This LEGO TECHNIC®-compatible double bevel gear has a rounded central hole so that it doesn't necessarily rotate with an axle passing through it.
Among other things, this allows for custom differentials to be built (high torque open differential, limited slip differential, ...), while using less space than a turntable.
This gear has a somehow special disposition of holes; it has pinholes every 30 degrees at a distance of 2 knobs from the center, as well as cross holes at a distance of 1.5 knob from the center, making it usable as a planet gears carrier for my
40 teeth ring gear.
A variation of this gear with a more trivial disposition of holes is also available
here. This is the second revision of the gear, which can withstand more torque than the first version without slipping when meshing with a bevel gear mounted on a perpendicular axle.
DISCLAIMER: This product is in no way endorsed by nor affiliated with the LEGO Group. The LEGO® and LEGO TECHNIC®brands are only used here to suggest this product is compatible with the LEGO® line of toys. It is an original design and not a copy of a branded product.