Grove Brake Issue

[robcaldwell]

Mark suggested that I call Bob, and I will do that tomorrow (5/3)... But also wanted to toss this one out to the rest of the Bearhawk community with substantially larger brains than me.

I have the 66-152 Wheel and Brake kit. On our first flight attempt on 4-23, the right brake caught fire and seized to the rotor during a high speed taxi test. After 15 minutes, it cooled and released and I was able to taxi back to the hangar. https://youtu.be/h7f_0nwvetQ

We thought we found the issue... The brake fluid reservoir was not venting properly. We vented it. I was delayed from performing another taxi test as I was involved in a few avionics issues that took me over a week to resolve. Yesterday I finally got my gadgets working and decided to try another HS taxi test. Again, the right brake overheated and seized to the rotor. Waited 15 minutes, the brake pad released and made it back to the hangar... No damage has resulted as far as I know. Possibly the rotor warped slightly, but not enough for the naked eye to see anything... and I'm guessing this heat isn't doing any good for the O-rings.

Before this, I properly seated the brakes per Grove's instructions. My feet are NOT riding on the toes brake levers. In fact, something I learned while taking Jared's BH Transition Training, was that my size 12's interact with the toe brake levers if I am only heals down. So I have to intentionally position my feet heals down AND heals slightly back to keep the toe of my shoes out of the brakes. I don't like that position and am not fully comfortable with it (need to come up with a solution for this in the future), but this makes it certain that I have no interaction with the brakes until I slide my foot up.

Anyway, I am headed to the hangar so I can stare at my right brake; my only obstacle to flight at this point. I might take it apart and see if there might be something pushing on the pad(s).

Just thought I would throw this out there in case anyone might have a suggestion.

Thanks!

UPDATED with Pics

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R Pads.JPG

Right Caliper.JPG
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[jaredyates]

Is there good freedom of movement of the caliper on the torque plate? I apply a dri-slide graphite lubricant there. Perhaps a manufacturing defect or a piece of construction swarf is in there preventing the float left-right? Also, what about the parking brake valve?

[robcaldwell]

Probably could use more graphite on the guides. We've looked at the parking brake several times. Seems like it is either full on, or full off.

[jaredyates]

You might try getting the right wheel in the air, while the plane is on the ground. Try turning the wheel, while someone inside presses and releases the brake. Try and see if you can reproduce the problem in the hangar.

[robcaldwell]

Good idea jaredyates. Will do that once we have it back together.

I just ordered new brake pads & rivets. Also ordered new O rings for the right side pistons in case the heat got em. Unfortunately, won't get the parts until Thursday.... and we wait.

[robcaldwell]

Also... I was able to taxi around all I wanted at 5 to 10mph for as long as I wanted. But, as soon as we increased the taxi speed to around 25 kias on the roll, the right brake got super hot. I read on the Grove website that it takes over 1,000F degrees before the brakes begin to fade.

[Aero_tango]

Hey Rob,
I’ve had a similar issue (not to your level) and I trace the problem back to a brake dragging. The problem in my case was that one of my master cylinder wasn’t fully releasing the fluid pressure. It was kind of intermittent, not every time. I’ve ended up installing exterior spring, which fully took care of the issue. You can visually check if the piston retract properly. Push the brake pedal with your hand, let it release. Then grab the pedal with your hand and pull to see if the piston further retract. Try few time, with different pressure. In my case, one of my master cylinder wasn’t retracting properly, not by much, maybe by 1/32, enough to make the brake to drag.
Look at the photo below, you see the spring around the piston shaft, sorry the photos isn’t great, but you get the idea...don’t pay attention to the tape measure...

Mike

[robcaldwell]

Thanks Mike. I will consider this.

[jaredyates]

Michel's comment reminds me of a master cylinder that stuck on occasion. The shaft was bent just enough to cause it to bind. Just one more thing to investigate.

[AZBearhawk272]

As said above. The smart money is on the Brake Master Cylinder. Mechanical binding externally or internally that keeps the inner piston / to input shaft seal in contact will prevent the pressure down stream of the master cylinder from releasing. Also using the wrong fluid can swell the seals so they do not release pressure. What is your brake fluid? Mil-H- 5606 or Automotive ATF?

Kevin D. #272
Phoenix, AZ

[robcaldwell]

Mobil 1 Synthetic ATF

[svyolo]

Rob, no pulsating at the pedal, or felt on the airframe?

[robcaldwell]

Felt no pulsating in any area.

[svyolo]

While you have the wheel in the air, you might want to try and rock the tire back in forth to check to see if the wheel bearing is seated correctly.

[Battson]

What brake-disc break-in procedure was used?

This sounds like a sub-systems issue, rather than a Bob Barrows / Bearhawk Aircraft issue. I would be talking to Grove, not because their product is at fault, but because they are likely to have seen this question before.

[robcaldwell]

No I don't have any thoughts that the issue is with Bob's design or the A/C. I used the recommended pad seating instructions per Grove. https://groveaircraft.com/brakeinstall.html.

[quadra]

Rob are your calipers mounted with the bleed screw at the bottom? that would make it tough to bleed a trapped air bubble in the caliper or the lower line if it drops below the top of the caliper.
I have seen similar behaviour in calipers that had air trapped in them; as the brake assembly heats up - the expanding air "applies" your brake and keeps it under pressure until the trapped air cools and contracts. If this is the problem, you can probably simulate this with the wheel in the air and a heat gun on the caliper/disc assembly.

[Battson]

This is my preferred way to mount them. Bleed from the top of the system by pumping in fluid from the bottom. Air rises so this is working with gravity not against it.

[JimParker256]

What Battson said! Bleeding from the bottom up is SO much easier!

[robcaldwell]

Mark encouraged me to call Grove and speak with Robbie Grove. Robbie was super nice and very helpful. He is 99% certain this is a master cylinder issue. With the push-type master cylinders, if the piston does not fully return, similar to what Mike described above by using an external spring under the clevis, pressure will continue on the caliper pistons. Robbie said as little as 1/32 will affect the system.

Some additional details I picked up from Robbie:

• The purpose for a vent in the reservoir is to allow heated brake fluid to expand.
• When my pads froze to the rotor, and later retracted was due to the over heated fluid. Once cooled, the pressure was relieved.
• Robbie said it's almost impossible for air to be trapped in the calipers, if fluid is bled from the bottom up.
• The O-Rings will deform if overheated, but mine did not. He thinks I probably saw around 300 degrees and would not be hot enough to affect the O-Rings. I will replace mine anyway.
• Robbie is not a fan of high speed taxi testing for the potential to "cook" the brakes. He says his brake system is designed for sustained braking of no more than 10 seconds. Then it must cool. Quick application after clearing the runway is ok. Just not continued engagement.

I have replacement master cylinders coming for the right side. Will install those, new pads, new O-Rings, refill the fluid. Will try another first flight after this task has been completed.

For more on Robbie Grove, here is a great article. https://www.kitplanes.com/landing-gear-guru/

[Bcone1381]

I am learning a lot from this..... the purpose of the vent, the caliper retraction and springs to assist that.

[robcaldwell]

When I mentioned using springs to assist the master cylinder piston, Robbie said this should not be necessary if the master cylinder is working correctly. If a spring is needed, there's a problem with the cylinder and it should be removed and overhauled.

[Nev]

Some additional details I picked up from Robbie:

That’s all really good info Rob. Thanks.

[schu]

https://bearhawkforums.com/forum/bearhawk-general-discussion-news/41782-matco-master-cylinders

[robcaldwell]

Apparently I'm not the only person hindered from a first due to master cylinder issues. No mention of Grove. Are Matco's preferred?

[huntaero]

Hi Rob also check the flex lines if a piece of rubber from inside the line got shaved off during the assy process it could be acting like a check valve in the system.