Quote:
Originally Posted by ThunderCactus
all pistons have at least one metal tooth on the end of the rack. not because of higher spring tension, but because of the final gear of the sector gear slipping off that tooth. Think of all that built up pressure on the very tip of the tooth, it would very quickly wear it away if it were plastic.
The sector gear with the piston all the way back isn't putting as much force on the bearing as you might think, and not in the spot you might think. Don't forget the spur gear is also pushing against the sector gear.
And as much as we know about what bearings fail in what place, I hate to say it but now that I think about it, none of that actually matters. The bearings that fail are always OEM or cheap replacement bearings.
When a bearing fails in a cheaper end CNC machine, it's not necessarily because something went wrong, sometimes it's just a really shitty bearing.
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Pistons that come with AEGs often only have 1 or two rack teeth being metal at the end due to the higher sheer stresses at the end of the compression, again as a result of the higher forces.
Reliable surface-to-surface sliding motion on the last teeth has more to do with hardness mis-match and resilience of both the gear and rack material. So its not necessary per se that the last teeth be metal to provide this capability. Metal is convenient and as that is also the gear of highest forces and since the gears are metal, making the last teeth metal ensures hardness matching. During the slip stage, you go from few tooth engagement during earlier stages (higher contact ratio) to single tooth at the end and with forces applied right at the tooth's cantilevered edge too, so the stresses build up immensely on that single tooth right before piston is released.
Some of the pistons you can buy as an upgrade either have full metal racks, or have partial racks that are more than just the final one or two teeth. The theory I guess for the partial metal ones must be again due to the higher forces as the spring is more compressed at the end of the cycle. So to save weight and cost, they only put metal where it counts.
For support bearings, a lot of times its axial misalignment that leads to premature failure. When you repair a CNC machine's bearings for their ballscrews and what not, how do you ensure they are axially aligned perfectly as well as to their linear guides? Do you put the bearings on first on both ends to allow natural alignment, and then tighten both gradually? Are there measurements that can be taken to verify good alignment? Maybe based on vibrations of the ballscrew?