During centrifugal blower operation, the inlet end(low pressure) of the blower draws in air/gas, while the outlet end discharges the air/gas with higher pressure.  This high pressure actually pushes the rotating assembly toward the inlet end.  The inlet end(fixed) bearing has to absorb this thrust.  On smaller blowers, the thrust is minimal and readily absorbed by the deep groove, radial ball bearing design. Lamson even added the wavy spring washer to the outlet end bearing to help absorb this thrust and extend the bearing life even more.   The larger blowers, with relatively large rotors, had increased thrust forces that resulted in premature, inlet end bearing failures.  

Blower manufacturers' determined the most effective approach was to add a balance piston to the high pressure end of the rotor. The outboard side of the piston is piped directly to the low pressure(inlet) end of the blower, thereby creating a pressure differential opposite to the direction of the impeller thrust.  Reducing the thrust pressure on the inlet bearing clearly extended the life of the bearing.   

The piston itself is keyed to the shaft and positioned after the last impeller, and held in place by the shaft locknut. The OD of the piston usually has labyrinth grooves to act as a seal between the piston and cylinder.  The piston cylinder can be a separate component that is bolted to the outlet head casting, or it can be machined directly into the head. Sometimes it is lined with lead, or lead babbitt, material.  The cylinder ID can also be grooved, rather than the piston.  The clearance between the piston and cylinder is minimal, so if there is a severe bearing failure whereby the shaft drops, the balance piston/cylinder is surely going to be damaged.  However, the softer lead/babbitt would prevent the steel piston from damage.  

You can easily determine whether your blower has a balance piston by looking at the exterior.  If there is a 1" - 2" diameter pipe running from the inlet head to the outlet head of the blower, then it is connected to a balance piston.