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

 

[ESPcomposites]

Are all the red circles pin joints? If so, the rod is a 2 force member regardless of what else is going on. Otherwise, you would have rigid body motion.

If however, there is a moment (i.e. they really are not pinned joints at the column ends) then would treat it as a so called "beam-column".

Brian

http://www.espcomposites.com

 

[kingnero]

Can we see the actual device?
Or a schematic that clearly shows the nature of the pin joints?

 

[rb1957]

a rod can only carry axial load. this bears repeating ... a rod can Only carry axial load.

the angles would experience a load along the line of action of the rod.

then draw a fbd of the angles to deermine the pivot reactions.

 

[desertfox]

Hi DiscipleofScience

The black arrows are the only force you need to consider acting in a plane of the connecting rod and placing it under compression.

desertfox

 

[DiscipleofScience]

yes the red circles are pins

The force placed on the rod has two components, one will be along the plane as desertfox states and the other will will place a torque on the rod I presume?

The cranks are just normal 90 cranks with a centre stud.

regards

John

 

[rb1957]

"The force placed on the rod has two components ..."

no, the force on the rod has one axial component, only.

the rod axial force reacts the torque applied. the pivot point reacts the two forces applied to the cracnk, the rod force and the main drive force.

 

[rb1957]

i see the problem you're having ... your green arrows are wrong ... the rod does not react the crank torque with a load at 90deg to the crank flange. the rod reacts axial load, so only a component of this (normal to the crank pivot, along the green arrow) is effective in reacting the applied torque.

similalry at the other crank, the load created in the output is the reaction to the torque applied to the second crank from the rod.

clear as mud ?

 

[desertfox]

Hi DiscipleofScience

Have a look at this example:-

http://wwwtw.vub.ac.be/werk/Mechanicasite/Statica/equilib/iv-iv.htm

the resultant forces act along the axis of the two force member, just like the black arrow forces in your diagram.

desertfox

 

[DiscipleofScience]

I am struggling yes, its been a long time since I did this last and it was an Electrical Degree too!

I think I understand in that what you are saying is that pin joints do not transmit moments. But what I am thinking about is how the off centre forces are dealt with

In attached diagram I have exaggerated the problem, the cranks are set up for mechanical advantage.

F2 imparts a force, but what happens to its components? Is there an F4? or does it all travel as F7? Then at the other end what happens to F7's components or is it all transmitted through F5?

Have I drawn forces F4 and F3 correctly - what is this doing to the rod, can it be resolved to a moment acting on the bar itself - tending to want to bend it?

http://files.engineering.com/getfil...-9f32-49aa-a059-e0a9b4643074&file=cranks2.jpg

regards

John

 

[DiscipleofScience]

thanks desert fox I'll shall read it through, I hadn't read your post whilst I was working on my picture.

 

[desertfox]

Hi

Yes look at the example of the two force member they resolve component forces horizontal and vertical and then resolve the resultant force along the axis of the strut.
Yes pin joints don't carry moments.

desertfox

 

[msquared48]

Technically, a rod can carry a moment, but is an inefficient shape to do so. It is usually designed as a tension only member, but, in this case, can see both tension and compression, depending on the direction of the forces applied. Whether or not it sees any moment or torque here depends on the pin connections and their ability to move with a minimum of friction. The more friction, the more torque, or moment at the connection which will be transferred to the rod.

Personally, unless the OP knows otherwise, I would ignore any torque here and just deal with the axial forces the rod sees.

Cheers.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rb1957]

no, as described ("pin ended") it can't. that description implies a single fastener attachment, possible with a spherical bearing. sure, you can have your static friction moment if you want, but i wouldn't.

 

[rb1957]

back to your 2nd pic ... F1*L1 and F6*L6 look clear enough !

the middle bit, loading hte rod looks messed !? There should be a single force in the rod (F2) offset L2 on one crank and L3 on the other ... F2*L2 does not have to equal F2*L3. At the end of the day, the sum of the input an output forces = the sum of the pivot reactions.

for a position of the cranks, you have a direction for the connecting rod, a line of action, and an offset from the pivots.

the crank pivot reacts the two loads applied to it. the crank arms will end up carrying axial load and bending (some transverse component).

do the cranks rotate, under load (changing the geometry) ?

 

[Ron]

rb1957...I think we are getting balled up on semantics. Yes, as Mike stated, a rod can carry a moment. With pinned connections, that moment is captive to the rod and is not transferred through the connection.

If the green arrows are acting on the pinned joint, then only their axial component transfers to the rod through the pinned connection. Unless the cranks can rotate in the opposite direction to each other (producing torsion in the rod), then I would do as you both agreed...analyze for the axial force only using k=1.

 

[rb1957]

sorry ron, don't buy "moment captive to the rod".

in a beam, transverse load is reacted by a moment at the far end, yes ?

in a pinned rod, there can be no end moment (as i said, you can have your friction moment, but i don't want it !) ... if there is no moment there can be no transverse force, 'cause there is no free body balance.

this is, IMHO, a really simple mechanism.

 

[Ron]

rb...I agree there's no end moment, but there can be a bending moment in the rod. It acts as a beam. If it is bending, it has a bending moment. With a pinned connection, that moment dissipates to zero at the ends.

 

[DiscipleofScience]

rb1957, yes the cranks move, the whole assembly moves in the 2D plane only.

It is a simple mechanism, but as can be seen from the diagram is the off centre forces causing a bending moment?

http://files.engineering.com/getfil...-b2a1-4782-8395-725ab55e3dbb&file=cranks3.JPG

thanks for your help so far on this.

John

 

[msquared48]

@ OP: The off-center forces you are concerned about will not cause a bending moment in the rod. Assuming frictionless pins, any torque due to any misalignment will be reacted out at the two supports.

@rb: Just because you do not want a moment to exist in a theoretical problem, does not mean that it will not exist in the real world. Parts do wear out and grease does fail, plus countless other imperfections of design. I am not saying that it is there. I am saying it can exist.

And steel rods can take moment, as can any section of any material (just depends how much). Just try to bend one with your hands. Cheers.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask