Archive for May 23rd, 2010

Brakes and Braking Systems

Sunday, May 23rd, 2010

     Imagine driving in your car, you’re traveling at a speed of 65 mph and you’re coming up on a curve.  You depress your brake pedal to negotiate the turn, and nothing happens…

     Scenarios just like this one have been in the news quite often lately, brakes which just aren’t operating correctly.  We’ve heard the tales of terror, recounted by those unfortunate individuals who have been placed in this situation, but have we reflected on just why their brakes might have failed?

     Put most simply, a brake is a device whose purpose is to stop a body in motion.  This important task is accomplished by converting the kinetic energy (energy of motion) into heat energy.  This can be accomplished by either of two methods, mechanically or electrically.  In today’s blog we’ll focus on the mechanical aspect.

     A simple mechanical brake is shown in Figure 1 below.  In this arrangement kinetic energy is converted into heat energy when force is applied to a lever, causing a brake shoe to meet up with a rotating wheel.  The brake shoe has a pad attached to its surface that makes direct contact with the wheel, and when the two come together great friction is produced.  It’s this friction that will ultimately stop the object in motion.  Friction turns the kinetic energy into heat energy.

Figure 1 – A Simple Mechanical Brake

     Friction at its simplest is a mechanical resistance to movement.  Whenever two materials in motion come into contact with each other there is always some degree of friction.  The extent to which friction is produced by their meeting is referred to as the “coefficient of friction.” 

     The coefficient of friction varies according to the surface character of the materials coming in contact.  For example, the coefficient of friction for the leather sole of your shoe on smooth ice is very low.  This means you’ll do a lot of slipping when you’re trying to walk, and that’s because ice presents little friction to resist a smoothly soled shoe.  But take this same shoe and apply it to the rough surface of concrete, and you’ll be walking quickly and efficiently.  Coefficients of friction between different materials have been duly measured in laboratories and are tabulated for easy access in engineering reference books.

     Based on our simple example above, one would easily come to the conclusion that a high coefficient of friction is desirable when talking about brake shoes, specifically the one represented in Figure 1 above.  The higher the coefficient of friction, the more the pad wants to grab the wheel, and the less force you will need to apply to the brake shoe to successfully come to a stop.

     That’s mechanical braking in a nutshell.  Next time, we’ll focus on an electrical braking system known as a “dynamic brake.”

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