The sifter or Vibrating Screen
Vibrations are not always eliminated in mechanical systems but are sometimes used for particular applications. One type of these mechanical systems is the vibrating screen system.
The vibrating screen is used when having various sizes of the same material (used in grain milling, in the crushing of aggregate for construction purposes, coal production etc.) that are in need to be separated. The particular item is placed on the screen (top part of the system) which looks like a mesh full of holes. (Wise Geek / ND)
Figure 13: Vibrating screen system in a batching plant Taken from http://www.actionconveyors.com/index.php/engineering/:
The whole machine will then start to vibrate to make the material move and pass over and through the screens and thus the different sizes of the material are separated. The screen has increasing size of holes such that as the material progresses over the screen, the smaller sized material falls first into the collecting hopper. The change in the screen hole size coincides with a separate hopper such that each size is collected in a different hopper. In this way material of different sizes is separated into a number of different sized groups which are then used for different purposes. The screen is mounted on flexible (normally rubber) mounts such that it can gyrate and cause the sifting to take place. The vibrations are caused by a motor with a flywheel with an unbalanced mass attached to it and rotated at a frequency which is close to the natural frequency of the sifter with the material on it. A number of vibrator motors are used along the whole sifter for better efficiency. (Wise Geek / ND)
Figure 14: Vibra...
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...on. The value for E can be estimated from the counted distribution of part orientations on the bowl track.
F = (E x V)/lavg (Eq. 7)
Measuring actual feed rate and efficiency
The actual feed rate and efficiency of a working bowl can be measured by running the bowl at steady-state conditions for a measured period of time, perhaps 20 seconds, and catching the parts as they leave the tooling. In this lab the correctly oriented parts, No, can all be caught as they feed off the end of the track, and the rejected parts, Nr , can all be caught as they fall off the edge of the track in front of the selector. The actual efficiency is found by substituting these counts into Eq. 1 as follows:
E = No/(Nr + No) (Eq. 8)
The actual feed rate is the counted number of correctly-oriented parts coming off the end of the feed track in a measured time.
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