Author Topic: How tight is tight  (Read 16480 times)

Offline Stuart

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How tight is tight
« on: January 12, 2014, 11:11:43 AM »
Ok here is the problem
On my Myford I have had one of the angular contact bearings fret and make a devil of a noise ,lack of lube was not a problem on these drippy beasts

Ok ripped it down to find these are the 7007 series's  with Linda,s house keeping now short of  £170 for the pair, I stared to look round for info ,note we are talking about the outboard bearings not the bronze one

The book of words supplied with the lathe is of little help as is for a small bore S7 they just say adjust preload ? To much will cause wear , so out with google to find out more but alas my Google fu failed me.

I have found a comment tighten up the coller hand tight ,that's not a lot of use as I have very strong grip so hand tight for me would be quite tight .

Another stated using the Allen key tighten the coller

Now Myford did sell a pin spanner for the big bores but it has a 100mm long handle on it. RDG still sell them, if did pick one up from Nottingham before the closed so I do have the tools.

For info there is a shim between the outer races to allow the pre load and let the oil in , it's split

So how much pre load to apply

I have looked around the sites on replacing mini lathe bearings with AC ones ( arc and others ) but all they say is set the pre load. the same info for my mill on the same site on the strip down page again just set the pre load ?

Ladies and gentlemen I await your response


Stuart
AKA lordemond
My aim is for a accurate part with a good finish

Offline steamer

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Re: How tight is tight
« Reply #1 on: January 12, 2014, 02:04:36 PM »
Stuart....Are these bearings sold as DU?   Put another way.. Do they have a bearing number and are they sold as a matched pair?....

If they are DU, they are a matched pair, and when assembled correctly, the races touch each other...and it really doesn't matter how tight you tighten them....

NOW.....if they are non matched pairs.....sometimes called "magneto bearings" and sold separately...then how tight is tight is a very real concern,

The short answer, just enough to take the play out...and no more.   As I recall the bearings on the Myford are right next to each other, so thermal expansion effects of operation should be negligible.

If they are separate bearings, and it sounds like it....put a dial indicator on the end of the shaft and adjust until it doesn't have any play axially...but no more.  I am sure there must be a minimum radial play dimension to allow for bearing clearance on the front bronze bearing...but I don't have the bearing adjustment procedure for a Myford in front of me.

Dave

« Last Edit: January 12, 2014, 02:17:17 PM by steamer »
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fcheslop

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Re: How tight is tight
« Reply #2 on: January 12, 2014, 02:05:39 PM »

Offline Graham Meek

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Re: How tight is tight
« Reply #3 on: January 12, 2014, 02:11:55 PM »
Hi Stuart,

The info given by Myford's handbook is not much use really when setting this part of the bearing adjustment, what you need to aim for is to adjust all the play out of the two angular contact bearing and no more. The angular contacts "Back to Back" as these are will give a satisfactory service with-out the spacer, BUT, without it, it will not allow the oil in. You could make a spacer for the inner two tracks of the angular contacts either the same thickness as the outer spacer or about 0.05 mm thinner if you want to add a small amount of pre-load, but I would not go any further. This last option was what I did when I had my Myford S7, the bearing pack is then a solid item.

My best regards
Gray,


Offline Stuart

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Re: How tight is tight
« Reply #4 on: January 12, 2014, 02:24:58 PM »
Thanks guys

Dave
There are two bearings back to back and come as a pair but you can get them as a single ,but i have them as a pair

fcheslop

As you have found out information is very sparse

Gray

Yes it's the shim that is used that upsets the apple cart but I think I will go with just take the play out using a dial indicator
To comment on making a shim for the inner race would require some preset fancy grinding to ensure that it is truly parallel and will not cause a twist

The ones that failed were constructed like a magneto bearing ie the race could be disassembled that how I noticed the fretting wear

Stuart
« Last Edit: January 12, 2014, 02:29:45 PM by Stuart »
My aim is for a accurate part with a good finish

Offline steamer

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Re: How tight is tight
« Reply #5 on: January 12, 2014, 02:31:43 PM »
Stuart,

A duplex pair can be disassembled also...but shouldn't be mixed up.  They are ground such that when you tighten the bearing nut, the outer races touch first, then the inner races at which point the exact amount of preload is established.    These are expensive!....and they are a truly Matched pairs...not just two single bearings which is what I was trying to get on about.

Did your bearings come in a common box and have a part number with a DU or DB letters in the part number?

Dave

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Damned ijjit!

Offline Stuart

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Re: How tight is tight
« Reply #6 on: January 12, 2014, 03:02:19 PM »
Dave

these are they
B7007CTP4SDUL Pair of Precision Angular Contact

and were sold as a pair in one box

Hope that helps you

Stuart
My aim is for a accurate part with a good finish

Offline steamer

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Re: How tight is tight
« Reply #7 on: January 12, 2014, 03:37:26 PM »
OK....DU...Universal

The preload is built into them   You can't over tighten them if installed correctly in an appropriately designed assembly.

The races touch....and that's it.

Got a drawing of the installation?

If not which one of these diagrams matches what you're doing?

http://www.br.nsk.com/upload/file/NSK_CAT_E1254f_148-151.pdf


Dave
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Damned ijjit!

Offline Stuart

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Re: How tight is tight
« Reply #8 on: January 12, 2014, 03:48:52 PM »
Dave

Back to back DB

But here's the rub as Gray has stated there is a shim between the outer races with a gap in the ring at the top to allow oil in , oil nipple at the top between the races . It's about 15 thou.

That's the quick version but I will post up a diagram asap
My aim is for a accurate part with a good finish

Offline Stuart

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Re: How tight is tight
« Reply #9 on: January 12, 2014, 04:04:09 PM »
Dave hope this explains it

note the spacer it has gap for lube oil





the shim is part 30
note this is a S7  the big bore is well bigger

race is 62 x 35 x 14 in metric bits

Stuart
« Last Edit: January 12, 2014, 04:32:11 PM by Stuart »
My aim is for a accurate part with a good finish

Offline steamer

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Re: How tight is tight
« Reply #10 on: January 12, 2014, 04:30:28 PM »
Well....I can understand the lack of clear instruction.   I would use the indicator method I described to take the lost motion out with the center adjusting nut...then use the threaded collars to set the gap on the bronze bearing.   Does that sound right Graham?    You're a machine tool guy from way back..and you've owned a Myford.

Dave
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Damned ijjit!

Offline Graham Meek

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Re: How tight is tight
« Reply #11 on: January 12, 2014, 04:31:37 PM »
Hi Stuart,

Is the shim exactly 0.015" ?

The reason I ask is that the following site has some shim washers listed, problem is they come in stock sizes and quantities of 100, (don't worry about the quantity for the moment), if you use 2 off 0.2 mm shims then using their tolerances this will suit 0.015" (0.381 mm) as the shims are rarely dead on size. As regards quantity just give them a ring,  tell them you are building a special piece of equipment and can you have a couple of free samples of the size shims that you need, offer to pay the postage, I have done this many times with them in the past when I was working, they are usually very helpful.

http://www.springmasters.com/sp/shims-support-washers.html

Shim size is 35 ID x 45 OD x 0.2 thick.

Hope this helps,

My best regards
Gray

Offline Stuart

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Re: How tight is tight
« Reply #12 on: January 12, 2014, 04:34:48 PM »
Dave I have uploaded the pdf to be a bit clearer

Gray

As I am in the house it was an approximation but Monday I will measure it and see what I can do

got down to WS  its 0.6mm


ouch £ 15 min order plus p&P plus vat

but the price of the bearings I will have to think

Stuart
« Last Edit: January 12, 2014, 04:50:03 PM by Stuart »
My aim is for a accurate part with a good finish

Offline steamer

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Re: How tight is tight
« Reply #13 on: January 12, 2014, 04:44:02 PM »
Thanks Stuart,

I understand it.   It eliminates the need to use matched pair bearings and lets the oil get in...which as Gray said, is why the shim is there   But....it also makes setting the "preload" rather subjective other than measuring the overall axial clearance with  a DTI.

But.....How many millions of these lathes are out there?.... 8)   So I guess they work right?

Dave
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Offline Stuart

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Re: How tight is tight
« Reply #14 on: January 12, 2014, 05:04:51 PM »
Dave
I have got to agree with you its the hairy fairy way they tell you how to do it , its subjective with their method but not with yours

but as you say there are a good few out there maybe I am being to cautious with the cost of the bearings , Yes I am glad its not a Colchester lathe with Gamet bearings @£4500 each or a Monarch 10EE at who knows the price but on a pension  :old: they are expensive to me and I do not what to be repairing my lathe but using it


Gray

I have found a site that has no min order for shims so a pack of 0.2mm ordered to do a test to get the outer and inner races to line up


Stuart
My aim is for a accurate part with a good finish

 

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