Types of Ballnut and preload, questions - pros & cons ?

[Jon S]

https://tech.thk.com/en/products/pdf/en_a15_035.pdf#1

So there is more than one way of making a ballnut, again I learn something new :whistling:

I am struggling to see how the "Deflector Type" recirculates balls. From the diagram it looks like it happens in the middle of the nut, surely the return path should be at the ends of the nut? If that was the case then what is the difference between this and the return pipe version? :confused:

The preload options seem pretty simple, push the two nuts apart slightly or pull them together slightly or mess with the thread pitch. Surely messing with the thread pitch will lead to a shorter service life? When would each of the three pre-load methods be of benefit or inappropriate and why?

Thanks,

Jon

[Jonathan]

I am struggling to see how the "Deflector Type" recirculates balls. From the diagram it looks like it happens in the middle of the nut, surely the return path should be at the ends of the nut? If that was the case then what is the difference between this and the return pipe version? :confused:

The return path just skips the balls back one 'row' (i.e the pitch of the nut assuming single start). It looks like the difference is just that for the 'Deflector Type' it happens inside the nut, wherease with the return pipe the pipe is on the outside so less compact. The RM series Chinese ballnuts that most people use have plastic return tubes inside (that's what the orange / white circular pieces are).

You can use Belleville washers to pre-load two single nuts to make a preloaded nut, but bear in mind if the force on the nut exceeds the pre-load you will get backlash.
In theory the error in the thread pitch of a low grade ballscrew C7 will cause different loading as the nut travels especially when it's a double nut that just uses a spacer. In practice for a DIY machine is this significant? Probably not.

Here's one way to do it:

http://www.mycncuk.com/forums/showthread.php/651-Warco-Major-CNC-build-log

You'll find plenty of long discussions on CNCzone and this forum about the relative merits of each type.