Clutches

Driveline components transmit and control power and motion. As simple as this sounds, it is necessary on every machine.
Brakes are basically a clutch with one member held stationary. The objective with both classes of hardware is to take two shafts rotating independently at different speeds and bring them into partial or total engagement.
Connecting shafts can be by direct mechanical lockup, mechanical friction, electromagnetic action, or hydraulic forces. Among the electromagnetic types, actual engagement may be mechanical, with electrical components used only for actuation.
Mechanical clutches generally are the simplest and normally used where an operator can actuate the clutch manually. Vehicles, for example, typically use mechanical clutches. Electric clutches are generally used where remote actuation is required (as on automatic machinery) or where special slip characteristics are required.
Hydraulic or fluid couplings are used in place of mechanical clutches where exceptionally smooth engagement is required or where it is desired to have the clutch automatically pick up a load with an increase in input speed. They are also used where constant engagement and disengagement would result in too much wear and maintenance.
Clutches rely on mechanical or electromagnetic action for torque transmission. However, they are usually identified by their mode of actuation: mechanical, electrical, pneumatic, or hydraulic.
Although the four operating modes are considered highly competitive, each mode actually is restricted to a fairly well-defined area of application. Within each area, one method provides definite advantages in terms of cost, response time, and torque transmission.
Mechanical actuation is the simplest mode, and mechanically actuated clutches generally are the least expensive. Mechanical clutches can be actuated through hand or foot-operated linkages or cables, which provide a "feel" for the amount of engagement.
Small mechanical clutches are actuated directly with cams or levers, while larger clutches are operated through compound linkages. Usually, mechanical actuation is feasible only when the lever or pedal can be located near the clutch. Some clutches can be actuated from long distances, but friction losses in the linkage or cable may be high.
Because mechanical actuation depends on hand or foot operation, actuation forces are limited to about 75 lb. This relatively low clamping force limits torque transmission to about 25,000 lb-ft and power transmission to about 2,500 hp (low compared with the tens of thousands of horsepower transferred by many machines). As a result, mechanically actuated clutches are restricted to vehicles and small industrial equipment such as hoists and cranes.
Besides low cost, the major advantage of mechanical actuation is the "touch control" the operator has over clutch engagement; he can closely control how quickly the output shaft comes up to speed.
The biggest disadvantage to mechanical actuation is the need for an operator. Hand operation not only limits clamping force and torque, it also limits response and cycling times. Normally, mechanically actuated clutches cannot be cycled more than a few times per minute without wearing the clutch elements or fatiguing the operator.
High temperature and contaminated atmospheres also are deterrents to using mechanical actuation. High temperatures increase clutch slippage, accelerating wear. Dirt can foul linkages (increasing required actuating force) and wear clutch elements. In some cases, using special housings and submerging the friction elements in an oil bath offsets the ill effects of contaminated atmospheres.
Pneumatic actuationis frequently encountered in industrial equipment. Air-actuated clutches transmit as much as 50,000 hp on machines such as rolling mills, grinding mills, and coilers. Air actuation also is common on vehicles large enough to accommodate an air compressor.
The reason for the wide range of uses is the general availability of pressurized air. Almost every factory has compressed air available that is easily piped to the clutch. In addition, air is a comparatively safe medium with which to work.
Generally, air-actuated clutches are mechanical friction clutches modified for air-pressure engagement, and spring disengagement. Shop air, usually 80 psi, is used as the actuation source. Cycle rates can reach 80 times per minute. Engagement usually is controlled through pistons and pressure plates. However, some air-actuated clutches use inflatable tubes or glands.
Air-actuated clutches transmit higher torques than equivalently sized mechanical or electric clutches. However, they fall short of the torques possible with hydraulic clutches.
Perhaps the greatest asset of air-actuated clutches is their thermal capacity. Unlike electricity, air does not generate heat during extended periods of clutch engagement. What's more, air can be directed across the faces of the clutch plates to cool them.
Air-actuated clutches can be used in almost any operating environment because the seals that prevent air leakage also keep out dirt. And actuation over long distances usually does not significantly lower air pressure at the clutch. Also, because static pressure maintains a constant force after the piston chamber is filled, power requirements for sustained torque transmission are almost negligible.
Air actuation generally is immediate, but in some cases it does provide some of the "touch control" of mechanically actuated clutches. For example, air can be channeled through a hand-operated throttle valve, allowing the operator to gain a feel for the amount of engagement. Also, air-actuated clutches can be operated through flow-control valves that engage the clutch over a specific time interval. Thus, acceleration of the output shaft can be smoothed over 100 to 150 msec.
In addition to remote control through hand-operated valves, air-actuated clutches can be operated automatically. Here, electric controls are incorporated to signal a solenoid valve to pressurize the clutch.
A possible disadvantage of air operation is the support equipment (compressor, valves, piping) which may increase space requirements and maintenance costs. Also, because air is exhausted to the atmosphere after each cycle, the pressurized air supply must be replenished constantly. Thus, system operating costs are increased by the need to run a compressor. The clutches themselves, however, are simple and durable, so clutch maintenance costs usually are not significant.
Electric clutches use two different operating principles. One type uses friction or tooth clutches engaged electrically and released by springs. The other type uses electric methods to engage input and output shafts without direct mechanical connection.
Electrically actuated clutches permit faster cycling times but they do not provide the torque range of air or hydraulic clutches. Electric clutches are more convenient for automatic machinery where control commands come as electric signals rather than as pedal or lever motions. Electric actuation also works better where the clutch is far removed from the control point and where mechanical linkages or pneumatic or hydraulic piping would be too cumbersome or expensive.
Some types of electric clutches provide closely controlled rates of continuous slip that would quickly wear out mechanical clutches. On the other hand, electric clutches do not provide the "feel" of engagement common to mechanical clutches.
The greatest advantage of electric actuation is the extremely fast response possible. For example, some of the smaller diameter clutches can respond in 1 to 2 msec. In many cases it is easier to wire an electric clutch than to pipe a pneumatic or hydraulic one. Also, various types of control switches are easily added to the circuit, permitting control by a variety of inputs such as photoelectric impulses, magnetic flux, and temperature.
In friction and tooth clutches, an electromagnet or solenoid replaces hand-operated levers and air or hydraulic pistons. The coil is stationary or rotates with the clutch. These clutches generally are used for full-engagement and minimal-slip requirements. Nonfriction clutches, such as hysteresis, eddy current, and magnetic particle, often operate with continuous slip but can lock up if required torque is less than the clutch torque capacity. Slip is controlled electrically to produce special operating characteristics.
Of the two types, electromagnetic clutches respond more quickly and transmit higher torques. On the other hand, noncontact clutches do not wear and dissipate heat better.
Hostile environments may be a deterrent to using electrically actuated clutches. For example, most electric clutches run dry and, therefore, do not have high thermal capacities. Thus, they may perform erratically at high temperatures. Because friction clutches sometimes spark due to metal-to-metal contact, they should not be used in explosive atmospheres.
Hydraulically actuated clutches deliver higher torque per unit volume than any other clutch. With this high capacity, hydraulically actuated clutches can be used on almost any size equipment from fans and blowers to construction and mining machines, where they are used most often.
Hydraulic clutches are mechanical friction clutches actuated with hydraulic oil acting on pistons. The oil is delivered at pressures as high as 500 psi, which accounts for the high torque-transmission capability. In general, operating pressure is a function of torque, speed, cycle rate, and B-10 bearing life.
Operating principles for hydraulically actuated clutches are similar to those for pneumatically actuated clutches. Friction elements can be immersed in oil to cool them, and the piston seals keep out dirt. However, if oil immersion is used, multiple clutch elements must be added to maintain the torque level.
Hydraulic actuation usually provides fast response, and smooth engagement can be produced by controlling the rate of pressure buildup with a pressure-control valve. Because fast response normally is a prime reason for using hydraulically actuated clutches, they require relatively short, large-diameter fluid lines. Remote locations or unusual control requirements may demand additional fluid-control devices to maintain oil pressure at the clutch.
The main disadvantage of hydraulic clutches is the installation and maintenance of the support equipment. This equipment generally is more sophisticated than that for pneumatic clutches and requires skilled maintenance personnel.

Brake Linings

Most brakes use some type of friction lining. Asbestos, once the accepted friction material, has been replaced in most cases by nonasbestos materials. Contents of the new material, largely proprietary, are also fibers molded or woven into pads or discs. Copper is sometimes added to improve heat dissipation. Cotton-based linings are sometimes used for light-duty service.
For heavy-duty service, sintered-metal linings perform better than fiber-based linings. Cermet linings are used for extremely rigorous service.
Linings are attached to the brake shoe either by riveting or bonding. Riveted attachment is simpler, but usable lining depth depends upon the depth of rivet-head countersink. Also, rivet heads may score the drum if the linings are not replaced soon enough.

Classical V-Belts

In addition to the flat leather and fabric belts used in belt drives of the early 1900s, industry used hemp or wire rope running in grooved pulleys. These rope systems inspired development of the rubber V-belt, which first appeared in the 1920s. V-belts make use of a wedging action between belt and sheave, thereby developing high driving forces with considerably less belt tension than required by flat belts. This feature reduces loads on bearings. Also, V-belts inherently track better than flat belts and, thus, do not have to be aligned as carefully.
When V-belts first appeared, most applications were for large industrial drives typically requiring 10 to 15 belts between a single pair of shafts. Thus, belts for these industrial drives became known as "multiple" belts, and today they are referred to as "classical multiple" or "heavy-duty conventional" belts. V-belts and sheaves have been standardized, with letter designations running from A through E. These standard sizes are recognized worldwide.
Classical multiple V-belts offer the broadest range of power ratings and generally are the first type of belt drive considered. They are readily available from local distributors and engineered and rated for long, low-maintenance service. Their only drawback when compared with more modern designs is relatively high weight and space requirements. The belt, being of heavy construction, generates high centrifugal forces that place relatively low limits on top speed. Also, the thickness of the belt limits bend radius, thus requiring the need for relatively large sheaves. In practice, these constraints rarely are serious limitations.
Most manufacturers offer two lines of multiple classical belts. One is the "standard" classification based on a fabric-wrapped construction. The wrap provides a protective envelope that tends to prevent damage and prolong life. The other classification is the "premium" or nonwrapped construction. The advantage of this design is that for a given standard size, none of the cross section is allocated to a wrap; thus, the total section consists of working tensile material. This allows rating belts of nonwrapped construction at higher power, section for section.
The lack of wrap makes a belt more vulnerable to damage; therefore, wrapped construction can be thought of as a more conservative selection. In OEM applications, however, service conditions tend to be well defined, and drive systems normally undergo thorough analysis and rigorous testing. Thus, these applications tend to favor nonwrapped construction, where the higher power rating can be used safely by virtue of the thorough engineering applied to possible environmental effects on the belt.
Belts for in-plant use, in contrast, are normally selected according to simple catalog ratings or to fit existing sheaves. Little, if any, attention is given to life testing or environmental effects, so wrapped construction is recommended.
Belts of nonwrapped construction, sometimes called bandless or raw-edge belts, are made with cogs or molded notches on the underside. Cogged belts permit more severe bends, thus allowing operation over smaller pulleys. Belts of standard wrapped construction are not available cogged because the protective envelope is both difficult to manufacture in cog form and also prevents severe bends. Notched belts are more expensive than those having a flat base. However, because they have a higher load-carrying capacity and run on less-expensive, smaller-diameter pulleys, the drive may require fewer notched belts.
Classical V-belts are frequently used individually, particularly in A and B sizes, smallest of the five cross sections. The larger sections, C, D, and E, generally are not used in single-belt drives because of certain cost penalties and inefficiencies encountered when scaling drives upward. The economics are such that for equivalent ratings, multiple belt drives based on A or B sections usually cost less than single-belt drives with C, D, or E sections.

Basic Design Engineering - Belts

Belts provide an efficient load transfer between shafts on a wide variety of applications. They also perform special tasks such a speed ratio variation, power transmission in more than one plane, clutching, torque limiting, and shaft synchronization.
Compared with most forms of power transmission, belts often provide the best overall combination of design flexibility, low cost and maintenance, ease of drive assembly, and space savings.
Disadvantages on some applications may include the need to retension belts periodically to avoid slippage (note: overtensioning can damage bearings on pulley system), deterioration because of severe exposure to chemicals and lubricants, and the requirement that damaged belts must be replaced, rather than repaired. Although V-shaped belts are not recommended for drives with a fixed-center distance or in low-speed, high-torque applications, synchronous belts will often solve these design considerations.
Many OEM products use commercially available belts and sheaves or pulleys. This facilitates mass-produced designs, and allows OEM customers to obtain stock replacement parts easily and quickly from local distributors.
Some OEMs have belts made especially for their use. Many times the belt is actually a stock item, with a standard construction, but with a special private brand label. In other cases, a special construction belt will be developed for unique application needs such as clutching, temperature extremes, idler use, extremely high loads, or small-diameter drives.
Advanced application methods and years of experience from reputable belt manufacturers generally assure good performance without extensive OEM field testing. This is especially true where operating characteristics are predictable and the load and speed range is rather small such as on electric motors in combination with fans, pumps, conveyors, and so on. However, suppliers recommend a field test to assure proper belt performance when speeds, loads, or both vary significantly, or when customer operating practices are not accurately predicted.
The following section describes belt types and cross sections generally available at the OEM level and in the marketplace. Most manufacturers have special belt sections or constructions which are developed for special applications, or which have higher performance characteristics. Belt manufacturers also offer special assistance to OEMs with unique applications. See the sections on Mechanical Fixed and Adjustable-Speed Drives for a complete discussion.