How much clamping surface to hold a 5lb weight 1

[Quickmode]

Hi, I'm building a small aluminum clamp that is holding a computer weighing approx 5lb. The computer uses a mount with a tang approx 3 inches from the back of the unit.

The tang gets clamped between two aluminum plates with a 3\8 bolt that goes through the assembly with a nut or threads in one of the plates.

I'm using a ratcheting handle to loosen and tighten the clamp since it has to be adjusted many times a day.
The thread in the handle is 3/8-16.

The end user wants the clamp to be as small as possible while maintaining a light effort needed to adjust the clamp.

Anybody have any ideas how I would calculate the area of the plates and tang needed to hold the weight horizontal?

I have tried a finer pitch bolt on the handle, but would like to use 3/8-16 because of the availability of those handles..

Also, does the clamping force on the plates start to fall off the further in diameter you go from the bolt, due to
flexing of the plate?

The first prototype is in the pic. Not able to tighten it enough to hold the application.

Thanks for any ideas

http://files.engineering.com/getfil...412f-45c6-a544-b115dbe97dac&file=IMG_2583.JPG

 

[hydtools]

The load capacity has little to do with the area of the plates as it does with the friction and clamp force of the plates against the tang. The friction force has to resist whatever moment the 5 lbs creates at its distance from the bolt hole. For example, 5 lbs 6 inches out creates a moment of 30 lb-in. Say that moment is resisted by a friction force acting at 0.5 inches. The friction force has to be 30/0.5 = 60 lbs. Say the friction coefficient is .3. The clamping force needs to be 60/.3 = 200 lbs.

Good luck.

Ted

 

[unclesyd]

Unless you can use a larger size bolt in the lever you will kave to make more force be applied to the mounting pin.
The pole clamp part of your part could be made with a thinner section where more force will go clamping the mounting pin.

Another way would be to tighten up the initial clearance to where more force can be applied to clamping the mounting pin instead of overcoming the spring in the clamp.

Are there threads in the opposite side of the pole mounting clamp or are you using an external nut?

If you are worried about stripping the threads in the pole mounting clamp you could use an insert or rivet nut to where you can apply more force to clamping the mounting pin without fear of stripping the threads.

http://www.reidsupply.com/GrpResults.aspx?pg=0&pid=1100000&aitm=KHF-154&apid=1100000

http://www.rapidrivet.com/

I think you have too many degrees of freedom in your clamping arraignment as you have to position the pole clamp, hold the computer and tighten the clamp at the same time.

 

[potteryshard]

You could also ream the hole in the tang to a taper, and put a hollow tapered plug in one side of the clamp, keyed so it is locked to the clamp. Then, the force applied to the clamp will be effectively multiplied by the taper creating much higher unit forces.

 

[dgallup]

I have seen similar clamping systems with radial ridges that interlock. The ridges positively lock together so the clamping force requirement is greatly reduced. However, it also means that you can not make fine adjustments, you have to go in ridge increments so if there are 30 ridges your angular adjustment increment is 12 degrees.

 

[hydtools]

If you add a counterbalancing spring you can reduce the effective load and reduce the required clamp force.
Or like on telescopes, add a counterbalancing weight.

Ted

 

[MintJulep]

Your design will never be reliable. You're trying to do too many things with one bolt and the performance is highly dependent upon part-to-part variations affecting the way everything will fit and draw up tight.

Separate the functions.

 

[Quickmode]

Thanks everybody, very interesting replies. I love this forum!

Ted-hydtools,
Thanks for your post, so if there was a way to shorten the distance from comp to clamp..interesting..like a 5lb weight attached to the clamp going the other way..
Also, you're saying the area of the plates doesn't contribute much too overall clamping force? If, say, I doubled the area of the plates, would this not spread the friction over a larger area, or more importantly larger diameter?

Dgallup,
Gone over the interlocking method, hard to manufacture, or too many pieces.. unless there is some kind of press fit insert I could find? Not too worried about the limited freedom of movement.

Unclesyd,
The second prototype has thinner sections with reliefs before the tangs as to relief any resistance caused by the first clamp section.
Also, the inside walls of the tang are tapered to be parallel +-.001" after the initial clamp is tight. I have a controlled dia on the first clamp.
And yes, the tang is threaded.

Mintjulep, what if the tolerances are controlled? Have tried to convince the other party to have two bolts, one for each clamp, but no go.. They want one handle to control all movement.

Potteryshard, Can you elaborate on that idea? If I understand correctly, the taper is to be pulled along the axis of the hole with the bolt handle??

 

[hydtools]

Quickmode,
I presumed that the friction is uniformly distributed from ID to OD. It acts at the average radius. Yes, you could increase the diameter of the clamp to increase the radius through which the friction acts. But to what effect within the limitations of your space? Doubling the area of a circle increases the diameter only 41%. Will that be enough to hold the computer?
It may be counter-intuitive, but you can increase the effective radius of the friction force by removing material, area. Cut a recess in the face of the clamp to create an outer ring of contact. Like clamp washers on a circular saw.
Average radius = (OD+ID)/4

Ted

 

[MintJulep]

what if the tolerances are controlled?

This is a hallmark statement indicating that you have a poor design.

Good design, especially of a simple thing like a clamp, will be highly tolerant of part-to-part variations, imperfections in alignment and fit.

"We just have to hold the tolerances closer" = "Our design is fundamentally flawed" This relationship is almost always true.

You have a bad design. Start over.

 

[unclesyd]

Depending on the degrees of freedom required you may want to have you mounting pin fixed straight up with threads on the top end, to allow your computer to rotate in the XY direction using a wave washer to hold it place. If you need tilt on the computer screen, again use a wave washer to maintain position.
By using your lever arm clamp instead of the 4 fastners show in this example you can still tighten the clamp to the pole. This will take the moment off the clamping mechanism.

It would look something like the one shown in the link below.

http://www.ergoindemand.com/cte_pole_mounts_short.htm

http://www.ergoindemand.com/pole_mounts_short.htm

 

[potteryshard]

Attached is a sketch of what I was thinking. The taper plug could be pinned to the tang as shown, or more elegantly, could be a square outline. The bolt, which I didn't show, would jam the taper plug into its seat creating the desired friction.

 

[dhengr]

Your small pin looks a lot like a shear pin to me and will prevent the tapered part from working the way you want it to.

Does the original design clamp tightly enough around the vertical post? I would lighten it up so as to soften its spring action w.r.t. your ratchet handle. Then I’d add a couple washers btwn. the tang and the clevis. This washer material should have a high friction coef. w.r.t. unmachined aluminum and a high shear modulus. It’s the shear (torsional shear) in the two washers which provides the moment resistance (10 or 15 in.lbs.) to angular motion. Should be some plastic or hard rubber material which fits that bill.

 

[dhengr]

My apologies Potteryshard.... as long as the clevis leg movement is from the left side of your sketch, pressing the tang onto the tapered part, it might work, and you would then need the shear pin to prevent angular motion. But, when you loosen the ratchet handle, what pulls the tang off the tapered part? Usually you need a hammer to do this.

 

[potteryshard]

A 15 degree engagement angle was pretty standard for cone clutches back in the day before disc clutches took over. Certainly jamming and possible galling could be an issue. Material selection would seem to be able to control both of these issues. If the natural resiliancy of the clevis is not enough to help pull the taper plug loose, the angle can be increased. In fact, it seems that the amount of clamping friction and the releaseability (if I can fabricate a word) could pretty much be tailored by experimenting with varying angles of the taper. Releasing from the tang could also be supplemented by placing a Belleville or other elastic washer between the clevis and the big end of the taper plug to help push the taper loose.

 

[Quickmode]

Thankyou everyone!
Potteryshard, I understand now. The anti-rotate pin would have to have enough allowance in its pocket to allow opening and closing of the clamp, correct?
Working on couple more prototypes...

 

[potteryshard]

It could probably work either way... If tight, the clevis springiness would provide the give; if loose, clevis springiness wouldn't be needed.

I would think that the best way would be to put a square head on the taper plug so that it would drop into a similar square opening in the clevis. The clamp bolt would then bear directly on the taper plug, so the clamping force would be independent of the clevis compression.