Thursday, September 4, 2014

Oil Hydraulic Struts... Repairs and Reassembly


Previously I posted about the disassembly of the "Oil Hydraulic Struts" as removed from a Curtiss Robin, NR59H.  Well, A few weeks ago we finally finished with the repairs and re-assembly of these strut assemblies.  Here are some photos of the process...I am happy with the results and believe we have a repair that should last for many years of continued service.

Portion of tube insert for repair. 
A decision was made to cut out the badly pitted portions of the upper tube.  We designed and fabricated a replacement tube to be welded in place of the removed portion. We found an "off-the shelf" material that was perfect for the job.  It is a close-tolerance seamless tube made of 1020 steel chrome plating applied during the raw material fabrication process.  This material is commonly used in the manufacture of hydraulic cylinders.  The chrome plating will hopefully resist the kind of pitting that caused us to tear down the oleos in the first place.

To ensure a proper repair, the design concept  for the repair 'insert' tube was to machine a step-down of the outside diameter and to give it a three-inch insertion length to allow the insert to self align to the existing tube. The insert was expertly machined by our friends at Madden Manufacturing, LLC, China Spring TX.  We were a little worried about causing too much stress in the upper tube and the potential for cracking during the press-fit-up operation.  The shop's owner, Dennis Thom, came up with the right approach for this by effecting a taper of the stepped-down OD from a 0.000 inch fit at the insertion point and gradual taper to 0.0025 interference fit at the last 1.5 -inch of tube to the bottom of its shoulder.  The results were spot-on.

At the shoulder, the repair was finished off by welding the interface joint all around.  Then the weld bead was ground and polished down smooth and flush with the tube OD.  Rosette welds were deemed unnecessary due to the interference fit design which pre-loaded beneficial stress into the assembly.

Tube 'insert' welded in place and polished smooth.
Re-assembly of piston on upper tube with spring-lock clip in place.
The original design of this oleo assembly did not utilize a rubber o-ring seal as would be expected in a similar/more modern oleo.  Instead, a packing material was used. Originally, this packing was most likely a degradable cotton or linen rope material of a round cross-section with graphite flake coating.  We found that the modern materials used in water pump packing applications are far better than that of the old-days.   We managed to find a synthetic graphite impregnated rope material with 1/4" x 1/4" square cross-section. This seemed perfect for the job (short of a re-design for o-rings).  We used 24 inch lengths of this material to seal each oleo. The slight compression of the packing material riding against the smooth chrome OD should be an ideal seal and wear-resistant surface.

Installing 'packing' seal.
 
Repaired oleo assemblies with removed portions of upper tube that were replaced.
New paint for the re-built oleo struts
 

New Lester tires being installed
 

Leann Bowden doing paint touch-ups on the Fischer 28" x 4" wheels

The Majestic Curtiss Robin proudly sits on its repaired landing gear




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