Mater Cylinder, How does it work

I recently had problems with the master cylinder (brake). It seems to be working now. But I have some question on the design. As you can see in the picture there are 2 springs, one for each plunger. The spring on the right is much weaker than the one on the left. The weak spring can easily be compressed with my fingers. The strong one will only compress if I put some of my weight on it. Question 1 is "Why are there 2 different springs and what are their purposes ?" The outlet port for the piston with the stronger spring is larger than the one for the weaker spring. Second question is "Why are there 2 different size outlet ports". As I said I currently do not have a problem I am just curious about how the master cylinder works.

Bob

 

http://auto.howstuffworks.com/master-brake1.htm
Two different size outlets prevents hooking lines up wrong

 

Seems to me that the "heavier" spring is pushing back twice as much stuff.

 

The right spring is the return spring for the ***embly, when you let off the pedal, this spring pushes the pistons back into battery position.

The left spring is a proportioning device. It is very likely that this MC came off a car that had disc brakes in front and drum brakes in the rear. Drums will take more volume of fluid to actuate than disc calipers. That little spring allows the rear most (left most) piston to continue to travel after the front piston has hydraulically "bottomed out" so to speak. When the pads have fully contacted the rotor, the front piston will stop moving forward, and start transmitting force. The spring in the center allows the rear piston to continue to pump fluid even after the front piston has stopped moving.

Now, I could be totally FOS (standard disclaimer), but that's sure what it looks like to me.

 

It is from a Drum/Drum. So how would work on a Drum/Drum system ?

 

both pistons are moving forward as the pedal is pushed. If one spring is twice as stiff as the other, then both plungers will displace the same amount of fluid if they are the same diameter as one is moving ahead as the other follows.

 

Yes, until the wheel cylinder or caliper cylinder stops moving and starts exerting force.

The brake fluid (which is just a kind of hydraulic oil), is essentially incompressible, so when the pad/shoe contacts the rotor/drum, the system stops displacing fluid and starts transmitting force.

If one output bottoms out before the other does, and the two pistons are rigidly linked (like machined as one piece), then only one output will ever generate any force, as the other hasn't bottomed out yet.

By decoupling the two pistons, you allow some tolerance for differential wear between the two outputs. If one bottoms out before the other, the spring allows the lagging piston to continue to displace fluid until it too bottoms out and can transmit force.

 

But that dose not explain why the springs are different.

It seems to me that the piston with the heavy spring is going to send less pressure to the wheel cylinders. For example let us say the plunger moves 1/2" with a force of 50 pounds. If everything were equal in the two systems each piston would move 1/4" and exert 50 pounds on their outputs. In this system the total pressure of each piston is the sum of the fluid pressure and the spring pressure. If you think of the fluid as just another spring inside the first spring the total force required to compress both springs is the sum of the forces to compress each spring. If the heavy spring requires 25 pounds to compress 1/4" then there will be only 25 pounds of fluid pressure in that output. If the weak spring requires only 5 pounds to compress then it will have 45 pounds at its output. This ignores any difference in the amount of fluid that has to be displaced in the front vrs the rear brakes. If the piston with the heavy spring requires more fluid movement to activate the brakes then the effect will be greater because the spring will have to be compressed even more which requires more force taking more pressure away from the output port. The reverse would make the heavy spring less effective. So why would you want the pressure to be less in one system than the other. And the proportion would change as you apply more pressure.

The above is a guess on my part. Anyone know for sure what is happening ?

Bob

P.S.
***le should read "Master Cylinder, How does it work?"

 

the light spring is there to return the master back to zero as it were. it has no effect on fluid pressure.

 

I'd agree. Seems the right spring for sure is a return spring, and the left/stiffer one is a shock absorber of sorts. Drum/drum, or disc/drum. My .02c

 

I do not think the amount of fluid required to actuate either system has anything to do with the heavy spring. As I see it the main result of the heavy spring is to reduce the pressure on one of the output ports. If it was a volume issue they would have just used a longer spring for that cylinder, thus increasing the available fluid, not use a stronger one. On a Drum/Drum system is there a reason for having more pressure at either the front or rear wheel cylinders ?

 

Bob, the thing you are missing is the fact that most fluids (including brake fluid, at least until it begins to boil), are incompressible.

That means that there is no second spring like you're thinking. When the shoe/pad contacts the drum/rotor, the fluid and the piston stops moving. They are, at this point, essentially, a single rigid body that acts to transport force from one point to another. A hydraulic system does not work as well if the fluid is compressible (see the limitations of most pneumatic actuators for proof of this concept). Water, oils, and brake fluid are for all intents and purposes incompressible, which means that their volume remains unchanged regardless of the pressure applied to them.

The easiest way to settle this is to hook up a gauge to each output port, and stroke the MC. Read the resulting pressures and you have your answer (BTW, the spring may very well be being used as a device to regulate applied pressure, but not in the way you are thinking). Volume per stroke could also easily be measured with a little bit of effort.

Also, when you stack two springs, the effort required to compress the combined stack 1" does not equal the sum of their two respective rates. The rate of a series spring system: (R1 x R2)/(R1 + R2) = Effective Rate

But that's another show.

EDIT: You do realize that the two pistons pictured are stacked one on top of the other inside a single bore, being actuated by a single push-rod from a single pedal, right? If not, I think I see where you're mistake is coming from. If each piston was in a bore of its own being actuated by a push-rod of it's own, then I could maybe see your theory holding some weight, if not for the springs being of such drastically different rate.

 

The pressure coming from the M/C is the same front and rear on a tandem unit, as pressure is relative to the bore of the M/C. If you want/need more pressure to the front, THAT is controlled by the diameter of the wheel cylinder (always bigger bore on the front). The front bears the grunt of the load (eng/trans weight, inertia forcing weight bias forward, etc)

 

Hi Bob, See you are quickly becoming a brake expert and you will see that there are some sharp guys on this site. Poster "Coolhand" has correctly answered your questions and very accurately too, I got my brake dynamics text out to check the accuracy and it was pretty much right on. While the softer spring is not a proportioning device exactly, both springs work in conjunction with each other per cylinder piston operation to control the balance of the pressure rise upon brake application and act as piston return springs with an override characteristic in case one side fails. Stiffer spring is the primary circuit.

Hey did that thing stop yet or not? Thanks for posting status reports.

 

I am not saying that the fluid is compressing and acting like a spring. You are 100% correct that the fluid does not compress. In fact my argument depends on the fact that the fluid is not compressible. What is compressing is the volume of fluid in front of the piston as the fluid is pushed out of the bore. To get more fluid out you have to apply more pressure. So the more the pressure the more the compression of the volume of fluid in the cylinder. That acts just like a spring. Maybe not with the same ratio of pressure to compression. My spring ****ogy is not good because it appears that I am saying the fluid compresses.

I am having a hard time explaining my point, maybe I am all wet but try this. What would happen if you put a stronger spring in the one piston. The strongest spring is one that would not compress at all. In that case there would be no compression around the spring and therefore no pressure out that port. However the other port with the weak spring will have full pressure. If you now replace the strong spring with one that is not as strong you will start to get some pressure out the port. It is not an all or nothing deal. There is not a certain spring strength where the pressure jumps to the same as the weak spring port.

Let me try another one. What if you had some big springs that are big enough for you to stand in and are 8 feet tall when uncompressed. OK you get inside the spring bend down and a 100 pound weight is put on top of the spring. Now you stand up and lift the weight over your head. The spring is going to help lift the weight and you will not have to lift the full 100 pounds. If you use a stronger spring you will have to do less lifting to get the weight over your head. With a strong enough spring you will not have to lift at all because the weight will not compress the spring enough to reach you. Again this is not a perfect ****ogy but I hope it helps explain.

Bob

 

I'm glad you guys got to this.
I would have said something stupid about it being a limited run on '57 Chevy Tow Trucks without hoods that only works backwards.

Re: Mater Cylinder,

 

The left spring is just there for the initial installation, or if the brakes fail. The right spring is the return spring, the left spring pushes the second piston back, if the master cylinder is empty. Once the brakes are full of fluid and bled, the left spring will just go along for the ride, it no longer comes into play. If the rear brakes fail, the left spring will compress, applying the front brakes

 

Ok, I'm starting to see where you're going here. Part of the mix-up is because you're using the wrong term to convey your idea. When you're using the term "compressing" above, you should instead be using the term "displacing". There basically isn't any pressure applied to the fluid until all the slack in the system is taken up. Until the pads bottom out on the rotor, the fluid flows through the system with a very low pressure (not zero pressure, but not a lot either).

I think what you're tripping up on here is that the springs in this case are not of such a high rate that it cannot be compressed.

The front spring is there to push the entire ***embly rearward when you let up on the pedal. That's all it's there for, which is why it's so weak, that's what make sure the MC always self resets. When combined with a one way check valve, this feature allows the brakes to "pump up". This is how, when bleeding the brakes, the pedal returns to rest and allows you to take a full new stroke, even though the last stroke expelled some volume of fluid. It transforms the MC from a simple sealed hydraulic cylinder into a piston type hydraulic pump.

The spring between the two pistons is there to allow the space to collapse (and thus displace fluid), and to maintain the separation of said pistons. Its spring rate is greater than the front one, yes. However, what happens is, as the front piston moves forward, it displaces fluid and compresses the soft spring, UNTIL the pad bottoms out on the rotor.

Up until this point, neither output port has seen any significant pressure, and only the front output port has had any fluid displaced through it.

After the front pad bottoms out against the rotor, that piston ceases to move, regardless of whether the spring has been fully compressed or not. The fluid now starts to exert force on front cylinder. At this time, since the front piston has stopped moving, the rear piston starts to compress the spring between them. This causes the rear piston to start displacing fluid.

At this point, the front output port is now seeing its operating pressure (whatever that ends up being), and fluid has stopped displacing through it. The rear port still has seen no pressure to speak of, but now has started to have fluid displaced through it.

When the rear pad bottoms out against the rotor, that piston too ceases to move, regardless of how much the spring between the two pistons has been compressed. This piston now starts to exert force on the rear cylinder. At this time, both pistons have stopped moving, and the system is now only transmitting force.

That concludes the downward stroke of the pedal. The upward stroke happens much the same as this but in reverse, right up until the residual pressure valves kick in and cause the MC to act as a pump. But at this point we don't care much about that.

More in another post in a min . . . . .

 

This one makes the most sense. I'm guilty of not giving it much thought. But, now that it's brought up, I'd like to know the answer. And my money, (as far as how I understand the theory of it all), is on this one

 

NO , To get more fluid out, you have to displace more fluid, that has nothing to do with how hard you push. The pistons in your master cylinder act jointly, the spring in most cases is used for two purposes,primarily to direct the flow to the primary brake circuit before initiating the flow to the secondary circuit and the difference in stiffness of the springs is computed to allow this differential. In other words to get the front brakes to work just before the rear. Upon release of pedal they aid in the return of the piston to its seat.

The spring doesn't aid the piston travel they work against the travel, controlling the pistons as they work together in a two system unit.

 

Now, here is where my deductive reasoning and my math and physics background starts to run me in circles (and thusly makes me want to find a reference text to consult). I would look up the answer, but my reference material is all in town at my office.

I am inclined to say that since there appears to be no rigid connection between the two pistons, the force transmitted to the front piston (before the rear pads bottom out at least) is some fraction of the force applied to the rear piston via the push-rod. Because their only connection is by way of a spring, some of the applied force is used to compress the spring. However, upon further reflection, I believe this thinking to be flawed. Let me reason through this here, and tell me what you all think.

If you hang a weight of 5lbs from a string anchored to the ceiling (just bear with me here), the weight of the object creates a downward force of 5lbs on the ceiling, since it's not moving the ceiling is exerting a reaction force of 5lbs upward, which creates a tension in the string of 5 lbs. This is all very calculable (and in this case, quite intuitive), so I know this to be true.

Now, say we cut that string in half, and install a pull type spring in the middle of it, with a spring coefficient (K) of 1lb per inch. Now, we have slightly more complex system, but it is still has very calculable solution.

The weight is still pulling downward at a force of 5lbs, but it is now acting on the spring (which is acting on the ceiling, but we'll get to that in a min). So, now the spring has a force of 5lbs pulling downward on it. Since it has a K of 1lb/in, that means the spring will get 5 inches longer than it would be at rest.

Now, let's look at the pieces one at a time. The spring sees 5lbs downward from the bottom string, and 5lbs upward from the upper string, which gives a tension of 5lbs in the spring, which then makes it stretch out 5 inches. The weight is the same as before, the weight is pulling 5lbs downward, and the string is pulling 5lbs upward. The first bit of string sees 5lbs downward from the weight, and 5lbs upward from the spring, still 5lbs of tension. The other bit of string sees 5lbs downward from the spring, and 5lbs upward from the ceiling, still 5lbs of tension. The ceiling still sees 5lbs downward from the string, and still exerts its reaction of 5lbs upward to maintain equilibrium, just as before.

The introduction of the spring into the system does not alter the forces in the system at all. A spring can store energy and do work by moving objects, but it does not alter the forces applied to it. It merely transmits them.

Therefore, I am now more inclined to say that the spring here does not change the force applied to either cylinder. What it will do is change the amount of pedal effort required before the rear piston starts to displace fluid. That is not to say that there could be an instance where only the front piston would be used for braking, since the force that you would apply to this ***embly to effect braking would be far above that required to begin compressing the spring between the pistons.

At this point, I can't really see much of a "tuning" use for the spring. I'm leaning more toward the notion that it is just there as a timing device, to make sure that the front brakes always react just a split second sooner than the rears, perhaps for stability's sake, I dunno. I've never designed a production car braking system before. On our race cars everything happened at the same time, and I would not say that had any effect on whether I was able to keep the car under me or not during hard braking.

I have enjoyed this discussion though, I thank everyone who's participated thus far.

 

There's one in every crowd. lol

 

OK I am close. I agree with one condition. If the fluid movement has the following properties (the fluid movement not the fluid). as the fluid moves out of the piston to take up the "slack" in the system the fluid pressure can be considered so low it is zero. Once the slack is taken up little if any fluid leaves the master cylinder. I thought that as more pressure is applied the wheel cylinders will move out a small amount and any rubber lines may expand a little. My mistake was thinking that there was enough to make a difference. From what you have said I made an incorrect ***umption.

Final question what is the reason for having one piston activate before the other?


Thank you guys for the information and for putting up with me.

Bob

 

My guess is that the front system supplies (typically...) 80% of the power to stop the car...perhaps the rear system starts to work before the front to stop that stand-on-the-nose, front brake lockup condition? Kind of an inbuilt proportioning valve.....or I may be overthinking things as usual...

 

CoolHand has it figure right! The stiff spring between the two pistons is needed to allow the pressure to rise in both the front and the rear brake circuits. If there was no spring, fluid would flow in the brake lines until the pressure rises at the time the first brake pads (or shoes) made contact (whether it's the front or rear), and then the piston in the master cylinder wouldn't be able to move enough to allow the other brake circuit to build any pressure! If this happens, you would only have two wheels with brakes that work.

Roy @ SAC Hot Rod Products

 

Bob, You are getting a brake education.

As the fluid is DISPLACED by the piston movement(moved from a to b)there is minimal amount of line pressure. As the brake shoes or pads meet a solid surface the flow of fluid stops. At this point the piston has moved past and closed off the fill port of the master cylinder reservoir and is a completely closed circuit. At this point the circuit transfers the pressure applied on the brake pedal to a clamping or expanding action at the wheel side. There is minimum travel in the master cylinder piston, what is now in effect is the ability of hydraulics to transfer a force. The pressure applied at the pedal is now magnified by the pedal ratio and bore size ratio to a mechanical advantage. Easily interpreted as how hard you have to push on the pedal to stop.

Any expansion of the brake lines would loose mechanical advantage. To control expansion of flexible hoses they are made from wrapped fiber materials to prevent expansion.

As other have posted, a single system can be stacked upon another creating a dual system for safety. The dual system requires the system to work in tandem and thus requires the braking action to be controlled front to rear maintaining stability during stopping. The basic principle is to have better stopping control with the majority of the braking force on the front. If it was reversed with the majority of braking on the rear, the rear would lock and have a tendency to place the vehicle out of control.

In this application both pistons act in conjunction with each other but with the primary acting just slightly before the secondary.

Thank "coolhand" for his explanation and excellent detailed post.

 

****,

First thanks for the information. You said

"As other have posted, a single system can be stacked upon another creating a dual system for safety. The dual system requires the system to work in tandem and thus requires the braking action to be controlled front to rear maintaining stability during stopping. The basic principle is to have better stopping control with the majority of the braking force on the front. If it was reversed with the majority of braking on the rear, the rear would lock and have a tendency to place the vehicle out of control. "

It makes sense that the majority of the braking force be on the front. I am unclear about how the front brakes get more force. Please explain.

Thanks
Bob

 

The front gets more pressure due to it's piston diameter. The bigger bore, the more clamping force with a fixed pressure coming from the M/C. To simplify, take a 4 wheel drum brake car with a single pot M/C (pre 1967 american cars). The front wheel cylinders always bigger. Let's say 1 1/8 front, and 7/8" rear for example. This gives more pressure to the front while sharing the same exact M/C pressure available to the rear. Make sense?