Prop shaft seals

I thought this was a topic worth discussing.

My approach is based on Norbert Bruggens book and I use a 4mm stainless steel shaft with a Simrit seal. (This is a lip type oil seal.)

They are very reliable and seem to last for years. Points to watch are to oil occasionally and try to keep the rotational speed below about 2000 rpm to minimise wear on the seal.

I "sandwich" the oil seal between two brass plates and incorporate a ball race just behind the seal on dry side.

Works well for me!

Any commercial products recommended?

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I'm sorry that this is an old photo but it gives you the idea. Fairly crude but works well.
It is basically a brass bolt with a brass plate, hard soldered to it.
The Simrit seal is sandwiched on the LHS.
I no longer fit the ball race as shown, because it can be inset into the head of the bolt so it lies just under the seal.
You can make it without the use of a lathe but drilling the bolt is much easier with one.

David,
your picture and explanation could not have come at a more timely moment as I am just about to commence building two shaft seals for a tube I am making. I do have two of your seals and drawings from you of three types of seal arrangement you have used.I think they are still in the foto section of the facebook site. Can you confirm for me that the smaller dia side of the seal (ie the thinest part of the cone) faces outwards.
many thanks Roger

Hi Rog,

Yes the conical "lip" side (with the spring) faces outwards to the water.

(i.e water pressure pushes the lips against the shaft.)

Well you may be showing your relative youth, Richard!
Stuffing boxes with oil or grease are good for near surface but seals get tighter the deeper you go.
Try the oil seal approach, though, you will like it.

Anyone else got some ideas I know Nigel has.

I didn't see them, sorry.

Yes O rings are another solution. May have more friction?? I don't have experience of them on prop shafts but have used them on control linkages when bellows can't be used. (e.g on the Holland where I was not using a WTC -- See forthcoming article in magazine- plug!)

...well, many years ago I had problems with shaft seals that leaked so I thought about eliminating the need for shaft seals.

The result was the idea of an electronically commutated DC motor with a magnet rotor beared and directly running in water (today you would call him brushless DC motor) and I build the first motor 1993 for my class 206A submarine model. This model hasn't any sliding seals and is in use since 1994 without any service for the propulsion system since then. I built similar motors for the SEAWOLF of my son and my class 212A submarine, mounted in the flooded rear section at the rear wall of the pressure hull (see photograph of the motor of the 212A sub)
They all perform well and there is no leakage at all.

Kind regards
Hermann

Thanks Hermann - that is beautiful work!

How does it work? It is a magnetic coupling, I assume. Are the dark circles magnets?

Hello David,

yes, the 7 dark circles you can see on the disc rotor are magnets indeed. They are very common ferrite magnets normally intended to hold/ stick sheets of paper at metal blackboards at the wall. These magnets have a special direction of polarisation, they are polarised in direction of the diameter so the half circle beneath the "aequator" is a south pole, the half circle above is a north pole. So the rotor (and the moulded stator also) has 14 poles. I took ferrite magnets because they had been easy to purchase (in 1993 when I built the first motor, rare earth magnets were hardly available for me). The energy density is much less compared to rare earth magnets but they are full resistant to corrosion in water.

It's not a magnetic coupling between two magnetic rotors (one inside, the other outside the pressure hull) but a real brushless DC motor where the inner rotor is replaced by a stator system equipped with coils and driven by switching transistors. That stator system (that narrow light grey ring) is attached to the aluminium motor housing and moulded with epoxy glued. The control electronics is located inside the cylindrical motor housing that is screwed to the rear wall of the pressure hull with its flange.

The motor has 4 terminals: 2 for power spply from the battery and 2 (signal and return) for connection to the radio control receiver.

And you can see - there is no seal for the 5mm propeller shaft. The whole rear section of the model is free flooded.

On the photograph you can also see the rods for moving the rudder blades. They are actuated by linear magnetic couplers, also eliminating sliding seals.

I must confess that this is an exotic practice and a some modellers here called me mad for doing this, but why not doing things in an unusual way? That hermetical concept was just an idea and I wanted to prove that it works.

Kind regards
Hermann

Very nice technology - and even easier and cheaper to do now (Better magnets and electronics in 2011)

Did you get any interest in this on a commercial basis? (Applications such as deep diving submersibles come to mind.)

Hello David,

when I came to a submarine modellers meeting with this motor for the first time (that was in Bremen in 1994) some modellers were interested but found it too complicated to build it also by themselves. They were more or less satisfied by their conventional solutions and saw no special need. I contacted some model companies in Germany (robbe, Engel) but they had no interest - I didn't get a reply even. Another one would take it - if it wouldn't cost more than 10 DM (that was before the EURO). For some time I thought about a small series production by my own but I realised that would overstress my capabilites besides my normal job. Nevertheless I will build two other motors soon for the next model of my son.

You are right, this motor is suitable for deep diving applications, and that's also one reason for this. The model you can see partly on the picture (a semi scale class 212A model built in aluminium) has deep diving capabilities and is designed for 100m (approx. 300ft) with a safety factor of 2, so it must be absolutely tight and withstand the enormous water pressure. A few years ago I had it in a lake in a depth of 35m (approx. 100ft) linked with a cable, and all went well. But some of my modeller colegues called me mad (maybe they are right).

Greetings from northern Germany
Hermann

Well, long may your "madness" continue!

We are all a bit mad in this hobby.

David

Let's thank Hermann for thinking out of the box, or should we say hull?
I think that this design is similar to a Grundfos circulator pump.It is a stator core design.It can be purchased in 24 volt version.
Giovanni

Hello everyone,

yes, modell submariners really seem to be somewhat mad, otherwise they wouldn't spend so much time in their workshops, face and fight a lot of self made problems they wouldn't have as a normal human. Or being at the lake even despite of bad weather like these:

Two submarine modellers are standing at the lake with their models running. It's raining cats and dogs and their clothes are already wet through and through. One starts to talk:"Just imagine - having this awful weather my wife tried to chase me out of the house to provide some shoppings for her!"

Concerning the wet running motors I have added a sketch for illustration (upload will succeed hopefully).
I have also added a sketch (text unfortunately in german, hope the picture will speak for itself) and pictures of another exotic device - magnetic rudder couplers to avoid sliding seals for the rods that operate the rudders.
Of course rubber bellows do also the job of avoiding sliding seals (if the depths are not so great).

Kind regards
Hermann

Very good and interesting engineering which suggests that it should be used in a really deep diving model submarine. (50m + ?)

Ever thought about trying one?

The problems would transfer to how you would control it at depth. (Radio would not get there.)

Hello David,

you are right - normal radio control transmission on 40MHz will not work in depths of more than a few metres so the only reliable way is using a cable (or using sound transmission in water but this will be a real technical challenge).

If you use a simple wire that is even loose coupled to your transmitting antenna and down with your model it is also near the receiving antenna the radio waves will travel along the wire as this is a much better propagation path. I did some experiments.

That class 212A submarine model is already designed for greater depths (50m+ shall be possible) and when I will have a camera and lights installed in the bow I will go again to the lake of Hemmoor (a former open mine, now flooded with ground water, about 100km from here, between Cuxhaven and Stade) and try to reach the bottom at approx. 60m.

You may ask why doing this. Well, most modellers are searching for their special challenges (like Jermy Clarkson, "Hamster" and "Captain Slow" from Top Gear at BBC), so why not exploring the third dimension - the depth - further? For the model designer this is an interesting challenge and requires reliable solutions.

Kind regards
Hermann

I'm impressed that you are tuned into "Top Gear!"

Very interesting plans you have for deep diving subs.

What with the rise of 2.4Ghz we have a need for a replacement control system in subs. (Sound, radio etc.???)

Anything coming along in Germany?

Hello David,

yes, I am a real enthusiast of "Top Gear"! But I must confess that it was my son Christian who discovered it first via internet. It is the very best motor TV series and so different (in a positive sense) from the motor TV series here where they make tests of new car models and it seems to be more like a promotion for the car manufacturer.
Unforgotten the items when the three converted a Renault Espace into a "convertible" or they modified 3 cars into amphibious cars (a "TOYBOTA", a narrow-boat-like VW camping bus and an Austin Herald sailing ship). And the race to the north between a motor bike, a Jaguar sports car and the new built Peppercorn pacific steam locomotive "TORNADO". Marvellous!

Well, radio control beneath the waves might become a serious problem for us model submariners if the 40MHz frequency range would be taken away for us by the authorities. 2.4GHz control sets are also extremly rising in Germany and perhaps this might accelerate the end of 40MHz here. But 2.4GHz are definetly no replacement for us! Although I considered from therory and by firsts reports from experience by a few other modellers I did an experiment in the laboratory where I work and made measurements of attenuation of 2.4GHz radio waves in normal water. As a rule of thumb I found an attenuation of approximately 1dB per cm depth; that would mean in 20cm you have 20dB (corresponding to 1/10 ) less signal strenght, and in 40cm 40dB less (means 1/100).

I hope 40MHz or at least 27MHz will stay a pretty long time!
Transmission by sound is imaginable from theory but there are a lot of difficulties to solve (for industrial or military applications real solutions exist but they require complex electronics for signal processing). Imagine that you will have high acoustic noise in water so you must filter your acoustic reception signal. Reflections will occur (from the surface, the bottom, ambient obstacles, nearby other models, and so on) and so multiple reception of the same control command wit delay times between must be considered.
As far as I know there are no special developments here for hobby model submariners (their number is not so great). But perhaps this would be a nice challenge for me after retirement...(I am 56 now).

Kind regards
Hermann

Well I only caught the end of last nights "Top Gear" but you have a treat in store (Rail mounted caravans!)

Interesting that you are experiencing the same rapid shift towards 2.4 GhZ in Germany.

Another huge advantage of 2.4 is the easy frequency sharing because of the large band width.

Much more of a sharing problem with other sound or radio systems.

(And sorry, Richard but we have drifted off frequency (topic?!) on this one. Do we have a section for "Top Gear?!)

...yes, we shouldt definetly have a "Top Gear" section. I just saw the last issue my son Christian has downloaded, with Jeremy's "sports train" - I had tears in my eyes from laughing!

But now for something completely diferent - e.g. model submarines.

At the moment we can have profit by the change to 2.4GHz of so many surface ship modellers. We may regain free frequency channels on 40MHz when we are at a lake with other modellers. My only fear is that the authority that is responsible for frequency licensing may close the 40MHz band for model radio control in the (perhaps near) future when it seems to become old fashioned. And how long 27MHz will last nobody knows...

That's not a big problem for deep diving models as you will still require a cable connection for this. And you will require reliable prop shaft seals that stay tight under pressure -or a solution with a wet running motor without seals. For normal depths of our models conventional seals as discussed and sketched before will satisfy in general.

Kind regards
Hermann