# Designing Coaxial Output Shafts: Fatigue Analysis

• CPP
In summary, the design project involves creating two coaxial output shafts, with one being a hollow shaft that shares its axis with the interior shaft. The current problem is that all fatigue analysis done so far is based on solid shaft design, and there is uncertainty about determining the appropriate sizing for the hollow shaft. The suggestion to reference ASME B106.1M and use past testing data to account for alternating bending and torsional moments has been considered, and another potential solution is to derive the goodman equation and insert the diameter cross section for the hollow shaft.
CPP
Part of my design project requires that I design two coaxial output shafts. I've gone through and calculated the inner shaft based on deflection and Goodman fatigue criterion. The problem now is this: the other output shaft must be a hollow shaft that shares its axis with the interior shaft. All of the fatigue analysis I've done to estimate shaft sizing thus far is based on solid shaft design where solid dimension 'd' is the parameter being solved for. I can determine an approximate sizing based on bearing deflections but that's it for now. Any suggestions?

Thanks,

There's not a whole lot of difference between the two cases. You have a cross sectional moment of inertia that drives your stresses in either case.

Have you had the opportunity to take a look at ASME B106.1M, Design of Transmission Shafting? In it there is a specified approach that uses past testing to provide an approach that takes into account both alternating bending moments and torsional moments.

I think I am going to chase down the derivation of the goodman equation and find where they insert the diameter cross section and then plug in my hollow shaft cross sectional area and find it that way. thanks

## 1. What is a coaxial output shaft?

A coaxial output shaft is a type of mechanical shaft that is used to transfer torque from one component to another, typically in a rotational motion. It consists of two concentric cylinders, with the inner cylinder (the drive shaft) rotating inside the outer cylinder (the driven shaft).

## 2. Why is fatigue analysis important in designing coaxial output shafts?

Fatigue analysis is important in designing coaxial output shafts because these components are subject to repeated and fluctuating loads, which can lead to failure over time. Fatigue analysis helps to determine the maximum stress levels that the shaft will experience and ensures that it can withstand the expected loads without failure.

## 3. What factors affect the fatigue life of coaxial output shafts?

There are several factors that can affect the fatigue life of coaxial output shafts, including the material properties, geometrical design, surface finish, and operating conditions. The material properties and design can determine the strength and durability of the shaft, while surface finish can impact the stress concentration and potential for cracks. Operating conditions such as temperature, speed, and load also play a role in the fatigue life of the shaft.

## 4. How is fatigue analysis conducted for coaxial output shafts?

Fatigue analysis for coaxial output shafts involves using mathematical models and simulations to predict the stress and strain levels that the shaft will experience under different loading conditions. This can include using finite element analysis (FEA) software to simulate the behavior of the shaft and determine potential failure points. Physical testing can also be conducted to validate the results of the analysis.

## 5. What are some common failure modes of coaxial output shafts?

Some common failure modes of coaxial output shafts include fatigue failure, bending and torsional stress, and surface damage such as wear and pitting. Fatigue failure occurs when the shaft experiences repeated loading and unloading, leading to cracks and eventual failure. Bending and torsional stress can cause the shaft to deform or twist, resulting in failure. Surface damage can occur due to poor lubrication or excessive friction, leading to wear and eventual failure of the shaft.

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