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    Drive Line

    May 21st, 2010

    A clutch package does much more than set your car in motion and allow the shifting of gears

    Given the opportunity, Confucius might have said, “Man who has no grasp on clutch fundamentals winds up with slipped disc.” And even if he might not have made such a statement, it could easily apply to our Shop Series. For regardless of how much power you’ve found a way to make, getting it channeled through the driveline and onto the ground can be the next most important thing to do. This month, we’ll explore the ways of accomplishing this by manually shifting gears, which again brings up the subject of clutch mechanisms.

    First of all, there are three types currently used in automotive applications. Known both by manufacturer and method of operation, they are as follows: (1) Borg & Beck, (2) Long and (3) diaphragm. Each has its own specific design features, but all rely on the transmission of engine torque into the driveline. And the ability of each type of clutch to do this is closely related to the amount and degree of friction involved. So before we get into the subject of clutch types, let’s spend a couple of minutes discussing friction and what it means to clutch stuff.
    Suppose you are standing in front of a cube of steel that’s resting on a concrete floor. Classroom science tells us that the amount of resistance to movement this chunk of steel will provide is based on its weight and the surface texture (or roughness) of the concrete floor. Figure A depicts this relationship and indicates that the amount of frictional resistance is the mathematical product of weight and friction coefficient (a big word meaning how rough the surface over which the steel cube will be moved actually is). In mathematical terms, this can be expressed in an equation as follows: F=uN, where F is the frictional resistance to movement, N is the weight of the steel chunk (or holding force as in the case of a clutch), and u is the coefficient of friction of the material (in this case it’s the frictional characteristic of the surface on which our chunk of steel is resting). Since F is the mathematical product of p multiplied by N, an increase in either u or N will correspondingly increase F (frictional resistance). More surface roughness or more weight can contribute to an increase in frictional resistance.

    In terms of clutch terminology, all this boils down to increased holding capability. The less a clutch slips, the more ability it has to transmit torque into the remainder of the driveline. So both clutch spring pressure and disc material friction characteristics play important roles in the performance of a given clutch package. With this in mind, let’s examine the basic parts of a clutch assembly.
    Essentially, there are three elements: (1) flywheel, (2) clutch disc(s) and (3) pressure plate assembly. Functionally, the flywheel provides a friction surface against which one side of the disc makes contact. It also is the carrier of the pressure plate, since this assembly bolts directly onto the flywheel.
    The disc is a flat surface normally faced with two pieces of friction material one side in contact with the flywheel, the other against the pressure plate ring. In the center of the disc there’s usually an arrangement of springs captured within the disc’s hub so that some amount of torque absorption (friction lag) is provided during clutch engagement.
    And finally, there’s the pressure plate assembly. This consists of a formed (or stamped) steel cover with recesses that hold springs against a ring (pressure plate) which is held against the clutch disc. Hopefully, Figure B will shed a little light on all this as you decipher the “what goes next to what” stuff.
    Also located in the pressure plate assembly is some sort of mechanism that allows the separation of clutch disc, pressure plate and flywheel surface. This disengagement lets you shift gears by breaking the connection between engine and driveline. These so-called release levers come in assorted sizes and shapes, but each is intended to pull the pressure plate back away from the disc so that some amount of air gap exists between disc and friction surfaces. As you will see In a moment, these release levers (there are usually three) can become rather numerous if the pressure plate is of the diaphragm design, but their job remains the same in terms of clutch disengagement (or engagement).

    Release levers do one other noteworthy thing: Since the pressure plate assembly is rotating during normal operation of the engine, centrifugal forces act on all parts of the package. And because of the way release levers are typically mounted, they can be used to contribute to the overall applied pressure of the complete assembly. Spring pressure is pretty much a function of spring stiffness and operating height. But pressure assist from centrifugally loaded release levers depends on the amount of weight on the outboard end of the lever(s) and how fast the engine is running. Figure C is intended to help you visualize all this.
    One nice thing about centrifugally loaded release levers is that they allow low spring pressures in the assembly (nice soft clutch pedal), but help keep everything held together when engine rpm builds and there’s more power to be transmitted into the driveline. It was this type of lever that opened the door to “slider clutches” in single-gear drag cars, but that’s a subject unto itself. We’ll touch on sliders a bit later in the story. They’re almost like automatic transmissions.
    Now it’s review time. Up to now, we have a clutch package that includes flywheel, disc and pressure plate assembly with the ability to come together (or apart) as release levers are operated within the pressure plate. It’s also been mentioned that the frictional characteristics of the disc and surfaces against which it operates affect the transmission of torque into the driveline. And we’ve indicated that some amount of torsional damping (torque “shock”) is provided by springs mounted around the center of the disc.
    Now let’s discuss the three basic types of pressure plate assemblies. Each has specific features that apply to both passenger car and race applications. Borg & Beck
    This design clutch relies primarily on spring pressure to prevent slippage. Sometimes (now you can refer to Figure D), rollers are fitted between the release levers and pressure plate cover so that additional pressure is applied to the clutch disc as a function of engine rpm—sorta like centrifugally loaded release levers—but the lever isn’t generating the load.

    But with or without rollers, Borg & Beck pressure plates are known for “stiff” clutch pedal pressures and rather quick engagement rates (rapid transfer of engine torque into the driveline). Twelve pressure plate springs are typically used in B&B designs, and release levers are stamped from steel instead of being cast (as in the case of most Long-style pressure plates).
    Since B&B pressure plates normally rely on high spring pressure for torque-holding capability, tensile loading of the release lever eyebolts can cause failure, especially at high engine rpm. The addition of rollers to generate centrifugal pressure assist can cause problems of clean clutch release at the higher engine speeds. B&B pressure plates used for racing should be checked with some degree of regularity for good roller and release lever condition (chips, broken rollers, wear ridges in the levers) to prevent failure.

    Long
    This design is characterized by nine pressure plate springs and, usually, cast release levers that provide some amount of centrifugal loading. Fewer springs also reduce the amount of static (spring) loading felt at the clutch pedal. And while this reduction in number of springs (for race applications) doesn’t necessarily mean low pedal pressure, it allows for pressure control relative to engine rpm. For example, a B&B pressure plate (with no centrifugal assist from the release levers) at a spring pressure of 3000 pounds will exhibit much more leave-the-corner pedal pressure than a Long-style with 2300 pounds spring pressure and centrifugal assist that depends on engine rpm.
    Because Long-style pressure plates frequently employ centrifugal-assist release levers, high-rpm applications of this design cannot use levers of heavy “bob weight” configuration. This would cause excessive holding pressure (the result of high centrifugal assist) and prevent easy or clean release of the clutch at the higher engine speeds. In fact, it might feel like you were pushing the brake instead of the clutch pedal. Unless, of course, your feet happened to be crossed!
    Diaphragm
    In this case, the “diaphragm” is more like a shallow cone-shaped piece of metal that has been cut (radially) to form fingers or release levers around the circumference of the diaphragm. This piece, commonly called a Belleville spring, takes the place of conventional pressure plate springs and holds the pressure plate against the remainder of the clutch package.

    A. The resistance to movement between two objects (surfaces) is a function of contact pressure and surface condition (coefficient of friction). Mathematically, frictional resistance (F) is the product of friction coefficient and object weight (load) that is perpendicular to the surface. As the friction surface is inclined from horizontal, a component of the object’s weight becomes the value of N. Regardless, in a clutch package there is a similar relationship between friction surfaces and load (spring and/or centrifugal pressure). Coefficients of friction may be different (u vs. u2), but the load and surface relationships still hold. B. In stark basics, this is an exploded drawing of fundamental pressure plate pieces. At attachment point (1), the centrifugal release lever bolts to the pressure plate cover. And since the cover is bolted to the flywheel, weight movement (increasing with engine rpm) as shown in the direction of (2) forces the pressure plate more tightly against the disc and flywheel.

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    Powershifting

    May 26th, 2010

    Whereas coil springs can be compressed to a point where “coil bind” (actual interference among all coils in the spring) takes place, Belleville springs are exempt from this condition, since they can be placed only in an over-center position. This means it is possible to depress (during clutch release) a Belleville spring beyond its design limit and cause it to not return to its original position after releasing the clutch pedal. You may know of this problem after “powershifting” a diaphragm clutch and finding that the clutch pedal stayed on the floor when you removed your foot. This “over-center” condition is a result of centrifugal forces acting upon the diaphragm, holding the release levers in their disengaged position. Improper clutch adjustment and/or clutch disc wear thickness can both contribute to this problem.
    But because diaphragm pressure plate assemblies seem to require less pedal pressure than spring-type units, more and more new-car manufacturers are using this design of pressure plate. And with the present trend toward lower engine rpm and reduced torque output of stock power plants, there is added indication that diaphragm clutches will be standard in the remaining stick-shift cars.

    C. Assume that this is a side view of a pressure plate. As dimension (1) is decreased, position of the bob weight is made farther from the pressure plate. This causes the clutch release lever to have more tendency to move toward the pressure plate (2). And since the pressure plate cover is bolted to the flywheel, lever movement pushes the plate harder against the disc and flywheel. Result? More holding pressure as a function of engine rpm. D. Another common method of centrifugal pressure assist is the roller mechanism often found in Borg & Beck clutch assemblies. As engine rpm increases, roller movement tends to be in the direction indicated by (1). Pressure plate cover attachment to the flywheel causes plate movement toward the clutch disc and flywheel (2). Only problem here is that rollers sometimes tend to stick between cover and roller stand, resulting in a stuck-to-the-floor clutch pedal. Graphite lube or a slight amount of moly paste will usually prevent this condition.

    E. Conventional clutch disc design includes pins (1) holding a damping spring (2) retaining plate that absorbs engagement torque through the transmission shaft hub (3). Not at all complicated. It’s much like wringing out a bath rag (unless you’re younger than 11 years). The torque or twisting motion between flywheel and disc cannot be instantly transferred into the pressure plate. It’s the job of the “friction lag” springs to dampen the shock of this transfer, even if the clutch is from a VW (as shown). F. Here you can see the multiple “release levers” of a diaphragm pressure plate (1). To prevent rotation of the pressure plate during clutch engagement, flat metal strips are fastened to the pressure plate cover (3) and anchored to the pressure plate (2) in a position (relative to engine crankshaft rotation) that keeps the metal strips in tension during torque transmission or complete clutch engagement.

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    Slider clutches

    May 26th, 2010

    There’s a story among drag racers about the origin of the slider clutch, it seems that a relatively inexperienced car owner incorrectly installed the clutch discs in a two-disc dragster clutch. By reversing the position of the discs, he caused a slight but additional amount of clearance oetween discs and the remainder of the assembly. Upon engagement, this Drevented immediate closure of the clutch, which was accomplished by Miction heat and centrifugal assist from the release levers at high engine rpm. The result was no tire slip-cage and an immediately lowered elapsed time.
    Today, much study has been made of how best to slip a clutch and what materials should be used to withstand the considerable heat from such action. Low spring pressures characterize slider clutches, in addition to carefully weighted release levers designed to slowly engage the clutch as engine rpm increase. Obviously, the object is to provide gradual transfer of torque into the driveline so that tire spin is minimized. The use of more than one clutch disc increases the amount of friction surface available to the spring pressure while giving more area over which neat can be dissipated. Tricky, but effective, especially for single-gear drag cars.

    Some summary thoughts
    In the selection of clutches, try to avoid high pedal pressures if at all possible. The higher the resistance to pedal movement the greater the load placed on the thrust surface of the main bearing that takes this pressure. And frequent use of stiff clutches can make you a little lopsided when comparing left leg to right. Clutch adjustment can be critical to clutch function, especially in high-rpm engines. Most shop manuals talk about free play at the pedal. Actually, what you’d like to measure is the amount of air gap between clutch disc and flywheel (or pressure plate) at maximum clutch disengagement. But, unless there’s an opening at the bottom of the bellhousing, this may not be possible.
    Even so, you might consider drilling such a hole and using an air gap of about .040-.050-inch for diaphragm clutches and .070-.090 for Borg & Beck and Long designs. Sometimes this causes pedal free play to be more than the usually suggested .750-1.5 inches, but you’ll do less damage to the pressure plate and avoid the problem of over-centering diaphragm clutches when engine speed gets high. Oddly enough, a properly adjusted clutch can help prevent sticking valve heads into piston tops if the pedal sorta stays to the floor. Even Confucius would understand that.

    REVIEW QUESTIONS: True or False
    1. Most centrifugally assisted diaphragm clutches have stiff pedal pressure when the engine is operated below 4000 rpm.
    2. Borg & Beck pressure plate assemblies are characterized by eight springs located between the cover and pressure ring.
    3. In the examination of friction surfaces, more holding pressure means a higher coefficient of friction.
    4. Friction lag is the amount of time required for a clutch to completely disengage.
    5. All clutches are required to slip in the normal process of shifting gears.
    6. in a complete clutch assembly, the pressure plate cover bolts directly to the engine’s flywheel.
    7. A Belleville spring is used to keep the pedal from accidentally sticking to the floor during high-rpm shifts and is connected to the shift linkage.
    8. Slider clutches require high static (spring) pressure in the pressure plate assembly.
    9. Long-type pressure plate assemblies with considerable centrifugal assist yield stiff pedal pressure at high engine rpm.
    10. Rollers work especially well for centrifugal assist in diaphragm pressure plates.
    11. Release levers for B&B clutches are usually cast iron.
    12. Long-style pressure plates with low static pressure and heavy centrifugal lever bob weights make good street machine clutches.
    13. Normally, diaphragm pressure plates have the lowest amount of pedal pressure (resistance to movement).
    14. The addition of clutch discs (two or three) to a clutch assembly automatically provides a desirable amount of slip.
    15. In terms of clutch language, air gap is the distance measured between clutch cover and disc with the clutch fully engaged.
    16. Confucius liked Powerglides.

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