bushing material choices 1

[Qcprick63]

I have a need for some advice. I have a carbide shaft that needs to side back and forth thru a bushing in a gage we are modifying. The carbide shaft is .375 " diameter. We have built one out of 932 (SAE 660). The shaft has to move pretty freely back and forth but has to be very tight side to side. We honed the bushing to .0001/.00015 clearance to the shaft. It still feels a bit "draggy" back and forth.
If we go bigger, I fear the side to side will be excessive. It's still an R&D project so I was wondering if a different material will be better. After pricing Aluminum bronze(954) it looks really expensive. Any alternate materials you guys can suggest? cast grey iron?

 

[Tmoose]

sliding axially?

Assembled dry?

How was the carbide shaft OD machined?
Can you check the shaft roundness and straightness to less than .0001" ?
Can you measure the the radial play of the shaft inside the bushing at many locations. Shaft slid axially, and rotated in a controlled sequence.


How long is the bushing engaging the shaft axially?
At .0001" diametral clearance the cylindricity of the shaft and bushing would need to be REAL good. down in the sub .0001' range.

Any nick or dent on the sliding surfaces will foul things up.
Dirt or even Honing grit embedded in the bronze bushing will foul things up.
The corner of the chamfer at the ends of the bushing ID needs to blended into the bore.

I'd rub a fine flat stone all around the carbide shaft to identify and knock down any high spots.
Then I'd polish it with a wide strip of 400 or finer grit wet.dry sand paper.

I'd used a stiff bristle brush and soapy water to scrub the bushing ID.

I'd assemble the shaft and bushing with some oil containing EP and anti-wear additives. I'm thinking Synthetic motor or gear oil or Engine assembly lube.
Then rig up a drill press etc to spin the oily shaft inside the bushing while stroking it in and out beyond the working zone for 5 minute.
Them I'd thoroughly solvent clean the shaft and the bushing ID, used a stiff bristle brush and soapy water to scrub the bushing ID.
Dry and assemble with something.

 

[geesaman.d]

I think I would start by searching for the lowest coefficient of dry sliding friction. Alloy steel vs. carbide is your current place.

What's your thermal expansion situation? With clearances that tight, the slightest change in temperature / COE differential will throw everything off.

 

[Qcprick63]

Thermal is actually a bit of an issue. Thats why I suggested possibly going to cast iron.

 

[EdStainless]

Any rotation or only linear motions?
If so the finish on the WC should be lengthwise.
What kind of loads?
Can you use a dry lube? If so go with WS2.
What temp range? CTE is super important with tight clearances.
My first thought is to use a ceramic bearing.
The ceramic bearing can be supported with wave inserts or even o-rings in the housing.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Qcprick63]

There is only axial load on the carbide shaft. It's for use in the shop, so ambient temps. I would prefer no lube if possible. Ceramic would be nice but there is a counter bore with ID threads and a pair of wrench flats on one end of the bushing. We are trying to keep costs at the absolute minimum.

 

[DAVIDSTECKER]

Have you looked at composite bushings? [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1683320326/tips/CIP_General_Engineering_Manual_-_like_Orkot_j1yrdd.pdf[/url]

 

[EdStainless]

So you designed a very complex part, to work with a very expensive shaft, and you want a low-cost solution.
Make the counterbore and thread part out of steel and use a engineered polymer bushing.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[WKTaylor]

The bushing and the shaft have to be made to complimentary/exacting tolerances and surface finishes... especially for 0.00010-to-0.00015 shaft-bushing clearances. I assume that the metals are 'super-fine-finish/roundness... and the shaft is flash-chrome or flash-nickel plated for wear/corrosion resistance??

CAUTION. With the 'isty-bitsy-teensy' clearance-range specified, even small expansion/contraction differences [dissimilar materials] can cause looseness or lock-up.

So-on...


Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Qcprick63]

David Stecker, I have contacted CIP , We are working towards a solution. Thanks!

 

[DAVIDSTECKER]

Qcprick63, You're welcome.
We've been using materials like this for decades and it has performed very well for us, even in direct sun light on the decks of ships.
The folks at CIP are great to work with.

 

[DAVIDSTECKER]

Something else to note, is that shafts can be too polished. A rougher surface can actually hold some of the lubricant in place making the interaction better.
One example is the homing pattern in an engine cylinder.
Another is the "Shark" skin bottom paint used on racing boats.