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Turbomolecular pump bearing question

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hboy007

Tue Jan 03 2012, 02:44AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

Now or never, I thought, when I started disassembling my broken Edwards EXT250H pump I got from a friend. I still dream of having a UHV chamber to do some vacuum experiments and apply coatings to surfaces, so here we go.

The pump lacks a bearing. I don't know what exactly to look for. The burn marks on the shaft and the spacers that came with it are misleading. The shaft itself has a diameter of 8mm, there is enough room for a 22mm bearing with a width of 7-11mm. The problem is... I have only a rough guess what bearing is supposed to fit in there...

Let's start with a cross-section drawing:

page 10

you can see that the upper bearing is a combination of magnetic and radial bearing (for safety) which gives you a heart attack when removing the rotor (those magnets are seriously strong in contrast to the fragile aluminium compressor vanes).

And there is the part which I'm pretty much stuck with right now: the lower bearing. The manual doesn't say what type of bearing it is and since Edwards also offers a repair service, they have little inclination to tell anyone. The magnetic bearing generates some axial force that may preload the shaft and the rotor must be controlled radially, so the load on the bearing is mixed. This rules out purely radial bearings like 608.

The 708A series looks more like it (that's an angular contact bearing) but the standby rpm is 42.000 and the normal operation is specified as 60.000 rpm. This would also rule out the 708As. (edit: there is a hybrid version with Si3N4 ceramic balls that runs up to 156.000 rpm:

)

So these are the specs: 8x22x11mm bearing, possibly with a split inner ring (there is a ring of burnt lubrication oil 4,5mm away from the 10mm->8mm shoulder of the shaft), four point contact or angular contact. The inner ring outer diameter in the proximity of the nut is 12mm (edit: not 10mm, corrected) so the nut reaches inside the bearing by ~1.5mm (this part scares me a bit).

The "11mm width" measurement is based on the distance between the spacer underneath the axial silicone seal and the threaded ring. The latter shows wear within a radius of 10.5mm (22mm outer ring with fillet) from the center, also indicating direct contact with a 22x11mm outer ring.

1325558665 1667 FT0 Ext250h 8x22 Bearing

any help is appreciated! Thank you!

Patrick

Tue Jan 03 2012, 04:23AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i fear you may be boned. with all those caveats you may need their proprietary bearing. is thatwhole yellow part the sealed bearing? or does it use loose bearings.

Experimentonomen

Tue Jan 03 2012, 04:26AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

As these pumps are very expensive to buy, i think it'd be worth it to send it in for refurbishment, i doubt you can get the correct bearings as an individual. Maybe Edwards sell them as spare parts, i dunno, but be ready to spend like 500 bucks or more as these are special bearings designed to operate under high vacuum.

The marks on the shaft you describe could be score marks from the bearing seizing up at operating speed, who knows.

hboy007

Tue Jan 03 2012, 11:48AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

The lower bearing is oil lubricated and sits inside a sump filled with felt discs to allow horizontal operation ( (9) in the figure on page 10), both bearings are only subjected to the roughing pump pressure (say 10^-2 mbar) which is well above the vapor pressure of common lubricants.

Experimentonomen wrote ...

As these pumps are very expensive to buy, i think it'd be worth it to send it in for refurbishment, i doubt you can get the correct bearings as an individual.

The safety bearing near the magnetic bearing assembly is just a 6804 bearing without seals and lifetime lubrication and it sits right inside the the upper end of the motor assembly. These guys have no qualms about fitting a $1 bearing inside a TMP where it doesn't matter.

Of course it would be worth sending it in for a major overhaul but I'm not the guy that speculates on getting the most out of it when I sell it again.
As I mentioned above, Edwards has (at least for some time in the past) followed an exclusive service policy. You'd decontaminate the pump, get a decontamination certificate and send it in for repair. With their improved designs, it now seems possible to exchange the bearing from the rear side only so they have on-site service and tool kits. This pump however is old and I see no other option as to remove the rotor/ stator assembly completely from the motor in the base and put it back when the bearings are in place.

I'm just collecting the facts and try to find a suitable bearing. The rpm rating dictates that the balls be ceramic. This leaves me with expensive hybrid or full ceramic bearings. Some hybrid bearings can handle the axial and radial loads to be expected in this setup so no full ceramic bearings are needed (this reduces the price from $140 to $90).
The bearing has a size of d=8mm, D=22mm and a more or less unknown height of 7-11mm. The 8x22 size is an absolute standard size for small bearings. We all know the 608ZZ from inline skates and stepper motors. 608 and all its variations (Z, ZZ, RS2, open) are deep groove bearings that handle mainly radial loads but can also handle lower axial loads when a few microns of shift is allowed.

708A with ceramic balls fits the 8mm, 22mm, handles the rpm rating of the pump, is available in an oil lubrication option and can be made to fit inside the given height. This leaves me with one critical unknown dimension and that is the mounting height of the rotor. However, the burn mark leaves me worrying that the bearing type might still be wrong.

hboy007

Fri Jan 06 2012, 06:58PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

I'm currently waiting for a 630/8-2RS1 bearing to arrive, I hope it will suffice to check the dimensions. There are also different tolerance classes, denoting positive and negative tolerances respectively. That explains why I can't get the standard bearings to fit onto the rotor shaft.

Let me sum up the facts for the EXT250H pump bearings once more:

upper safety bearing:
6804 type (20x32x7mm ultra thin section deep groove bearing),
grease lubricated,
no seal,
no special tolerance requirements.

The rotor does not touch this bearing under normal operating conditions and it is centered by the magnetic bearing assembly.

lower "main" bearing:
ID 8mm (+0.008 mm positive tolerance for 8.00mm shaft)
OD 22mm (standard tolerance, not critical)
W 11mm (standard tolerance, not critical)
oil lubrication
single non-contact seal
inner and outer rings : stainless steel
cage: Polyetheretherketone (PEEK)
ceramic balls (Si3N4)

Problem is, I have no idea where to get these. Most suppliers' online search quit once I specify hybrid bearing / ceramic balls and non-contact seal because these are mainly unsealed. Can anyone please help me find this thing?

ps. ok now I'm really in trouble: according to

and confirmed by a glance into the manual there is a piece of metal that participates in the transfer of oil from the reservoir to the bearing nut. That one is missing :( The oil to be inserted into the pump is something like this stuff:

( for something between $25-$100 a shot it's fairly cheap ), the "15PK Replacement Wicks" is $89.

By the way, I found the official "bearing kit":

the price tag is $525 :(

hboy007

Tue Jan 17 2012, 11:00PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

Update: I've managed to get a 608ZZ hybrid bearing and a 603/8 22x8x11 bearing. After disassembly, I fitted the 603/8 chrome steel rings with the 5/32" Si3N4 balls. Without and even with the cage back in place and everything regreased, the bearing does not run smooth. I'm sure I didn't damage the bearing surfaces because the same inner and outer ring runs smoothly again with the old steel balls back in place. Maybe I just bought cheap hybrid bearings :(

By the way... I found the hybrid version of the 603/8-2RS1 :

and even better: The 87038 is an 8x22x10.319mm bearing. I've put a 608 inside the hole and filled it with spacers to dissipate the remaining space. It seems to fit just fine with a little more than 10mm.

Let's see if I can manage to lay my hands on one of these:

(8x22x10.319mm / 8x22mmx0.406")

updated specs:

87038 bearing (8x22x10.319mm)
single sided shield / unshielded depending on cage
ISO-P4 / ABEC-7
outer/inner ring: stainless / 521000 chrome steel
Si3N4 balls
PEEK cage

Ash Small

Wed Jan 25 2012, 07:40PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

These people are the ones to speak to about parts for Edwards pumps:

(They are nothing to do with Edwards themselves, but have huge supplies of spares)

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