EPAIII
Diamond
- Joined
- Nov 23, 2003
- Location
- Beaumont, TX, USA
I have never designed a solenoid, but working from first principles I see this.
First, you do not state why you want a lighter plunger. From the discussion I can assume it is to get a faster motion from the overall assembly. But a plain and complete statement of what you want and why is the first step in any design process. That should be done.
1. With just a coil, as your photo seems to show, you have a coil with a hole in it. The magnetic plunger does try to center itself inside that coil when it is energized. Why? There is no stationary frame that the plunger is attracted to. What is happening is the magnetic lines of force are traveling through the plunger and they want to become as short as possible. So the plunger seeks a centered position where this is true. And this is stronger if more of the magnetic field passes through it.
2. Magnetic materials have a saturation point where they are as magnetized as is possible. They are saturated with those lines of force and more will pass around the magnetic material, not through it. So, for a given current in the coil, there is a certain volume, a certain cross sectional area which will contain all the lines of force being generated by the current/coil. If you want more force, then you will need a larger volume/cross section of the magnetic material as well as more current. A hollow tube would need to have a larger outside diameter if it were to have the same cross sectional area as a solid. When the cross sectional areas as well as the lengths are the same, then the volume and hence the weight or mass of the two plunger will be the same. It would seem that you do not gain anything from using a hollow plunger. If you want less mass, then just use a smaller OD.
3. If you use a hollow tube with the same mass, then the OD will be larger and then each turn of the coil will be LONGER. So more copper will be needed for the same amount of current. Remember that the magnetic field of a coil is primarily dependent on the number of turns and the current. Conversely, if you use a hollow plunger with the same coil you should expect less force and probably slower operation.
4. A further effect of a larger OD is that the larger length of each turn of the wire will mean more resistance per turn and you may need to step up to a larger wire gauge to maintain the same current.
All of the above seems to argue against making the plunger of a solenoid hollow. Once you determine the amount of force you need, then that would determine the cross sectional area of the plunger as well as the number of turns and the current through them.
In the above, it is assumed that the length of the plunger is roughly the same as the length of the hole in the coil. This would provide a more or less ideal pattern for the lines of force to pass through the hole (and plunger) before fanning out to wrap around the outside of the coil. If the plunger is significantly shorter, then the force would not be any greater after the trailing end of the plunger enters the hole and the centering action would lessen, producing a shorter stroke. On the other hand, if the plunger were significantly longer than the hole in the coil, then the force would reach a maximum when the end of the plunger reaches the far end of the hole in the coil and again would be reduced as both ends of the plunger stick out.
I suspect that the OEM solenoids have aluminum extensions because aluminum is non-magnetic and this allows the magnetic plunger to be just as long as the hole, thereby producing the maximum force at all points in it's travel.
If you want a one piece plunger, including the extension, then it would be best to lighten it by making the EXTENSION hollow instead of the main, functioning part of the plunger. But perhaps a better idea for reducing the weight would be to turn the extension to a smaller diameter. I don't know your dimensions, but if the main part of the plunger is 0.5" in diameter then the extension would be around 0.2" in diameter. It could have a larger tang at the end for the attachment point. I would not use stainless at all unless as a replacement for the aluminum extension and then I would select a non-magnetic alloy. And if you do it with one piece, be careful not to introduce a stress riser at the junction of the two ODs. In other words, use a generous fillet.
In support of the above is the fact that I have never seen a solenoid with a hollow plunger. The companies that make them day in and day out must know something. In short, I do not think you are right about using a hollow plunger.
First, you do not state why you want a lighter plunger. From the discussion I can assume it is to get a faster motion from the overall assembly. But a plain and complete statement of what you want and why is the first step in any design process. That should be done.
1. With just a coil, as your photo seems to show, you have a coil with a hole in it. The magnetic plunger does try to center itself inside that coil when it is energized. Why? There is no stationary frame that the plunger is attracted to. What is happening is the magnetic lines of force are traveling through the plunger and they want to become as short as possible. So the plunger seeks a centered position where this is true. And this is stronger if more of the magnetic field passes through it.
2. Magnetic materials have a saturation point where they are as magnetized as is possible. They are saturated with those lines of force and more will pass around the magnetic material, not through it. So, for a given current in the coil, there is a certain volume, a certain cross sectional area which will contain all the lines of force being generated by the current/coil. If you want more force, then you will need a larger volume/cross section of the magnetic material as well as more current. A hollow tube would need to have a larger outside diameter if it were to have the same cross sectional area as a solid. When the cross sectional areas as well as the lengths are the same, then the volume and hence the weight or mass of the two plunger will be the same. It would seem that you do not gain anything from using a hollow plunger. If you want less mass, then just use a smaller OD.
3. If you use a hollow tube with the same mass, then the OD will be larger and then each turn of the coil will be LONGER. So more copper will be needed for the same amount of current. Remember that the magnetic field of a coil is primarily dependent on the number of turns and the current. Conversely, if you use a hollow plunger with the same coil you should expect less force and probably slower operation.
4. A further effect of a larger OD is that the larger length of each turn of the wire will mean more resistance per turn and you may need to step up to a larger wire gauge to maintain the same current.
All of the above seems to argue against making the plunger of a solenoid hollow. Once you determine the amount of force you need, then that would determine the cross sectional area of the plunger as well as the number of turns and the current through them.
In the above, it is assumed that the length of the plunger is roughly the same as the length of the hole in the coil. This would provide a more or less ideal pattern for the lines of force to pass through the hole (and plunger) before fanning out to wrap around the outside of the coil. If the plunger is significantly shorter, then the force would not be any greater after the trailing end of the plunger enters the hole and the centering action would lessen, producing a shorter stroke. On the other hand, if the plunger were significantly longer than the hole in the coil, then the force would reach a maximum when the end of the plunger reaches the far end of the hole in the coil and again would be reduced as both ends of the plunger stick out.
I suspect that the OEM solenoids have aluminum extensions because aluminum is non-magnetic and this allows the magnetic plunger to be just as long as the hole, thereby producing the maximum force at all points in it's travel.
If you want a one piece plunger, including the extension, then it would be best to lighten it by making the EXTENSION hollow instead of the main, functioning part of the plunger. But perhaps a better idea for reducing the weight would be to turn the extension to a smaller diameter. I don't know your dimensions, but if the main part of the plunger is 0.5" in diameter then the extension would be around 0.2" in diameter. It could have a larger tang at the end for the attachment point. I would not use stainless at all unless as a replacement for the aluminum extension and then I would select a non-magnetic alloy. And if you do it with one piece, be careful not to introduce a stress riser at the junction of the two ODs. In other words, use a generous fillet.
In support of the above is the fact that I have never seen a solenoid with a hollow plunger. The companies that make them day in and day out must know something. In short, I do not think you are right about using a hollow plunger.
Like you think the bar should be spinning, not moving sideways. 
and the more you read, the more you think this can't possibly be right. 
I mean do these guys actually do anything other than sit around in classrooms being creepy? And then at the end of the book you get to these parts where it says that nothing can move faster than light and the universe is going to eventually transmute into a field of iron spheres and then you are like, this is just so wrong. 
