Clutches and Brakes

Design and Selection

William C. Orthwein

• Publisher: CRC Press
• Year: 2004
• ISBN:
  9781135533069
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Mechanics

Conveniently gathering formulas, analytical methods, and graphs for the design and selection of a wide variety of brakes and clutches in the automotive, aircraft, farming, and manufacturing industries, Clutches and Brakes: Design and Selection, Second Edition simplifies calculations, acquaints engineers with an expansive range of application, and a

1
Friction Materials

Curves of the coefficient of friction as a function of load and of the speed differential between the lining and facings and their mating surface are no longer available from many manufacturers. Perhaps this is a consequence of the ease with which trial lawyers in the United States can collect large financial rewards for weak liability claims based upon often trivial, or unavoidable (due to physical limits on manufacturing tolerances), differences between published data and a particular specimen of the manufactured product. Furthermore, differences between published and operational coefficients of friction are beyond the control of the manufacturer because comparison of laboratory and operational data have shown that temperature, humidity, contamination, and utilization cycles of the machinery using these linings and facings can cause significant changes in the effective coefficient of friction at any given moment. Consequently, the coefficients of friction mentioned are nominal, the following discussion is in generic terms, and all curves shown should be understood to represent only the general character of the material under laboratory conditions.

The value of laboratory data is twofold, even though the data should not be used for design purposes. First, the data provides a comparison of the performance of different lining materials under similar conditions, such as given by the SAE 661 standard. Second, comparison of the laboratory data with field data for a particular type of machine for several different linings may suggest an empirical relationship that yields an approximate means of predicting the field performance of other lining materials based upon their laboratory data. A history of the comparison of field and laboratory data may, therefore serve as a starting point in the design of the prototype of a new machine of the same or similar type.

Field testing of a new machine by customers under the most adverse conditions is still necessary. Users often seem to devise abuses not envisioned by the design engineers.

I. FRICTION CODE

The usual range of the dynamic friction coefficients for those friction materials normally used in dry brake linings and pads is given in the Society of Automotive Engineers (SAE) coding standard SAE 866, which lists the code letters and friction coefficient ranges shown in Table 1 [1]. According to this code the first letter in the lining edge friction code indicates the normal friction coefficient and the second letter indicates the hot friction coefficient. Thus a lining material whose normal friction coefficient is 0.29 and whose hot friction coefficient is 0.40 would be coded as follows:

Temperatures for the normal and hot friction coefficients are defined in SAE J661, which also describes the measurement method to be used.

TABLE 1 Friction Identification System for Brake Linings and Brake Block for Motor Vehicles

Static and dynamic coefficients of friction are usually different for most brake materials. If a brake is used to prevent shaft rotation during a particular operational phase, its stopping torque and heat dissipation are of secondary importance (i.e. a holding brake on a press); the static friction coefficient is the design parameter to be used. On the other hand, the pertinent design parameters are the dynamic friction coefficient and its change with temperature when a brake is designed for its stopping torque and heat dissipation when a rotating load is to be stopped or slowed.

Most manufacturers will provide custom compounds for the linings and facings within the general types that they manufacture if quantity requirements are met. In almost all applications it is suggested for all of these materials that the linings and facings run against either cast iron or steel with a surface finish of from 30 to 60 micro inches. Nonferrous metals are recommended only in special situations.

Effects of heating on the linings and facing discussed are expressed in terms of limiting temperatures or limiting power dissipated per unit area at the surface of the brake lining or clutch facing. Time is usually omitted, even though the surface temperature is determined by the power per unit area per unit time. This is because it is assumed that the power dissipation occurs over just a few seconds. More precise estimates, and only that, of the heat generated by the power dissipated in particular cases maybe had by using one of several heat transfer programs from suppliers of engineering software. It is for these reasons that prototype evaluation is always recommended.

II. WEAR

Hundreds of equations for wear may be found in the literature. These equations may depend a variety of factors, including the materials involved, the temperature, and the environment under consideration, i.e., the liquid or gas present, the formation of surface films, and so on [2]. Two of the relations that pertain to the following discussion are the specific wear rate and the wear rate.

The first of these, the specific wear rate, or wear coefficient, is a dimensional constant K that appears in the relation

From which υ may be written as

In these relations, δ represents the thickness of the lining material removed, υ is the volume material removed, K is a dimensional constant that is termed the specific wear rate or the wear coefficient, and p is the pressure acting over the surface area A that is in contact with the lining material. Force F is given by integral of the pressure acting on the specimen integrated over the area A over which it acts. Upon rewriting equation (2–1) to evaluate K we have that

Hence the units of K are lt²/m where l,t, and m denote length, time, and mass, respectively. As a practical matter, if υ is millimeters cubed (mm³), if force F is in newtons (N), and if the distance d is in meters (m), then the units of K become mm³ N–¹ m–¹, which explicitly shows the physical quantities involved, as in Figure 3.

The second relation that may be used by brake and clutch lining manufacturers to describe wear is

in which Γ represents the wear rate, P is the power dissipated in the lining, and t is the time during which volume V was removed at temperature Θ The units of Γ in equation (2–3) are those of the work (ml²/t²) required to remove a unit volume of material multiplied by the volume (l³) removed.

Whenever the temperature is held constant during a test, the temperature variable Θ is suppressed. Since brake testing according to the SAE 661b standard is done at 200°F, the wear rate is often given by Γ=υPt and presented in the form υ=Γ/(Pt) Again, to be practical the wear rate divided by the product horsepower hours (hp hr) may be given in cubic inches (in.³), as in Table 2 near the end of this chapter.

III. BRAKE FADE

Brake fade is a term that refers to the reduced effectiveness of many dry brakes as they become heated. A standard test described in SAE J661 outlines a procedure that uses controlled temperature drums and controlled brake lining pressure to stimulate brake fading as a basis of comparison of the brake fading characteristics of various lining materials. The equipment and temperatures are essentially identical to those used in estimating the coefficient of friction as a function of temperature. Only the presentation of the data is different, as shown in Figure 1. The fade test mode of presentation of data provides another indication of the recovery capability of the various lining materials. As with the previous test data, the fade test results are limited to a comparison of different lining materials for the test conditions only.

Limitation of the application of these data to preliminary design is emphasized because the friction coefficient is dependent upon the pressure, the temperature, and the relative velocities of the contracting surfaces, as noted earlier. Field tests are recommended before the production of any brake design because of the uncertainty usually associated with the variables involved in lining heating and in the cooling capability of the brake housing and any associated structure.

FIGURE 1 Display of brake lining fade test results. (Courtesy of Scan- Pac, Mequon, WI).

IV. FRICTION MATERIALS

Friction materials may be classified as either dry or wet. Wet friction lining materials are those that may operate in a fluid that is used for cooling because of the large amount of energy that must be dissipated during either braking or clutching. The fluids used are often motor oil or transmission fluids. Lining materials that cannot operate when immersed in a fluid are known as dry lining materials.

A. PTFE and TFE

At this time it appears that PTFE (polytetrafluoroethylene) and TFE (tetrafluoroethylene), both included under the trade name Teflon, are commonly used for brake linings [3]. PTFE exhibits a low coefficient of friction and is mechanically serviceable at about ±260°C, is almost chemically inert, does not absorb water, and has good dimensional stability. Its weakness in shear stress is greatly improved by the addition of fillers, such as glass fibers. These fibers also increase its wear resistance and strengt...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Mechanical Engineering
• Preface to the Second Edition
• Preface to the First Edition
• Introduction
• Chapter 1 Friction Materials
• Chapter 2 Band Brakes
• Chapter 3 Externally and Internally Pivoted Shoe Brakes
• Chapter 4 Linearly Acting External and Internal Drum Brakes
• Chapter 5 Dry and Wet Disk Brakes and Clutches
• Chapter 6 Cone Brakes and Clutches
• Chapter 7 Magnetic Particle, Hysteresis, and Eddy-Current Brakes and Clutches
• Chapter 8 Acceleration Time and Heat Dissipation Calculations
• Chapter 9 Centrifugal, One-Way, and Detent Clutches
• Chapter 10 Friction Drives with Clutch Capability
• Chapter 11 Fluid Clutches and Brakes
• Chapter 12 Antilock Braking Systems
• Chapter 13 Brake Vibration
• Chapter 14 Engineering Standards for Clutches and Brakes
• Bibliography