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# Telescopic beam - moment fixity?6

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## Telescopic beam - moment fixity?

(OP)
Do you think a telescopic beam achieves moment fixity?

Considering a mobile crane with a telescopic boom, this would be a cantilever with a telescopic concept right?

Understand deflection may be an issue.

### RE: Telescopic beam - moment fixity?

From a strength perspective yes, as presumably if it can’t cantilever then it falls over
As you say though, deflection could be an issue. Some fancy maths involving the tolerance within the tubes combined with length of cantilever and idk this is what maths people are for

### RE: Telescopic beam - moment fixity?

(OP)
Great, thanks for your input.

### RE: Telescopic beam - moment fixity?

I’m not sure what exactly you’re doing but it may be worth considering that the fixity mechanism is different in the telescopic situation than in a regular beam. In a regular beam we just have accumulations of axial force etc. In the telescopic case, the transfer from the telescopic boom to the main boom is actually compression bearing on the inside of the tubes (or something similar depending on how exactly it is built). So local buckling or compression failure could be worth checking?

### RE: Telescopic beam - moment fixity?

(OP)
Yes, thanks I was aware of this.

No worries

### RE: Telescopic beam - moment fixity?

#### Quote (LR11 ...Do you think a telescopic beam achieves moment fixity? Considering a mobile crane with a telescopic boom, this would be a cantilever with a telescopic concept right? Understand deflection may be an issue.)

The necessary resisting moment SHALL develop for the equilibrium .. The following doc. is useful to get the structural aspect of the boom ...

### RE: Telescopic beam - moment fixity?

(OP)
HTURRAK, thanks this document looks to be useful.
Scuka, I was going top use stainless steel as sliding surfaces (acknowledge differential corrosion).

### RE: Telescopic beam - moment fixity?

is the telescoping segment bearing on the surfaces ? or are there some "point" bearings providing the contact.

another day in paradise, or is paradise one day closer ?

### RE: Telescopic beam - moment fixity?

(OP)
Thank you phamENG.
rg1957, there will be plates in bearing.
There's a gap for the extension/retraction, the resulting deflection which I'll make up somehow.

### RE: Telescopic beam - moment fixity?

Careful, some stainless steel surfaces do not slide well.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

### RE: Telescopic beam - moment fixity?

(OP)
OK thanks for advising.

### RE: Telescopic beam - moment fixity?

LR11:
Please explain your second sentence, it doesn’t make any sense to me. That looks like an adjustable length simple beam to me. What is it being used for, how is it adjusted in length, what means or mechanism , how often is the length changed, how easy does this have to be? What are the shapes, the make-up of these two beam sections, how do they nest? Fab and rolling tolerances vs. straightness for easy operation and nice running of the bearing points (pads) are always in competition.

### RE: Telescopic beam - moment fixity?

(OP)
Hi, which second sentence?
Note: In answering your queries, the sketch in the original post is a simplification, it only shows the concept, which I wanted to know if it was an acceptable mechanism, which is seems it is. The details are in progress.

- That looks like an adjustable length simple beam to me ... IT IS A SIMPLE BEAM.
- What is it being used for ... SUPPORT A MOVING LOAD, ON CASTOR WHEELS, THERE WILL BE RAILS ABOVE TELESCOPIC ITEMS, AND OFFSET RAILS ON THE TELESCOPIC MEMBERS.
- how is it adjusted in length, what means or mechanism , how often is the length changed, how easy does this have to be? ... MANUALLY, OFFSITE, NOT VERY OFTEN, DOESN'T HAVE TO BE EASY. THE MAIN REASON FOR THE QUERY WAS TO SEE IF IT WAS A "DONE THING" OR AN ACCEPTABLE STRUCTURAL MECHANISM, WHICH IT SEEMS TO BE.
- What are the shapes, the make-up of these two beam sections, how do they nest? ... RECTANGULAR HOLLOW SECTIONS WITH PLATES OUTSIDE INNER MEMBER, AND INSIDE OUTER MEMBER. NOTE, OUTER MEMBER IS SHOWN AS SIMPLIFIED IN THE ORIGINAL POST, I DO HAVE SPACE TO WELD PLATES ON THE INSIDE.
- Fab and rolling tolerances vs. straightness for easy operation and nice running of the bearing points (pads) are always in competition ... THANKS FOR POINTING OUT, I WILL TAKE INTO ACCOUNT THIS AND THE DEFLECTION DUE TO GAPS WHEN FITTING, TO ACHIEVE A LEVEL RUNWAY.

### RE: Telescopic beam - moment fixity?

#### Quote (Careful, some stainless steel surfaces do not slide well.)

If I didn't mention it, EdStainless would have...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

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