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Thread: Machinists- press fit tolerance question

  1. #1
    Scootertrash's Avatar
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    Machinists- press fit tolerance question

    What type of tolerance should I have between the shaft OD and the bearing bore ID for a press fit?
    Quote Originally Posted by fabiodriven View Post
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    I’ve always used .002”’to .004”, but I suspect the diameters, materials and application involved play a role in determining exactly what you should use.

    Ironchop is the guy I would ask if it’s a critical application.
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    Thanks for the reply El!

    Not super critical, fan support bearing for a Case 1840/Cummins 390-4 engine.

    What happened was I was having a radiator leak fixed by the local radiator repair shop, so I was performing maintenance, checking other things I had access to with the radiator out, fan bearing was super gritty and rough when spun, so I opted for a replacement. Well, Case doesn't offer a replacement bearing, you have to buy the complete "fan support bearing housing assembly" for the modest price of 410.00, + tax of course.

    I jumped online and after perusing several forums I ordered a replacement bearing. Yesterday I pressed the bearing out of the housing and pressed in the replacement. I got the fan flange/shaft lined up and started pressing it into the bearing and it seemed to be extremely tight. Just a couple more pumps on the press, right? Well, before I started the replacement process I failed to verify the bearing ID and the fan shaft OD for proper fit. I just assumed it was the right bearing After pressing the fan shaft back out of the bearing, I found that I had damaged the fan shaft because the bearing ID was about .040 smaller than it should have been.

    So now my plan is to turn down the fan shaft to remove the damage I created and allow it to fit the bearing I have.

    So as a result I have learned once again, as I have a couple times over my lengthy career of wrenching, never assume, always verify before proceeding........

    I'll post some pics later, and I when I posted I figured if 'chop didn't chime in I'd send him an IM.
    Quote Originally Posted by fabiodriven View Post
    Trick the people into thinking they're enacting their own will and you have willing slaves.

    Liberalism suspends the intellect of its victims, while at the same time tricking them into believing that they're smarter than everyone else.


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  4. #4
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    That is actually a very complicated question with no single correct answer because it depends on many factors like the internal bearing clearance and the id of the bearing and the width of the bearing and the type of use and even the ra (surface roughness) of the shaft or bore the bearing will be pressed on to or in to. If the od is being pressed, it also depends upon the type of material it is being pressed into because different materials expand a different amount with heat etc.

    Below is a press fit calculator that can give you an idea, but for you to get the most accurate info from it, you should at least know the bearings internal clearance, and if it is not an SKF bearing, you can cross reference your bearing to an SKF one that has the same internal bearing clearance and use the SKF number in the calculator.

    http://webtools3.skf.com/BearingCalc...Product.action


    Below is a forum for machinists that you can google search or join to get some info at, however, I can tell you from experience that you can sometimes get several different answers on there to a question leaving you with no real definitive answer at all, and below is one example in a thread about bearing press fit tolerances, lol.

    https://www.practicalmachinist.com/v...uestion-76903/


    There is a section in the machinists handbook for bearing press fit tolerances as well, however, all bearing mfg's (except possibly chinese ones) have recommendations for the press fit of every bearing, so you can often simply call them and ask for their tech line to get some info.


    As far as motorcycle crank bearings go, .001" of interference fit on a shaft that is around 1" in diameter is typical, and is exactly what it is on my 1969 Triumph 250. The original English bearing and the new Kawasaki TECATE...The Most Powerful 3 Wheeler In The World crank bearing I am going to replace it with both have the exact same id.

    .

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    I should add that whenever you're concerned that you're a little loose on fitment copious amounts of green Loctite is great insurance. I'm told there is also a purple Loctite available for these things, but its as permanent as a weld.
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    You kinda waste locktite on a press fit. With no room for the locktite to reside, what can it do? When you press the two parts together, it just pushes the locktite out. ...but locktite is cheap and it's not gonna hurt anyhting either.

    Larger diameters have more bearing surface so you don't need as much interference.
    KNowing what materials you are working with helps. If you get too much you risk cracking if the parts are exposed to any real amount of heat. If the shaft expands at a different rate than the mating part that is.

    .002 should be plenty for most anything. About the only thing it's doing in a bearing application is keeping the bearing from moving. If the bearing is doing it's job there should be very little pressure on it.
    I was born and raised on Venus & I may be here a while.....

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    Quote Originally Posted by Gabriel View Post
    You kinda waste locktite on a press fit. With no room for the locktite to reside, what can it do? When you press the two parts together, it just pushes the locktite out. ...but locktite is cheap and it's not gonna hurt anyhting either.
    A friend of mine engineers repair processes for military helicopters. He’s the one that told me about the use of purple Loctite on inference fits.

    I use it on tapered shafts that don’t use key stock and also on some that do after lapping them with valve grinding compound

    As far as non-tapered applications you have to keep in mind that you can only get away with so many press fits before you lose the tolerance and end up with a big hole, or a small shaft, especially if the guy before you didn’t use heat to open the hole up before he forced in the bearing in. This is especially relevant when pushing steel into aluminum.

    I like to rub a little anti-seize in the edge of my case hole before I heat it and push in a cold bearing. Makes getting it out later easier. However, on a 30 year old 2 stroke you often find that the bearing drops right in and that when some Loctite is a good idea to prevent it from spinning in the cases.

    While we’re on the topic you should always take a really good look at the cases before you bother putting a new bearing in, as every once in a while there is evidence that a bearing has spun on the outer race inside the aluminum. Providing that the bearing is not slapping around in the hole this is a great application for green Loctite and don’t think that just because you have an inference fit that none of the Loctite is making it down into the hole. Under a microscope the shiny surfaces of your bearing and hole look like a mountain range, there is plenty of room for the chemicals to enter and form a bond between the two surfaces.
    Last edited by El Camexican; 05-27-2019 at 01:57 PM.
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    Scootertrash's Avatar
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    Thanks for all the posts guys!!
    Quote Originally Posted by fabiodriven View Post
    Trick the people into thinking they're enacting their own will and you have willing slaves.

    Liberalism suspends the intellect of its victims, while at the same time tricking them into believing that they're smarter than everyone else.


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  9. #9
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    I just made some bushings for a steering shaft and .004 was a really nice slip fit and .002 was a little tight. .001 would have been a nice press fit. This was delrin on a steel shaft mind you so your mileage may vary.

  10. #10
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    I would turn down the shaft to just to where its good enough to get a bearing to go past the damage then bring it to a bearing shop and ask them to size it up for you... then if you must, you can turn the shaft down to fit the closest size to the one you had originally. Is the shaft that bad? can you just turn it down where the damage is and go past it to "Good shaft" with a new stock sized bearing? i would think that would be the way to go.

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    As far as non-tapered applications you have to keep in mind that you can only get away with so many press fits before you lose the tolerance and end up with a big hole, or a small shaft, especially if the guy before you didn’t use heat to open the hole up before he forced in the bearing in. This is especially relevant when pushing steel into aluminum.

    I like to rub a little anti-seize in the edge of my case hole before I heat it and push in a cold bearing. Makes getting it out later easier. However, on a 30 year old 2 stroke you often find that the bearing drops right in and that when some Loctite is a good idea to prevent it from spinning in the cases.
    I kept waiting for the sexual innuendo punchline here but you left me hanging
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    There are many classes of press fit as well....size on size, +.001, +.002 (both of which you would use heat, I never press anything bigger than 'size on size', cold)

    Type of material is also a big factor as something mentioned above

    We are a support/design and build shop for most of the major plants in the areas. About 85% of our work is repair related. I use green Loctite on slightly stretched bearing bores all the time. It absolutely works. I would prefer to machine new parts, but when Smuckers Uncrustables machine spins a bearing and they make 4 million of those little sandwiches in a week, you need a quick fix to keep the line rolling and forty employees in work. True story.

    Isn't Purple Loctite supposed to be "wicking"?
    Last edited by ironchop; 05-30-2019 at 09:42 AM.

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    Quote Originally Posted by ironchop View Post
    Isn't Purple Loctite supposed to be "wicking"?
    Sure looks like it.

    http://henkeladhesivesna.com/blog/th...threadlockers/

    I need to ask my buddy why I understood from him that the purple stuff was so permanent.
    It sucks to get old

  14. #14
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    So what happened was they sent the wrong bearing and I made a lot of assumptions. Usually I'm pretty anal when I wrench, but I got in a hurry and let some things glide by that I normally don't.

    I froze the bearing and pressed the bearing into the cast housing whjilst heating the housing a little. All good. I let everyhting come to room temp while I froze the fan blade mount shaft.n Didn't heat the bearing, I figured freezing the shaft would be sufficient. I start pressing the fan mount shaft into the bearing and it seems too tight, it's not like I haven't done this before I figure just one more pump on the handle, still don't seem right. So I flip it over and press out the shaft. OOOPs! Bearing bore seems a bit small. I measure the bore and it's .040 smaller than the shaft. OOOHhhhhhhhh ffuuuuuudddggge!

    Like I said before, Case does not sell the bearing by itself, It was an NTN Double Row Angular Contact bearing. I talked to some buddies who work on old tractors and they gave me the name of a bearing place about an hour west of me. They cross referenced the bearing and gave me the choice of a Chinese bearing for 10 bucks or National brand for 32 bucks. It looks like the National brand is manufactured in Germany (From the quick search I made) so I went that route.

    Case only sells the complete bearing/bracket assembly---410.00 plus tax
    Amazon had the NTN bearing for 72.00

    I'll throw the other bearing in one of my lockers "just in case"

    I turned down the damage to the shaft, I believe there is plenty of good shaft left for support. I'll just keep an eye ion it for a bit after it's back together.
    Quote Originally Posted by fabiodriven View Post
    Trick the people into thinking they're enacting their own will and you have willing slaves.

    Liberalism suspends the intellect of its victims, while at the same time tricking them into believing that they're smarter than everyone else.


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    Quote Originally Posted by 86T3 View Post
    I kept waiting for the sexual innuendo punchline here but you left me hanging
    There's plenty of sexual innuendo in his post. Subtlety is an art
    Quote Originally Posted by fabiodriven View Post
    Trick the people into thinking they're enacting their own will and you have willing slaves.

    Liberalism suspends the intellect of its victims, while at the same time tricking them into believing that they're smarter than everyone else.


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