Tender brakes air pressure and leverage

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DaveD
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Tender brakes air pressure and leverage

Post by DaveD »

I'm fabricating my 1.6" B&O P7 tender brakes. They will run on air pressure, as will the locomotive brakes. I need to design to an air pressure and cylinder-to-brake pull rod lever ratio. The loco brakes are set up with two scaled prototype cylinders and levers. The cylinder area is around five square inches total and the lever ratio is about 4.5 to 1, so full air pressure of 50 psi would apply over 1200 pounds of total force to the six locomotive brake shoes, 200 pounds per. The tender will use a single brake cylinder with about the same area, around 2.5 square inches, and the prototype ratio appears to be about 1 to 2.5 (2.5 times more force at the brake cylinder than to the brakes). There are brake shoes on each of the eight wheels that are much smaller than the loco drivers.

I'm having trouble understanding how to apply this in the model world. I understand that it is important to have effective tender brakes, so they need to work well. And they will be running on the same pressure as the loco brakes. Most live steam tender brake levers that I've managed to have any kind of look at are at about a 1 to 1 ratio, with the outer link running to one truck and what amounts to the fulcrum running to the other truck, halving the motion of the cylinder rod to each truck. That is my initial inclination to design to, but I would like to know just a bit more about best practice here. It would be a simple task to change the ratio by changing the point that the inner link connects to the lever--outward to increase leverage ratio, inward to decrease. Again, these brakes will operate on air pressure, not steam or vacuum.

Edit: I forgot to mention that there is an additional lever on each of the truck brake beams--one beam per axle--with a 2 to 1 ratio. So the force per brake is doubled there.
Last edited by DaveD on Fri May 27, 2022 6:46 am, edited 1 time in total.
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Bill Shields
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Re: Tender brakes air pressure and leverage

Post by Bill Shields »

I struggled with this and after changing brake pad material and linkage pivots...I eventually went with foot powered tender brakes like on a motorcycle...figuring that after I knew what pressure was required I would work backwards from there.

That was 15 years ago and the foot brake is still there..it is so darn convenient to just tap the brakes here and there..
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amadlinger
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Re: Tender brakes air pressure and leverage

Post by amadlinger »

I have always aimed for the brake shoe force that would cause the wheels to lock up and start sliding as the starting point for the design, and then experimentally adjusted up/down from there with a target of just less than locking up the axle. Of course with air that's pretty easy to do with a pressure regulator in the line. On my 2-8-0, I sleeved down the locomotive brake cylinders considerably to avoid the excessive forces I would have otherwise gotten (like you mentioned), while still looking the part externally.

The generic rule of thumb for factor of adhesion is about 4 for steam locomotives, aka 25% of the weight on the given wheel, but this article on the IBLS site breaks it down even more (not sure about the source of this info or its accuracy, but it's a starting point).

http://ibls.org/mediawiki/index.php?title=TLS_Sander
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Re: Tender brakes air pressure and leverage

Post by Pontiacguy1 »

In theory: the coefficient of Friction of cast iron on aluminum is 0.25, or 25% of your weight on the wheel. That's in Lab conditions, though. What I've actually observed over the years trying to do calculations for my own stuff or stuff I was working on, those coefficients given in the above article are pretty darn close, and I wouldn't hesitate to use them.

Here is what I have actually used when trying to do a force calculation before: 0.22 for cast iron wheel on aluminum rail, 0.25 for steel wheel on aluminum rail. That's pretty well in the ball park. You can take away another 10% or or more if the rail is wet. I try to get my braking force to about 85% to 90% of the force it would take to slide the wheel using the above coefficients. That seems to provide very good braking on dry track. with bad track conditions, it will sometimes still lock up and slide, but again it depends.
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Re: Tender brakes air pressure and leverage

Post by Berkman »

it'll be easy to over power the brakes on it. Especially if you are running 80-100 psi air, I think it will need less force on the brake shoes than you might think. For MCC/titan brakes, I think 40-50psi is a significantly strong brake application.
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Re: Tender brakes air pressure and leverage

Post by Steggy »

Bill Shields wrote: Thu May 26, 2022 5:22 pmThat was 15 years ago and the foot brake is still there..it is so darn convenient to just tap the brakes here and there..

Also, they work even when the air compressor goes on the fritz.
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Re: Tender brakes air pressure and leverage

Post by Bill Shields »

Never installed an air compressor...😀
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DaveD
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Re: Tender brakes air pressure and leverage

Post by DaveD »

Thanks, so far, to all who responded. I have a much better understanding of the direction to take.

Bill's foot brakes are obviously the simplest and probably the easiest and most effective, as you apply brake pressure until right before they lock (like tightening a bolt until right before it breaks! :mrgreen: ). But this isn't going to happen on my loco.

I may have a plan, subject to revision. Since I really don't have to worry about maximum compressor pressure until I get the loco and tender on the road for testing, I believe the best start is to try to equalize the amount of force required on both loco and tender to have them both lock up at about the same pressure--I know what the weight on the drivers and on the tender with full tank and operator will be. The loco is fixed, but I can vary the force to the tender brakes easily enough by changing the main lever leverage point. Once about even, the same pressure can be applied to both and adjusted through a regulator.

I will have a separate propane car behind it all and that is where I will place the compressor and battery. I fabricated the three compressor tanks from DOM tubing and silver soldered the ends on. They are functional, tested to 60 psi, and would probably hold enough pressure for loco/tender brake application should the propane car separate. I also have my Westinghouse two-stage compressor hooked to the tanks and it actually functions, but I would not depend upon it to provide working pressure. It's slow... Plus, these things typically give a lot of trouble.

Keep the comments coming. This is helpful!
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Re: Tender brakes air pressure and leverage

Post by ccvstmr »

Dave,

My Allen 10-wheeler was originally equipped with an ejector for vacuum brakes on both the loco and tender. After the loco was purchased, found out the vacuum diaphragm on locomotive was shot. The diaphragm has since been replaced and the ejector cleaned, but I rarely use the ejector. In the Amtrak RR world, they would apply the train brakes and "bail off" the loco brakes. This was referred to as "power braking" as the train brakes are set while the loco is pulling. Wears out brake shoes. But on the flip side, kept the train slack stretched for a smoother ride.

Built a stock car to drag along behind the tender (with a coupler drawbar). Not only did the stock car carry a horizontal propane tank for loco fuel, it also carried a battery module for 12 volt power, a compressor module for the air brakes with self contained air reservoir and a CD module for...well, let's just say, entertainment.

The tender was converted from vacuum to straight air brakes. The stock car trucks were equipped for straight air brake operation. Air reservoir pressure is train-lined up to the loco cab where there's a 3-notch brake stand: release (off), service (10-20 psi application) and emergency (50-60 psi). In actuality, there's a linear regulator under the cab floor that's depressed by a cam on the brake stand shaft. The brake pipe is train-lined back thru the train to control brakes on the tender, stock car and any car (usually a riding car) coupled behind the stock car.

When coasting downgrade, can "feather" the brakes in the 10-20 psi range. Only under extreme conditions would the full pressure be applied to the brakes. Even then, the train won't stop on a dime and the last thing you would want is a lock up and slide the wheels on the rails.

Have pulled 11-car trains (approx. 1100 pound load) with this brake arrangement and still able to control train speed downhill on 2 to 2.5% grades. If I have more riding cars coupled in the consist for riders...will reduce the number of cars in the train by 2 for each rider. In short, need to estimate the train load before I start rolling. So a question for your consideration would be...how much train are you looking to pull?

Have found over the years, it's more important to be able to STOP the train...than to get it going. Help any? Carl B.
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NP317
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Re: Tender brakes air pressure and leverage

Post by NP317 »

Carl is spot on!

My Ten Wheeler also has separate brake controls for the locomotive, and the tender plus cars behind. Prototypical.
The engine brake is steam actuated; the brakes behind are vacuum from an ejector.

When sitting at the station, I can keep the engine brakes applied which consume no steam (static),
and for controlling train speed the vacuum brakes perform that work. I can "feather" the brakes by pulsing the vacuum brake valve.
And both brakes valves can be applied by one hand movement in an emergency. (Picture of cab layout is below.)
This combination has worked excellently even when navigating the steep grades at Train Mountain, while hauling several passengers.

I do recommend separating the engine and tender brakes, per prototype practice.
RussN
317 Cab 2small.jpg
Moron
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Re: Tender brakes air pressure and leverage

Post by Moron »

NP317 wrote: Sat May 28, 2022 10:29 am Carl is spot on!

My Ten Wheeler also has separate brake controls for the locomotive, and the tender plus cars behind. Prototypical.
The engine brake is steam actuated; the brakes behind are vacuum from an ejector.

When sitting at the station, I can keep the engine brakes applied which consume no steam (static),
and for controlling train speed the vacuum brakes perform that work. I can "feather" the brakes by pulsing the vacuum brake valve.
And both brakes valves can be applied by one hand movement in an emergency. (Picture of cab layout is below.)
This combination has worked excellently even when navigating the steep grades at Train Mountain, while hauling several passengers.

I do recommend separating the engine and tender brakes, per prototype practice.
RussN

317 Cab 2small.jpg


Just to clarify, on the prototype, the engine and tender were both part of the Indy brake, and everything behind the tender was auto. When you bail the Indy, it releases engine and tender brakes. On some mountain railroad, especially in freight service, they had what was nicknamed a “mountain valve” which would cut out just the locomotive brakes but keep the tender brakes cut in, so you could make an application, not bail, and have the tender brakes help with braking forces without overheating the tires on steam locomotives.
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NP317
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Re: Tender brakes air pressure and leverage

Post by NP317 »

Thanks for the clarification on the steam locomotive tender brakes being applied with the Independent brake valve. I was not clear on describing that prototype operation.
Except that my model steamers are different with the separated engine/tender brake applications. I find it easier to balance the brake applications that way.

I still have the muscle memory of applying the train brakes and immediately releasing the independent brake valve.
So many years and miles in the cabs of steamers...
RussN
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