JeffHs
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| posted on 16/10/16 at 03:12 PM |
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Hydraulic seals again
Any ideas welcome.
We're still having problems with the hydraulic flap unit on the Auster. It's a hand operated 2 way jack to drop and lift the flaps
depending on the position of a lever that operates a shuttle valve, so there's all the usual things you'd expect - pump, ram NRV, PRV,
fluid reservoir and hosts of little seals. It has to overcome massive airloads so it works quite hard for such a small device and in our case not hard
enough because the flaps blow back. The unit was built in the 50s by Lockheed AP (Automotive Products), the same people who competed with Girling on
braking systems in old British cars.
Why did they choose to use square section seals? Do they make a better seal than O rings? With the exception of the pump seal which is a cup or lip
seal every other seal in the thing is symmetrical and square section and not easily sourced. We can get seals cut from solid but we have to specify
the sizes and some of the small seals inside housings are virtually impossible to measure.
Until some detailed measurements this morning I was tempted to rebuild the unit with O rings and see what happened, but in many instances that's
not possible because the seal grooves are machined deeper than the width so an O ring that fits the groove would drop below the shaft and not seal.
We've concluded from measuring some old intact seals and good hardware that the size of a seal for a 1/2 shaft inside a 3/4 inch groove needs
2.5 thou oversize on OD and the same undersize on ID to make a proper seal. Does that sound about right?
Am I right in thinking that if it was being designed today it would use O rings?
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JeffHs
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| posted on 16/10/16 at 03:56 PM |
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I've just answered most of my questions using a different tack on Google search. Lathe cut seals are widely available. It seems like my only
remaining question regards seal sizing, i.e. given dimension of groove and bore, how much under and over size to specify. I can't find that yet.
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MikeCapon
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| posted on 16/10/16 at 04:19 PM |
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Jeff. Take a look at quad rings manufactured by Trelleborg. They will suit a square housing and are much higher performane.
Trelleborg
www.shock-factory.co.uk
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JeffHs
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| posted on 16/10/16 at 05:07 PM |
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quote:
Originally posted by MikeCapon
Jeff. Take a look at quad rings manufactured by Trelleborg. They will suit a square housing and are much higher performane.
Trelleborg
Mike, thanks for that. Looks like a fabulous resource.
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Dingz
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| posted on 16/10/16 at 05:16 PM |
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Quad rings are designed to fit into O ring grooves usually BS1806 sizes so you have the same problem as with a circular cross section. If you have
dimensions of the parts, grooves and photos could you mail them to me? I may be able to help, I spent the last 20 odd years of my working life in
hydraulic and pneumatic seals.
Phoned the local ramblers club today, but the bloke who answered just
went on and on.
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pewe
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| posted on 16/10/16 at 07:03 PM |
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See U2U.
Cheers, Pewe.
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ianhurley20
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| posted on 16/10/16 at 08:32 PM |
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I used to fly one of these when I was a member of the South Wales Gliding club in the 70's. I would never have thought of asking such a
technical, off topic question on this forum. Wow! What a response and source of information. Next time I have a question on anything the world can
throw at me this is where I will come. Well done those who can answer all the questions randomly thrown!
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JeffHs
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| posted on 16/10/16 at 08:44 PM |
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That's what is so brilliant about this forum. There are some very clever people on here!
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ReMan
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| posted on 17/10/16 at 09:41 AM |
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quote:
Originally posted by JeffHs
That's what is so brilliant about this forum. There are some very clever people on here!
Agreed
Hip hip hurrah etc etc 
www.plusnine.co.uk
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