Is There a Missing Load in My Statically Indeterminate Shaft Calculation?

In summary, the conversation discusses the difficulties in determining the moment and reaction forces for the bottom shaft in a gearbox design. The calculated values for torque and moment seem unlikely, and there is concern about the load distribution on the bearings. The possibility of a missing load or incorrect load model is also mentioned. The conversation ends with a suggestion to use the "unit load" method to resolve the issue.
  • #1
yonese
15
1
Homework Statement
I'm trying to solve for this intederminate beam for an output shaft for a gearbox I'm designing. I have 3 bearings (1 deep groove, 1 needle and 1 cylindrical) and I have assumed that axial load is negligible.
Relevant Equations
P = A+B+C
I have drawn the bending moment diagram to try to help me determine the moment, but I am certain my values are wrong. I have included a picture of my gearbox design, where the bottom shaft is what I'm trying to calculate. I've been able to calculate the top shaft moments and reaction forces with no problem. (Bending moment = 30.63 Nm, ball bearing = 850.8 N, roller bearing = 928.2N, Torque = 55.2 Nm)
Screen Shot 2021-05-24 at 18.10.19.png

Screen Shot 2021-05-24 at 18.11.27.png

For the bottom shaft, I am given that the torque is 107.8 Nm and I managed to get moment = 2.98 Nm, which just doesn't seem likely. My calculated reaction forces for the bearings are
Ball = 17 N
Needle = 1700 N
Roller = 98 N

It doesn't seem likely that the needle bearing would take so much of the load, considering the magnitude of my previous answers.

Is there a second load that I am missing? Or prehaps I should be taking the shaft and gear as a UDL?
 
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  • #2
I'm not into gears and such, but your load diagram shows the concentrated load of 1779 N practically right on top of the needle support B, implying B will take most all of the up reaction load, with small down reaction at A, and small up reaction at C, and there will be very little bending moment in the shaft, so your analysis is correct for the loading shown. But whether your load model is correct is another story.
 
  • #3
Are you familiar with the "unit load" method? If so, that will resolve this problem rather quickly.
 

FAQ: Is There a Missing Load in My Statically Indeterminate Shaft Calculation?

What is a statically indeterminate shaft?

A statically indeterminate shaft is a type of structural element that cannot be analyzed using basic statics equations due to the presence of multiple unknown forces and reactions.

How is the degree of statical indeterminacy determined for a shaft?

The degree of statical indeterminacy for a shaft is determined by counting the number of unknown forces and reactions in the system, and comparing it to the number of available equations of statics.

What are the common methods of solving for the reactions and forces in a statically indeterminate shaft?

The common methods for solving for reactions and forces in a statically indeterminate shaft include the method of consistent deformations, the slope-deflection method, and the moment distribution method.

What are the advantages and disadvantages of using the moment distribution method for analyzing a statically indeterminate shaft?

The advantage of using the moment distribution method is that it can handle any degree of statical indeterminacy. However, it can be time-consuming and complex for more complex systems.

How does the presence of internal hinge and/or roller supports affect the statical indeterminacy of a shaft?

The presence of internal hinge and/or roller supports can reduce the degree of statical indeterminacy of a shaft, making it easier to analyze using basic statics equations.

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