By | April 28, 2015

A bolt and nut in combination is a fastening device used to hold two parts together. The body of the bolt, called shank is cylindrical in form, the head; square or hexagonal in shape, is formed by forging. Screw threads are cut on the other end of the shank. Nuts in general are square or hexagonal in shape. The nuts with internal threads engage with the corresponding size of the external threads of the bolt. However, there are other forms of nuts used to suit specific requirements.

For nuts, hexagonal shape is preferred to the square one, as it is easy to tighten even in a limited space. This is because, with only one-sixth of a turn, the spanner can be re-introduced in the same position. However, square nuts are used when frequent loosening and tightening is required, for example on job holding devices like vices, tool posts in machines, etc.

The sharp corners on the head of bolts and nuts are removed by chamfering.

#### Methods of Drawing Hexagonal (Bolt Head ) Nut

Drawing hexagonal bolt head or nut, to the exact dimensions is labourious and time consuming. Moreover, as standard bolts and nuts are used, it is not necessary to draw them accurately. The following approximate methods are used to save the draughting time :

#### Method 1

Empirical relations :

• Major or nominal diameter of bolt = D
• Thickness of nut, T = D
• Width of nut across flat surfaces, W = 1.5D + 3 mm
• Radius of chamfer, R = 1.5D

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### Procedure

1. Draw the view from above by drawing a circle of diameter, W and describe a regular hexagon on it, by keeping any two parallel sides of the hexagon, horizontal.
2. Project the view from the front, and the view from side, and mark the height equal to D.
3. With radius R, draw the chamfer arc 2-1-3 passing through the point 1 in the front face.
4. Mark points 4 and 5, lying in-line with 2 and 3.
5. Locate points 8,9 on the top surface, by projecting from the view from above.
6. Draw the chamfers 4–8 and 5–9.
7. Locate points 6 and 7, lying at the middle of the outer two faces.
8. Draw circular arcs passing through the points 4, 6, 2 and 3, 7, 5, after determining the radius R1 geometrically.
9. Project the view from the side and locate points 10, 11 and 12.
10. Mark points 13 and 14, lying at the middle of the two faces (view from the side).
11. Draw circular arcs passing through the points 10, 13, 11 and 11, 14, 12, after determining the radius R2 geometrically.

It may be noted that in the view from the front, the upper outer corners appear chamfered. In the view from the side, where only two faces are seen, the corners appear square.

#### Method 2

Empirical relations :

• Major or nominal diameter of  bolt = D
• Thickness of nut, T  = D
• Width of the nut across corners = 2
• D Radius of chamfer arc, R = 1.5 D

Figure below illustrates the stages of drawing different views of a hexagonal nut, following the above relations, which are self-explanatory.

Selection of Views in Orthographic Projections

#### Method of Drawing Square (Bolt Head) Nut

A square bolt head and nut may be drawn, showing either across flats or corners. Following relations may be adopted for the purpose:

• Major or nominal diameter of  bolt = D
• Thickness of nut, T = D
• Width of the nut across flats, W = 1.5 D + 3 mm
• Radius of chamfer arc, R = 2 D

Figure below illustrates the method of drawing views of a square nut, in two orientations.

#### Hexagonal and Square Headed Bolts

Figure below shows the two views of a hexagonal headed bolt and square headed bolt, with the proportions marked.

#### Washers

A washer is a cylindrical piece of metal with a hole to receive the bolt. It is used to give a perfect seating for the nut and to distribute the tightening force uniformly to the parts under the joint. It also prevents the nut from damaging the metal surface under the joint. Figure below shows a washer, with the proportions marked.

Figure below illustrates the views of a hexagonal headed bolt with a nut and a washer in position.

#### Other Forms of Bolts

• Square Headed Bolt with Square Neck – It is provided with a square neck, which fits into a corresponding square hole in the adjacent part, preventing the rotation of the bolt.

• T-Headed Bolt With Square Neck – In this, a square neck provided below the head, prevents the rotation of the bolt. This type of bolt is used for fixing vices, work pieces, etc., to the machine table having T-slots.

• Hook Bolt – This bolt passes through a hole in one part only, while the other part is gripped by the hook shaped bolt head. It is used where there is no space for making a bolt hole in one of the parts. The square neck prevents the rotation of the bolt.

• Eye Bolt – In order to facilitate lifting of heavy machinery, like electric generators, motors, turbines, etc., eye bolts are screwed on to their top surfaces. For fitting an eye bolt, a tapped hole is provided, above the centre of gravity of the machine.

• Stud Bolt or Stud – It consists of cylindrical shank with threads cut on both the ends. It is used where there is no place for accommodating the bolt head or when one of the parts to be joined is too thick to use an ordinary bolt. The stud is first screwed into one of the two parts to be joined, usually the thicker one. A stud driver, in the form of a thick hexagonal nut with a blind threaded hole is used for the purpose. After placing the second part over the stud, a nut is screwed-on over the nut end. It is usual to provide in the second part, a hole which is slightly larger than the stud nominal diameter.

#### Other Forms of Nuts

• Flanged Nut – This is a hexagonal nut with a collar or flange, provided integral with it. This permits the use of a bolt in a comparitively large size hole.
• Cap Nut – It is a hexagonal nut with a cylindrical cap at the top. This design protects the end of the bolt from corrosion and also prevents leakage through the threads. Cap nuts are used in smoke boxes or locomotive and steam pipe connections.
• Dome Nut – It is another form of a cap nut, having a spherical dome at the top.
• Capstan Nut – This nut is cylindrical in shape, with holes drilled laterally in the curved surface. A tommy bar may be used in the holes for turning the nut. Holes may also be drilled in the upper flat face of the nut.
• Slotted or Ring Nut – This nut is in the form of a ring, with slots in the curved surface, running parallel to the axis. A special C-spanner is used to operate the nut. These nuts are used on large screws, where the use of ordinary spanner is inconvenient.
• Wing Nut – This nut is used when frequent removal is required, such as inspection covers, lids, etc. It is operated by the thumb.

#### Cap Screws and Machine Screws

Cap screws and machine screws are similar in shape, differing only in their relative sizes. Machine screws are usually smaller in size, compared to cap screws. These are used for fastening two parts, one with clearance hole and the other with tapped hole. The clearance of the unthreaded hole need not be shown on the drawing as its presence is obvious. Figure below shows different types of cap and machine screws, with proportions marked.

Cap screws are produced in finish form and are used on machines where accuracy and appearance are important. As cap screws are inferior to studs, they are used only on machines requiring few adjustments and are not suitable where frequent removal is necessary. These are produced in different diameters, upto a maximum of 100 mm and lengths 250 mm.

Machine screws are produced with a naturally bright finish and are not heat treated. They are particularly adopted for screwing into thin materials and the smaller ones are threaded throughout the length. They are used in fire-arms, jigs, fixture and dies. They are produced in different diameters upto a maximum of 20 mm and lengths upto 50 mm.

#### Set Screws

These are used to prevent relative motion between two rotating parts, such as the movement of pulley on shaft. For this, a set screw  is screwed into the pulley hub so  that its end-point bears firmly against the shaft. The fastening action is by friction between the screw and the shaft.

Set screws are not efficient and so are used only for transmitting very light loads. For longer life, set screws are made of steel and case hardened. Further, for better results, the shaft surface is suitably machined for providing more grip, eliminating any slipping tendency. Figure given shows different forms of set screws.