by James Wonder
Hello again, for my second installment I will talk about
bolts once again. I would really like to know if you find anything else
interesting so I can make a post on that as well. Things like wheel alignment,
etc might be interesting so let me know. If you want to write one feel free,
the forum is open!
Bolt stretch:
Yes, bolts stretch! In fact, high-end bolts are made specifically to stretch.
In some racing and aircraft applications there is a "stretch gauge"
and it measures the amount of bolt stretch in thousands of an inch or tenths of
millimeters. In these applications bolt stretch is used in place of torque for
a tightening protocol. A very good writeup and more
particulars are at this
web page if you are really interested.
The main point for our use though is that bolts do stretch,
and that if you over-tighten them they will stretch to a point where they are
now very weak and/or brittle. So, making your bolts too tight will cause them
to get loose, get ruined or possibly even fail. The cheaper grade, grade 5 or
8.8, will stretch more easily and fail quicker due to over-tightening. So, that
is a big worry because our karts are full of those bolts!
Thread warp:
When a bolt stretches, it usually does it at the weakest point, which in this
case is the thread area. Have you ever had a nut that was hard to thread into a
bolt, even when it was a new nut? The threads on that bolt are stretched, and
the bolt is garbage. We call this "thread warp" and it is not good.
Most bolts fail in the thread or at the point where the thread
meets the shank.
Re-tightening:
Every time you yank on the nut and bolt to make sure it is tight you simply
increase the torque on the bolt, hence making the problem worse. Secondly you
disturb the wear pattern that the bolt/washer combination has made on the
secured part. Both of these are not good.
To properly check bolts that are not marked (we will get into that later) you
should put a wrench on it and apply a little pressure to one side. If the nut nor the bolt move then it is still tight. We are
only using one wrench for this test and we are trying not to rotate the bolt
unless it is loose. Not only is it good for the bolt, it is quicker!
Nylon locking nuts:
These are your standard locking nuts, DO NOT USE STAINLESS! Yes, stainless
looks nice, but it is not strong! Please make sure that you are not buying
stainless hardware in your local hardware store, most of them sell stainless.
Put a magnet on it if you don't know, stainless is not magnetic.
This next statement will not be popular, but use these nuts ONLY ONCE! Do not
re-use a nylon locking nut because it is very hard to tell when it will not do
its job. Get them at McMaster and save a bunch of money so you wont mind throwing them out. Click
here for all the nuts, 100 M8x1.25 are about $8, cheap enough at 8 cents
per nut.
Grip length:
This is a very interesting topic and I will not cover in much length, but in
racecar and aircraft applications we suite the "grip length" to the
application always. The grip length is the length of the non-threaded part of
the bolt. The best way to fasten a part is to find a bolt with the proper grip
length. If your part is on the threads it will eventually both ruin your part
and let the bolt get loose. Military, NAS and other speciality
bolts come in hundreds of grip lengths. Standard metric bolts do not, so we are
limited in our choices here.
I would simply say that you should always try to use a bolt that has the
correct grip length whenever possible, especially in the serious areas like the
bearing carriers.
How tight should they be?
This is very subjective, and it really matters what
you are trying to clamp and/or fasten and what material, etc etc etc. I would say the best
rule of thumb for standard bolts is tighten it until
it will not turn and then a little bit more. Yes, it's not that tight! It's a
great deal looser than what you are used to, but that is about all you need.
Most bolts operate fine with 10-20 foot pounds of torque which is much less
than what most people do with their wrenches. If you have a torque wrench, try
20 pounds and see how it feels.
If the bolt is in single shear then you should tighten to 30 pounds of torque,
maybe even 40 in hard working applications. Click
here to see an image of what single and double shear is. The top image has
the bolt in single shear and the bottom is in double.
So, single shear applications which is what we see in
our karts are not very good, and need tighter bolts. Double shear which is
better only require that you hold the nut onto the bolt without rotating. The
bolt is doing the work, not the nut. But is most
applications we ALL over-tighten our bolts! Don't do it!
Safety wire:
I will keep this really short. Safety wire does NOT keep a bolt tight! It only
keeps a loose bolt from falling out. In single shear applications it is
useless. In most double shear applications it is almost equally useless. Do not
use it in place of regular maintenance, and only use it if you know what you
are trying to accomplish.
Inspection seal:
There is special paint that is called "inspection seal", "safety
seal" or sometimes "torque seal" that allows you to put a dab of
this heavy paint on the nut or bolt head and check for it moving. It needs to
be put in the crotch of the head or bolt head and will turn to dust if the seal
is broken.
This allows for visual inspection of the bolt, hence not forcing you to pull on
the bolt with a wrench, and it is much quicker to ensure that the bolts are
tight. This is really the way to go and I use it on everything I do!
One company that I like to use is the Organic Products Company that
sells the torque seal by small tubes at the previous link. No, I don't get a
cut!
Any questions please ask, and if you want to see something that I can do please
ask, I might need help on some subjects though!
