Despite what you may think, the two-inch stroke limit on a standard Type-30 brake chamber isn’t arbitrary. With a Type-30 chamber, 0.66 inches of pushrod travel is used to move the brake lining from its resting position to the point of contact with the drum. This is sometimes referred to as “free stroke.” The next 1.15 inches is called the power stroke. Here we have the foundation brake components twisting, bending, and stretching as the slack-adjuster arm exerts more force. Component deflection varies with different axle and brake setups: the short cams found on drive axles don’t twist as much as the long cams found on some trailer axles with brake chambers mounted near the middle of the axle.
So now you’re left with 0.19 inches of reserve stroke. If everything is perfect–no worn bushings, the linings are in reasonable shape, and your brake drums are cold–you’ve got less than one-fifth of an inch between a green light and red ink.
The chambers are designed to produce a consistent force across the full length of pushrod travel. As the diaphragm stretches out to its limit, the force begins to drop off until it bottoms out against the front wall of the chamber, where the power output becomes zero.
Once a pushrod reaches its maximum stroke length, it’s deemed to be out of service.
The slack adjuster is designed to compensate for lining and drum wear by taking up the “slack” in the rotation of the cam between the resting position of the pushrod and the point of lining-to-drum contact. Were that not to happen, the pushrod might travel 1.5 inches or more before contact was made; then, when full pressure was applied, the pushrod would travel out beyond the point where the power output drops off.
The reason the system is designed the way it is, and the tests are applied the way they are, is to ensure there’s some reserve power output for the brake when the drums have heated and expanded, which allows for longer pushrod travel. This is the sensation of “brake fade,” and tests of downhill braking events have shown properly adjusted brakes can exceed the stroke limit by a half-inch or more at the bottom of a long grade. That’s why they build some reserve into the allowable stroke limit before requiring a readjustment.
Checks & Balances
Let’s face it, there are about a dozen drivers in the country who ever do a full mark-and-measure brake inspection-according to the trip-inspection guidelines-every day, and I sure wasn’t one of them. So in lieu of that dirty and near-impossible task, many drivers rely on add-on stroke indicators or coloured markers on the push-rod shafts.
That’s a start, but you can’t assume the brakes are properly adjusted simply by verifying the stroke length using the output force of the parking-brake spring. Provided the indicator gauge is properly installed, if the indicator suggests the brake is one-quarter of an inch under the limit with the force applied by the parking spring, you’d likely be over the limit with a 90- to 100-psi application, as is required by the CVSA roadside inspection criteria.
The biggest problem is the way we check brake stroke on a parked vehicle, says Vic Wintjes, president of VW Transportation Consulting and the former maintenance supervisor at Canadian Tire in Brampton, Ont. “We need to check brake stroke using the same application pressure inspectors use at roadside,” he says. Drivers need to check their brakes by using a stick to hold the brake pedal all the way down while the system pressure is at 90 psi. Using the spring-activated parking brakes to determine pushrod travel isn’t accurate enough.
“There’s no way to tell how much force the spring is producing, but I can tell you, it’s nowhere near the equivalent of a 90-psi application,” Wintjes advises. “You’ll always get a longer stroke with full pressure. If your system is in poor shape, you’ll see the pushrod move out beyond the two-inch point every time.”
A parking spring, in good condition, delivers the equivalent of a 50- to 60-psi application, or somewhere between 1,500 and 2,000 pounds of force, to the pushrod. A standard Type-30 chamber delivers 3,000 pounds of force at 100 psi.
Now, multiply that force by the length of the slack-adjuster arm, which acts as a lever, multiplying the brake force in proportion to its length. For example, a six-inch slack adjuster with 3,000 pounds of force at the pushrod exerts 18,000 pounds of torque on the cam shaft (3,000 x 6 = 18,000). Compare that to the force exerted by the spring (2,000 x 6 = 12,000), and you can imagine what happens to the brake hardware downstream of the slack adjuster.
Once the brake linings have made contact with the drum, the rest of the movement in the hardware results from bending, twisting, and stretching. The brake shoes will bend outward to force the ends of the lining against the drum, the spider and foundation brake will flex slightly, and the camshaft itself will twist like a piece of rope. The longer the camshaft, the more it will twist.
The window of compliance here is incredibly slim. If your free stroke (the distance from rest to the point the lining contacts the drum) is anywhere from 3/4 to one inch of travel, you’ll go beyond the limit with a 90- to 100-psi application (applied stroke). A good parking brake spring might push out to half of the applied stroke, but there’s no way of guessing what the spring output might be. The only way to verify pushrod travel is to check it with a full-pressure application (90 to 100 psi).
So why measure stroke at 90 psi when an application of about 60 psi can lock up a wheel? Because 90 psi represents about the best output the brake can hope to produce, Wintjes says.
“The slack they give us here is the difference between 90 and 120 psi, the normal system operating pressure,” he explains. “Yes, in most cases, 60 psi will lock up a wheel. But the extra 30 psi applied during the test represents the less-than-ideal situation, like when the drums are hot and expanded. Or when one or more brakes are doing more than their share of the work, as on a trailer with lift-axles.
“To be sure the brake has some reserve braking potential, we test them beyond what’s normally expected, like when we test a radiator cap to 40 psi when the operating pressure is only 10 psi. There has to be some redundancy in the system.”
Wintjes, like most other brake experts and enforcement personnel, says the driver is only responsible for checking the equipment, not for fixing it. Drivers need to be able to recognize a problem and have it corrected before heading out.
Fleets need to help drivers recognize problems during trip inspections, Wintjes says. If the drivers don’t know what to look for, they can hardly be expected to play a role in preventive maintenance.
If the truck is equipped with brake-stroke indicators, a 90-psi application is what you need while checking pushrod travel. If your ABAs are not adjusting properly, make a few full-pressure applications and check them again. If they haven’t readjusted themselves, head for the shop.
Don’t take the chance of coming up short on brake performance: the fine is a pittance compared with the potential for an accident, and the litigation that’s sure to follow.Long-stroke brake chambers or LSBCs go a long way toward reducing the
frequency of stroke-limit violations. The useful stroke range on an LSBC is from 0.75 to 2.5 inches, and because of its consistent output, regulators let the LSBC to go out to 2.5 inches before requiring re-adjustment. That extra half-inch compared with the limit on a Type-30 chamber can make a huge difference at a roadside brake inspection.
Long-strokes cost about $40 more per unit–over the counter–compared to a premium Type-30. There are no real issues with reservoir capacity when spec’ing long-strokes, and in all but the rarest of cases clearance isn’t a problem during installation. The long and short of it is, there isn’t much of an excuse for not spec’ing LSBCs.When an automatic brake adjuster or ABA–correct terms for what most people call an “auto slack”–isn’t working as it should, there’s something wrong with it. Period. It needs professional attention and possibly replacement.
Yet upon recognizing an out-of-adjustment condition on an ABA installation, lots of drivers and mechanics will try to re-adjust the device.
The fact is, the adjuster nut on an ABA should be used only during the initial set-up following its installation. Depending on a number of factors, including the length of the pushrod, the type of clevis, and the position of the ABA body relative to the cam orientation, the ABA is designed to self-adjust to the way it was originally set up. If it wasn’t installed properly, it will keep trying to reset itself to that orientation.
ArvinMeritor’s Prakash Jain says that manually adjusting an ABA may temporarily solve the problem, but the unit will soon return to its previous state. “That’s what it was designed to do,” Jain says. “The ABA cannot compensate for other deficiencies of the foundation brake.”
It’s a great point. Check to see that the other parts of the braking system are within tolerance. Worn bushings between the slack arm and the clevis, for example, could translate into improper adjustment of the slack. Or, if the return springs at the wheel end have lost tension and aren’t fully returning the cam to its resting position, the adjuster mechanism may not adjust properly (on a clearance-sensing ABA).
Indeed, the way to “adjust” ABAs is to use them. It’s been noted that drivers who consistently make light applications–not forcing the system anywhere close to its limits–don’t allow the slacks to “stretch” past their adjustment increments. Over time, light applications and gradual lining wear can increase the gap between drum and lining, without the ABA compensating for the wear.
So, while you’re stopped with the parking brakes released, make several full-pressure applications at least once a day to cinch everything up tight. If that fails to produce satisfactory results, have the ABA checked by a qualified brake technician.
ABAs may work automatically, but they still need to be greased. Use a grease gun to apply grease until it starts to purge from around the camshaft or some other grease relief point.
But apply the grease sparingly. Too much can cause the ABA to back off on its own. Be sure to inspect the seals and dust boots (if equipped) for damage after lubrication. And use the recommended lubricant so you don’t damage the internal clutch mechanism.
Save your neck. Keep a good head on your shoulders. Get a qualified brake technician to work on your spring brake chambers.
Brake chambers house the actuation spring that applies the parking brake, and it’s held within the assembly under considerable force–enough to kill anyone who accidentally lets it get loose.
Brake chambers are identified by their size. A Type 24 has a 24-square-inch diaphragm; a Type 30 is 30 square inches. The force they exert on the pushrod depends on the area of the diaphragm. At 60 psi application pressure, a Type 24 exerts 1,440 pounds of force in the pushrod (60×24). A Type 30 exerts 1,800 pounds force. Obviously, the two are not interchangeable.
Maintenance involves checking brake stroke and adjustment at regular intervals. At the same time, make sure there is no contact or binding of the pushrod at the chamber hole. Check the mounting nuts for tightness and make sure cotter pins are in place. While you’re at it, check lines for any signs of deterioration or abrading.
Diaphragms should normally be replaced at 100,000 miles, although you should check to see what the manufacturer recommends. If one diaphragm fails, it’s essential to replace the diaphragm at the other end of the axle, and it would probably pay to replace all diaphragms in similar-sized chambers of the same age.
Brake chambers are now available with the brake-stroke stages marked on the pushrods. This gives you a better idea of when adjustment is needed, but it also helps pinpoint out-of-adjustment brakes at an inspection.
You can make indicators yourself. A nylon washer fitted tightly on the pushrod is ideal. To make one fit, just cut through one side of the washer. Fit it over the pushrod and push it back against the chamber face. As the brake is applied, the distance between the chamber face and the washer is the stroke. (It still takes two people, though, to check the stroke.)
That aside, if you have stroke adjustment indicators on the maxi brakes, they allow a quick visual check of brake adjustment on a walkaround. Just take a close look at the spring-brake chambers with the parking brake set.
Remember, if you see the warning colour or grooves, or if the indicators you’ve added show a large stroke, it’s past time to adjust. The spring system only exerts a force equivalent to an air pressure of about 65 psi, and that’s way below the 90-psi test brake pressure used in a roadside inspection.
Also, use the stroke indicators to watch for balance. The pushrods on an axle should be within 1/8 of an inch of each other-given you always service the brakes on an axle as a unit. Any variation indicates a problem, and the wheel ends should be disassembled to track
And remember: even removing the assemblies can be risky. If you do have to remove the chambers, make sure you cage the spring using the caging bolt, apply the parking brakes, and back off the slack adjusters before removing the mounting bolts.
Have your say
We won't publish or share your data