Rotary shaft seal with concentric tolerance?

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liveaboard
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Rotary shaft seal with concentric tolerance?

Post by liveaboard » Sat Oct 19, 2019 1:13 pm

While in the Netherlands recently, I removed a thrust bearing block from my canal boat; I noticed the shaft had an mm or two of lateral movement, so I brought it home to fix it.

It turns out that it's made that way; very odd. The shaft will be bolted to the prop shaft but the bearing housing can be just slightly off line.

It's supposed to have gear oil in it but it leaks out; the thing is made without seals.
I would like to add seals.
Shaft diamiter is 55mm, movement is 1.5mm max.
Is there any such thing as a rotary shaft seal that can tolerate being mounted 0.7mm [.03 inch] off center??

There are other solutions, but the easiest would be to obtain 2 such seals, in the unlikely event that they exist.
DSCN2337.jpg
The center bearing has a spherical outer ring. I've never seen anything like this before.
bearing block shaft + bearings.jpg
Last edited by liveaboard on Sat Oct 19, 2019 3:06 pm, edited 1 time in total.

John Evans
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Re: Rotary shaft seal with concentric tolerance?

Post by John Evans » Sat Oct 19, 2019 2:46 pm

I doubt any lip type seal will work with than much deflection from center even if it is constant. Try using a flow able grease ,here in the US we call it picker head grease. NLG 0 is the spec. Farm supplies should have it. If the shaft deflection is constant try a medium hard felt for a seal with the flowable grease.
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Re: Rotary shaft seal with concentric tolerance?

Post by liveaboard » Sat Oct 19, 2019 6:16 pm

Grease is an interesting idea I hadn't thought of; but I'm afraid it wouldn't stay in the flat thrust bearing races.

I'm thinking a seal set into a collar that would be movable like the shaft, tighten it down last after installation, when positions are set.
Gear oil will make this thing happy inside for a long time, if I can get it to stay in there.

If I'd understood how it worked before removing it, I would have paid mare attention to how it lined up.
The center bearing is a very strange thing, I've never seen anything like it.
bearing block center bearing.jpg
The outer spherical ring cages the middle ring [which would be the outer race on a normal bearing], the two parts can't be separated without destroying one.
I wonder how it was manufactured, and when?
It was old when I bought the boat 35 years ago, already dismantled at least once. It was made for a much larger vessel.


I need to make a new prop shaft too. The stern tube is also totally oversized; 45mm.
30mm would be normal for this type of vessel.

choprboy
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Re: Rotary shaft seal with concentric tolerance?

Post by choprboy » Sat Oct 19, 2019 6:29 pm

Weird. I can't think of a modern seal that would work off hand. Could the housing originally have had a rope seal (possibly with a hard grease ring like John suggested)? That would deform to fit the shaft and take up an offset.

...Edit... Re-reading, maybe I'm misunderstanding, this has 0.7mm of static radial offset from the center bore (and 1.5mm of axial play)? Or it has 1.5mm of radial play?

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Re: Rotary shaft seal with concentric tolerance?

Post by John Evans » Sat Oct 19, 2019 7:04 pm

Flowable grease at normal room temp is about the same as STP or ,molasses at just above freezing. I do think a adjustable flange with a seal would be a good repair where you can get the seal close to center once the thrust assy is installed.
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Re: Rotary shaft seal with concentric tolerance?

Post by liveaboard » Sun Oct 20, 2019 2:48 am

choprboy wrote:
Sat Oct 19, 2019 6:29 pm
Weird. I can't think of a modern seal that would work off hand. Could the housing originally have had a rope seal (possibly with a hard grease ring like John suggested)? That would deform to fit the shaft and take up an offset.

...Edit... Re-reading, maybe I'm misunderstanding, this has 0.7mm of static radial offset from the center bore (and 1.5mm of axial play)? Or it has 1.5mm of radial play?
The total movement of the thrust bearing in the housing is 1.5, so if a seal were fixed in the center, the most it would need to deal with is 0.75mm offset.
bearing block thrust bearing gap1.jpg
bearing block thrust bearing gap2.jpg
There are grooves where the shaft exits the housing, and I suppose a rope seal might have been there once; but they're quite narrow, like 5mm.
I thought the drain passages were blocked, but there are none.
bearing block oil rings.jpg
The bearing shaft bolts solid to the boat's prop shaft; so if the prop shaft is true, the bearing shaft will not wobble. That big center bearing won't allow any wobble of the prop shaft.
The bearings are designed to accommodate slight misalignment of the housing where it bolts to the cross member of the boat.

It sort of makes sense; there's no way to make the cross member machinist-correct. But I asked on a boat mechanic group and no one has ever seen a thing like this.
Usually, a big effort is made to line up the components when a boat is first put together.
I think the center bearing was specially made for it [by FAG], so there must have been many of them.

Someone has definitely been in here before!
bearing block ugly nut.jpg

When I search for rotary shaft seals, there is a huge variety but little information on tolerances. The only info I found stated 0.15mm as the maximum offset for that particular size / type.

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Re: Rotary shaft seal with concentric tolerance?

Post by Russ Hanscom » Sun Oct 20, 2019 8:44 am

At the RR, the journal box seal we now use is hard felt. We buy sheet felt and cut to fit, the stock we use is almost 1/2" thick. Most of the original seals were leather. Both should be tolerable of the offsets you mentioned, offsets with the RR axles can be in excess of 1.5 mm. Might consider using either or both and cutting to fit in the recess you have indicated. Neither will provide a tight seal but would serve to keep most of the grease in.

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Re: Rotary shaft seal with concentric tolerance?

Post by GlennW » Sun Oct 20, 2019 3:03 pm

Felt seals should be treated before use with a dunk in a melted mix of Petrolatum (Petroleum Jelly) and Paraffin. (1:1 mix) It gives the seal some light rigidity and will not absorb the grease or oil it is sealing.
Glenn

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Re: Rotary shaft seal with concentric tolerance?

Post by liveaboard » Sun Oct 20, 2019 5:46 pm

I took a 22mm seal I have and put a 22mm shaft in it; it's really pretty flexible.
Definitely more than the spec sheet says, for that size around 0.05mm; That would be pretty tight machining.

So I ordered a pair of large seals with large profile mountings. When I have them I'll try to judge whether they might handle the offset.
It might have once had felt seals, that would explain the ring cavities at the necks of the housing.
There are no remnants to be found now though.

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warmstrong1955
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Re: Rotary shaft seal with concentric tolerance?

Post by warmstrong1955 » Sun Oct 20, 2019 6:54 pm

See attached, from a C/R Seal Engineering Handbook. (Now part of SKF)
The misalignment, what seal guys refer to as 'STBM', varies by seal type.
.64mm seems to be the greatest amount, on CRW1, and CRWA1, which are the most common seal types.
That is dangerously close to what you have.....so ....that type, or similar, may work. Emphasis on 'may'.
:)

Bill
C-R Handbook,58.jpg
C-R Handbook,59.jpg
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Re: Rotary shaft seal with concentric tolerance?

Post by liveaboard » Mon Oct 21, 2019 5:50 am

Thanks for that, but the images are just a little too fuzzy to read.
The holes in the case are 1mm larger than the shafts, so I have to amend my first measurements from 1.5 to 1mm total movement, or 0.5mm from center at most.

I've come to suspect that the seal manufacturers are a bit conservative with their published tolerances [when I found any].
Looking at a couple of leftovers in my parts drawer, it's clear that the seals have quite a bit more movement than they say, and the deeper the case, the longer the support rubber, the more flex it will have.

I'll decide what to do when I have the parts in my hands.
I could bench test it by spinning it with an electric motor for a couple of days.

What is the seal type you mention that can tolerate the greatest error?

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Re: Rotary shaft seal with concentric tolerance?

Post by warmstrong1955 » Mon Oct 21, 2019 7:08 am

I tried making a pdf before, but it was over the attachment size allowed. Sorry about that.
New plan.....different freeware.....
Hope you can read these pages. I could here.
Look toward the top, and you'll see the seal types which allow the most error.
Page 1: CRW1, CRWH1, and CRWA1, CRWHA1 Page 2: All allow .64mm STBM

Bill


Scan0001.pdf
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