Why are shafts hardened at the location of bearings?

  • Thread starter k.udhay
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In summary: It is a localized heat treatment that is very hard and wears very slowly. The main reason for induction hardening is to prevent the mating surfaces from slipping. It also makes the shaft more durable and less likely to fret.
  • #1
k.udhay
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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.
 
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  • #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?
 
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  • #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:
 
  • #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
 
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  • #5
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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1. Why are shafts hardened at the location of bearings?

Shafts are hardened at the location of bearings to increase their strength and durability. Bearings are used to support rotating or moving parts in machinery, and the hardened shaft provides a strong and wear-resistant surface for the bearings to rest on.

2. How are shafts hardened at the location of bearings?

The hardening process involves heating the shaft to a high temperature and then rapidly cooling it, which changes the molecular structure of the metal and makes it stronger. This is typically done through techniques such as induction hardening or quenching and tempering.

3. What are the benefits of hardening shafts at the location of bearings?

Hardening the shafts at the location of bearings increases their resistance to wear and fatigue, prolonging their lifespan and reducing the need for frequent replacements. It also helps to prevent the shaft from deforming or bending under heavy loads, ensuring smooth and efficient operation of the machinery.

4. Are there any disadvantages to hardening shafts at the location of bearings?

One potential disadvantage is that the hardening process can make the shaft more brittle, making it more susceptible to cracking or breaking under extreme conditions. However, this can be mitigated by carefully selecting the appropriate hardening technique and monitoring the process closely.

5. Can shafts be hardened at locations other than the bearing points?

Yes, shafts can also be hardened at other points where they experience high stress or wear, such as at keyways or splines. This can further increase their strength and durability, but it is important to carefully consider the design and application of the machinery to determine the most effective locations for hardening.

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