Rotating Ballnut - design ideas

[mutzy]

Reefy86, how are you? I am no expert in this forum like Boyan, M_C and Jonathan are. They truly understand the science behind the build. I was told once that once you start geting more than about 1574.8 mm the possible whipping action of the ballscrew may have an effect on the life of the bearings... take this into consideration and maybe the professionals could answer the question more directly. Mutzy

[reefy86]

That's what i was thinking so in general then i may need 3 of these ordering which i know these are not cheap but what is lol

[magicniner]

If it's only whipping in the ball screws and not rotational mass there have been some very elegant, low cost solutions to that,

- Nick

[mutzy]

Hello magicniner, forgive my limited intelect, but what is the difference between "whipping in the ball screws and not rotational mass"? what solutions are you referring to? Thanks for the info. Mutzy

[magicniner]

Hello magicniner, forgive my limited intelect, but what is the difference between "whipping in the ball screws and not rotational mass"? what solutions are you referring to? Thanks for the info. Mutzy

The difference between 1-"whipping in ballscrews" and 2-"Rotational mass is that

1. "Whipping" is when the length allows sag and rotation then "Whips it around" like a skipping rope resulting in a lot of off-centre mass and the associated vibration and potential damage.
and
2. "Rotational Mass" refers in this case simply to the mass which requires acceleration and deceleration to control a rotating screw assembly.

One of the simplest solutions I've seen is like a sprung saloon swing door with a slot in it, the slot fits the ballscrew and supports it in 3 directions, when the carriage passes it "pushes the door open" and the door swings back once it passes, a few of these spaced out along the screw on alternate sides will control whipping very effectively.

There are other more complex solutions but they can approach the cost of a rotating mount for a ball nut and as stated address only whipping and not rotational mass,

Regards,

Nick

[Neale]

2. "Rotational Mass" refers in this case simply to the mass which requires acceleration and deceleration to control a rotating screw assembly.

It's not obvious until you do the arithmetic but the inertia of a rotating screw like this can be high - you can easily get to the point that the inertia of accelerating the ballscrew exceeds the inertia of the gantry. As the length of screw goes up, the critical speed - the speed at which whip in the screw becomes excessive - goes down. So you need a bigger diameter screw, which increases critical speed again. But the rotational inertia of the screw goes up with something like the 4th power of diameter. In other words, you can increase the screw diameter to increase critical speed and hence gantry speed, but the increase in inertia will have a massive effect on acceleration and acceleration in turn has a big effect on cutting performance if you are doing anything other than straight cuts. This is what is behind Nick's comment - whip, critical speed, machine speed and acceleration, screw diameter - these are all linked and the art of the engineer is to find an acceptable compromise between them. Or use a different solution, which is why bigger machines use rotating ballnuts or rack and pinion.

[mutzy]

Thanks Magicniner, maybe you can give some names/part numbers and some places where to get these types of ball screw adapters. I'm going for the rotating ballnut myself.

Thanks Neale, It's awesome to have so many smart people to help us little brains get through the learning process.

Cheers to all.

Mutzy

[magicniner]

It took me a while to find this but I knew I'd seen it somewhere

https://www.youtube.com/watch?v=NWB6FAJCPhA

And there's this, a more technical but more expensive system

https://www.youtube.com/watch?v=kkcVWIsXGjk

I'm sure there are other ways to skin this particular cat but these are the only two I've come across so far ;-)

- Nick

[m_c]

Here's a vid of an Igus solution for supporting long screws - https://www.youtube.com/watch?v=Duq163m59TU

And I'm struggling to find a video of the system magicniner mentioned, but I'm sure I've seen one at some point.