# Combined Force And Stress Calculations On Marine Propeller Shafts

Marine propeller shafts are very important structural elements in mechanical engineering that must be designed properly to overcome the required work. Marine propellers are very good examples of combined loading: torsion, bending, and direct thrust. A safe design of shafts that undergoes these loading conditions must be established with the wise consideratiın of these loads. Here, this stuff is explained with basic calculators that you can use them.

## Combined Torsion, Bending, and Direct Thrust Image Source: D. K. Singh – Strength of Materials-Springer, 2020, pg.641.

In the case of marine propellers, torsion takes place because of the rotation of the shaft due to the power transmission. Bending takes place because of the self-weight of the shaft which is fixed with bearings. Direct thrust force is the result of the rotation of the propeller at the edge of the shaft, which applies an axial force along the shaft.

### Calculation of Twisting Moment Effect

Because of the rotation, a twisting moment occurs on the shaft. The effect of the twisting moment is the shear stress which is the maximum at the surface of the shaft. This equation is used to calculate the effect of the twisting moment;

In here, ‘T’ is the twisting moment, ‘d’ is the diameter of the shaft.

You can use the calculator below to calculate shear stress due to the twisting moment effect to make more fast calculations.

#### Torsional Stress Calculator

The use of this calculator is very simple. You just need to enter the required values inside the brackets then click on the ‘Calculate’ button to see the result. If you want to make another calculation, just click on the ‘Reset’ button then re-enter the new values.

### Calculation of Bending Moment Effect

Bending moment occurs because of the own weight of the shaft. The bending moment must be calculated according to the placement of the bearings. The formulation to calculate the tensile or compressive bending stress is like this;

In this equation, ‘M’ is the magnitude of the bending moment.

Use the calculator below to calculate the tensile or compressive bending stress due to the bending moment.

### Calculation of Vertical Shear Force Effect

Shear force is not produced at the top-side surface of the shaft. The maximum shear stress is occurred at the side surface of the shaft because of the shear force effect. This shear stress is calculated via this formulation;

In this formulation, ‘V’ is the vertical shear force. You can use the calculator below to swiftly calculate the shear stress due to the shear force.

### Calculation of Direct Thrust Stress Effect

Compressive stress occurred on the shaft because of the thrust force produced by the propeller. The calculation is made with this equation;

In this equation, ‘t’ is the thrust force.

Use the calculator below to calculate the normal compressive stress due to the thrust force.

### Find The Critical Point

In terms of the direct stress effects, thrust and bending stresses are superimposed. The critical point is where the maximum direct stress occurs on the shaft. It is generally the top surface of the shaft. The effect of shear force is the same for all the points on surfaces.

After finding out the critical point and the stresses on that critical point, you can calculate the maximum principal stresses on that point.

## Conclusion

As you see above, an engineering approach to combined force and stress conditions is like that.

Strength of Materials, Third Edition

If you are interested in the guidebook that is used for this article, click on the given link above or the ‘Shop Now’ button to check it from Amazon!

Do not forget to leave your comments and questions below about the combined force and stress calculations on shafts!

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