Reflected Inertia -- correct concept?

[JohnnyAction]

Hi all,

First time posting to this forum. I have a mechanical engineering credential from long ago, but the following ideas were never dealt with in great detail in my studies. As of now this is sort of hobby / avocation for me.

My questions relate to reflected inertia of a geared system. I understand the basics of inertia and mass moment of inertia, and that load inertia is what the motor sees and that total system inertia includes the load inertia, plus all gears, shafts, linkages and the motor inertia itself.

Further, I understand that (for example --- in a simple system) a load driven by, say, a 3:1 gear set demonstrates a ratio of N = Nl/Nm (60 teeth driven load gear / 20 teeth pinion driver). Torque is increased at the driven gear, and this increased torque is carried forward to the payload.

My question is this: given that reflected load inertia is (Jr = Jl/N^2); the denominator of the gear ratio squared (N^2) implies that the MOI is acting in reverse. That is, it is feeding back to the motor such that N = 1:3 thereby decreasing the torque at the pinion gear. So, I am imagining that, in reverse (if you will), the concept of reflected inertia carries with it the idea that the load inertia is now "felt" through the system, but directed in reverse, and it is tending to drive the pinion-motor rather than the motor driving the load.

Is this correct thinking? Any additional thoughts would be appreciated. Thanks for reading.

JA

 

[mikekilroy]

First time posting to this forum. I have a mechanical engineering credential from long ago, but the following ideas were never dealt with in great detail in my studies. As of now this is sort of hobby / avocation for me.

My questions relate to reflected inertia of a geared system. I understand the basics of inertia and mass moment of inertia, and that load inertia is what the motor sees and that total system inertia includes the load inertia, plus all gears, shafts, linkages and the motor inertia itself.

Further, I understand that (for example --- in a simple system) a load driven by, say, a 3:1 gear set demonstrates a ratio of N = Nl/Nm (60 teeth driven load gear / 20 teeth pinion driver). Torque is increased at the driven gear, and this increased torque is carried forward to the payload.

My question is this: given that reflected load inertia is (Jr = Jl/N^2); the denominator of the gear ratio squared (N^2)

upto here makes sense; rest does not. so I guess I would say in response to your question, I dont understand it.

reflected J is just that; reflected J. MOI, J, whatever you want to call it, is just reflected inertia. It doesn't do anything other than act as an inertia the motor needs to move.... yes, what the motor sees goes down by the square of the ratio while the torque increase goes up linearly with the ratio. but what is your question then?

 

[JohnnyAction]

Thanks mikekilroy.

Your reply has been helpful.

JA