Seat Track - Boundary Conditions 1

[irq]

I am designing some components, which are connected to the seat track introducing high amount of moment into it. What constraint conditions do you usually apply to simulate seat track connections? Are the translational directions enough or should I also include rotations (it is not too conservative?)

Thanks for answers.

 

[johnhors]

You should be using a free body diagram with each component to determine what forces to apply to them. From that you can then decide how to support the model.

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[rb1957]

is the seat track fitting capable of transferring moment ? (or is it just part of your model ?) maybe you have something like a double stud track fitting ??

if the moment is real then you have to design for it ... maybe you need to reinforce the seat track to help it distribute the load the the fuselage frames supporting the seat track. if the loads are so different to what the OEM would have considered, maybe you should chase the loads into the fuselage shell ?

 

[irq]

You are right - I have somethink like double stud track fitting (a bolted connection between sandwich panel and seat track, see attached picture). The aim is not to design seat-track, but the bolt size. I'm trying to find if the moment is real or not to decide if it is necessary to design a bolt for high moment or simply treat this issue as simply supported beam (deformation of the seat-track legs).

 

[rb1957]

if the moment would create a couple between the two studs, then it probably is real. if the moment is "merely" bending the bolts, then a simply supported connection is an option; but either ...
1) you completely ignore the moment (and release it in your model), so what you're modelling is the situation after the bolt has yielded [a simple approach, but unconservative; but if you've got a large MS ...], or
2) apply both moment and axial loads to the bolt, assume the moment is equal to the limit moment strength of the bolt and this'll reduce the axial load you can apply; i'd combine as Rl + Rm = 1 (or Rl^2+Rm^2 = 1) Rl = applied axial load/ult allowable, Rm = applied moment (= limit)/ult allowable moment. to model this i'd model both full fixity and simply supported and combine the results (if yield moment is 70% of the moment generated in the fully fixed model, then the combined internal loads are 70%*fixity + 30%*SS)

clear as mud ?

 

[irq]

Thanks for this interesting approach. Finally I estimated studs flexibility of the seat-track and used this data to reduce the moment. The difference was significant and moment about 40-50% lower than in fully fixed model, but still there! Better to avoid the simply supported idealisation ...

 

[rb1957]

so you modelled a finite stiffness connection ?

 

[irq]

I modeled stud track "T" fitting by two cbeam elements and applied certain stifness on both ends of the lower beam via 2 spring elements (up/down)

 

[rb1957]

ok, i thought we were talking about bolt bending