Hi everyone
Carlos Pinto signing in again. The case of the ever loosening screws on
a 2400.
Although I know very little about electronics, and spelling
( no designer I know is a great speller). Industrial design and
plastics, manufacturing technology is something I am very involved in.
The factors involved in a 2400's assembly fasteners is quite complex.
Although I agree with allot of Sydney's assessments , there are other factors.
While I cannot be certain about the decisions of a design & engineering team,
I can comment on the factors in some decisions.
Why are there brass thermally inserted "nuts" in the 2400's plastic shell ?
Usually in home electronics you will find a fastener screwed into a plastic channel .
This works well in a product that sits on a shelf or counter.A screw very rarely
comes loose on even a toaster that experiences great temperature changes, more
than our beloved 2400 . Now the reasons for the brass "nut".
The shell of the 2400 is made of ABS not polycarbonate as some people have stated.
If you heat ABS it will deform . ABS will also shrink with age., as the plastic dries
out.
As we all know plastics are based on petroleum extracts. This means that lubricants
or any other oil based liquids will deteriorate or break down the plastic. Add to
this a
certain flexibility and you are creating a fastening problem. Why ?
Fasteners are metal. they are either molded or cast and cut . In the manufacturing
process of a fastener lubricants are used to keep the metal from sticking to molds
and
to enable cutting to be done at a high speed as well as other reasons such as
prolonging
mold life and cutting tool life etc...
So now you are going to fasten a motherboard to a plastic shell using a screw that
still has some lubricant residue (even if put through a cleaning process) . The
problem is obvious.
The plastic will deteriorate over an extended time and as you insert and remove the
screw the
channel will strip,as plastic has a much lower strength or shear factor. Add to
this, chassy
flex caused by the inherent properties of all plastic, and you have a need to change
how you
fasten your motherboard.The fact that these things (powerbooks) are opened up for
repair
and upgrade due to their high cost, defines a more serviceable fastening .Now we have
the
reason for the brass nuts. But wait, the nuts are made of brass. Isn't brass a metal,
and wouldn't there
be lubricant residue on them ? Yes , but all design is a case of compromise between
what is ideal and
what is possible . The nuts should be placed using thermal insertion. One way is
while the
shell is formed the nut is in it's final place in the mold . This is rarely done well
as the plastic flow
pressure has to be turned down and there are cooling problems etc etc....
So my guess is they are placed after molding of the case. The nut is heated, it has
a thread on
the outside so that it cuts it's way into the plastic and seats itself in the shaft
as a screw would.
The heat allows the plastic to deform around the nut., locking it in place. This is
the ideal method.
The method that was used on the 2400 is just screw the nut into place , I suspect no
heat to avoid
a reaction between the fastener and the plastic as well as quality control factors(
wasted shells)
Remember , there is lubricant still on the nut.
So now we have an Idea of how the fasteners work on a 2400. Why do they come
loose?
Another and perhaps the biggest factor is the case rigidity. If you look inside a
2400 you will find
circuit boards attached to inner strengthening frame made of plastic with the same
nuts inserted.
The fact that the outer case flexes keeps the 2400 from smashing into a million
pieces when drooped,
which occasionally happens . Maybe not to you or me , but this is a part of a design
teams decision on how
a product is made. This combination of flexing shell combined with a bunch of
different circuit boards
attached to an inner frame that in my opinion is too flexible, and a constant
vibration coming from
the hard drive, results in a lose screw waiting to happen.
Well why do the screen screws loosen near the hinges on the screen assembly? One
factor is because a kind of sliding
action happens between the two halves of the screen plastics as most of us close the
screen using the top .
This puts a stress load on the lower screws as they try to keep everything fastened .
There is slight movement
in and around the screw .As this happens the pressure on the areas that cause the
friction that keeps the screw
tight, changes . This is what causes these screws to release. The nut also unscrews
ever so slightly.
What would solve some of this ? A stiffer , heavier more fragile powerbook .
Some manufacturers use a screw glue to keep fasteners in place as plastics shrink
and products get
banged around. This causes other problems such as broken circuit boards cracked welds
as well as other
fastening complexities.
In conclusion , a powerbooks design as in any products design, is always a
compromise of different factors.
I would prefer a more rigid frame for the 2400 , but this would ultimately be a more
fragile and heavier 2400.
I suspect that it is the flexible light construction of the 2400 combined with
other circuit design factors
that have caused the 2400 to be such a problematic machine.
This said , I still love it.
My company gave me a Pismo G3 powerbook that is absolutely beautiful and I guess a
progression of
design from previous experiences. Doesn't change the fact that I love my 2400. And
prefer to carry it
around and make my G3powerbook a desktop.
There is a glue made for screws . It is made by loctite. I used to use it on my
racing bike . It is made
for more durable bolts that don't have the fragile philips screw design.
If you decide to use the loctite product make sure you use the version that can be
undone.
Keep in mind that all the movements that I talked about inside the powerbook are
minute and acrew
over time to cause the problems that both Sydney and myself have talked about.
As to the lubricant on the screw issue , this is a proven fact that has already been
documented at the
professional level. I don't care to argue with any engineers as has been the case in
the past. I know this
sounds like a strong statement . I apologize in advance for the tone.
As well I would like to add that not every 2400 will react to use the same way. As
we all move our
powerbooks around differently and with varying frequency . I stand by the fact that
the two powerbooks
that I oppened up had no loose screws.
I know that some of the statements I have made may sound strange to someone
buying a $4000 dollar
laptop, but life is a wonderful strange exploration of experiences .
Can't say it enough , Love the List!!
By the way Sydney , I agree with what you wrote , this was just my addition of what I
know from experience.
Cheers
Carlos
Sidney Ho wrote:
> This loose screw has also found loose screws in various places in 2400s.
> I've concluded that this is NOT the work of sloppy technicians, but seems so
> at first sight.
>
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