Noah
Well Known Member
I've been considering performing a hydrostatic pressure test on the brake system prior to putting in service. I'd rather find out now that I have a problem with a leaking joint (or worse). I recall reading sometime back that the Vans brake tubing might not really be up to the task, hence my concern and desire to perform a system-level test.
How to apply this pressure, and what pressure to apply?
I happen to have several compressed nitrogen cylinders in my shop, so developing whatever pressure is necessary isn't a problem. Filling the lines first might be a good idea to prevent a huge energy loss in case of a failure, as is done whenever hydrotesting high pressure stuff.
But what pressure to apply? The mechanical advantage of the brakes is 5:1.5 when pushing on the top of the pedals. Thus whatever force you apply at the pedals is multiplied by 3.33 at the master cylinder. I got my bathroom scale and put it against the wall, sat on the bathroom floor and pushed on the scale with my toes on one foot. I tried to put the amount of force I think I would use when doing a run-up, a healthy amount of force, but well below any significant amount of strain. That force was around 140 Lb. Then I figured I'd see how much force I could put on the scale with one foot which would be about the max that could be applied without really straining so much that I might hurt myself. This could be the force you might use while taxiing if somebody ran in front of your path and you needed to make an emergency stop. I measured that force at about 250 Lb.
So at the master cylinder, these forces are multiplied by the mechanical advantage of the pedal moment arms to yield 466 Lb and 833 Lbs. The matco master cylinder is 5/8 ID which works out to .31 square inches. Thus the fluid pressure at "run up" is 466/.31 = 1521 psi and at "maximum" is 833/.31 = 2713 psi.
OK, not a huge pressure for a hydraulic system. The Bonaco brakelines are made for 3000 psi operating pressure, no problem there. Hmm, better check that Vans 3003-O 1/4" tubing. Hoop stress in cylindrical tubing = Pr/t (Pressure * tubing radius / wall thickness). Vans tubing is .032 wall, so r/t is 3.9. Thus the stress in the tubing is 3.91 * the fluid pressure, or 5941 psi & 10,610 psi, respectively for my "run up" and "maximum". What's this, fully annealed (O condition) 3003 aluminum alloy has a yield stress of only 6000 psi, meaning that at that stress metal is bending permanently and not returning to its original shape?
Wow, so at my standard run-up pedal force of 140 Lbs, I am right on the hairy edge of permanently deforming my brake tubing. If I apply the maximum force I can "comfortably" apply (250 Lb), I will exceed the yield stress by (10,580/6000) or 76%! This entire discussion thus far has not even included a typical minimum safety factor of 2 for fluid applications - in fact there appears to be negative safety margin in the Vans brake tubing design. Indeed, Spruce lists 795 psi as the "max working pressure" for 1/4"OD .032 wall 3003-O tubing. So to stay under THIS number (includes safety factor of 2), you should NEVER put more than a 73 Lb force on your brake pedal - YIKES!
What to do? 5052-O tubing has a much higher yield stress of 13ksi, and comes in .049 wall standard, which will lower the stress in the tubing. This results in a tubing stress of 3880 psi for my run-up case, and 6929 psi for my "maximum can comfortably apply" case. With the higher yield 5052 tubing, these result in MUCH more comfortable safety factors of 3.35 and 1.88, respectively.
Looks like I will be ripping all the Vans supplied brake tubing out of my airplane before doing my pressure test since being on (over) the "hairy edge" is not comfortable for me!
Anybody want to buy some installed but never used 1/4" tubing?
How to apply this pressure, and what pressure to apply?
I happen to have several compressed nitrogen cylinders in my shop, so developing whatever pressure is necessary isn't a problem. Filling the lines first might be a good idea to prevent a huge energy loss in case of a failure, as is done whenever hydrotesting high pressure stuff.
But what pressure to apply? The mechanical advantage of the brakes is 5:1.5 when pushing on the top of the pedals. Thus whatever force you apply at the pedals is multiplied by 3.33 at the master cylinder. I got my bathroom scale and put it against the wall, sat on the bathroom floor and pushed on the scale with my toes on one foot. I tried to put the amount of force I think I would use when doing a run-up, a healthy amount of force, but well below any significant amount of strain. That force was around 140 Lb. Then I figured I'd see how much force I could put on the scale with one foot which would be about the max that could be applied without really straining so much that I might hurt myself. This could be the force you might use while taxiing if somebody ran in front of your path and you needed to make an emergency stop. I measured that force at about 250 Lb.
So at the master cylinder, these forces are multiplied by the mechanical advantage of the pedal moment arms to yield 466 Lb and 833 Lbs. The matco master cylinder is 5/8 ID which works out to .31 square inches. Thus the fluid pressure at "run up" is 466/.31 = 1521 psi and at "maximum" is 833/.31 = 2713 psi.
OK, not a huge pressure for a hydraulic system. The Bonaco brakelines are made for 3000 psi operating pressure, no problem there. Hmm, better check that Vans 3003-O 1/4" tubing. Hoop stress in cylindrical tubing = Pr/t (Pressure * tubing radius / wall thickness). Vans tubing is .032 wall, so r/t is 3.9. Thus the stress in the tubing is 3.91 * the fluid pressure, or 5941 psi & 10,610 psi, respectively for my "run up" and "maximum". What's this, fully annealed (O condition) 3003 aluminum alloy has a yield stress of only 6000 psi, meaning that at that stress metal is bending permanently and not returning to its original shape?
Wow, so at my standard run-up pedal force of 140 Lbs, I am right on the hairy edge of permanently deforming my brake tubing. If I apply the maximum force I can "comfortably" apply (250 Lb), I will exceed the yield stress by (10,580/6000) or 76%! This entire discussion thus far has not even included a typical minimum safety factor of 2 for fluid applications - in fact there appears to be negative safety margin in the Vans brake tubing design. Indeed, Spruce lists 795 psi as the "max working pressure" for 1/4"OD .032 wall 3003-O tubing. So to stay under THIS number (includes safety factor of 2), you should NEVER put more than a 73 Lb force on your brake pedal - YIKES!
What to do? 5052-O tubing has a much higher yield stress of 13ksi, and comes in .049 wall standard, which will lower the stress in the tubing. This results in a tubing stress of 3880 psi for my run-up case, and 6929 psi for my "maximum can comfortably apply" case. With the higher yield 5052 tubing, these result in MUCH more comfortable safety factors of 3.35 and 1.88, respectively.
Looks like I will be ripping all the Vans supplied brake tubing out of my airplane before doing my pressure test since being on (over) the "hairy edge" is not comfortable for me!
Anybody want to buy some installed but never used 1/4" tubing?
