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................................ Troubleshooting

Length "L"
Diam. (between Radial Bearing & Impeller oenterlines)
"D"
i
(ins.)! 6.0 6.5 7.0 75 8.0 8.5 9.0 9.5 10.0
! , ,
1 135 172 214 264 320 384 456 536 625
1 1 ! 88 112 140 173 210 251 299 351 410
1 8 3- 60 77 96 118 143 172 204 240 280

1 1 43 54 68 83 101 121 144 169 198
,
1 ~ 31 39 49 61 73 88 105 123 143
9 , ,
1 4 3- 23 29 37 45 55 56 78 91 106
1- 7 17 25 28 34 ' 41 50 59 69 81

I
2 14 17 21 26 32 38 46 54
2-1 11 14 17 21 25 30 36 42 49
2-1 8 11 13 16 20 24 28 33 39

Figure 11.5 Shaft slenderness ratio chart
The same thing is true of a packed pump which cannot maintain a
minimal amount of leakage for any length of time and seems to be
constantly leaking excessively regardless of the amount of time and
expertise that is spent on minimizing the leakage. This problem is
frequently blamed on the last individual that repacked that pump, or
even on the type of pacldng that is used; resulting in many different
packing styles being tried. The underlying source of the difficulty is also
an undersized shaft that is being subjected to excessive deflection.
The table shown in Figure 11.5 identifies the values of the Shaft
Slenderness Ratio (SSR) for varying sizes of pump shaft diameters and
the distances between the impeller and the radial bearing. This is a
dimensionless number that is derived from the equation shown in
Chapter 2.5.2 for calculating shaft deflection. The purpose of this SSR
is to provide a relative value for the condition of the overhung section
of the shaft in an end-suction pump.
The wealmess will escalate as the shaft diameter decreases and the
overhung length increases. Reducing the value will therefore improve
the strength of the pump shaft.

As an example, consider a pump with an overhung length of 8 inches
and sleeve diameter of l S/8 inches. This reveals a value of SSR of 73

m