# How To Fit A Bearing On A Shaft Or A Bore?



## prasad (Aug 4, 2015)

Guys

I am running into difficulty while trying to fit ball bearings. The bearing I have is about 5.5" in diameter. 
I want the bearing to stay in its place once mounted on the shaft or bore but I always end up with it sliding away. As I get close to the desired diameter I reduce the cut to bare minimum steps. The shaft is still not accepting the bearing. So I take one more tiny cut using the same depth setting on my cross slide dial with a hope that it will give me the required diameter. Then I find the shaft is too loose for the bearing. I have the same exact experience in boring too. I dont know how else to do it. One moment it appears too tight and next time it is too loose. When am I going wrong? What is the right method to get it right? I do not want to use loctite to get over the loose sliding fit. 

Thanks
Prasad


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## JimDawson (Aug 4, 2015)

prasad said:


> As I get close to the desired diameter I reduce the cut to bare minimum steps.




There is your main problem.  Take the last few cuts at about 0.010.  Take a cut, measure, take another cut at the same amount, measure again.  How much was actually removed?  That is amount that is also going to be removed on the finish cut if you do everything the same each time.  Consistency is the most important here.

With a little practice, this will allow you to hit +/- 0.0002 every time.  Don't try to sneak up on the dimension.  If needed, take the last 0.0005 with emery cloth or a file. (use good safety practices when using emery or a file on a lathe)
.
.


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## Ulma Doctor (Aug 4, 2015)

i don't want to over-simplify...
if the bearing's id is slightly smaller (.0005") than the shaft, you will have a light press fit and the bearing will not move without encouragement.

something else to consider,
make the shaft to .001" oversize dimension, then freeze the shaft in your freezer.
warm the bearing (unless it has neoprene seals) to a maximum temp of 250*f (for most bearings)
install warm bearing on frozen shaft and allow to equalize
baddabing- a press fit, sans the pressing


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## Firestopper (Aug 5, 2015)

You need to practice your technique and confirm that your dials reflect what you removed. Like Jim stated, the last few cuts should be varified so you know exactly where your at. Other influences can result in a poor fit, such as work piece too hot at time of measuring. If too hot to touch with bare hand, then your measurement will change when piece cools to touch. 
Ulma Doc also bring up a great tip for assembly of interference fitment.  
Machining has and continues to teach me patience.  Adding a DRO to my machine was a game changer.


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## zmotorsports (Aug 5, 2015)

JimDawson said:


> There is your main problem.  Take the last few cuts at about 0.010.  Take a cut, measure, take another cut at the same amount, measure again.  How much was actually removed?  That is amount that is also going to be removed on the finish cut if you do everything the same each time.  Consistency is the most important here.
> 
> With a little practice, this will allow you to hit +/- 0.0002 every time.  Don't try to sneak up on the dimension.  If needed, take the last 0.0005 with emery cloth or a file. (use good safety practices when using emery or a file on a lathe)
> .
> .



^^This.  It takes practice and experience but get used to your machine so you know EXACTLY how much it will actually remove when you get down to the finish cuts.  If I get to the point where I didn't take quite enough and am within a thou or so, I will use a file followed by a piece of emery cloth to "sneak" up on the final dimension.  I will usually take my roughing cuts and leave one or two final cuts of about .010" to do just as Jim recommended.

Also depending on how tight of an interference fit I want, knowing what the actual dimension you want is important as well.  I generally don't like creeping up on the dimension merely by fit and feel.  I will accurately measure the part and then depending on the actual fitment I am after I will calculate and work to an actual dimension.  For light interference fits I shoot for .0003" per inch of diameter.  If I want a good pressd fit I use .0013" per inch.

Maybe others will have difference numbers but those are what I use, can't remember where I came up with them from but they have served me well over the years.

Mike.


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## Kernbigo (Aug 5, 2015)

Mike is write on the amount of press, .005 is to much press


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## prasad (Aug 5, 2015)

Thank you guys, 

Many points learnt by reading your replies. I list them below. 
1. Determine the exact shaft diameter you want to get based on "light interference fit"
2. Take very small cuts 
3. Measure after each cut
4. When it is close to desired diameter go more slow in taking cuts. 
5. Measure but remember the temperature can cause issues. Wait for cool down if necessary. 
6. Never try to insert the bearing by hand - it may not go straight and give you an erroneous feeling that there is more machining needed. In reality the shaft may be ready to take the bearing. Depend on the measurement than you insertion by hand. 
7. When it read for final mounting freeze the shaft and heat the bearing. (I am not sure if opposite of this will work for inserting bearing into the bore. I remember reading somewhere that bore shrinks when heated. Would it still shrink if the bore is about 5.5"?)

Thanks
Prasad


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## John Hasler (Aug 5, 2015)

prasad said:


> Thank you guys,
> 
> Many points learnt by reading your replies. I list them below.
> 1. Determine the exact shaft diameter you want to get based on "light interference fit"
> ...


A bore expands when heated.


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## RJSakowski (Aug 5, 2015)

prasad said:


> Guys
> 
> I am running into difficulty while trying to fit ball bearings. The bearing I have is about 5.5" in diameter.
> I want the bearing to stay in its place once mounted on the shaft or bore but I always end up with it sliding away. As I get close to the desired diameter I reduce the cut to bare minimum steps. The shaft is still not accepting the bearing. So I take one more tiny cut using the same depth setting on my cross slide dial with a hope that it will give me the required diameter. Then I find the shaft is too loose for the bearing. I have the same exact experience in boring too. I dont know how else to do it. One moment it appears too tight and next time it is too loose. When am I going wrong? What is the right method to get it right? I do not want to use loctite to get over the loose sliding fit.
> ...



I don't believe that anyone mentioned making a spring cut yet.  Any time you turn a piece you will experience deflection of the tool and/or workpiece.  The amount of deflection depends upon the depth of cut, the relation of the turned length of the workpiece to its diameter, additional support (tailstock center, follower, etc.), workpiece material, and tool geometry and sharpness.

When you get close to your final dimension, make a second (spring) cut at the same setting.  Since the tool loading is lighter, the amount of deflection or spring is less and you will remove a small amount of additional material.  This will often bring you in to your final dimension.  If not, then make a slight adjustment and repeat.

Taking a .01" depth of cut and getting close to the point where you only have a thousandth or so to go and making an adjustment without a spring cut often will give you an undersized piece.  Every situation is different.  After many iterations, you can develop a sense  of the expected behavior.  My routine for critical diameters is to stop short by a a suitable amount and make a trial finishing pass.  That will tell me how the turning is behaving for a given situation.  It does take a little practice to dial it in.  

Bob


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## hanermo2 (Aug 5, 2015)

Lots of good advice.
There are 2 schools of thought .. and both are correct.

Spring cuts, ie advance nothing, will usually take off minuscule amounts of material in the 0.01 mm - 0.0005" range.
Will work well, and can get you there when you should be at the correct size, and it more/less measures correctly, but "it just wont fit".

The other option.
Take off about 0.1 (-0.06mm) / 0.004-0.005"  as the minimum amount.
Calculate your near- to- last cuts, so that the last 2-3 are about this amount.

Both work, both are correct.

What happens is that everything has slop, ie minuscule clearance, unless its preloaded.
EVERYTHING bends.
EVERYTHING yields.
You add up all 3 and thats your wobble/slop/backlash. It contributes to all three.

On small lathes, the slop is usually 0.01 mm (-0.03) mm.
BUT,they easily bend about 0.05 mm (or more) with the small loads of finishing cuts. About as much as one-finger pressure .. circa 10 kg.
(A 1000 kg bridgeport will bend near/about 0.04 mm with one finger).

When doing spring cuts, the parts settle into the middle range, and typically keep taking off a little, for 2-3 passes.
When doing calculated cuts, the error is already added into the results. This error is very consistent (to less than 0.01 mm variance).

On really good industrial lathes, or cnc lathes, where there is no slop (much less than 0.01 mm), and very little bend, other techniques can be used.
There, the dials will actually tell you where the controlled point is, and you can just advance to that point, and you are done.
They are extremely consistent.


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## zmotorsports (Aug 5, 2015)

Good advice from above but everyone's machines are different to some degree, even machines of the same make/model will have their own personalities.  This is why I try to encourage people to get to know YOUR machine.  Only you will know how your machine reacts to a spring cut after just taking off .020" vs. a spring cut of removing .050" pass. 

Mike.


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## Jeff White (Aug 5, 2015)

prasad said:


> Thank you guys,
> 
> Many points learnt by reading your replies. I list them below.
> 1. Determine the exact shaft diameter you want to get based on "light interference fit"
> ...



On your Item #7    Actually    when heated,   holes   Dia  gets smaller      .........   Rings     Dia gets bigger.      It kind of depends on the direction of  least resistance to all the material expanding.    Also,   I would not heat a bearing very much.


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## RJSakowski (Aug 5, 2015)

Jeff White said:


> On your Item #7    Actually    when heated,   holes   Dia  gets smaller      .........   Rings     Dia gets bigger.      It kind of depends on the direction of  least resistance to all the material expanding.


This has been a heated discussion many times in the past. Almost all materials increase in length when heated, i.e, a positive thermal expansion coefficient.  If what you say is true, a steel shaft in a steel plate which had a tight slip fit would seize up if the assembly were heated.  It doesn't happen.  Another way of looking at it is to heat a solid plate and bore a hole.  The plate had expanded outward as a solid.  when the plate cools down, it will shrink and the hole with it.

Bob


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## Andy Rafferty (Aug 5, 2015)

I read many excellent points, I can only share one or two more. When you are turning your shaft heat builds in the part not a lot but trust me it does. When you have snuck up on your measurement, before you take a final cut shut the lathe down and step away for a sweet tea. give the work time to cool off and measure again. I always do this on close stuff and I sometimes don't need another pass. The second suggestion is use a chip brush or clean paper towel to clean /admire the finish not your hand. The oil in your hand and the heat sink properties can skew your results including an odd cut on your next pass. Just my .02 cents worth. yrmv


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## Bill C. (Aug 5, 2015)

A lot of good advise. Sometimes you need to stop and walk away for a few minutes.


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## chips&more (Aug 5, 2015)

A tooled finish from a HM lathe is almost never as good as a ground finish. I would get to within say 2 tenths of actual and try and burnish the finish to size. If the fit is a little loose, then try a Loctite bearing mount product or similar…Good Luck, Dave.


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## Andre (Aug 5, 2015)

When fitting bearings I like using a fine (Smooth) file and scotchbrite to polish the shaft/bore. Heat shrinking is also a miracle!


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## martik777 (Sep 3, 2015)

You could advance with your compound (instead of the crossfeed) set at various angles  to reduce the amount taken off, for example, at 5.7°  .001 on the compound will move the tool bit in  .0001


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## stupoty (Sep 3, 2015)

I find press fit bearings at the smaller diamitors are realy hard to hit.  I spent ages boreing some holes for 10mm od bearings, ahhh 

Stuart


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## T Bredehoft (Sep 3, 2015)

Steel changes size in the following way, there is .0000066 expansion/contraction for each degree inch of diameter. 10º on a 10" shaft equals .00066 inches. 100º equals .0066 inches.  When you're talking about a foot, that's a lot, when  you're talking about 10mm, not so much.


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## RJSakowski (Sep 3, 2015)

martik777 said:


> You could advance with your compound (instead of the cross-feed) set at various angles  to reduce the amount taken off, for example, at 5.7°  .001 on the compound will move the tool bit in  .0001


+1 on a great old idea.

Just make sure that it is set at 5.7 deg. to the spindle and not to the ways.  i.e., if your compound reads 0 deg. when the compound advances parallel to the cross feed, set at 84.3 deg.; if it reads 90 deg., set at 5.7 deg.

Bob


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## Bill Gruby (Sep 3, 2015)

When using the heat and cold method of mounting  a bearing on a shaft you HEAT THE BEARING and COOL THE SHAFT. I do this all the time mounting bearings on crank shafts in Model Engine Repair. I have a toaster oven for the bearing and a small freezer for crank shafts.

 "Billy G"


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## Bill C. (Sep 3, 2015)

I remember our shop made steel wheels with brass tires. Those who put them together would freeze the wheels and heat the tires causing a hard press fit when they cooled. I remember grinding the pressed parts flat on a surface grinder. Then someone else probably stacked them together and took a skim cut to make all of them perpendicular. We must have made 100s over the years. The were used in seam welders to weld appliance cabinets.


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## Downunder Bob (Apr 2, 2017)

prasad said:


> Guys
> 
> I am running into difficulty while trying to fit ball bearings. The bearing I have is about 5.5" in diameter.
> I want the bearing to stay in its place once mounted on the shaft or bore but I always end up with it sliding away. As I get close to the desired diameter I reduce the cut to bare minimum steps. The shaft is still not accepting the bearing. So I take one more tiny cut using the same depth setting on my cross slide dial with a hope that it will give me the required diameter. Then I find the shaft is too loose for the bearing. I have the same exact experience in boring too. I dont know how else to do it. One moment it appears too tight and next time it is too loose. When am I going wrong? What is the right method to get it right? I do not want to use loctite to get over the loose sliding fit.
> ...



Generally bearings are fitted with either the shaft or the housing an interference fit. The most common approach is if the shaft is rotating then the that will be the interference fit, and generally 0.001 per inch of dia. 

The housing should be a firm sliding fit, and the bearing held in place with a circlip or a plate held on by screws. leaving a small end float clearance, at least on one end of the shaft. to allow for thermal expansion, sometimes a spring might be used to keep it loaded in one direction.

In the case where the housing is rotating then the interference fit should be between the outer race and the housing, otherwise it's the same procedure. 

When fitting an interference fit bearing onto a shaft, do NOT bash it on, the preferred method is to heat bearing in good machine oil to about 70c or 160f if the bearing has rubber seals consult bearing supplier for max temperature. You may also cool the shaft if possible.

If heating is not possible you can press it on, with a driving sleeve on the inner race only, also use plenty of good machine oil on the shaft to prevent scoring. If you have no alternative but a hammer make up a sleeve to fit over the shaft and drive on the inner race only. 

When fitting to a housing the same principals apply except you must only apply pressure to the outer race. Never apply axial pressure to a ball race that will be transferred though the balls.


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## Kernbigo (Apr 2, 2017)

I worked machine repair rebuilding high speed spindles, and always used a heat lamp to heat up the bearings to drop on the shaft.


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## Hellkell (Apr 2, 2017)

bobshobby said:


> In the case where the housing is rotating then the interference fit should be between the outer race and the housing, otherwise it's the same procedure.



Can you explain why you want to do this?  I thought it would be the same as with a shaft moving because the bearing would expand from the heat and tighten itself up in the housing. 

Thank you.


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## Hellkell (Apr 2, 2017)

bobshobby said:


> In the case where the housing is rotating then the interference fit should be between the outer race and the housing, otherwise it's the same procedure.



Can you explain why you want to do this?  I thought it would be the same as with a shaft moving because the bearing would expand from the heat and tighten itself up in the housing. 

Thank you.


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## Downunder Bob (Apr 2, 2017)

Hellkell said:


> Can you explain why you want to do this?  I thought it would be the same as with a shaft moving because the bearing would expand from the heat and tighten itself up in the housing.
> 
> Thank you.



It is common engineering practice, I thought I had explained it, what bit do you not understand?


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## Downunder Bob (Apr 2, 2017)

Kernbigo said:


> I worked machine repair rebuilding high speed spindles, and always used a heat lamp to heat up the bearings to drop on the shaft.



Yes that will work, any process that gives a gentle even heat is ok. Do NOT use an oxy torch. Some people put them in an oven and set the thermostat, that is ok. 

As a marine engineer where we wanted maximum service life under extreme conditions we always kept a tank of light machine oil fitted with an electric element and thermostat, bearings were put in the tank when we started pulling a machine down so they were ready to simply drop on at the right time.

Most of the time when that machine was due for an overhaul after five years, the bearings were still fine, but we always replaced them anyway. Bearings are cheap compared to a breakdown and the labor to replace them. 

Sometimes the replacement bearings for very special highly rated equipment would come in a sealed tin that was already full of the oil; that they would run in, much like a large jam tin, special heating instructions were printed on the label. You've gotta believe it, if they weren't followed, then premature failure was very high on the cards.


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## Hellkell (Apr 2, 2017)

bobshobby said:


> It is common engineering practice, I thought I had explained it, what bit do you not understand?



Maybe you're misunderstanding my question. I was talking about the bearing in use heating up and expanding not heating it in oil to install it.  

I am wondering why in both cases you wouldn't have a tight fit on the shaft so as the bearing heated up from use it would expand and tighten up inside the housing. Seems no different to me whichever race is moving or stationary. 

Just wondering the theory behind it. Thank you.


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## Downunder Bob (Apr 2, 2017)

Hellkell said:


> Maybe you're misunderstanding my question. I was talking about the bearing in use heating up and expanding not heating it in oil to install it.
> 
> I am wondering why in both cases you wouldn't have a tight fit on the shaft so as the bearing heated up from use it would expand and tighten up inside the housing. Seems no different to me whichever race is moving or stationary.
> 
> Just wondering the theory behind it. Thank you.



The theory is based on the idea that with a rotating shaft then the inner race becomes part of the shaft. In the case of a wheel then the outer race is part of the wheel. It's common engineering practice.


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## fitterman1 (Apr 2, 2017)

@ Hellkell - The bearing in use will heat up and expand, and so will every other part in contact with it. This maintains equilibrium in the assembly during operation. That's the whole point of bearing fits.
Also I haven't seen this mentioned either - that if you leave your measuring tools on a shelf with sun streaming through a window onto them or some other heat source nearby, your tools will measure your work undersize because they are in an expanded state. The error may be minor but for bearings its crucial in a high speed environment. Keeping your instruments at an ambient temperature and recognising the fact your workpiece is different and will require stabilizing is a sign of experience and the tighter the tolerance the more vigilant you have to be.
No good in a machineshop where you turn 10 shafts for one that is correct.
Even I failed to notice the age of this thread, hopefully there will be lessons learnt.


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## gheumann (Apr 3, 2017)

Prologue - the most recent post brought this thread to my attention. I didn't see the original post was many months ago. But I'll leave my answer here in case it helps someone down the road.

You certainly can't do an accurate press-fit sizing by feel. The bearing won't go on at all until the shaft is already too small. The heating and cooling method is good to know but I wonder if you're even attempting to get to a known diameter by measuring as opposed to "by feel". You can't use a digital calipers that's reads to  .0005 and is only accurate to .001" for this. You need to use a micrometer. And on that topic while I own the classic $150+ Mitutoyo digital calipers I just bought an "iGaging" 0-1" digital micrometer for $40 on Amazon to see how good it is. Know what? It is worth the $$. It takes a little more care to measure accurately but I measured several different gage blocks many times and feel its real world accuracy is good to about +/-.00015", good enough for this job and good enough for most of the jobs I do.

When I'm sizing a bore or a shaft for a press fit I always try to stage my passes so that the last 3 passes are .005" each. Heavier cuts will bend the tool and work away and the diameter from one end to the other will vary. Never count on being able to take .001 or less. The spring cut is about the only way I know to do that - so if you CAN stop and measure your final diameter BEFORE moving the tool, do. If it is the right size, retract the tool. It is slightly over, start back up and take the spring cut.

But back to my original intent - if you want to size stuff for press fits you need accuracy to about .001" - and so you need measuring tools that measure to .0001" or you're going to be fighting measuring error every time.


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## jamby (Apr 3, 2017)

Just a random thought but if you are using high speed steel or carbide to cut with they take very different spring cuts.  Carbide cuts with a lot more pressure and will take a deeper spring that high speed.   I've had problems with carbide spring cuts tearing the surface and giving more that expected removal.  Also carbide will some times leave a harder surface.   I prefer to use high speed for close fits.

Jim


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## fitterman1 (Apr 3, 2017)

Jim, I like to use CCGT tips designed for Aluminium, regardless of material (in unhardened state). They have nice sharp positive rake and give an excellent finish even on stainless. Very good for finishing cuts down to 001"


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## Tozguy (Apr 3, 2017)

prasad said:


> I remember reading somewhere that bore shrinks when heated. Would it still shrink if the bore is about 5.5"?)
> Prasad





John Hasler said:


> A bore expands when heated.



I believe the confusion comes from a specific situation like when the bore is in a large casting or hub. If you heat the bore with a torch from the inside the metal immediately surrounding the bore will expand. But because the metal farther out is still cooler, then the expanding metal has no place to go but inwards, making the bore temporarily smaller. But if the whole part is heated evenly or from the outside, the bore will expand.


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## FanMan (Apr 3, 2017)

bobshobby said:


> The theory is based on the idea that with a rotating shaft then the inner race becomes part of the shaft. In the case of a wheel then the outer race is part of the wheel. It's common engineering practice.



Correct.  To visualize it better, picture if the bearing is a press fit on the shaft and very loose in the hub.  As the hub rotates, the bearing would roll around inside the hub.  Now picture it the other way, tight in the hub and loose on the shaft.  The bearing won't move relative to the hub, and since the load direction isn't changing relative to the shaft, it won't move there either.

For a rotating shaft in a fixed hub, opposite fits, same principle.

Another aspect of it is that bearings are designed with a certain amount of internal clearance, which closes up slightly (and predictably) when the bearing is pressed onto the shaft or into the hub.  If you had a press fit on both sides, it would close up too much and possibly bind.  If a press fit in both is necessary, bearings with increased internal clearance are available by special order.

I'm too much of a machining novice to advise on _how_ to achieve the required fit, but as an engineer who designs machines and specifies bearings I will say that the bearing manufacturers publish tables of recommended fits and tolerances for their bearings, and those fits will give good results in almost all situations.


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## Downunder Bob (Apr 3, 2017)

FanMan said:


> Correct.  To visualize it better, picture if the bearing is a press fit on the shaft and very loose in the hub.  As the hub rotates, the bearing would roll around inside the hub.  Now picture it the other way, tight in the hub and loose on the shaft.  The bearing won't move relative to the hub, and since the load direction isn't changing relative to the shaft, it won't move there either.
> 
> For a rotating shaft in a fixed hub, opposite fits, same principle.
> 
> ...



Excellent advice from FanMan. For more advanced information on bearing fits, interference and clearances email a major bearing manufacturer, I'd be very surprised if they didn't send you some literature on the subject. Also google the question "Ball race interference and clearances.


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