I will expand on DKNguyen answer, because to my knowledge also the two reasons are:
reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation)
change the joint tightening characteristics (see joint diagram).
reduce contact stresses on surfaces.
The basic idea is that since contact stress is defined as:
$$\sigma = \...
It is for spreading out the stress.
But it is also for giving the bolt a bearing surface to turn on. The washer always goes on the side (nut or bolt) that is being turned. It prevents it from marring up the work surface and also changes the tightening characteristics. I don't know the specifics of that though but that's what I was told by a toolmaker. Always ...
Except for special applications, most washers are made of dead soft steel, which deforms under the compressive load imposed by a tightened bolt head. As the washer smooshes, it minimizes stress concentrations caused by bumps under the bolt head and surface flaws in the part the bolt is running through.
To visualize part of Nmech's answer: in the image, the washer actually greatly increases the contact area of the bolt head.
The bolt head looks pretty big:
But most of that is the shaft, which obviously does not spread out load on the material. So the actual contact area looks like this:
Comparatively, the bolt head on the washer looks like this:
That's a ...
Another important part of the answer is the symmetry of the stress pattern. The stress caused by a bolt head varies greatly between the points of the bolt head and the straight sides. As a result local stresses, which are what you really care about because those are what the materials have to withstand, can be much higher than the average stress. A washer's ...
The image shows what is often known as a zerk fitting, aka a grease fitting. The ball seals the passage from the outside to the internals which require grease.
A grease gun with a properly sized connector will snap onto the zerk fitting. The gun is used to pump grease under pressure to the area described in the image as "no space, grinds," likely ...
You have to show two reference lines for tolerance measurements. One is the longitudinal axis of the linked parts. The other one is the line through the center of the pin as shown below.
Does this answer your question?
You have to remember that you can control independently (or with a fixed gear ratio), the rotational velocity for each roll.
So, in order to decrease the tension, what you do is you decrease the rolling speed progressively in the output zone.
A very similar concept which is under the same restrictions, is the belt pulley system. Below it shows the tension ...