Mon Nov 22 2010

Changing o-rings on brake components

OK, I'm going to use automotive brake fluid instead of aircraft brake fluid in my plane. Why you would I want to do that?

The original mineral oil aircraft brake fluid (MIL-H-5606) has a very low flash point. A few planes have been consumed by fire in the following way:

  Heavy braking on taxi, landing, or aborted takeoff
  Brakes (including calipers) overheat
  Caliper o-rings start leaking fluid
  Fluid catches on fire
  Wheel pant catches on fire
  Aircraft consumed in fire

There are basically four options:

  1. Use aircraft brake fluid
    • Mil-H-5606 - low flash point ~194 F
    • Mil-H-83282 - better, but still flamable. Flash point ~390 F
    • ATF - pretty much the same stuff. Flash point ???

    Pros: Compatible with Buna-N (nitrile) o-rings and hoses which are standard on aircraft brake systems

    Cons: Flammable

  2. Glycol based brake fluid such as DOT-3, DOT-4, and DOT-5.1

    Pros: Non flammable. Hygroscopic (absorbs water) - this is good because any water in the system will stay suspended in the fluid rather than going to the low point of the system.

    Cons: These act as a good paint stripper. Hygroscopic (absorbs water) - this is bad because it means you need to seal your system from the atmosphere. Not compatible with Buna-N (nitrile) o-rings and hoses...these need to be changed to EPDM and Teflon, respectively.

  3. Silicone based brake fluid - DOT-5

    Pros: Non flammable. Compatible with all seal materials. Doesn't damage paint.

    Cons: Doesn't absorb water, so that any water which gets in the system will run to the low point (the calipers) and cause corrosion. Not in common usage.

  4. Skydrol - A phosphate ester based fluid - Flash point ~880 F

    Pros: High flash point.

    Cons: Not common for experimentals. Mildly hygroscopic. Not compatible with Buna-N (nitrile) o-rings and hoses...these need to be changed to EPDM and Teflon, respectively. Expensive (~$35 / quart)

I'm going with DOT-4. I purchased EPDM o-rings a while back to change my parking brake, master cylinder, and caliper seals. This week I got around to changing them and I found one more seal that needs changed. It's called a stat-o-seal and is internal to the Matco master's basically a washer with rubber around the inside diameter. The current one is Buna-N and needs to be changed out to EPDM also. Well I had a heck of a time finding an EPDM stat-o-seal, but finally found a source.

FYI, EPDM stands for ethylene propylene diene monomer and is also sometimes called EPR for ethylene propylene rubber. NAS1613 as well as the obsolete NAS1611 and NAS1612 specifications describe EPDM requirements.

In case you want to change your o-rings out to EPDM material, here is a table of the various o-rings in use on the brake system of a Van's aircraft:

ComponentSubcomponentQtyPart or dash numberMcMaster-Carr partPrice
Master cylinderCap seal1-1129557K473$9.23 / 100 pk
Rod seal1-0129557K464$4.65 / 100 pk
Piston seal1-1119557K472$8.51 / 100 pk
Parking brake valveShaft4-0109557K462$4.34 / 100 pk
Poppet2-0069557K457$3.41 / 100 pk
Fitting2-0129557K464$4.65 / 100 pk
CaliperPiston1-2189557K487$5.06 / 100 pk

First item to work on was the parking brake valve. This is the newer Matco design, the PVPV-D which is sold by Vans under their part number PV-2. It is supposedly less prone to leaks than Matco's original design PVPV-1 which was sold by Vans under their part number PV-1.

Here it is partially disassembled.

Old o-rings on the right, new on the left.

I had an old container of DOT-3 brake fluid sitting around which I figured would be good enough for lubricating the seals for installation.

After installing the four new shaft o-rings and reassembling the shaft, I looked in the ports and found this fleck of rubber. Arg! One of the o-rings got sheared during the installation.

If you click on the full sized picture, you can see that o-ring nearest to the body has had a chunk sheared out of the side of it. This is completely a consequence of poor design. Trust me, I know what I'm talking about here. I've designed multiple o-ring applications before and there are very specific requirements for chamfer lead-in angle and size which were totally ignored in this case. Because of the sharp corners (having been only deburred) leading into and out of the bore, there is a high likelihood of the o-ring getting pinched.

Well, I destroyed three shaft o-rings in the process, but finally got it reassembled without any nicked o-rings.

Next up was the master cylinder. I'll have four of these on my plane since there will be brakes on both the pilot and passenger sides.

Comes apart easily enough. Two of the three o-rings are visible here. The third (the rod seal) is on the inside diameter of the cap.

...but down at the piston end, I found this little guy. It turns out it's called a stat-o-seal. I'm surprised I haven't seen these before in 20 years of doing engineering. Huh.

It looks like a washer with an o-ring molded around the inside diameter.

I pulled off the rubber to expose some teeth that allow firm attachment to the elastomer.

Last (and easiest) was the caliper. I had not unpacked these from the box until now.

Just had to take out two bolts to allow access to the piston. Then a puff of air in the port to pop the piston out. There apparently have been times when people have inadvertently reinstalled the piston backwards, so the o-ring is closer to the pad. That'll cause leaks. My pistons were marked "THIS SIDE OUT", so it was easy to not get mixed up.

I decided to do a little experiment and soak one of the old Buna-N seals in some DOT-3 brake fluid. I'm expecting it to swell up or something...we shall see.

Update 3/17/2011: I can see no effect of the DOT-3 brake fluid on this o-ring. I wonder why they give a poor rating to Buna-N seals with DOT-3 or 4 brake fluid? Shrug.

Update 5/11/2020: I'm bringing an end to this experiment. I removed the O-ring from the jar and measured it. This is a 2-111 O-ring. The cross section diameter should be .103 +/-.003. The ID should be .424 +/-.005. The actual cross section diameter is .1065 (i.e. just barely out of tolerance). The actual ID is .437 (i.e. .008 larger than the max permissible size). I didn't measure these dimensions before starting the experiment, but let's compare the nominal and actual volumes. The nominal volume would be 0.01379509890976 cu in. The volume of the O-ring at it's actual (after testing) dimensions is 0.015210325850418. Dividing one by the other shows that the O-ring gland fill would increase by about 10% as a result of this swelling. Having spent the past eight years dealing with hydraulic equipment, and having made a detailed analysis of O-ring gland fill tolerances and calculations, it seems that the effect of swelling alone is unlikely to cause a problem. Of course, there could be other effects I cannot measure such as degredation of abrasion resistance or a change in the durometer. I leave it up to the reader to determine whether the observed changes are enough to warrant changing out the O-rings as I did.

This picture is copied from a future date, but here are the EPDM stat-o-seals. They look pretty much like the other ones, except the metal on these appears to be black oxide plated rather than bright zinc plated. It'll be good to get the master cylinders re-assembled so I don't have to worry about getting the parts contaminated what with miscellaneous sawing, filing, and grinding in the workshop.