Master Cylinder Design

[Arthur Beloso]

Hello everyone. I’m part of a Baja team from Brazil and I am having some problems on the design of our new master cylinder. In the last prototype we used PTFE o’rings to seal the brake fluid. But the lifetime of the o’ring is way too low, like 4h in the maximum. I though in use some U-ring’s but I don’t have sure if this change can improve the lifetime. I saw that commercial master cylinders use piston cup and some kind of u-ring.
Usually the pressure it is around 8 Mpa (1160 Psi), but it can be higher since it is used in racing conditions. We use an 11.5mm bore diameter, but it is changeable to any size avaible of seal. The master cylinder is made of aluminum.
There is any better type of seal that I can use? Do you now any book that can give me more information about it?

 

[MikeHalloran]

You would take advantage of decades of expensive development by using commercial seals and known materials for use with brake fluid. Or just use a complete commercial master cylinder and adapt the linkage to your special requirements.




Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

A close cousin of the phrase "don't re-invent the wheel" ... Never redesign something that you can buy off the shelf. It's generally worth designing your system around off the shelf subassemblies - such as a complete master cylinder with all these issues already worked out for you.

 

[TugboatEng]

PTFE has no resilience (it's plastic not rubber) so it's a poor choice for dynamic seals. EPDM rubber is the correct material if using standard brake fluids. The seal geometry for a master cylinder is usually a u-cup seal. It has a high tolerance for wear and has little leakage.

 

[Arthur Beloso]

Oh, sorry, i wrote wrong the material. Yes, we actually use EPDM o'rings like TugboatEng said. About buy it ou design it. Well, since it is a racing competition I think is good to design our system, first because we reduce mass but also because we have a lot of more points to talk about in the presentation. I agree that the issues may not compesated all the work we have, but for now, we are taking the risks.

Obs: We also design our calipers, but we buy a motorcycle caliper seal ( because it is avaiable ), so we dont have problems with this one.
Obs : Sorry for any english mistakes.

 

[BrianPetersen]

You can buy master cylinders (and brake calipers) intended for racing applications.

Even major manufacturers buy components from companies in the business of making those components. My high performance Kawasaki motorcycle has Brembo brake calipers and master cylinder, an Ohlins steering damper, etc., and those are all original-equipment to the bike (I didn't change them).

 

[MikeHalloran]

for now, we are taking the risks.

It is not "WE" who are taking the risks, it is the poor schmuck who volunteers to drive the vehicle.

Do not let your hubris and your eagerness to have something to discuss put that person's life in peril.

Buy some nice, shiny 'racing' brake parts, analyze the hell out of them, and test them to failure in a static test rig. Then do the same for your homebrew stuff before you start thinking you can do better. You will probably want to race with the commercial parts, and you will be able to explain why in your presentation.

Take some time to review the development of disc brakes, taking special note of how complex the early ones were. Later ones are much simpler, but there are subtleties that you might miss, e.g. square section seals and subtly tapered groove faces on caliper pistons to provide retraction.






Mike Halloran
Pembroke Pines, FL, USA

 

[Arthur Beloso]

I get your point MikeHalloran. Actually, some years ago we used to buy our parts, and do tests with them, just like you said. But with the time, we thought in designing by ourselves, not only to ( maybe) improve the perfomance, but also to develop a better understaning of the manufactiring and producction of the brake parts ( just like this one in question ). Me and my team are really concerned about the driver security and we are studying ways to have a project that avoid problems and ensure that nothing bad happens.

 

[Tmoose]

How do your Teflon O-rings fail ?
Wear, extrusion or tearing ?
What finish are you putting on the MC bore ?
Aluminum MC I AssUME.

 

[Arthur Beloso]

We believe the o'ring fail because of excessive wear ( it is EPDM ). For the finishing we use the recommened by our supplier ( Rf < 4µm for the bore and Rf<16µm for the piston ). Yes, the MC is made of aluminium ( SAE 6351 ).

 

[TugboatEng]

Are you using a backup ring?

 

[Arthur Beloso]

No. Just the o'ring by itself.

 

[BrianPetersen]

An O-ring as a dynamic piston seal in a brake master cylinder??