Here is some background info: 1. Caliper failed hydraulically during run-up at Buchanan Field, Concord, CA 2. Repairs by Sterling Aviation 3. O-ring was found to be partially extruded/damaged. 4. No measurements of piston or caliper bore were taken 5. Repair consisted of o-ring replacement and pressure testing by brake application in the aircraft.
I have two brake assemblies that I purchased from a VAF member sitting on the shelf. I disassembled one to take measurements. Part of my interest was to see if I could replace the circular section o-rings with square section o-rings, which are used in 100% of automotive brake applications today. Assisting me was the supervisor of a machine shop that specializes in the repair of hydraulic cylinders. Using his equipment, we measured the piston to bore clearance at .010 by measuring piston OD and bore ID. From our experience, this struck both of us as excessive clearance. Not wishing to rely on either his or my assumptions, I contacted Parker-Hannefin and requested dimensional information for the two components I was promised a response and have heard nothing for three weeks.
FWIW, numerous posters on this website have had similar failures and recommend packing spare o-rings
Here are answers to posts by #
#5 &6 I was surprised, too. However, despite my written explanation that the information was requested to inspect components on my own aircraft, I did send the e-mail from my work computer, which identifies me as a forensic engineer. Yes, the piston was correctly installed.
#7 I previously looked at the maintenance and the service manuals. There is plenty of info re bolt torques, lining thickness and rotor thickness. Unless I missed it during several reviews, there is no dimensional info re piston OD and caliper bore ID.
#8 Dan, both the piston and caliper are aluminum. Heat path into the piston is from the inside backing plate and heat into the caliper is from the outside backing plate. I was a Sr. Brake Engineer at Kelsey-Hayes. All new designs were tested to hydraulic failure with a minimum factor of safety of 4x the maximum in car working pressure. My 48mm piston diameter brake had .004 -.007 clearance because it had a phenolic piston. For a metal piston, the clearance would have been .003 - .005. My brake was tested at the .007 upper clearance limit. Failure was at 9200 psi. Failure mode was partial extrusion of the square cut o-ring. The 30-9 caliper uses a circular section o-ring. The auto industry switched from circular section 0-rings to square cut o-rings decades ago.
My own personal interest in this investigation is to determine whether a square-cut o-ring would be a better choice in this application, but it is hard for me to do any testing until I am knowledgable about the design intent of piston - bore clearance.
Larry