Blower Minimum Deflection Guide

When blowers are 60 HP (45kw) or larger, select deflection requirements for next larger span. A minimum of 2.5” (64mm) should be used unless larger deflections are called for on the chart or these fans are located in the lowest sub-basement or on a slab on grade.



1. Minimum deflection called for in this specification are not ‘nominal’ but certifiable minimums. The 0.75”(19mm), 1.5”(38mm), 2.5”(64mm), 3.5”(89mm), and 4.5”(114mm) minimums should be selected from manufacturers nominal 1”(25mm), 2”(50mm), 3”(75mm), 4”(100mm) and
5”(125mm) series respectively. Air spring isolation specifications 8 & 9 may be substituted for steel springs above in highly sensitive noise free locations.

2. Vacuum, Condensate or Boiler Feed Pumps shall be mounted with their tanks on a common spec. 21 base with deflections as specified for base mounted pumps.

3. The base described in spec. 20 is used under the drive side. Individual mountings as described in spec. 6 are used under the Cooler and Condenser.

4. This type of compressor is highly unbalanced and sometimes requires inertia bases weighing 5 to 7 times equipment weight to reduce running motion.

5. Limit deflection for utility sets 18” (450mm)wheel diameter and smaller to 1 1/2” (38mm).

6. FLOATING CONCRETE INERTIA BASES. Floating concrete inertia bases do not reduce vibration transmitted to the structure through the mountings. These bases will reduce vibratory motion, provide a very rigid machine base and minimize spring reactions to fan thrust. Engineers preferring steel bases rather than the concrete mentioned above in specification 5-21 should change the designation to 5-20. Concrete is preferred for all fans operating at static pressure above 4” (100mm) and on roof tops.

7. LIGHT FLOOR CONSTRUCTION. When floors or roofs are lighter than 4” (100mm) solid concrete it is desirable to introduce a localized mass under the vibration mountings in the form of a sub-base. This sub-base should be 12” (300mm) thick and 12” (300mm) longer and wider than the mechanical equipment above it. When this mass is provided the 30’ (9m) minimum static deflection requirements will suffice even in longer bays. The mass is also useful for unusually large bays over 50’ (15m). When floors are lighter than the 4” (100mm) concrete or the location is in a particularly sensitive area and the mass described above cannot be introduced, select deflection requirements for the next larger span.

8. For equipment where increased resiliency and decreased accelerations are required, change specification 16 snubbers to specification 17 snubbers.