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| Brushless DC Motor Spindle by c raynerd |
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| raynerd:
Hi Guys I`m making a spindle using a brushless DC motor and I`m using the idea that someone has come up with on mycncuk. You basically take a brushless DC motor with a 8mm shaft. Purchase an 8mm straight shank er collet chuck, build up a bearing housing on the front of the motor and and replace the shaft with the chuck. Something like this: http://www.mycncuk.com/forums/attachment.php?attachmentid=4786&d=1286643523 I`m sure many many of you on here understand these much better than me, but this evening I took apart the outrunner motor and now have an idea on how it works. Basically, the outside of the motor "case" itself actually spins and this is locked to the shaft, which therefore forces the shaft around. In this picture: http://www.mycncuk.com/forums/attachment.php?attachmentid=4609&d=1316538477 ...you can see how the ally bearing housing is screwed to the motor winding section. It is then the blank looking piece to the top right, filled with the magnets that spins and by being connected to the shaft, causes the shaft to rotate. The bearing house as per the original idea shows the ally house bored to a deep shoulder, a washer, then bellvelle washers and then finally a second washer with the entire housing being screwed to the four mounting points of the motor. Although these means that the belleville washers will squeeze together when clamped to the motor and put tension against the washers against the motor and bearing house, it doesn`t put any tension or load on the spindle! The spindle can freely move up and down through the washer and is clamped with the grub screw at the bottom end. In my opinion, would I not be better boring a piece of ally straight through to act as a guide to centre the washers. Inserting the washer, belleville washers and then last washer. Screwing the housing to the motor and then .... pushing the er chuck hard against the top bearing, consequently compacting the washers and then tightening the top grub screw to hold this tension on the bearings AND spindle. Any help appreciated! Chris |
| raynerd:
I`ve just re-read last nights late woffle I wrote above and I think perhaps I need to simplify my question. When making a spindle, should there be tension on just the bearings in the housing and the spindle just runs between these loaded bearings OR should the spindle actually be clamped up and be the thing that is actually putting the tension on the bearings? I have two option in my head. 1. Is to make a bearing housing that is just a little bit too short with a blind hole into which the bearings and washers slide. When the housing is clamped up against the motor body, the outer bearing which will be pretruding just a small amount will compresses the beveled washers and puts tension on the bearings inside the housing. 2. The other option is to have a straight through bore in the housing. Bolt it upto the motor and slide in my bearings, beveled washers and last bearing. The bearing will then be protruding my a tiny amount on collet side. I`ll then push the chuck hard up against the bearings, compressing them a little and then tighten the grub screw to hold it in place. The bearings will then be loaded and the chuck will be holding this tension. I`m not clear which I am aiming for...if either!! Chris |
| John Rudd:
I'm a little surprised to see such a large motor with what would seemingly be having a low kv used as a spindle driver... It is usual to find the smaller motors having higher kv ratings than the bigger ones...Still if it does the job why not....and a novel idea of attaching the shaft to the front.. |
| David Jupp:
I wouldn't trust a grub screw to put any pre-load on the bearings. Definitely need something more positive than that. Your plan 1 for compressing the belleville washers sounds good. Careful control of dimensions (and/or shims) would allow you to choose the pre-load level. |
| John Hill:
I have some old tape drive capstans (1" mag tape) and they each have a knurled ring threaded on the shaft to put end load on the two ordinary-looking ball bearings. |
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