help analyzing clamp block overturning forces

huangyhg

help analyzing clamp block overturning forces
i am looking for some guidance/suggestions on how to solve the following problem.
i have a clamp block which is used to hold hydraulic tubing and electrical wires at various locations along their run.  a typical clamp block installation is shown in the attached figure.
the clamp block assembly is subjected to high g loading in each of the three critical axes.  the problem is that based on my calcs, the bolts fail in bending.  this assumes that the bolts are the only loadpath and that the attachment into the nutplates is robust enough to carry the moment (which i really don't think is the case).
another approach i'm exploring with a co-worker is to assume the bolts are tightened down onto the spacers and thus some of the load is transfered through the spacer on the compression side only.  however, i am not sure how to treat this condition to analyze the tensile force in the bolt.
suggestions/comments are accepted.
thanks,
jetmaker
looking at the tensile force in the bolt: if all parts (channel, washers, spacers, clamping blocks & support bracket) are free to slide along the length of the bolt, the nut at the bottom is the only element that can resist tensile force in the bolt. to maintain static equilibrium tensile force in the bolt must be uniform along the entire length of the bolt no matter what the clamping force on the tubing/wires.
slideruleera,
do not disagree with you in that only the bolt is able to carry the tensile load.  
now if the force is normal to the face of the block (ie along the axis of the tubing/wiring), the bolt is put in bending due to an overturning moment.  however, if the bolt is installed such that the head bears down on the spacer (putting the spacer in compression and bolt in tension), the spacer helps the bolt react some of the overturning moment.
how do i account for this effect in the analysis?
thanks.
jetmaker - i don't know how to precisely analyze this situation, but i can tell you what i would have done (with a slide rule) in my "old days" in aircraft r&d - bound the problem.
you have already looked at what happens if the spacer did not "contribute" anything - that is the worst case and the bolt fails.
take a look at the hypothetical best case - assume the spacer is an integral part of the bolt, and see what happens. if the calcs show the bolt still fails, you may just have to use a larger (or stronger) bolt. if the bolt "passes" the best case, then you have to start using engineering judgment to look at the details of each component, such as crossectional area of the bolt versus the spacer, clearance the bolt and the inside wall of the spacer, etc. if the results are really close, you may have to do some load tests on actual units.
hi jetmaker
yes its a pretty difficult one to analyse, i have uploaded a file with the way i would look at it, its not perfect by any means but gives some idea of the loads due to moments.
i like you,let the bolt take all the load and gave no credit
to any other part taking some of the load is this how you calculated it?
once you have the max bolt load due to moment you can determine a preload to counter this and subsquently calculate the compressive stress in the spacer.
finally the force as shown in the file would have a shearing action across each bolt which in this case would be 10n in addition to the tensile force due to the moment.
regards
desertfox
here's another way to look at it.  consider the spacer as the load carrying element and the bolt as a prestressing element.  analyze the spacer for the bending and shear.  this will give you a tension stress on one side of the spacer.  provide enough prestress to overcome the tension, so all parts of the spacer are in compression.  be sure your maximum compression stress is within limits.  use the prestress to carry the shear in friction.
thanks all,
the why i have choosen to approach this is the method outlined by desertfox and miecz.  i'll update you on the results soon.  desertfox, i have yet to look at your spreadsheet, but will do so this week.  thank you for your time and effort in producing this work.
jetmaker