friction wheel drive, which are respectively used for parallel shafts andCross axisTransmission between.The failure modes of friction transmission are as follows:
1. The change of overload, compression force and friction coefficient will lead to slipping, which will cause local friction and burns on the wheel surface.
2. High alternating contact stress leads to fatigue pitting and surface collapse of the working surface.
3. High speed operation under the action of high-pressure tightening force leads to the instantaneous temperature rise of the friction surface and the wheel surface gluing.
When both wheel surfaces are metal, they are usually calculated according to surface fatigue strength;One of the wheels isNon metallic materialsIn most cases, conditional calculation is carried out according to the pressure on the unit contact length.[3]
Sliding of friction drive
Announce
edit
slideIt has a great influence on the performance of friction transmission.The types of sliding can be divided into the following three types: elastic sliding, geometric sliding and sliding.
1. Elastic sliding
The sliding caused by the elastic deformation of the material when the friction pair is working is called elastic sliding.The elastic sliding area is located at the outlet of the contact area.The human end in the contact area does not slide.That is, the whole contact is divided into static zone and sliding zone.In the sliding area, the driving wheel is ahead and the driven wheel is behind, and there is a "slip difference" between the two.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 is working, the sliding caused by geometric shape is called geometric sliding.For example, when the cylinder rolls around the center of the disk at the end of the disk, the velocity distribution on the contact line is "vortex", and there is only one point for pure rolling, which is called a node.The size of geometric sliding is only related to the shape and relative position of friction pair components.Point contact friction pairs also have geometric sliding.Cylindrical friction pairs or conical friction pairs with common top have no geometric sliding.Geometric sliding is not common to friction pairs.
3. Slip
When the load is so large that the whole contact area slides, the friction transmission will slip.Slip is an overload phenomenon.When there is geometric sliding, the influence of elastic sliding and geometric sliding should be considered at the same time.
Slip is a form of friction transmission failure.It will not only reduce the transmission efficiency, work is unreliable, and even cause wear on the working surface, which will cause serious gluing.Appropriate safety factor shall be adopted in design.Slip cannot be designed as overload protection measure.
The factors that affect slip are: friction coefficient orTraction coefficientToo small, the normal pressure is too small, the elastic modulus of the friction pair is too small, and the design of geometric shape and relative position is unreasonable.During oil film traction, the traction coefficient is related to the sliding rate. To ensure sufficient traction coefficient, there must be a certain sliding rate, which is not sliding at this time.Slip not only refers to the speed loss of the driven shaft, but also the unstable movement.[3]
