Distance Between Bearings on Shaft Design Formula and Calculator

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Distance Between Bearings on Shaft Design Formula and Calculator

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The shaft which the bearings are installed on are potentially subject to both bending and torsion forces and must be accounted for in the design installation and spacing of bearings. Also, a factor of or margin of safety must be applied. The maximumbearing distance of separation may be calculated as follows

Instead of computing the transverse deflection, the maximum distance between the bearings (in meters) may be computed by the empirical formula to limit the transverse deflection to 0.8mm/m of length

Distance between bearings (Reference 1)
Eq. 1
L = 1500 / ( n + 1500 ) c D^{( 2/3 )}

Values of constant c

Type of shaft loading	Coefficient c	Allowable stress
Type of shaft loading	Coefficient c	MPa	kpsi
Shaft heavily loaded, subjected to shock, or reversed under full load	0.82	17	2.5
Line shafts and countershafts,	1.1	27	4.0
Line shafts or bar with pulleys close to the bearings	1.56	44	6.4

Alternatively

The shaft-bearing spacing formula is based on the formula for critical speed for a shaft having fully flexible bearings at both ends.

Eq. 2 (Reference 2)
L = [ 3.21 D ) / n ]^(1/2) ( E / W₁ ]^(1/4)

Where

D = diameter of solid shaft, m (in)
L = Unsupported distance between bearings
n = speed, rpm
E = Modulus of elasticity
W₁ = weght in lbs of shaft

Note: The minimum minimum spacing for RIGID bearings should exceed 20 shaft diameters when possible to facilitate alignment

Figure 1 Bearing Spacing Design Graph - Click on Image to enlarge

References

1. Lingaiah, K., and B. R. Narayana Iyengar, Machine Design Data Handbook, Engineering College Cooperative, Bangalore, India, 1962.
2. Aqualoy Propeller Shafting

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