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change C3 clrearance to CN (by change the bearing seat )
8

change C3 clrearance to CN (by change the bearing seat )

change C3 clrearance to CN (by change the bearing seat )

(OP)
We have a centrifugal water pump, 2900 rpm speed.
The water temperature is 20-30 centigrade degree.
In original design (KSB Germany) this pump has two 6413 C3 bearing with 0.023-0.043 mm radial clearance.
Now we can't find C3 bearing. Can we use CN (0.008-.0.028 mm radial clearance) bearing
by change the bearing seat tolerance on shaft/housing? The shaft tolerance on shaft is 65k6 (+0.002, +0.021m) and in
housing is 160K6(-0.021,+0.04mm).we use induction heater for mounting bearing on shaft. Thank you

RE: change C3 clrearance to CN (by change the bearing seat )

From your data it is seems that the CN bearing is more accurate therefore you can use it without any change to the shaft and housing dimension. When you heat the bearing you should be very carefull not to temper the bearing steel heat treatment.

RE: change C3 clrearance to CN (by change the bearing seat )

2
Whoa!
C3 bearings (i.e. radial internal clearance greater than standard) are usually fitted to equipment where large temperature differences bewteen inner and outer ring can be expected on start-up. The inner ring usually expands a little quicker than the outer, therefore the internal clearance of the bearing can drop. Without sufficient clearance to start with (i.e. C3), the bearing may end up in a preloaded condition, which will create heat, therefore even more expansion etc etc until early failure.

If your machine has a C3, I would suggest that you get some.

You can relax the shaft and housing fits, but that seems to be a 'long' way of solving the problem.

Can't believe that 6413 C3 are not available to you. There are hundreds listed on the BearingNet.

Hot fitting will not damage the bearing steel in any way properly controlled. If you have to heat your bearing much more than 80C above ambient I would be surprised.

Hope this helps!

Lester Milton
Telford, Shropshire, UK

RE: change C3 clrearance to CN (by change the bearing seat )

smaty
You should read carefully what Hippo 41 says " Without sufficient clearance to start with (i.e. C3), the bearing may end up in a preloaded condition, which will create heat, therefore even more expansion etc etc until early failure."
There was good reason the original design incorporated C3 and Hippo 41 nailed it.
Good luck

RE: change C3 clrearance to CN (by change the bearing seat )

(OP)
Dear Hippo41, thanks for your help.
you Can't believe that 6413 C3 are not available, but believe because I live in a conutry that cant find C3.
Here in markets there are various kinds of bearings with different marks that may be many of those counterfeit and we can’t trust to those. I have problem to find C3 really, and we haven’t any possibility to buy from foreign countries. In your opinion, if a pump has 1450 RPM, Does it need C3 clearance for start-up?
Is pump among equipments where large temperature differences between inner and outer ring can be expected on start-up?
thank you
 Smaty




RE: change C3 clrearance to CN (by change the bearing seat )

Can you tell me where you are?  

If you really can't get C3 then you have no choice other than to use what you can get. You may not get the problem developing - but if you do you'll know why!

Can you monitor the bearing temperatures on startup at all? Then you will some idea of what is going on. Remember the bearing temperature will climb rapidly on start-up, hopefully reach a peak (I would suggest you set a limit of less than 100 Celsius otherwise you will be cooking your lubricant!) and then settle back down to a normal operating temperature - possibly around the 40 Celsius mark. (or wherever it was before if you have that information).

Lester Milton
Telford, Shropshire, UK

RE: change C3 clrearance to CN (by change the bearing seat )

(OP)
I'm form Iran.
I try to monitor the bearing temperatures on startup.if i get an aanswer i'll tell you.

RE: change C3 clrearance to CN (by change the bearing seat )

2
Is this pump a one-off unit undergoing maintenance ?  If so, the chances are very high that fitting a CN will be quite satisfactory. Its unlikely that your shaft and housing will be in 'maximun metal condition' and the bearing bore and OD certainly wont be. Do the sums for the actual parts and you'll find at worst probably only a few microns of negative clearance in the brg after fitting. This is quite ok. Thermal runaway needs a lot more than this to give trouble. (If your housing is aluminium then its even better when your operating temperatures are taken into account.)

As regards fitting temperatures, there lots of unnecessary worries about this too. Most brgs are tempered to operate at around 125C. Above this, and you will start to create dimensional changes within the brg rings. However depending on the temperature, you will have to maintain it for hours or days to get any significant growth or shrinkage effect. Days and hours are easy to achieve in an application, but I doubt very much that you fitting process will take more than a few minutes !

Unfortunately, there are lots of old wives tales circulating in the bearing world, some of them even proliferated in bearing manufactures catalogues !

Gerry



RE: change C3 clrearance to CN (by change the bearing seat )

3
At the risk of being branded "an old wife", I disagree with Gerry45. I would not be so brazenly confident that "only a few microns of negative clearance is quite ok" and that "thermal runaway needs a lot more than this to give trouble". The inner race expands much more rapidly than the outer, especially during transient start up conditions. My experience, although somewhat anecdotal, is that once you run out of radial clearance at high speed,  rolling element bearings are likely to become toast, although every application is different. But perhaps Gerry45 has some detailed theoretical and/or experimental data to back up his claim.

RE: change C3 clrearance to CN (by change the bearing seat )

Gerry45 delivers an interesting discussion point. By definition, old wives last longer. I cannot resist speculating that it is so because they stay with what they know @:)

RE: change C3 clrearance to CN (by change the bearing seat )

Smaty says quote "We have a centrifugal water pump, 2900 rpm speed." end quote.  Indicates a shaft speed above 9m/s. Even neutral clearance at start up is an accident waiting to happen.
Hippo41 & English Muffin are correct.

RE: change C3 clrearance to CN (by change the bearing seat )

I have watched this thread with interest.  My approach has always been that internal clearance and fits should be strictly per OEM or other standard recommendation unless there is a thorough evaluation of the risks of changing it. With that said, I was also under the vague impression that C3 was intended for applications where higher temperatures may be expected, and the CN might[/] be acceptable alternative here.  

On the subject of clearance or interference during operation, skf offers the following:

‘The radial internal clearance of a bearing is of considerable importance if satisfactory operation is to be obtained. As a general rule, ball bearings should always have an operational clearance which is virtually zero, or there may be a slight preload. Cylindrical and spherical roller bearings, on the other hand, should always have some residual clearance – however small – in operation. The same is true of taper roller bearings except in bearing arrangements where stiffness is desired, e.g. pinion bearing arrangements, where the bearings are mounted with a certain amount of preload, see under "Bearing preload".’

EM/rnd2 – what is the source of expansion during startup you mention?  Bearing frciton heat?
Also, I have never heard speed mentioned in the context of requiring looser internal clearance.  Is that also due to bearing friction heat?
just curious to learn a little more.

RE: change C3 clrearance to CN (by change the bearing seat )

please ignore the bolds.

RE: change C3 clrearance to CN (by change the bearing seat )

Electricpete is correct. A slight preload is best. but slight is the main word. It quickly moves to a less life condition. C3 is also normally recommended when the speed exceeds around 75% of the speed rating of the bearing.

RE: change C3 clrearance to CN (by change the bearing seat )

Electricpete:
The thing is that if the speed is higher, the heat generation rate will be higher. But at high speeds, ball bearings generally have a greater heat generation rate on the inner race, if there is a contact angle involved, because the spin velocity is higher there. There is also usually less opportunity for heat dissipation on the inner race. So consequently the relative expansion of the inner race will be greater and the operating preload will increase. As has been mentioned above, this can then lead to a vicious circle of "thermal runaway". As has also been mentioned, the fit of the outer race can also come into it. Using FEA, it is possible to study this effect analytically in any given case, using FEA, but it gets rather involved becuase it's a coupled field situation. Poplawski associates has developed a program that runs in conjunction with Ansys, to solve this problem, and they have done a number of studies for people in special cases. There is a much older program called Shaberth, which used finite differences to do a similar thing. Most of my experience in this field has been with machine tool spindle bearings, where the speeds tend to be higher than run of the mill electric motors. I have come across cases where even spring preloaded angular contact bearings ran out of radial clearance at high speed - and they have failed instantly.
You can certainly run with preload in the case of angular contact bearings, of course. But your SKF excerpt is a bit of a cop-out where they say "As a general rule, ball bearings should always have an operational clearance which is virtually zero, or there may be a slight preload". The use of the word "operational" lets them off the hook, rendering the statement almost valueless from a practical standpoint.

RE: change C3 clrearance to CN (by change the bearing seat )

thx em. You get a star.

RE: change C3 clrearance to CN (by change the bearing seat )

star to Hippo 41 for making the call
star to English Muffin for the elegant explanation
star to Gerry45 for the phrase "thermal runaway"

RE: change C3 clrearance to CN (by change the bearing seat )

Being and 'old wife' is really nothing to be ashamed of. I have been one myself on numerous occasions. But as with alcoholism I tell myself, admission is the first step to recovery! So anyway, back to CN versus C3 debate and what is the basis of my brazen statements .....

1)Deep groove / Conrad type ball brgs made by good manufacturers generally have bore and OD tolerances at the middle of the tolerance band (+/- a couple of microns). There is a little more 'spread' on the RIC but even this wont reach anywhere near 'catalogue' limits.

2)The processes used to make shafts and housings are by no means as well controlled as brgs, but the likelihood of a single machine taken in for repair having both shaft and housing features on a 'max metal' condition is quite remote.

3)normal surface quality on shafts and particularly housings means that, during the brg fitting process, significant amounts of the theoretical interference is lost.

4)Machines which have been in service will often exhibit additional wear/smoothing of the seating surfaces.

5)Even in a brg that is 'theoretically tight' after installation, normal elastic characteristics mean that when an external load is applied, a clearance zone is re-created within the brg.

6)In my experience, assumed shaft~hsg temperature differentials of often exagerated for safety due to the difficulty in attempting to predict or measure them.

Ok these are just general points which may or may not be applicable in every case. But certainly not taking into account the 'real' dimensions of the parts when doing fit calcs can be misleading.

The one issue I haven't addressed yet is the conditions required for 'thermal runaway' and my statement that 'a few microns' will not cause trouble. This is probably because I dont really understand the situation myself!!! But it is the case that 'moderate' speeds and 'moderate' negative clearances in ball brgs can and do work successfully. If they didn't, lots of other things wouldnt work either.  Take machine tool spindles. These are often fitted up with pairs of angular contact brgs clamped together in an axially preloaded condition. From the rolling elements point of view, they cant tell the difference between an axial or radial preload, a zero or 15 deg contact angle. All they know is that they are trapped tightly between raceways. After startup, temperature differentials develop across the brg and things, theoretically at least, should go from bad to worse....except that it doesnt !

In the automotive world, there are lots of applications where conventional theory has been ignored without disasterous consequences. Some alternators for example, incorporate C2 clearance brgs(ie less than normal) heavily interference fitted into aluminium housings (to make sure that they dont become loose at 100 degs C). According to the theory, this arrangement shouldn't work either, but is does do, even when you freeze them down to -40C !

Im not suggesting that 'thermal runaways' never happen, just that they dont happen as frequently as the world would generally predict. I remain convinced that provided nothing dodgy happens on the lubrication side, Smaty will get away with fitting a CN. He'll probably also find that his pump becomes smoother and quieter than its ever been!

Gerry


RE: change C3 clrearance to CN (by change the bearing seat )

Smaty will need to get the best lubricating oil that is available.  

RE: change C3 clrearance to CN (by change the bearing seat )

"Take machine tool spindles.........After startup, temperature differentials develop across the brg and things, theoretically at least, should go from bad to worse....except that it doesn't !"
"From the rolling elements point of view, they can't tell the difference between an axial or radial preload, a zero or 15 deg contact angle."

I don't agree with either of these statements. In the case of the first, I can assure you that "it can and often does". That is why many high speed spindle bearings are spring or hydraulically preloaded, even though this unfortunately reduces stiffness. Even allowing the whole bearing to float in the housing can still lead to failure if the outer race expands enough, and spindle manufacturers have had to resort to mounting floating bearings in linear roller cages. However, in many of the less demanding cases you can get undoubtedly get away with it. There are also a number of cases that I have come across in gearboxes where any amount of excess preload does not seem to lead to thermal runaway, apparently because of the flexibility of the casing. Every situation is different, and I just don't think you can make blanket statements like that.
In the case of the second statement, there is often a big difference in practice between a zero contact angle and a non-zero contact angle, because bearing assemblies are usually much stiffer radially than axially. I stand by my comment that once you run out of radial clearance at high speed (ie achieve a zero contact angle) - watch it !

RE: change C3 clrearance to CN (by change the bearing seat )

"Blanket statements", steady on old chap, I admit to being an old wife on occasions, but please dont accuse me of making blanket statements too ! I thought I had inserted enough 'qualifiers' to avoid such slurs on my good name, eg.
when I refered to brg arrangements in machine tools, I did say 'often'. I appreciate there are some sophisticated arrangements out there, but the ordinary ones do exist, and do work.

As for the bit about contact angles, some confusion here I think. The point I was trying to make, (but perhaps not very well), was that when considered as a system, two lightly and solidly preloaded small-angled A/C brgs are in precisely the same thermally 'dodgy' situation as a deep groove brg installed with a negative clearance. They both have the potential for a thermal runaway, ie no clearance to allow for that inherent temperature differential across the rings. Its quite strange, people hear the words a/c and dont question the situation. But as soon as one suggests some preload on a deep-groove, there's almost mass hysteria ! Funny old world isnt it!

Finally E-M, you say  "once you run out of radial clearance at high speed, watch it !"  I'd say on Smaty's 6413@2900rpm, watching it will be as exciting as watching the grass grow.... cos nothing will happen !

RE: change C3 clrearance to CN (by change the bearing seat )

Gerry45 :"When considered as a system, two lightly and solidly preloaded small-angled A/C brgs are in precisely the same thermally 'dodgy' situation as a deep groove brg installed with a negative clearance".
Now while it is certainly the case that two solidly preloaded bearings of any type are often in a "dodgy" situation, especially when the axial spacing is small, I do not agree with the contention that they are "precisely the same".
Kinematically, both arrangements obey the same rules - ie they are both really angular contact bearings. However, in terms of thermal runaway there is a difference between a bearing with an operating contact angle of say, 15 degrees, and one with an operating contact angle of close to 0 degrees, which is what you have in the case of a deep groove bearing which is lightly axially preloaded. If the radial preload increases, the contact angle decreases, and the bearing becomes stiffer, thus generating even more internal load and more heat. Once the contact angle becomes zero - and it will if the differential race temperature of the bearing gets high enough - the non linear radial stiffness v load is now in a somewhat different regime, where the mitigating effect of the contact angle v stiffness relation - although it admittedly has already become small - no longer exists at all, and the highly non-linear load v stiffness relationship inexoribly will lead to thermal runaway if the speed is high enough. It only takes a tiny change in radial clearance to go from a contact angle of 15 degrees to zero. With angular contact bearings, the bearing spacing also has a large effect on the situation, since axial expansion has the effect of relieving the preload. Once the contact angle has become zero, any beneficial effects of this are also lost. An exact analysis in any given case is obviously very complicated, but it can be done using FEA, although it is not straightforward - that's why nobody can usually give a definitive answer to the question of preload, and bearing manufacturers are undoubtedly conservative in many cases, or occasionally even just plain wrong in their recommendations. Ultimately, their business is selling bearings, not applying them. So your contention that "nothing will happen" in the case of this particular bearing may or may not be true. It depends on how things fall in the tolerance zones, and the exact nature of the housings etc. But there are good reasons why increased radial clearance is often specified for motor bearings, or indeed deep groove bearings in general, and I stand by my comments.

RE: change C3 clrearance to CN (by change the bearing seat )

EM - You really don't like manufacturers do you? They'll do all the research you want - as long as you want to pay for it.

Manufacturers remarks are made with the best intentions and are intended to solve 99% of real world cases, and as such have to be rather general in nature.

Yes we are in businesss to sell bearings - but we are also heavily involved in making sure that bearings perform well under both test and real-world conditions (not always the same thing, despite lots of free training given out) and most if the information we dole out is FREE. Unfortunately having senior engineering staff work on FREE projects can have a negative efect on profitability and so we have to stop being engineers and start being realists sometime and draw the line somewhere.

Lester Milton
Telford, Shropshire, UK

RE: change C3 clrearance to CN (by change the bearing seat )

Hippo41:
On the contrary - I have the greatest respect for "manufacturers", having spent most of my life working for them, athough they have not been specifically bearing manufacturers. I quite agree with you that "they will do all the research you want - as long as you want to pay for it". The problem is that most people can't afford to pay for it, or won't pay for it, or are actually in the position of being better able to do the research themselves for less money than it will cost the bearing manufacturer. Bearing manufacturers are in business to make money by selling bearings, and they put their engineering resources to work where they will yield the biggest return. And if you study bearings and understand them well enough, then as the user you frequently end up in a better position to get to the bottom of problems, since you are in possession of the equipment on which they are being incorporated.
So I stand by the point that I am trying to make, which is that you often hear people say "rely on the bearing manufacturer's recommendation", but this is not always the best thing to do - although getting as much information as you can from the manufacturer is obviously the first step. Frequently, you will get different opinions from different bearing manufacturers, even though all the products are often essentially the same. I believe that the same goes for all purchased products - it is inevitable that as a user, you can frequently become more expert in your own specific area of application than the manufacturer.

RE: change C3 clrearance to CN (by change the bearing seat )

I agree with that.

On a lighter note I'm reminded of a definition of specialization that said something like "you end up knowing more and more about less and less until you know everything about nothing".

Interesting discussion. Thanks!

Lester Milton
Telford, Shropshire, UK

RE: change C3 clrearance to CN (by change the bearing seat )

I reckon Smaty has moved on, hasn't told us what happened and left us in a rut. With every pun intended, I came across this magnificent post and I hope you will all get as much of a chuckle as I did:

butelja (Mechanical) May 29, 2002
Here is a little light hearted story that explains many things.

The US standard railroad gauge (width between the two rails) is 4 feet, 8.5 inches. That's an exceedingly odd number. Why was that gauge used? Because that's the way they built them in England, and the US railroads were
Built by English expatriates. Why did the English build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that's the gauge they used. Why did "they" use that gauge then?  Because the people who built the tramways used
the same jigs and tools that they used for building wagons which used that wheel spacing.

Okay! Why did the wagons have that particular odd wheel spacing? Well, If they tried to use any other spacing, the wagon wheels would break on some of the old, long distance roads in England, because that's the spacing of the wheel ruts. So who built those old rutted roads?

The first long distance roads in Europe and England were built by Imperial Rome for their legions. The roads have been used ever since. And the ruts in the roads?

Roman war chariots first formed the initial ruts, which everyone else had to match for fear of destroying their wagon wheels. Since the chariots were made for (or by) Imperial Rome, they were all alike in the matter of wheel
spacing.

The United States standard railroad gauge of 4 feet, 8.5 inches derives the original specification for an Imperial Roman war chariot. Specifications and bureaucracies live forever!

So the next time you are handed a specification and wonder what horse's Ass came up with it, you might be exactly right, because the Imperial Roman war chariots were made just wide enough to accommodate the back ends of two war horses. Thus, we have the answer to the original question.

Now the extraterrestrial twist to the story...... When you see a Space Shuttle sitting on it's launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are solid rocket boosters, or SRBs. The SRBs are made by Thiokol at their factory in Utah.

The engineers who designed the SRBs might have preferred to make them a Bit fatter, but the SRBs had to be shipped by train from the factory to the launch site. The railroad line from the factory had to run through a tunnel in the mountains. The SRBs had to fit through that tunnel. The tunnel is slightly wider than the railroad track, ..... and the railroad track is about as wide as two horses' behinds.

So, the major design feature of what is arguably the world's most advanced transportation system was determined over two thousand years ago by the width of a horse's ass.

And you wonder why it's so hard to make things change ...

RE: change C3 clrearance to CN (by change the bearing seat )

It is worth noting that there were a number of other wider gages in use in the early days of the railroads, both in Britain and the US. It is probably unfortunate that they all lost out to Stephenson's narrow gage, apparently because it minimised the interchangeability problems at the time. This seems to have happened in much the same way that VHS won out over Betamax, QWERTY keyboards became the standard, AutoCAD beat out many far superior CAD programs etc. The least desireable lowest common denominator solutions usually become the de facto standards.

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