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"PRINCIPLES OF HYDRAULICS EXPLAINED, CENTERING ON THE VALUE FOR SAFETY & COMFORT OF HYDRAULIC BRAKES."
NEW VERSION with improved video & sound: https://www.youtube.com/watch?v=mTO7vlITLek
Public domain film from the Library of Congress Prelinger Archive, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum.
The term "drum brake" usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disk brake, it is sometimes called a "pinch drum brake", although such brakes are relatively rare. A related type of brake uses a flexible belt or "band" wrapping around the outside of a drum, called a band brake.
The modern automobile drum brake was invented in 1902 by Louis Renault, whose unique genius inspired him to use woven asbestos lining for the drum brakes lining as there were no other alternatives that dissipated heat like the asbestos lining, though a less-sophisticated drum brake had been used by Maybach a year earlier...
Some of the major components of the drum brake assembly are the back plate, the brake drum and shoe, the wheel cylinder, and various springs and pins.
The back plate serves as the base on which all the components are assembled. It attaches to the axle and forms a solid surface for the wheel cylinder, brake shoes and assorted hardware. Since all the braking operations exert pressure on the back plate, it needs to be very strong and wear-resistant. Levers for emergency or parking brakes, and automatic brake-shoe adjuster were also added in recent years...
The brake drum is generally made of a special type of cast iron which is heat-conductive and wear-resistant. It is positioned very close to the brake shoe without actually touching it, and rotates with the wheel and axle. As the lining is pushed against the inner surface of the drum, friction heat can reach as high as 600 °F (316 °C).
One wheel cylinder is used for each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. When hydraulic pressure from the master cylinder acts upon the piston cup, the pistons are pushed toward the shoes, forcing them against the drum. When the brakes are not being applied, the piston is returned to its original position by the force of the brake shoe return springs. The parts of the wheel cylinder are as follows:
Brake shoes are typically made of two pieces of sheet steel welded together. The friction material is either rivetted to the lining table or attached with adhesive. The crescent-shaped piece is called the Web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage and self-adjusting components. All the application force of the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table generally has three "V"-shaped notches or tabs on each side called nibs. The nibs rest against the support pads of the backing plate to which the shoes are installed. Each brake assembly has two shoes, a primary and secondary. The primary shoe is located toward the front of the vehicle and has the lining positioned differently than the secondary shoe. Quite often the two shoes are interchangeable, so close inspection for any variation is important.
Linings must be resistant against heat and wear and have a high friction coefficient unaffected by fluctuations in temperature and humidity. Materials which make up the brake shoe include, friction modifiers (which can include can include graphite and cashew nut shells), powdered metal such as lead, zinc, brass, aluminium and other metals that resist heat fade, binders, curing agents and fillers such as rubber chips to reduce brake noise.
The self-adjuster is used to adjust the distance between the brake shoe and the drum automatically as brake shoes wear...