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Inverted strut loads

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CHagen

CHagen

Mechanical
Jul 3, 2011
29
Ive got a free body diagram set up, but Ive confused myself into balancing it out a few different ways if you can believe it. Im trying to determine the lateral loads on the internal bushings under a load by applying a torque at the lower ball joint. Basically I am trying to get a ballpark estimate by viewing it as an equivalent of a lower spherical joint at the bottom of an open tube with another tube just like it upside down and slipped inside it with another spherical bearing at the top. Where they overlap are two bushings in reality, but I'm also considering them as point loads for simplicity. Can someone help me with this fbd?
 
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Quite why in the days of universally available digital cameras people persist in describing diagrams verbally is beyond me.

Anyway, there is not (in the simple case) a torque at the upper ball joint, but a pair of forces in opposition at each ball joint.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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CHagen, Greg's right that unless the spherical joints are binding, they cannot transmit a torque. In PRINCIPLE all loads go straight through the joints. The force between two spherical joints go from joint center to joint center. A straight strut between the joints in principle sees only a force aligned with its center. Hence, concentric sliding bushings in this strut have no side forces.

This is like a MacPherson strut. In reality, if the strut carries a spring there will be a bending force which will load the bushings due to high force from the spring and any slight misalignment. There will also be some friction in the spherical joints. An initial calculation would not include these. To allow for departures from ideal behavior designers regularly use guidelines, fudge factors and rules of thumb.
 
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I suspect your strut bushing load problem is really two of the three supports for a two span beam and therefore not statically determinate.


Norm
 
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Here is the picture Greg. BTW LBJ is 0 on the length scale.
The two bushings are being simplified as point loads at their centers. I don't think its statically indeterminate, I could see that if the bushings were connected to the same ground as the LBJ and top bearing, but they are internal to the strut itself. I almost want to think that the bushing forces are equal and opposite, which is one solution I came up with, but I fear it was more self satisfying the way I came up with it.
 
 http://files.engineering.com/getfile.aspx?folder=994c1da5-818e-406a-abeb-bf27250d6618&file=Inverted_Damper.png
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Without wishing to be obtuse, that diagram is pretty poor. I'd also like to see YOUR working on this problem.

Big hint I'd expect to see separate FBD for each part, not just the assy.

As a matter of interest, if you have a rigid rod with a spherical joint at one end and you apply a torque to the other, what do you need to do to maintain equilibrium?

Second big clue 140Airpower's post includes at least one sentence of negotiable truth value, there may be more but I stopped reading at that point. There are internal horizontal forces at points 2 and 3, that is, you get side thrust on the rod guide and piston.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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Isn't there an axle with wheel and tire in there somewhere?
 
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Not necessarily, for example a shock absorber installed in a typical Short Long Arm suspension looks roughly like that, except the spherical joints are rubber bushes. I suspect part 2 of this question is going to be about the sideloads caused by the spring.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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I guess when I read "strut" in the title, and 140's post included MacPherson, I just leaped to a conclusion and add MacPherson.

Sorry

Dan T
 
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Greg and Tmoose, my assertions are based on the OP's use of ball joints and wanting to calculate side loads on a telescoping strut. In fact, an ideal ball joint is frictionless and cannot transmit torques, only linear loads in perfect alignment with their centers. The only way to calculate a side load is to know the friction in the joints, which is usually dependent on the linear load. Without knowing or specifying that, I assume there is a working rule of thumb for designing such struts. I mentioned MacPherson struts because one can calculate a side load based on the spring pressure alone plus an assumed amount of offset from concentric alignment and as far as I know, that will be the dominant force on the bushings.
 
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Seems to me if a moment large enough to evaluate for is being applied at a ball joint, that's only a simplification of the true loading. Such as from loads coming through a spindle that's assumed to be close enough to the LBJ for first cut purposes to put the moment at the LBJ in this FBD.

CHagen - the lower portion of your strut has three supports (points A, 3, and 2), making it a two-span beam which very definitely is statically indeterminate. It's why you're having trouble getting a solution that fully satisfies static equilibrium (hint: there will be lateral forces at all three points on the lower portion regardless of where you locate your moment).


Norm
 
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Overall from A to B for the reactions at those two points, I agree.

But I don't think that the internal forces between two elements of a mechanism such as this one can be determined from a single FBD of the entire thing without making assumptions about the deflected shapes of the two tubes that may or may not be appropriate.


Norm
 
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