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

[smaty]

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

 

[Gerry45]

"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 !

 

[EnglishMuffin]

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.

 

[Hippo41]

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

 

[EnglishMuffin]

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.

 

[Hippo41]

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

 

[rnd2]

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 ...

 

[EnglishMuffin]

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.