Bending Moment
Bending Moment
(OP)
Hi everyone,
Wasn't really sure where to post this. I have a problem I'm having trouble wrapping my head around. I have a piece of glass that is clamped around the entire outer edge. The frame is then secured down to a surface. The glass is circular with a flat (OD of 7.95", 281 deg 18'30" arc with a flat 5.04" long). It is flat, 0.197" thick. The frame has a circular OD, with a bolt pattern OD of 8.375". There are several variables I am looking at. Two different tightening patterns (going around the pattern vs. across), different torques (the correct torque of 9 in*lbs vs an over-torque of 15 in*lbs), and a raised fastener pressing up on the bottom of the frame. The fastener is 1/16" raised and at -22.5 deg on the bolt pattern. Basically, to start with, I want to look at modeling the bending stress created by these different conditions. Can anyone give me a good starting point? Thanks for any help.
Wasn't really sure where to post this. I have a problem I'm having trouble wrapping my head around. I have a piece of glass that is clamped around the entire outer edge. The frame is then secured down to a surface. The glass is circular with a flat (OD of 7.95", 281 deg 18'30" arc with a flat 5.04" long). It is flat, 0.197" thick. The frame has a circular OD, with a bolt pattern OD of 8.375". There are several variables I am looking at. Two different tightening patterns (going around the pattern vs. across), different torques (the correct torque of 9 in*lbs vs an over-torque of 15 in*lbs), and a raised fastener pressing up on the bottom of the frame. The fastener is 1/16" raised and at -22.5 deg on the bolt pattern. Basically, to start with, I want to look at modeling the bending stress created by these different conditions. Can anyone give me a good starting point? Thanks for any help.






RE: Bending Moment
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Bending Moment
Post a sketch please it speaks a 1000 words
RE: Bending Moment
Is this thing working in a constant temperature environment, or do you need to work out differential thermal expansion? The first indication you are doing anything wrong will be when the glass shatters.
When I clamp pieces of glass, I always include an elastic element to provide controlled force. Elastic elements like gaskets and O-rings also provide a seal, which you may need.
Read up on screw torques. I would not rely on screw torques to provide a precise compression force.
--
JHG
RE: Bending Moment
RE: Bending Moment
OD = 7.95", with a flat 5.04" long (dia?) ... that`s a pretty big flat.
"281 deg 18'30" arc" ... ?
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
RE: Bending Moment
RE: Bending Moment
RE: Bending Moment
alternatively model the plate and the ground (with the protrusion) and have the plate resting on the ground, then apply bolt loads progressively.
if you have to live with this protrusion (obviously), i'd suggest either ...
1) add a spacer so that the glass is supported on a flat surface, or
2) add shims at each fastener, to gradually remove the protrusion height.
tell us more about what this glass is doing ... architectural detail ?, window in a bathysphere ?, window into a pressurised compartment ??
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
RE: Bending Moment
how about grinding the glass ?
how about reseating the protruding screw ?
how about deleting the protuding screw ?
sure you can model it, but living with this is a Bad way to resolve the problem. if somebody's pooched it, fix it.
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
RE: Bending Moment
it sounds like someone has gone and installed this light filter over the raised head and didn't stop to think. and now the customer has said "that looks like a dog's breakfast, show me it's good". personally, i think you're skrewed ... there's no way to show it good ... modelling the installed stresses will take a month (maybe a year) of sundays ...
1) how will you model the stiffness of the supports (not rigid, but maybe rigid is a conservative assumption) ? as i posted above, you can model the ground plane (with the protrusion), model the flat glass, then load the individual fasteners, forgot to mention you'll need to run non-linear. I've got no idea about writing a model from a MatLab script ... i`d use a pre- and post-processor like FeMap.
2) this'll get you the installed stresses (which are clearly ok as the filter hasn't cracked), but this is when the real questions start ...
a) what about thermal effects (cold at altitude, hot on the ground?) ?
b) fracture at altitude (cold generally reduces the fracture toughness) ?
c) thermal fatigue ?
...
personally, the customer has called your bluff, and you should fold ... "ok, we'll replace it; it was bad workmanship in the first place"
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
RE: Bending Moment
like i said, don't how how MatLab scripts would create a model.
thinking about the model, if you model the plate sitting on the ground-plane (with the protusion), it is a little easier (i think) to apply a load at each fastener to see the deflection induced, to see when the plate (at the fastener) touches the ground. playing with this will give you different results ... next question, how did they actually tighten it down ? another scenario is applying a proportion of load at all fasteners, and seeing how it deforms.
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
RE: Bending Moment
i didn`t realise that when a part, obviously installed incorrectly, fails in service wwe need an FEA to tell us what we did wrong !
but at least that simplifies the modelling ... add fastener loads like they tightened the fasteners. you should be able to show that it's "naff" pretty easily ... much easier to show something down when it already is!
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
Large bolt tensions will be fine, but you will need a hard surface between the bolt and the glass like a fiber gasket so the bolt does not detension. Is the glass laminated? If so, you need to model the interlayer, and you can expect the bolt to detension relatively quickly. Is the glass tempered or chemically strengthened? Do you have good load parameters?
RE: Bending Moment
another day in paradise, or is paradise one day closer ?
RE: Bending Moment
RE: Bending Moment
- glass to metal contact
- tensioning bolts through laminated glass. The interlayer will squish and give a local stress concentration.
- holes in non-tempered glass
- stress concentrations at re-entrant corners
- movement of the substrate
- edge damage
- thermal stress
- adhesive shrinkage
- impact (obviously!)