Posted by #1 Son on August 24, 04 at 11:49:40:
Friends,
My previous post on this subject lost the diagram due to change of Server. I am reposting it below for your reference.
#1 Son
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Nick,
The left panel of the above diagram is a rough sketch of an automotive drum brake system. It is not to scale and proportion. Some of the parts are missing. Notably, for clarity of presentation, the two top springs that pull the two brake shoes toward each other is not shown. Still I hope it can be used to explain to you how a drum brake system works.
1. When the brake pedal are pushed, more brake fluid is forced into the "brake fluid chamber", thus expanding its volume. This increase in volume will force the pistons in the brake cylinders to push against points X & Y, spreading the brake shoes apart. The brake shoes will then push against the inner surface of the brake drum, stopping its rotation, and hopefully the car.
2. As the brake linings are worn, the GAPs between the brake drum and the linings will increase. The brake pistons will have to travel a greater distance before the brake linings will make contact with the drum surface. This condition shows up as "low pedal feel" to the driver.
3. To correct this "low pedal feel" and to compensate for the effects of the wearing of the brake linings, an adjuster is incorporated as shown at the bottom of the sketch.
4. When the brake lining are first installed, the "length" of the brake adjuster assembly, A-B, is adjusted so that there is a very small gap between the lining and the brake drum surface without actually touching each other. As the linings are worn, this gap becomes larger, as explained above.
5. To reduce the size of this gap when the linings are worn, we need to spread the two brake shoes further apart by increasing the length of the brake adjuster assembly. This is accomplished by turning the Star Wheel to lengthen the adjusting screw.
6. To accommodate manual adjustment of the brakes, a "HOLE", located directly behind the Star wheel, is punched into the backing plate of the brake assembly. A tool or screw driver is inserted through this hole to turn the Star Wheel to spread out the brake shoes just as I have described above. Refer to the right hand panel in the diagram above.
7. To make the system self-adjusting, another mechanism, shaped like a lean-back "L", not unlike a lawn chair, is cleverly designed and positioned such that the tip of the lower right hand leg of the "L" rests on top of the Brake Adjustment Star Wheel. Each time the car is reversed and stopped, this "L" shape piece will rock about its lower left hand corner. Since the tip of its lower right hand leg rests on top of the Brake Adjustment Star Wheel, his action will turn the Star Wheel IF AND ONLY IF there is any excessive gap between the brake lining and the brake drum (otherwise it will just "slip"). This IS the self-adjusting action.
Please note the direction to turn the star wheel at the upper right hand corner of the gaphics above when doing manual adjustment yourself.
#1 Son
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