A screw pitch gauge is a precision instrument. It is used for measuring diameter of circular objects mostly wires, with an accuracy of 0.001cm. It consists of a hollow cylinder mounted on a U frame. The hollow cylinder leads to a ratchet which is meant for fine adjustment. The U frame consists of a flat end known as stud and a screw on the other side. This screw can be moved inside the nut by fitted in the U frame by rotating the hollow cylinder called the thimble. This is called the main scale. The hollow cylinder or the thimble is graduated into 50 or 100 equal parts. This is called the circular scale.
Principle of Screw Gauge
1)The screw gauge works on the principle of screw.
2) When we rotate the head ‘H’ by means of safety device ‘D’ through one complete rotation, the distance moved by the screw for every complete rotation is constant.
3) This constant distance moved by the screw for one complete rotation of head ‘ H ‘ is called Pitch of the screw.
4) If the head scale has 100 equal divisions, then the distance moved by the screw for even 1/100 of a complete rotation can be measured accurately,this is called Least count of screw gauge.
Least count=Pitch/No of division on the circular scale
How to use Screw Gauge
Lets take the example of Determination of Diameter of a Wire
The wire whose thickness is to be determined is placed between the anvil and spindle end, the thimble is rotated till the wire is firmly held between the anvil and the spindle. The rachet is provided to avoid excessive pressure on the wire. It prevents the spindle from further movement.
Reading = Linear scale reading + (Coinciding circular scale x Least count)
Linear scale reading = 1mm or 0.1cm
Circular scale reading = 26, so 26 x 0.001 = 0.026cm
So observed reading = 0.1 + 0.026 = 0.126cm
Viva Voice Questions for Screw Gauge physics experiment
What is back-lash error?
Within a nut there is a little space for the play of screw. Due to continuous use this space increases. Thus when the screw is turned in one direction the stud moves as usual. However, when the screw is rotated in the opposite direction, the stud does not move for a while. This error is called Back lash error. In short “Back lash error is the error introduced on reversing the direction of rotation”.
What is a screw?
Screw is a simple machine related to inclined plane.
Why is the screw used in screw gauge called a micrometer screw?
It is called as it can measure distance correct upto a micrometer
What is meant by “gauge”?
The gauge means device or instrument
Name two main parts of a screw-gauge?
(a) A nut
(b) A bolt or screw
What is pitch of a screw?
Pitch is the distance between two nearest (consecutive or successive) threads along the axis of screw.
How is the pitch found?
By dividing the distance covered by the screw in a known number of rotations by the total number of relations.
What is the least count (L.C.) of the screw gauge?
L.C. of screw gauge = 0.001 cm.
What is the Least count? How the L.C. of a screw gauge is found?
Least count is the distance moved by the screw when it is rotated by one circular scale division.
It is calculated by using the relation: L.C. = (Pitch of the screw / No. of circular scale divisions)
What is meant by zero error of a screw-gauge?
The error which arises when the zero of circular scale does not coincide with the zero of the main scale upon joining the two studs.
When the zero-error is positive?
If the zero of the circular scale lies above the reference line, provided that the fixed and movable studs are in contact.
what metal used in making screw and why?
The screw is made of gun metal to avoid wear and tear after long use
What is the degree of accuracy of the screw gauge?
Degree of accuracy = L.C. or Reading power = 0.001 cm
What is mechanical advantage of a screw gauge?
Like a screw jack mechanical advantage of a screw gauge is 2? r/h; where ‘r’ is the radius of cylinder of the screw and ‘h’ is the pitch.
What is meant by range of the screw gauge?
The maximum length of the main scale.
How back lash error is avoided?
By turning the screw in one direction only.
What are “precision instrument”?
The instrument that can measure up to a fraction of a mm, e.g., vernier caliper, screw gauge and spherometer.
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