Cast iron, multistage, centrifugal air & gas blowers are by design, flex mount.  There are applications where the baseplate is bolted down tight, or even grouted into place, but all of the blower manufacturers will recommend mounting the base on solid level ground and positioned on flexible "vibration" pads. Although each manufacturer uses a different type of pad, they are all specifically sized and designed to handle the weight of the blower/motor assembly.   

Whether there are 4, 6, 8, or more pads, the placement of the vibration pad is critical to reducing the overall vibration level of the motor/blower unit.  I've moved vibration pads mere inches along the baseplate and experienced major changes in vibration levels.  When new installations report excessive vibration levels upon startup, this is one of the first things I look at.  Sometimes a bolt, or other piece of hard material, will get jammed under the baseplate effectively negating the 'flex' of the vibration pad. 

The pads do wear out.  They can also become so compacted with dirt & grime that they are 'hard as rocks'.  Most often, however, the flexible material degrades over time, appears to become flattened, and needs to be replaced.  When troubleshooting for the cause of excessive vibration, worn, or missing, vibration pads is high on the list.  I had one WWTP where I had to replace the vibration pads at the blower end of the base every year for 3 years in a row.  He would report high vibration, I would replace the 2 pads at the blower end, and the vibration levels would decrease by half! 

Another WWTP reported high vibration, and after not finding any other cause, we determined the motor end of the baseplate was dangling in the air!  It was not even touching the pads!  Never did figure out why that happened, but after adding steel shims beneath the base, with new pads, the unit has been running trouble free for years now!  

Although it is best to go back to the blower manufacturer to purchase the correct, original size pad, some suppliers, such as STS, can cross-reference and provide an equivalent set of vibration pads for any brand of blower.  The condition of the relatively inexpensive vibration pad can adversely effect the operation of the much more expensive blower & motor bearings.  Just a little Preventative Maintenance can save 1000's of dollars!