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Impeller Stress Solutions

See also Impeller Stress Calculations

Depending on the fan model there are a number of ways to reduce the stress levels.

The following are listed in order of preference

In general grade 350 material is stronger than grade 250 especially at higher temperatures and is almost the same price.

Airfoil Blades :

1) Add an outer speed ring if available.

2) Increase the number of spars.

3) Try a stronger material (remember that this may increase thickness due to availability.

4) Increase thickness (may reduce max permissible number of spars)

Single thickness blades (curved or backward inclined):

1) Try a stronger material (Beware of special handling, preheat and NDT for some materials. Option 2 or 3 may be better)

2) Increase thickness (could also affect inertia and starting time)

3) Add Outer speed ring if available (May also enable a reduction in blade thickness).

4) Add Inner speed ring if available (May also enable a reduction in blade thickness).

5) Add Outer and inner speed ring if available (May also enable a reduction in blade thickness).

6) Add Full speed ring if available (May also enable a reduction in blade thickness)

Backplates :

1) Try a stronger material (Beware of special handling, preheat and NDT for some materials.)

2) Increase thickness (will increase starting time)

3) If you are using a bolted hub try an integral hub.

4) Increase the hub cone or support disk thickness in conjunction with an integral hub.

Shrouds (front plates)

1) Try a stronger material (Beware of special handling, preheat and NDT for some materials.)

2) Increase thickness (will increase starting time)

3) Try a speed ring if available (in same order as for laminar blades).

Open Paddle fans

In general stresses are not affected by changing thicknesses.

1) Try a stronger material (Beware of special handling, preheat and NDT for some materials.)

Frequency problems:

1) Increase thickness. It may not be possible to eliminate the frequency problem by continually increasing thickness. Special modifications can be made to change the natural frequencies but detailed engineering is required using FEA (finite element analysis). This can be done at a later time but ensure that the stresses are within limits and that you have allowed for extra mass by increasing the thickness at least one gauge size over standard.