I would like to submit an application for a new mechanism to be patented. Do I have to attach the complete mechanism including screws, nuts, housing and general electric components like DC motors in the illustration drawings? Or it's just sufficient to attach drawings for the main mechanism components that i would like to grant?
To supplement another answer - You mention a "main mechanism". A critical point before drafting a patent application is to determine what you think that the invention is. If you have invented a new carburetor you do not need to show a whole car and you might discuss variations of the carburetor that make it suitable for a lawnmower.
An invention needs to be enabled in a patent application meaning you need to teach someone skilled in the art how to make and use the invention. Usually that does not include specifying where all the screws go.
I'm not an attorney, but generally you submit drawings that illustrate just the specific mechanism involved in the invention with enough context to explain how it works. Ordinary elements like motors and gears can be illustrated generically. You can get a pretty good idea of what patent drawings look for by looking at other patents. You can use Google Patents to search for some comparable device and look at some of the patents that show up.
It's important to note that the drawings and the text of the application work together to explain how the invention works. You really want to think about alternative variations in addition to the preferred implementation to make sure that you can cover enough breadth in the claims. I always suggest working with a patent attorney or agent even though it costs money. My experience is you are much more likely to end up with an effective patent that way.