Reed type Mechanical Comparator and Sigma Comparator

Reed type Mechanical Comparator

  • The instrument used to measure linear displacement of spindle with the help of the reed mechanism is known as reed type Mechanical comparator.
  • It is a device magnifying small motions of spindle.

Constructional details :


  • The reed mechanism is frictionless device for magnifying small motions of the spindle
  • It consists of a fixed block `A` which is rigidly fastened to the gauge head case
  • The floating block `B` carries the gauging spindle
  • The floating block is also connected horizontally to the fixed block by reeds `C`
  • A vertical reed is attached to each block with upper ends jointed together
  • These vertical reeds are shown in fig by letter `D`
  • A pointer is attached to this joint

Working principle

  • A linear motion of the spindle moves the floating block vertically upwards
  • This vertical movement causing the vertical reed on the floating block to slide past the vertical reed on the fixed block
  • The movement causes both reeds swing through an arc
  • As the point is nearly an extension of the vertical reeds `M` it swings through a much wider arc

Sigma Comparator

The Sigma Comparator is a mechanical comparator in which the amplification is obtained by means of a compound lever
  • Advantages: It has got a bold scale and larger indicating pointer
  • Dis Advantages: Due to motion of parts ,there is wear in the  moving parts


Constructional details :

  • The vertical beam is mounted on flat steel springs connected to fixed members
  • The vertical beam which in turn are screwed to a back plate
  • The assembly provides a frictionless movement with restraint from the springs
  • The hinged assembly carrying the forked arms incorporates a hardened fulcrums  
  • The metal ribbon attached to the forked arms passes around the spindle
  • This spindle is to rotate in specially designed miniature ball bearings
  • The indicating pointer is secured to a boss on the disc

Operating Principle :

  • The various movements in the Sigma Comparator are shown in figure.
  • The plunger is mounted on a pair of slit diaphragms in order to have frictionless linear movement
  • A knife edge is mounted on it and bears upon the face of the moving member of a Cross strip hinge
  • The cross strip hinge consists of the moving component and a fixed member
  • The cross strip hinge then connected by thin flexible strips alternately at right angles to each other
  • Thus if an external force is applied to the moving member it will pivot as would a hinge about the line of intersection of the strips
  • To the moving member an arm `Y` shape having the effective length `b` is attached
  • If the distance of the hinge from the knife edge be `a` then the magnification of the first stage is `b/a`
  • A phosphor-Bronze strip is attached to the two extremities of the `Y` arm
  • And is passed around a radius `r` attached to the pointer spindle
  • If the length of pointer be `R` then R/r is the second stage of magnification
  • The total magnification is
        

    • In order to adjust the magnification ,distance `a` must be changed by slackening and tightening the two screws attaching the knife edge to the plunger
    • In sigma comparator a damping device must be employed to prevent oscillation of point before coming to rest.

    Comments

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    2. Edward Brown5 October 2019 at 18:43

      This comment has been removed by the author.

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