ceramic/hybrid roller/needle bearings

Discussion in 'Propulsion' started by sigurd, Oct 5, 2011.

  1. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    Hello, anyone knows who makes or sells such?

    Been through a dozen bearing makers catalogs, and googled my eyes out, no luck. Need several sizes up to about 30mm OD.
     
  2. Frosty

    Frosty Previous Member

    That would comfirm my suspision that they dont yet exist.
     
  3. sigurd
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    sigurd Pompuous Pangolin

    that would be annoying.
     
  4. powerabout
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    powerabout Senior Member

    assuming you want something to go fast then its usually the cages that is made from exotic material
     
  5. sigurd
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    sigurd Pompuous Pangolin

    No, I want heavy radial load and slow - less than 1000 RPM. Lack of lubrication, or water lube, and durability is the reason for ceramic. From what I gather so far steel races and ceramic rollers seems to be almost as good for this as full ceramic but better for shock load, and cheaper.
     
  6. Yellowjacket
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    Yellowjacket Senior Member

    High speed is the biggest incentive to go to ceramic bearings. Bearings with ceramic rolling elements have numerous advantages when applied to high speed applications. The amount of heat that the bearing creates due to Hertzian stress is reduced and the bearing runs much cooler than a metal rolling element.

    While the heat evolved in the rolling elements themselves is obviously lower in a ceramic rolling element, at very high speeds, a major part of the load in the bearing is the centrifugal load of the rolling elements on the outer race. The ceramic rolling elements weigh about half that of the steel elements, so at high speeds the radial load on the outer race is reduced, which also reduces the heat generated in the outer race.

    Bearing engineers generally talk about bearing speed as DN or the diameter of the bore multiplied by the rpm. As you go up in speed more heat and more centrifugal load in the rolling elements creates heat. The typical phenolic cages are good for about 1.8 million DN, after that you need steel cages, but with proper lubrication silver plated steel cages can go to close to 3 million DN. Cage speed limits are based on the centrifugal stress in the cage, and while you have to go to better cage material than phenolic cages, high speed cage materials aren't that sexy.

    While ceramic balls have a higher load capacity than metal balls, at lower speeds the races still limit the load capacity of the bearing. Put in too much loading and the races still spall on you. Ceramic rolling elements are better and improve the life, but most of that gain is at higher speeds.

    If you want to lubricate with water you are still going to have problems because bearing steels don't have any corrosoin resistance, ceramic rolling elements or not.
     
  7. powerabout
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    powerabout Senior Member

    sounds like you want boat stuff
    plastics/carbonfibre/titanium with delrin bearings
     
  8. sigurd
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    sigurd Pompuous Pangolin

    I forgot to mention 150C + temperature. Thanks for the tips.
     
  9. Yellowjacket
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    Yellowjacket Senior Member

    :eek: That's gonna be a problem....

    You are pushing it in terms of bearing temperature. Bearing materials don't like to get much hotter than that and you can't use phenolic cages at that temperature. You are limited to a brass or steel cage, but actually you shouldn't have a cage at all.

    Since you are at low speed and want high load capacity you don't want a caged bearing, you want a "full compliment" roller bearing. That is one with no cage, full of rollers. This gives you the highest load capacity, but doesn't work at high speed.

    Another problem is that you are getting up into the temperature range where lubricant coking can be a problem. If you get oil to too high a temperature, for too long, the oil starts to deteriorate and the result is small bits of carbon (called coke) that form in the oil. These bits of carbon are very hard and chew up the bearings. If you are running that hot you really want to use a very high efficiency oil filter (like 3 microns) to keep the oil clean. We went from a 20 micron filter to a 3 micron filter on a well known gas turbine engine that had consistently high oil temperatures and it improved bearing life by a factor of 3.

    I am assuming that you are using a recirculating oil system, but if you are using grease it is even worse, since you don't get to filter the lubricant and the bits of carbon stay in the bearing. Something like that isn't going to last a long time. There are some high temperature greases, but you are pushing it at that temperature.

    If you have water available, you need to think about a water jacket around the bearing compartment to cool things off, and then you can probably grease pack the bearing, get a full compliment roller bearing and that will maximize your load capacity.

    Also, be careful and fully calculate the bearing life, don't just use the load ratings in the bearing catalog. Generally bearing manufacturers publish a load rating in the catalog, but they have a procedure for calculating the life at the load you are applying from the load rating. The acceptable load for the life you require may be significantly different from the load rating, considering that you are running this at very low speed.
     
  10. michael pierzga
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    michael pierzga Senior Member

  11. Frosty

    Frosty Previous Member

    There is a bearing for most situtations, what situtation have you got that needs a new type of bearing.
     
  12. rxcomposite
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    rxcomposite Senior Member

    I have seen those sizes for bicycle bearings. Try searching for bicycle bearings. They have ceramic bearings.
     
  13. powerabout
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    powerabout Senior Member

    I wonder what the lunar rover used?
     
  14. sigurd
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    sigurd Pompuous Pangolin

    I can use 90C water to pressure lubricate the synthetic bearings. How will they do under full load at start and stop? I've heard oil lubricated journal bearings have problems with this.
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    Depending on how much pressure and area you have it is possible to avoid the stop/start isssue with hydrodynamc bearings. Hydrodynamic bearings support the shaft on a film of fluid, much the same as a water skier is supported on the water. The problem is, at a dead stop there isn't any lift and before the bearing can provide lift you need some motion. Hydrodynamic bearings using oil, because it has higher viscosity and better lubricity, can survive the startup problem.

    Some bearings are designed to be pre-presurized with a fluid, and this pressure is sufficiently high (when multiplied by the cross sectional horizontal area of the bearing) to lift the shaft off of the bearing. These hydrostatic bearings don't have a problem with starting and stopping, so that is an advantage. The need for pressurized fliud to be applied BEFORE the bearing starts to rotate sometimes makes this difficult to do.

    You noted that this bearing had a relatively high load, and this generally requires large area and big diameters for hydrodynamic bearings. This is particularly true when you have low speeds. Hydrodynamic bearings need speed to work and with a lower viscosity fluid (like water) you need more speed and more area to support the load.

    Perhaps if you gave us some more information about what you are trying to do we could be of more assistance.
     
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