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# MOI on two axes thru rt angle gearbox

## MOI on two axes thru rt angle gearbox

(OP)
Hi all,

Question about MOI. The system consists of an electric motor, coupled to a right angle gearbox by way of a torque limiting clutch. The output of the gearbox turns a drum.

Is it valid to add the individual MOIs to find total MOI, even though the drum rotates around an axis that is perpendicular to the motor's axis of rotation? I feel like it's not, but that's how our calculations are set up.

Thanks!

### RE: MOI on two axes thru rt angle gearbox

You are wrong.

Mike Halloran
Pembroke Pines, FL, USA

### RE: MOI on two axes thru rt angle gearbox

(OP)
Mike,
I don't doubt you're correct but a little detail about why I'm wrong would be appreciated!

### RE: MOI on two axes thru rt angle gearbox

The addition as such is correct. The MOI represents the kinetic energy of the rotating masses. The perpendicular redirection through an appropriate mechanical linkage (i.e. bevel gear stage within the gearbox) does not change the energy.
However, in order to do the drive calc's, one needs to consider the acting gear ratio to refer the single MOI's as if acting on the motor shaft. So, the MOI of drum is to be divided by the square of the gear ratio. And so on, for all resp. instances. The gearbox manuf. usually give the MOI of the whole gear train already as "referring" to the input shaft.
A reference would be Shigley 7.2.1 on sizing of flywheels, or else you bing on <MOI refer to shaft> or in short: https://www.engineersedge.com/motors/gear_drive_sy...
Regards

Roland Heilmann
Lpz FRG

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