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Posted: August 1, 2014

By Jim Park. From the March 2004 issue

Despite what you may think, the 2-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.

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 when I was driving. So in lieu of that dirty and near-impossible task, many drivers rely on add-on stroke indicators or colored markers on the pushrod 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 fleet-maintenance supervisor at Canadian Tire in Brampton, Ont.

“We need to check brake stroke using the same application pressure that 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 1500 and 2000 lb of force, to the pushrod. A
standard Type-30 chamber delivers 3000 lb 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 3000 lb of force at the pushrod exerts 18,000 lb of torque on the cam shaft (3000 x 6 = 18,000). Compare that to the force exerted by the spring (2000 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 at 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

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.


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