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View Full Version : angular contact bearings - back to back, further apart



dsc
04-01-2014, 07:39 PM
I've decided to ditch the double row AC + floating bearing approach and simply use two single row angular contact bearings mounted back to back:

11185

Am I correct to think that the bearings need to be a slide fit on the shaft and a press fit in the housings? preload will be via top cap, with the mounting bolt torqued to ##Nm, not sure on the value though. That or I'll use a locknut with a side grub locking screw.

I'd like for this design to offer as much stiffness as possible, will it work, or is there something I'm missing?

Regards,
dsc.

m_c
04-01-2014, 09:14 PM
The main thing is whatever bit gets adjusted floats.

However that drawing won't work. Tightening the screw will simply pull the inners towards each other and preload nothing!
You'll have to flip both bearings, which given your application, will probably be better in terms of where the main forces go...

dsc
04-01-2014, 09:43 PM
Indeed I've screwed up the lines showing the centers, so the above shows a face-to-face rather than back-to-back (I think). Re done here:

11188

Original I had tapper rollers in the design, the issue with those was no seals. ACs offer seals as well, but the idea stays the same.

Thanks m_c.

Regards,
dsc.

JAZZCNC
04-01-2014, 09:52 PM
M_C beat me to it about wrong way around but I would also had that you'll want the locknut approach with locking grub screw rather than the just tightening a thread to a torque value.

johnsattuk
04-01-2014, 10:59 PM
One of the snags with separated bearings in that configuration, is that the preload will change if there is differential expansion between spindle and housing :concern:

dsc
05-01-2014, 12:07 AM
Jazz I do have a fancy SKF locknut with a grub screw (pricey little bugger) which I could use, the only issue is accessing the grub screw as it's on the side. The cap / bolt is easier, as I can bolt it up from the top, but proper preload doesn't guarantee the bolt staying where it should without threadlock (I think).

As for expansion John, I'm guessing you meant thermal expansion, which would force the housings further apart and increase the preload? this shouldn't be an issue as all the parts will stay close to room temperature all the time.

Regards,
dsc.

johnsattuk
05-01-2014, 01:55 AM
As for expansion John, I'm guessing you meant thermal expansion, which would force the housings further apart and increase the preload? this shouldn't be an issue as all the parts will stay close to room temperature all the time.

Regards,
dsc.

Would depend on the materials of the housing and spindle and what the source of the heat was. If the spindle is hotter it will decrease the preload.

Is not usually a problem with slower running spindles like lathe headstocks or gearboxes which are oil filled as the oil evens the temperatures out. More of a problem with HS spindles where a lot of the heat is generated by the bearings and the rotor and the housing is cooled from the outside.

johnsattuk
05-01-2014, 11:44 AM
I'd like for this design to offer as much stiffness as possible, will it work, or is there something I'm missing?

Some car differentials and wheel bearing designs have a single use deformable spacer tube between the bearings. It has the advantages of gripping both sides of the bearings, the compressed tube and the tensioned shaft add to the stiffness of the assembly, and the high bolt up torque required to deform the spacer, make the nut more secure. Usually adjusted by measuring the increase in torque required to turn the spindle whilst tightening the nut.

Jonathan
05-01-2014, 01:03 PM
Back to back they faced each other, ... (http://www.folklore.bc.ca/Onefineday.htm#Onefine)


I'd like for this design to offer as much stiffness as possible, will it work, or is there something I'm missing?

If you want it to be as stiff as possible, then you could go back to using tapered roller bearings and add shaft seals, e.g:

Bearing Station | Oil Seals (http://www.bearingstation.co.uk/products/Oil_Seals)

dsc
05-01-2014, 01:58 PM
Pretty much everything is aluminium, apart from the shaft which is 316 SS. Heat sources will be far away, so the only way the housings can get warm is through consistent running, which is rather unlikely (operation will be more start-stop than on forever). In between the bearings there's a pulley, so the tube idea is not possible.

Cap vs. locknut - still not sure, both have pros and cons, with no access from the side, the locknut is a) harder to torque to the right value (needs a locknut socket) b) harder to secure in place via grub screw (would need a longer thru hole on the mounting plate for allen key access). Cap / bolt has top access which is good, easy to torque to the right value, although would need Loctite or some threadlock to hold in place without undoing itself as I doubt the bearings can take huge preload needed to stop the bolt from undoing itself.

With tapered roller bearings, I'm guessing I'd need to periodically re-apply grease? I can fit grease seals, but it complicate things a bit more from a design point of view. With sealed single row AC bearings, I can just whack them on the shaft and not worry much. With tapered rollers vs angular contacts - is there really that much more stiffness offered by the rollers?

Just one addition check before I start doing proper drawings, both single row angular contact bearings should be an interference fit in the housings and a close slide fit on the shaft?

Regards,
dsc.

johnsattuk
05-01-2014, 02:11 PM
In between the bearings there's a pulley, so the tube idea is not possible.

A tube each side of the pulley would also fasten the pulley to the shaft :thumsup:

dsc
05-01-2014, 04:31 PM
John, wouldn't a tube cause more problems, if it's length is not super precise?

I was asking about tolerances on the bearing fits, as I'm not quite sure how ACs are normally fitted. I've seen interference fits mentioned a few times in technical literature and I can understand that on the bottom bearing, but should the top bearing be an interference fit as well? the bottom can be easily pressed against the shoulder, so that's not an issue, but I guess the top should simply slide over the shaft? if so, isn't there a risk of the shaft rotating within the top bearing inner ring? I guess preload would create friction between the top of the inner ring and locknut / cap, but is that enough?

Regards,
dsc.

johnsattuk
05-01-2014, 05:09 PM
John, wouldn't a tube cause more problems, if it's length is not super precise?

I have been talking about collapsible tubes as used extensively in some applications.

Because your tubes would be relatively long a pair of thin wall aluminum tubes would compress the amount needed, a bit of trialing to get the right combination of force/distance needed, perhaps a bit OTT for you.

johnsattuk
05-01-2014, 06:38 PM
Oldish American video, they call it a crush sleeve.

Setting pinion bearing preload on the Ford 8.8" axle - YouTube (http://www.youtube.com/watch?v=8APYO2sZyJU)11193

LAND ROVER FREELANDER REAR DIFF DIFFERENTIAL COLLAPSABLE BEARING CRUSH SPACER | eBay (http://www.ebay.co.uk/itm/LAND-ROVER-FREELANDER-REAR-DIFF-DIFFERENTIAL-COLLAPSABLE-BEARING-CRUSH-SPACER-/321097188467?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ac2e2447311192 (http://www.ebay.co.uk/itm/LAND-ROVER-FREELANDER-REAR-DIFF-DIFFERENTIAL-COLLAPSABLE-BEARING-CRUSH-SPACER-/321097188467?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ac2e24473[ATTACH=CONFIG)

dsc
05-01-2014, 07:51 PM
I guess the tube acts as an additional way of protecting from too much preload, right? without the tube, there's no way to stop the bearings from being practically destroyed by over tightening the locknut / cap-bolt?

I'll probably be less scientific with my approach and tighten the top until there's no play and maybe go 1/8 turn further.

Regards,
dsc.

johnsattuk
05-01-2014, 08:25 PM
I guess the tube acts as an additional way of protecting from too much preload, right? without the tube, there's no way to stop the bearings from being practically destroyed by over tightening the locknut / cap-bolt?

Well not quite, if you get too enthusiastic with the wrench and collapse the tube too much, you can still destroy the bearings. If you exceed the preload on assembly the recommendations are that you use a new sleeve, which of course they always do :highly_amused:

What it gives you is a high torque retaining nut/bolt with the ability to set a small preload and a very rigid assembly. In practice it is quite easy to feel the spindle start to take a preload and is also possible to adjust later if required, not so easy with fixed length spacers.

Differential pinions do have stringent requirements, high loads from the hypoid gear at perhaps 200hp 6000+ rpm and must run quietly.

dsc
05-01-2014, 09:59 PM
As this is for a 240RPM spindle with rather low load requirements, I think a sleeve will be a bit OTT. All that is required is for the whole assembly to be as stiff as possible and to be assembled with the least amount of effort. So far two single row AC bearings win on all levels, with taper rollers in second place.

I've found some additional info on CNCzone regarding the fittings of the single row AC bearings, the bottom one against the shoulder should be an interference fit, the top will be a slide fit. Hopefully that's the right way to do it.

Regards,
dsc.