Under Car Linkage

Let's talk about the linkage under the Car Frame that conveys the Air Cylinder's force to the Pull Rods attached to the Trucks.

An important Goal is to have the maxinum amount of braking without sliding the wheels.

One of the Great advantages of a Center Pull Brake System is the fact that we have some additional mechanism that allows us to alter the Mechanical Advantage of the Air Cylinder to the Brake Shoes. This is in addition to the fact that there's only One Air Cylinder and it's up off the Ground, out of the Dirt and the Harm that might happen during a Derailment. Another advantage is that we have a much wider choice of Cylinder sizes, because we typically have plenty of room under the Car Frame compared to that available under the Trucks.

One thing that is almost impossible to measure is the actual force applied to the Brake Shoes themselves. It's hard to get a little bitty scale in between a brake shoe and a wheel in order to accurately measure this force. And, even if we could get a scale that small, how could we do Dynamic testing with a rolling car, without losing our scale ?

We can’t !

What we want, is enough force to stop the car without sliding the wheels on the Rail. Braking force is Friction, but we DON'T want the Friction of a Cast Iron wheel sliding against an Aluminum Rail as our Brakes. Worse Yet !, We don't want our cast iron wheels sliding against Steel Rail. What we really want is the Friction of a properly lined brake shoe sliding against the cast iron or steel wheel.

In order for the Air Pressure from the Reservoir to effectively apply the right amount of pressure to the Cylinder, we need to incorporate an in-line PRV (Pressure Regulator Valve), to give us a point of quick, easy and relatively accurate adjustment.

If we knew exactly how much Brake Shoe pressure it was going to take, then we wouldn't need the PRV, but we just Don't.

Now ! ......... Ideally, we'd like for the car to start braking as soon as the Engineer makes an application. To accomplish this, we need to have the linkage setup so the PRV is operating at, or near it's upper limit. This means that the lengths of the levers used is an important factor here.

These two levers are identical in construction with the exception that one might have an extra hole for a Return Spring. (This is Optional.)

The Mechanism basically consists of the Air Cylinder that’s pivot mounted to the Car Frame, two Levers, two Pull Rods to the Trucks and a Connecting link. Optionally, we can add a Return Spring to the Air Cylinder Lever to make sure the Cylinder always retracts. The Lever that’s not connected to the Air Cylinder is Pivot mounted to the Car Frame. (see figure 1, below)

We’ve already decided that the Train Line Air Pressure is going to be 50 PSI. The inline Check Valve from the train line to the reservoir requires about 2 pounds to open, so the actual Air Pressure inside the Reservoir is more like 48 PSI. That’s fine, just as long as we know this.

There are tons of Formulas to figure all these Forces, but without knowing what exact force we need on the Brake Shoes for any given car, they are basically meaningless. The easiest, fastest and most accurate way to determine what we need is to do Dynamic Testing on each car.

Assume now, that during Dynamic testing, we find that in order to stop a given car without sliding the wheels, we have to set the PRV way down to say 29 PSI. ( Keep in mind that the Air Cylinder has an O-Ring around it’s piston inside the cylinder. This O-Ring sliding against the cylinder walls is "Friction" and therefore means that the differential air pressure on both sides of the piston needs to be greater than about 3 pounds before the piston begins to move in one direction or the other. ) This means that during normal Train operation, this car doesn’t begin to start braking until the Train Line pressure is reduced to around 26 PSI. Which is almost half the normal Train Line pressure.

We’d like to setup our linkage on this car, to decrease the Mechanical advantage of the Cylinder such that we have to increase the PRV pressure and get it closer to 48 PSI, again, without sliding the wheels.

This is Easier than it sounds if we make our linkage a certain way.

What we can do is to make our linkage levers with several (if not many) adjusting holes. This gives us the opportunity to find an adjustment point at, or near the desired leverage point. We could, at this time, if we so desired, completely remove the PRV, as it wouldn’t be needed for this particular car. This might be more difficult than would be profitable, however.

Another advantage in this scheme is that all the cars could have the same size Cylinders and the same type of linkage levers, just adjusted to different points for the given weight of each car.

Notice in figure 1 that there are numerous holes to allow the connecting link to be moved up and down. In this example, the Higher we move the connecting link, the more Mechanical advantage we have. Conversely, the Lower we move the connecting link, the less Mechanical advantage we have and therefore can adjust our PRV up closer to the maximum pressure.

You can see in the figure that there are formulas to figure out the forces, but without knowing exactly what force we need on the brake shoes these are somewhat meaningless.

One thing to be aware of here, is that the forces resulting from the movement of the connecting link is Not a linear function. It’s more logarithmic than anything.

In the figure you’ll notice that the C5-D5 position of the Connecting link is given in Red. This is a point at which the distance from points A to B is equal to the distance from points B to C.

In this position the Mechanical advantage is 2 to 1. If the link is moved up to position C1-D1, the Mechanical advantage is 3 to 1. If the link is moved down to the extreme position of C13-D13 then the Mechanical advantage is 1 ½ to 1.

You could, if you liked, make the levers with only the holes at point A, Point B, Point C1, Point C5 and Point C13. This would give the three choices of 3:1, 2:1 and 1 1/2:1 ratios. If you decide to do it this way, you can put all the holes on the same centerline. In the figure, the adjusting holes are offset back and forth to get them close together without breaking through the material.

You can see that if you were to make these levers even longer, you could continue to Decrease the Mechanical advantage to a point near 1 to 1. (Never exactly 1 to 1)

We also have to keep in mind that the psychical width of the car limits how long that we can make these levers. If we can't achieve proper braking with the connecting link all the way out while the PRV is set for 48 PSI, then our other option is the reduced the size of the Cylinder.

Another thing to keep in mind, is the fact that as we increase the mechanical advantage by moving the connecting link (Up) closer to the pull rods, then we are requiring more stroke from the cylinder. At some point we will run out of stroke unless we have chosen a cylinder with considerable length.

I have found that it's a good practice to only use about 75% of the cylinder's stroke during normal operation. This allows for plenty of wear on the brake shoes before readjustment is required.

On my cars I use a Cylinder with a Bore of 1 1/16 inch and a Stroke of 2 inches.

A Cylinder with a Bore of 1 1/16th inches has 0.8866 square inches of Piston Area. With 48 PSI pushing against this surface, it has the force of 42.559 pounds. At a Mechanical Advantage of 3:1, this means that the Pull Rod force will be 127.676 pounds. Which, by my experience is way more force than is required on a car that weights in the neighborhood of 250 pounds.

With a 2:1 force factor it would be 85.118 pounds of force, which is just about right for this particular car. Again, it's important to do Dynamic testing to setup a car for Maximum braking.

So ....... The bottom line is, adjust the Mechanical linkage and the PRV to acheive Maximum braking with the PRV set at around 45 PSI. This is a nice, safe range to be in.

Here in lies a problem. Earlier I said that the Return Spring was optional. If we are going to operate the PRV at a pressure of 45 pounds then we will need a Return Spring on the Cylinder to assure effective Retraction when the Train Line is set for 50 pounds.

The reason for this is that the Rod inside the Cylinder takes away force from the Retract side of the Piston. The 1 1/16th inch Cylinders that I use have a 5/16 inch Rod. That is 9% of the Area of the Extend side of the Piston. I also mentioned earlier that the Piston needs about a 3 pound differental before it moves in either direction. This means that with No Return Spring, the maximum pressure that we could set the PRV to would be 42 pounds.

By using a rather soft Return Spring on the Cylinder's Lever, we can operate the PRV at about 45 PSI and still make sure that the Piston Fully retracts when the Train Line is at 50 PSI. I would recommend a spring with about 4 pounds per inch tension, about 3 to 4 inches in length. These are readily available at most hardware stores.

One thing to add ! Setup your Riding cars with No Load. Meaning, nobody in the car. If you setup a car Loaded then the wheels will slide if no one is seated in them.

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