Why 0.002" clearance ?

[Ian S C]

Jeff, Loctite, depends where you get it
                                                  Ian S C

[Brian Rupnow]

I use Loctite a lot. It works extremely well, and as pointed out, is horribly expensive. The only saving grace is that a small bottle of Loctite 1.69 ounce (50ml) for which I pay $70 Canadian lasts for a loooooong time. However, they recommend that you don't use it after 2 years as it loses its effectiveness after this time.---Brian

[steamer]

Additionally, Loctite needs some clearance to cure and bond effectively.   if it's a tight fit, you just push the loctite out.


On a 9" motor shaft ( 14 horsepower machine tool drive)  , the required clearance was .002" per side or .004" on diameter.   Strong enought to shear the shaft off too.

Dave

[moerman]

I found a nice piece about clearance on the model Engine News website: http://modelenginenews.org/~modeng74/ed.2006.08.html#tt. Half way the article there is a part about clearances. I don't understand the calculation he does though. Is the notation of the calculation wrong? Maybe the  = 0.00005 should be left out? That will make it:
0.0625 + 0.0005 + (0.0625 * 0.00075) = 0.06305 which makes sense I think.

[mechman48]

Hi Moerman

Have a look at the pocket bible 'Zeus' book; it gives a good couple of pages on limits & fits according to BS4500-1969, the diagrams give a good indication of how the holes & shafts relate to each other so you can formulate a picture in your mind     I, as many others, many, many, moons ago during my apprenticeship, was taught to work to 1 thou' per shaft diameter for all general engineering tolerances if nothing was specified on dwgs... I still do it now where I can on my stuff as most of the drawings I have do not specify a tolerance (& where I can still see it...  ), just enjoy.

George

[RonGinger]

However, they recommend that you don't use it after 2 years as it loses its effectiveness after this time.---Brian

We had a Loctite sales engineer speak to a meeting of the New England Model Engineering Society. He was asked about the life and said they did not think it had any limitation, but the government purchase specs insisted on an expiration date on every product, so they labeled it as 2 years, the longest time the spec allowed. As far as he was concerned there was no limit. I have had a bottle much longer than 2 years and  it still seems fine.

[Dave Otto]

Same here Ron,

We had a Loctite Sales rep tell us pretty much the same thing; he said if it still comes out of the bottle it will work. I would bet some of my stock is over 20 years old.

Dave

[Brian Rupnow]

Dave, that is good to know. I have a bottle of Loctite #648, which is labeled as "press fit" which is about 5 years old. I use it all the time, but I wasn't sure if it was any good or not.---Brian

[Meldonmech]

   

     Another unmentioned 0.002" clearance was the backlash between two mating gears.

                                                                   Cheers David

[Alan Haisley]

It's obvious.    .002" is the thickness of doubled up cigarette paper.  Wrap a shaft without overlap and see if it will fit. If it fits without the paper but not with the paper you know that you got it right.

Alan

[Brian Rupnow]

Alan--That's the best answer so far!!--Brian

[moerman]

0.002" is the thickness of doubled up cigarette paper.  Wrap a shaft without overlap and see if it will fit. If it fits without the paper but not with the paper you know that you got it right.
Alan

  I like that one too! It makes me wonder, is this cigarette paper test something which is used in real life engineering?

Wout

[fumopuc]

Hi Wout, this method was also always a good edge finder. If you stick a piece of cigarette paper on you work piece, with some spittle and than  the running cutter will mop it away without making some scratches in the work piece, than you wil be very close to the point you want to be.

[Jo]

I like that one too! It makes me wonder, is this cigarette paper test something which is used in real life engineering?

Yes, in the old days 

Jo

[moerman]

Slowly I am getting a better fit between the shaft and the bearing for my engine, but I think it still is more than 50 µm. I have a brass tube with a 3,85 mm ID and I am turning a steel shaft to fit. I encounter a few difficulties:

- It is hard to measure the ID of the brass. I now use the turned steel as a measuring device and turn a slightly bigger OD each time. I use an old iron nail as material, so the material costs are negligible and it is good to gain experience. But each time I get an almost perfect fit the brass seems to get wider if I move it around a bit. The ID at the ends is probably less than further down in the tube, caused by parting the tube. I don't have a ID micrometer and won't have one anytime soon. What is the best way to do this? Drill a hole with a known diameter drill and copy this diameter for the shaft?

- If I have turned to the right diameter the surface is still a bit rough. Polishing or lapping reduces the diameter and spoils the good fit. So I have to learn how much the OD is affected by polishing or lapping the shaft. Are there any pointers or rules of thumb?
 
- I used the compound slide to move the cutter and the shaft was tapered. I checked the squareness and found it was a few degrees of. Very easy fix. But now I started using the carriage for sideway movement and I think that method is better. How is your opinion?

Wout