[Ashish B]

hI

I have a shaft & wanted to suspend them into 2 plain bearings at the end. Functionwise, it is really important to keep the shaft suspended in high accuracy.

My question was what should be the clearence which i should keep between shaft & bearing to achieve a precise performance?

Need Help.

Thanks
Ash

[HuFlungDung]

The rotational speed of the shaft has a significant effect on the desired clearance. As a rule of thumb, I'd suggest that the minimum film thickness for grease or oil would be about .001" and part of this requirement is due to inevitable particle contamination of the lubricated zone. Once some wear particles start to accumulate, they'll quickly lead to seizing of the shaft in the bushing. So that means the bushing should be .0015 to .002" over the shaft size. If you give it even more clearance, it probably does not help, because then the excessive grease or oil simply gets pumped quickly out the end of the bushing because of the loss of capillary action, which is part of what keeps a film intact.

[Ashish B]

Hi

Thanks for the reply.

It is actually a low load application & the load suspended by the shaft is very less. But the accuracy of shaft movement is accurate & precise.

Also i don't prefer any lubrication zone into these assembly because of low loads.

Any idea about clearence in absence of lubrication?

Thanks

[HuFlungDung]

Okay if no rotation is required, then the clearance can be reduced to a close sliding fit or even to a permanent fit with zero clearance. That would depend on how much hassle you encounter trying to slide the shaft through the two bushings, with the bushing brackets already installed. If you can install the brackets on the shaft first, then you could have zero clearance, and heat them slightly, slide them on, and they will cool and grasp the shaft tightly, and the accuracy will be as good as the shaft straightness.

If you must slide the shaft through the bushings, then probably .0005" diametral clearance is a practical minimum for hand fitting. Any clearance will permit some misalignment, of course, but if the seperation between the two bushings is quite far, then the angular misalignment is going to be very small.

[timdgoodpaster]

You wouldn't want the bearing to slip on the shaft. I use .0005 clearance and 680 Loctit. To remove heat till you see just a small bit of smoke and slide the assembly apart.
Tim

[Ashish B]

Thanks all for giving that valueable information...

Hey but whats loctit?

[HuFlungDung]

Loctite is a trade name for an anerobic adhesive. It is used to fill tiny gaps, such as in screw threads, or to fill the clearance space between a bearing or shaft and a housing or hub. It solidifies in the absence of air, permanently locking the parts together by virtue of simulating 100% contact between the two parts.

[timdgoodpaster]

sorry for the spelling
Tim

[Ashish B]

Wonderful,

But does loctite leaves some residual sticky material after evaporation ( or in other words when the liquid loctite 680 gets solidify ) ?
I am eager to know this because this is a common problem while using feviquick.


ALSO I WANTED TO ASK THAT IF I NEED TO ATTACH 2 PARTS ( WEIGHING AROUND 5-10 Grams ) AND IF THESE PARTS ARE SUSPENDED VERTICALLY (without any support), THAN CAN I USE LOCTITE?
I somewhat feel that the gravity force will tend to seprate these 2 parts one or the other day.

Thanks
Ash

[HuFlungDung]

Loctite does not have solvent that evaporates (at least I don't think it does). It needs constant bombardment by oxygen to stay liquid. When the oxygen is kept away, then it solidifies. It is necessary to heat the assembled joint to about 400°F to melt the solidified Loctite and pull it apart while hot. It leaves a fairly messy residue, it may help to wipe quickly while still hot or you will have to scrape the residue off.

I use Loctite 680 extensively in an application where a very smooth stainless steel tube fits in a reamed aluminum socket, 6mm diameter by 16mm deep. The overall length of the tube is 130mm in the application. The clearance of the fit is about .05mm and the parts are cleaned in solvent before final assembly when the Loctite is applied.

In destructive tests we have done, it is impossible to pull the tube (without destruction) out of the hole after the the Loctite has cured. It is equivalent to a light press fit.

I would not use it for a glue, as such, if you are thinking of gluing flat parts together, because of the necessity to keep air away from the joint (except for the edges of the film) until it has cured.

[Ashish B]

Hi,

Thanks for the wonderful respone.

[timdgoodpaster]

We assembles a shaft one inch OD into a piece one inch thick ID. Light press fit. It took five ton to break bond.

Have uesd 680 on a part to EDM Flat piece to table. Hold is very good. Light tap with hammer and part is off.
Tim

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