Adjustable robotic links 5

[FeX32]

Simple question guys.

Anyone come across legs or links that have an adjustable length?
For example, there exists synthesis methods for 4-bar linkages in which adjustability of a link provides several advantages.
Has anyone seen a practical version of an adjustable link? For mechanisms or robots.

Thanks and cheers!


Fe

 

[MikeHalloran]

See "Johnson rod". Once ubiquitous on British motorcycles.

Practical? No. They were always falling off, which is part of the reason there are now so few British motorcycles.





Mike Halloran
Pembroke Pines, FL, USA

 

[FeX32]

Thanks Mike!
Is there another name for it? I can't get a decent description of one online it seems.


Fe

 

[FeX32]

I forgot to mention in my original post that I meant for things other than the obvious like double side threaded rods ect.
I never knew about the Johnson rod, so that's a good start.

Basically, I want to know if something I can use exists or not. Otherwise I wil simply design a custom part. I may be forced to modify a linear actuator of some sort.



Fe

 

[GregLocock]

You mean like an in line screw jack? Displacement or force controlled?

Guessing you want displacement, I'd use a custom designed ballscrew setup.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[FeX32]

You guessed exactly right Greg.
Adjustable displacement. Basically, the robot I am working on right now needs one of its links to be adjustable in length. But not actively (as the robot is being used). It can be adjusted offline for different duties ect. It is a custom robot, but I thought I might be lucky to get some parts that might exist.
So it will have to be rigid and reliable.
Custom ballscrew setup is very feasible.
So I guess not much exists in this domain of mechanisms in practicality.

guys


Fe

 

[GregLocock]

Well, you could have a look at what racing yachts use to fiddle with their standing rigging. I'm guessing it is more likely to be hydraulic than electrical.

The usual manual method I've seen is a locknutted turnbuckle with left and right hand threads, like a tie rod in a car.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MintJulep]

How about:

http://www.hizook.com/blog/2010/11/07/spiralift-ultimate-telescoping-linear-actuator

or

http://www.hizook.com/blog/2010/11/07/spiralift-ultimate-telescoping-linear-actuator

If it doesn't need to be power driven how about something like a camera tripod leg?

 

[FeX32]

Thanks guys.

like a tie rod in a car.

I am leaning towards something like this in the preliminary design stage. Thanks Greg.

Mint,
Very interesting links you provided. I never knew those existed. I will look into their feasibility into my design. Thanks.




Fe

 

[Compositepro]

Mint, that is a pretty cool device.

FeX32, everything you've specified would seem to indicate that a turnbuckle or tie-rod type of device would be ideal for your application, yet you say you want something else. Why? Turnbuckles can use left-hand/right-hand threads for "coarse" adjustment, or differential thread pitches of the same handedness for very fine "micro" adjustments.

 

[sreid]

Race car suspensions often use Rod Ends with left and right hand threads [one each end of the attaching rod] to make the link length adjustable.

 

[rb1957]

you say "other than double side threaded rods" ... why ? what's wrong with a tube with an internal thread, two threaded rod ends, and two jam nuts ?

 

[ornerynorsk]

Telescoping tubing with snap spring buttons. Downfall is strictly manual manipulation, coarse adjustment, but is very robust and inexpensive. Linear electric actuators with built in braking mechanism make wonderful mechanical linkages (the actuator itself is the linkage). These are used extensively in the medical equipment industry for precisely that purpose (of adjusting linkage length).

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[FeX32]

Thanks for the responses!

I was just wondering if there was some other sort of link or mechanism that could get the job done.
The tie-rod now mentioned by many of you suggests to me that this is likely the best way to go.
One of the reasons I was wondering if I could get something else is due to the large difference in link adjustability. For example, lets say I want the link to go from 500mm to 1200mm (or 100mm to 200mm). The male thread may end up protruding too far into the female part making the lesser length not practical. Maybe there is a way to work around this. I will do have some simple modeling to find out.

Telescoping tubing with snap spring buttons.

Interesting. I have never seen this used.

Thanks again guys for the responses!


Fe

 

[sreid]

Very common now to adjust the height of crutches maybe in one inch increments.

 

[sreid]

And it suddenly occured to me that crutches are what you use after the Johnson Rod fails on your motorcycle.

 

[MintJulep]

Eccentric cam on the end of the smaller tube of a telescoping tube set.

 

[unclesyd]

You didn't mention size or load capacity so here goes.
The first is a Push Pull type screwdriver, Yankee, For light loads and somewhat limited reach.

Next is the Spieth locking bushing, etc where you can use a tube for almost unlimited adjustment. We use this arrangement for heavier loads and lengths.

The Serapid product is for very heavy loads.

Not referenced is the tube clamp nut as seen on the above mention crutches and on adjustable camera legs. This approach comes in many flavors. We use precision telescoping tubes with a lock nut similar to aforementioned products.

http://www.thefind.com/appliances/info-yankee-push-drill

http://www.spieth-maschinenelemente.de/cms/p@c=19&l=5.html

http://www.serapid.com/gb/new_products/electric_spindle_actuator.html

 

[unclesyd]

Just looking and I think the adjustable metal walking cane/stick might be a good configuration if your loads will allow it's use. The walking cane has both spring detentes as posted above and a compression locking collar. It's possible to take the metal part right out of the plastic handle for a ready made arm.
I think a trip to to the drugstore might be worth the effort.

This is the type I had in mind.

http://www.physioonline.ie/catalogu...-walking-aids/metal-adjustable-walking-stick#

 

[FeX32]

unclesyd,

Thanks for those 3 first links. I think I'm going to get one of those yankee screw-drivers for my personal "screwing around" .

I like the way an adjustable walking stick type mechanisms works with a locking collar. It seems stable enough for lighter applications. Easy to manufacture. The only drawback might be the precision of the adjustability. But I am sure there is a way to get around this.
Thanks for the ideas. Much appreciated!




Fe