Shaft Length Between Bearings

[bridgey]

I have searched extensively and yet can't seem to find this common calculation. I want to calculate the maximum distance allowable of a shaft of a given diameter, rotation speed. type of material and horsepower. Does overall length of the shaft depend on spacing between bearings. I would suspect can you extend a shaft length by just adding more bearings at regular intervals. Can anyone please help me with this quest?

Thanks.

[tom kane]

The 2o 40 rule applies,Bearings spaced no closer than 20 times the shaft diameter.And no further apart than 40 times the shaft diameter.Of course there will always have to be exceptions.

[Rick Willoughby]

You might find something of use if you Google shaft critical speed.

I found this calculator. If the disc is set to zero mass it is the same as your shaft. I am sure this it is set for steel. The type of steel will not make much difference to critical speed. If you use another material like aluminium then it will be different.
http://www.denysschen.com/rotating/critSpeed.asp

It looks to give a reasonable result without going into the maths. The analysis would be similar for an overhangling load like a prop.

This formula only applies in air. You get substantial damping if the shaft is partially in water. Also a pushing prop has very high stabilising forces that work to align it to the flow. If the shaft is angled then there are large out-of-balance forces that have to resolved. They will cause vibration and bend a light shaft if it is not supported near the prop.

Rick W.

[FAST FRED]

Simple table in Skeen's Elements of Yacht Design,by F. Kenney along with hundreds of other goodies.

FF

[Greybarn]

Bearing spacing is a function of shaft material, diameter and RPM. The formula that Aquamet uses is
L=(3.21*D/N)^(0.5)*(E/W)^(0.25) where L is the maximum length between bearings, D is the shaft diamter, N is the shaft RPM, E is tensile modulus of elasticity in PSI, and W is the density in pounds per cubic inch. This length can be increased by 50% if it is in a rigid coupling at the engine end and the other bearing is in a strut
I bet if you google Aquamet that you can find their data.
Good luck.