 |
|
| Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
| Reply |
|
|
LinkBack | Thread Tools | Display Modes |
|
|
|
#1
Posted to rec.crafts.metalworking
|
|||
|
|||
Bursting speed of flywheels (and overspun ball bearings)
Machinery Handbook has long had a section of flywheels. In the 20th
edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, and is impinging on the balls, the outer race will go twice the speed of sound. If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Basically, it all fits together. Then it bursts. Joe Gwinn |
|
#2
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
Joseph Gwinn wrote:
The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Basically, it all fits together. Then it bursts. Think of rotation as the average motion and resonant acoustic vibration as the instantaneous motion. The stress from the maximum vibration excursion can be much greater than that from the rotation. -- Fred R "It doesn't really take all kinds; there just *are* all kinds". Drop TROU to email. |
|
#3
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
On Mon, 17 Apr 2006 22:05:57 -0400, Joseph Gwinn
wrote: Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, and is impinging on the balls, the outer race will go twice the speed of sound. If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Basically, it all fits together. Then it bursts. Joe Gwinn I'd love to use a high speed camera to take a picture of the burst. I wonder if a light beam could be used for this. I'm picturing (sp?) 3 mirrors, a laser pointer, and a photodetector of some sort to trigger the shutter. Maybe use a very bright halogen light to illuminate the area. Use the mirrors to make a box shaped area with the laser. Maybe use more than three mirrors to make a cube shaped area. I guess if the bearings are exploding at 25000 rpm then if my math is right the pieces will be moving at about 218 feet per second. With a 1/1000 shutter speed it looks like the parts would travel about 2.6 inches. Maybe a better solution is to leave the shutter open and use a flashlamp instead. Hmm. ERS |
|
#4
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
Eric R Snow wrote: On Mon, 17 Apr 2006 22:05:57 -0400, Joseph Gwinn wrote: Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, and is impinging on the balls, the outer race will go twice the speed of sound. If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Basically, it all fits together. Then it bursts. Joe Gwinn I'd love to use a high speed camera to take a picture of the burst. I wonder if a light beam could be used for this. I'm picturing (sp?) 3 mirrors, a laser pointer, and a photodetector of some sort to trigger the shutter. Maybe use a very bright halogen light to illuminate the area. Use the mirrors to make a box shaped area with the laser. Maybe use more than three mirrors to make a cube shaped area. I guess if the bearings are exploding at 25000 rpm then if my math is right the pieces will be moving at about 218 feet per second. With a 1/1000 shutter speed it looks like the parts would travel about 2.6 inches. Maybe a better solution is to leave the shutter open and use a flashlamp instead. Hmm. I think the guy claiming 20,000 rpm max is wrong, and the actual rotational rate is ten times that. So, it would take a microsecond light pulse to stop the pieces, which are moving at Mach 2. This is high-powered rifle bullet speed. An easier way to measure the speed of the fragments is two curtains of fine wire - measure the time delay between disruption of the inner curtain and the outer curtain. One can also use disruption of a curtain to trigger a flash. Joe Gwinn |
|
#5
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
Joseph Gwinn wrote: Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Joe Gwinn - IF - The surface speed is what really matters, not the diameter, what would be the force with a surface speed of 1047 MPH or 1535 feet per second? An intersting number that is... It is very roughly (check my math) the surface speed at the equator. How much reduction in weight do we expect with that much surface speed? Aparently, 1535 feet per second is not enough speed at that diameter(?) Thus the diameter -And- the surface speed are important, or: Something strange needs to be explained. Someone please educate us all. Pete |
|
#7
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
|
|
#8
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
says... In article , "Ken Davey" wrote: wrote: Joseph Gwinn wrote: Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Joe Gwinn - IF - The surface speed is what really matters, not the diameter, what would be the force with a surface speed of 1047 MPH or 1535 feet per second? An intersting number that is... It is very roughly (check my math) the surface speed at the equator. How much reduction in weight do we expect with that much surface speed? Aparently, 1535 feet per second is not enough speed at that diameter(?) Thus the diameter -And- the surface speed are important, or: Something strange needs to be explained. Someone please educate us all. Pete From Machinery handbook - 22nd edition. (page 226) "The bending stresses in the rim of a flywheel may exceed the centrifugal (hoop tension) stress predicted by the simple formula s = V(squared) divided by 10 by a considerable amount. See relevant section for further edification. That's if you try to change the axis of rotation while the flywheel is madly rotating. No, that's if you're talking about a real flywheel with a thick rim and spokes. The formulas we've been bandying about are only strictly applicable to thin cylinders. Ned Simmons |
|
#9
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
Ned Simmons wrote: In article , says... In article , "Ken Davey" wrote: wrote: Joseph Gwinn wrote: Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. - IF - The surface speed is what really matters, not the diameter, what would be the force with a surface speed of 1047 MPH or 1535 feet per second? An intersting number that is... It is very roughly (check my math) the surface speed at the equator. How much reduction in weight do we expect with that much surface speed? Aparently, 1535 feet per second is not enough speed at that diameter(?) Thus the diameter -And- the surface speed are important, or: Something strange needs to be explained. Someone please educate us all. Pete From Machinery handbook - 22nd edition. (page 226) "The bending stresses in the rim of a flywheel may exceed the centrifugal (hoop tension) stress predicted by the simple formula s = V(squared) divided by 10 by a considerable amount. See relevant section for further edification. That's if you try to change the axis of rotation while the flywheel is madly rotating. No, that's if you're talking about a real flywheel with a thick rim and spokes. The formulas we've been bandying about are only strictly applicable to thin cylinders. Hmm. On second thought, I think you're right. With balls rolling on the outer ring at very high speed, we should see some metal fatigue effects from the cyclic bending seen as the balls pass by. Wonder if we are running through the fatigue life of the steel, which then cracks, precipitating the burst? One problem with this theory is that one would not expect this mechanism to lead to the essentially symmetrical explosions that have been universally reported. Joe Gwinn |
|
#10
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
says... Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, It's not. Show me a reference to a blowgun that produces a supersonic air jet with shop air and I'll reconsider. and is impinging on the balls, the outer race will go twice the speed of sound. Okay, since this keeps coming up, despite what seems common sense to me, I set up a test myself. That's a 6204 bearing with the seals removed, the grease washed out, and relubed with a few drops of light spindle oil. There's a paint mark on the race and the ball cage. http://www.suscom-maine.net/~nsimmon.../Bearing01.JPG http://www.suscom-maine.net/~nsimmon.../Bearing02.JPG I spun the bearing up and measured the difference between the speed of the ball cage and the outer race at several speeds between 1400 and 5500 RPM. The difference in the angular velocity in all cases was 100~200RPM, i.e., the race was going 5-10% faster than the balls. Exactly as you'd expect in an unloaded bearing with internal clearance where the balls are free to slip relative to the races. As I've said at least twice before, this clearance will only increase with speed. If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. I checked this as well just to make sure the assumption that the frequency of the sound from the bearing does in fact correspond to the ball passing frequency. I got my teenage son, who's involved in composing synthesized music, to set up his laptop with an FFT to monitor the bearing siren tone. Agreement was within a few percent, probably as good as could be expected with me getting a strobe fix while asking him to read the frequency. The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Because it's well known that, for the sort of bearing we're talking about, speeds in the few tens of thousands of RPMs are the lubrication limit for properly mounted bearings with elaborate mist lube systems. It's no surprise at all that a loose bearing that's just had all its lubrication removed would fail at somewhat higher speeds. Ned Simmons |
|
#11
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
Ned Simmons wrote: In article , says... Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, It's not. Show me a reference to a blowgun that produces a supersonic air jet with shop air and I'll reconsider. I didn't say supersonic, I said sonic (as the upper limit). The airflow chokes in the orifice, being limited to the speed of sound, so this is the upper limit (unless one has a nozzle that looks like the back end of a rocket engine, with an expansion bell). and is impinging on the balls, the outer race will go twice the speed of sound. Okay, since this keeps coming up, despite what seems common sense to me, I set up a test myself. That's a 6204 bearing with the seals removed, the grease washed out, and relubed with a few drops of light spindle oil. There's a paint mark on the race and the ball cage. It appears to have eight balls. What's the OD? http://www.suscom-maine.net/~nsimmon.../Bearing01.JPG http://www.suscom-maine.net/~nsimmon.../Bearing02.JPG I see one problem in the photos: That long thin copper tube will not achieve anything like the airspeed that a proper nozzle will achieve. I would suggest using a piece of 3/8" tubing with a machined brass nozzle hard soldered into one end. The brass nozzle would have a 60-degree (included angle) cone inside, going from 3/8" to 0.014" diameter at the face. I would put around the bearing a piece of heavy metal pipe lined on the inside with wooden staves, to stop the shrapnel. Even if you believe that the bearing won't burst. I spun the bearing up and measured the difference between the speed of the ball cage and the outer race at several speeds between 1400 and 5500 RPM. The difference in the angular velocity in all cases was 100~200RPM, i.e., the race was going 5-10% faster than the balls. Exactly as you'd expect in an unloaded bearing with internal clearance where the balls are free to slip relative to the races. As I've said at least twice before, this clearance will only increase with speed. These are very low rotational speeds. As the speed increases, won't centrifugal force pin the balls against the inside of the outer race, reducing or eliminating slippage? If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. I checked this as well just to make sure the assumption that the frequency of the sound from the bearing does in fact correspond to the ball passing frequency. I got my teenage son, who's involved in composing synthesized music, to set up his laptop with an FFT to monitor the bearing siren tone. Agreement was within a few percent, probably as good as could be expected with me getting a strobe fix while asking him to read the frequency. Good. The "siren" theory is confirmed. So, how do we explain the reports that the tone went ultrasonic just before the bearing exploded? With eight balls, this implies 20000/8= 2,500 rps, or 150,000 rpm, a factor faster than the 20,000 rpm discussed here. Also unexplained is the essentially perfect symmetry of the explosions. The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Because it's well known that, for the sort of bearing we're talking about, speeds in the few tens of thousands of RPMs are the lubrication limit for properly mounted bearings with elaborate mist lube systems. It's no surprise at all that a loose bearing that's just had all its lubrication removed would fail at somewhat higher speeds. While I don't doubt that being run bare at such high speeds chews the bearing up pretty fast, the guy I was mentioning also used the long thin air tube, and so didn't achieve full airspeed. Joe Gwinn |
|
#12
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
says... In article , Ned Simmons wrote: In article , says... Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, It's not. Show me a reference to a blowgun that produces a supersonic air jet with shop air and I'll reconsider. I didn't say supersonic, I said sonic (as the upper limit). The airflow chokes in the orifice, being limited to the speed of sound, so this is the upper limit (unless one has a nozzle that looks like the back end of a rocket engine, with an expansion bell). and is impinging on the balls, the outer race will go twice the speed of sound. Okay, since this keeps coming up, despite what seems common sense to me, I set up a test myself. That's a 6204 bearing with the seals removed, the grease washed out, and relubed with a few drops of light spindle oil. There's a paint mark on the race and the ball cage. It appears to have eight balls. What's the OD? 47mm - 1.85" http://www.suscom-maine.net/~nsimmon.../Bearing01.JPG http://www.suscom-maine.net/~nsimmon.../Bearing02.JPG I see one problem in the photos: That long thin copper tube will not achieve anything like the airspeed that a proper nozzle will achieve. I would suggest using a piece of 3/8" tubing with a machined brass nozzle hard soldered into one end. The brass nozzle would have a 60-degree (included angle) cone inside, going from 3/8" to 0.014" diameter at the face. I would put around the bearing a piece of heavy metal pipe lined on the inside with wooden staves, to stop the shrapnel. Even if you believe that the bearing won't burst. I have no interest in exploding a bearing g. I just wanted to get it spinning fast enough to run the tests described. I spun the bearing up and measured the difference between the speed of the ball cage and the outer race at several speeds between 1400 and 5500 RPM. The difference in the angular velocity in all cases was 100~200RPM, i.e., the race was going 5-10% faster than the balls. Exactly as you'd expect in an unloaded bearing with internal clearance where the balls are free to slip relative to the races. As I've said at least twice before, this clearance will only increase with speed. These are very low rotational speeds. As the speed increases, won't centrifugal force pin the balls against the inside of the outer race, reducing or eliminating slippage? Even at these speeds I don't imagine there's much slippage between the balls and the outer race - clearly there isn't. Where the balls *are* slipping is relative to the inner race, minimizing any speedup due to planetary action. If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. I checked this as well just to make sure the assumption that the frequency of the sound from the bearing does in fact correspond to the ball passing frequency. I got my teenage son, who's involved in composing synthesized music, to set up his laptop with an FFT to monitor the bearing siren tone. Agreement was within a few percent, probably as good as could be expected with me getting a strobe fix while asking him to read the frequency. Good. The "siren" theory is confirmed. So, how do we explain the reports that the tone went ultrasonic just before the bearing exploded? With eight balls, this implies 20000/8= 2,500 rps, or 150,000 rpm, a factor faster than the 20,000 rpm discussed here. I can't. Maybe Eric's hearing is worse than he thinks. I wouldn't know how low my upper limit is in one ear (starts rolling off at a few kHz) if I didn't fail the hearing test in grammar school every year. Also unexplained is the essentially perfect symmetry of the explosions. As I said before, it's easier to explain the lack of serious injuries if the available energy is much lower. Perhaps the bearings that exploded with bad consequences has selectively thinned the reporters g. The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Because it's well known that, for the sort of bearing we're talking about, speeds in the few tens of thousands of RPMs are the lubrication limit for properly mounted bearings with elaborate mist lube systems. It's no surprise at all that a loose bearing that's just had all its lubrication removed would fail at somewhat higher speeds. While I don't doubt that being run bare at such high speeds chews the bearing up pretty fast, the guy I was mentioning also used the long thin air tube, and so didn't achieve full airspeed. I agree that higher speeds than 20 KRPM should be attainable, but the deterioration will limit the speed, and that may be why he couldn't go faster. As the bearing gets beat up it'll take more power to keep it spinning at a given rate. Ned Simmons |
|
#13
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
In article ,
Ned Simmons wrote: In article , says... In article , Ned Simmons wrote: In article , says... Machinery Handbook has long had a section of flywheels. In the 20th edition, on page 346, states that all that matters is surface speed at the periphery, and the tensile strength of the steel, and gives a formula: V=Sqrt[10*s], where V is surface speed in feet per second, and s is tensile strength in pounds per square inch. Let us assume that s= 300,000 psi, the cited strength of ball bearing race steel. Sqrt[10*300000]= 1732 fps. A bearing 1.75 inches in diameter will have a circumference of (1.75)(3.1416)/12= 0.4561 feet, so 1732 fps implies 3,781 rps, or 226,832 rpm. The speed of sound is about 300 meters per second at sea level, or about 900 feet per second, so the surface speed of the outer race is 1732/900= 1.92 times the speed of sound at sea level. If the airjet is at the speed of sound, It's not. Show me a reference to a blowgun that produces a supersonic air jet with shop air and I'll reconsider. I didn't say supersonic, I said sonic (as the upper limit). The airflow chokes in the orifice, being limited to the speed of sound, so this is the upper limit (unless one has a nozzle that looks like the back end of a rocket engine, with an expansion bell). and is impinging on the balls, the outer race will go twice the speed of sound. Okay, since this keeps coming up, despite what seems common sense to me, I set up a test myself. That's a 6204 bearing with the seals removed, the grease washed out, and relubed with a few drops of light spindle oil. There's a paint mark on the race and the ball cage. It appears to have eight balls. What's the OD? 47mm - 1.85" http://www.suscom-maine.net/~nsimmon.../Bearing01.JPG http://www.suscom-maine.net/~nsimmon.../Bearing02.JPG I see one problem in the photos: That long thin copper tube will not achieve anything like the airspeed that a proper nozzle will achieve. I would suggest using a piece of 3/8" tubing with a machined brass nozzle hard soldered into one end. The brass nozzle would have a 60-degree (included angle) cone inside, going from 3/8" to 0.014" diameter at the face. I would put around the bearing a piece of heavy metal pipe lined on the inside with wooden staves, to stop the shrapnel. Even if you believe that the bearing won't burst. I have no interest in exploding a bearing g. I just wanted to get it spinning fast enough to run the tests described. How fast did you get it to go? I spun the bearing up and measured the difference between the speed of the ball cage and the outer race at several speeds between 1400 and 5500 RPM. The difference in the angular velocity in all cases was 100~200RPM, i.e., the race was going 5-10% faster than the balls. Exactly as you'd expect in an unloaded bearing with internal clearance where the balls are free to slip relative to the races. As I've said at least twice before, this clearance will only increase with speed. These are very low rotational speeds. As the speed increases, won't centrifugal force pin the balls against the inside of the outer race, reducing or eliminating slippage? Even at these speeds I don't imagine there's much slippage between the balls and the outer race - clearly there isn't. Where the balls *are* slipping is relative to the inner race, minimizing any speedup due to planetary action. So the balls and outer race spin together more or less as a unit? I guess I don't quite trust that so small a clearance will really always do the job. All it would take to get some real traction would be for the human holding the inner race to move slightly, twisting the axis of rotation a few degrees, causing the balls to come up against the sides of the groove in the inner race, pushing against the gyroscopic forces keeping the outer race from turning with the inner race. The lack of lubrication will make for more traction, especially if the balls are galling with the races. A test jig where the bearing is clamped to a bench would never see this effect. If the bearing has ten balls, the siren tone will be at 3,781*10= 37,810 Hz, well into the ultrasonic, as people have observed. I checked this as well just to make sure the assumption that the frequency of the sound from the bearing does in fact correspond to the ball passing frequency. I got my teenage son, who's involved in composing synthesized music, to set up his laptop with an FFT to monitor the bearing siren tone. Agreement was within a few percent, probably as good as could be expected with me getting a strobe fix while asking him to read the frequency. Good. The "siren" theory is confirmed. So, how do we explain the reports that the tone went ultrasonic just before the bearing exploded? With eight balls, this implies 20000/8= 2,500 rps, or 150,000 rpm, a factor faster than the 20,000 rpm discussed here. I can't. Maybe Eric's hearing is worse than he thinks. I wouldn't know how low my upper limit is in one ear (starts rolling off at a few kHz) if I didn't fail the hearing test in grammar school every year. I bet your Son can test your hearing limit. Also unexplained is the essentially perfect symmetry of the explosions. As I said before, it's easier to explain the lack of serious injuries if the available energy is much lower. Perhaps the bearings that exploded with bad consequences has selectively thinned the reporters g. Darwinism in action. But don't you think we would have heard the stories, if there were stories to be heard? The guy that did the experiment showing a max speed of ~20,000 rpm for whatever reason did not achieve full speed, as 20,000 rpm isn't nearly enough, and yet people have no problem causing bearings to burst from overspeed. Because it's well known that, for the sort of bearing we're talking about, speeds in the few tens of thousands of RPMs are the lubrication limit for properly mounted bearings with elaborate mist lube systems. It's no surprise at all that a loose bearing that's just had all its lubrication removed would fail at somewhat higher speeds. While I don't doubt that being run bare at such high speeds chews the bearing up pretty fast, the guy I was mentioning also used the long thin air tube, and so didn't achieve full airspeed. I agree that higher speeds than 20 KRPM should be attainable, but the deterioration will limit the speed, and that may be why he couldn't go faster. As the bearing gets beat up it'll take more power to keep it spinning at a given rate. Will deterioration really be that much of a limit on an unloaded bearing, especially if it isn't in contact with the inner race all that much? And, the airjet has plenty of power. Joe Gwinn |
|
#14
Posted to rec.crafts.metalworking
|
|||
|
|||
|
Bursting speed of flywheels (and overspun ball bearings)
|
| Reply |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Hybrid Mode
