carbon fiber used in center posts
Posted 20 February 2005 - 12:40 AM
When I look at the carbon fiber center posts made by Tiffen , XCS, and MK-V; each looks different. MK-V has that distinct fiber grain that my back mounted vest has. Tiffen?s post on the other hand looks and feels very smooth. So does the XCS, but it looks different too. I am looking to learn a little more about how the carbon fiber posts are made and what the different manufacturing approaches bring to the table.
I DO NO WANT TO GET A GEAR ARGUMENT STARTED, so I am not looking to say anything is good or bad. As a matter of fact?
There is a picture on MK-V?s web site where Howard Smith is standing in the center of his 4 stage carbon fiber post which is bridged across two chairs. It looks real strong in this demo. [http://www.mk-v.com/...copic_post.htm] The caption that goes with the picture says Howard weighs in over 250lbs too!
In the same strength demo vein, but with a different style post:
Last summer I flew out to LA for both Cinegear and to meet Greg Bubb with XCS. He showed me another cool demo of carbon fiber post strength. He put a tube of the carbon fiber he uses in his posts and placed it on a concrete floor. Then he stood up on it and started to roll back and forth over it with his shoes on. Then he picked it up and banged it across a wooden workbench. After that, when you looked at the tube closely, there were no cracks or chips visible.
What I would like to learn about is what is it that MK-V does to get that fiber weave design in their center posts, or should the question be ?do Tiffen and XCS do something to get rid of that fiber weave? (And does this ?in theory-- have any effect on the post?) And what are some of the less obvious differences between the Tiffen style and the XCS style post?
Because XCS and MK-V both use 2 inch tubes, physics say that their posts should be stiffer [as in less flex] then a smaller diameter carbon post. But if everyone made 2 inch posts, and if they were all made in the same length with just one tube of carbon fiber, how would the manufacturing process of wrapping the carbon fiber make them differ? Or would they be almost the same?
Does anyone have a good resource so I can learn more about this topic?
Posted 20 February 2005 - 02:16 AM
Posted 20 February 2005 - 05:12 PM
Posted 21 February 2005 - 11:40 PM
thanks for that tip off on bike design. I did not think of that. I know there might be info in race car design, but i totaly forgot about bikes!
Posted 22 February 2005 - 11:34 AM
Having lived through the evolution of post design (Aluminum to Titanium to Carbon Fiber) I have learned there is a lot of different approaches to tackling this issue.
The posts are manufactured by winding a carbon fiber thread over a mandrel (a metal core that determines the inside diameter of the tube). The flexability of the post is determined by the angle that the fiber is wound. If the angle is too shallow (more wraps per length) the post will flex too much from the camera to the base. If the angle is to sharp (less wraps per length) the post will have too much twist in the pan axis.
For instance...if you were to make a tube with fiber sheets and the grain was running the length of the tube, the post would not flex (Camera to base) very much, but the twisting of the post during a whip pan would be intense. This would cause a visible "ringing" in the image where the camera would bounce left to right at the end of the pan until it settled.
So...there is a compromise that needs to happen to manufacture a post with the least amount of flex and twist. This can also be dealt with by varying the direction of the fiber windings and the angle in which they are wound. Remember a post is many winds thick.
The finish on the posts is due to a grinding process. In order to get the posts to maintain uniformity so the gimbal will move the entire length, the outer layers of fiber are ground down to a very tight tolerance. You can see the effect of this grinding with some of the posts that have a camouflage texture. This grinding is considered in the post design and does not compromise the strength of the post...much.
Regarding the banging and standing on a post to show durability...this is mostly for show. Considering the post is made up of continuous lengths of thread, breaking one of these threads is nearly impossible. If you do see a carbon fiber product with chips in it, this is because the manufacturer has used a material that is made up if chips of carbon fiber. But this type of manufacture is normally done with molds and not with tubing.
I hope this helps,
Posted 22 February 2005 - 02:03 PM
Brett (Brrrr) Manyluk
Posted 07 April 2005 - 11:15 PM
Carbon fiber composites by nature do not slowly bend or develop hairline cracks that we can see. A drawback has always been that they suffer what is known as catastrophic failure in the field. This is to say that a post may have suffered repeated strikes ( or, to use the phrase, offenses ) and not show much externally to indicate internal stress fractures in the layering of the weaves and epoxies.
Until....it simple cracks into pieces. Something as simple as docking your rig into a normal docking bracket. ( No offense to folks using the GoreLock, and other high-tech docks. I mean by normal to say a regular metal "U" - shaped dock with a pin to hold the post in ). You dock and undock dozens of times a day- really, hundreds of times a day on a busy day. The centerpost will slowly chafe along identical lines, each time. Eventually without realizing it you have weakened the centerpost. You go to low-mode, and the top of your centerpost ( which is now hanging just above the camera near the floor ) cracks off suddenly.
A sobering reality. I'm not saying that carbon fiber centerposts aren't amazing- they are. But it is surely worth considering a fairly serious hidden flaw potential. Unless you carry around a portable x-ray machine and keep count on how the inner layers of your centerpost are faring, you may well slowly suffer internal fractures in the weave, and epoxy resin layering.
p.s. for my two cents, they do beat the heck out of titanium or aluminum as centerpost materials.
Posted 07 April 2005 - 11:17 PM
Wooden bench, eh? The wood is slightly soft, and transfers force and vibration much more forgivingly than metal does.
Posted 09 April 2005 - 02:18 PM
Are there any examples of this having happened to people? like the rod end bearing incidents (socket block plague of 2004).
Posted 09 April 2005 - 02:47 PM
Oh, and I watched a race car crash against the outside wall at Nazareth Speedway once, on a shoot. The pieces of the car did not crunch or fold over, as would metal. They exploded outwards all over the place. Zero flex means that when sudden forces are introduced, there is no place for the forces to be "spent".
I will admit that I am aware that such amazing items as the US military Stealth Bomber make extensive use of this technology. Then again, if a Stealth Bomber wing strikes something repeatedly in the exact same place, I am guessin' that it'd be taken care of before fracturing within the wing took place.
Posted 10 April 2005 - 03:20 PM
Often the topstage is simply glued into the post on carbon fibre. Back when I was more of an idiot than I was now, I had not realised that loosening off the gimbal would make the 2ndstage of my post come out easier, so I was using a lot of twisting force on the topstage to get anything done, eventually The glue came loose and I was able to simply rotate my topstage within my post! good job I never had any low mode to do!
Now I make sure that my docking collar is over a section on the glued in central bit. For that extra grip.