"fftt" wrote in message
...
On Oct 8, 11:09 pm, "CalifBill" wrote:
"fftt" wrote in message

...
On Oct 8, 6:35 pm, "CalifBill" wrote:



"fftt" wrote in message

...
On Oct 7, 11:14 pm, "CalifBill" wrote:

wrote in message

...
On Oct 7, 3:32 am, "Buerste" wrote:

Hexheads are cheaper and grade 2 is just fine for most stuff. But, I
DO
know not to micromanage. It's when they hijack my designs and add
foof
that
I speak up. I started doing prints in PDF.

I agree with your approach. But the difference in price for grade 2
and grade 5 is very small. So I would vote for grade 5 bolts. Using
grade 5 bolts you might be able to use one less bolt. And using grade
5 bolts might mean the bolt can be backed out five years later.
Instead of having the head shear off and all the extra pain that
involves.

Tell your engineers that one definition of an engineer is someone that
can do what any damn fool can do, but the engineer can do it for less
money.

Dan

Use the correct grade bolt. Grade 8 / 5 ain't always the best. You
need
a
bolt that will spring stretch in preload more than the movement of the
bolted pieces. Otherwise the bolt loosens.
Use the correct grade bolt. Grade 8 / 5 ain't always the best.

True...

but.......

You need a bolt that will spring stretch in preload more than the
movement of the

bolted pieces. Otherwise the bolt loosens.

The design / assembly concept to which you refer is not accomplished
by using lower grade bolts.
Perhaps a refresher on mechanics of materials or machine design is in
order.

cheers
Bob

Actually it does require a lower grade bolt sometimes. Years ago they
used
a Grade 8 on a Hopto's arm, and was always loosening. You could not get
enough preload on the bolt to stop the loosening, and have threads still
in
the arm. So a Grade 5 worked much better. You have to stretch the bolt
at
least the movement length of the part and not be in the plastic range of
the
bolt. Grade 8 is a stronger bolt, but it can go in to the plastic range
earlier in the tightening stage.

Something else going on there.....urban legend perhaps?

The "spring rate" / "axial stiff" of two geometrically identical
bolts is the same....only the yield loads are different.

The elastic moduli for alloy steels are all pretty much the same which
explains why the bolts perform the same until one yields.

Are your trying to tell me that a Grade 8 bolt will go "plastic" at a
lower load than a Grade 5 bolt?

......I don't think so, show me the data.

better yet, here's the data

http://www.boltdepot.com/fastener-in...-and-Grades/Bo...

If the bolts are short, there isn't going to be much stretching going
on.
If want to get some stretch, you have to reduce the bolt's "spring
rate" (aka "axial stiffness")

this can be done by:

going to a smaller diameter bolt,
a longer bolt,
adding some bolting compliance, like a disc spring (aka cupped washer,
Belleville washer)
using a different bolt material (copper based, Ti)

not by using a lower strength steel alloy

cheers
Bob

No, I am saying the Grade 8 will not stretch as much in the elastic range
before getting into the plastic range. The Grade 8 will hold a lot more
force for the same diameter. And if you are going in to internal theads,
threads are not normally grade 8 and will pull out before you can get
enough
preload on the bolt to stretch it the length needed.


For identical bolt / assembly geometry, a Grade 8 & a Grade 5 bolt,
when preloaded to the same load, will stretch the same amount.

Since the yield point (either measured in load or stress) of the Grade
8 bolt is higher than that of the Grade 5 bolt....the Grade 8 bolt
will actually stretch (elastically) more before yielding than the
Grade 5 bolt. Of course, at the same load (as long as the load is
below the yield of the Grade 5), both will stretch the same amount.

for example; consider a "mystical" bolt that has a cross-sectional
area of .1 sq inch.

uisng the material properties data from the "bolt chart"

the Grade 8 bolt with yield at ~ 13,000 lbs
the Grade 5 bolt with yield at ~ 9300 lbs

so if each bolt is taken to a load just their respective yield points;
the Grade 8 bolt will actually strecth 1.4x as much as the Grade 5
bolt.
The actual bolt stretch will be driven by the active length of a
bolt.....that's why longer bolts perform better where one needs to
maintain a pre-load.

wrt base material threads not being "Grade 8", just increase the
thread engagement length. Also that's what thread inserts (aka
Helicoils) are for.

Cheers
Bob


Why would you say it stretches 40% more? Because there yield points are 40%
different. Load both bolts to 9000# and how much will it stretch? Do the
same for Grade 8. How much will it stretch? A lot less.