Most mixer manufacturers,
committed to solid shafting even on larger mixing equipment,
use cast impeller hubs with bolted blades. These
keyed-in-place designs have many disadvantages including
weight, high stresses at the point where the bolted blades
attach, and problems with removal. These keyed hubs also
prevent the use of more rigid hollow shafting.
Sharpe Mixers overcomes these inherent
disadvantages with a family of hub designs that are unique
in the industry. Depending on the application requirements,
Sharpe impellers are normally attached to the mixer shaft by
one of the following means:
 SPLIT
HUB - Not available from other
manufacturers, the Sharpe Mixers Split-Hub design allows for
a stronger, lighter, more adjustable and more easily
removable impeller. Furnished on most of Sharpe's larger
(over 24" dia.) impellers, the two-piece hub clamps onto the
mixer shaft much like a split rigid shaft coupling. The
clamping force transmits the torque without the use of a key
on all but the highest torque installations. This means
infinite adjustability of the impeller position anywhere
along the shaft, without cutting and weakening the shaft
with a keyway. Fail-safe operation is guaranteed on
higher-torque impellers by welding "torque pins" on the
shaft that fit into matching sockets in the bore of the
impeller hub. This pin-and-socket joint prevents movement of
the impeller in any direction, and welding the pins to the
shaft prevents them from falling off and causing damage to
pumps, as loose keys have been known to do. The Sharpe
Split-Hub design also allows for a strong welded attachment
of the blades to the hub. This joint is normally the
highest-stressed part of an impeller, and is often the point
of failure on bolted-blade designs. Another strong advantage
of Sharpe's Split-Hub impellers is the ease of disassembly.
Keyed, one-piece hubs can get stuck on the shaft and be
difficult (or impossible) to remove. The Split-Hub impeller
is easily removed by using a crowbar (if necessary) to pry
the two halves of the impeller apart.
Split hubs are available in standard cast
form (where cleanability is not a problem) and polished &
gasketed "block" style for a more sanitary design.
ONE-PIECE -
Sharpe Mixers normally reserves one-piece hubs for smaller,
one-piece impellers (without bolted blades) when solid
shafting is used. These impellers, usually 24" diameter and
below, can pass through an 18" manway without the need for
removable blades. The
strength of a welded blade-to-hub attachment is retained and
ample clearance in the hub bore allows for easier
removal from the shaft. Torque is transmitted thru a
standard setscrew & key design or a torque "divot" in the
shaft. On the "divot" design, one setscrew in the impeller
hub extends into one of many sockets machined into the
shaft. This pin-and-socket design is similar to the torque
pins used on larger split impellers, preventing slippage of
the impeller and removing the concern of loose keys damaging
downstream pumps.
ALL-WELDED - There are many
specific reasons for permanently welding the impeller to the
mixer shaft. These include Halar, Kynar, rubber and other
coated impeller assemblies and sanitary and
pharmaceutical-grade applications that cannot have any
cracks or crevices on any wetted parts. Sharpe Mixers
specializes in these more "exotic" designs and has the
experience to prepare the equipment properly for the coating
(or polishing) and to handle and package the equipment
following specific procedures to prevent damage to the
coating or the surface polish.
Bolted Blades
- Sharpe impellers on larger designs (shafts greater than 6
1/2" dia.) are usually built using a welded hub and bolted
blades. This is due to the weight problems associated with
casting a hub for an 8" or 12" diameter shaft. These
impellers are designed specifically for the application,
carefully engineering every weld, bolt and section of the
impeller to handle the torque, thrust and bending moments
generated by the blades. The impellers pictured above are
179" in diameter mounted on a 40' long 12.8" diameter shaft
operating in a lime slurry.
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