Fig. 2.17. Methods of measurement of the external friction

The first group of methods are those in which one plane progressive moves relative to the other plane (Fig. 2.17, a, b, C). Under the scheme and the product goes on moving the studied surface, while the friction force is measured by the dynamometer any type. Therefore, the most common method used to determine the friction Tverdovsky products: biscuits, bread, meat, fish, grains, etc. On the chart b product is transported by cargo on investigated static surface. The friction force in this case is equal to the minimum amount of equipment required for a uniform product flow. This method determines the coefficient of friction solid products such as cheese, fish, confectionery. According to the scheme in the product is sliding down an inclined surface. The friction coefficient is determined by the minimum angle α the surface of the analyzed structural material, on which the slides food product. This method has allowed to determine the static coefficient of friction of cheese, crackers, dry food. The determination of the coefficient of friction on the angle of the plane, which begins the movement of the product on surfaces that are not well as the angle of inclination of the plane at the moment of movement is approximately determined, moreover, it is impossible to establish the change in the coefficient of friction with a change in velocity of sliding.

The second group includes such methods under which one of frictional pairs rotates. Under the scheme g determine the friction coefficient of the candy mass. Product lies on a rotating disk, studied structural material slides on a rotating product. The friction force is determined by torque transferred from the disk to the sample. Scheme d product enshrined not elastic beam onto the rotating surface of the material under investigation. This method determines the coefficient of friction of meat, fish, tablets of coffee and tea, bread plates, tin bread.

The third group includes methods in which is generatrix of the cylinder is in contact with the plane. Diagram e - beam performing oscillatory motions on two rotating towards each other rollers.

The fourth group includes methods, in which a cylindrical or flat flexible surface moves on a cylindrical surface (diagram f).

The change in pressure allow the methods depicted on the schemes except W; change in the amount of surface methods a - d; change the speed of sliding allow for all methods except century

On the external friction is affected by the same factors as for adhesion. Therefore, static friction should be more dynamic. In the initial shift part of the efforts spent for output of particles or molecules of the same body of irregularities or molecular Yam another, and the rest to break molecular and other contacts that while driving do not have time to fully recover. Quite a low rate of relative motion of the rubbing bodies can cause surges, i.e. intermittent motion: the body goes from one static location to another where the newly formed contacts with the sliding surface. At high sliding velocities and unchanged condition sliding surfaces such effects are not usually observed.

Date: 2015-12-24; view: 811