Class 12 - Holiday Home work for the coming week (25th May to 29th May 2020)

Dear Students,

The following is the holiday homework for the week - 25th May to 29th May 2020

You will learn about KEYS. Let us understand them in detail.


What is a KEY?

A key is a piece of metal which is used to fasten two parts together. More specifically, a key is used to join or tighten two circular moving parts together. For example, moving parts such as pulleys and flywheels are joined to the shaft by means of a key. A key prevents relative motion between the two parts so that these can rotate as one composite part. The key can be unfastened and removed. Therefore it is a temporary fastener.

There is a groove cut out on the shaft as well as the pulley, within which a key is seated. The groove is called as the key way.

Types of Keys

1. Rectangular Sunk Key
2. Woodruf Key

Feather Keys are of two types

3. Gib Head Taper Key
4. Double headed feather key
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Lets us learn about Rectangular keys

1. Rectangular Sunk Key

The rectangular sunk key has a rectangular cross section, and a taper that runs along its length. The typical taper is 1: 100. Refer to figure 1 below

What is Taper:  Gradual reduction in the thickness of the key as we move along its length is called as Taper. 


       
Fig 1: Rectangular Sunk Key

Supposing, the key is used on a shaft with diameter, d. Then:

1. Width of the key will be = d/4
2. Thickness of key, t = d/6
3. Length, L = 1.5 d
4. Taper = 1:100

If the key is not provided with taper then the key will remain lose fitted in the key way and it will disengage when the shaft rotates at high angular velocity.


2. Wood ruff Key

Wood ruff key is a special sunk key. Its looks like a segment of a circular disc. The seat of the key is semi circular in shape. The top of the key way is rectangular. The key way is smaller than the key seat. The advantage of the wood ruff key is that it can be easily adjusted in the recess. It is largely used in machine tools and automobiles.








3. Gib Head Taper Key

A Taper key is uniform in width but tapered in height. The bottom surface of the key is straight and the top surface is tapered. Sometimes, taper keys also have a gib head for easier removal of the key during disassembly. The gib head prevents relative motion between the hub and the shaft. 

   








4. Double headed feather key with Gib Head

A feather key is a kind of sunk parallel key. In parallel key, the thickness of the key remains the same throughout the length of the key. A key that has gib head at both the ends is called as the Double headed feather key with Gib Head. The hub is gripped between its gib heads.








Exercise

Q 1. Sketch freehand the FV, TV and Side View of a double headed gib key, suitable for a shaft of diameter 60 mm. Give standard dimensions.


Q 2. Sketch free hand rectangular sunk key, in position, on a shaft of diameter, d = 40 mm keeping the axis of the shaft parallel to the VP and the HP.


Q 3. Sketch freehand the FV, TV and Side View of a wood ruff key, suitable for a shaft of diameter 40 mm. Give standard dimensions.