Anatomy of a spur gear.

Create a Polygon Primitive Gear.  Create > Polygon Primitives > Gear 

*Do not change any of the options this time, they should still be saved.  If pGear2 does not look identical to the last one.  The settings may have been changed.  Now in the Outliner, a object appears, pGear2

In the Outliner, select, pGear3 and then pGear2 - order is important.

Then Mesh>Boolean>Difference, this will subtract the pGear2 from pGear3.

Move (translate) pGear1, Translate X = -2 units.  This is pGear1 radius  + pGear2 radius, the gear should fit snugly .

Select pGear1 and group it, the pivot point moves to the origin. Name the group "Center_Gear_Group".

In the Outlier,  select the Center Gear Group selected, open the Graph Editor.  Windows > Animation Editors > Graph Editor  This curve should appear.

Choose > Infinity This shows you how the curves repeats

The curve remains at a constant.

With the Center Gear Group still selected, set Curves > Cycle with Offset  The curve should repeat itself from the last point.

With the Center Gear Group still selected, set Tangents > Linear  The curve should switch from Plateau to Linear or a straight line. It should look like this.

In the Outlier, toggle the plus sign next to the Center_Gear_ Group to show the child objects.  Select pGear1.

Make sure you are on frame 1 in the time slider. 

Right-Click on Rotate Y and choose Key Selected

Change Rotate Y to -1080 and then  Right-Click on Rotate Y and choose Key Selected

With the pGear1 selected, open the Graph Editor.  Windows > Animation Editors > Graph Editor  This curve should appear.

With the Center Gear Group still selected, set Tangents > Linear  The curve should switch from Plateau to Linear or a straight line. It should look like this.

With the Center Gear Group still selected, set Curves > Cycle with Offset  The curve should repeat itself from the last point.

Go back to frame 1 by selecting this.  pGear1 should return to its original position.

Right-Click and assign a new Lambert material.

This could be any color.

Move pCylinder1 only in the Translate X somewhere on the pGear1.

It could look like this.

Select pCylinder1 first, then {Shift} + pGear1 and then {p} to parent the pCylinder1 to pGear1.  Now what ever pGear1 does pCylinder1 follows.

Now pCylinder1 should be a child of pGear1.

Select pCylinder1, Change the Menu Set to Animation.