preload

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jack_h
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preload

Post by jack_h » Fri Jul 28, 2006 9:06 pm

what does the preload do. and do you need one

jthompso
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re: preload

Post by jthompso » Fri Jul 28, 2006 9:53 pm

Most small gas turbines have a spring that touches the turbine (rear) bearing exerting a preload on this bearing. The loading is there to limit the rpms that the bearing will turn which prevents the turbine from spinning too fast and running away or excessively wearing the bearings. While not essential to the operation of the motor it is a good idea to have a preload on your bearing as a safety feature.

skyfrog
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re: preload

Post by skyfrog » Sat Jul 29, 2006 2:17 am

Hi Preload is used to remove radial clearance which has bad influences on bearing life as radial clearance will cause vibration and noise.

So it is preload that made the bearings to last longer in high rpm.
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jthompso
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re: preload

Post by jthompso » Sat Jul 29, 2006 3:39 am

I'm going to assume that you meant axial slop when you were talking about radial clearance. There is a common misconception that the thrust spring in many engines is there to counteract the axial frontward force exerted on the shaft by the turbine wheel, but this just isn't true. A thrust spring is a type of spring, the use of this type of spring in this position is purely coincidental. There is no forward force acting on the turbine wheel, the only load on it acts to the rear of the engine as the exhaust air hits the turbine blades. If the preload spring were there to counteract this force than it would have to push from the other side of the bearing.

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re: preload

Post by jthompso » Sat Jul 29, 2006 3:53 am

I suppose just to be on the safe side I should mention that some ball bearings actually require a small axial preloading to center the balls in the race and absorb slop, if you're using this type of bearing (manufacturer's literature should be able to tell you) then it would definitely be advisable to put in a preload spring. Also, in general a flanged bearing requires more preload forced than a standard bearing. Good luck!

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re: preload

Post by skyfrog » Sat Jul 29, 2006 8:01 am

Hi radial clearance of course refers to clearance in the ball races. The existance of radial clearance is a big problem in high speed, especially when we are using the ball race bearings to run far beyond its rating RPM. We are saying to run at 150-170K RPM.

The axial force on the bearings is not used to limit the RPM, on the contrary, axial force must be applied adequately or bearing cannot go up to such high speed. If axial force is too big of course it will act like a brake so a lot of heat will be generated, and too much heat will cause the bearing to fail.

Use throttling to the fuel pump instead of using axial force in the ball races to limit the turbine speed.
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Mike Everman
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re: preload

Post by Mike Everman » Sat Jul 29, 2006 9:12 am

Eliminating clearance, vibration and helping all the balls get in the load path are main goals of a spring preload. The third, yet probably most important is thermal compensation. If bearings are mounted hard, without one sprung toward the other, the thermal expansion of either shaft or housing will make short work of the bearings.
There are configurations of angular contact bearings with the proper diameter, spacing and contact angle that can be mounted hard and not fail due to thermal loading. Al Belli would like this one. Perhaps I'll sketch it up, so you see what I mean...
Mike
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