Shaft Bearing Spacing in feet.

Formula;

• Ft = square root ( ( 3.21 x D ) / RPM ) x 4th root ( E / density )

Where D = shaft diameter in inches, RPM = shaft RPM, E = modulus of elasticity of shaft material, and density =  density of shaft material in pounds per cubic inch.

For our example vessel using a Tobin Bronze shaft of one inch diameter rotating at 2000 RPM, the modulus of elasticity is 16,000,000 p.s.i. and the density is 0.304 lbs/cubic inch.

This gives a result of 3 feet and 5 inches shaft bearing spacing.

If spacing is much less than this then we are wasting power with too many bearings, if spacing is any more than this there is a real danger of damage occurring to the shaft.