Heavy Duty Single Shear Joint 2

[kgwhipp]

I'm trying to find the best configuration for a joint on a lift we're designing. This is a heavy structure but I want the joints to be field installable, something that a tech can wrench and hammer into place.

I want to keep the two 1018 elements held together very closely so that there isn't misalignment, but I also don't want to wrench down so hard that I bind the joint.

I've tried looking for similar joints in use to no avail. My attached picture describes most of what I'm doing. The key reason I like this design is that the polyurethane acts as a spring but doesn't wrench down too tight.

Then again, the joint only moves slowly about 20deg, I might be overthinking this entire endeavor, would other manufacturers simply put a clevis pin and a grease port?

Thanks for any insight.

-Kevin

 

[kgwhipp]

An easier link:

http://i.imgur.com/bfxN9bV.png

-Kevin

 

[desertfox]

Hi kgwhipp

Well I can only see the pin and the joint and have no idea about mass or size of your structure but depending on how often the joint rotates and whether it's easy to get to in service,I would drill some holes in the pin and fit grease points.
The other thing I would be worried about is about the nut on the end of the bolt coming loose, I would recommend locking the nut.

 

[MikeHalloran]

I can't see what the PU 'spring' adds to the assembly.

E-rings are available in large sizes, so the pin could be 2-1/4" TGP Stressproof or equal with two grooves.



Mike Halloran
Pembroke Pines, FL, USA

 

[dhengr]

Kgwhipp:
I’m not sure that is a hinge detail I would use if the loads where great enough that I needed a 2.25" dia. pin. With CAD, you guys can draw way too many things that look o.k. on the screen, and appear to work, but probably won’t work too well in reality. And, the damn CAD program doesn’t even know the difference, nor do its users. With the large loads I would try hard to put those pins in double shear, make the hinge joint more symmetrical, and take the torsion or most of the eccentricity out of that joint. You say you want a tech, in the field with a hammer and a wrench to be able to assemble this joint. When a 3/32" thick by 2 & 7/16" o.d. sleeve bushing spans the joint, I wonder how you are going to press that into the joint and achieve alignment. There is a pretty good chance that the threaded end of the pin will chew up the bushing in the driving of the pin into the joint. This thread dia. should be small enough so it never touches the bushing. And, I doubt that the nut will be an off-the-shelf item. It might be good to step the pin, with a taper btwn. the steps. And, it might be good to fix the pin to one half of the hinge in some way and then bush the other half of the hinge. You should lock the nut as Desertfox suggested. There are bushing systems which do not require grease. And, your plastic spring just looks like a wear part which will allow the joint to become loose laterally. Why do you need that? What is this piece of equipment, what does it do. How does this joint work, how frequently, under what loads? Show some dimensions, loads, etc. on your sketch so we get some feel for proprtions.

With that pin size, and those kinds of loads, the two offset arms (probably about 3" x 6" HSS tubes) and (about 3 or 4" eccentric w.r.t. each other) which support the two hinge round tube halves will tend to twist, and this will bind up the hinge and cause uneven wear. I would try to put 2 arms on the top structure so that the hinge is in double shear and more symmetrically loaded.

 

[MikeHalloran]

Dhengr made excellent suggestions, most of which you would be well advised to adopt, unless you've already painted yourself into a corner, in which case I'd advise you to throw away what you've got and start over.

FWIW
Grainger keeps local stocks of ferrous nuts up to at least 1-3/4" nominal thread diameter; I was shocked by overnight delivery. For five bucks you can get a really big one for your desk.

The online catalog crapplication doesn't allow a search by thread size, but I found some even bigger ones in there by looking at 'width'.



Mike Halloran
Pembroke Pines, FL, USA

 

[kgwhipp]

This has been excellent feedback, thanks. I certainly understand the limitations of CAD vs Reality which is why I came here for advice on something!

The structure is a standard duty automotive lift. Non ductile components must have a Safety factor of 5+ which explains the large size of the pin. Main beams are 4"x8". I mentioned slow before but didn't quantify: The joints cycle 55° over a timespan of 120 seconds as the structure lifts. It will cycle, at most, a few times a day.

I'm limited to a single shear at this local joint because of how small of a space everything must compact into. See:

http://i.imgur.com/madtxuz.png

The joints do come in pairs and have a stiffener which should help with torsion. See:

http://i.imgur.com/V6NNwol.png

MikeHalloran: I too was shocked to find this off the shelf, but McMaster never ceases to amaze me:

http://www.mcmaster.com/#93126a150/=oucgwt

(item depicted in the original design).

I'm definitely going to ditch the PU spring. (It was appealing before because of the economical price and immediate availability vs Belleville washers.) I think I'll just live with a bit of play but design so that it won't wear out over time.

My next steps will be to redesign as such:

Use a stepped shaft with a head that will fix to one half of the joint. The other half will have a larger bore to include a bushing. The pin will feature a precisely machined shoulder with a tapered entry for installation and the threads will have a hole for a castle nut pin. I'd like to find an option that doesn't require grease as the owner will likely never maintain the unit.

-Kevin

 

[desertfox]

Hi Kevin

if you make the stud with the threaded end slightly longer than your joint and put a shoulder on it, the joint cannot then be clamped solid and will be free to rotate.
Also instead of fixing the large bolt with castle nuts and smaller screws, just put a pair of nordlock washers under the bolt head and nut and tighten up and the nut and bolt won't come loose.

http://www.nord-lock.com

 

[dhengr]

Kgwhipp:
Take a look at usa@GGBearings.com, for example. There are others. You can get fiber reinforced teflon rings too, Fiberglid used to be one name there. These come as cylinders with a single longitudinal slit in them. They snap into a wide few thousandths deep groove, actually .003" deep x 2 or 3" long/wide machined into the structural round tube which makes up the bearing or half the hinge. On your last sketch, on the right structural arm, at the bearing, there are two shoulders on the left and right ends of the stl. round which makes up the body of the bearing, and they are about .25" wide and then drop into the above groove about .003". These bearing pieces then snap into (spring into) the groove and are retained there.

 

[rb1957]

it looks to me (FWIW, which probably ain't much) that the head and nut are the weak links in this design ?

you want to align the two pieces by the overlapping faces ? you want to clamp the faces, but not too tightly ??

Quando Omni Flunkus Moritati

 

[Tmoose]

Have you talked to your suppliers about making this, especially whether welding the bearing housings to the arms finished, or machining the bores after welding? I expect the fit to the pin will need to be quite loose unless they will be line bored after welding.

I'd be thinking about moving the tubular crossmember (is this your torsional stiffener?) and combining it with the joint.
And providing some self aligning capabilities either by -
1 - using self aligning bearings

http://www.rbcbearings.com/sphericalplainbearings/index.htm

2 - making the end of the "other" component a thin plate with some length to allow flexibility for assembly and even in service. Like the axle end of this motorcycle swing arm.

http://ultimatemotorcycling.com/wp-content/uploads/2011/04/vintage-motocross-1965-cz-twin-port-2.jpg

 

[tbuelna]

kgwhipp-

While you mentioned that you feel that you are limited to a single shear arrangement with this hinge joint, I would heartily recommend taking a second look at it to see if it is possible to design the joint with a double shear pin arrangement. This looks like a hinge joint for a scissor lift mechanism. If this is correct, then using double shear hinge joints would be mandatory. With a cantilevered, single shear hinge joint there will be a fair amount of torsional deflection produced in each of the beams. And the total accumulated hinge deflections within the scissor beam mechanism will create lots of friction and will make the lift mechanism motion unsteady.