In Engineering (Mechanical works) What is the function of washers when using bolts?

Beautiful answer given by Mr. Irfan Ali. Tank you for participation. 

The purpose of installing washers in a typical bolting system is to distribute the loads under bolt heads and nuts by providing a larger area under stress. Otherwise, the bearing stress of bolts may exceed the bearing strength of the connecting materials and this leads to loss of preload of bolts and creeping of materials. Alternatively, flanged fasteners instead of using washers could be adopted to achieve the same purpose.

a function of the washer is not to damage the part that you are connecting and easily slide over it to connect

As mentioned by Irfan, there are different types of washers with different functions. Torque washers are to give an idea about how much torques has been applied. Washers also prevent relative movement between assembly and fastener head.

• To distribute the pressure of the nut or bolt evenly over the part being secured, reducing the chance of damage thereto, and
• To provide a smooth surface for the nut or bolt to bear on, making it less likely to loosen as a result of an uneven fastening surface.
• There is the locking washer to prevent it of getting lose due to so many reasons such as mechanical vibration.

Washers in bolt in used to equalized/distribute the holding/tightening force  locked item by increasing its surface area.

this quation has mor ansewer . and defrent status( bolt need sharpness , wash with kerosen , etc )

In engineering when there is a bolt screewing the washers support the bolts and the object being screwed to become very tight and firm.

Washers help to have a firm grip of bolt with the nut and also avoid surface damages when tightening the nut. Washers also avoid nut to loose during movements or vibration. EG.palin washers & spring washers.

1. 'plain washers', which spread a load, and prevent damage to the surface being fixed, or provide some sort of insulation such as electrical;
2. 'spring washers', which have axial flexibility and are used to prevent fastening loosening due to vibrations; and
3. 'locking washers' which prevent fastening loosening by preventing unscrewing rotation of the fastening device; locking washers are usually also spring washers.

REDUCING THE OVER TIGHTENING AND BREAKING OF MATERIAL

A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers usually have an outer diameter (OD) about twice the width of their inner diameter (ID).