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MarshSt
09-28-2011, 12:08 PM
I'm replacing the bearings that support the Vari-Drive shaft in the top end of a Bridgeport clone. The factory bearings are xxxZZ with steel shields. Why would it have the shielded bearings instead of RS sealed bearings? Is this a cost thing or is there an engineering reason for the selection? Has anyone tried one type over the other? Any feedback would be appreciated.

Thanks,
Steve

flathead4
09-28-2011, 12:43 PM
From what I understand, the shielded bearings keep out the big chunks but allow external lubrication to flow through the bearing. Sealed bearings are factory lubed and sealed. So which bearing type used depends on the design of the machine. I don't know what your machine calls for.

Tom

ironnut
09-28-2011, 02:39 PM
Sealed bearing will have a bit more drag. In some situations it could be significant at start-up.

In another life time I worked as the spindle support engineer for a maker of disk drives. These disk drives had 14" diameter platters in them. The spindle was massive for a disk drive as it had metric 208 bearings in it. Originally the bearings were sealed on both sides. A Japanese customer had issues with the drives not starting in the winter time. Their computer room, at night, was allowed to cool (night set-back). During operation of the previous day the air inside of the bearing would expand due to the heating from spinning and the warmer ambient temperature. The seals acted like one way valves and allowed the air to escape from the inside the bearing. Then at night when the disk drives were turned off and the room cooled, the air inside the bearing contracted and pulled the seal down tight against the inner race. The increased drag was sufficient so that the spindle motor (~1/4 hp) could not start and spin up the disks. Customer didn't like that. The fix was to put a seal on one side and a shield on the other.

I don't think that in your application you would ever notice the increase in drag if that situation ever occurred.

I believe shielded bearings are a bit less expensive. Shielded bearings would be good in a very dusty environment. The head of a milling machine is not normally in a dusty environment, so a shielded bearing should work just fine. Sealed bearings will eventually become shielded bearings as the pliability of the seal will decrease over the years.

philbur
09-28-2011, 02:50 PM
Here's what SKF says about the subject:

Bearings with shields
Bearings with shields, designation suffix 2Z, are produced in two different designs. The sheet steel shields used in smaller bearings form a narrow gap with the land of the inner ring shoulder (fig). Larger bearings as well as all SKF Explorer bearings have recesses in the inner ring side faces into which the shields extend (fig).

Shielded bearings are primarily intended for applications where the inner ring rotates. If the outer ring rotates there is a risk that grease will be lost from the bearing once it reaches a certain speed.

Bearings with seals
Bearings with seals, designation suffix 2RS1, use a acrylonitrile-butadiene rubber, sheet steel reinforced seal that fits against a recess in the inner ring side face (fig). The lip of the seal exerts a light pressure against the inner ring for a positive seal. The periphery of the seal engages in a recess in the outer ring to provide a good sealing. The permissible operating temperature range for these seals is 40 to +100 C and up to +120 C for brief periods.

Grease may be lost from sealed bearings at the inner ring under extreme operating conditions, e.g. at high speeds or at high temperatures. For applications where this could be a disadvantage, special design steps should be taken to prevent this. For additional information please contact the SKF application engineering service.

Ref: http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_3_22a

Phil:)

ikdor
09-28-2011, 03:12 PM
Interesting trivia: some automotive gearboxes use sealed bearings on the inside, where most gearboxes use open bearings. This is exactly for the reason to keep small metal particles out of the bearing and prolong life.
The grease inside these bearings is slowly replaced with the transmission fluid.

Applying this to your application I'd take the sealed ones to prolong life, even if there is other lubrication provided for the bearing.
(assuming the lubrication is not used to cool the bearing.......)

It is often overlooked that the service life of many bearings is much lower than the fatigue life because of particles/water ingress.

Igor

JoeLee
09-28-2011, 03:17 PM
I don't know if I can fully agree that sealed bearings create enough drag to really affect any motor, unless were talking fractional HP.
I've even seen them on grinder spindles with as low as 1 HP motors. I can't see where a 1 1/2 - 2 HP or larger would be affected by seals. I have heard that they do generate a bit of extra heat due to the friction of the seal but not enough to cause any problems to a motor.
When I replace any bearing I always replace it with a sealed type provided the bearing I'm replacing is available that way. Some bearings with special extended shoulder type inner races may not be available in a sealed type. Somewhere I have a list of all the Fafnir prefixes and suffixes, if I can find it, I'll post it. I also like the sealed bearings mainly due to the fact that you can remove the seal, clean and grease the bearing and pop the seal back in. You can't really do that with a metal shielded bearing, and seals keep the bearing cleaner and they last longer.

JL.................

Tanto
09-28-2011, 07:18 PM
Seal drag is a very real problem in certain applications, this is not one of them. I have used bearings with the requirement of exceptionally low bearing losses and the solution was to pop the seals out, flush the grease, and run them on oil.

Steve I'd expect the shielded to be used in your application due to the lubrication setup of your mill's head. Sealed bearings do not require any further lubrication as they are "sealed for life" ... which may not be very long, but that's another story. I don't know the lube configuration of your mill, but basically if there is lube provided for these bearings use shielded bearings, if there is no additional lube provided use sealed. As to why shielded are used v sealed, that's a complex question, and many of the reasons have been touched on below.

chipmaker4130
09-28-2011, 10:29 PM
Another point: You don't need to use both sides of any type seal. I often use a sealed bearing on projects where oil is provided, and pull the seal on the lubricated side. The remaining integral seal in conjunction with an external seal provides extra protection on the outside of such things as rototillers, farm implements etc.

darryl
09-29-2011, 12:11 AM
Aside from the reasons given so far, another reason to avoid seals where practical would be to minimize heat build-up where it will cause parts to expand. A spindle would be a good place to keep heating to a minimum-

Peter S
09-30-2011, 08:16 AM
All of the standard deep-groove bearings we use (NTN and NSK) come with three options - shields (ZZ), non-contact seals (LLB) or contact seals (LLU). There is little sealing difference between shield and non-contact seal, however the contact (or rubbing) seal is a much better seal. The extra drag is noticable when you spin a bearing with contact seals by hand. We only use contact (rubbing) seals on our equipment (dusty).

My guess is that when most people (as per this thread) ask for a "sealed" bearing they are probably given a non-contact type which is no better than a shield....

Chris S.
09-30-2011, 09:44 AM
Many years ago I had a discussion with a bearing supplier about this. He noted that the shielded bearings are usually rated at a higher maximum rpm than it's sealed counterparts.

Chris