free body diagrams guidelines

8/10/2019 Free Body Diagrams Guidelines

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Guidelines for Drawing Free Body Diagrams (FBDs)

Importance of Proper FBDs

Free Body Diagrams (FDBs) are an important tool in engineering analysis and a cornerstone ofsystematic implementation static and dynamic analysis for many machine design calculations. I believe that about 80% of errorsin machine design calculations are due to incorrect FBDs,which lead to inaccurate equations and analysis.

Wat is a FBD!

FBD is a drawing of an ob!ect or system of ob!ects that shows all vector forces and momentsapplied onto that ob!ect. Below is an FBD of a system consisting of a bicycle with rider."ote that all forces shown are the e#ternal forces acting on the system, and the forcevectors are drawn in the direction in which they are applied onto the system. By drawingforces properly the equations of motion and$or equilibrium directly correspond to what isshown in the FBD.

Forces that are internal to the system being analy%ed are "&' shown. For e#ample in the FBD

of the"yclist on Bicycle, pedal forces are not shown since they are internal forces.owever in the FBD of te "yclistthe pedal forces are shown, since they are e#ternalforces applied onto the yclist. "ote, that the *edal forces are an equal and opposite forcein the FBD of te Bicycle, since contact forces always have an equal and oppositereaction force.

FBD of "yclist on Bicycle FBD of "yclist FBD of Bicycle

Fg" yclist gravity force P *edal ForceFgB Bicycle gravity force #$ +eat Force -

& 'raction force #' +eat Force

& "ormal force / * andlebar Force -

+ "ormal force 0 ' andlebar Force 1

#$ $

#' '

FG" P& & Fg, +

& Fg" FgB +

P

#' '#$ $

P

&

P


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Bicycle e#ample inspired by +tatics2 nalysis and Design of +ystems in 3quilibrium by+heppard and 'ongue

FBD Guidelines

Draw a separate figure for each FBD.

'itle your FBD with the name of the part or system of parts being analy%ed, since a single

analysis problem can have multiple FBDs. +ee the e#ample FBDs of yclist on Bicycle,yclist, and Bicycle.

Draw only e#ternal forces being applied onto the ob!ect being analy%ed. 'he direction of the

force vectors should always be in the direction it is applied onto the ob!ect being analy%ed.'his will ensure that that you avoid sign errors in your equations.

t every point where the ob!ect being analy%ed touches an e#ternal part, there is the

potential for an applied force at that point. onsider all such contact points, and draw all

possible forces at those points. 'he only e#ception is where you can show that no force isbeing transferred, such as with a frictionless surface which will have no force tangential tothe surface. In the BicycleFBD above, there is a hori%ontal force at the rear wheel due todriving traction between the ground and wheel, but there is no hori%ontal force at the frontwheel since it is assumed the front wheel rotates without friction.

Draw the force vectors at the locations where the force is being applied, or for a distributed

load a force vector can be drawn at the center of pressure.

Include a coordinate system on each FBD. 'his will ensure the equations match with the

figure.

For dynamic analysis, always show the enter1of14ass of the ob!ect.

large ma!ority of machine design analysis occurs in 0D. 'hus one needs to select theappropriate plane to draw your FBD. 4ost 5D analysis can be done with a combination of0D planar analysis.

Do not draw 6imaginary7 forces such as inertial or equivalent forces. 8hile some te#ts do

so, they often lead to incorrect equations. In the FBD of the Bicycle above the weight of thecyclist is not shown. In this e#ample, we should not draw an 6imaginary7 cyclist weight, butrather reali%e that the weight of the cyclist is reflected in the contact forces at the seat,handlebars, and pedals.

9eoccurring cases (for e#amples see +ome "otes on Free1Body Diagrams by allett)

*inned and bolted !oints can have forces in both the # and y directions.

ables and belts apply only tension forces aligned with the cable or belt.

-isual "ec. of 'our FBD

:isually add the forces in the # and y direction, and estimate cloc;wise and counter1cloc;wisetorques. If your ob!ect is in equilibrium there should be balanced forces and torques. If yourob!ect is undergoing acceleration, there should be forces$torques in that direction.
http://www.genie.uottawa.ca/profs/hallett/gng1100/gngfbdnotes.pdfhttp://www.genie.uottawa.ca/profs/hallett/gng1100/gngfbdnotes.pdf


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omments and +uggestions to ". Delson, ndelson

free body diagrams guidelines

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