## Inverted strut loads

## Inverted strut loads

(OP)

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?

## RE: Inverted strut loads

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

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## RE: Inverted strut loads

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.

## RE: Inverted strut loads

Norm

## RE: Inverted strut loads

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.

## RE: Inverted strut loads

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Inverted strut loads

## RE: Inverted strut loads

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Inverted strut loads

Sorry

Dan T

## RE: Inverted strut loads

## RE: Inverted strut loads

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

## RE: Inverted strut loads

Greg Locock

## RE: Inverted strut loads

Greg Locock

## RE: Inverted strut loads

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

## RE: Inverted strut loads

Greg Locock