Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Mechanical engineering general discussion FAQ

Structural Calculations

How do I construct Free Body Diagrams? by GregLocock
Posted: 22 Sep 12 (Edited 24 Sep 12)

/////DRAFT////// v0.03

Greg Locock

0) Assumption is that you are trying to solve a quasi static 2D system of forces moments and accelerations on one contiguous body, but the principle holds for 3D. For multiple bodies you will end up with multiple FBDs.

1) Identify the location and magnitude and direction of all forces and moments of interest. Usually these should only be the forces and moments acting ON the body. You could use just the forces the body is exerting on its constraints, but by convention we don't. The location of the point of application of a moment to a rigid body makes no difference.

2)Identify the accelerations of all 'large' inertias. Add D'Alembert pseudo forces (inertia * acceleration) to the centre of gravity in opposition to the accelerations. You may need to include pseudo moments due to rotational accelerations.

4)Sum all forces and pseudoforces in some obvious direction. Since the system is in equilibrium the total should be zero.

5)Sum all the forces and pseudoforces in some other direction that is not parallel to that in (4)(typically, but not necessarily, at right angles to it). Since the system is in equilibrium the total should be zero.

6)Take moments about a cunningly chosen location. Since the system is in equilibrium the total should be zero.

//At this point, if your system is well mannered, you should have a solvable solution//

If it is not fully defined you have to add additional constraints. Alternatively it may be over defined, for example, a car, sitting on four springs, is not solvable by the above, unless you know the profile of the road surface, and the lengths of the springs. That is called compatibility.

See also http://en.wikipedia.org/wiki/Free_body_diagram


Back to Mechanical engineering general discussion FAQ Index
Back to Mechanical engineering general discussion Forum

My Archive


eBook - Mastering Tolerances for Machined Parts
When making CNC machined parts, mastering tolerances can be challenging. Are general tolerances good enough? When does it make sense to call out for tighter tolerances? Do you need a better understanding of fits, datums, or GD&T? Learn about these topics and more in Xometry's new e-book. Download Now
eBook – How to Choose the Correct Corrosion Testing Method
When designing a metal component, engineers have to consider how susceptible certain alloys are to corrosion in the final product’s operating environment. In a recent study by NACE (National Association of Corrosion Engineers), it was estimated that the direct and indirect costs of corrosion in the United States is approximately 6.2% of the GDP. In 2016, that cost exceeded $1 trillion dollars for the first time. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close