I need some guidance in determining whether or not I need to lock a shaft in a bearing. We have a bearing block that has two deep groove ball bearings installed (one on either side). This block houses a 12mm shaft. The shaft is positioned in place by securing a collar on both sides of the shaft. The shaft rotates at 1000 rpm...
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When is it necessary to Loctite a shaft in a bearing 1
- Thread starter tlyter
- Start date
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1
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EnglishMuffin
Mechanical
It is usual to arrange for a shaft that sees a rotating load to be connected to its respective race in such a way that relative motion is prevented, otherwise fretting can occur. There are various ways of doing this. The most common is to follow the manufacturers guidelines for race fit up. It will usually be found that they recommend some type of transition fit, which means that some clearance could occur. At low speeds, this usually seems to work. At high speeds, (which you don't really have), I have found that a definite interference is preferable (otherwise the race can separate from the shaft due to centrifugal force). Recommendations in manufacturers catalogs are usually inadequate in this respect. When you use interfernce fits, it modifies the internal geometry of the bearing, and this must always be born in mind - with deep groove ball bearings you are stuck with whatever internal clearance the manufacturer has provided. Another method of preventing relative rotation is to axially clamp the bearing race (with a locknut, for example). Again - I have found that this is not always sufficient to prevent fretting at high speeds. It is not clear whether your "collar" has axial clamping capability. Regarding Loctite, I personally would only do this in an emergency, but if combined with the correct fit, it would certainly prevent fretting. The problem may come when you need to disassemble it.
LOCTITE is generaly not recommended by bearing companies as we suggest that you get hings like size and concentricity right in the forst place. However, in a maintenance (i.e. real world) situation it often helps get a machine running quickly.
You need to determine the load condition, i.e. which ring sees the load rotating. That will be the ring that requires a tighter fit. e.g. standard electric motor running a V-belt system - the inner ring see the load rotaing. Outer ring sees a stationary load. Hence the tight fit on the shaft.
If you are geting creep, then you don't have enough interference.
Internal clearance will be modified, depending on the degree of interference, butyou can overcome that easily by using a C3 or even C4 bearing.
Lester Milton
Telford, Shropshire, UK
You need to determine the load condition, i.e. which ring sees the load rotating. That will be the ring that requires a tighter fit. e.g. standard electric motor running a V-belt system - the inner ring see the load rotaing. Outer ring sees a stationary load. Hence the tight fit on the shaft.
If you are geting creep, then you don't have enough interference.
Internal clearance will be modified, depending on the degree of interference, butyou can overcome that easily by using a C3 or even C4 bearing.
Lester Milton
Telford, Shropshire, UK
EnglishMuffin
Mechanical
The only thing about bearings with extra clearance is that you can't always get them on short delivery (at least in my experience).
It's always difficult to know how to respond to a comment like that, but in all honesty if you can't get at least a good selection of deep groove ball bearings in standard or C3 clearance, sealed, shielded, grooved or otherwise modified ball bearings off the shelf, change your supplier.
And if they are critical to your plant stock them in your stores.
Lester Milton
Telford, Shropshire, UK
And if they are critical to your plant stock them in your stores.
Lester Milton
Telford, Shropshire, UK
EnglishMuffin
Mechanical
Don't take it personally - I've probably just been unlucky. There are so many bearing types being made today, with more being developed all the time, that to expect to get them all with three or four internal clearance grades "off the shelf" is probably unrealistic. On the subject of bearing fits, there is another thing that has happened to me on occasion. When using tight inner race fits, I have had races snap in two due to the high hoop stress. In one case, it happened in service, and in the other, at assembly. They were SKF bearings, and although the fits were tighter than standard, the hoop stresses were not excessive, at least theoretically (maybe around 15000 lbf/in^2). In both cases, it only happened once, and never occured with the replacement bearings. With rock hard 52100 steel, it only takes a small surface imperfection to cause this to happen.
electricpete
Electrical
C3 fit is pretty much the standard for most electric motor bearings.
electricpete
Electrical
I'm sorry, bad terminology on my part.... C3 internal clearance is pretty much the standard for electric motros.
No offence taken, believe me.
CM now becoming a 'standard' for electric motors, especially ones that have to run quietly. This is a clearance range somewhere between normal and C3; i.e. slightly tighter on average than C3.
Lester Milton
Telford, Shropshire, UK
CM now becoming a 'standard' for electric motors, especially ones that have to run quietly. This is a clearance range somewhere between normal and C3; i.e. slightly tighter on average than C3.
Lester Milton
Telford, Shropshire, UK
- Thread starter
- #10
I appreciate all the input! We have radial loads on both sides of the shaft...which I believe will not allow for relative motion of the shaft in the bearing. I personally think the loctiting the shafts is extreme overkill, but I have been unable to convince our operations manager. Hopefully your input will give me a winning edge in the debate!
EnglishMuffin
Mechanical
What is wrong with Loctite :
1. If you have a tight fit (which ideally you should) and press the bearing on to the shaft there will be no clearance for the Loctite
2. There is always a danger that you might get Loctite in the bearing
3. Bearing removal may require heating or excessive force
4. Before Loctite was invented, bearings were successfully installed without it
5. No bearing manufacturer recommends it, as far as I know.
(This does not necessarily mean anything - but could give them an excuse if anything goes wrong)
1. If you have a tight fit (which ideally you should) and press the bearing on to the shaft there will be no clearance for the Loctite
2. There is always a danger that you might get Loctite in the bearing
3. Bearing removal may require heating or excessive force
4. Before Loctite was invented, bearings were successfully installed without it
5. No bearing manufacturer recommends it, as far as I know.
(This does not necessarily mean anything - but could give them an excuse if anything goes wrong)
Ah yes, that word ideally again. I went down and asked one of our older fitters about why he used the stuff when he could have tight fits. He made some remarks about tight fits that I can't print but then went on and said "If at 2 o'clock in the morning I find out the tight fit is not quite a tight fit I put a drop of loctite on and it never gives any trouble. If I don't it probably will come back and bite me." Now I have never ever met a fitter who would admit he was wrong but there seems some pragmatic logic here.
My work area is absoloutely not a perfect world
My work area is absoloutely not a perfect world
diamondjim
Mechanical
I orginally mentioned loctite several months
ago and was wondering if anyone recommended it
to keep the bore or od from creeping around
the circumference. I do not know if creep is
a bad thing on is expected in all applications.
Maybe with a press fit, this does not happen.
ago and was wondering if anyone recommended it
to keep the bore or od from creeping around
the circumference. I do not know if creep is
a bad thing on is expected in all applications.
Maybe with a press fit, this does not happen.
EnglishMuffin
Mechanical
Diamondjim : If you have clearance between the bearing and the member that sees a rotating load, the bearing race may creep around as you say. It will not necessarily do so, however, because the races are often axially clamped. But even when they are axially clamped, you can still get fretting - which with steel parts looks like rust, as you probably know. I have noticed this often on machine tool spindles disassembled for maintainance - even German built ones. The funny thing is that you don't see fretting on the spacer faces - probably because the spacers vibrate radially with the bearing because they are long enough to flex. Does this really matter? It bothers me when I see it, but in all honesty I can't say I have ever seen a case where it was unequivocally responsible for a failure. If the rust particles can get into the oil, it will certainly shorten the bearing life, but in the case of machine tool spindles just mentioned, the rust particles seem to remain trapped. In the case of todays machine tool spindles, they often rotate so fast that centrifugal force can eliminate the initial interference fit, assuming you have one.
rnd2 : You are obviously a Brit, since you said "fitters". In the UK, (except in mass production), it still seems to be tacitly assumed that things will have to be adjusted at assembly, just as they were in the days before interchangeability was developed in the US. But in the US, there are only "assemblers" - a rather different mind set. Now if you look at the last sentence of my very first post, right after the initial question, you will see that I basically don't totally disagree with this "fitter" that you mention, if you interpret his being confronted with an undersized shaft as "an emergency". This fitter of yours brings to mind my very first undergraduate summer job in a machine tool company in the UK (it was a subsidiary of a US company and has long been defunct). Being one of the "lowest of the low", I was put to work helping a fitter, whose job was to assemble gearboxes for a line of large machine tools. This guy had been a plumber in his previous job, and I later came to recognize that some of his practices were outrageous, although I didn't know any better then. He hand polished the bearing location diameter of nearly every precision ground shaft that went into a gearbox, until the bearings would just slide on (he didn't "approve" of tight fits either). It wasn't long before he had me doing this for him, and of course I went a bit too far in some cases, being relatively unskilled. These shafts were quite complicated and expensive, so when this happened, to save my butt, he would furtively look around the shop to see if anyone was looking, and then put three pricks with a center punch on the shaft surface so that the bearing would appear to be tight. I have often wondered what happened to those gearboxes! We did have Loctite in those days, but it was relatively new then and wasn't quite "acceptable". If he had used it at that time, and it had been discovered, we would both have been in big trouble. But it would certainly have been better than the prick punches, except for the problem of disassembly.
rnd2 : You are obviously a Brit, since you said "fitters". In the UK, (except in mass production), it still seems to be tacitly assumed that things will have to be adjusted at assembly, just as they were in the days before interchangeability was developed in the US. But in the US, there are only "assemblers" - a rather different mind set. Now if you look at the last sentence of my very first post, right after the initial question, you will see that I basically don't totally disagree with this "fitter" that you mention, if you interpret his being confronted with an undersized shaft as "an emergency". This fitter of yours brings to mind my very first undergraduate summer job in a machine tool company in the UK (it was a subsidiary of a US company and has long been defunct). Being one of the "lowest of the low", I was put to work helping a fitter, whose job was to assemble gearboxes for a line of large machine tools. This guy had been a plumber in his previous job, and I later came to recognize that some of his practices were outrageous, although I didn't know any better then. He hand polished the bearing location diameter of nearly every precision ground shaft that went into a gearbox, until the bearings would just slide on (he didn't "approve" of tight fits either). It wasn't long before he had me doing this for him, and of course I went a bit too far in some cases, being relatively unskilled. These shafts were quite complicated and expensive, so when this happened, to save my butt, he would furtively look around the shop to see if anyone was looking, and then put three pricks with a center punch on the shaft surface so that the bearing would appear to be tight. I have often wondered what happened to those gearboxes! We did have Loctite in those days, but it was relatively new then and wasn't quite "acceptable". If he had used it at that time, and it had been discovered, we would both have been in big trouble. But it would certainly have been better than the prick punches, except for the problem of disassembly.
EnglishMuffin
Close, but no cigar. We have fitters in Australia too.
I now understand the remedy for removing a loctited whatever is to capitalise on two things:
1) Over 75 deg.C bond strength reduces significantly and is virually non-existant above 100 deg.C.
2) Loctite apparently has low impact resistance.
Applying this logic suggests increments of increased heating and belting err bashing err tapping til it comes off.
Close, but no cigar. We have fitters in Australia too.
I now understand the remedy for removing a loctited whatever is to capitalise on two things:
1) Over 75 deg.C bond strength reduces significantly and is virually non-existant above 100 deg.C.
2) Loctite apparently has low impact resistance.
Applying this logic suggests increments of increased heating and belting err bashing err tapping til it comes off.
EnglishMuffin
Mechanical
rnd2: I had come across that actually - I was in Australia last year and discovered it then. Sorry I forgot. They probably have them in the Falkland Islands and parts of Africa and India too. As you say, heating is probably necessary for removal. But in many cases in an assembly, in order to get the temperature high enough at the inner race you would have to get the temperature somewhere else extremely high, and this could easily cause damage. Some bearings have plastic cages which can only stand about 220 deg F, so in this case if you are disassembling to get at some other part you could wipe out one or more perfectly good bearings.
diamondjim
The rolling components are supposed to roll, and the stationery components are not supposed to move. Anything that keeps it that way and does not impede the rolling components has got to be beneficial.
As far as Loctite goes there seems to be two mind sets:
Don't use it: It's not required if the job is perfectly engineered (OEM) and later the stuffed bearing will be easier to replace. When it is time to replace the bearing heat is'nt necessary so it is entirely possible to save other bearings that may be in the train.
Use it: It is required if the job is'nt quite perfectly engineered (maintenence) and the stuffed bearing and all the others in the train are going to be replaced anyway because its not worth risking leaving old bearings in the train.
Hope this helps
The rolling components are supposed to roll, and the stationery components are not supposed to move. Anything that keeps it that way and does not impede the rolling components has got to be beneficial.
As far as Loctite goes there seems to be two mind sets:
Don't use it: It's not required if the job is perfectly engineered (OEM) and later the stuffed bearing will be easier to replace. When it is time to replace the bearing heat is'nt necessary so it is entirely possible to save other bearings that may be in the train.
Use it: It is required if the job is'nt quite perfectly engineered (maintenence) and the stuffed bearing and all the others in the train are going to be replaced anyway because its not worth risking leaving old bearings in the train.
Hope this helps
grandlemans
Mechanical
As someone who used to work for a bearing manufacturer, I thought I'd add my two cents.
Reasons for not using loctite:
-could contaminate bearing
-could cause bearing not to seat properly against shoulder
-could actually build up more under some areas of the bore causing deformation of the bearing ring and pinch points(most important)
Reasons to use:
-only if shaft fit and finish is poor or if the bearing is being replaced and the worn shaft is not being re-done.
There are definitely occasions when using a retaining compound is going to help. The most important suggestion I have is that the proper compound be used. Loctite and others make a compound for this express purpose. Using threadlocker is a bad idea and is more likely to cause problems.
Hope it helps
Reasons for not using loctite:
-could contaminate bearing
-could cause bearing not to seat properly against shoulder
-could actually build up more under some areas of the bore causing deformation of the bearing ring and pinch points(most important)
Reasons to use:
-only if shaft fit and finish is poor or if the bearing is being replaced and the worn shaft is not being re-done.
There are definitely occasions when using a retaining compound is going to help. The most important suggestion I have is that the proper compound be used. Loctite and others make a compound for this express purpose. Using threadlocker is a bad idea and is more likely to cause problems.
Hope it helps
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