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Why are shafts hardened at the location of bearings?

  1. Sep 12, 2013 #1
    Hi,

    All the shaft drawings I have seen are induction hardened at the location of bearings. What is the need for that? The shaft and bearing are going to rotate in the speed and hence no relative motion (rubbing) between them. Does it have something to do with strength? Thanks.
     
  2. jcsd
  3. Sep 12, 2013 #2
    The reason why they do it in just the locations of the bearings is normally to act as the actual journal of a bearing surface but if what you say is true about them not slipping at all then it probably has something to do with strength or stiffness. What are the shafts used for, what loads do they see? It also could be that they used to have it slip in older revisions and just never changed it back?
     
  4. Sep 12, 2013 #3
    Thanks abrewmaster...
    Generally all the automobile drivetrain shafts that I have seen either come with full hardened or at least the bearing locations induction-hardened. In many of the shafts the bearings are press fitted and hence no slip is possible. In other shafts, it is a transition fit and hence hardly there will be any slip.
    About stiffness - I think stiffness is a function of geometry and young's modulus of the material. As, none change with heat treatment, I don't think stiffness plays a role here.
    Maybe strength is what is doing something here... :confused:
     
  5. Sep 12, 2013 #4
    k.udhay
    A shaft in torsion has as certain amount of twist along its length. At the bearing the hub will not twist as much as the shaft and there will be relative motion beteen the two, usually at one end. You can simulate this by grabbing tightly a finger from one hand and forming a fist from the other hand around the finger especially the finger end. Now try to rotate the finger back and forth in the fist. You will see the relative motion of the finger where it exits the fist.

    For a shaft undergoing variable torsion, the same thing happens to the mating surfaces of the shaft and bearing hub. Although the relative movement is minute, after repeation a condition called fretting can occur, where microscopic particles tear loose.

    An induction heat treatment can improve the endurance limit for the shaft by, if done correctly, setting up a residual compressive stress on the outer fibers or layers of the shaft. That is the usaual remedy to combat fretting, if the relative movement cannot be reduced by other means such as decreasing the load or number of oscillations.

    Other ways to combat fretting by setting up residual compressive stresses on the outer fibers of the shaft are shot peeing , surface rolling, flame hardeneing, special coatings( molybbdenum disulfide, baked corn syrup, grease or a mixture of for mating parts not necessarily force fit shafts and bearing hubs but also bolted connections, splines for example ), carburizing. nitriding. and anodizing aluminium.

    If not done correctly, the fatigue strength of the material can decrease with some of the operations so you have to have good control on temperature increase and quenching the heated materail.

    The fatigue strength of the part is increased.

    I would think that should be the reason for the heat treatment.
    You can look up some of the terms such as fretting, etc to gain more insight.

    cheers
     
  6. Sep 13, 2013 #5

    Baluncore

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    Science Advisor

    Bearings are made from very hard material and are designed to be replaced. When a bearing is pressed on or pulled off the shaft it will damage the shaft if the shaft is not hardened. It is significantly more expensive to replace the shaft than the bearing, a disposable commodity.

    When pressing a bearing onto a soft shaft the bearing will often become slightly diagonal and shave the shaft. That damages what was intended as an alignment reference surface.

    The critical part of a shaft is the outer annulus. Damage to, or embrittlement of that critical area must be avoided. Induction hardening is very thin. It does not change the deeper bulk properties of the shaft material or cause significant distortion of the machined part.
     
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