I am mounting a trailer hitch on 1-ton truck. Trailer weight will be up to
>15,000 lbs.. My first thought is to use Grade 8 bolts. (Tension and shear)
>But I notice most applications use Grade 5. Is this a matter of cost? I
>will only be using 8 or 10 bolts some the price difference does not matter.
>I know #8 is supposed to be better than #5, but is that true in all
>situations? Something in the back of my brain tells me #8 might be more
>brittle than #5. Also worried about vibration and shock from potholes.
Would rather overbuild it. Any
thoughts?
Also, are there grade 8 nuts to go with the bolts? Thanks, Chief
# 8 bolt is stronger but corrodes more readily in the atrmosphere than grade 5.
Most nuts are less strong MATERIAL than the bolts, but the length of grip for the fastener diameter makes it stronger than the bolt, even with that weaker material - by having a weaker nut material, the nut thread area deforms to fit the threads of the bolt, rather than the other way around .. Using the same grade of nut and bolt material together can break threads on the bolt.
as an engineer, I am hard pressed as to where one would use a grade 8 nut in a design.
Not true....name one reason why this would be true or why this is supposed.
Most nuts are less strong MATERIAL than the bolts, but the length of grip
Internal threads are stronger than external threads. Since they are mating components wouldn't the length of grip be the same for both components in the joint? Do not confuse the strength of material with the strength of the part.
Internal threads are stronger than external threads. Permanent deformation of threads in a bolted joint is a failure mode. Remember, the objective is to develop "stretch" in the bolt. If the threads in the nut give way to the threads in the bolt, the joint is poorly designed.
Grade 8 nuts are designed to be used with grade 8 bolts. See SAE J429 and SAE J995. This is not a coincidence. However, over-torqueing can break the threads on a bolt or strip the threads of the nut if of lesser grade or quality.
It is desirable, if a failure is going to happen, that the bolt fails before the nut. By under-grading the nut, it is more likely that the opposite occur.
Good Luck.
PS: In the original application, I would use a grade 8 nut and bolt.
The hardening process causes formation of smaller grains, with lots of internal stresses. If the grade 8 is made of unplated carbon steel, carbon, that which makes it "tough", makes it prone to corrosion.
You may be thinking of alternate materials, or plating processes to protect the basic bolt?
Galvanic corrosion between disimilar material, ie the link you provied, is not directly relevant to this topic. Grade 5 bolts are also made of unplated carbon steel. Heres how rust occurs: When a drop of water hits an iron object, two things begin to happen almost immediately. First, the water, a good electrolyte, combines with carbon dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen. The free oxygen and dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the piece of iron itself.
1) After 35 years of engineering, writing codes and regulations, and inspecting fasteners in the field, my personal experience, research, and tests says it is correct -
the higher carbon content in grade 8 steel used to help give it strength. (if you are not familiar with metallurgy and the corrosion mechanism of steel at usual structural ambient temperature - it uses the carbon atom. Grade 8 has more carbon atoms to use in making oxide, pound for pound, than grade 2. - and the grade 8 and 5s definitely absolutley rust faster than grade 2 in tests and fiueld expereince - I have never seen it NOT happen on mixed grade bolts on the same component/vehicle )
2) >Most nuts are less strong MATERIAL than the bolts, but the length of grip
Using the tensile area of the bolt at the threads, determine the bolt's tensile strength.
then compare that strength to the strength of the shear area of the engaged area of the threaded nut. The nut shear area is stronger than the bolt tensile area at the threads
The grip of the threaded section of the bolt compared toi the nut has nothing to do with it.
3)
no, incorrect - the area of the bolt at the thread on the bolt limits its strength, not the location of the thread. If you limited the nut to the same tensile area, it would be equally as strong as the bolt.
4) the capacity of a 10-24 thread at 2/3 the way from the tip of the thread itself is the same whether the thread shape is cut internaly or externally - the internal thread has exactly the same amount on metal as the external thread at the same distance from the axis
5) Permanent deformation
If the nut does not deform, it does not engage the thread on the bolt - except in a math book. The tolerance on thread manufacturing is such that it not perfect enough to mate threads if one or both do NOT deform. One must be softer to allow full engagement of the other. Since bolts are limited in tensile strength by the tensile area under the thread and nuts are not, the nut is made of softer material.
see shigley. Why this does not happen is well explained.
I don't think this is the reason, since cast iron is made of small grains with and has so many internal stresses we don;t use fatigue factors, and it is fairly corrosion resistant
this is indeed the reason, in part because carbon in the matrix is a source of carbon, used in forming iron oxide, for water to dissolve; and in part because it is a galvanic bridge. (There is a paper regarding tests of lubricants on bolts and corrosion they create, done in the 70s, which pretty much defintively shows the galvanic bridge accelerator effect of carbon, in and on the metal )
When I first read Rudy's post (below) something sounded not quite right. It is usually easy to tell the phonies by their mock-superior attitude, but Rudy's tone was quite polite and rational.
I suppose I should have drawn the appropriate conclusion from this line: "">Not true....name one reason why this would be true or why this is supposed.""
This is a flat denial followed by a demand for justification of the opposing position.
I should have remembered, if one denies a factual assertion, he is obliged to offer support for that position.
Anyway, I was glad that two engineers who support this newsgroup DID find time to set the record straight.
So now I try to remember, "Nobody can tell if you are a talking dog, on a newsgroup!"
Cold working decreases grain size, which increases *total* grain surface. There is more surface available to start corrosion, even if this "surface" is distributed inside the material.
The grain structure of cast iron is *huge*, and carbon has plenty of time to diffuse into the iron matrix while molten.
Cold working *exposes* carbon, as sites where dislocations are formed.
Hob, while I appreciae your experience and knowledge, I must press on with my case...
Carbon steels vary in the percentage of carbon they contain. The amount of carbon affects the properties of the steel and its suitability for specific uses. Steels rarely contain more than 1% carbon. Structural steel contains about 0.1-0.2% carbon by weight; this makes it slightly more ductile and less apt to break during earthquakes. Steel used for tools is about 0.5-1 % carbon, making it harder and more wear resistant. (see
formatting link
Further, I believe that
1010 steel has .10% carbon content; 1028 has .28% carbon content; 1035 has .35% carbon content. I would like to make two points:
1) Fasteners are graded by their strength not neccesarily by the steel they are made from. The original question compared grade 8 vs. grade 5. Per SAE J429, the minimum carbon content of a grade 5 fastener is .28% and the maximum carbon content of a grade 5 fastener is .55%. A grade 8 externally threaded fastener has the exact same min/max values. Grade 5 bolts need not be heat treated (mfr's option) and grade 8 bolts must be heat treated. The heat treatment process should not induce carbon into the treated part. Therefore, I conclude that a grade 8 bolt does not neccesarily have a higher carbon content than a grade 5. Your inclusion of a grade 2 fastener into this argument is interesting but according to the specifications, grade 2 externally threaded fasteners have a max carbon content of .55% but they have no minimum requirement. In practice, you won't commonly find a grade 2 with .28% minimum carbon content but to do so would not be out of spec.
2) Further, I do not believe the carbon content affects the rate of rust (see carbon fiber and or carbon dating). However, ceding that point to you as well as the point made in #1 above, in a practical sense, how muh difference would .13% make and how often would an unplated fastener be used in a trailer. As a selection criteria between using a Gr5 or Gr8, the supposed negative effects of the added carbon in a Gr8 fastener seems to miss the point. Far more consideration should be given to the strength of fastener relative to its intended application than its predisposition to rust at some unquantifiable rate faster than a similar sized lower grade fastener.
PS: For rust to occur, three things must be present: A) a cathode, B) an anode and C) an electrolyte. Carbon is not one of these conditions. Iron acting with oxygen is the cause of iron oxide which is rust.
Next..
Do you agree that internal threads are stronger than external threads? Forgetting the mathmatical interpretaions of your argument, the practical matter is as follows:
1) Internal threads are stronger than external threads
2) Manufacturers of fasteners are aware of this.
3) the geometrical advantage of an internal thread is considered when specs are written and fasteners are produced.
4) A grade 8 nut is manufactured to mate with a grade 8 bolt
5) A grade 8 nut might be made from 1022 steel while a grade 8 bolt might be made from 1035 steel.
6) The goal is to produce a product that meets cerrtain strength requirements.
7) using a grade 5 nut with a grade 8 bolt is "under-grading" the nut.
8) Interestingly a cold formed grade 2 nut will meet all strength requirements of a grade 5 nut up to about 1 inch or M24. A part machined from bar is a different story.
9) To use a grade 5 nut with a grade 8 bolt because of your understanding of the mechanical dynamics inherent in a bolted joint and to presume that those who wrote the specifications and manufacture the parts do not fully understand those same mechanical dynamics seems to me to be asking for a field failure.
Would you use a grade 2 nut with a grade 8 bolt? I can tell you that one of the largest USA fastener manufacturers made both grade 2 and grade 5 finished hex nuts exactly the same way. The only difference was the inclusion of a grade marking on the grade 5. From raw material through forming, tapping and packaging, the production process was identical but for a mark indicating the parts to be grade 5. This was an entirely appropriate thing to do and met all specifications.
Finally, my point is to use appropriately graded fasteners whenever possible.
The heat treatment process involves quenching to produce very small grain size. The grains form with carbon at interstitial sites. Carbon is both its strength, and its weakness (corrosion).
"It isn't just how much you've got, its where you've got it located."
Please don't top post. This newsgroup is archived, and people following up have difficulty understanding your response, if what you are responding to is elsewhere...
I certainly apolgize for offending your delicate sensibilities for asking for justification of a position that I do not intuitively understand. Would I have not given myself away as a talking dog if I somehow begged or pleaded for that justification. Or would it be more appropriate to guess why a grade 8 bolt would corrode faster than a grade 5 bolt. As you can see from further posts, upon learning the logic behind the comment, I did offer further support for my position.
I am not an engineer nor am I unknowledgeable. A question was posed about what strength fastener should be used to pull a 15,000 pound trailer behind a truck. I can assume this could be at speeds of up to 70 to 80+ mph. An answer which well may be right was offered that a grade 5 is better than a grade 8 for this purpose. And the reason given is that a medium carbon alloy steel bolt should not be used because its relative carbon content is greater than that of a medium carbon steel bolt and will corrode faster in an unplated condition.
If this type of logic and discussion is what your after in this forum, again I apologize for offending your delicate sensibilities. The trailer may pass you on the highway at 75 mph but damnit the bolt won't be rusty. When the trailer hits a bridge abuttment, we can be proud of the shiny bolt. Of course, it won't be shiny because it, for some reason, won't be plated. But it damn sure won't be rusty.
My intention was never to have a mock-superior attitude and I focused on substance rather than on style. Your dismissal of my argument and the subsequent criteria used to conclude a position on the matter, speaks volumes about either your intellectual capacity or to a condition of knee-jerk reactionary commentary based solely on some inferred belief of motivation on my behalf. Somehow, my question when confronted with the sophisticated rules you impose of first guessing the intended meaning of a comment, second, rebutting that comment, and third offering justification for said rebuttal, reveals me a talking dog? (PS: you might not have noticed but this very paragraph is written acccording to your rules as I have inferred them. I am guessing what you meant, I have rebutted your claim of my "mock superior attitude", and third I have offered a longwinded justification for said rebuttal. I conlude with my question, "You believe me to be a talking dog?" It seems inefficient to me and of course calls into question your intellectual capacity, but rules are rules)
What I am missing is the effect of carbon on the formation of rust. I agree that there is more carbon in a higher grade part as a rule. I can also understand that heat treating a part relocates the carbon "in very small grain sizes....at interstitial sites". But how does the presence or location of carbon increase the rate of the iron/oxygen reaction to form iron oxide.
Does the tighter grain structure of the carbon expose the anodic iron more readily? My understanding is that the electropotential difference between anode and cathode determine the rate of corrosion. As electrons move from the anode to the cathode, which can occur within the same piece of metal, the anode begins to disintegrate. In a heat treated bolt, how is carbon acting to influence this process?
Secondarily, should this understanding affect fastener selection in a trailer hitch or are we answering the question of what time it is with plans to build a watch?
It depends on the quenching fluid. As you pointed out, grade 8 material can *start* with "standard" carbon levels. It is where the carbon ends up being located...
Galvanic corrosion (because there is always also water). Electronegativity of carbon is: some value available on the internet. Electronegativity of iron is: some lower value.
Iron loses, when carbon is exposed. Such as at the grain boundaries of hardened carbon steel.
And when you've exposed "all" the carbon, as opposed to "less than half" the carbon...
And when those little "batteries" are lined up millions per inch, rather than thousands per inch (as just numbers)...
And when the current paths (and hence the resistance to current flow) are reduced by orders of magnitude...
When arguing that hardened materials (unplated, unprotected) should not be used because they are prone to increased corrosion, no. When arguing that the nut should/should be a lower grade for very good reasons, no.
When arguing that they are NOT prone to corrosion, because we don't have a clue why it WOULD corrode, and challenge everyone to PROVE why it would corrode, we are neither building a hitch nor a watch. We are posting to hear our own "voice".
I'm not thrashing, I'm encouraging a change in "tone".
If you want to *know*, ask. There are some really bright people here, and many that like to share. But the knowledge is easier to take if it is not filled with bile, if you catch my drift.
In my opinion, there was no bile or anything inappropriate in my initial comments. Until the talking dog comment, nothing said was meant in any way other than an inqusitive dialog. I could have added IMO more frequently. I did not demand anyone participate nor that they justify their positions. To ask why something would be so, or supposedly so is a simple question not an attack.
In my opinion, there was no bile or anything inappropriate in my initial comments. Until the talking dog comment, nothing said was meant in any way other than an inqusitive dialog. I could have added IMO more frequently. I did not demand anyone participate nor that they justify their positions. To ask why something would be so, or supposedly so is a simple question not an attack.
AN bolts used on aircraft are weaker than grade 8 bolts. But they can be folded double at a fairly small radius without fracture. A grade 8 bolt has increased strength at the cost of decreased ductility so that it tends to fracture if bent double.
But the cost of a long grade 2, 5 and 8 bolt is so low that you might find it interesting to try the experiment yourself. A tube over the shank of a bolt held in a meaty vise would provide the leverage to bend a slender bolt, perhaps 1/4 or 5/16 OD X 5 or 6 inches long . Bolts that you would prefer to hold on even when abused might be better selected as grade 5. If you can guarantee that any load on a bolt is in tension, then a grade 8 might be better.
Because higher strength bolts tend to be susceptible to corrosion weakening, you would of course choose a zinc passivated, cad plate, or like surface coat as a first line of protection. But then most bolts these days are plated in this way.
A paint cover would be the next line of insurance.
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