symmetric drive

The following discussion assumes a turntable is being operated with the platter in the horizontal plane.

When using the turntable there are two sources of friction to be considered. The first (and most obvious) is that caused by the weight of the supported structure. Very little can be done about this friction in a small to medium model apart from squirting graphite powder into the cracks. In larger models I have seen the weight supported by outrigger wheels. The second and less obvious source is horizontal. When a single gear drives the turntable, the turntable platter is pulled towards the driving gear causing increased drag and even a grabbing effect under larger loads. Driving the turntable symmetrically provides equal and opposite forces on the platter that reduce the horizontal friction to near zero.

Unless some structural consideration makes it impossible, I like to use symmetric drives even if load is not large. I have used the simple drive train presented below in a number of models. The main consideration when constructing the drive is that corresponding gears be aligned in the same way. It is possible to get the 8t gear on one side out of phase with the other 8t gear by 1 tooth. This situation actually increases the friction markedly and is to be avoided.

	A general view of a symmetric drive.
	Top down view. Not a lot to show in this structure so I had to take a photo of something.
	Underside. Note that the gears meshing with the 24t crown gears must be at least 16t gears. This is because the hub of the 24t crown gear extends too far to allow an 8t gear to mesh - the supporting axle bumps into the hub.
	End view showing the detail of the vertical axle bearings. The yellow liftarms can be mounted horizontally but in this position they help to lock the main beams together.