swing arm design question

I am using a swing arm for my home made device. It was inspired by the swing arm lamp like this one

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My swing arm is made of 3/8 square steel tubes and Delrin plates. Tubes are attached to Delrin plates by 10-32 bolts. I use steel washers against Delrin. That should give me low friction.

The problem I have is that it's hard to control bolts tightness. If it's not tight enough the whole swing arm has a significant "play" If you tight it to much it's hard to move due to high friction.

Is there a good solution for this problem?

BTW: what should I use to make sure bolts are not getting loose? Retainer nuts, lock washers, jamming nuts?

Thanks, Alex

Reply to
Alex
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BTW Thrust ball bearing is not an option since I hope to eventually mass produce this device so it must be really inexpensive

Thanks

Reply to
Alex

If you use a nut with a nylon locking patch, you can control the screw tightness without the screw working loose. The other approach is to use a stepped bolt (aka shoulder bolt or stripper bolt) and a wave washer to control the load. A shoulder bolt can be replaced by a regular bolt and a length of tubing.

Erich

Reply to
Kathy and Erich Coiner

"Alex" wrote in news:FKsdb.6838$a46.2950 @newssvr27.news.prodigy.com:

Nylon locknuts would be my choice for the bolt tightness issue. (Nylock?) I'm afraid with the delrin plates, control of the tightness is going to be an issue. It is too slippery and deformable, in my opinion, to retain a very specific clamping force. Probably why the lamp manufacturers use metal plates for this.

Reply to
Anthony

Thats what I would do. The problem with using only delrin, is its got an odd "crush" limit and its either too slippery when loose, or it suddenly reaches its elasticity limits and starts to crumple (sorta kinda)

Gunner

"Anyone who cannot cope with firearms is not fully human. At best he is a tolerable subhuman who has learned to wear shoes, bathe and not make messes in the house." With appologies to RAH..

Reply to
Gunner

How about using a spring washer of some sort (cone or wave type) in the assembly to help you control the compression better? I would try this in conjunction with an elastic stop nut (nylock).

Your Delrin washers may cold flow after awhile and get thinner though, which might make it difficult to keep the compression constant.

-- Cheers,

--Mitch

Reply to
Mitch

What is the optimal solution for this problem than? I guess thousands of people before me encountered this problem before and successfully(at least some of them) solved it.

Reply to
Alex

I hate those damn lights. They work well brand new but after a few doses of chems on them they start to sag, droop and everything inbetween.

I dont know what kind of environment this is going to be used in but delrin aint gonna last long if its going into a machine shop. :)

In any event you may want to look at hemi-spherical washers, nylock bolts and such. Also, to where you are bolting it may make a difference. The swing arm is bolted from the side which makes it conveinant but distributes the clamping force from the side of the movement of the arm. To me it would make more sense to have the bolt in parallel to the axial movement of the arm, not 90 degrees from it.

Sort of along the lines of a wriggler set.

HTH

Bing

Reply to
Bing

"Bing" wrote in news:JkBdb.23088$ snipped-for-privacy@nwrddc01.gnilink.net:

I'm afraid with using the spring washers, (whether it be bellvelle or wave), they will cut into the delrin over a short period of time.

Bellvelle washers are a great asset in many types of clamping fixtures, especially if you want to control the clamping force accurately, or if the parts are odd shaped. The variety and accuracy of the spring rates available allow you to find just about whatever you need, (relatively cheap too)

Reply to
Anthony

Alex, You won't ever get good bolt stretch using special washers, plastic, etc.

This is the same problem you are having with the Delrin. You need to remove it from compression in the joint. It's stiffness is low compared to the bolt, so you are not getting any bolt stretch (the goal).

A steel sleeve on the bolt to put stiff material between both ends is best.

A shoulder bolt will work, but you better use a lock nut, because you're depending on the ends of the threads to hold the nut on, rather than a good joint. Most lawn mowers do this for their wheels. Lock nuts are great for keeping you from digging all over the back yard looking for the nuts after they fall off, but don't contribute to bolt stretch either.

Reply to
Dr Jimmy

Why not just use some springs to counter the torque like on the lamp shown? You are working with controlled opposite forces. There's a good reason for all those stringy things in your arms and hands. If you don't like springs you could use hood lifter nitrogen shocks. Or larger diameter knuckles so the load at each joint is spread out and easier to adjust

Reply to
dann mann

Leather and cork are 2 traditional materials for this application.

Reply to
Jim Stewart

Sounds like a fishing reel drag, although that would be somewhat overbuilt. Still, a drag washer between flat metal surfaces, with tension adjusted by threaded nut and belleville washers, would work well.

Pete Keillor

Reply to
Peter T. Keillor III

Note that most commercial versions I have seen have a thumbscrew or wingnut for adjusting the compression/ stiffness to taste - which also serves as a periodic "tightener" when things start to wear a bit...

-- Cheers,

--Mitch

Reply to
Mitch

I don't think you want low friction as much as wanting dynamic friction and static friction coeficients to be fairly close to each other. And the other thing you want is a spring element so that the bolt does not have to stretch to provide the spring. A bellville washer and a plain washer ( so the B. Washer does not cut into the friction material ) should work well. Dan

Reply to
Dan Caster

Alex, The belleville washers others have suggested are a good option. A few things to know. They come in different spring rates, by varying the thickness. They also can be stacked. When stacking two with the cones the same way the spring rate will double. Flip one over so the cones are opposite and the travel doubles while the spring rate is cut in half. This will keep a more constant pressure over a greater range of wear. You want to use them with a flat washer against the delrin as the brake surface. That way the bellevilles won't dig in and the tension will stay constant for a very long time. Use the Nylon licking nut to keep the nut from working loose.

Gary H. Lucas

Reply to
Gary H. Lucas

Nope. Consider: Spring rate is force change per unit distance. When you put two springs of the same rate in series, be they Bellville or coil, each one only moves half as far for a given total displacement. The delta-F is therefore half.

Ted

Reply to
Ted Edwards

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