Thickness of gasket influence on preload?
Thickness of gasket influence on preload?
(OP)
Does the thickness of the gasket influence the preload when temperature is changed?
I have a bolt connection where everything is steel except the gasket (it is actually a composite part). Everything is preloaded so that the bolt is under tension. When temperature is changed the gasket will expand differently than the steel due to the different thermal expansion coefficient and stiffness. Since the therm. coeff. (alpha) is length/length/temp; does it matter if the gasket is 1mm or 0.1mm? What am I missing?
I have a bolt connection where everything is steel except the gasket (it is actually a composite part). Everything is preloaded so that the bolt is under tension. When temperature is changed the gasket will expand differently than the steel due to the different thermal expansion coefficient and stiffness. Since the therm. coeff. (alpha) is length/length/temp; does it matter if the gasket is 1mm or 0.1mm? What am I missing?





RE: Thickness of gasket influence on preload?
And the second /length/ in the units refers to the gasket thickness, so a thinner gasket will affect bolt preload less. There may be other reasons for preferring a thick gasket, but bolt preload is not one of them.
One decent example of a thin gasket used to maintain bolt preload is in manual transmission gearboxes, e.g. between case and tailshaft housing, or between case and input shaft housing; traditionally made of specialized thin paper.
Mike Halloran
Pembroke Pines, FL, USA
RE: Thickness of gasket influence on preload?
I'd be frightened of preload loss in a structural joint that includes a gasket, or whose mating faces are even painted.
RE: Thickness of gasket influence on preload?
If the changes exceed the original stretch then all preload is gone.
RE: Thickness of gasket influence on preload?
RE: Thickness of gasket influence on preload?
I can't really let go of the problem. Your arguments feels logic but I just can't prove it... What I'm looking at is a bolt connection of composite panels so the thickness of the "gasket" is quite high.
If I assume for one material: F = -k*dx (a linear spring).
Let's say dx = a*dT*t, which is the distance, delta, the material compresses or extends (and where a*dT is the strain [m/m]). And finally t is the thickness [m].
I then say the stiffness k = E*A/t for a typical bar (t being the length of the bar), E the stiffness [N/m^2] and A the cross section [m^2].
Then I get F = (E*A/t)*a*dT*t = E*A*a*dT
Which step am I missing here?
RE: Thickness of gasket influence on preload?
RE: Thickness of gasket influence on preload?
If you've got a soft gasket, two composite flanges, and two thin steel flange reinforcing compressed, that's a whole nother thing.
... like valve cover gaskets, which are basically impossible.
Mike Halloran
Pembroke Pines, FL, USA
RE: Thickness of gasket influence on preload?
This site might help:-
http://
desertfox
RE: Thickness of gasket influence on preload?
The idea is that for daily fatigue loading all in-plane loads are transferred by friction, hence I need some preload.
rb1957; You are probably right, the values will be very small. I just need to document that they are :)
Thanks for all inputs
RE: Thickness of gasket influence on preload?
Mike Halloran
Pembroke Pines, FL, USA
RE: Thickness of gasket influence on preload?
RE: Thickness of gasket influence on preload?
Mike Halloran
Pembroke Pines, FL, USA
RE: Thickness of gasket influence on preload?
In broad strokes, soft materials clamped in a bolted joint are generally bad. Then tend to "ooze", getting thinner over time.
If the get thinner enough much or even all of the stretch of the bolt that was introduced by the initial preload will be lost and the joint will fail.
Long stretchy bolts are good. Short stiff bolts are bad.
Work out the bolt stretch from whatever preload you feel you need.
Figure out how much loss of thickness of the panels is possible over time. This may be a difficult thing to do.
If the bolt stretch is very much larger than the potential amount of relaxation then all is good. If not you've got a problem.