Transmission Tech

On 05/26/2010, In Uncategorized, By admin

Chances are good that you are standing or sitting fairly motionless. Based on the fact that you are situated in a gravitational field (most of us are), your body has weight and some resistance to motion. Some of us have more resistance to motion than others, which has nothing to do with weight. But that’s another story.

Now let’s suppose you decide to cross the room. Having been at rest, and being of some amount of weight, you must overcome the inertia (resistance to movement) to cover the distance. The relative ease (or difficulty) with which you do all this relates to how much work is required to overcome the inertia, and how fast you go from one side of the room to the other. But it’s safe to say the ability to change inertial resistance into motion depends upon effort expended (power) and how fast you did it (time). And now before you glance back at the title for this month’s Series, we’ll tie this concept to your car and its transmission.

First, the vehicle is at rest. So it presents the engine with some amount of inertial resistance. If the engine, now at an idle, were suddenly coupled directly to the driveshaft and rear wheels, we’d probably have a restart on our hands. The inertial load presented upon coupling the engine to the rear wheels would likely be too much for it to overcome, even with generous slipping of the clutch just to maintain engine speed.
So some sort of mechanism is required which allows the engine to gradually convert maximum inertial resistance (vehicle at rest) to a combination of momentum and inertial conditions. (For now, let’s define momentum as the moving force of a vehicle in terms of what it would require to bring velocity back to zero; in other words, to stop vehicle movement. Try to stop it with your hands and you’d feel the effects of momentum, among other items.)
Since most of us have used a “pry bar” or “simple lever” or other form of mechanism that provided a measure of mechanical advantage (see Figure A), it should be obvious that the transmission of engine torque is best performed through a “mechanical advantage” device, hereinafter called a transmission.

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Clutches are a positive connection between engine and transmission. This means that, with the exception of slippage expected during normal engagement of a clutch, there is a one-to-one correspondence between engine rpm and drive-shaft rpm (based on interconnecting gear ratios). But if there is a substitution made for a clutch such that a positive connection is not provided, we enter the world of “fluid clutches” in which two or more finned, concave shells face each other in the presence of a fluid, such as oil. And while all this may seem a bit complicated, let’s examine it in terms of coffee-break terminology.

A. The “actual mechanical advantage” of a system can be defined as work divided by the force required to produce this work. The degree of mechanical advantage is related to the ratio of distances (as in the cases shown) Y:X as in illustration (a) and L:M as in (b). For an increase in either Y or L, with no change in X or M, the mechanical advantage increases proportionately. Pulling the nail or lifting the box gets easier. B. Little needs to be said here. This is a typical spur gear, but the terminology is applicable to other types of gears. Not shown, but worth mentioning, is the relationship between tooth space width and the thickness of a meshing tooth on the pitch circles. This additional space is called “backlash” and is a normal feature of meshing gears, just about regardless of design or type. C. This is an example of external spur gear mesh and direction of rotation (a & b) and internal spur gear mesh and rotational direction (c & d). Note that, for external mesh, the direction of rotation is opposite, one gear to the other. For internal mesh, rotational direction is the same. This comes into play in the section on planetary gear trains. Watch for it as a coming attraction in local theaters.

Visualize a grapefruit cut in half as you would if ordering in a coffee shop. Also picture what you’d have if all the “pulp” was removed and only the “fins” were left in each half. Now consider what we’d have if both halves were placed back together (with minimal air gap between them) and submersed in a bath of lightweight oil. Assume that one half of our transmission “grapefruit” is connected to the engine’s crankshaft (or bolted to some form of ring gear carrier) and the other to the driveshaft. Depending upon such variables as oil viscosity (fluid “thickness”), engine rpm, fin design and number, and proximity of the two halves to each other, the engine side of the system will begin to transmit torque to the driveline side as rpm builds.
In actuality, this is the basis of either a’ fluid coupler or torque converter with the basic difference being fin design, pitch relative to the axis of the converter housings, and how oil is directed out of the driving half of the system into the driven half. Basic fluid couplers have fins with little or no pitch (fins that do not line along radial lines of the converter housings) and rely simply on the centrifugal force (on the oil) to impart motion to the driven member as the oil passes from the driving to the driven member.

Conceptually, it’s like placing an electric fan in front of a second fan, using air movement from the driving fan to turn the second fan not connected to a source of power. And while oil viscosity has an effect on the amount of torque transmitted, it is the centrifugal forces on the oil passing from driving to driven member (in conjunction with fin pitch) that is the more influential. If, however, we were to design a system of vanes or fins that had both pitch and specific pitch direction relative to other fins in the system, we could arrive at a fluid coupler that provided some amount of torque multiplication from driving to driven member. Such units multiply torque as a function of the difference in rotational speed between the driving and driven housings (sometimes called the “pump” and “turbine”). And the greater the speed difference, the greater will be the torque multiplication. These are called torque “converters” and are commonly found in most of today’s automatic transmissions. So to this point, we have discussed

two basic types of clutching mechanisms located ahead of whatever gearing stands between engine and drive (or propeller) shaft. Now suppose we consider some of the fundamentals of gears as could be applied to both automatic and non-automatic (standard shift) transmissions. For the sake of simplicity, let’s examine the standard shift gears first. They’re easier for us to understand. And if you’re confused at the end of this section, you can skip over to the True or False question section—we probably will.
Some common gear terminology can be found in Figure B. Here you can see the relationships among base circle, pitch circle, face and flank. This is a basic involute tooth design for which your dentist probably has little interest, and space doesn’t warrant further explanation. But it’s a common, frequently used design in transmission gearing (standard and automatic).

D. Although no gear teeth are shown in this schematic of a planetary system, visualize teeth on the inside diameter of the outside gear, on the outside diameters of the planetary gears, and on the outside diameter of the sun gear. This gear train forms the basis for a majority of automatic transmission gear reduction. Ratio changes are accomplished by control of the rotation of either sun or outside gears. Usually, an automatic transmission will contain two or more planetary gear trains. This is also a common method for accomplishing overdrive gear reduction on the output side of a transmission. E. The ability to stop, hold and release the outer gear of a planetary system is normally accomplished by a spring steel band (internally lined with a friction material) operated by a combination of hydraulic and spring pressure working on a “servo diaphragm” connected to one side of the band. In conjunction with clutch packages that affect sun gear movement, gear ratio changes are made to suit a variety of driving conditions.

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The basic types are comprised of (1) spur gears, (2) helical gears, (3) bevel gears, (4) spiral gears, (5) hypoid gears and (6) worm gears. Essentially, spur gears have teeth cut parallel to the axis of gear rotation and straight across the gear’s face. Helical gears have teeth cut along the line of a helix (a straight line marked on a rotating cylinder). Bevel gears have teeth cut on the surface of a cone and are sometimes called “bevel spur” gears. This is a common design for differential gears. Spiral gears have teeth cut on a line traced on a rotating cone. And worm gears have teeth much like the threads on a bolt.
Of this group, most standard shift transmissions are a combination of spur, helical and sometimes spiral gears. These are typically arranged on both common and adjacent shafts. Gear sets or systems (trains) in which some of the gears rotate on fixed axes while others turn on axes which are themselves in motion and are called “epicyclic” gear trains. You might want to keep this term in mind when we get into the typical sets of drive gears employed in automatic transmissions.
In the conventional standard shift transmission, there are combinations of gears (thus providing different gear ratios) arranged in either constant mesh or “selective sliding” systems. By gear ratio, we mean the rotational relationship between two or more gears in which one gear turns through a certain number of revolutions for some amount of rotation of the gear with which it is meshed.

For example, let’s say that we have two meshed gears whereby one turns through two revolutions while the other revolves through one. This would give a ratio of 2:1. Stated another way, one of these gears might have 15 teeth and the other 30. The ratio would remain 2:1, since arithmetically we simply divide the smaller number of teeth (driving gear) into the larger (driving gear) and compare the answer in a ratio to 1. If the larger gear had 35 teeth and the smaller (pinion) had 10, the ratio would be 3.5:1 (as in rear gear ratios).
By further definition, sliding gears move back and forth along a splined shaft until in full contact with another gear. This engages the gears. Disengagement is accomplished when the sliding gear slides completely out of mesh with the other gear.
Constant mesh gears never come out of mesh. Their ability to transmit torque is accomplished by means of a coupling device which either allows the gear in constant mesh to be locked to its shaft or free to turn without transmitting any power. But in either case, it stays in mesh with another gear all the time, waiting for the coupling device to hook it into the power transmission system. And finally, the last common gear function in the standard transmission is the so-called “idler” gear.

Such a gear is normally located between driving and driven gears so that the ratio relationship between these two gears is not altered, regardless of how many teeth a given idler may have or how many are in the system (train). Changes in gear rotational direction are accomplished by reverse idlers, but ratios do not change as a result. In combination with the number of forward and reverse (a very important transmission function) speeds desired, standard shift gears are fitted into a gear case and operated by shifting “forks” or similar yokes designed to move sliding gears and couplers (synchronizers) back and forth. Synchronizers (or syn-chromesh couplers) normally move on splines on the transmission’s main shaft and accomplish two jobs. First, they allow the changing of gear ratios (shifting gears) without the sliding of gears into mesh. Remember the constant mesh gears? And they also synchronize the rotational speeds of the shaft and gear before mesh is accomplished.
Power flow through a standard shift transmission is therefore achieved by the movement of sliding gears or activation of constant mesh gears, both of which allow gears of different size (or tooth number) to transmit torque from input shaft to output shaft.

F. Schematically, this is the “two halves of a grapefruit” analogy of a fluid converter. Fins are positioned so they lie on radial lines inside each “dish” so there is no pitch to fin position. Centrifugal action of the oil inside the driving half of the coupler causes rotation of the driven half. And while this illustration may not be as descriptive as the two grapefruit halves, it contains far fewer carbohydrates and isn’t as sticky. G. This is called a “three-member” torque converter. In it you can see that a third member (as compared to the fluid coupler) has been added in the form of a “stator” which is mounted on a sprag-type clutch (it can turn in only one direction). The stator’s function is to alter the direction of oil flow from converter pump to turbine. It also reacts to oil delivery from the converter pump in such a way that torque multiplication is enhanced.

Rotational direction reversal of the output shaft is usually accomplished by the use of a reverse idler gear. Fully synchronized standard shift transmissions use synchronizer couplers for all forward speed gears, and employ helical gears in constant mesh (except for reverse gear, and low and reverse sliding units). Faster shifting up and down, reduced gear wear, and generally smoother operation characterize fully synchronized standard shift transmissions.
Overdrive units, intended to reduce engine speed to something less than a 1:1 final-drive gear ratio at the transmission output shaft, employ a gear arrangement frequently called a “planetary” system (see Figure D). And since this arrangement of gears is also often used in the gearbox of automatic transmissions, it’s a logical bridge in our discussion from one trans type to the other. But first, a word about what a planetary system actually is.
By definition, we have a gear with internally (or externally) cut teeth around which one or more smaller gears track while rotating about its (their) own axis (axes). Such “planetary” motion sees small gears rotating about their own axes while also rotating about another common to all the smaller gears. If you’ll refer to

Figure D for a moment, note that if the sun gear was held stationary and the pinion carrier (planetary gear carrier) rotated (let’s say clockwise), the direction of rotation of both the planetary gears and outside gear would also be clockwise. Study this for a couple of minutes, for it is this inter-gear relationship that forms the basis for gear ratio up and down in an automatic transmission. This way (planetary method), it’s possible to have both the input and output shaft rotating on the same axis, yielding a good selection of gear ratios in a relatively small space.
By providing some external means for holding the outside gear stationary (see Figure E), a change in gear ratio by utilization of planetary gear size with respect to sun gear size can be made. This typically involves the use of hydraulic pressure, a control valve (sometimes called throttle or shift valves), a servo mechanism, an oil control system or circuit, and bands and clutch discs designed to stop outer gear rotation and transmit power from an input to output shaft (or change ratios). Bands are normally constructed of spring steel, formed in a circle, lined on the inside surface with some form of friction material, and designed to fit around an outer gear drum. Apply and release pressure work against the servo piston’s diaphragm which, in turn, causes the band to stop or release the rotating outside gear ring.

Clutch packages, on the other hand, are used to stop, hold, or release gears within the planetary system (frequently the sun gear) so that changes in gear ratio can be made for different driving conditions. Hydraulic pressure is applied to the clutch pack in most cases, but sometimes spring pressure holds the pack together and hydraulic pressure is used to disengage the unit.
Power flow through an automatic transmission varies somewhat with origin of manufacturer and is beyond the scope of this particular Series. But it essentially involves the use of a fluid coupler (converter) which damps power pulses from the engine while transmitting torque into the “gearbox” segment of the transmission. From here, planetary gear arrangements (usually more than one) are operated by a combination of hydraulic and spring pressure to engage or disengage sun and/or outer gears in order to provide desired gear ratios, both through the trans and at the transmission’s output shaft.
Due to the buildup of transmission fluid heat, resulting from mechanical shear within the converter and frictional heat among clutch packs, some form of external cooling is generally required for automatic transmissions. This can be accomplished by means of an air-washed heat exchanger or by passing the hot fluid through cooling coils located inside a tank of the engine’s radiator. As you might expect, some amount of transmitted torque efficiency is lost in the converter, depending upon its design and point where converter “lockup” takes place (relative to engine rpm). But with the sophistication of today’s automatic transmission technology, it’s a fairly safe guess that these types of transmissions will soon be as efficient (in terms of impact on fuel economy and torque loss efficiency) as a standard shift unit of comparable capability. And unless you’re suffering from a light case of temporary brain fade, it’s far easier to avoid missed shifts if it’s being done for you. The shifting, not the missing, right?
By the way, if you didn’t cross the room at the beginning of this episode, perhaps you’ll want to now. It’s our way of breaking the inertia at the end of this transmission.

REVIEW QUESTIONS: True or False
1. Torque converters are normally designed to convert horsepower into torque as applied to automatic transmissions.
2. Inertia is a term describing how much moving energy a body or system has after the body or system has been set into motion.
3. By “mechanical advantage” we mean your transmission knows something you don’t.
4. The basic difference between a fluid coupling and a torque converter is the design and arrangement of the fins used to move fluid from the driving portion of the unit to the driven portion.
5. Helical gears have teeth cut along a line that lies in the surface of a cone.
6. Spur gears have teeth cut with the face of each tooth lying perpendicular (90 degrees) to the axis of the gear.
7. Planetary gear trains can also be called “epicyclic” gear trains.
8. Gear trains are a common means of transporting unfinished gear blanks from foundry to manufacturer.
9. Standard transmissions are often combinations of helical, spur, and sometimes spiral, gears.
10. Gear ratios are mathematically determined by dividing the number of driven gear teeth by the number of driving gear teeth.
11. Synchronizers are used to engage (or disengage) constant mesh gears with a shaft with which the constant mesh gear is splined.
12. Idler gears are used for small changes in effective ratio between two or more gears.
13. Planetary gear trains (or systems) are characterized by two or more sun gears.
14. In an automatic transmission, clutch packages are used to transmit torque, and hydraulically operated bands are used to keep gear ratios constant.
15. Fourteen questions would have been sufficient for this month’s Series.

 
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