PDA

View Full Version : ballnut housing and floating end ballscrew assembly



dsc
09-07-2013, 12:15 AM
Gents,

excuse the rather massive thread attack, but I'd rather ask than be sorry later.

Any reasons why ballnuts are normally mounted inside a housing rather than bolted on top? See attached drawing.

9220

I've got limited space between the rails, linear bearings and mounting plates and a 1204 ballnut fits nicely with it's 25mm depth. Anything over this, like a 30mm deep housing in order to fit the ballnut inside, will make things bad.

Second question is for a 10mm ID bearing on a 12mm ballscrew, doable? I know not enough material to hold the bearing in place, so I'm planning to use a circlip to rest the bearing on, again see attached drawing. Stupid?

9221

Thanks in advance for any advice.

Regards,
dsc.

John S
09-07-2013, 12:46 AM
Gents,

excuse the rather massive thread attack, but I'd rather ask than be sorry later.

Any reasons why ballnuts are normally mounted inside a housing rather than bolted on top? See attached drawing.

9220


Regards,
dsc.


Unless I'm missing something very obvious the two drawing will take up exactly the same space.
It's the flange that controls how much room you have.

m_c
09-07-2013, 12:48 AM
thanktheidiotwebmasterfortheremoval

dsc
09-07-2013, 09:37 AM
John, I should've attached a drawing showing the ballnut, this is something I got from Chai and it shows a flange which is 25mm deep. Perfect for my application, but looking closely at the drawing, you'll see that the 'barrel' of the nut is 24mm, which means there's 0.5mm radius difference between the outside of the round part of the nut and the flange. If I go with a housing that is 25mm deep and bore a 24mm hole in it to fit the ballnut inside, I'll be left with 0.5mm of aluminium on both sides, unless of course I make the housing deeper and I don't really have space for that, which is what triggered my question.

9222

m_c:

1) Hmmm, care to expand on why the bolts would be hard to get in?

2) Sh*te, that's what I was afraid of. I'd happily go with 16mm ballscrew, but the ballnuts for those are 40mm deep, which means there's no way to get it in. Another idea would be a 16mm trapezoidal screw, but again I need nuts which are max 25mm deep. Bearing wise I wouldn't want to go lower than 10mm, as it's flimsy and thin at 10mm anyway (plus manufacturers like SKF / Timken, don't go below 10mm with angular contact bearings which one typical uses for ballscrews).

Regards,
dsc.

John S
09-07-2013, 10:07 AM
OK got it now. What machine is it for ?

dsc
09-07-2013, 10:11 AM
I'm guessing we're talking loads here? it's a rather non-standard application with a z-axis alone, nothing else, as I need precision positioning in the vertical. Max loads would be maybe 1-5% of the maximum load capacity of the ball screw given to me by Chai (quoted figures where 4kN for static load and 3.5kN for dynamic load). The Z-axis runs on 15mm Hiwin rails, which are a pain in the ass to use, as their profiles are super low, which is why I've got this depth problem.

Regards,
dsc.

John S
09-07-2013, 10:49 AM
Put some gauge plate packers under the hiwin rails to get more clearance ?

m_c
09-07-2013, 11:14 AM
1) Hmmm, care to expand on why the bolts would be hard to get in?


Look at the drawing, and think about where the bolt head will go with the housing the wrong way around.

dsc
09-07-2013, 11:51 AM
There's 28mm between the base of the rails and the base of the blocks. 1204 ballbut is 25mm deep, but 1605 is a whooping 40mm, so I'd need 12mm more clearance, it's almost like adding an additional 12mm plate just to get clearance. How do people normally get around this? is it packers / pockets and double plates to get more clearance?

The bolt head can go from the top on both versions of the housing, it's an M4 bolt and I have 8mm clearance on top for the head. Standard head on a caphead M4 is roughly 7.2mm.

Regards,
dsc.

irving2008
09-07-2013, 12:21 PM
...

The bolt head can go from the top on both versions of the housing, it's an M4 bolt and I have 8mm clearance on top for the head. Standard head on a caphead M4 is roughly 7.2mm.

Regards,
dsc.
Standard BS EN ISO 4762 spec caphead for M4 is 6.64 - 7.00mm dia.

There's 8mm to play with... seems to me there's plenty of room...

m_c
09-07-2013, 12:36 PM
thisonecouldofbeenleftuneditted

dsc
09-07-2013, 02:48 PM
Thank you gents, this is exactly why I've asked the question, I've never thought about it to be honest.

Regards,
dsc.

dsc
12-07-2013, 09:22 AM
One more question regarding the fitting of the 'barrel' of the ballnut inside the housing. Normally the non-flange part of the ballnut is marked as 'g6' tolerance and if I'm not mistaken it's a slide fit in the housing. Any reason for this other than space saving and making the bore in the housing an almost exact fit? I'm asking because the mounting holes on the flange are 4.5mm dia, which means there's some wiggle room to align the ballnut with the ballscrew, but of course in order for that to be possible, the bore for the body of the ballnut needs to be slightly bigger than g6.

Regards,
dsc.