By Daniel P. Duffy

A vehicle's power train consists of an engine (possibly with a variable geometry turbocharger), an automated or manual transmission, flexible driveshafts and wheels, and a chassis.

There are three basic types of transmissions: manual, automatic, and auto-shift manual - each typically used for a different type of vehicle or equipment. The transmission is the most complicated system in most vehicles and is more prone to wear, tear, and breakdown than any other component, so proper care and maintenance are essential. Because transmissions can be used to do more than simply drive vehicles or other types of equipment, aftermarket add-ons, including electronic shifters, retarders, and power take-off (PTO) units, are common.

What Does a Transmission Do?

The transmission is the intermediary between the engine and the driveshaft, delivering power to the wheels. Cars need transmissions because of the heat and operational limitations of a diesel engine. Any engine has a maximum revolutions per minute (rpm) value above which it cannot go without exploding. This is the engine's redline. Engines have narrow rpm ranges when horsepower and torque are at their maximum. Horsepower is the engine's rating of the amount of work it can perform, equivalent to lifting 550 lb. a height of 1 ft. in one second. Torque is defined as the force exerted on or by a driveshaft without reference to the speed of the shaft's rotation. In other words, torque is the force causing the rotation (often measured in foot-pounds). If the horsepower produced by an engine is constant, the torque delivered by the engine to the driveshaft will be inversely proportional to the rpm of the driveshaft.

The transmission consists of a series of mounted gears of varying diameters and numbers of teeth. Gear teeth have two primary functions: preventing slippage between the gears and allowing for exact gear ratios (the ratio between the number of teeth of two connected gears; e.g., if one gear has 60 teeth and another has 20, the gear ratio when they are connected is 3:1). Note that the number of teeth determines the gear ratio, not the diameters of the gears.

Operating the transmission changes the gear ratio (and resultant rpm) between the engine and the driveshaft. Hypothetically, a gear ratio of 3:1 would reduce the rpm of an engine operating at 3,000 rpm to a delivered rpm at the driveshaft of 1,000. If the gear ratio between the engine and the driveshaft changes as the velocity changes, the engine can operate in its most efficient range while staying below the redline. A five-speed transmission applies one of five gear ratios to the input shaft to produce a different rpm value at the output shaft. Here are some typical gear ratios:

Note that a lower gear setting results in a higher gear ratio. This reduces the rpm delivered by the engine to the driveshaft, resulting in a lower operating speed but a higher torque. This is suitable for the initial movement of a vehicle (as there is no momentum to assist the engine's applied torque). It is also important for situations in which delivering power to the driveshaft is more important than increased speed, such as climbing a steep hill with a heavy load). A higher gear setting results in a lower gear ratio. This increases the rpm delivered by the engine, resulting in a higher speed but a lower torque. This is suitable for vehicles already in motion that require speed more than torque, such as vehicles traveling over highways and roads.

The clutch is the mechanism that connects the engine and the transmission. By pushing down on the clutch pedal (disengaging the clutch), the operator disengages the engine from the transmission. When the clutch is engaged, the input shaft of the transmission turns at the same rpm as the engine. The clutch allows for the gradual application of engine power when a vehicle is starting. This same gradual application force helps prevent the grinding of gears when shifting. The basic components of a clutch are the flywheel, clutch disc, pressure plate, and throw-out bearing.

The flywheel is attached to the engine's crankshaft and provides a friction surface that the clutch can contact. Its main function is to transfer engine torque from the engine to the transmission. The clutch disc is a steel plate located between the flywheel and the pressure plate. It is covered with a frictional material that allows it to transfer torque from the flywheel to the pressure plate when the clutch is engaged. The pressure plate is a spring-loaded clamp that squeezes the clutch disc and flywheel, allowing the transfer of torque from the engine. The throw-out bearing releases the pressure applied by the plate, interrupting the flow of power from the engine. The throw-out bearing is activated when pressure is applied to the clutch pedal; this pressure is transferred from the pedal to the throw-out bearing along a hydraulic pressure line.

Manual Transmissions

With a manual transmission, the driver manually selects the proper amount of power and torque linking the power of the engine to the wheels. This operation involves a stick shift to change gears and a clutch to release the gear setting, which allows for the linking of a new gear set. Most manual transmission automobiles have four, five, or six forward gears and one reverse. Trucks and heavy earthmoving equipment can have as many as 20 forward gears and five or six for reverse. Manual transmissions allow greater control over the vehicle or equipment operation, are much less complicated than automatic transmissions, result in lower initial purchase and maintenance costs, and maximize fuel economy.

The simplest manual transmission is the sliding gear design. As described above, power enters the transmission via the clutch. An initial drive gear is used to turn a series of gears along a layshaft. All the gears on the layshaft turn at the same rpm but are used to turn gears with differing ratios. These gears spin on ball bearings attached to a third shaft, the main shaft, which directs power to the differential to turn the wheels. Directly attached to the main shaft with dogteeth is the collar plate, which is moved into place by shift linkages and forks attached to the gear shifter. By sliding the collar into place, the driver connects the gears with differing gear ratios to the differential. The engine power transmitted via the gears attached to the layshaft can then be transmitted along the main shaft to the differential. This allows the wheels to move at an rpm different from the engine's.

Although the sliding gear manual transmission provides the basis of all transmission operations, it has been made obsolete by the constant mesh transmission. The constant mesh uses a similar gear arrangement as the sliding gear type, but all the main shaft differential gears are in constant contact with the cluster gears and are free to rotate on the layshaft. All the gears in a constant mesh transmission are constantly turning, even when the transmission is in neutral. The heart of a constant mesh transmission is the dog clutch (not to be confused with the main drive clutch). Each gear has a dog clutch that is splined to the differential and has teeth to engage the main shaft gear when it is moved into contact with the gear. Each dog clutch is equipped with a synchronizer (a system of shifter plates, lock springs, and blocking rings) that prevents the gears from grinding.

Automatic Transmissions

Because automatic transmissions do not require either a clutch pedal or a manual gear shifter, they have become the system of choice for most collection fleet operators. Unlike a manual transmission, an automatic transmission achieves multiple gear ratios with a single set of gears. These gears are arranged in a device called a compound planetary gear set, which has three main components: a sun gear, planet gears (and their carriers), and a ring gear. Each can be the input or the output or can be held stationary. By varying their function, different gear ratios can be achieved, as shown in the following table:

A transmission with four forward and one reverse gear is summarized below:

The planetary gear set is driven by the torque converter, a fluid coupling that allows the engine to operate at a speed independent of the transmission. This eliminates the need for a drive clutch. A slowly turning (idling) engine transmits very little torque to the transmission, which is why only light pressure on the brake pedal is required to stop it. A rapidly turning engine (traveling at high speed) transmits a great deal of torque, so heavier brake pressure is required to stop a speeding vehicle. A torque converter consists of three parts: turbine, stator, and centrifugal pump. The turbine is attached to the engine's flywheel, transmitting its power to the torque converter. The pump flings transmission fluid to the perimeter of the converter, where it is directed back into the converter by way of the turbine. The stator, which is located at the center of the torque converter, uses a blade design that almost completely redirects the flow of transmission fluid. It is connected to a fixed shaft in the transmission by a one-way clutch, so it cannot spin with the fluid; it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades.

A hydraulic system is used to operate the clutches that engage the various gears within the compound planetary gear set. The hydraulic system is a maze of passages and tubes that sends transmission fluid under pressure to all parts of the transmission and torque converter. Transmission fluid serves a number of purposes, including shift control, general lubrication, and transmission cooling. Every aspect of a transmission's functions is dependent on a constant supply of fluid under pressure. In fact, most of the components of a transmission are constantly submerged in fluid, including the clutch packs and bands. The friction surfaces on these parts are designed to operate properly only when they are submerged in oil.

Clutches in an automatic transmission are more complicated than dog clutches in a manual transmission and are arranged in clutch packs. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Clutch packs consist of alternating layers of clutch friction material and steel plates. Springs make sure the clutch releases when the pressure is reduced. The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches and bands are energized at any given moment.

Auto-Shift Manual Transmissions

An auto-shift manual transmission (AMT), unlike a standard manual version, does not require either clutch actuation or gear shifting by the driver. These functions are controlled by a hydraulic system or an electric motor with the help of electronics and sometimes a computer. The clutch control depends on the position and movement of the selector lever. The shaft and fork (or dog clutch) connection between selector lever and transmission is eliminated, and the transmission is controlled electronically via shift-by-wire. Advantages of the AMT over automatic transmissions include greater efficiency, lower weight, and lower initial cost because manufacturers can use existing manual-transmission facilities. An existing manual transmission can be modified into an AMT by the adding on of automation components. The expense for automation, however, should not be underestimated. Many components are necessary to compensate for the omission of the clutch pedal and mechanical connection between the shift lever and transmission. Even so, an AMT is slightly less expensive than an automatic transmission.

Proper Care and Maintenance

Most transmission troubles are with overheating. Because they have more moving parts, automatic systems are much more likely to succumb to wear and tear; manual transmissions are more prone to breakage from misuse. Transmissions can overheat because of heavy loads, use in mud or snow, continuous stop-and-go traffic in hot weather, and so on. At high temperature, transmission fluid burns, losing its lubricating qualities, becomes oxidized, and leaves deposits inside the transmission. Exposed to the heat, the rubber seals and gaskets inside the transmission become hardened, causing leaks; the metal parts warp and lose their strength. This eventually results in transmission failure.

According to the Web site www.familycar.com/transmission.htm, "Transmission fluid should be changed [as recommended by the owner's manual, which could be intervals] anywhere from 15,000 miles to 100,000 miles. Most transmission experts recommend changing the fluid and filter every 25,000 miles. Few transmissions have drain plugs to drain the old fluid. In order to get the fluid out, the technician removes the transmission oil pan. Even if the transmission has a drain plug, the only way to also change the transmission filter is to remove the pan. When the pan is down, the technician can check for metal shavings and other debris, which are indicators of impending transmission problems. In most cases during these transmission services, only about half the oil [can] be removed from the unit. This is because much of the oil is in the torque converter and cooler lines and cannot be drained without major disassembly. The fluid change intervals are based on the fact that some old fluid remains in the system. Each transmission manufacturer has their own recommendation for the proper fluid to use and the internal components are designed for that specific formula."

Aftermarket Equipment

With aftermarket add-ons, transmissions can be modified to assign power to other work than driving a vehicle; modified to allow for pushbutton, electronic shifting of gears; and adjusted to minimize the range of duty cycles.

PTOs are used to transmit the drive power of the transmission to an auxiliary work unit, such as a power winch. There are three general types of PTOs:

Engine-dependent PTOs are used to deliver high levels of constant power in continuous operating mode at very high torque. They can be started and stopped while engaged in work and can be operated while stationary or while the vehicle is in motion, though typically the vehicle is stationary as the power delivered by the engine via the PTO reduces the power available for movement. Applications include high-pressure sewer pipeline rooting equipment, high-pressure pumps for fire-fighting vehicles, high-pressure drain-cleaning and suction vehicles, drilling rigs, cement mixers, and nonsubmersible pumps.

Clutch-dependent PTOs come can be operated intermittently and continuously, with the vehicle in motion or stationary. A transmission can be fitted with several clutch-dependent PTOs, each of which can operate independently by use of the standard gearshift. Applications include all types of pumps (water, slurry, hydraulic, and air), air compressors, and such mechanical devices as winches, fire-fighting ladders, and lifting platforms.

Road speed-dependent PTOs are used on multiaxle highway trucks as a safety backup system. A road speed-dependent PTO supplies the hydraulic system on multiple steering units with necessary operating pressure. Should the truck experience engine failure, its several steered axles allow the driver to retain steering control.

Electronic shifters are noncontact devices that allow the driver to simulate exact shifting parameters of his vehicle. Some semiautomatic transmissions come with an optional handlebar-mounted electronic shifter to eliminate the need to use the foot to shift. A high-speed electronic shifter can reduce by 80% the time required for numerical shifting and eliminates numerous tubes and program units of the vehicle's hydraulic control system. Electronic shifters can be mounted in the same locations as a stick shift: either next to the driver or (less frequently) on the driving column.

A retarder is a shift-lever handle that attaches to the steering column of a vehicle and gives an electronic pulse-width-modulated output signal that relates the position of the lever. It provides a secondary means of slowing a vehicle, reducing brake wear. A retarder is connected to the throttle, but it brakes only the drive axle. It is activated when a driver releases the throttle. The transmission retarder functions by creating resistance to slow the transmission output shaft, which is connected to the main driveshaft. A retarder provides continuous and limited braking torque, the amount of which depends on its setting and the vehicle's driveline speed. Though there is a slow reaction time (more than one second), the retarder allows the brake force to be smoothly transmitted through to the driving wheels only. This allows for greater control when braking. Retarders are not normally used in icy road conditions, however, because its link to the drive axle could reduce driver control. For the same reason, if a retarder is designed to activate 100% when the foot is removed from the throttle, the back end of a truck or bus or truck could slide when turning a corner.

Daniel P. Duffy, P.E., is a professional environmental engineer in Cincinnati, OH.

MSW - July/August 2003