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Old 23rd January 2015, 04:20 PM   #1
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Hollow bolts

Looking for an education here. What is the purpose of these, and when would one use them. I saw some 1/2" hollow bolts the other day, and upon close inspection could see they were simply tubes with a hex head welded in place, and rolled threads. Obviously not something that would withstand a normal 1/2" bolt amount of torque or shear strength. I was extremely skeptical that a welded head would not pull loose with any decent amount of torque applied to the fastener.

So the obvious question to myself was, why would anyone us such a large weak bolt, when a smaller diameter solid bolt (probably same weight and probably stronger) could be used instead?
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Last edited by blueovalz; 23rd January 2015 at 04:54 PM.
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Old 23rd January 2015, 04:56 PM   #2
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Re: Hollow bolts

Terry,
Was there a hole in the hex head? Could a smaller diameter bolt be installed through this hollow bolt? Sometimes in aircraft applications where a single point attachment was required, a bolt in bolt configuration was used to prevent single element failure of the joint.
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Old 23rd January 2015, 06:51 PM   #3
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Re: Hollow bolts

A hollow shaft can be stronger than a solid one. Normally when used in shear as apposed to tension.
Motorcycle axles are an example.
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Old 23rd January 2015, 10:38 PM   #4
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Re: Hollow bolts

I need to think about that. Shearing failure is a function of area, and there is no way a hollow or tube bolt has more area than a solid bolt for the same diameter. I'm not familiar with motorcycle parts, but I can't image these hollow bolts being the same diameter as would be applied with a solid bolt.

Yes, the hole runs through the entire bolt on the ones I looked at. If the bolt was actually experiencing shear, I can see where an inner rod or bolt would help to in essence make it solid, but shouldn't the normal friction force of a clamped assembly be more than the shear force of the bolt itself? With a welded-on head, I don't believe this could possibly happen.
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Old 24th January 2015, 04:19 AM   #5
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Re: Hollow bolts

Hello

In the 68/69/70's Porsche racing cars bolts where holow in order to minimise final load on their prototype cars .
On some "factory" cars bolts where hollow "and" titanium material !

Diameter of suspension bolts are linked to diameter of spherical bearings diameter and the thread being installed into the whihsbone welded bushing

A sample ; if you have an 1/2 "spherical bearing and you do not use a reduced diameter ball you need an 1/2" bolt !!! plus 1/2 washer and nut !! these are far too big ( even for a 1500kg car ) specially if you are using hight grade steel bolts. in this case there is absolutly no problem to use reasonably hollowed , Why ?
Because for most of these suspension bolts ( beside the transmision bolts !!) the working load is not at all in torsion but in "perpandicular cut load"
( apologise I don't remind the right english word!!)
For such effort the center " neutral fiber" is not for use because it does not enter in the calculation
So it's better to have a big diameter with a hole than a smaller diameter ; this one being possibly bended much more quickly

Again this is not the case for transmisson bolts ; because they are submited mainly on torsion failure ( this is why Factory Porsches used titanium hollowed due to the very higth tensile and torsion ratio this material carry)

Another sample ; in some aeronautics application the bolts are hollow, not only, but nuts are in aluminium !! They are in place just to tighten the bolt and the arrangement and they do not suffer any lateral load

Torsion or "trench" calculations are different , in the formula's calculation , material thickness is used differently ; this is why ( after considering the situation of that bolt) you can take off the center off these .

But there is an important advise to remember ; those hollow bolts are done very industrially , means the hole is done very clean and not "badly drilled"
so there is no on the inside surface any circular mark that can be the start of crack failure !!!!!!!!!!!!!!!

So using already aero and race hollow bolts YES, but drill your own standard bolts HUMMMMMMM !!! I will not be confident offff !!!!!!!!!!!!!!

Hope this helps ( beside my so bad English !!)

And if you want some more "formula's" information just click on there ( but beware of any head sick LOLLLLLLLLLLLLL!!!!!!!)
https://www.google.fr/search?q=shear...C&ved=0CC4QsAQ

Last edited by Mic; 24th January 2015 at 04:29 AM.
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Old 24th January 2015, 10:36 AM   #6
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Re: Hollow bolts

Terry,
To state some obvious things. Bolts are used for tension loads and shear loads. Bolts have "allowable" loads for each. Bolts designed for tension are different than those designed for shear. Tension bolts have heads with a generous radius between the head and shaft and must be installed with the appropriate washer that provides clearance for this radius. Preload (installation torque) is important for a tension bolt. Bolts in shear are limited by shear strength of the bolt, alloy, temper, size and bearing strength of the material being bolted, alloy,temper and thickness of bolted material. "Clamp-up" friction is not normally used in evaluating the strength of a bolted shear joint. If a joint is loaded to failure, bearing failure is usually preferred because it is "benign", that is the joint continues to carry load. Fastener shear failure is complete, the joint does not carry load. Hollow bolts might be used to try to balance shear and bearing capability but I am not aware of any such application. Rolled threads are generally used on tension bolts because they improve the tension load transfer to the nut. I have trouble combining this with a welded on head.
Don't know if this is useful.
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Old 24th January 2015, 11:55 AM   #7
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Re: Hollow bolts

I was touring my local Ace Hardware store that has millions of bolts. I found some hex head bolts that were hollow and am using one as a vent for my 930 trans. Had to weld a cover over the bottom hole to keep it from slinging the gear lube out.Put it on the inspection cover(inverted trans) on the top of the trans. This looks like a standard hex head and the same metal as the regular bolts of similar make.

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Old 26th January 2015, 11:03 AM   #8
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Re: Hollow bolts

I found some more info that makes me feel better about using these "tubular" bolts for specific applications. The welded hex head is actually threaded onto the DOM tube (threads are rolled onto both ends of the tube), and then the nut is welded in place on the end.
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Old 26th January 2015, 04:54 PM   #9
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Re: Hollow bolts

Explosive bolts are hollow!
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Old 26th January 2015, 05:09 PM   #10
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Re: Hollow bolts

Quote:
Originally Posted by Scott Calabro View Post
Explosive bolts are hollow!
So are Banjo Bolts... And I have yet to see one break or strip...
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Old 26th January 2015, 05:24 PM   #11
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Re: Hollow bolts

An example of what I found was a 1/2" bolt, 42-45 lb/ft torque limit, and with the .095" wall thickness, made it comparable to a 3/8" solid bolt in terms of total square inches of steel (for shear strength reference).
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Old 26th January 2015, 08:41 PM   #12
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Re: Hollow bolts

Now you really have me wondering...

Where did you find these bolts and the torque specs? Any pics?
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Old 26th January 2015, 11:53 PM   #13
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Re: Hollow bolts

Racebolt.com (they call them tubular bolts)

They had a lot of information about them, except the shear values, which I'm deducing by using total square area of the bolt if the wall was .095", which is what the tech told me they were today. A 1/2" bolt is about .196"^2, but hollow, with a .095" wall, becomes about .120"^2, which is very close to a solid 3/8" bolt. Now whether a tube of the same square area shears differently that a solid of the same area; I don't know that.
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Old 27th January 2015, 12:23 AM   #14
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Re: Hollow bolts

Really there's no benefit to be had from a hollow bolt other than weight reduction. Shear values tend to be less with a hollow bolt for obvious reasons.

There are applications where weight makes a real difference under certain situations...rod bolts, etc. Generally a racing application where rebuild and inspection times are short and frequent. A hollow bolt that has been properly machine drilled is going to be better than one with a welded on head.

Hollow drive axles are not better simply because they are hollow. Rather, they can be better because for roughly the equivalent weight of a solid axle they are larger in diameter and thus better able to handle large torque loads.
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Old 2nd February 2015, 02:37 PM   #15
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Re: Hollow bolts

I am dealing with a hollow bolt right now. It is part of the parking/emergency brake system on my Cobra replica. Where the hand lever receives the cable from the E-brake disc there is a hex-headed hollow bolt through which the cable courses on its way to the attachment location on the parking brake lever mechanism. It mounts through a home-made bracket and has the capabliity of being adjusted to take up slack in the cable, much like one would find where a clutch cable attaches to the throw-out bearing fork, just in smaller scale.

My problem is that the inner cable on this new industrial strength cable I had made won't fit through the hole that goes through the bolt...I tried to drill a new "index" shoulder with a high-quality 1/2" drill bit and the bolt appears to be Grade 8, it just smoked the bit tip and I only got about 1/8" depth before it started sparking and quit cutting. I am going to attempt to enlarge the hole in the bolt, but since it's grade 8 I wonder whether or not that will be successful.

We may be able to adapt a -4size A/N Bulkhead fitting....but it sure would be easier if I could just get a larger bolt with a larger hole for the cable to pass from the head to the top of the bolt. I see someone posted a link for tubular bolts, I'll check that out.

[EDIT---after looking at the Race-bolts offerings, it appears that their product is exactly what I need. I called and spoke to Karen and she informed me that I can order directly from them and that it's OK if all I need is just one...no minimum order. The only measurement she needs is the size of the cable inside the housing so that we can make sure we get the tubular bolt with the right size passage. Thank goodness this thread appeared when it did, I have been struggling with this issue for about a month and had almost given up hope for getting the e-brake operational again without having to go through great expense. All I'll need to do for this option is drill a larger hole in the existing bracket ]

Cheers!

Doug
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Last edited by YerDugliness; 2nd February 2015 at 02:56 PM. Reason: The link had exactly the part I need for my application
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Old 2nd February 2015, 06:06 PM   #16
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Re: Hollow bolts

Doug,
That sounds like the cables for the brakes on a bicycle. Might have been able to source one through them. But you got it worked out.

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Old 2nd February 2015, 07:28 PM   #17
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Re: Hollow bolts

Thanks, Bill. I did consider a bicycle cable. In fact, most of my bicycles use a sort of a "hollow bolt" with a slot along the length...used to adjust the tension on the cables for the derailleurs and brakes, but I think bike cables would be too small for what I needed. What I think might have worked would have been something from a custom cable maker intended for a motorcycle, but even in a town as large as Houston, TX there didn't seem to be any custom cable manufacturers for cars or motorcycles, at least not that I could find. I thought about Lokar and some of those manufacturers, too...a cable for shifting a transmission might well have worked. In the end, the product I found from the link posted by Terry will let me use my high dollar brake cable.

Hard to believe that a 45" long eBrake cable would cost $66 but this thing would stop a Freightliner...so it'll last much longer than the Cobra replica it'll be parking.

Cheers!

Doug
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Old 3rd February 2015, 04:48 PM   #18
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Re: Hollow bolts

Just finished ordering from Karen at Race-Bolts. They are sort of expensive (about 4.75 each for the size I needed), but their products are either Grade 8 or 4340 steel...I told her the story about how the 1/2" drill bit just melted and she was not surprised. She accepted my order without having me prepay and said they would just include an invoice with the parts (I bought two just in case I lost one...wouldn't be the first time I lost the only one I bought and had to go get a part the second time!!).

Cheers!

Doug
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Old 3rd February 2015, 06:31 PM   #19
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Re: Hollow bolts

They really emphasize the strength gained in the DOM tubing used to fabricate these bolts versus drilling a normal bolt, with wall surface cuts and imperfections that would weaken an otherwise smooth interior tube (or tubular bolt).
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