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Socket bloc on my arm is disfigured


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#1 Ramon Engle

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Posted 24 September 2004 - 09:49 AM

The tension knobs on my BMV are gouging the surface of the socket bloc on my arm. I spoke with George Paddock and he attributes this to the arm not being originally designed to be mounted in this fashion. Apparently it's an inordinate amount of stress on the tension knobs.
The area where the knob(threaded part) makes contact with the socket bloc has become knurled and gouged. The pic is a bit small but the gouging is apparent on the left side. Yeah, you'll have to zoom in on your own to see it. Anyway
the arm mounts perfectly and no play has developed as a result. But it still concerns me. I'm thinking of using small nuts on the end of the threaded section to increase the surface area. Mcmaster Carr sells a great selection of half size nuts. Is anyone else having this problem?


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#2 George Grammatikos

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Posted 24 September 2004 - 02:06 PM

hi Ramon
my sugestion is to change the socket block ,don't risk it ,it look like that you will change it soon or later so do it now to avoid any problem in the future ,and of cource change your steering screws in the female socket block

Fly safe ! ! !
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#3 Ramon Engle

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Posted 24 September 2004 - 10:37 PM

George, I considered changing the socket bloc. I asked George about it and descrided in detail what was going on. After much deliberation hes said I don't have any worries. I truely believe it's OK. If I thought there was an ounce of real risk I would take action.

Thanks.
R
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#4 Lawrence Karman

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Posted 25 September 2004 - 10:17 AM

I'm not an engineer, but I just don't understand why there is additional stress on the arm block because it is pointing straight forward with a back mounted vest instead of pointing left or right with the traditional vest. As I recall with my old PRO vest the ends of those screws became deformed or flattened to the point where I could not extract them from the vest socket block. Perhaps the screws on the back mounted vest are a higher grade than the ones used by PRO and the steel is harder than the Titanium and digging in. Perhaps Brant or Jerry or Tom could comment on this.
I did get a call from another operator a few days ago who said the Titanium block on his PRO arm split in half. That's three I personally know about and he said there was a fourth. Make you nervous?
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#5 Mitch Gross

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Posted 25 September 2004 - 12:51 PM

I'm not an engineer, but I just don't understand why there is additional stress on the arm block because it is pointing straight forward with a back mounted vest instead of pointing left or right with the traditional vest.

As I understand it (and I'm no metalurgist or engineer either), it is a question of static pressure v. torque force. The arm and sled put a lot of energy into the socket block. In the traditional mount the static pressure of the weight load is horizontal and the torque force perpendicular to the mount, but with the back mount design these are reversed. I think the only true way to test what this means is to take a rig into a metal stress lab and have them abuse it in all possible configurations until they blow it apart and determine what loads are really appropriate. Any volunteers? :lol:
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#6 RobVanGelder

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Posted 25 September 2004 - 07:45 PM

I did get a call from another operator a few days ago who said the Titanium block on his PRO arm split in half.

That´s the hidden danger in the use of this metal: it´s extremely tough, but when it goes, it goes without warning.
It doesn´t crack or bend like more "elastic" metals, it just goes from new to ruins immediately.

Maybe titanium is not the right choise for this kind of mounts......
but it´s probably perfect for tubing like the (extendable) post.
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#7 Eric Fletcher S.O.C.

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Posted 26 September 2004 - 02:31 AM

That´s the hidden danger in the use of this metal: it´s extremely tough, but when it goes, it goes without warning.
It doesn´t crack or bend like more "elastic" metals, it just goes from new to ruins immediately.

Ummmm, NO, that would be incorrect. Ti is a spring metal, that's why the springs in the arm are made from it.
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#8 Brad Grimmett

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Posted 26 September 2004 - 06:45 AM

Ummmm, NO, that would be incorrect. Ti is a spring metal, that's why the springs in the arm are made from it.

I've always believed the same to be true about titanium snapping but not bending. Are the springs possibly made of some kind of blend of titanium and other metals?
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#9 RobVanGelder

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Posted 26 September 2004 - 07:55 AM

Ummmm, NO, that would be incorrect. Ti is a spring metal, that's why the springs in the arm are made from it.

Eric, I disagree here, almost every material is "flexible" also concrete and glass. It´s the shape of the application that will determine if it is "crackproof".

With Titanium, I am not so sure. For instance, if you overstetch a iron spring (I know, the Pro system is a compressing spring so I am not referring to that) it will just stay in that overstretched position, it will not return to the original shape. I am not completely sure if this also happens with Titanium.

Another concern is this: relatively soft metals will just dent when a point-pressure is applied, but titanium and also cast iron (without carbon in its atomic structure) will withstand until a certain point and than just give in, in an explosive manner.

This is probably what happens to the socket block.
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#10 Eric Fletcher S.O.C.

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Posted 26 September 2004 - 01:10 PM

With Titanium, I am not so sure. For instance, if you overstetch a iron spring (I know, the Pro system is a compressing spring so I am not referring to that) it will just stay in that overstretched position, it will not return to the original shape. I am not completely sure if this also happens with Titanium.

Disagree all you want but it doesn't change the fact that Ti is considered a spring metal. Higher in strength then steel but not as stiff. You can always Google "6al Materials properties"

If Ti was not able to handle elastic deformation (ie Be a spring metal) then you could never use it as a suspension "Flexure" in Formula 1 cars (Great example on Pg 139 of "Ferrari Formula 1" by Peter Wright)

It's also on of the reason's that Ti has been a preeminent structural metal in aerospace.

BTW springs are not made from Iron but from various grades of either steel or Ti. Ti springs have the advantage in that Ti is not only lighter but the spring constant is higher allowing you to wind a smaller lighter spring then one wound from steel. Steel springs also fail catastrophically when overloaded.
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#11 DavidWest

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Posted 26 September 2004 - 02:20 PM

just curious,,,, how many people here have heard of the steel socket blocks "blowing up"???


Ti is great for springs, bicycles and race cars,,,, but looking at Young's modulus might indicate that if you are "shoving" high psi bolt heads into a curved plate that steel might be a better choice....
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#12 Stephen Murphy

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Posted 26 September 2004 - 06:24 PM

just curious,,,,  how many people here have heard of the steel socket blocks "blowing up"???

Hasnt hapeened to my arm but it did happen to the brand new socket block on my garfield last year. sheared cleanly in half. it was in storage in a peli case when it happened and had only been used a half dozen times.
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#13 Ramon Engle

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Posted 26 September 2004 - 06:27 PM

Quite a thread. I'm constantly amazed at how much peripheral knowledge most steadiphiles have locked away in thier noggins.
The majority of the deformation is due to the threaded protion of the tension knob being milled and the threads are removed. This is to prevent disfiguring the end of the threads to the point that you cant remove them. I've had break out the dremel tool to remove a few bent tension knobs. I was using a thread stock that was too soft. Now I'm using one that's too hard! Who am I....Goldy Locks?


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#14 RobVanGelder

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Posted 26 September 2004 - 08:45 PM

Eric, it was not my intention to disagree with the statement of Ti being a spring metal, I think that is a well proven point.

But as others also mentioned and I made clear in my post, it is resistant to pointed pressure until a certain limit after which it suddenly gives in, where as other metals like stainless steel will show deformation first.

It is this explosive behaviour that determines if Ti is good or not in a certain application.

In a similar case:
As I do a lot of designing of stuff myself I once got some beautiful aluminium plate, 3mm thick, the type they use for aircrafts.

It is much tougher, but where you can bend normal Al-plate around, this stuff will only go to exactly 90 degrees and than just break!
So I could not make a construction by bending as you always have to "over-bend" (not the right word but you get the meaning I hope) to have a certain angle as a result.
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#15 Eric Fletcher S.O.C.

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Posted 26 September 2004 - 09:38 PM

But as others also mentioned and I made clear in my post, it is resistant to pointed pressure until a certain limit after which it suddenly gives in, where as other metals like stainless steel will show deformation first.



Ti shows deformation first, if it did not how could it be elastic enough to be a spring metal....

It is this explosive behaviour that determines if Ti is good or not in a certain application.



Now your confusing Ti with Carbon. Carbon fails catastrophically where as Ti, Al, or various grades of steel will all deform reach their elastic limits and then fail when overloaded

As I do a lot of designing of stuff myself I once got some beautiful aluminium plate, 3mm thick, the type they use for aircrafts.

It is much tougher, but where you can bend normal Al-plate around, this stuff will only go to exactly 90 degrees and than just break!
So I could not make a construction by bending as you always have to "over-bend" (not the right word but you get  the meaning I hope) to have a certain angle as a result.



First off you comparing very disimilar materials. Secondly without knowing the material, it's hardness and it's working limits your going to have problems working it. (And BTW a little heat goes a LONG way) From the sounds of your description you were trying to fold it around too tight of a radius.

Just as an aside I designed and built Race car suspension (On a pro level) for several years, I've got a decent foundation and understanding of materials properties.
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