Sliding, Pivoting Joint design 1

[battleship81]

I need to design a joint that slides while it is pivoting. I can provide more details if needed, but here are the basics:
-This is part of a precision motion imaging system, so eliminating slop or play in the joint is highly desirable.
-The system will be driven by a reciprocating motor, so, back-and-forth motion is what would produce the slop.
-I need the joint to be relatively small: the initial concept is a .5" cam follower in a slot that is just slightly wider.
The main problem I see with the initial concept is making the slot sized perfectly, so that there would be minimal to no play of the follower in the slot. I considered mounting some sort of bushing in the body of a small linear guide/track system (i.e. Igus Drylin T miniature), this seems a bit too cumbersome and may be too big still. Maybe it could be as simple as a precision ground aluminum pin in a precision machined aluminum slot.

I'm not necessarily looking for a solution, but if anybody could point my in a general direction or show me existing examples of this type of joint, that would be great!

Many thanks!

 

[MintJulep]

Rack and pinion with anti-backlash gear as pinion.

 

[Jboggs]

You are trying to combine two separate types of motions, and that is always a problem. Find a way to separate the two motions, one linear mechanism and a separate pivoting mechanism. It is easier to eliminate free play in each separate motion rather than a combined motion.

 

[KENAT]

What levels of accuracy and precision do you require?

I recently did something like this just using a fairly precise SST shoulder screw and machined slot in aluminum 'stage'. The stage was sprung against 2 screws for X & Y adjustment.

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[imcjoek]

Linear bearing (you can get them in very small sizes, 1/4" bore or so) pressed into a small housing that has a shaft of some sort on the side for ball bearings.

 

[EngineerErrant]

Ball spline.

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams

 

[dvd]

Sounds like a barrel cam. You might look at these folks equipment:

http://www.camcoindex.com/CAMS.HTM

You might also have use for these:

http://www.pbclinear.com/Simplicity-Precision-Sleeve-Linear-Plain-Bearings

Cam follower cross-over is an issue to understand. Cam followers on both sides of the slot might alleviate the issue.

 

[rmw]

Here's one:

http://www.youtube.com/watch?feature=endscreen&NR=1&v=hWxAVqzUtPY

Sliding and pivoting.

rmw

 

[MotionGirl]

First of all, what are your criteria for success: Before a recommendation, one would need to know both static precision and the dynamic precision you would need. Statically, how much error in position could your design tolerate before it becomes inefective. Are there particular positions that the system needs to be accurate or does it need to be precise throughout the mechanism cycle. Having said that, my approach would be to get a linear slide with two slide blocks. Fix the blocks in the mechanism frame as far apart as you can so that rail slides in them without dropping out of the blocks of course. Then fix a pin to the rail. The pin should press fit into a bearing mounted to the part you want to slide and rotate. Try to keep the force of the sliding rotating part through the centre-line of the rail. This arrangment is virtually backlash free. As imcjoek said, you can pretty small linear slides these days.
I'd be interested in the mechanism you are designing as it is always possible that another mechanism could be used to eliminate your problem...

 

[chicopee]

I would not use aluminum for material.

 

[Cockroach]

Great thread, RMW, that machine is a thing of beauty. Star for you!

Regards,
Cockroach

 

[battleship81]

Thanks everyone for the great input!

For my design tho, I guess I took the most from KENAT's suggestion, as small size and simplicity are very important.

As it's modeled currently the joint will utilize a 2 piece plastic (nylon 6-6 probably) slide. The assy. is about 3X longer than it is tall and will slide along its length in a machined slot. The parting of the 2 pieces of the assy. is along the assys' length., below the center line for the overall height. between the 2 pieces (blocks essentially) will be either 2 springs or set screws. In this way I'll be able to take out slop in the linear part of the motion, by adjusting the contact pressure of the slider against the machined slot. The pivoting comes from a hole through the taller of the 2 blocks, as close as possible to the center of the slider assy., in order to not apply moment load to the slider. In the hole, I'll either use a precision shoulder screw, or a press fit cam follower. I have a cool way to control to keep the slider capture the slider in the slot as well, but that would take too much more effort to describe.

chicopee, you said to not use aluminum - why do you say this? Would SS be a better material for the machined slot?

 

[KENAT]

If it's under any significant load neither Aluminum (soft) or SST (galling) are necessarily fantastic options.

In my case there was limited loading (the spring only gives around 1 pound of load) so I used hard anodized Aluminum for the slotted stage.

In terms of accuracy & precision, while I haven't done extensive testing I'd say that even without any 'heroic' measures to perfect the motion etc. I'm able to adjust the stage around +-.010 micrometer accuracy. Maybe a bit more but let me put it this way, I'm trying to position a 200 micron 'spot' on the cross hairs of a magnified view and don't have any real trouble.

I had trouble following your description of your stage so can't comment more, sometimes a picture paints a thousand words but I know there can be IP issues. I know there is at least one point on my design I can't discuss until our IP guru confirms my suspicion that no I didn't come up with some great new invention.

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[battleship81]

Kenat-
I attached a pic for you to check out. The 2 springs you see in the slider may be replaced with set screws for better control.