Hose Problems

Hose Problems

Yesterday a friend brought by some hoses off of his racing car for pressure testing. Normally, I only do aircraft hoses, but for a friend --OK

So the hoses fail. Look carefully at the hose to the left and you can see that the hose

pulled out of the socket. The hose was only engaged into the socket 1/4 inch or so. This is why he had a hose leak - it wasn't assembled properly to begin with.

Of all the different types of aircraft hoses I build, this style is the most tricky to assemble correctly. It is also the style of hose that race-car and custom car builders love to use. We use very little of this stuff on aircraft and in general don't like it - except it is "pretty" so I guess that is all that matters.

There are multiple ways you can get into trouble screwing the fittings onto this style hose. Here is just one tip:

After you screw the hose into the socket to the correct depth, place a "back-out" mark on the hose. Then when you screw the fitting together go back and make sure the hose hasn't pushed out of the socket as you screwed the nut-nipple Assembly into the socket.

Also, don't flood the cavity with oil as you can create a hydraulic lock that prevents the rubber from sliding into the recess.

Here is a link to another article on hose assembly at my web site:

http://www.mechanicsupport.com/aircraft_hose_assembly.html

AN Thread Size and AN Fitting Size Chart

Measuring Inside Diameter 

Measuring outside "major" diameter

AN Dash Size Chart

AN fittings differ from commercial threads in that they are class 3 instead of class 2 and generally UNJF rather than commercial UNF. Threadform: UNJ -3A or 3B

Note: There is an entire industry supplying so-called "AN" fittings that do not meet aerospace standards. See my article at http://www.mechanicsupport.com/articleStronger.htmanicsupport.com/articleStronger.htm

Web App version of this chart. Bookmark the link below in your mobile device:

http://www.mechanicsupport.com/apps/Threads/index.html

Dash and Thread Size Chart
Click on the Right button for next chart. Handy reference for sizing fittings and fasteners.

Use of crush washer on AN Fitting Connections

An "aluminum crush washer" or conical seal is sometimes placed between the flare surfaces. These are mechanic band-aids to be used when you have a leaking connection and no replacement fittings. The usual problem is using a new hose fitting against an old male nipple whose sealing surface has been damaged. It's easier to drop a conical seal into the hole than to replace the damaged fitting. Going back 10-15 years no one had ever heard of them.





Conical seals getting more popular now, in my opinion, because of the increased usage of steel on steel mating surfaces. Traditionally, AN plumbing mating surfaces were aluminum to aluminum or steel to aluminum. To seal any surfaces, one or both surfaces must conform (yield) slightly under pressure to seal any microscopic gaps between the surfaces. It helps if one or both surfaces are relatively soft. The more recent switch to steel on steel requires increased nut torque to get the two hard surfaces to yield sufficiently to seal. This should not be a problem with good surfaces and proper torqued.

Bastardized AN fittings - Which AN fitting goes into the hole?

Automotive racing has adopted the aircraft AN fitting technology, bastardized it, and now sent it back into the aircraft industry. I've talked about the differences before in my article "What is the difference between aircraft AN and JIC fittings". The result of this bastardization is confusion and extra expense for the aircraft industry. Let me try to clear up some confusion when it comes to screwing fittings into straight-thread ports.

In non-aircraft applications ports are mostly straight thread "ORB" O-ring Boss. The automtove industry (heck I don't know what to call you guys - "race industry", is that better) has plenty of adapters to adapt AN to the port - pictured below.

There is no AN number for this adapter. It does not exist in the AN series - for good reason - it is not needed in aircraft. In aircraft, the traditional straight threaded boss is called a "AND" port (AND10050 or MS33656) and doesn't require an adapter fitting. This should be the end of the story for us aircraft people but it isn't.

Some automove style components with ORB ports are being used in aircraft. These require the adapter shown above. So now us aircraft people must be able to look at the port and tell what kind it is.

Is it a tapered pipe thread port (NPT), an ORB port or a AND style port? Curse you automotive people for bringing us your ORB crap! NPT is bad enough!

Here is a aircraft brake caliper with a male AN nipple sticking out of the port. It looks like out adapter above. But it's not.

Here is a picture of the entire fitting.

Notice that this is a standard AN fitting. Below is a picture showing how it installs with a boss o'ring.

Simple, just install a a boss O'ring onto the end of a standard AN nipple fitting and screw into the port. You can also use a bulkhead fitting and special nut for high-pressure hydraulic applications.

Here is a picture below.

This port (AND10050 - MS33656) has stronger (greater shear strength) threads (class 3 versus class 2). and can accomidate the extra length of the nipple. It is adaptable to a wide range of AN fittings and pressures.

AN fittings that can be screwed into AND ports are what the drawing calls a Type E style and can be used to seal on the flare OR seal on the nut with an O'ring. Not all AN nipple fittings are Type E. Below is a non-type E fitting.

Notice that there is no nut hex and no circumferal groove above the last thread. If you wish to use a Type E angle fitting then use the bulkhead series with a nut. You can now point the fitting any direction you wish.

What does a AND10050 port look like?

A AND port has a countersink around the top edge forming a 120 degree included angle. This port is also sometimes called a "Military Straight Tread Port". The latest drawing for this port is AS5202.