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# Driveshaft Critical Speed

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This page contains the engineering information related to driveshaft design. The inputs are shaft dimensions and material properties.

Dimensional Inputs
Tube Outside Diameter:   inches
Material Thickness:   inches
Tube Length:   inches

Material Property Inputs

Common Driveshaft Material :
(click the button for that material)

 Metal Matrix Composite Aluminum Mild Steel

Or you can use the inputs below if known

Modulus of Elasticity:   million pounds/inch2
Density:   pounds/inch3

Outputs
Tube Weight: pounds
Critical Speed: RPM
Torsional Yield: pound-feet

## Glossary:

Modulus of Elasticity:
The Modulus of Elasticity is a material property that describes the strength of the material. The formal description is the stress that it would take to stretch a piece of the material to twice it's length (realizing that of course in the real world very few materials will deform anywhere near that far without breaking).

Critical Speed:
This is the probably the most influential factor in driving the purchase of an aftermarket driveshaft. The critical speed of a spinning shaft is described as the lowest speed which excites the shaft at it's natural frequency of vibration. This will cause the shaft to bend under the stress of vibration coupled with the centrifugal forces due to the rotation. In english, this means the shaft will vibrate very badly and possibly take a permanent bend (if not destroy the car in the process) as the critical speed is exceeded. If you use the calculator to predict the critical speed of a shaft, you can clearly see that lighter materials with higher modulus of elasticity give higher critical speeds. So do larger diameter shafts with thinner material thicknesses. Unfortunately, there is a limit to the diamter of shaft you can put in the car, so stronger lighter materials were developed to make stronger and faster spinning shafts.

Torsional Yield:
This is the torque value that will cause damage to the shaft. So you could say it is basically the maximum torque rating. Remember that horsepower is just torque at a certain speed. There is a horsepower to torque conversion calculator here.

Density:
The density is the weight of the material in pounds per cubic inch for the.

Weight:
This is the weight of the tubing that makes the shaft and not the end yokes or u-joints. Keep in mind that the material selection will affect the type of end yokes since they are welded (i.e. 6061 and MMC shafts use lighter forged aluminum yokes than steel shafts). This makes it pretty difficult to predict the total weight of a shaft with this simple calculator.