Saturday, February 8, 2014


MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.


1. Introduction
                  A key is piece of mild steel inserted between the shaft and hub or boss of the pulley to connect these together in order to prevent relative motion between them. It is always inserted parallel to the axis of the shaft. Keys are used as temporary fastenings and are subjected to considerable crushing and shearing stresses. A keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key.

2. Types of Keys
The following types of keys are important from the subject point of view:
1. Sunk Keys
2. Saddle Keys
3. Tangent Keys
4. Round Keys
5. Woodruff key

Sunk Keys
The sunk keys are provided half in the keyway of the shaft and half in the keyway of the hub or boss of the pulley. The sunk keys are of the following types.
a.  Rectangular sunk key:  A rectangular sunk key is shown in Fig. The usual proportions of this key are, Width of key, w = d / 4 and thickness of key, t = 2w /3 = d / 6      Where d = Diameter of the shaft or diameter of the hole in the hub.
The key has taper 1 in 100 on the topside only.

b. Square sunk key: The only difference between a rectangular sunk key and a square sunk key is that its width and thickness are equal i.e. w=t=d/4.

c. Parallel sunk key: The parallel sunk keys may be of rectangular or square section uniform in width and thickness throughout. It may be noted that a parallel key is a taperless and is used where the pulley, gear or other mating piece is required to slide along the shaft.

d. Gib-head key: It is a rectangular sunk key with a head at one end known as gib head. It is usually provided to facilitate the removal of key. A gib head key is shown in Fig.(a) and its use in shown in Fig (b).

The usual proportions of the gib head key are: Width, w=d/4;
And thickness at large end,    t = 2w /3 = d / 6

5. Feather key:  A key attached to one member of a pair and which permits relative axial movement is known as feather key. It is a special type of parallel key, which transmits a turning moment and also permits axial movement. It is fastened either to the shaft or hub, the key being a sliding fit in the key way of the moving piece.
The feather key may be screwed to the shaft as shown in Fig. (a) or it may have double gib heads as shown in Fig, (b). The various proportions of a feather key are same as that of rectangular sunk key and gib head key.

2. Saddle keys:
The saddle keys are of the following two types:
a. Flat saddle key, and b.Hollow saddle key.
a) Flat saddle key is a taper key which fits in a key way in the hub and is flat on the shaft as shown in Fig.  It is likely to slip round the shaft under load. Therefore it is used for comparatively light loads.

b) Hollow saddle key is a taper key which fits in a key way in the hub and the bottom of the key is shaped to fit the curved surface of the shaft. Since hollow saddle key hold on by friction, therefore these are suitable for light loads. It is usually used as ‘a temporary fastening in fixing and setting eccentrics, cams etc

3) Tangent Keys
The tangent keys are fitted in pair at right angles are shown in Fig.  Each key is to withstand torsion in one direction only. These are used in large heavy duty shafts.

4. Round Keys:
The round keys, as shown in Fig.(a), are circular in section and fit into holes drilled partly in the shaft and partly in the hub. They have the advantage that their keyways may be drilled and reamed after the mating parts have been assembled. Round keys are usually considered to be most appropriate for low power drives.

5. Woodruff key:  The woodruff key is an easily adjustable key. It is a piece from a cylindrical disc having segmental cross-section in front view as shown in Fig..  A woodruff key is capable of tilting in a recess milled out in the shaft by a cutter having the same curvature as the disc from which the key is made. This key is largely used in machine tool and automobile construction.

         The jigs and fixtures are the economical means to produce repetitive type of work by incorporating special work holding and tool guiding devices. The following are the advantages of employing jigs and fixtures in mass production work.
1.It eliminates the marking out, measuring, and other setting methods before machining.
2. It increases the machining accuracy, because the workpiece is automatically located and the tool is guided without making any manual adjustment.
3. It enables production of identical parts which are interchangeable. This facilitates the assembly operation.
4. It increases the production capacity by enabling a number of workpieces to be machined in the single set up, and in some cases a number of tools may be made to operate simultaneously. The handling time is also greatly reduced due to quick setting and locating of the work. The speed, feed and depth of cut for machining can be increased due to high clamping rigidity of jigs and fixtures.
5. It reduces the operator’s labour and consequent fatigue as the handling operations are minimized and simplified.
6. It reaches semi-skilled operator to perform the operations as the setting operations of the tool and the work are mechanized. This saves labour cost.
7. It reduces the expenditure on the quality control of the finished products
8. It reduces the overall cost of machining by fully, or partly automatising the processes.
The definition of jigs and fixtures are given below :
Jig : A jig may he defined as a device which holds and locates a workpiece and guides and controls one or more cutting tools. In construction, a jig comprises a plate, structure, or box made of metal or in some cases of non-metal having provisions for holding the components in identical positions one after the other, and then guiding the tool in correct position on the work in accordance with the drawing, specification, or operation layout.
Fixture : A fixture may be defined as a device which holds and locates workpiece during an inspection or for a manufacturing operation. The fixture does not guide the tool. The tools are set at the required position on the work by using gauges or by manual adjustment.
 The following are the fundamental differences between fixture and a jig:
1. A fixture holds and position the work but does not guide the tool, whereas a jig holds, locates and as well as guides the tool.
2. The fixtures are generally heavier in construction and are bolted rigidly on the machine table, whereas the jigs are made lighter for quicker handling, and clamping with the table is often unnecessary. -
3. The fixtures are employed for holding work in milling grinding, planning, or turning operations, whereas the jigs are used for holding the work and guiding the tool particularly in drilling, reaming or taping operations.

No comments: