### Author Topic: Swinging rod Pivot  (Read 995 times)

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#### sherby

• Jr. Member
• Posts: 22
##### Swinging rod Pivot
« on: January 07, 2014, 09:05:48 AM »
Hi,
I was looking for a pivot mechanism for a freely swinging rod, and the pivot should be frictionless.
Its something like this:
http://1.bp.blogspot.com/_ERSGg8pg_sk/SxKWhgEORBI/AAAAAAAAAcI/rao6rbX-XIs/s1600/pendulum+2.jpg

Imagine the top pivot point for that.
However its replaced by a thin rod of around 0.3-0.5 inch.
I looked into using a ball bearing, but we dont have ball bearing of a smaller size, as a rod going through it will be more smaller, and will require a small bearing.
The rod is required to just swing in 1 plane.
For comparison, you can also imagine it to be something like a pulley(https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcQ50XT-2WxRB0NDUZm-vx0LW3fm_cct8pdc3D9LOpgP9OBtjYS8),
but with the string replaced with a thin Rod.

Any suggestions on how I can achieve the same?
Thanks for any help in advance.
« Last Edit: January 07, 2014, 09:08:09 AM by sherby »

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: Swinging rod Pivot
« Reply #1 on: January 07, 2014, 09:54:33 AM »
First, there is no such thing as a frictionless pivot. There is always friction between two moving parts.

To minimize the friction look into the "knife edge pivot" that is commonly use for clock pendulums.
Next is the needle pivot that is commonly use for model airplane propeller balancers.
The latter will not support much weight and are very delicate. The Former will support more weight and is my recommendation.

#### sherby

• Jr. Member
• Posts: 22
##### Re: Swinging rod Pivot
« Reply #2 on: January 07, 2014, 10:38:50 AM »
Thanks, I couldnt find much on Knife edge pivots.  Is there another name for it, or somewhere you can point me to look at?

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: Swinging rod Pivot
« Reply #3 on: January 07, 2014, 12:41:21 PM »
Why wouldn't you just buy a small ball bearing (or two) and use that to suspend the of swing?

#### sherby

• Jr. Member
• Posts: 22
##### Re: Swinging rod Pivot
« Reply #4 on: January 07, 2014, 02:27:53 PM »
Hi jwatte, the ball bearings are not available for such small sizes.

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: Swinging rod Pivot
« Reply #5 on: January 07, 2014, 02:34:13 PM »
Sherby, Move the PM to this thread so that others can take a guess on what kind on bearing was used.
It looks like its just a precision, low friction (ball) bearing in the pendulum arm.

I just put "Knife edge pivots" in Google and found lots of info. Oh, and precision Balance scales typically use knife edge bearing (found that from the google search). Also be sure the check out the 'images' from the google search.

Very small Ball bearings are available. You are just looking in the wrong place.
One source I can think of is almost any old CD, floppy or hard drive or small PC fans and if you look around lots of other places.

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: Swinging rod Pivot
« Reply #6 on: January 07, 2014, 04:47:07 PM »
Quote
ball bearings are not available for such small sizes

Think of the ball bearings used in slot cars. They have axles that are 1/16th of an inch or smaller.

I typically get my bearings from VXB. For example, they carry a 1mm inner diameter bearing for \$6: http://www.vxb.com/page/bearings/PROD/1mm/Kit19424
If you want high-quality bearings (ABEC-3/5/whatever) then you'll have to pay more, of course :-)

The draw-back with knife edge bearings is that they typically rely on gravity to keep things "down," so you don't have any stabilizing force vertically. That may not matter to you. More information here: http://goo.gl/Rt26EL
« Last Edit: January 07, 2014, 04:48:41 PM by jwatte »