Need help in re-designing rotating joint
Need help in re-designing rotating joint
(OP)
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



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








RE: Need help in re-designing rotating joint
Barry1961
RE: Need help in re-designing rotating joint
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
RE: Need help in re-designing rotating joint
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
RE: Need help in re-designing rotating joint
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
RE: Need help in re-designing rotating joint
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-- 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.
RE: Need help in re-designing rotating joint
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
RE: Need help in re-designing rotating joint
I uploaded the pictures to my server then used the [img] 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
RE: Need help in re-designing rotating joint
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).
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
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.
RE: Need help in re-designing rotating joint
A) Add more bolts
B) Increase bolt diameters
Both will increase the clamp load in direct proportion.
- BT
RE: Need help in re-designing rotating joint
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
RE: Need help in re-designing rotating joint
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
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
Not sure if you can understand this but it may be clear in my mind only!
RE: Need help in re-designing rotating joint
"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
RE: Need help in re-designing rotating joint
We need more accurate leveling that what a lovejoy coupling would provide. Smaller cogs wouldn't work either because we are using aluminum.
RE: Need help in re-designing rotating joint
2. Barry's ideas.
Sltg
RE: Need help in re-designing rotating joint
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.
RE: Need help in re-designing rotating joint
OK, then i don't think increasing the bolt size is going to matter much to us since the applied load is governed by what the person can apply to the quick adjust handles or with an L-key allen wrench. The 3/8" bolts we are using now have been able to handle these loads.
In fact, I was wanting to go down to a 5/16" so I could have more room to work. As long as the 5/16" bolt can handle the load then I should be ok, right?
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
I know you mentioned a desire to test a new prototype this weekend. Please keep us updated dkmulford.
Ray Reynolds
"Computers in the future may weigh no more than 1.5 tons."
Popular Mechanics, forecasting the relentless march of science, 1949
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Need help in re-designing rotating joint
Instead of having the pivot in the middle of the disk can you move it to the top of the disk? This would give you a longer lever. If you moved the horz. pivot sleeve up you might have room for 3 bolts at 4,6 and 8 o'clock.
I think a larger bolt may help since in aluminum it is the yielding of the aluminum that limits the clamping force, if you are coming up against that limit. Also make sure your washer is not sinking in.
A shallow taper would clamp great but want to stick. Also, I am not sure of the longevity of a taper in aluminum.
Great problem in a small box!!! Good Luck!
Barry1961, Titular Despot Emeritus
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
After this disappointment, we decided to take another look at the OD clamping design that we had previously made (see below). I made the clamp out of round stock instead of square and designed a slot and pin to limit the rotation. See below. The slot and pin are not shown. When going to round clamp, we had to counter sink the quick adjust levers (also not shown) so they had a flat surface to clamp against. There was barely enough space for the steel keenserts.
We ensured that there was enough space to sufficiently clamp the two halves together (without the halves touching) and we also ensured we tapered the sharp ends around the od of the male piece (where the two 45 degree angles meet) and at the front of the female piece. During our test, we used the quick adjust levers.
During our first test of this prototype, the joint slipped with little weight applied to it. This was VERY upsetting. We decided to use a red magic marker to paint the internal components to see where the parts rubbed and it was deterined that we had good contact around the entire parts. After this we decided to rough up the surfaces by sand blasting everything.
After blasting, it held like a charm. However, the problem we now had was that when we wanted to rotate it (to level it up), we had difficulty getting it to turn even when the bolts were loosened. The pieces were just too rough. In order to rotate the joint, you had to use the arms for leverage. Although this might have been acceptable, we decided we didn't like the feel of things.
We have to get an order done THIS WEEKEND for 15 of these. In order to get this order out the door, we are going to go back to the old design and rough up the surfaces by sandblasting. As long as the customer uses a long allen wrench to clamp the two plates together, he can provide enough leverage to prevent the plates from slipping. Going from a 3.0" diameter to 3.25" should help also.
After this order is out the door, I'm going to further investigate the worm gear. This was the ideal design that we identified long ago, however, we knew it would add cost and weight to the design. I'm guessing I'll need a 1.5" to 2" diameter main gear in order to get it and it's worm into a 3.25" diam. housing. I'm not sure if the teeth on a worm this small will hold up to the force applied with a 240 ft-lb moment (80 lbs on a 3 foot arm)? I guess I'll tackle that problem after this order.
Darren
RE: Need help in re-designing rotating joint
Again, just a thought.
Jomor
RE: Need help in re-designing rotating joint
Revisit knurling (actually a form thereof on flat surfaces according to ANSI std). Knurl one side of the joint or one surface only. I never met a machinist that did not like to knurl things. Have it hard coated (a deep anodize that puts a hard garnett casing of a few thousandths into the surface, not decorative anodizing). One would probably redesign the part with the slotted holes so no welding is required (a machined block, with a cross drilled hole for the vertical axle, unlike what you have now). After hard coating the part can only be ground, no more machining or welding. Now you have hundreds of super hard little pyramids biting into the opposite soft surface, giving ample friction. Make sure there is a close slip fit with the 1/2 dia. center shaft, so it takes all the radial and most of the bending load across the joint.
An alternative would be to add a piece: a 3" dia. flat washer form 1/16 to 1/8" thick, with 1/2" clearance center hole and two 3/8" clearance holes that inserts between the existing joint. The washer would be knurled on both sides. It could be made of steel or hard coated aluminum.
At 240 ft.lbs. I would be concerned with structural integrity throughout, including the tree. It may see that much now periodically in the field as people plop down their heavy cameras on the mount. However, if you rate it for 80 lbs weight on the end, it will probably see more than that from the handling. Next, you will have people building articulated tree stands out of two of them.
RE: Need help in re-designing rotating joint
2 rotating plates sandwiched between 3 stationary plates will give 4x the holding torque. Of course the locating pins on the plates will reduce some of the effective area.
Another simple thing would be to relieve the inside of your friction faces so that all your clamping force is applied to the largest diameter possible.
Jeff
RE: Need help in re-designing rotating joint
We used 10 discs (10 inners and outter)with one screw at the center.
We bought the friction discs (not the whole assembly) from : http://www.moenninghoff.de/
The moment generated by the preloaded multi discs can be found in "shigley".
Lasker
RE: Need help in re-designing rotating joint
Torque is what you are trying to hold, a larger clamping diameter will give the most for the least, and, or try a fine thread screw for higher clamping force.
RE: Need help in re-designing rotating joint
I beg to differ. Multiple discs give multiple friction faces. Each face has the same force applied to it. Each face has the same coefficient of friction, so the torque is a direct function of the number of friction faces.
If anything, heat dissipation is worse because the heat must travel through the entire disc stack.
Jeff
RE: Need help in re-designing rotating joint
We designed a brake to hold 50,000 ft lbs torque a few years ago and found this formula:
F = µ * N F is force, µ is the coefficient of friction, and N is normal force. The force is independent of the pressure per unit area.
If a brake or clutch is going to slip then the area of contact becomes another problem. The surfaces become a bearing surface under load. At this point in design, the area subjected to friction becomes important and it will need to large enough for adequate life.
In the case of this product, it should not be under load when being rotated.
RE: Need help in re-designing rotating joint
Maui
RE: Need help in re-designing rotating joint
The basic equation for torque in a brake or clutch is:
1) T = F * R = µ * N * R where R is the effective friction radius.
Now, in a clutch such as an automotive clutch, the driven plate is sandwiched between the flywheel and the spring loaded driving plate. There is friction material on both sides of the driven plate, both of which are loaded by the driving plate, so the equation becomes:
2) T = n * µ * N * R where n is the number of friction faces, in this case 2, making the equation:
3) T = 2 * µ * N * R
If there are multiple driven plates on a common splined output shaft sandwiched between multiple driving plates driven from a common housing, all loaded by the one set of springs, equation 2) applies.
Multi disc clutches are commonly used to transmit large torque loads in a confined space. Because heat dissipation is a problem they are often enclosed in an oil bath. The resulting reduced value of µ is more than compensated for by the greater torque capacity.
You might have been able to greatly reduce the size of your 50,000 ft lb brake with a multi disc design, although it might have caught fire if it ran too long.
Jeff
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
I think you could use a toggle or over center coupling/clamp to grab the outside edges of the mating surfaces. You might put a little taper on each half to made the clamp much more effective.
There are several types used in vacuum service and can be found in any alloy.
I have an SS one for 3" SS pipe that uses a wing nut instead of a toggle. This one clamps with a rubber friction ring inside the metal clamping part that grabs the pipe, it will slip. Once clamped you will collapse a Sch 10 pipe before it slips. It only has a part number and I have no idea who made it.
This is just one type
Take a look at the toggle coupling in the middle of the page
http://www.intercast.au.com/cast_standards.php
Worst comes to worst you could make one. There all types of things you could do with this clamp.
Taper the edges of the mating parts.
Make a taper fit male /female of the two components.
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
RE: Need help in re-designing rotating joint
http://cf.victaulic.com/products/results.cfm?nowild=yes&style=Style%2078&keyword=
RE: Need help in re-designing rotating joint
This has been a messy description, but if you are interested email me at
jeffstanton@optusnet.com.au
and I will send you a sketch
I don't know what machining facilities you have available, but a simple way to machine the cotter is to tighten a bolt into the boss then bore both the boss and bolt together. Cut off the bolt head to finish the cotter.
Easy to make and clamps very tight, I first saw it on a spot welder being used to clamp the electrodes into the arms.
Jeff
RE: Need help in re-designing rotating joint
Is it possible to enlarge the spikes on the mounting base and provide a spirit level (as suggested before) and simply instruct the user to attach the base to the tree in a level fashion while strapping it into place? By enlarging and possibly sharpening the spikes the tightening process would be less likely to cause the base to shift off level. Also by providing a smooth surface for the attachment strap to slide thru by possibly filletiing the slots may also help to keep the base positioned while tightening.
It seems like this would reduce the complexity and weight.
If you were intent on providing fine adjustment your rotating joint could be modified with two jack screws. One on each respective disk - pushing on a stop on the opposite respective disk. Although this would not allow a lot of rotational adjustment it would provide enough to compensate for initial missalignment of the mounting plate and would certianly be capable of handling the torque. Helicoil inserts in the aluminum would also help with longeivity of the joint.
If you are interested in the equal and opposite acting jack screws e-mail me with some dimensions and I will be happy to model something up quick for you.
RE: Need help in re-designing rotating joint
In my opinion leveling should be done closest to the point where the level surface is required.The camera mounting plate.
Provide some sort of adjustment to the mounting plate attatchment. Maybe a simple ball joint with
the adjustment mechanism under the plate so that the camera can be easily held while the fine leveling adjustments are being made.
RE: Need help in re-designing rotating joint
Jeff