Why do trucks have air brakes? 6

[pietro82]

I discovered that trucks are fitted only with air brakes? What is the reoson of that? What are the main advantages?

thanks

 

[btrueblood]

I believe most (all?) mobile home manufacturers use electric brakes for their trailer axles. I believe this because my dad would use the surplus axles for building trailers for heavy equipment, livestock, etc. The axles were cheap to make, and originally intended to be "disposable" - but (of course) a market quickly developed for rebuild kits for them.

 

[bigTomHanks]

I know this is old but one of the main reasons that air / drum brakes are still used on tractor trailers and trains is because they have a "self-applying" characteristic. After you engage the pad to the drum, minimal pressure is needed to maintain contact due to the pad "biting" into the drum.

 

[JayMaechtlen]

Mobile homes don't generally go very far - a trip to intermediate storage, then a trip to point of use.
Then it goes up on stands, the axles go to get reused on the next unit.
I'll bet a modern mobile/modular home doesn't even have its own axles.
Anyway, electrics work well enough for that application.
Look inside, you wouldn't want to scale that up for a semi grossing 80,000 lbs.

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[tbuelna]

bigTH- It's true that drum brakes are designed to be "self-energizing". The approach contact friction causes the brake shoe to push harder against the drum ID surface, producing higher braking forces. This makes using a pneumatic brake actuator, with far lower circuit pressure than a hydraulic system, much easier.

However, there are now electric disc brake calipers (from Siemens and other OEMs) that employ the same "self-energizing" principle as common drum brake systems. So we may soon see these electric disc brake systems used on commercial trucks.

 

[JayMaechtlen]

a comment on disks versus drums -
try going down a very steep hill very slowly (10-15mph) for an interesting experience with disk brakes!
If the vehicle doesn't have really good compression at low speeds, all the work has to be done by the brakes.

slow speed = no air flow = faded brakes
disks or drums, your choice - exciting either way!

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[RossABQ]

try going down a very steep hill very slowly (10-15mph) for an interesting experience with disk brakes!
slow speed = no air flow = faded brakes
disks or drums, your choice - exciting either way!

Low speed = very little energy to dissipate

On another note, I was surprised to find that many big-truck brakes are not self-energizing. The Lockheed double-anchor type has one shoe that will tip-in, the other does not, both being anchored at a common point. Only explanation for this I could find is that braking in reverse was considered as important??

 

[MikeHalloran]

The castellated nuts hold the shoes to fixed anchor posts.
Between the other ends of the shoes is an S-cam. Its central shaft, not visible here, extends out the far side, and rotates in bearings, but is otherwise fixed in position. An arm attached to the far end of the shaft is pushed on by the pneumatic brake actuators. This sort of drum brake, typical on trucks, cannot be self energizing. Note that the entire mechanism is simple and sturdy.

Typical car drum brakes are much more complicated. There is one anchor pin. The double piston hydraulic actuator is adjacent, and pushes the shoes apart. The shoe that is being pushed away from the anchor pin at the time pushes on the end of the other shoe through a self-adjusting screw/ratchet mechanism. The point is that one shoe's drag force is used to apply the other shoe, the one that's being pushed toward the anchor pin. The 'servo' action reduces the pedal pressure required without the use of a master cylinder booster.

Not all drum brakes are self-energizing, and self-energization is not in any way intrinsic to drum brakes; it's an evolutionary enhancement to the apply linkage, appropriate for passenger cars.








Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

MikeHalloran- your comment about self-energization not being intrinsic to drum brakes is correct. Here is an electric disc brake caliper design from Siemens that is self-energizing:

 

[NormPeterson]

Chrysler offered mechanically self-energizing disc brakes in the early 1950's.


Norm

 

[JayMaechtlen]

I don't see any aspect of the above disk that looks like 'self energizing'.
Since the truck doesn't use the driver's effort to apply the brakes, Self energizing brakes for them would be a matter of economics: can you save enough money on actuators and related if you make it self-energizing?

Re: slow downhill - yes, slow rate of energy release. But no heat dissipation except by conduction and radiation.
One vacation, I took my Aerostar (with family on board) up a fire road (steep, bumpy, unpaved) to gather some firewood. Coming back down, the road didn't permit travel fast enough for compression braking in first gear. (4-speed automatic)
When I realized the brakes were getting hot, I had a very difficult time getting it stopped. There was a lot more road ahead of me, it could have been ugly if I didn't stop it then.
Sure, I was well outside the usual operating parameters for that vehicle. But disks or drums - they can still fade without cooling!


Jay Maechtlen

http://www.laserpubs.com/techcomm