Chapter 7: Keys, Cotter Joints, Pin Joints – Machine Drawing with AutoCAD

Chapter 7

Keys, Cotter Joints, Pin Joints

Chapter Outline

 

Key, cotter, and pin are employed as temporary fasteners to join two components to transmit motion and forces from one element to another. Perhaps the most common of these motion-transmitting connections are keys. Keys are usually driven parallel to the axis of the shaft that are subjected to torsional motion. Cotters are normally driven at right angles to the axes of the connected parts.

KEY JOINTS

The major function of keys is to prevent relative rotation between the members connected by keys and keyways, for example, motor shaft and pully, gears, couplings crank, crankshaft and so on. In addition to relative rotational motion, keys also prevent relative axial movement. The extensive use of key joints is largely due to their simple yet robust design, convenience of assembly and disassembly, low cost, and so on.

KEY

Key is usually a rectangular or slightly tapered piece with a square, rectangular, or round section—Fig. 7.1a. Keys are usually made of cold finished low-carbon steel, though heat treated alloy steels are also used when keys are subjected to considerable crushing and shearing load. Fig. 7.1b illustrates the application of a round key (pin) to connect two components.

 

FIG. 7.1   Application of round key

 

A key should be designed in such a fashion that it becomes the weakest component of the assembly and thus acts as a mechanical fuse. In order to accommodate the key, a seat or groove is cut in the shaft as well as on the hub (keyway) as shown in Fig. 7.2. The groove on the shaft makes its effective cross-sectional area smaller, in addition to the development of stress-concentration at the sharp corners. This may often cause failure of the shaft or axle. The figure shows a rectangular cross-section of a key without any tapering. If a taper key is employed, then the keyway depth (d) will not be uniform along its length to accommodate the increase in thickness (t) of the key due to tapering.

 

FIG. 7.2   Key and keyway

 

A large number of standard keys find use in engineering application. The choice of a particular key depends on the load to be transmitted. Keys can be broadly classified into the following catagories.

Sunk key   It is a type of key which goes partly in the key seat, machined in the shaft, and partly in the key way, machined in the hub. The majority of the keys belong to this category among which rectangular and square sunk keys are commonly used in industry. Fig. 7.2 shows the arrangement of a rectangular sunk key with parallel sides. The cross-section of a square key (not shown) is d/4 × d/4 where d is the diameter of the shaft.

A sunk taper key is used for large power transmission, having either a rectangular or a square cross-section. The depth of the keyway is uniform inside the shaft but there is a tapering in the hub as shown in Fig. 7.3. The approximate proportion at the thicker end of a sunk taper key may be taken as width W = 0.25d and thickness, T = 0.66W, for a rectangular key and W = 0.25d and T = W, for a square key where d denotes the diameter of the shaft.

 

FIG. 7.3   Sunk taper key

 

Saddle key   A saddle key may be of two types. A flat saddle key–Fig. 7.4a—used for light duty only, is a rectangular piece with a taper of 1 : 100. It is inserted within the keyway made on the hub. No key seat or groove is cut in the shaft.

A hollow saddle key—Fig. 7.4b—is similar to a flat saddle key and suitable for the same kind of work. A concave surface is made at the bottom of the key with a radius slightly less than that of the shaft. The proportion of the key in terms of the shaft diameter d can be given as follows.

 

Width of the key        W = 0.25d + 2 mm

Thickness of the key   T = 0.8d + 1 mm

 

FIG. 7.4   Saddle keys

 

FIG. 7.5   Woodruff key

 

FIG. 7.6   Tangential key

 

Woodruff key   It is extensively used in the automobile industry and machine tools industry where low power needs to be transmitted. It has the form of a segment of a circular disc with uniform thickness. The curve portion is placed within the seat cut in the shaft while the flat portion goes into the hub as shown in Fig. 7.5. The disadvantage of this key is that the shaft becomes weaker due to removal of material to make room for the keyseat. Various dimensions of the key in terms of shaft diameter (d) are mentioned in the figure.

Tangential key or kennedy key   It is used for high power transmission as in the case of rolling mills. The keys are tapered and driven tightly. To lock shafts subjected to reversal of direction, two keys are placed 90° or 120° apart as shown in Fig. 7.6.

Gib headed key   It is an ordinary, regular key with a gib at one end so that it can be easily withdrawn with the help of a wedge between the gib and the hub at the space marked X in Fig. 7.7 The proportion of the gib head in terms of the shaft diameter is also mentioned in the figure.

 

FIG. 7.7   Key with gib head

 

Feather key   A feather key is similar to a parallel sunk key secured to the shaft by means of a cap screw as shown in Fig. 7.8c. The hub is free to slide axially though relative rotational motion between the hub and the shaft is not possible. The cross-section of a feather key may be rectangular (Fig. 7.8a), square, or dove tail (Fig. 7.8b). Easier sliding is obtained with two keys spaced 180° apart. The ends of a feather key are sometimes rounded (Fig. 7.9).

 

FIG. 7.8   Feather key

 

Splines   Splines are multiple feather keys with the keys machined integral with the shaft (Fig. 7.10a). They are used primarily, when a single key is not good enough to withstand the stress. The spline may be of involute shape or straight sided. It is mainly used in gear boxes of automobiles and machine tools where sliding or axial movement between the mating pieces are essential from a functional point of view. The keyways are provided on the sliding part (Fig. 7.10b). Both parts are developed using the AutoCAD solid modelling features.

 

FIG. 7.9   Feather key (solid model)

 

FIG. 7.10   Spline shaft (solid model)

COTTER JOINT

A cotter (Fig. 7.11a) is a flat key-like component with a rectangular cross-section of uniform thickness but tapering to one side in general. This can be very efficiently used to connect two rods subjected to axial load either compressive or tensile in nature. The cotter is inserted perpendicular to the axes of the rods. Fig. 7.11b clearly illustrates the arrangement of a cotter joint in parametric view. It should be noted that a cotter joint cannot transmit rotary motion from one shaft to another.

 

FIG. 7.11   Cotter joint

 

The joints are simple in design and allow a convenient and quick assembly and disassembly. The main shortcomings of a cotter joint are that it is difficult to manufacture and that a locking arrangement for the cotter is necessary for important cases.

The taper in a cotter is generally 1 in 30. If for some reason, a larger taper is provided, a locking arrangement is made so that the cotter does not come out or become loose. If the diameter of the rod is given by d, then the other proportions of the cotter are as follows.

 

Width of the cotter at the center, W = 1.3d

Thickness of the cotter, T = 0. 31 d

 

There are some variations of cotter joint to suit different purposes. Two of them are discussed below with figures.

Socket and Spigot joint   Here, one end of the rod is formed into a socket that fits into the other and having a larger diameter called a spigot. Both the ends are formed by forging. The dimensions of both the ends are made such that the socket can slide easily within the spigot. A slot, to accommodate the cotter, is provided in the socket and the spigot. The position of the slots are so adjusted that the cotter can be driven through them.

There is a clearance between the cotter and the slots. But the cotter comes in contact with two rod ends on opposite sides so as to leave clearance on both sides. In fact, the clearance is very essential for the proper functioning of the cotter. Since a cotter either pulls or pushes (depending on the nature of the load) the slots along the axis, it makes the joint perfectly tight and rigid. The orthographic projection views of a typical socket and spigot cotter joint with all dimensions are shown in Fig. 7.12.

 

FIG. 7.12   Socket and Spigot joint

 

Sleeve joint   Sometimes, instead of making a spigot and a socket on each end of the rod, a separate sleeve or muff fits over the end of each rod as shown in Fig. 7.13. This facilitates the manufacturing of the joint as both the rod ends are circular in section. The position of the clearances should be noted as they are important to ensure a tight joint.

The detailed steps for the drawing of this joint in AutoCAD is described at the end of this chapter.

 

FIG. 7.13   Cotter joint with sleeve

 

Gib and Cotter joint   This type of joint is generally used to connect shafts with square rod ends as shown in Fig. 7.14. One end of the rod is converted into a fork or strap in which the rod end fits. Rectangular slots are provided at both ends to accommodate the gib and cotter, keeping space for clearances in rod ends. The depth and the width of the gib heads are usually equal. The outer sides of the gib and the cotter are made parallel while the inner mating sides have matching taper as shown in Fig. 7.14a. The orthographic half-section view of the gib and cotter joint is shown in Fig. 7.14b.

Pin joint or knuckle joint   This joint is used to connect two rods whose axes intersect at a point and are subjected to tensile or compressive load. The joint is not rigid as it allows small relative angular motion between the rods. Different components of a knuckle joint along with their assembly view (Fig. 7.17) are developed in AutoCAD solid models and are presented here. The operational principle of the joint can be understood from these figures.

 

FIG. 7.14   Gib and Cotter joint

 

A pin joint is necessary to convert reciprocating motion into rotary as in the case of a slider crank mechanism. Application of this joint is found in valve and eccentric rods, lever and pump rod joint, tie rod joint for roof truss and so on. The pin is kept in position by means of a collar and a taper pin. Fig. 7.15 shows the orthographic views of the knuckle joint with all the dimensions.

 

FIG. 7.15   Pin-joint or knuckle joint

 

FIG. 7.16   Assembled view of a knuckle joint

 

FIG. 7.17   Exploded view of a knuckle joint

EXAMPLE OF ORTHOGRAPHIC PROJECTION DRAWING USING AutoCAD

Cotter Joint with Sleeve

Now that you are familiar with AutoCAD drawing commands required for 2-D drafting, let us make an attempt to develop an orthographic drawing of a cotter joint with sleeve starting from scratch. Since the object is complicated and symmetric, the inherent advantages of AutoCAD over the manual mode of drafting will be appreciated more by the readers.

The detailed working principle of the joint is already mentioned earlier in this chapter. Here, we shall concentrate only on the drawing part. The dimensions of different components of the joint are shown in Fig. 7.18. Our objective is to reproduce the drawing shown in Fig. 7.18 using AutoCAD drawing commands.

The detailed procedure of preparing the complete orthographic views is illustrated in the following steps. It is advisable to follow the same procedure for any drafting work with AutoCAD.

Step 1   Fix up the units of measurement and choose from the Metric or English options when you open a new drawing file. For this example, the option will be Metric.

Step 2   Next set the Limits of your drawing area. This will depend upon the basic dimensions of the object and the number of projection views to be represented. You may have to set aside some space for a clear margin all around the drawing and also space for the title block, parts list and so on. Based on the dimensions of different components of the joint mentioned in Fig. 7.18, the two corner points are chosen such that the entire drawing can be accommodated in full scale (1:1). For the present case, the following limits are provided.

 

Limits: Lower left corner 0, 0

Upper right corner 450, 300

 

FIG. 7.18   Dimensions of different components of knuckle joint

 

Step 3   You should create suitable layers so that the outline of the object, center lines, hidden lines, dimensions, text objects, borders, and so on can be drawn on separate layers. Each layer should have different linetypes with specified thicknesses (and colors, if applicable). For beginners, it may be advisable to create one layer for construction lines (Geometry) which may be turned off in the final drawing. Using the layer command, create the layers shown in Table 7.1.

 

Table 7.1   Different Layers Created for the Example

Name Linetypes Line Thickness
Object outline Solid line 0.50 mm
(Ooline)
Center line Center 0.25 mm
(Center) (Default)
Hidden ISO dash 0.25 mm
(Hidden) (Default)
Hatch Solid line 0.25 mm
(Hatch) (Default)
Dimension Solid line 0.25 mm
(Dim) (Default)
Text Solid line 0.25 mm
(Text) (Default)
Construction Solid line 0.25 mm
(Default)
Border Solid line 0.35 mm

Step 4   First of all, it is recommended to draw a rectangle (in construction layer) with lower left and upper right corners as 0, 0 and 450, 300 respectively, indicating the limits of the drawing area. Then draw the border (in Border layer) taking offset distances as per the margins to be left. Next, decide on the allocation of space for the drawing area and title block. In Fig. 7.19 a rectangular border (ABCD) is drawn in the construction layer where different views are to be drawn. Standard title box space (185 mm × 65 mm) is also located at the bottom right corner. The layout of the drawing is thus complete.

 

FIG. 7.19   Layout of the drawing

 

Step 5   The Title Block is generally a standardised format. It can be drawn separately or it may be kept ready as Wblock for insertion at the appropriate places (discussed in the Block section). Here, a specimen Title Block following BIS guidelines which may be drawn or inserted as shown in Fig. 7.20.

 

FIG. 7.20   Standard Title box

 

After placing the Title block and turning off the construction layer, your drawing will resemble Fig. 7.21. Remember, the shape of or the text in the Title block can be changed by exploding it (created by Wblock command).

 

FIG. 7.21   The drawing layout with Title box

 

Step 6   The general principle for drawing any machine element may be listed as given below.

  1. Draw center lines in all the views. If the cylindrical part or a hole is viewed as a rectangle in a particular view, then draw only one center line along its axis (Fig. 7.22). However, if it is seen as a circle, draw two center lines intersecting at right angles at its centre.
  2. Develop details of all the views in the sequence as mentioned below.
    1. Circle and arcs.
    2. Straight lines generating the shape of the object.
    3. Minor details.
    4. If the object is symmetrical, as in the present case, then one half may be drawn in the initial stage and the other half may be generated by mirroring the formerly drawn portion.
  3. Wherever necessary, add sectional lines (hatching).
  4. Put up dimensions.
  5. Insert necessary text.

Following the above principles, draw the center lines for the side and front view of the cotter and sleeves. The outlines of the object in the side view will be circles with specified diameters. After this step your drawing will resemble Fig. 7.22.

 

FIG. 7.22   Drawing the center lines

 

FIG. 7.23   Projection lines before trimming

 

Step 7   Since the front view is symmetrical about an axis, develop the shape of the right hand part of the sleeve, shaft, and cotter. The left hand part may be generated by mirroring the right hand part. The projection lines may be drawn initially by construction lines and later the final object outline parts (after trimming) may be converted into object lines in Ooline layers. The right hand part should be drawn first because, if you notice, the front view is actually a symmetrical view and therefore draw half of the object first and then use mirror command to get the other half. After drawing the right hand part partially, the drawing will resemble Fig. 7.23. For clarity, only the drawing of the object is given. Note that the taper side of the cotter has been drawn by giving proper offset distances and then connecting the upper left point with the right bottom point with a line. The two lines showing these extremities will be eventually erased.

Step 8   Now use the Trim command to delete all the excess lengths of the projections so that only the outline of the object remains in the drawing (Fig. 7.24).

 

FIG. 7.24   Object outlines after trimming

 

Step 9   Now fillet all the ends (fillet radius = 3.0) that require filleting. Also, draw the upper and lower curved parts of the cotter in the front view and complete its projection to the side view. Also show the broken shaft by drawing arcs at the extreme right end of the shaft. Your figure will now look like Fig. 7.25. Note that the edge lines of the sleeve on the left have been removed to accommodate the hatching. At this stage, convert some object lines into hidden lines as shown in Fig. 7.25.

 

FIG. 7.25   The completed front view before mirroring

 

Step 10   Use the mirror command to create the left hand view of the shaft, sleeve, and cotter at one stroke. Next, put all the essential dimensions in the dimension (Dim) layer. Finish your drawing by hatching (in Hatch layer) in the required zones. The shaft is provided with local section only. The final version of the completed drawing is as shown in Fig. 7.26.

 

FIG. 7.26   The final view of the drawing

EXERCISE
  1. Explain, with sketches, the use of (i) flat saddle key (ii) sunk key with gib head (iii) woodruff key and (iv) feather key.
  2. Explain, with sketches, the arrangement of attaching a feather key with the shaft and the hub of a pulley or gear.
  3. What is the difference between a key and a cotter? State the purpose for which each is used.
  4. Explain the difference between a cotter joint and a pin joint with sketches. Why is clearance provided in a cotter joint?
  5. Draw the two views of a cotter joint for joining two 30 mm diameter rods. Mention the necessary dimensions in the drawing.
  6. Two rods of diameter 25 mm are to be joined by an arrangement of a cotter joint with a sleeve. Draw two views in full scale.
  7. Why is a gib used along with a cotter? Draw the arrangement of a gib and cotter joint with the gib and cotter to connect two shafts having a square section of 40 mm on each side.
  8. Draw the necessary views of a knuckle joint to connect two rods, each with 40 mm diameter.
  9. Draw the detailed views of a knuckle joint shown in Fig. 7.15.
  10. Draw two views of a 6-spline shaft, taking the outside diameter as 80 mm.
  11. Fig. 7.17 shows different components of a knuckle joint created in solid model. Develop the isometric views of the component using the isometric settings provided in AutoCAD.
  12. Draw the isometric views of the components of the cotter joint with sleeve described in Question 6.
  13. Develop the pictorial view of a gib and cotter joint.
  14. Develop the isometric view of a cotter joint described in Question 5.