Rotating Ballnut - design ideas

[Jonathan]

I've been thinking about how to attach bearings and a timing pulley to a ballnut such that it can be rotated with a stepper motor, instead of rotating the screw.

I've already done this on my router with an M20 screw since the long X-axis means that screw whip and inertia would have limited the speed significantly:



This is, I think, the most promising design I've come up with for rotating a ballnut. The chosen ballnut is the standard RM2510 ballnut from linearmotionbearings eBay seller (among others). The diameter of the ballnut and screw would makes angular contact bearings quite costly. At first I drew it with the ballnut inside the bearings, however the following design allows for smaller 6007 bearings and also gives the option of putting 2 ballnuts in to eliminate backlash. One ballnut attached to either side of the shaft.

Here it is:
(I'll draw it in 3D if necessary)



Black is ballnut/screw/bearings
Red is shaft, steel or aluminium - probably aluminium since lower moment of inertia.
Orange is thingy that fits on the end of the shaft and attaches to the ballnut via the 6 holes in the ballnut.
Green is some aluminum plate to hold every thing. One plate will be extended to accommodate the stepper motor.
Brown is timing pulley. I could machine the pulley on to the shaft, however that means making a new shaft if I get the number of teeth wrong.
Blue is a collar bolted to the aluminium plate to preload the bearings.

Any comments/ideas/criticism welcome!

[m_c]

Have a look at my Harrison conversion thread for pics of how I done it -http://www.mycncuk.com/forums/showth...ill-Conversion
Everything is big, but that's only because it needs a big screw. I should still have the solidworks files somewhere, which I'll have a look for shortly.

[Jonathan]

Have a look at my Harrison conversion thread for pics of how I done it -http://www.mycncuk.com/forums/showth...ill-Conversion
Everything is big, but that's only because it needs a big screw. I should still have the solidworks files somewhere, which I'll have a look for shortly.

Nice design, if you could find the solidworks file that would be much appreciated. I had drawn something similar previously, but with two angular contact bearings. I've now redrawn it based on yours with a single 5207 bearing. It's going to cost about £32 for two of those bearings, so quite a lot more than the previous design. It also means the pulley has to be at least 60T (assuming XL pitch), which is getting a bit large if I need a bigger pulley on the stepper motor. I've drawn two versions with the pulleys mounted using different methods.



I'm not sure if where the force from the tension of the timing belt is ok?

The problem for me with the above design is making the thread / nut. It's not something I've had much luck with before. Unless I spend £20 on a die and can buy the nuts:

http://cgi.ebay.co.uk/35mm-35-x-1-5-...item1c1a2cb180

[m_c]

The main reason I ended up with my design, was it was the best way I could find to fit everything into the available space, plus I was aiming for a pretty large ratio, so pulley size wasn't really an issue. Everything was made from hot rolled steel, due to it having to carry the full weight of the knee.


That was my 3rd revision of the design. Prior ones used two single row angular contact bearings, but they were too tall to fit in the available space.
Once I get the Conect lathe up and running, and get the Triumph manouvered into place, I'm hoping to get back to the milling machine.

If you want the actual solidworks files, PM me your email, and I'll send them over.

[Jonathan]

The main reason I ended up with my design, was it was the best way I could find to fit everything into the available space, plus I was aiming for a pretty large ratio, so pulley size wasn't really an issue. Everything was made from hot rolled steel, due to it having to carry the full weight of the knee.

I see, so out of the designs I've posted so far which do you think is the best option? Space is no problem at all for me.

I'm just redrawing the latest one to use a 40mm bore angular contact bearing since that means I can bolt a sort of collar on to the end of the shaft so that I don't need to do any thread cutting.

[m_c]

Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.

I got my bearing and locknut from www.simplybearings.co.uk (for locknuts, click bearings on the front page then scroll down).

As for thread cutting, sharp tool (I bought a HSS insert one from greenwood tools, but I see chronos/glanze have them listed now), slow speed, and remember don't disengage the leadscrew feed once you've made the first cut when doing metric! Reverse the lathe back to the start.

[Jonathan]

Personally, if space wasn't an issue, I'd take your top design, replace one of the normal bearings with a pair of angular contact bearings, and put a lock nut onto the shaft to adjust play out the bearings. That way endfloat is adjustable, the normal bearing provides extra support for the belt tension, and it allows for smaller pulleys.

By a pair of angular contact bearings are you referring to something like 2 of 7207, or a single 3207? Either way that's getting a bit expensive, plus cost of threading tool though I would use that for other things. I don't want to compromise on the design due to cost, but there are limits. Another thing with that design is since the bearing supports are separate pieces I've got to get the centre heights very accurate, and mount them on a good flat surface for the bearings to be concentric. Bear in mind I'm probably going to be machining this on my mini lathe, which isn't great.

How much end float can I expect in a 5207 bearing if I went for the other design? These are the cheapest I can find:

http://cgi.ebay.co.uk/5208-ZZ-DOUBLE...item53e20e2fe6

This is the other design modified for no thread cutting:



Thanks for the link to the lock nuts, I'll get them if necessary.

Decisions decisions...

[Jonathan]

I could use the original design but with tapered roller bearings - 32007 or L68149.

[m_c]

The bearing I used is a 3208 double row angular contact.
Normal bearings, which includes double row ones, aren't really suited to applications with lateral loads. Deep grove bearings are better, but still not ideal.
However, most common angular contact bearings are the same physical size as normal bearings, so you can always switch to angular contact at a later date.

[Jonathan]

The bearing I used is a 3208 double row angular contact.

Almost the same as I drew in post #7 then.

Normal bearings, which includes double row ones, aren't really suited to applications with lateral loads.

Yes, however I looked up on the SKF site before using those and is say that the bearings I chose for the initial design are OK for half the radial load rating in the axial direction. That's 5100N, which is far higher than I will get on the router - unless I crash it, and the force is shared over 4 bearings so my thinking is that that might be the way to go as they're so cheap and, as you say, I can either replace with angular or the same if they wear out. I wouldn't be able to fit double row in though.

Is there a reason why you have not mentioned tapered roller bearings? They would fit well.