I just came across the doubt about how the space between the stator and the rotor in Brushless DC motors affect their performance.
Of course the further apart coils and magnets are, the least torque it provides, but, smaller gaps do really mean more torque? Is there a point where some other things begin to counteract the motor performance? (not counting the obvious case where the stator literally touch the rotor)
In any given motor, a higher field means better efficiency, with a lower current needed for any given torque, so lower copper losses. It also means better power per volume/cost/weight of motor.
The permanent magnets have to set up that field across the airgap. With a longer airgap, you need stronger (aka more expensive) or longer (aka heavier, more expensive and bulkier) magnets than with a motor that maintains a shorter airgap.
To maintain a small airgap without clashes between poles requires a more accurately made (more expensive) and stiffer motor.
There is a small penalty in very tight airgaps that the mechanical loss due to air viscosity increases, but that's generally outweighed by the reduction in electrical losses.
The design of any given motor is therefore a compromise between several different expensive parameters
Heat drives up the resistance of the windings, thus you loose torque. Excessive heat can weaken the permanent magnets, thus loss of torque. Air gaps of 10 mils are common. The issue with very tight 1 mil air gaps is when bearings wear out, the rotor has a slight wobble and if it hits the stator that is bad news.
To compensate ultra-hard bearings can be used, like the silicon nitride bearings used in the Space Shuttle, 1/10th the cost of synthetic sapphire bearing and lasting 5 times longer.
But this makes for a very expensive motor, and marketing often has more say than engineering. Bearings made of hard nickel-steel alloys would be more common.
