Most motor sizing programs deal with time and torque analysis.  The traditional tradeoff is more torque for less time.  As an aside, the increase in electric motor torque comes with increased motor inertia, so it’s not for free.  And the motor costs are always a factor.

But this assumes that inertia is fixed.  And that’s an OK assumption as long as the assumption is made consciously.  Because, it’s only an assumption for the convenience of doing a time/torque tradeoff analysis, not reality.

The fact is that there are lots of options on inertia, whether the mechanism is new or existing.  If the parts are existing, you know what your starting parameters are.  But the real issue is to NOT ignore the options.  There’s a lot of performance bandwidth to be had by exploring materials and inertia options.

Here are some typical material properties; steels are approximately 7.86gm per cubic centimeter.  Good old steel, cheap, strong, dense.  Typical strength is 50-60kpsi without getting into the exotic alloys.   This is where most mechanical designs start because of steel’s low cost.

Machinable grades of aluminum alloys are routinely available in the 30-40kpsi range.  And the density is only 2.7gm/cubic centimeter.  Roughly one third of steel.  That means only 1/3 the inertia.  And only 1/3 the amount of torque needed to achieve a given motion.  So even though aluminum costs three times as much as steel, the cost of the motor to drive the load is reduced significantly.  In addition, most machine shops can run aluminum parts twice as fast, so it costs less to machine.  A lot less.  So at the end of the day, even though we could start the comparison of steel versus aluminum at direct material cost, that comparison wouldn’t take into account all the benefits.

Then there are the engineering plastics which have gotten better over the years.  Polycarbonate, for example, has strength in the range of 10-13kpsi with density of 1.3gm per cubic centimeter.  Half the density of aluminum, good strength and very inexpensive.  So in some cases, you could use more polycarbonate volumetrically to replace aluminum and reach comparable strength requirements while reducing inertia at the same time.

This is all based on common off the shelf materials.  The  options get even more interesting when you start exploring more exotic materials.

Titanium is a great alternative to steel when high strength and light weight are required.  But it’s expensive material to buy and because it is so hard, the machine costs are typically much higher than steel.  But when you have to have it, you have to have it.  And I have had projects where we needed the inertia advantage, and the premium paid for the material made possible some applications that couldn’t have been done any other way.

A friend of mine developed a material called AlBeMet, a blend of aluminum and beryllium.  The beauty of this material is that it has the strength of titanium at the weight of aluminum, beryllium being much harder and lighter than aluminum.  Beryllium doesn’t alloy well, but that’s the part the folks at Brush Wellman were able to get done.  And the results are phenomenal.  Again, it is expensive material, but you don’t need much of it to get the job done.  And where strength and light weight is needed, this stuff is incredible.

But the real point is, keep your options open when working on mechatronic designs.