brake master cylinder / pedal attach clevis

Oy vey. Not sure I want to go here, but at the risk of kickback I’d like to comment further on the brake discussion, as I think there may be some confusion when it comes to matching the master cylinder to the wheels and brakes. Not regulatory confusion, but the hydraulic system itself.

We have a closed hydraulic system on the Bellanca brakes. In a closed system, pressure is equal throughout. Since fluid is not compressible, once the brake pedals are depressed to the point where the brakes are in contact with the wheels, no further pedal movement is theoretically possible. In effect there is a solid connection between the brake pedal and the brakes and any further force on the brake pedal results in increased braking force at the wheels. The energy required to stop our airplane is based on several factors - most significantly the speed and the weight of the aircraft. The wheels and brakes are sized by the manufacturer to meet those requirements, and as noted in Blimpy's post the Cleveland Wheels and Brakes requires 400 to 500 psi at full gross weight to stop the Bellanca.

This is where it gets interesting. When matching the master cylinder to the wheels and brake requirements, any master cylinder that fits in the airplane and can deliver 400 to 500 psi at the outlet port meets the requirements and is satisfactory.

Here is why.
There are three factors to consider:
Pressure (P) which is the internal system pressure expressed in pounds per square inch (psi);
Force (F) which is the force exerted against the brake pedal and ultimately against the brakes, expressed in pounds;
and Area (A) which is the surface area of the piston (Bore) in the master cylinder or of the brake cylinder at the wheels, expressed in square inches.

The relationship between these factors is expressed in the formulas:
Pressure equals Force divided by Area or P = F/A
so we can solve for the Force as F = P*A
or we can solve for the Area as A = F/P

We know that we need 400-500 psi, therefore let P = 500 psi.

Any combination of Force and Area that will produce 500 psi is theoretically acceptable. Using the example of the Cleveland Master Cylinder given in Blimpy's post, we can look up the P/N 10-35 and see that it has a piston diameter (Bore) of 0.625”.

Using the formula for area of a circle (Area = pi * radius squared) (A = π * rˆ2) we know the area of piston is 0.306 sq. in.

Solving for Force (F = P * A) we get (500 psi * 0.306 sq in) = 153 lbs. The Force required is a measure of how much pressure you can exert on the brake pedals. We need 153 lbs. force to generate 500 psi system pressure. With the same master cylinder (same size piston) you would need 122 lbs. force to generate 400 psi.

This correlates very favorably with a human factors study done in 1971 at Wright-Patterson, where foot forces were measured from 100 pilots of various sizes and weights to determine the affects of brake pedal angles and seat positions on foot force. Neutral pedal forces were around 140 lbs. at a 25˚ pedal angle and extended leg forces were as high as 185 lbs. at 25˚ pedal angle.

The full study is available at http://dviaviation.com/files/38801004.pdf

Since the design of the Bellanca brake system is already established, I would look for a master cylinder that would provide 400 - 500 psi system pressure with a 120 - 140 lb. pedal force.

Using the formula A = F/P you would need a master cylinder with an area between 0.24 sq in (F = 120 lb / P = 500 psi) and 0.35 sq in (F = 140 lb / P = 400 psi).

This means that any master cylinder that fits in the airplane and has a piston diameter (Bore) between 0.552” and 0.667” will do the job. What about the stroke you ask? It isn’t relevant to determining the Pressure since this (our application) isn’t a pressure compensating or power brake system - the stroke only comes into play to seat the brakes against the wheel, and our needs are small.

I think this brief analysis, along with the concepts outlined in AC43.13-1B (Acceptable methods, techniques, and practices  aircraft inspection and repair) along with AC 23-27 (Parts & Material Sub for Vintage Aircraft) would form the basis for any proposed STC, 8110-3 or Form 337 to use any master cylinder that fits the need.

Any comments, corrections, questions or challenges are accepted and appreciated.
 
Because of this thread I crawled into my cockpit and did a close exam of my brake pedal cylinders.

I have Bodel brakes and my cylinders appear to be the same as Art's (from his post pictures).

I found that one cylinder's single nut on the threaded clevis post that extends out the top of the cylinder and attaches to the brake pedal was loose and was allowing the clevis rod to be pushed slightly off-axis when the brake pedal is pushed. This was obviously putting a bending load on the clevis rod at the point that it enters the end of the cylinder piston rod, the load may be slight but done many times probably adds up.

It took about 2 complete turns of the nut to tighten it down to the top of the piston rod.

This could conceivably cause the clevis rod to break. Worth looking at during the annual. Also, would be prudent to add a second nut on each rod for a stop nut.
 
Jeff, from the only photo that I saw on Arts post, it would be difficult/impossible to determine if it is a Scott or a Wheeler Dealer. They look the same from the top. The difference shows up in the area that the cylinder reduces in size. The Scott reduces in a gentle flowing manner, and the Wheeler Dealer reduces in a clear step. If you cant see the lower part of your cylinder, just run your finger down the side and it will be easy to tell. Your explanation and the linked study are really great. I always wonder why, when there is old established information out there, that the FAA insists on reinventing the wheel before we can make simple changes to our planes. OH WELL. ________Grant.
 
Grant,

I looked at mine > they are the Scott cylinders as they are reduced in diameter via the curved reductions.

now i know.
 
ArtL. We explained and well reasoned ! Most excellent post.

Re: broken clevis. pedal etc. Looseness is one explanation . My plane actually has the corner of one pedal broken off.. and I am sure the reason is panic and frantic pumping trying to get a bad brake to do something..
anything !

My plane was in that condition when I bought it, and has reverted to it a couple of times, once with
expensive results. Here are multiple reasons for Hayes / Scott Brake Failures ( from experience):

1. Old or mixed brake fluid the consistency of honey.

2. Single broken spring brake shoe retaining clip ( these are flat blued metal springs).
Symptom will be all or nothing.. grap or nada braking.

3. Air in brake lines. Because of the routing of the brake line with a horizontal run inside the wing, the brakes must be bled by forcing fluid UP from the bottom of the system.

4. Failure of Expander Tube. Small leaks in the expander tube can be the cause of loss of fluid ( frequent top ups) with no apparent leak, and diminished brake action. This happened to me.. no leaks in wings, no external leaks... but small puddled of drying brake fluid inside the brake hub. Expander tubes are unobtanium.. the clips were $12 each a few years ago. So, when you lose an expander tube you have to get another or Sheiss Can the hayes system. I have kept mine going robbing parts from 1 NOS brake. But done with that
 
This is a great topic and, sorry, I just couldn’t let it die. The original post provided a clear picture of the failed clevis fork (or whatever you want to call it). Many great replies but nobody commented about the design of this part. The failed part is a machined component consisting of the fork and the threaded stud – all one piece. From the picture it appears that the threads were “cut” instead of “rolled”. Cut threads are generally weaker than rolled threads as the machining process severs the grain of the steel. Of greater significance is the fact that cut threads are far more susceptible to fatigue failure than rolled threads. Heat treating is another variable that can either have a positive or negative impact on a threaded stud. Most clevis forks manufactured today have internal threads for a good reason: attaching the clevis fork to a rolled thread on the end of a piston – or a threaded rod – is a stronger and superior solution to this type of assembly. Tomorrow I am going to look at the master cylinders in my bird and figure out what I have. Rob
 
Rob58 - when you check your master cylinders, I suggest you also look at these two items. As the pedal attach clevis adjusts the angle of the brake pedal, you might verify that the pedals are adjusted equally and at approximately a 30˚ angle from vertical. This assures maximum force application to the master cylinder and a nearly linear force on the attach clevis. Second, take a look to verify that the entire assembly has clearance to any structure when the rudder pedals and brake pedals are at their forward most location. This is adjusted by the turnbuckle at the rudder control horn. These adjustments will help prevent or minimize any bending force introduced into the attach clevis, and help avoid the type of failure shown in the original photos.

artL
 
scott 4000

I will post scott, wheeler dealer ( two types), and grove side by side soon.

Scott upper diameter is about 1 3/4 " and wheeler dealer is about 1 inch.
The scott has a pronounced flair in the body casting.. large reservoir
the W.D. does not have a large reservoir..
 
working on the masters in the airplane is quite do-able.

Only 2 bolts.. one to the pedal, and one at the bottom, and the hose connection
attach the master cyl.

The "difficult" part is the MESS. And the ergonomics of working under the airplane.

It is easy to remove the plunger of the Scott 4000.. from the cockpit.. without disturbing
the hose, or attachment to the airframe. Doing so will give you Positive ID of your master cylinder,
but so will measuring the body at the top.. or comparing photos of the tops.. which I will provide
"bye and bye". The Scott uses a snap ring keeper and the W.Dealer uses a real snap ring ( with holes in the tits for snap ring pliers). As I said the Scott is much larger in diameter.

to inspect master, renew O-rings, determine type of master :
you need:

rags, disposable gloves, small end wrench, needle nose pliars,
small screw driver or snap ring pillars ( depending on type of master cyl)
Good Light !
small suction device
something waterproof to protect carpet from wet tools
protection for your seats.


a. disconnect pedal
b. put rags around master cylinder to keep any spilled /dirbbled fluid off rugs and out of area under the master cyl.
c. get a small suction device.. turkey baster, large eye dropper, syringe etc.
d. remove fill plug, and if possilble suck out fluid from reservoir
c. alternately.. remove snap ring.. slide the flat cap up.. and suck out the fluid
d. very slowly remove the plunger catching spilling fluid with rags.
e. place wet nasty plunger in a dish, can, or in soft dry absorbant rags... so you can get it out of
the airplane without sliming anything with 5606 ( which I think is the nastiest crap in the known universe)

Now.. if your plunger has two main O-rings and two plunger sections.. it is a two stage master and thus a high pressure type ( Wheeler Dealer, Cessna, Grove, Cleveland, or even a Scott 4350 from a aircoupe)
If it has a single plunger section and one main o-ring.. and a large reservoir.. it is a Scott 4000.

Remove plunger to clean workbench and inspect. Measure and renew O-rings.
Don't USE Any Solvents like Stoddard or paint thinner will destroy your O-rings.. don't use it !
It will contaminate your system !

AN O-rings are available from AC Spruce.
A hydraulic supply store can also help.
No Ace Hardware plumbing parts please !

Re-assemble using clean brake fluid only to lubricate new O-rings and plunger.
I have previously showed how to polish the plunger and hone the cylinder.. if it wont
work properly. Don't fix what isn't busted.

After plunger is re-installed ..you can fill the reservoir in the usual way.. I use a pump
type oil can dedicated to 5606.. but there is a plastic squeeze bottle with a very fine spout
which is ideal.. if you can find one.

No bleeding of the system should be necessary.. any small air bubble with work themselves
out the fill cap or breather hole with some foot pumping.

Test well on ramp before flying !
 
oh.. while the plunger is out.. remove the last of the fluid from the cylinder proper,
( suction, rags) and use a bright light and mirror to visually inspect the cylinder for scoring, and scuffing.

a Bent shaft is probably accompanined by scuffing or scoring in the cylinder and on the wide part of the plunger.
this can be polished and honed.. but you MUST remove the cylinder from the airplane to hone it..
must must must.
 
In the scott.. don't overlook the very small o-ring that seals the shaft at the top.
you must dissemble to top to renew this... it is beetch to get the right size.. to control leaks at the shaft.

as the whole thing wears overtime, due to all the sins that metal is heir to.. and the fundamental
goofy nature of how pedal mounted master cylinder work.. things are just going to get sloppier sloppier
benter and benter.. until you have an expensive aviation ashtray in a matched set.

these will keep your unrepairable hayes brake assemblies from getting lonely and dispondent in the long cold winter. but if they throw them selves off a high shelf .. that's why. :mrgreen:
 
View attachment 1Here is are pictures which will positively ID the Scott Master cylinder.
Note the type of keeper on top of the Scott is diagnostic, as is the larger reservoir diameter compared to Wheeler Dealer. This is easily seen inside the cockpit.. with flashlight and crawling in on your belly.

Overall pic for reference.
 
Hi...first time post for me..I have a 50 Cruisemaster and it has the Paramount Brake Cylinders installed and they work great, however, the pedal angle for the pilot's side is too far aft and makes it difficult in operation...I have read from existing posts on this website that that seems to be a problem..2 questions: 1) are the Paramount cylinders original equipment for the 50 Cruisemaster, and if not what is? 2) If the Paramount is the right cylinder, then how can one adjust the angle of the the toe pedal?...thanks...Bullockdr
 
The clevis that attached to the pedal should be adjustable. Remove the pin attaching the clevis from the brake pedal, then screw the clevis in or out of the master cylinder piston shaft to adjust the height of the clevis and therefore the angle or the brake pedal.
You might find that the clevis is already fully screwed into the piston, meaning the brake pedal is already as adjusted "forward" as it will get
 
You must log in or register to reply here.
Back
Top