How do hand brakes work?

[MMLDelete]

Thanks to everyone who responded. I get it now. 

[Overmod]

Lithonia Operator

Chain is taken up when the handbrake is applied. Something has to act to retract the chain when the brake wheel is moved to the released position.

The weight of the chain 'fall' itself is sufficient to keep it feeding as the wheel is released (or the handle switched to un-ratchet).

Keep in mind that the closer the shoes ride to the wheels, the less air is required to apply them.  The automatic slack adjusters will try to keep the foundation jointing 'tight' so there is minimal free play in the rigging itself.  Note that the brake beams are designed to 'float' slightly free of the wheels (clasp brake hangers also have a 'center of mass' that allows them to fall slightly away from the tread).  In a relatively slack linkage with multiple pin joints and pull rods it would be difficult to provide a positive spring action to push the shoes/beams away, on the same principle as your 'it's hard to push on a chain'.

As another consideration:  many classical drum-brake systems are set up to provide 'servo action' when applied: they are hinged so normal rotation of the drum forces the shoes against pivots to increase leverage when the braking friction applies.  It would not be wise to mirror this with railroad shoe brakes, as anything that happened to swing the brake beam into contact with the treads might cause self-application. 

[MMLDelete]

Yes, I read that. I just wasn't sure that that would be enough force to pull the chain back. You can't push a chain.

[Deggesty]

Quoting Euclid:" The brake shoes tend to fall away from the wheels as the brake beam on the brake hangers swings to a balanced condtion.   "

WHen there is no pressure that forces the shoes against the wheels, they tend to fall away.

[MMLDelete]

Thanks, Euclid. I appreciate your spending so much time on that.

I think I’ve basically got it now. But what pulls the chain back? The weight of the shoes falling away from the wheels?

Chain is taken up when the handbrake is applied. Something has to act to retract the chain when the brake wheel is moved to the released position.

 

[Euclid]

Lithonia Operator

So, on the typical freight car, the handbrake chain will move the same mechanism that the brake cylinder does, but can do that without releasing any air from the cylinder?

And when a handbrake is released, some type of spring action pulls the chain back to the “released” position?

 

Both the handbrake and the air cylinder move the linkage of rods and levers that apply the brake shoes to the wheels.  But the handbrake and the air cylinder work independently of each other.  If they are both applied at the same time, they duplicate their effort.  But either one can be applied without applying the other.

There is a retraction spring in the air cylinder that pulls the piston back in when its air pressue is released.  However, this retraction spring does not pull the entire brake linkage (foudation rigging) into a released condtion.  And likewise, the handbrake has no spring action that pulls the brake shoes back from the wheels when it is released.  The brake shoes tend to fall away from the wheels as the brake beam on the brake hangers swings to a balanced condtion.   

Look at page 6 of the manual shown in this link posted above:

Freight Car Brake Rigging Arrangements

 

You can see the brake cylinder with a callout for the Hollow Rod and the Push Rod.

The hollow rod is connected to the pistion and keeps the pistion perpendicular to the cylinder walls.  The hollow rod is a thin-walled steel cylinder with the outer end open.  The cylinder retraction spring (not shown) inside of the cylinder forces the pistion and the hollow rod to retract when air pressure is released. 

The solid rod is not attached to the hollow rod or the pistion.  It is simply a lose push rod that runs all the way through the hollow rod toward the pistion.  When the air pressure pushes the piston out, the face of the pistion pushes directly against the end of the solid rod.  This pushes out the solid rod, which they moves the foundation linkage to apply the brake shoes to the wheels.

So if the air cylinder is despressurized, the piston and the hollow rod are fully retracted.  Then if you set the hanbrake, that action pulls the entire foundation rigging to move the brake shoes against the wheels.  It also pulls the solid rod outward from inside of the hollow rod.  But since the solid rod is not connected to the piston, and is only a push rod, the handbrake pulling the solid rod out does not pull the hollow rod and piston out.  So if the air cylinder has no air pressure, its piston and hollow rod remain fully retracted as the handbrake is applied and the solid rod is pulled out as a consquence.    

 

[MMLDelete]

So, on the typical freight car, the handbrake chain will move the same mechanism that the brake cylinder does, but can do that without releasing any air from the cylinder?

And when a handbrake is released, some type of spring action pulls the chain back to the “released” position?

[zugmann]

zardoz

My memory of locomotive hand brakes is that the handbrake was merely sufficient to keep only the locomotive from rolling, and that the handbrake only pulled one cylinder on one of the trucks. Perhaps on the more modern locomotives the handbrake does the entire truck--my memories are from the days of mostly SD40-2's (back then it was a real treat to get a SD50 or even more rarely a SD60, and I cannot remember the brake configuration on them).

Every engine I've seen* has the handbrake pulling one piston on one axle on one truck.  BAsically enough to hold itself only (and sometimes not enough if the brake shoes are crap, or it's on a good grade.) Always test your engine handbrakes, folks!

*- although it's been a very long time since I seen an old GE in use, so mayeb they're different.

[BaltACD]

zardoz

 

BaltACD

The TSA Lac Megantic also highlighted the difference in hand braking 'potential' between rail cars and locomotives - with the hand braked locomotive having less braking force than a hand braked rail car.  Locomotive being hand braked on one truck, a rail car being hand braked on all wheels (in most cases). 

My memory of locomotive hand brakes is that the handbrake was merely sufficient to keep only the locomotive from rolling, and that the handbrake only pulled one cylinder on one of the trucks. Perhaps on the more modern locomotives the handbrake does the entire truck--my memories are from the days of mostly SD40-2's (back then it was a real treat to get a SD50 or even more rarely a SD60, and I cannot remember the brake configuration on them).

Without going back to the report, I don't recall if the locomotive hand brake applied to one truck or one axle - I do recall that the published braking value of a locomotive was a fraction of that of one of the cars. 

[zardoz]

BaltACD

The TSA Lac Megantic also highlighted the difference in hand braking 'potential' between rail cars and locomotives - with the hand braked locomotive having less braking force than a hand braked rail car.  Locomotive being hand braked on one truck, a rail car being hand braked on all wheels (in most cases).

My memory of locomotive hand brakes is that the handbrake was merely sufficient to keep only the locomotive from rolling, and that the handbrake only pulled one cylinder on one of the trucks. Perhaps on the more modern locomotives the handbrake does the entire truck--my memories are from the days of mostly SD40-2's (back then it was a real treat to get a SD50 or even more rarely a SD60, and I cannot remember the brake configuration on them).

[BaltACD]

Overmod

As noted, the action on locomotives is different, and in some cases requires that a brake cylinder actually be cut out to apply the brake completely.  You may remember that this was a 'topic of interest' for the Lac Megantic investigators, in part because there was some question whether MM&A employees were fully trained on the special procedures involved.

The TSA Lac Megantic also highlighted the difference in hand braking 'potential' between rail cars and locomotives - with the hand braked locomotive having less braking force than a hand braked rail car.  Locomotive being hand braked on one truck, a rail car being hand braked on all wheels (in most cases).

[Overmod]

Lithonia Operator

But just for now: the hand brake activates AIR braking? The linkage lets the air pressure out, and so the brakes apply? (I always assumed the hand brake was a strictly mechanical system, nothing to do with the air brakes.)

This is a surprisingly rich question.

In most cases the handbrake is not like the system on cars (where it is a mechanical linkage that operates small shoes or pads in the rear brake mechanism); it is, as noted, a manual method for moving the foundation that the air cylinder 'powers'.

Which brings up the immediate question: can you apply handbrakes 'harder' with the air fully applied than you could if you started winding on the handle, even with a brake club or stick, with the air not applied?  And the answer is a resounding 'yes'.   (This also raises some potential follow-on questions about how slack adjustment is performed, etc.)

A drawback to this is that, because the handwheel or lever mechanism has to have a fairly high mechanical advantage or gear ratio, it will usually take more turns or pumps to snug the brakes on.  Another potential disadvantage is that, if the pressure in the air cylinder is partially or wholly lost, the brakes may be so firmly applied that it is difficult to release them, even with a stick; you might think that this would be easily covered because 'you need the brakes pumped up before releasing handbrakes' but it may take an actual recharge and reapplication to get enough cylinder 'action' to permit release.

As noted, the action on locomotives is different, and in some cases requires that a brake cylinder actually be cut out to apply the brake completely.  You may remember that this was a 'topic of interest' for the Lac Megantic investigators, in part because there was some question whether MM&A employees were fully trained on the special procedures involved.

[gregc]

Paul of Covington

Thanks, Greg.  I'll spend some time reading that.

thank paul for suggesting where to find it

[BaltACD]

Lithonia Operator

I will use the links and peruse the information. Thanks all.

But just for now: the hand brake activates AIR braking? The linkage lets the air pressure out, and so the brakes apply? (I always assumed the hand brake was a strictly mechanical system, nothing to do with the air brakes.)

We go to an ice cream stand which is an old MEC caboose (a unique one, made from a boxcar). I saw that the chain connected to a rod which seemed to run to the longitudal center of the car. I was surprised. That got me thinking about how I really don't understand hand brakes. I was expecting some kind of purely mechanical linkage running directly to the truck at that end.

Hand and Air brakes are separate means of applying the same brake shoes and use the same levers and rods to get it done.

If a hand brake is applied while the air brakes are also applied, the additional power of the air brake application will inturn make the hand brake being applied with somewhat more force that if the hand brake had been applied by itself.

[MMLDelete]

I will use the links and peruse the information. Thanks all.

But just for now: the hand brake activates AIR braking? The linkage lets the air pressure out, and so the brakes apply? (I always assumed the hand brake was a strictly mechanical system, nothing to do with the air brakes.)

We go to an ice cream stand which is an old MEC caboose (a unique one, made from a boxcar). I saw that the chain connected to a rod which seemed to run to the longitudal center of the car. I was surprised. That got me thinking about how I really don't understand hand brakes. I was expecting some kind of purely mechanical linkage running directly to the truck at that end.

[Paul_D_North_Jr]

What about the two-car "multi-levels" (auto racks) and articulated intermodal well cars? 

- PDN. 

[Paul of Covington]

   Thanks, Greg.  I'll spend some time reading that.

   And thanks, Overmod.  I was thinking of standard boxcar, and forgot that "unit body" tank cars and some others were exceptions.

[Overmod]

Lithonia Operator

And, if you set a car's hand brake, are you setting it on one truck only?

The answer to this differs by the type of car.  Where there can be no easy or cost-effective way to install a full foundation, as for example on many tank cars, arrangements have to be made to apply the brake on each truck individually.

Modern passenger cars have truck-mounted brake cylinders with short linkage, and usually here too a given handbrake wheel will only affect the truck on that end of the car.  Over the years there have been a couple of attempts to put truck-mounted cylinders on freight trucks, including three-piece trucks, to get around the 'problems' associated with foundation rigging, and the ones I have seen required handbrake application by individual truck.

A reasonably good description of locomotive handbrake arrangements was included in the TCA (Canadian) accident report for Lac Megantic, and we had some discussion about it in various threads here.

[gregc]

Freight Car Brake Rigging Arrangements

[Paul_D_North_Jr]

On the recent (Jan.? to date) thread for the CP runaway, someone - SD70dude? - posted a pretty good diagram or a link to one. 

- PDN. 

[Paul of Covington]

   There is one brake cylinder on each car which activates the brakes on all wheels through a system of rods and levers.  When you set the hand brake, you wind the chain around the shaft, and the chain pulls on the same set of rods and levers as the brake cylinder, so you set the brakes on all wheels just like the air brakes.  This is a sort of simplified explanation.  I remember seeing a pretty good diagram of the layout of the linkage, but I can't find it now.  Maybe someone else here can come up with a good illustration.   At the link below, scroll down a little to "Straight Air Brake" and click on the image on the right to enlarge it.  It will give you a rough idea of the linkage, but remember that the levers are horizontal, not vertical as they appear in the drawing.

https://en.wikipedia.org/wiki/Railway_air_brake

[Lithonia Operator]

It occurred to me that I have no idea how railcar hand brakes work.

I see that the brake wheel is connected to a chain. What all happens between the wheel and the brake shoes? And, if you set a car's hand brake, are you setting it on one truck only?