Brake System Design

Discuss park gauge trains and large scale miniature railways having track gauges from 8" to 24" gauge and designed at scales of 2" to the foot or greater - whether modeled for personal use, or purpose built for amusement park operation or private railroading.

Moderators: Glenn Brooks, Harold_V

Forum rules
Topics may include: antique park gauge train restoration, preservation, and history; building new grand scale equipment from scratch; large scale miniature railway construction, maintenance, and safe operation; fallen flags; track, gauge, and equipment standards; grand scale vendor offerings; and, compiling an on-line motive power roster.
Bob D.
Posts: 381
Joined: Thu Jun 21, 2012 10:43 pm
Location: Saco, ME. USA

Brake System Design

Post by Bob D. »

My freelanced large scale loco has progressed to the point I'm designing the mechanical brake system.
My question to all of you is this. How do you determine the force required to be applied to the brake shoes against the wheels to have effective braking? My assumption is you don't need any more brake force than the loco weight per wheel.
Anyone have any guidance on this?
3/4" Juliet II 0-4-0
3/4" Purinton Mogul "Pogo"
3/4" Hall Class 10 wheeler
3/4" Evans Caribou/Buffalo 2-8-0
3/4" Sweet Violet 0-4-0
3/4" Hunslet 4-6-0
3/4" Kozo A3. Delayed construction project

1 1/2" A5 Camelback 0-4-0
Pontiacguy1
Posts: 1565
Joined: Thu Apr 26, 2012 10:15 am
Location: Tennessee, USA

Re: Brake System Design

Post by Pontiacguy1 »

That's a good starting point, but you also have to figure in the coefficient of friction for the material that you are using for brake shoes. What I've always tried to shoot for is a force that applies the brakes hard, without sliding the wheels. About 80-85% of your tractive effort would be a good place to start on that. steel wheels on steel rail should have a coeff. of about 0.4 when dry and clean, at least that's what I looked up. So if your wheel has 100 lbs. of locomotive weight on it, it would give you roughly 40 lbs of tractive effort. Your brake force should be about 32 to 35 lbs on that same wheel to keep it from sliding. Friction =(coeff. of friction)x(normal force) So if my brake shoe had a coeff. of 0.6, I would need about 20 lbs of force on that shoe to achieve effective braking.

Some will say use higher or lower brake force, but this should be in the ballpark. Even going by this formula, there will still be times when track conditions aren't ideal and you'll slide your wheels. It's a trade-off between maximum braking and trying to not lock up your wheels. Hope this helps you out.
Bob D.
Posts: 381
Joined: Thu Jun 21, 2012 10:43 pm
Location: Saco, ME. USA

Re: Brake System Design

Post by Bob D. »

Thanks for the reply. Tractive effort comes down to a line of contact between the rail head and wheel. My cast aluminum brake pad has 3 sq inches of contact area against a cast iron wheel. The wheel diameter must come into play too as it is a lever from the pad to the axle.
So I'm unsure how to figure these into the mess. Larger pad increases required force? Or does increased friction area decrease needed force? Is the wheel radius taken as a lever multiplying the force?
3/4" Juliet II 0-4-0
3/4" Purinton Mogul "Pogo"
3/4" Hall Class 10 wheeler
3/4" Evans Caribou/Buffalo 2-8-0
3/4" Sweet Violet 0-4-0
3/4" Hunslet 4-6-0
3/4" Kozo A3. Delayed construction project

1 1/2" A5 Camelback 0-4-0
User avatar
NP317
Posts: 4551
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon, USA

Re: Brake System Design

Post by NP317 »

Best solution:
Be able to lock up your wheels, and modulate the brake forces to apply only as much force as needed.
That's the most important job of the Engineer!
Prototypical.
RussN
Bob D.
Posts: 381
Joined: Thu Jun 21, 2012 10:43 pm
Location: Saco, ME. USA

Re: Brake System Design

Post by Bob D. »

I get being able to lock up the wheels and modulating from there..... At this point I'm trying to figure out how much mechanical advantage I need to design into the linkages for the shoe operating system so a pneumatic cylinder(s) can either operate it or overcome the spring force that applies brake pressure. The loco also has 3600 watts of dynamic braking available during movement. The mechanical brakes are to supplement this and be used as a parking brake.
3/4" Juliet II 0-4-0
3/4" Purinton Mogul "Pogo"
3/4" Hall Class 10 wheeler
3/4" Evans Caribou/Buffalo 2-8-0
3/4" Sweet Violet 0-4-0
3/4" Hunslet 4-6-0
3/4" Kozo A3. Delayed construction project

1 1/2" A5 Camelback 0-4-0
Kimball McGinley
Posts: 809
Joined: Wed May 31, 2006 2:13 pm
Location: Laguna Niguel CA

Re: Brake System Design

Post by Kimball McGinley »

The brake pad area does not figure into this problem. A 1/2 x 1/2 bar or a 2 x 2 bar would brake the same with the same force applied. The area is important for wear and therefore brake life, however. Same as pad thickness.
Bob D.
Posts: 381
Joined: Thu Jun 21, 2012 10:43 pm
Location: Saco, ME. USA

Re: Brake System Design

Post by Bob D. »

Thanks for the brake pad info Kimbal.
3/4" Juliet II 0-4-0
3/4" Purinton Mogul "Pogo"
3/4" Hall Class 10 wheeler
3/4" Evans Caribou/Buffalo 2-8-0
3/4" Sweet Violet 0-4-0
3/4" Hunslet 4-6-0
3/4" Kozo A3. Delayed construction project

1 1/2" A5 Camelback 0-4-0
Pontiacguy1
Posts: 1565
Joined: Thu Apr 26, 2012 10:15 am
Location: Tennessee, USA

Re: Brake System Design

Post by Pontiacguy1 »

I guess it depends on your system type. I was ASSUMING that you had a simple on/off brake system like a lot of steam locomotives have with a brake valve in the cab that pressurizes a cylinder or cylinders, that then applies the force to your wheels. For a system like that, I usually try to set it up so that my wheels don't lock up when the brakes are applied. Nothing is perfect, however, and you can still lock up and slide the wheels on slimy or wet rails sometimes.
The brake pad material has a LOT to do with your required brake force, so figure out what your shoe material is, and then work from there.
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Brake System Design

Post by rkcarguy »

I'm going with springs to engage the brakes and using air cylinders to pull the pads off the wheels for an extra measure of safety. My 12" RR build is going to have HEAVY trains and will be working 2-3% grades so I can't have anything get away from me.
Unfortunately you do lose the ability to adjust the air pressure to adjust braking force, and have to resort to changing the springs or adjusting their fulcrum points to get the amount of braking correct. I'm planning on having a locomotive brake and a full train brake, so when I'm empty or just want to ease to a stop I can just use the locomotive brakes, and when loaded or in an emergency I can dump the air pressure and engage the train brakes on locomotive and cars.
User avatar
NP317
Posts: 4551
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon, USA

Re: Brake System Design

Post by NP317 »

You might consider a mechanical way to release the brakes, too.
Then you can move the equipment when it is not running...
RussN
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Brake System Design

Post by rkcarguy »

I actually drew up a little ratchet mechanism and brake wheel to have laser cut that will allow me to attach a cable to the brake linkage and "undo" the brakes with the brake wheel on each car so I can roll them around manually if needed.
Johnny O
Posts: 68
Joined: Wed Jul 20, 2011 5:02 pm
Location: Havertown

Re: Brake System Design

Post by Johnny O »

I have a question on brake material. I built my A3 tender with pull lever mechanical only operated brakes per Kozo drawings. The difference was in the shoes I made. Their cast iron backings with an actual brake shoe material bonded to them. Its a heavy black composite material from replacement shoes for the coal cars that would line the site of the old coal plant I worked at. I cut it up and formed it to the shoe curvature. It seems to work great but do I need to go to that much trouble up front. Relining is easier of course. What is a common brake shoe material used on cast iron wheels and do I need to cut the groove profile on tire rim into shoe? I guess it keeps them from drifting sideways on wheel taper.
Thanks
John
Post Reply