Washers and nut-bolt assembly

The essentials

A plain washer is a thin flat ring placed under a nut (or bolt head). It gives a smooth, flat bearing surface, spreads the clamping load over a larger area, and protects the surface of the part from being scored when the nut is turned. A nut and bolt assembly joins two parts, usually plates or flanges, through drilled clearance holes, with or without a washer.

Key points

• Washer proportions commonly used in school drawing: hole diameter D + 1 mm, outside diameter 2D + 3 mm, thickness 0.15D. For M20: hole 21 mm, outside 43 mm, thickness 3 mm.
• The clearance hole in the parts is slightly larger than D so that the bolt passes freely.
• Bolt length must cover the thickness of the parts (the grip) + washer + nut, with a small extra length projecting beyond the nut.
• In a sectional assembly view, the bolt and nut are not sectioned when cut along their axis; the parts (plates) are hatched, with adjacent parts hatched in opposite directions.
• The thread end of the bolt is shown, and the threaded portion should end within the clearance hole, so that the nut can tighten fully.
• Without a washer, the nut bears directly on the part; the washer is preferred when the surface is soft or rough.

Exam tips

• Draw the parts first, then the bolt through the holes, then the washer and nut; keep the centre line throughout.
• Show the small gap between bolt and clearance hole clearly.

Common mistakes

• Hatching the bolt in a longitudinal section.
• Making the bolt so short that the nut is not fully engaged.