# Ultra-Low Friction Pivot Bearing?

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I need to make a pair of very low friction pivots for a measurement
system. I don't need much range of motion, say +/- 20 degrees. It's the
sort of application where a knife edge would work. I've seen reference
to using carbide for knife edge bearings, and was thinking about using
carbide lathe inserts for the "knives". However, that still leaves me
with what sort of material to groove for the knives to rest in. This
supposed to be a "quick and dirty" job, and I'd prefer to avoid having to
heat treat the grooved parts.
I'm basically building a pendulum system for measuring the moment of
inertia of some irregularly shaped objects. The items to be measured
will be suspended just below a pair of pivots, and set swinging. By
knowing the mass and location of the center of gravity, I can estimate
the moment of inertia by measuring the period of the resulting pendulum.
Any drag will damp the swing and screw up my readings. I've thought
about just hanging them by a string, but that seems a bit too crude.
Anyone know what sort of string has the lowest flexing losses?
Any suggestions for the knife edges, or some other sort of low friction
pivot? The things I'm testing weigh several pounds, so the forces on the
pivots aren't tiny, but they aren't enormous either.
Thanks!
Doug White
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Two balls, or a roller from a ball bearing will make a "knife edge" (use epoxy to hold them in locating holes / grooves) to run on a hard flat, perhaps a carbide insert could be used for the flat? You can get precision sapphire balls as used in touch-trigger probes at a price, but rolling element bearings are a relatively cheap source of very hard precision steel parts. Or Google for "flexure pivots".
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Different solutions to your knive: 1.) Use some thin flat spring steel like the ones you find in feeler gauges. The error can be calibrated out.
2.) Depending on the shape, you can also use some length of round spring rod that gets twisted. That method is used when you want to know the inertia along an axis.
3.) Completele different: Re-model the shape in CAD and let it do the math. Accurate and fast if you have a decent CAD.
HTH, Nick
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Although the error will be small, if the pivot moves back & forth due to the rolling action of the bearing, it muddies the waters for the calculations. The inside of a bearing might make a good surface to run the carbide against, however.
The problem with flexure pivots is that now I have a restoring force which is the combination of the pivots spring constant plus gravity. I'm actually planning on building a horizontal torsional pendulum using flexure pivots eventually, but at ~ \$100 each, I want to do some rough tests using gravity first to see what sort of moments I get so I can pick the right strength flexure.
Doug White
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Do-able, but a bit of a pain to do the calibration. I can use a standard bar, but it would be easier if I had a calibrationless system.
Also do-able, but to some extent the calibration then depends on the weight of the object, which complicates things further. This assumes you suspend the things from the spring/rod.
OK for simpler shapes, but the things I want to measure are pretty irregular. I was thinking about learning Solid Works for this, but enterring all but the simplest structures would be a pain.
I'm beginning to think that just stringing the things up with cotton twine may be my best bet...
Thanks!
Doug White
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One classic solution is elastic flexures, which are basically a bit of steel shim stock clamped to the stationary and the moving part, and held in tension. So, instead of being held up by an upward facing knife, the moving part hangs down, suspended by the flexure.
Joe Gwinn
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Do they have to be in grooves?
Traditional scales use flat agate bearings, hard steel knives I think.
If you want a "groove", could you simulate with say 3 more carbide flats in an interleaved V formation?
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That's true of the student analytical balance I have.
Harold
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Consider hanging it from two bits of Spectra line. This stuff is extremely flexible (limp), very little stretch, small diameter per lb of tensile strength. For a few lb I might use a couple of pieces of 30 lb or 50 lb test braided Spectra musky line. 30 lb line is about .011" dia. Two brands are Power Pro, and Tufline from Western Filament.
Also, see
.030" dia spectra has 158 lb breaking strength.
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Hi Doug, Spinning disc type kWH meters sometimes use magnetic bearings to reduce friction. These use the repulsion between two magnets to suspend the disc is space, no contact with anything. You could set up something with magnets to give you near zero friction.
regards,
John
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If you determine that you'd want to use a sharp carbide edge, you can get carbide paint scraper replacement blades (about 3" in length, about 1/2" wide and maybe 1/8" thick), at retail stores (about \$7 at Lowes, or maybe H-Depot).
Richard K had previously recommended these blades as good source of carbide stock for cutting into smaller, machine scraping blades.
WB metalworking projects

Doug White wrote:
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Carbide point on carbide anvil.
Fine nickel-chromium resistance wire, 34 gauge wire is 0.006" thick and holds several pounds. Also good stuff to have for some clever repairs with epoxy like wrapping broken castings or plastic parts.
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Dental floss is strong & flexible. For more strength try snare wire, it's plastic coated multi-strand stainless abour .015 OD and if you try to break it like the thread it looks like it will cut your fingers off first ;)
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Chemical balances used agate, if I remember, for this. So how about a peice of polished granite? ...lew...
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Mine are quartz. (man-made solid quartz, abrasively machined)
LLoyd
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Hardened steel knife-edges on agate is the old-time analytical balance pivot, water-hardening drill rod full-hard should do it for knife-edges. One middle-ages book on assaying recommended using an old sword for knife-edges, high-carbon steel in other words. Take a look at a powder scale for shape and angle. Size to fit the load. You might be able to substitute TC flats for the agate, would have to be polished, though. Fused aluminum oxide "sapphire" or "ruby" blocks and rods used to be available from various supply houses, would be a good substitute for polished agate if they could be found. If you had some method for restraining the knife-edges from twisting in the vertical plane, you wouldn't need to notch your support material, stops or some such.
Some first-year college quantitative analysis books have extensive sections on balance construction and design analysis of same. One thing about torsion pivots on balances is they aren't much good for measuring anything but light weights, as the weight increases, the axis deforms from the ideal straight line and accuracy declines. Probably the same would happen with your variable mass pendulum on a cross wire support.
Stan
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LDI in Minneapolis (laserdesign.com) will scan the part for you with a laser scanner and give you back a CAD model. Not sure of the cost.
If I'm not too far out of line, would you mind explaining how the pedulum gives you moment of intertia? Seems like all you would get from a pendulum set up is mass.
Bob
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Way too expensive for what I have in mind, and some of the objects have hollow bits that won't scan well.
If you know the mass & the distance from the pivot to to the CG, the period of the pendulum is related to the moment of inertia. There are sites with formulas on-line that go through all the details. Basically, for a given mass & distance to CG, the longer the period, the higher the moment of inertia must be. You start by calculating the moment of inertia at the pivot, and then use another formula to translate thsi to the moment about the CG.
Doug White
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That sounds like it would work real well. Time to head for the local tackle shop.
Doug White
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Thanks! That certainly beats the price of most inserts. I've been scoping them out on eBay, and this sounds better (and much simpler) than messing with inserts.
Doug White

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