friction...

ChrisKaswer · ‎Nov 10, 2009

Paul,

If you turn-on the "infinite friction", you can also provide a friction
coefficient that will be used to calculate IF the bodies would have
slipped. This would at least tell you if you had enough pre-load to
prevent shear sliding. If it turns out they can't slide, then the
assumption of "infinite friction" does not "hurt" or take away from the
analysis. In addition, it would be more accurate to model the bolts
with preload by using an enforced displacement. You would need to cut
the bolts in half and apply an enforced displacement to each half toward
one another (PL/AE). This is because bolt preload is really a bolt
"pre-stretch". If a constant load is applied, it will never change
through the analysis when the application load is being applied. The
bolt's load does change when application loads are applied. This is how
other codes implement their bolt load feature. This will have an effect
on your calculation for shear sliding if the clamping load changes
enough to allow the parts to slip.

Regards,

Chris
Christopher Kaswer, M.Sc.
Principal Engineer
Jacobs Vehicle Systems
22 East Dudley Town Rd
Bloomfield, CT 06002-1002

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jholst-2 · ‎Nov 10, 2009

I agree with Chris about the friction.

The way I do bolt preload is to use the material orthotropic CTE and apply a
global temperature change to get the bolt to shrink more than the parts. I
have a description of this in the 'tips&tricks' section of my web site
www.TSDengineering.com . Sorry that the web site is a little dated. I plan
to update it this winter when I have more time.

---Jim Holst

ptcuser11 · ‎Nov 10, 2009

Chris,

How does this method of sectioning the bolt handleany shearing loads if present? In mymind I see the bolt as two seperate bodies with theprescribed displacement as you mentionedat the sectioning surface. While I can see how the preload works I am having a hard time seeing how the two seperate bodies could carry any shear.

I do think this is a novel idea in Mechanicathough. I wishMechanicatook the approach that Ansys workbench does regarding the application of preload on 3Dboltgeometry.Workbenches internal routines perform the sectioning automatically, thus the only step for the useris to select the cylindrical boltsurface and enter the preload magnitude.

Steve

In Reply to Christopher Kaswer:

Paul,

If you turn-on the "infinite friction", you can also provide a friction
coefficient that will be used to calculate IF the bodies would have
slipped. This would at least tell you if you had enough pre-load to
prevent shear sliding. If it turns out they can't slide, then the
assumption of "infinite friction" does not "hurt" or take away from the
analysis. In addition, it would be more accurate to model the bolts
with preload by using an enforced displacement. You would need to cut
the bolts in half and apply an enforced displacement to each half toward
one another (PL/AE). This is because bolt preload is really a bolt
"pre-stretch". If a constant load is applied, it will never change
through the analysis when the application load is being applied. The
bolt's load does change when application loads are applied. This is how
other codes implement their bolt load feature. This will have an effect
on your calculation for shear sliding if the clamping load changes
enough to allow the parts to slip.

Regards,

Chris
Christopher Kaswer, M.Sc.
Principal Engineer
Jacobs Vehicle Systems
22 East Dudley Town Rd
Bloomfield, CT 06002-1002

ChrisKaswer · ‎Nov 11, 2009

Steve,

My suggestion was to mimic the approach ANSYS takes. They split the
bolt body and apply the proper enforced displacement to achieve the
given preload. This condition can then be held during all other load
applications so it will act just like a real bolted joint. You can do
the same thing in Mechanica by creating these boundary conditions
yourself. It's possible this approach could help if the working load
applied causes a significant enough change in joint load to allow
slippage between the bolted bodies.

Regards,

Chris

PAULKORENKIEWIC · ‎Nov 11, 2009

Interesting point... Earlier, I had thoughts of maybe modeling the
shoulder bolt as a two separate pieces. One would be a simple hollow
pin, and the other would be a Mechanica fastener feature with the screw
head diameter adjusted to the larger shoulder bolt head diameter. The
fastener feature does a nice job of handling the contact and pretension.
I thought there might be some issues with the "only two parts" rule of
the fastener function, or maybe it wouldn't like having the pin sort of
wrapped inside it. By that I mean that the hollow pin would sit under
the head of the fastener...

Paul Korenkiewicz
FEV , Inc.
4554 Glenmeade
Auburn Hills, MI, 48326-1766

ptcuser11 · ‎Nov 13, 2009

I just read a post on another engineering forum website where a person gave another novel method for applying preload to 3D bolts. Their method was to adjust the amount of initial interference between the bolt head and mounting surface to acheive the desired preload force. He indicated that it would take some trial and error to get the preload forces right.

This is a pretty good idea. One unfortunate drawback I could see would be that it would automatically make your problem a non-linear contact problem, which could slow things down. Another would be the trial and error part to get the interference such that the preload in the shank of the bolt was at the desired value. I guess you could use an optimization study, but that may take a long time too.

Onepositive thing about this guy's method would be that the bolt would definately carry shearing (or transverse) loads.

In Reply to Paul Korenkiewicz:

Interesting point... Earlier, I had thoughts of maybe modeling the
shoulder bolt as a two separate pieces. One would be a simple hollow
pin, and the other would be a Mechanica fastener feature with the screw
head diameter adjusted to the larger shoulder bolt head diameter. The
fastener feature does a nice job of handling the contact and pretension.
I thought there might be some issues with the "only two parts" rule of
the fastener function, or maybe it wouldn't like having the pin sort of
wrapped inside it. By that I mean that the hollow pin would sit under
the head of the fastener...

Paul Korenkiewicz
FEV , Inc.
4554 Glenmeade
Auburn Hills, MI, 48326-1766