Need help in re-designing rotating joint 1

[dkmulford]

Hello everyone. I am a civil engineer by education, however, I have a side business that manfactures amateur and professional camera mounts.

My professional camera mount, called the Pro-Mount, is used by professional videographers to videotape from within trees (i.e., whitetail deer and bear hunts for the Outdoor Channel, Sportsman Channel, ESPN Outdoors, etc.).

My Pro-Mount is able to level vertically and horizontally. I am trying to redesign the "shoulder joint" which provides the horizontal leveling. Please see the pictures below.

Currently, the shoulder joint is two 3 inch diameter plates. The front plate has two slots with two 3/8" hex bolts passing through the slots and into keenserts located in the back plate. There is a stainless steel 1/2" diameter shaft extending through the center of both plates.

My problem is that when the arms are fully extended 90 degrees to the tree (and in line with the rotation of the shoulder joint), the joint can slip. Obviously slipping is dependent upon the force applied to the two bolts and the load applied to the end of the arm. Altough the TV type cameras weigh about 30 to 40 pounds, I would like the arm to be able to handle about 80 lbs without slipping. Additionally, I would like to incorporate a handle in place of the allen bolts so the user doesn't have to carry an allen wrench. Doing this reduces the amount of force the user can place on the bolts.

Is there a book out there that might help us design this joint to maximize the clamping effort?

By the way, we can't increase the diameter of the two plates and the two materials must be aluminum. Additionally, the joint must provide enough rotation to allow leveling (say minimum of 15 degree each direction).

Thanks for your help,
Darren

 

[Barry1961]

If at all possible I would move that axis joint to the end of the arm directly under the camera mount plate. If you can do this it should drop your torque holding requirement to where you could use a variety of toggle or cam type clamps.

Barry1961

 

[Barry1961]

Just to throw out of few clamping methods.

Clamping the OD. A strap wrench or hose clamp type clamp.

A worm gear type joint. There maybe something in that size already mass produced that could make it viable. I have seen worm gear joints on telescopes.

Pull pin incremental adjustment. Probably would not be accurate enough.

Toggle clamps. Destaco is one mfg. that has a fairly wide selection.

Disc brake type clamp.

Of course most of these would clamp better with sticky or rough surface.

Barry1961

 

[dkmulford]

Barry,

Thanks for the reply. We actually tried clamping around the outside. We had a square 3x3inch block with a 2 inch diameter shaft in the middle. The edge of the 2 inch round was angled on the front and back (looked like a diamond).

The problem with it was that when the bolts were loosened, the arm could rotate completely around (there was no stop). Although we could have added a stop, we also didn't like the having the tightening bolts coming out of the bottom. The natural way to tighten was from the front. One other thing was that we liked having a round plate and not a square. Would could have machined a plate that was round with two flat spots for the bolts, however, this required more machine time (versus just cutting a 3" diameter round). One last thing we didn't like was that looseing only one bolt slightly made the entire thing rotate around. We liked the built in redundance of the two bolt clamp of the old version AND the built in safety stop provided by the slots (the joint could not completely rotate around due to the slots).

You are correct about the pin adjustment. Murphy's law says that we'd never get it leveled.

We've also considered a worm gear but have not pursued this further. On this I am thinking I could use a 2 inch diameter stainless gear and bring the worm out the bottom. As you can see from the picture below, I designed the base to fold up, so with the worm out the bottom, we would still be able to fold. I'm not sure if I can fit everthing into a 3" diameter housing though. Do you have a source for worm gears this small?

We also considered increasing the friction of the surfaces. However, my machinist was liking this idea. We figured knurling would eventually smooth out since alum. is so soft. We ruled out a layer of rubber because we thought it would not be rigid enough. The last thing I considered was painting on some rino-liner stuff. However, it would give an uneven surface resulting in loss of intimate contact.

I'll check out the toggle clamps. I'll have to think about the disc brake method.

Darren

 

[nbucska]

Interleaved non-magnetic sheets ( e.g. brass-foils) between
an iron sheet and a permanent magnet -- like
ironS
brassR
brassS
brassR
brassS
etc
magnet

The "S" and "R" indicate stator and rotor.
The distance of the magnet and the last -- heawier --
plate is adjustable.

Think it and two interleawed pack of cards...


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[DesignerMike]

You could use a V-band clamp around the OD of the two disks. The sanitary piping industry uses this method everywhere to quickly clamp two fittings together. Typically there is a gasket that gets sandwiched between the fitting faces. The back side of the flanges (in this case your disks) would be tapered, and the inside of the clamp would have the reverse taper. Normally there is a single wing-nut type screw to clamp everything down. It is a strong joint due to the angles, and you could still keep the bolts in place for added friction.

I'm guessing that you are getting some distortion in the aluminum during tightening. This clamp would apply the clamping force on the largest part of the disk, on both the front and back of the disks. I have some 400 pound valves hanging with these clamps so I'm sure they would be strong enough.

The clamps are Stainless Steel, and reasonably priced. Page 153-157 of McMastercarr catalog--

http://www.mcmastercarr.com

-- has them listed so you can see what I am trying to explain. Their prices aren't great, but it is an easy way to see how they work, and they have cad drawings available.

 

[Barry1961]

I think the important thing is to move the problem joint to the other end of the arm. If your arm is 30" long and the camera is 40lbs you have 1200lb-in of torque. If you move the joint to the end of the arm you will probably have less than 200lb-in of torque.
With the "tilt" joint mounted under the camera and only tilting 15 degress off top dead center your torque should be very low.
With this arrangement you should be able to use a wider variety of clamps.
I made a sketch of this but can't seem to paste it in hte message. How did you put the jpegs in?

Barry1961

 

[dkmulford]

Barry,

I uploaded the pictures to my server then used the tag to show them.

If you don't have a place to upload your picture, email it to me and I'll put it on my server then post it for you.

Darren

 

[dkmulford]

If I understand you correctly, you are suggesting to put the horizontal leveling out underneath the camera?

If so, I don't think this would work because I need the two arms (each 1.5 feet long) to be able to rotate around the tree. Click the link below for a plan view of the rotation. The 2 foot diameter circle you see in the picture represents the tree.

http://www.huntingfootage.com/images/armswing.jpg

If you put the horizontal leveling plates out underneath the camera, then I would not be able to level the joints that allow the arms to swivel (there is one joint where the arms meet the base and another joint where the top arm meets the lower arm - known as the elboe joint).

 

[plasgears]

I have had success using aluminum washers between metallic surfaces. The clamping is very positive.

 

[sreid]

Two thoughts;

1) Redesign the two plates so jacking screws can be installed. Sort of a poor mans' worm gear.

2) Make the rotating joint a taper. This will increase the clamping force. 15 degrees is common for non lock up.

 

[btrueblood]

Simple solutions:

A) Add more bolts
B) Increase bolt diameters

Both will increase the clamp load in direct proportion.

- BT

 

[griffengm]

DK,
I think the rotating plates can be eliminated altogether if you arranged to move the top and/or bottom end of the diagonal by using a screw and lock nut. Should also eliminate the need for setup tools.

Griffy

 

[dkmulford]

sreid, Do you have an example of a "jacking screw".

Regarding your taper suggestion, one of our ideas was to try to incorporate a taper like you suggest. The idea was to bowl out one of the plates and make the other plate the reverse with a 1/16" gap between the two at the flat interface so it will wedge together. In addition to this, use two T-slots in the back plate. This is one of the ideas we were planning on trying out this next weekend. See below:

We previously tried a modified T-slot, but instead of a T, we had a 15 degree angle on one side. See below:

Once it was clamped, it didn't budge, however, in order for it to lock into place, the system had to "seat" itself. This made the system unlevel which defeated the whole purpose. We figured the problem was with the bolts cocking to the side due to the void of material around it in the slot. One solution might be to surround the bolts with a steel bushing that would extend down into the slots? I came up with this idea but my machinist didn't like it for some reason.

We also replaced the allen bolts with quick adjust screws. With the slots in the back plate, the two screws in the front were always in the same spot (which I liked). I don't have a picture of this.

btrue, We also thought about adding bolts but this really makes things congested on that 3" plate. Additionally, it makes it user-unfriendly.

How will making the bolt larger help?

griffengm, I don't follow. I assume you are referring to the vertical adjusting rod? Somewhere, somehow, I have to have horizontal leveling ability for the arms. Even if you put this motion on the vertical adjusting rod, or on the base, or below the camera, you'd still have something that works in a similar fashion to what we have now.

Maybe I'm missing something? By the way guys, I really appreciate your comments. I work with dirt and water in my day job so some of this stuff takes a while to sink in.

Thanks,
DM

 

[dkmulford]

By the way, I did the drawings above real quick. There was also a dowel pin the second design.

 

[ruble3]

What if you tried making the plates like a Lovejoy coupling (with two or more teeth or cogs) - don't use the rubber insert part-then if you can, make the as shaft that goes right to the end of the horizontal arm that comes out from the tree ( can't tell if its hollow or a piece of square stock) - put a spring on the shaft so when you want to move the position, you pull out on the horizontal arm that pivots, the teeth disengage,but the spring locks them up again when swivelled and released)
Not sure if you can understand this but it may be clear in my mind only!

 

[griffengm]

Destructotron,
"I assume you are referring to the vertical adjusting rod? Somewhere, somehow, I have to have horizontal leveling ability for the arms. Even if you put this motion on the vertical adjusting rod, or on the base, or below the camera, you'd still have something that works in a similar fashion to what we have now."
If our terms are the same, yes.
Picture a vertical plane intersecting the center of the tree and running through the center of the horizontal link as well as the vertical adjusting rod. In your present design, you are splitting the plane at the rotation joint to get your adjustment.
If you think of being able to tilt the vertical plane by moving the bottom or top of the vertical adjusting rod it will tilt the end of the articulated arm when left or right.
I’m not sure how much adjustment you need so your method may be better in that respect.
It appears that you are building adjustment similar to a transit when that of an older level(theodolite? plane table? prior to dumpy levels)might work.

Griffy

 

[dkmulford]

What if you tried making the plates like a Lovejoy coupling....

We need more accurate leveling that what a lovejoy coupling would provide. Smaller cogs wouldn't work either because we are using aluminum.

 

[SLTG]

1. clam those 3 inch OD plate with clamping mechanism like the one in the bike seat or wheels.

2. Barry's ideas.

Sltg

 

[btrueblood]

How will making the bolts larger in diameter help?

Simply that you can support a higher clamp load with a bigger bolt. Of course, this takes up room on your 3" plate, and the torque required to generate clamp load goes up with increasing diameter.