since 1940

Hose Mandrels

The function of the mandrel is to compress the bulge of rubber that forms in front of the nipple during assembly. A secondary function is to hold the hose straight as the fitting is assembled onto the hose.

As the fitting nipple screws into the hose, the rubber is compressed and a bulge forms directly in front of the nipple. By compressing the bulge we prevent the nipple from cutting through the bulge and cutting a rubber flap from the hose inner tube. If the mandrel was the same diameter as the nipple then the bulge is fully compressed and the nipple cannot cut a flap.

 Unfortunately, the mandrel must be made slightly smaller so that it fits through the nipple. Here lies the problem; mandrels sold to individuals are too small. For example, the mandrel shown above is for -8 hose and has a diameter of .375 inch. The professional mandrel (shown below) is part of $10,000 socket machine and has a diameter of .387 inch. The difference, .012 inch is the thickness of the rubber flap shown above!

When using a undersized mandrel even if the nipple compresses the bulge and doesn't cut a flap, it's going to take more force to turn the nipple. This causes two  problems:

• mandrel slips and you cannot get the nipple screwed all the way into the socket

• turning force is so great that you rip the rubber inner tube from the hose braid and it wraps around inside the hose and forms an obstruction.

So why are store-bought mandrels too small? Why don't you make one the correct diameter?

If I did you wouldn't like it. Most mandrels are also used to remove fittings from old hoses. Unfortunately most fitting nipples distort during the years they are in use (especially -8) so that a properly sized mandrel will no longer fit.  A smaller diameter mandrel allows you to remove and reuse the old fitting without worrying about the mandrel being too large and not being able to slide into the nipple.

But using the small diameter mandrel creates  two problems:

• undersize mandrel doesn't compress the bulge, and

• distorted nipple being driven into the bulge acts as a saw blade and cuts thru the bulge.

By making the mandrel smaller it allows you to use fittings that otherwise would not be used because they have distorted and shouldn't be used.

Hose mandrels, like the one shown on the right are used for field assembly of Stratoflex 111 or Aeroquip 303 style hose (MIL-8794).
Mandrel with nut and fitting installed ready to screw into socket.
Professional  (the kind hose shops use) version of above mandrel.
Fitting nipple sliced a piece of rubber from the bulge. Mandrel's that fit loosely in the nipple do not compress the bulge and allow the nipple to cut into the hose inner tube.
Bulge gauge used by professional shops to detect excessive bulge
Bulge occurs at the end of the hose nipple

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.