User:Thepigdog/Continuously Variable Transmission
Continuously Variable Transmission is transmission converts rotation at one speed to rotation at another speed where the relative speeds may be varied continuously through a range, while the rotation continues.
There are an intriguing number of devices that achieve this in some way. These may be classified as,
- Frictional Partial Contact - Use friction over a point or line to transmit the rotation.
- Frictional Full Contact - Use friction over an area of matching shape to transmit the rotation.
- Cogs - Use cogs or teeth, to transmit rotation.
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[edit] Refining the task
There are many ingenious devices based on partial contact. The materials involved may use oil or deformation of the materials to achieve a finite area of contact. The smooth vibration free nature of these devices may make them the best solution for implementing CVT.
But thats no fun. Let us restrict our attention to Frictional Full Contact and Cogs.
[edit] Vibration Free
In practice vibratation may waste more energy than the effects of friction. So in practice a CVT that uses cogs and full friction contact that radiates energy through vibration will not be practical. So is it possible to build a device that is vibration free? These conditions may be simplified down to,
- No part hits another part.
- There is no unbalanced movement.
There is a certain amount of approximation used here. Materials in the real world are not rigid and do transmit energy through waves. So instead of an exact measuring stick lets consider this as more of a quantitative score base system.
[edit] Spirit of the Challenge
I must admit this is a bit of fun or a diversion for me. So in that spirit of good fun let us consider possible designs. You are invited to add your own.
[edit] Design Issues
[edit] Non integer ratios of teeth
If there are gears or chains used for the transmission, there is a problem if the gearing ratio is not a ratio of integer numbers of teeth. However this problem may be resolved if the teeth are not engaged for the full cycle. While the teeth are disengaged the cog may turned an extra amount to adjust the teeth into the correct position to re-engage.
If this movement were always in the same direction then we would be left with the same problem. However the position may be an oscillation, adjusting the position forward on one cycle and back on the next. ???? There is still a net rotation needed ????
The shape of the oscillation may be relative to a cylinder, rotating at half the speed of the output cog. Two wheels could follow each side of the shaped cylinder giving the movement.
- Fixed while teeth engaged
- Smooth acceleration and then deceleration to move the cog forward relative to the output.
- Fixed while teeth engaged
- Smooth deceleration and then acceleration to move the cog backward relative to the output.
To reduce vibration in the drive we could duplicate the drive but with the opposite phase.
- Smooth deceleration and then acceleration to move the cog backward relative to the output.
- Fixed while teeth engaged
- Smooth acceleration and then deceleration to move the cog forward relative to the output.
- Fixed while teeth engaged
This oscillation may then be added relative to the output drive speed using planetary gears.
[edit] Placing Teeth on a Cone
A cone shape with a disk spinning against it gives gearing that changes as the disk moves against it. We want to replace the disk by a cog and place teeth on the cone. There are two problems.
- Equally spaced teeth can only be added when they fit around the cone in integer numbers.
- Some teeth will need to be angled relative to the axis of the cone to provide the equal teeth spacing.
The first problem is resolved by placing teeth on only one side of the cone. The second problem is resolved by not placing the teeth directly on the cone. Instead of teeth slots are put in the cone that carry the teeth. The teeth are linked together with a hinge mechanism. Some mechanism would need to be provided to achieve the stability and positioning of the teeth.
Straight teeth cause a whining sound as they engage. This problem may be resolved by having the teeth on the drive cog on the inside of a circle. This would give smoother engagement.
[edit] Vibration due to the Oscillation
There must be some vibration caused by the oscillation needed to position the teeth to engage. Because of the continuous shape of the cylinder this oscilation would occur even at integer teeth intervals.
When the gearing is an integer ratio, the cone may be disengaged, and a stepped cone engaged. A clutch could then disengage the oscillation driven to the planetary gear. This would eliminate the vibration in this case. There would then only be oscillation when moving between integer gears.
