by ArtF » Tue Feb 18, 2025 1:06 pm
>>is kicked by the escapement and sends it spinning , what causes it to reverse direction? (is it that the vane assembly is asymmetrically balanced
Yes. But not by much. Imagine your vanes have just 1/4 of a gram extra on one arm. You spin the vanes by hand with some amount of thrust , the vanes will spin and spin and spin.. but, in the end when it stops spinning, that 1/4 gram weighted arm will be near 12:00 so long as the asymmetric weight is more than the friction of the bearings that oppose the rotation. This is because in an asymmetric system , the weight is removing rotational energy more than any other thing in the system, and it removes energy only on the upswing of the weight, once over the top it adds energy to the rotation, so it wont stop after 12:00 but only just before 12:00. This means that gravity, after it stops, must make it rotate backwards to pull that weight by gravity to the bottom.
The trigger is arranged so that it triggers when the weight gets to 6:00. You are ensured by gravity and physics that you will have almost 1/2 rotation backwards. So imagine the initial kick makes it spin 5 times, or 10, or 100, doesn't matter, the heavy arm will always be near 1:00 ( if spinning CCW ). and so , after any number of rotations the arm is between 12:00 and 2:00, this means it WILL now spin in reverse until the weight hits the bottom at 6:00 ( and momentum will carry it past 6:00 to about 8 or 9:00. ), The device re-kicks when it hits the trigger at 6:00. And so, the cycle continues.
SO the best scimitar 3 arm vanes revolve really easy and multiple times if you kick it. The offset of weight on one arm is just enough to push the trigger as it rotates backwards. If you dont have enough weight, it will hit the trigger and bounce back. The secret is to set the trigger sensitivity to be as hair trigger as you can make it, and the mass of the entire vane system is enough to press that trigger. This means the vanes are "almost" balanced.
Consider any pendulum, you pull it 25 degrees to one side and let it go, it will swing to "almost" 25 degrees the other way, its the same with the vanes, once the weight goes over the top during rotation, its a pendulum, and will swing to "almost" 12:00 as it goes around. If it has enough momentum it will go over the top again and slow a bit, this repeats until not enough energy is there to go over the top, and that's the point where it will reverse direction and try to go to "almost" 12:00 in the other direction, but it will trigger when it hits 6:00 because scimitar only triggers in that direction, and only at 6:00.
I can see on yours the offset is way too much weight, make it much more balanced, it will seem to you as it it isn't unbalanced at all as it rotates, and will not show its unbalanced nature until it stops, ( at which point that heavy vane will be at 1:00 or so.. as long as the weight is more than frictional energy.
Try this:
Mark one arm, remove the trigger and simply spin it with your hand, note the thing always ends that direction of spin with the heavy arm upwards near 1:00. If it doesn't, say it stops at 4 - 5:00, lower the weight. The perfect asymmetric weight is when one arm is just overweight enough to end at 12:01. It can then swing backwards to the trigger at which point the mass of the vanes trigger it from momentum, its not the weight per say that triggers it. Its the angular momentum. If it doesn't trigger, add weight to all three arms to keep it still just off balanced, but adds mass to the entire vane system.
If I spin mine by hand without triggers, it will spin 10 times, but always ends at 1:00 no matter how many times it rotates. It has enough mass to always trigger when it hits 6:00. Remember, the laws of physics says that , after stopping at 1:00, it will always try to rotate backwards to 11:00, but gets kicked at 6:00.
Adjust your weighting to "near" perfect balance. You'll find it starts to spin many times. Its all a matter of balance.
Regards
Art
(Sorry for the verbosity)
>>is kicked by the escapement and sends it spinning , what causes it to reverse direction? (is it that the vane assembly is asymmetrically balanced
Yes. But not by much. Imagine your vanes have just 1/4 of a gram extra on one arm. You spin the vanes by hand with some amount of thrust , the vanes will spin and spin and spin.. but, in the end when it stops spinning, that 1/4 gram weighted arm will be near 12:00 so long as the asymmetric weight is more than the friction of the bearings that oppose the rotation. This is because in an asymmetric system , the weight is removing rotational energy more than any other thing in the system, and it removes energy only on the upswing of the weight, once over the top it adds energy to the rotation, so it wont stop after 12:00 but only just before 12:00. This means that gravity, after it stops, must make it rotate backwards to pull that weight by gravity to the bottom.
The trigger is arranged so that it triggers when the weight gets to 6:00. You are ensured by gravity and physics that you will have almost 1/2 rotation backwards. So imagine the initial kick makes it spin 5 times, or 10, or 100, doesn't matter, the heavy arm will always be near 1:00 ( if spinning CCW ). and so , after any number of rotations the arm is between 12:00 and 2:00, this means it WILL now spin in reverse until the weight hits the bottom at 6:00 ( and momentum will carry it past 6:00 to about 8 or 9:00. ), The device re-kicks when it hits the trigger at 6:00. And so, the cycle continues.
SO the best scimitar 3 arm vanes revolve really easy and multiple times if you kick it. The offset of weight on one arm is just enough to push the trigger as it rotates backwards. If you dont have enough weight, it will hit the trigger and bounce back. The secret is to set the trigger sensitivity to be as hair trigger as you can make it, and the mass of the entire vane system is enough to press that trigger. This means the vanes are "almost" balanced.
Consider any pendulum, you pull it 25 degrees to one side and let it go, it will swing to "almost" 25 degrees the other way, its the same with the vanes, once the weight goes over the top during rotation, its a pendulum, and will swing to "almost" 12:00 as it goes around. If it has enough momentum it will go over the top again and slow a bit, this repeats until not enough energy is there to go over the top, and that's the point where it will reverse direction and try to go to "almost" 12:00 in the other direction, but it will trigger when it hits 6:00 because scimitar only triggers in that direction, and only at 6:00.
I can see on yours the offset is way too much weight, make it much more balanced, it will seem to you as it it isn't unbalanced at all as it rotates, and will not show its unbalanced nature until it stops, ( at which point that heavy vane will be at 1:00 or so.. as long as the weight is more than frictional energy.
Try this:
Mark one arm, remove the trigger and simply spin it with your hand, note the thing always ends that direction of spin with the heavy arm upwards near 1:00. If it doesn't, say it stops at 4 - 5:00, lower the weight. The perfect asymmetric weight is when one arm is just overweight enough to end at 12:01. It can then swing backwards to the trigger at which point the mass of the vanes trigger it from momentum, its not the weight per say that triggers it. Its the angular momentum. If it doesn't trigger, add weight to all three arms to keep it still just off balanced, but adds mass to the entire vane system.
If I spin mine by hand without triggers, it will spin 10 times, but always ends at 1:00 no matter how many times it rotates. It has enough mass to always trigger when it hits 6:00. Remember, the laws of physics says that , after stopping at 1:00, it will always try to rotate backwards to 11:00, but gets kicked at 6:00.
Adjust your weighting to "near" perfect balance. You'll find it starts to spin many times. Its all a matter of balance.
Regards
Art
(Sorry for the verbosity)