Direct Driven Versus Gear Driven Mixers, A comparison:
Why is it that 95% of all portable mixers are gear driven designs rather than direct driven mixers? There are numerous reasons for this result, many of which lie beyond applying simple logic. It is a fact that the initial cost of most direct driven designs are less expensive as compared to a heavy duty gear driven mixer design. So if initial cost alone was the key factor, then you would think that a direct driven mixers would outsell gear driven designs, but this couldn't be further from the truth.
Torque, which is a ratio of horsepower divided by the mixer output RPM's, ultimately defines a mixers ability to generate a combination of flow and fluid shear. Said another way, the greater the torque, the greater the mixers capability to mix. So why would a heavy duty gear driven designs be selected over a direct drive design, even though the torque ranges may overlap, or in some cases be exceeded by direct driven designs?
(3 HP/1725 RPM)63,025 = 110 in-# >> (0.25 HP/350 RPM)*63,025 = 45 in-#
Again, a 1 horsepower direct driven design is very similar in torque generated by a 1/4 horsepower gear driven design, where the direct driven design is, on paper, much less expensive. Now explain why, in this torque range, 9+ out of 10 mixers that will be selected will end up being heavy duty gear driven designs?
To cloud and confound the issue even further, in some applications, direct driven designs outperform gear driven designs. The mixers power that is available is generally split into two commodities, flow and fluid shear. The later is used to break-down or degrade materials such as for dispersers and homogenizers. Again, direct driven designs have another advantage, but in this torque range, they are dominated by gear driven designs.
To begin to answer some of these questions, we'll need to generally apply the 80/20 rule of thumb to most mixing applications. What we find is that 95%+ of these applications are flow controlled.
Flow is proportional to (mixer RPM's) * (impeller diameter)3
So as you lower the mixers RPM's, at constant torque, you increase this mixers ability to generate flow, by increasing the impellers diameter. Unfortunately, there is no free lunch, because the impeller is generally constructed of an alloy such as stainless steel, but this is only the tip of the iceberg.