rod that is pin jointed at both ends 1

[DiscipleofScience]

Hi

I am using a rod in compression and am fairly comfortable with using Eulers theorem.

However how could I resolve the situation where a compressive force is at an angle - see picture

I believe that this would actually force a torque on the rodding and perhaps should be more of a concern than Euler.

Would I resolve the force into the x and y components relative to the centre line of the rod and the force that is not resolved to the centreline is basically the torque that is acting on the rodding itself rather than transmitting force.

http://files.engineering.com/getfil...4-a853-4515-89fa-c2fa4d7cb705&file=cranks.jpg

I am trying to understand the forces in this rodding as presently it is bending.

regards

John

 

[DiscipleofScience]

zekeman - The mechanism spends majority of its time in static equilibrium. Your last comment is bang on ist Euler buckling due to single compression, probably very worn cranks and friction - I just wanted to understand the magnitude forces in the rod. I understand Euler formula is for ideal conditions too (perfect surface, no in-built stresses, perfect axial load etc), so it is reccomended to use a factor of safety.

rb1957 - I understand what you mean about negative and positive forces, and yes FBx will be negative to how I have drawn it, since FCx >> FAx I can virtually eliminate FAx.

I have now at least arrived at a Free Body Diagram which provides the compression force in the rod, thanks to all who pointed me in the right direction and having patience!! Especially rb1957, desertfox, Terio and zekeman. The rear crank will follow the same pattern.

Once I understand more I can search for relevant information and found some great resources:

How to add moments in a 4 part series:

http://youtu.be/MUMcn-VJsOc,

use the simpler method!!

How to solve FBDs:

http://youtu.be/vNwOcEUU7Jg

I have now answered my original question of what is the force in the rod, which is really determined by the load, the actuating force just increases to meet the demand. It is my opinion that the off-axis load tends to accelerate the situation once the critical Euler load has been exceeded.

thanks all

http://files.engineering.com/getfil...b-94b3-4449-a512-8d5c72c15ec7&file=img005.jpg

 

[desertfox]

Hi DiscipleofScience

Sorry I thought the thread had ended which is why I didn't post anymore, anyway if your happy with your anwsers now goodluck and your welcome.

desertfox

 

[rb1957]

well I Really dislike neglecting Fax ... why ?

it's really simple to solve the problem correctly ...
determine Fc from sum of moments about B.
determine the pivot reaction from Fa and Fc.

neglecting terms as early as you have will get you into trouble. And these no reason to.

sorry, but IHMO your final sentence shows you haven't learnt anything. there is No off-axis load in the rod.

and you penultimate sentence implies that maybe the applied loading is a displacement (rather than a defined load) and a seized crank2 could explain the behaviour of the rod.

desertfox,
three times "your" should be "you're"

 

[rb1957]

ok, 2 out of 3 !

 

[DiscipleofScience]

ok rb1957, I have effectively done what you have said - I have summed the moments about B to arrive at a value for FC

Removing FAx is to help me investigate the suitability of the crank pin.

Again, I agree, you can't have an off axis load through a pin - I do learn rb1957! Have faith ;-) I do think though that when the rod (carrying FC) is actually bending its a different matter.

 

[rb1957]

"Removing FAx is to help me investigate the suitability of the crank pin" ... don't understand ? the best i can think of is since Fax and Fcx act in opposite directions then it's conservative to say Fbx = Fcx ('cause it's really = Fcx-Fax) ... but that's not what you wrote.

still wonder if this column buckling ... if the rod was buckling then the rod would lose axial stiffness and crank1 would rotate freely.

but maybe this is a displacement load ? does a move x" controlled by the linkage attached to it ??

 

[DiscipleofScience]

no x is static, it can be moved for setting up, but after that it remains 'as is'.

I know that angle thetaA is probably 5 to 10 degrees at the most so FAy is relatively small, similarly with thetaB.

 

[rb1957]

still don't think euler buckling explains the rod bending.

are the fasteners at the ends of the rod clamped up tight ?

really dumb question ... is the rod too long ?

 

[desertfox]

Hi rb1957,
Thanks for correcting my mistakes - didn't know this was an English class, thought we were in an Engineering forum (grin). By the way "and you penultimate sentence" should be "and your...."?
Regards,
desertfox

 

[rb1957]

touche (imagine the accent)