5ton Converter Dolly

[RobertoGatos]

Spent the last few days making a converter dolly out of a 5ton bogey and a MK back half towbar. Have most of it finished up now except for hooking up the brakes, and thats where I'm needing some help.



The bogey had no relay valves, or plumbing besides the lines that ran from the top of the chunk to the brake pots when I got it. The bogey was from a non ABS truck, so I started robing parts off another non ABS chassis. Now all the lines are hooked up between the relay valves and the brake pots.

Where I'm stuck is getting the service and supply lines properly hooked. Whats really got me turned upside down is the brake brake system has provisions for the "spring brake override" button on the dash. Most brake pots have two ports, but ours have three. I think it is because of this feature?

Also, does anyone know the model of these relay valves so I can look up a diagram?

Thanks for any help!

 

[Floridianson]

That's going to be called more than a 5 ton and good job so far. My little dolly is called a 6 ton for the 6 ton trailers. Think your pintle rating is what you should call it.

 

[Swamp Donkey]

Also, does anyone know the model of these relay valves so I can look up a diagram?

If I'm understanding correctly, you want the model numbers for the non-ABS relay valves? If so, they are in the screenshot below from the old -24P-1 TM on actual page 281-1. For non-ABS trucks it'd probably be best to use the older TMs for information since they pre-date the ABS MWO. The newer TMs have all the ABS components in the diagrams which would probably just complicate things. The P2P program would probably be helpful for figuring out the brake chambers. ABS components will be included but it'll give you a good idea of where air moves in the system from the animated diagrams. I really like that program but haven't found anything that'll let me run it on my phone.

 

[Castle Bravo]

...Most brake pots have two ports, but ours have three...

All the M939s that I have come across have 2 ports for a service only chamber and 3 lines for a spring brake/service chamber.

On the spring brake/service chamber, one of the ports is to charge the spring brake chamber and release the spring brakes. The other port is to charge the service brake chamber to apply service brakes. The "3rd" port is for the service brake release and functions through the use of the inversion valve on under the driver's side step. I'm not 100% sure, but I think it works something like this - when you let off the brake treadle, air goes to the "other side" of the service brake diaphragm and moves it back away from the push rod, allowing it to release the brakes. I think there is also a return spring, but they use air to assist brake return for whatever reason.

On the service only chamber, it is the same, less the spring brake charge line.

You might need to plumb an inversion valve onto the dolly.

 

[Floridianson]

Or just do away with the parking brake system on the dolly. The only problem I could see is the dolly has to fit the lunette to pintle correct height The single axle dolly can hook to any height pintle but the tandem has to have a arm the is free to go up and down with different pintle heights.

Here is the inversion valve talked about and protects the system in a trailer air problem. Yes it would be a good thing to have in the system but the military I don't believe the military worried about that when the dolly was used. I will look at mine but I think just a quick air release on the dolly. http://www.haldex.com/en/North-Amer.../Product-Information/Valves1/Inversion-Valve/

 

[USAFSS-ColdWarrior]

Subscribed opcorn:

You are pursuing a project I have only dreamed about.

Thanks for sharnig.

 

[M813rc]

I, too, shall watch carefully, as I'm thinking of putting my now-spare military 5th wheel on my M200 (I put a Fontaine slider on one of my M931A2s to reduce trailer height, and to allow it to tow my "shorter necked" trailers like the M146).

Cheers

 

[RobertoGatos]

Thanks for all the input.

What I'm wondering with the inversion valve is this: It uses the secondary air system to back up the primary in the event of a primary system failure to maintain modulated brake control. However, there is no secondary air system on a converter dolly, since there is no front axle...

I would like the brakes to function like normal trailer brakes, because this will not just be used as a converter dolly, but also as a rear steering bogey on a M270 step deck - turned lowboy, that gets towed backwards with a Tru Hitch. (That was a mouth full). In this instance, does the inversion valve seem like a liability?

Got the air tanks mounted, but now have to relocate the relay valves. Hah. All the major fab work is finished now though.

 

[Csm Davis]

Roberto I would forget about the inversion valve and look a the air routing on a regular dolly and your springbrakes will need to be run to the main line to the tank.

 

[Castle Bravo]

Perhaps I am incorrect about the function of the inversion valve - nevertheless, won't you need some way to release the service brakes? I doubt the wedge brakes have that additional air line for no reason at all...

 

[Two Stroke Madness]

Spent the last few days making a converter dolly out of a 5ton bogey and a MK back half towbar. Have most of it finished up now except for hooking up the brakes, and thats where I'm needing some help.

View attachment 614715 View attachment 614714 View attachment 614716 View attachment 614717 View attachment 614718

The bogey had no relay valves, or plumbing besides the lines that ran from the top of the chunk to the brake pots when I got it. The bogey was from a non ABS truck, so I started robing parts off another non ABS chassis. Now all the lines are hooked up between the relay valves and the brake pots.

Where I'm stuck is getting the service and supply lines properly hooked. Whats really got me turned upside down is the brake brake system has provisions for the "spring brake override" button on the dash. Most brake pots have two ports, but ours have three. I think it is because of this feature?

Also, does anyone know the model of these relay valves so I can look up a diagram?

Thanks for any help!

Nice welds. Do you plan on having that dolly powered, if not you could save weight by replacing the axle centers with square tube. But a powered dolly would be awesome.

 

[Two Stroke Madness]

That's going to be called more than a 5 ton and good job so far. My little dolly is called a 6 ton for the 6 ton trailers. Think your pintle rating is what you should call it.

Oh no, now I want one. Better get out the welder.

 

[Swamp Donkey]

Perhaps I am incorrect about the function of the inversion valve - nevertheless, won't you need some way to release the service brakes? I doubt the wedge brakes have that additional air line for no reason at all...

Service brakes are air applied. Remove air pressure and they release by way of the two shoe springs. Service brakes use both shoes in each drum. There is a dedicated service brake chamber and the spring brake chamber also has a service side to it.

Spring brakes are air released. Remove air pressure and they apply. They only operate one shoe in each drum.

Jeff is correct on how the inversion valve functions. From Haldex's website:

Inversion Valve

Straight trucks and some tractors use an Inversion Valve which does not function during normal operation of the vehicle’s brake system but operates in the event of a primary (rear service brake) system failure. When the Inversion Valve senses an imbalance of the primary and secondary delivery pressure during an application of the foot valve, it inverts that pressure to release air from the spring brake chambers. This permits the modulated application of the mechanical spring brake chambers on the rear axle(s) to assist the front brakes in stopping the vehicle safely.

The three ports on the spring brake chambers have the labeling cast into the housing next to the ports. The inversion valve turns the spring brakes into service brakes in the event of pressure loss on the service side by changing how the air goes to the chambers. It is not used during normal brake function.

I agree with Jeff. Ditch the inversion valve idea and run it like other trailers. The service line from the truck works the service side and the emergency line from the truck keeps a tank charged to keep the spring brakes released. When pressure from the spring brake side is removed (in the event of a break away or for parking) the spring brakes apply the same as they do on the truck.

 

[RobertoGatos]

The plan is to power the dolly at some point, but its a pretty low priority. In the meantime the axles just got chopped.



Agree the inversion valve needs to go, this looks like the third port on the spring brake chambers will be left unused. I'll just leave it open to vent?

 

[Castle Bravo]

I was running on the incorrect assumption that the inversion valve applied air to the "back side" of the service diaphragms to help release service brake applications, in addition to the return spring - not the case.

To me, it doesn't seem like the additional ports have anything to do with the inversion valve function, however. The TM refers to these 3rd lines as vent lines and they are not explained very well. The vent line is common to all 10 brake chambers on the 939 as well as almost all the brake valves...

 

[RobertoGatos]

On all brake pots, parking and service, air moves in both directions though the ports. No vent on the brake pots. I'm guessing the 3rd port on the spring brake pot is for the valve on the dash that says "emergency spring brake override" and if this is the case, I'll just leave it vented.

 

[rosco]

I like air brakes & air brake problems but I throw my hands up on this thread. I'd strip all the old valves off the dolly. Save the various air hoses & fitting - you might find one you can re-use. You will need two air tanks with related check valves. A quick release valve, & a Relay/Emergency Valve. I would keep the spring brake parking feature, as its comforting to know that if you do have to dolly-off in a less then desirable area, that your trailers will still be there when the air bleeds off. Suggestion: Don't try to speculate on the brake features of the Tow Vehicle, but just its Service & Emergency lines & what they do. Various brake parts mfgs. have simplified diagrams of brake systems at your local "heavy truck store" & Dollys are part of it. Don't forget to "Y" off (with valves) to control the semi trailer too.

A simple way to handle spring brakes is to mount dedicated tank w/check that charges from the emergency side. As you charge the system, the air begins to charge the spring brakes & by the time your up to about 60 PSI, the springs are released. You just have to bleed the air off manually, if you want to park it.

Note: That lunette looks rather wimpy! The more complex your outfit looks, the more likely you are to attract DOT attention.

 

[Floridianson]

Guess this is the problem building a dolly. We look at the air system but in most cases it goes that a tractor is used. I might be wrong but would seem all that is needed is a relay and a quick air dump plus air tank on the dolly. As said do away with parking brake or as said use emergency air to release but then you need to install a valve you open by hand on the dolly to relieve the pressure to be safe.
The you need two sets of lines for service and emergency at the front of the dolly. One for dolly then one for trailer.

 

[Castle Bravo]

Plumb it like a regular air brake trailer, for sure.

I was just trying to get to the bottom of the 3rd ports. If they're vents, like the TM says, just put a vent cap at the top of the line somewhere on the dolly. If its unnecessary in your application, ignore them.

 

[Floridianson]

Also thinking about it I myself would not want the parking brake on the dolly. Bad things happen when a parking brake pancake blows.