Hollow Shaft vs Solid Shaft- Which Is Better?

Before Selection of Shaft you must be considered whether to choose hollow shaft or Solid Shaft depending upon the type of application, power Requirement, torque transmission requirement, weight, cost, machining time, Material. 

Difference /Comparison Between Hollow Shaft and Solid shaft as Below: 

• Weight: In comparison to a solid shaft, a hollow shaft is of less weight, for a given length and diameter. This is pretty obvious and wont be requiring any further explanation. Moreover it’s a good idea to go ahead with hollow shafts, if we emphasize on weight reduction and cost-cutting.
• Hollow shafts are much better to take torsional loads compared to solid shafts. As shown in the figure, shear stress in a “shaft subjected to torsion” varies linearly from zero at the center to the maximum at the boundary. Inside a solid shaft, most of the material experiences / carries a shear stress whose value is much below the maximum shear stress [Interior portion of the shaft]. But at the same, they are adding to the weight, without contributing much to the capability of the shaft to carry the torsional load.
  Shear stress at a point in a hollow shaft (τ)(τ) is given by , τ=TrIpτ=TrIpwhere τ=τ=Shear stress in a hollow shaft

  TT = Torque in shaft (N-m)

  

  rr = radial distance from the center to the point of interest (m)

  Ip=Ip=Polar area moment of inertia (m4m4)

  Average shear stress[ (Min. Shear stress + Max. Shear Stress))/ 2 ] in a hollow shaft will be higher compared to a solid shaft and its value is closer to the maximum shear stress.
  Hollow shaft has a greater strength to weight ratio.

• Wall Thickness: If we reduce the wall thickness, then there are chances of wrinkling or buckling of the shaft wall. So we have to be careful about these things.
• Cost Consideration:  Cost of Hollow shaft is more than solid shaft because of machining is needed for hollow shaft which is time-consuming.
• Hollow shaft exhibits high resistance to stresses while the solid shaft may tend to fracture at a certain limit.

Selection Of Hollow Shaft or Solid Shaft Depends On : 

• For a load-bearing type of shaft, then its always better to choose a hollow shaft because it has higher stiffness and rigidity and can resist slightly higher bending moments.
• A shaft for torque transmission like crankshafts, drive shafts it is always better to choose a solid shaft because it has higher torsional stiffness.
• Hollow shaft is not always better compare to a solid shaft. It depends on our uses. When there is no space constraint, then for the same mass, the hollow shaft is better than the solid shaft. But if we have a radial space constraint then the solid shaft is preferred since, in this case solid shaft has more power transfer capacity than the hollow shaft.
• Practical use: In some mixing applications, gas is fed through a hollow shaft to aerate the product. Other fluids can be supplied this way, either directly or inside a tube-like compressed air, hydraulic fluid, or coolant.
• Assembly: Sometimes, it is simpler to affix the appropriate ends to a hollow shaft for custom length shafts, similar to RWD vehicle drivelines. One could stock tube and the end pieces, cut the tube to whatever length is needed, and attach the ends. This can be simpler for some shops and applications than building a custom solid shaft.
• Weight: The strength of circular members is mainly contributed by their outer surface. The core of a solid shaft is essentially wasted material in some cases. It may be helpful from a mechanical standpoint to reduce rotating mass, or it can simply be a matter of value.
• Vibration: Natural frequency of hollow shafts differ from solid shafts. Typically, a hollow shaft can increase the critical speed about 20% compared to a solid shaft

Advantage of hollow shafts:

1. Lightweight comparable to the solid shaft of the same diameter.
2. Low rotational inertia.
3. Cheaper.
4. High tolerance to rotational imbalance due to low weight.
5. Also, hollow shafts are tolerant to high temperature because of low specific mass.

Disadvantage of hollow shafts:

1. Less tolerant to over-torque.
2. Prone to bending especially when exposed to lateral stress.
Ex. boat propeller shaft.
3. Even though it is tolerant to mild imbalance, this however increases as rotational speed exceeds the critical limit.

Advantages of solid shafts:

1. Suitable for high torque applications.
2. Very resistant to bending.
3. When perfectly rolled it has a very smooth operation even at high speeds.
4. High rigidity even at a small diameter.

Disadvantages of solid shafts.

1. High cost.
2. It must be perfectly straight from the manufacture. A slight imbalance is not tolerable and very hard to deal with.
3. High rotational inertia especially on larger diameters.
4. On application where the shaft is exposed to very high temperatures, solid shaft made of mild steel bends even at rest. Special metal is used for this application thus adding cost.

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