LECTURE #29 BRAKES Course Name : DESIGN OF MACHINE ELEMENTS Course Number: MET 214

Brake: A device used to bring a moving system to rest, to slow its speed or to control its speed to a certain value under varying conditions. From previous lecture, the energy absorbed or dissipated by the unit per cycle is equal to the change in Kinetic energy of the components being accelerated or stopped. where mass moment of inertia of system being de-accelerated initial angular velocity of system being de-accelerated final angular velocity of system being de-accelerated change in energy associated with de-accelerated system If then

The change in energy associated with de-accelerating a body must be absorbed by the material adjacent to the brake lining. (usually a metal drum) The temperature increase of the brake is related to the energy that must be absorbed by the brake. where frictional energy the brake must absorb weight density of the drum material lb/ in 3 volume of drum material absorbing the energy Specific heat of brake drum material (lb f -ft/lb m 0 F) increase in temperature of brake drum Specific heats of a few materials: 1) Cast iron =101 2) Steel= 93 3) Aluminum = 195

The following table list maximum drum temperatures for some commonly used brake and clutch materials Most brake lining manufacturers include the effect of the rate of energy/ dissipation by giving limiting values of Pv for given materials where P= pressure lb/in 2 v= velocity = ft/ min Pv= ft-lb/in 2 m or hp/in 2 To develop an appreciation for the numbers involved, the following Pv values are typical. MaterialMaximum Drum Temperature, 0 F Coefficients of friction, f Maximum Allowable Pressure, psi Metal on metal Wood on metal Leather on metal Molded Blocks Asbestos on metal in oil Sintered metal on cast iron in oil

1) Less than 28,000 in applications involving continuous operations and inadequate heat dissipation. 2) Less than 55,000 for intermittent operation and poor heat dissipation but with long periods of rest. 3) Less than 83,000 for continuous applications with good heat dissipation.

Band Brakes: A flexible material, usually made of steel, is faced with a friction material that can conform to the curvature of the drum. The application of a force to the lever establishes tension in the band and forces the frictional material against the drum creating a torque that slows down the drum

The relationship between the tensions of the brake band, designated as P1 and P2 in figure 1 and the torque applied to the brake drum are the same as the relationships used with pulleys. where braking torque applied to brake down by brake band (ft-lbs) Maximum band tension (lbs) Minimum band tensions (lbs) radius of brake drum Since the brake band is not rotating, the terminology used to describe the brake is different from the terminology used to describe the belt and pulley. tight side tension -> Maximum band tension slack side tension -> Minimum band tension As was the case before: Where angle of wrap (coverage) of the band in radians coefficient of friction of brake band

Similar the disc clutches and/or brakes, the maximum pressure that can be applied to the brake band is determined by the material. The maximum pressure that is associated with the band and drum contact occurs at the end of the band where the maximum in band tension exists. The actuating force w 1 required to operate the brake can be related to the tension in the brake band and the geometry of the lever. For the simple brake band shown in Figure 1(a) sum moments about the pivot point of the lever to relate w to P 2 For the geometry illustrated in figure 1(b), sum the moments about the pivot point of the lever to relate w to P 1 and P 2.