chapter 15 bennet

Chapter 15

Standard Transmissions

Objectives (1 of 3)

• Identify the types of gears used in truck transmissions.

• Interpret the language used to describe gear trains and calculate gear pitch and gear ratios.

• Explain the relationship between speed and torque from input to output in different gear arrangements.

Objectives (2 of 3)

• Identify the major components in a typical transmission including input and output shafts, mainshaft and countershaft gears, and shift mechanisms.

• Describe the shift mechanisms used in heavy-duty truck transmissions.

Objectives (3 of 3)

• Outline the role of main and auxiliary (compound) gear sections in a typical transmission, and trace the powerflow from input to output in different ratios.

• Describe the operating principles of range shift and splitter shift air systems.

• Define the roles of transfer cases and PTOs in heavy-duty truck operation.

Gears

• A gear can be used in any of the following roles: – The shaft can drive the gear.– The gear can drive the shaft.– The gear can be left free to turn on the shaft (it idles).

• Sets of gears can be arranged to do the following:– Multiply torque and decrease speed– Increase speed and decrease torque– Transfer torque and speed unchanged

Gear Design (1 of 2)

Gear Design (2 of 2)

Gear Ratios

Speed Versus Torque

Idler Gears• An idler gear may be used

to transfer torque without changing the direction of rotation.

• Idler gears are also used to provide reverse gearing.

• If two idler gears are used, the driven gear will rotate in the opposite direction of the drive gear.

• Idler gears can also transfer power in place of a chain drive or belt drive.

• Idler gears do not affect the relative speeds of either the drive or driven gears.

Spur Gears• Teeth are cut straight,

parallel to the shaft. • Only one tooth is in full

contact at any given moment.

• Spur gear teeth minimize the possibility of popping out of gear.

• For this reason, spur gears are often used in the reverse gear train.

• A disadvantage of spur gears is noise.– At higher turning speeds,

their clicking noise becomes a constant whine.

• Figure 15–12 here

Helical Gears• Teeth are cut at an angle.

(helical to the axis of rotation) • Two or more teeth may be in

mesh at the same time providing more evenly distributed load.

• They are useful in applications requiring high torque to transfer loads.

• They perform more quietly than spur gears because they mesh with mating gears with a wiping action.

• The main disadvantage of helical gears is the longitudinal thrust they create during operation.

Gear Train Configurations• Twin-countershaft

transmissions deliver torque equally to two countershafts with each gear set carrying only half of the load.

• Torque path travels through the countershaft(s) until it reaches the selected gearing.

• This gearing routes the torque path back to the mainshaft and from there to any auxiliary gearing present.

Sliding Gear Shift• Gears on the mainshaft are

moved until they mesh with the desired gear on the countershaft.

• Spur-cut sliding gears are needed.

• Shifting is unsynchronized; grinding and gear clash are a problem.

• Sliding gears are prone to gear chipping and fracture.

• Currently, the only gear ratios using sliding gears are first and reverse.

First Gear (1 of 3)

• When the shift fork or yoke is moved by the gearshift lever in the cab, it slides the collar and gear either to the front or rear of the transmission housing.

• Sliding it forward (to the left) engages the first and reverse sliding gear on the mainshaft with the first gear on the countershaft.

First Gear (2 of 3)

• This results in directing powerflow through the first gear as shown.

• The torque path is as follows: It flows from the engine flywheel through the clutch plate splines to the transmission input shaft, then (2) through the input shaft gear (5) to the countershaft-driven gear (6).

First Gear (3 of 3)

• Powerflow is then transmitted through the countershaft to the first gear (15) and up to the first and reverse sliding gear (18) on the mainshaft (16).

• Because the first and reverse sliding gear is splined to the mainshaft, powerflow is directed through the mainshaft and out to the vehicle driveline.

Reverse Gear• The shift fork forces the first

and reverse sliding gear backward until it engages with the reverse idler gear.

• The reverse idler gear allows the first and reverse sliding gear to rotate in the same direction as the reverse gear (20) on the countershaft.

• Powerflow now runs from the input shaft (2) to the input shaft gear (5) and countershaft-driven gear (6), then down the countershaft to gears 20 and 21.

• From the first/reverse sliding gear (18), torque is transferred to the mainshaft (16) and out to the driveline.

Collar Shift (1 of 7)

• In a collar-shifting arrangement, all gears on the countershaft are fixed to the countershaft.

• The mainshaft gears are free to freewheel (float) and do so around either a bearing or bushing.

• The mainshaft gears are in constant mesh with their mating countershaft gears.

Collar Shift (2 of 7)

• The input shaft (2) rotates at engine speed any time the clutch is engaged.

• The input shaft gear (5) is integral with the input (clutch) shaft, so it has to rotate with it.

• The input shaft gear meshes with the countershaft-driven gear (6), the countershaft, and all the gears fixed to the countershaft also have to rotate.

• The countershaft gears transfer torque to their mating gears on the mainshaft.

Collar Shift (3 of 7)

• But mainshaft gears 8, 11, and 13 all freewheel on the mainshaft.

• Because they are freewheeling, they cannot transmit torque to the mainshaft, so it does not turn.

• This means nothing is output to the driveline.

• To enable torque transfer to the mainshaft, one of the freewheeling mainshaft gears must be locked to it.

Collar Shift (4 of 7)

• The shift gear is internally splined to the mainshaft at all times.

• The shift collar is splined to the shift gear. • The main gears have a short, toothed hub. • The teeth on the main gear hub align with the teeth

on the shift gear. • The internal teeth of the shift collar mesh with the

external teeth of the shift gear and hub.

Collar Shift (5 of 7)

• When a given speed range is not engaged, the shift collar simply rides on the shift gear.

• When the driver shifts to engage that speed range, the shift fork moves the shift collar and slides it into mesh with the teeth of the main gear hub.

• At this moment, the shift collar rides on both the shift gear and main gear hub, locking them together.

Collar Shift (6 of 7)

• Power can flow from the main gear to the shift gear, then to the mainshaft and out to the propeller shaft.

• A second, more common method of locking main gears to the mainshaft does not use a shift gear.

• Instead, the shift collar is splined directly to the mainshaft.

• This shift collar, also called a clutch collar or a sliding clutch, is designed with external teeth.

Collar Shift (7 of 7)

• These external teeth mesh with internal teeth in the main gear hub or body when that speed range is engaged.

• Most shift collars or sliding clutches are positioned between two gears so they can control two-speed ranges depending on the direction in which they are moved by the shift fork

Third Gear Power Flow

• Moving from neutral to third gear moves the second and third shift collar (or sliding clutch) (12) forward toward the third gear (11), locking it to the mainshaft.

• Power flows from 2 to 5 and 6, along the countershaft to 10, up to 11, through the shift collar (12) to the mainshaft and out to the driveline

Fourth Gear Power Flow

• After shifting from third to neutral, the neutral to fourth gearshift causes the shifter fork to move the fourth and fifth shift collar (or sliding clutch 7) into mesh with the fourth gear (8).

• Power now flows from 2 to 5 and 6, along the countershaft to 9, through 8 and 7 to the mainshaft, and out to the driveline

Fifth Gear Power Flow• The shifter fork moves the

fourth and fifth shift collar (or sliding clutch 7) into mesh with the input shaft gear (5).

• This locks the input shaft (2) directly to the mainshaft (16). Input and output speeds are the same.

• The power flow is from 2 to 5 through 7, then to the mainshaft and out.

• The countershaft and its gears are all turning. The mainshaft gears 8, 11, and 13 are also freewheeling on the mainshaft, but have no effect on the powerflow.

Shop Talk

• The clutch brake is used to stop gear rotation in order to complete a shift into first or reverse when the vehicle is stationary. The clutch brake is actuated by depressing the clutch pedal completely to the floor. For normal upshifts and downshifts, only partial disengagement of the clutch is needed to break engine torque.

• The 750 rpm drop used in the description of shifting procedure varies according to engine-governed speed and torque rise profile.

Block or Cone Synchronizers (1 of 4)

• The synchronizer sleeve is splined to the clutch hub.

• The clutch hub is also splined to the transmission output (main) shaft.

• The synchronizer sleeve has a small internal groove and a large external groove in which the shift fork rests.

• Three slots are equally spaced around the outside of the clutch hub.

Block or Cone Synchronizers (2 of 4)

• Inserts fit into these slots and are able to slide freely back and forth.

• These inserts are designed with a ridge in their outer surface.

• Insert springs hold the ridge in contact with the synchronizer sleeve internal groove.

Block or Cone Synchronizers (3 of 4)

• Brass or bronze synchronizing blocker rings are positioned at the front and rear of each synchronizer assembly.

• Each blocker ring has three notches equally spaced to correspond with the three inset notches of the hub.

• Around the outside of each blocker ring is a set of beveled dog teeth, which are used for alignment during the shift sequence.

• The inside of the blocker ring is shaped like a cone.

Block or Cone Synchronizers (4 of 4)

• This coned surface is lined with many sharp grooves. • The cone of the blocker ring makes up one-half of a

cone clutch assembly. • The second or mating half of the cone clutch is part of

the gear to be synchronized. • The shoulder of the main gear is cone shaped to match

the blocker ring. • The shoulder also contains a ring of beveled dog teeth

designed to align with dog teeth on the blocker ring.

Plain Synchronizers

• It is like a block synchronizer with fewer parts.

• The hub is internally splined to the mainshaft.

• Mounted on the hub is a sliding sleeve controlled by the shift fork movement.

• The friction generated between the hub and the gear synchronizes the speeds.

• Pressure on the sliding sleeve prevents it from engaging the gear teeth until sufficient pressure has caused synchronization.

• The sleeve teeth then engage the gear teeth.

Shift Bar Housing (1 of 2)

• The shift bar housing contains the components required to convert gear stick movement into shifts within the transmission.

• This is an exploded view of a typical shift bar housing assembly such as one commonly used for five-speed main box.

• This transmission is usually coupled to an auxiliary box or compound (used to multiply the number of available gear ratios).

Shift Bar Housing (2 of 2)

Operation (1 of 2)

• After a shift has been effected, the shift bar must be held in position with a detent mechanism.

• The detent mechanism consists of a spring-loaded detent steel ball or poppet.

• The spring loads the steel ball into the recess in the shift bar.

• The detent ball holds the shift bar in position and prevents unwanted movement of the other bars.

Operation (2 of 2)

Shop Talk

• In troubleshooting a transmission complaint of slipping out of gear, one of the first things you should check is the detent assemblies.– Broken springs and seized detent balls can

result in unwanted shift rail movement.

Twin Countershaft Transmissions (1 of 3)

• Most heavy-duty truck standard transmissions are compounded, usually with a single auxiliary section.

• Some have a main box and two auxiliary sections.

• Twin countershaft transmissions having nine to eighteen forward speed ranges are among the more common heavy-duty truck transmissions.

Twin Countershaft Transmissions (2 of 3)

• The countershafts on either side of the transmission split input torque equally.

• Because of this, the face width of the gears can be narrower.

• The mainshaft gears float between the countershaft gears when disengaged, eliminating the need for gear bushings or sleeves.

• When disengaged, the mainshaft gears freewheel around the mainshaft because they are in constant mesh with the countershaft drive gears.

Twin Countershaft Transmissions (3 of 3)

• The motion is not transferred to the actual shaft itself, however, until the sliding clutch gear is moved into engagement.

• The output shaft will then turn at the same speed as the mainshaft gear.

• The sliding clutch gear that engages with the mainshaft gear is typically splined to the mainshaft.

Powerflow in Low Range

• The input shaft and drive gear are in constant mesh with both countershaft drive gears.

• The countershaft gears are in constant mesh with the floating mainshaft gears.

• The mainshaft gears freewheel on the mainshaft.

• A sliding clutch gear splined to the mainshaft is engaged into the internal clutching teeth of the mainshaft gear, coupling it to the mainshaft.

• The mainshaft will now be turning at the selected gear ratio.

5 Speed Main + Auxiliary

• Two- or three-speed auxiliary section• Main shifted manually• Auxiliary air shifted

– Selection of the gears in the auxiliary section is made by a driver-actuated, air-operated piston.

• The driver uses a pneumatic switch, usually located on the gear lever, that moves the auxiliary section into low- or high-range ratios.

• The driver controls this range selection mechanism through the use of a master control valve switch mounted on the gearshift tower in the operating cab.

Auxiliary Gear Sections (1 of 2)

• Power is directed through the high-range (direct-drive) gearing of the auxiliary section.

• In this range, the sliding clutch gear locks the auxiliary drive gear to the output shaft.

• The low-range gear on the output shaft is now allowed to freewheel.

• The five-speed shifting pattern is used twice–the first time with the auxiliary section engaged in low gear or low range; the second time engaged in high gear or high range.

Auxiliary Gear Sections (2 of 2)

• By using the same shifting pattern twice, the shift lever position for sixth speed is the same as first, seventh the same as second, eighth the same as third, ninth the same as fourth, and tenth the same as fifth.

• This illustrates the gearshift lever pattern and range control button positions for this model transmission.

High-/Low-Range Shift Systems

• An air-operated auxiliary section gearshift system consists of the following:– Air filter/regulator– Slave valve– Master control valve– Range cylinder– Fittings and connecting air lines

Air System

• A typical air-operated gearshift control system used to engage high- and low-range gearing in the auxiliary section– Note the location of the range and splitter cylinders and how

they connect with the control pneumatics.

Air Filter/Regulator

• The air filter/regulator assembly:– Minimizes the

possibility of moisture-laden air or impurities from entering the system

– Reduces chassis system air-supply pressure to the range valve and the slave valve

Range Air System (1 of 3)

• Filtered: Air from the chassis air system is supplied to the air supply port on the air regulator.

• Regulated: When filtered, the air is then routed to the air regulator. Transmission air pressure is typically regulated at between 57 and 62 psi.

Range Air System (2 of 3)

• Slave valve: Next, the air passes through the 1/4-inch supply air line and 1/8-inch OD (overdrive) range valve supply air line to the supply ports of the slave valve and range valve.

• Range valve: Depending on the position of the gear shift-mounted range valve, air will pass through either the low-range air line or the high-range air line to the range shift cylinder.

Range Air System (3 of 3)

• Pre-selecting: Range shifts can be made only when the gearshift lever is in, or passing through, neutral. The driver can pre-select a range shift while in gear.

• Actuating plunger: As the gear lever is moved through neutral, the actuating plunger in the shift bar housing releases the slave valve, allowing it to move to the selected range position.

Slave Valve

• The slave valve can be of the piston or poppet type.

• The slave valve distributes inlet air pressure to both the low- and high-range air circuits

• The piston controls when and where air pressure is distributed.

Slave Valve In Low Range

• Slave valve operation in low range is illustrated.• An air valve shaft protruding from the shift bar

housing prevents the actuating piston in the slave valve from moving while the gear shift lever is in any gear position.

Slave Valve in High Range

• Slave valve operation in high range is illustrated. • An air valve shaft protruding from the shift bar

housing prevents the actuating piston in the slave valve from moving while the gear shift lever is in any gear position.

Slave Valve In Neutral Position

Range Valve

• Constant air pressure is supplied to the inlet port.

• In low range, this air passes through the valve and is routed to the slave valve end cap or P-port.

• In high range (control switch up), the valve slide prevents the air from passing through the range valve.

• Air pressure that was in the outlet line is now exhausted.

• This means that the transmission defaults to high range.

Split Shifting

• A typical splitter air system is equipped with both the high/low range selector and splitter selector mounted on the gear shift lever.

• The splitter gear system in a thirteen-gear transmission is used only while in high range and splits the high-range gearing into either direct or overdrive ratios.

• Splitter systems used on eighteen-gear transmissions are used to split both high- and low-range gearing.

Splitter Cylinder • Constant air is supplied to

the splitter cover and acts on the front side of the piston.

• An insert valve directs the air.

• In overdrive, air is routed through the shift tower valve and is supplied to the left port of the cylinder cover.

• In direct, the S-port of the shift tower valve is closed and no air is supplied to the left port of the splitter cylinder cover.

Eighteen-speed Transmissions (1 of 2)

Eighteen-speed Transmissions (2 of 2)

• See Table 15-1, page 453 of text book.

Low-range, Overdrive Powerflow

• The auxiliary drive gear splits torque between the two auxiliary countershafts.

• Torque is delivered through both countershafts to the low-range gear output shaft.– The high/low synchronizer is

used to lock this reduction gear to the output shaft.

• Torque is transferred to the output shaft through the sliding clutch of the synchronizer.

• Torque is delivered to the driveline as low-range overdrive.

High-range, Direct Powerflow

• In these gear selections (eleventh, thirteenth, fifteenth, seventeenth, and #3 Reverse), powerflow is through the rear auxiliary drive gear.

• This gear is locked to the auxiliary output shaft by the front/rear sliding clutch and the high side of the high/low range synchronizer.

• This locks the rear auxiliary drive gear directly to the output shaft.

High-range/ Overdrive

• In twelfth, fourteenth, sixteenth, eighteenth, and #4 Reverse, powerflow is through the front auxiliary drive gear, which is locked to the output shaft by the front/rear sliding clutch.

• Torque is then delivered through both auxiliary countershafts to the rear auxiliary drive gear.

• The rear auxiliary drive gear is locked to the output shaft by the high/low synchronizer.

Thirteen-speed Transmissions (1 of 4)

• Similar to the eighteen-speed transmission

• The auxiliary section contains:– A high-range gear – A low-range gear – An overdrive gear

• In some models, this overdrive gear is replaced with an underdrive gear.

Thirteen-speed Transmissions (2 of 4)

• The first five gear ratios occur with the range selector in its low-range (down) position.

• Torque is delivered along both countershafts to the engaged low-range gear on the range mainshaft or output shaft. – This creates low- range

power flows through the auxiliary gearing for each of the five speeds of the main section.

Thirteen-speed Transmissions (3 of 4)

• The driver shifts to the high range by pulling up on the range selector.

• This action moves a sliding clutch that locks the auxiliary drive gear directly to the range mainshaft or output shaft.

• Torque is delivered through the range mainshaft and/or output shaft as high-range direct power flows for the next four gear ratios—fifth, sixth, seventh, and eighth.

Thirteen-speed Transmissions (4 of 4)

• While in the high range only, the gear ratios can be “split” by moving the splitter control button to OD.

• This moves a sliding clutch that locks the overdrive splitter gear in the auxiliary section to the output shaft.

• Torque is delivered along both auxiliary countershafts to the auxiliary overdrive gears to the output shaft overdrive gear and out through the output shaft.

Deep-reduction Transmissions (1 of 3)

• The forward gear ratios are low-low, low, and first through eighth.

• Low-low is a special deep-reduction gear for maximum torque. – It is used to produce

maximum drivetrain torque for high-load, standing starts, using a deep-reduction gear in the auxiliary section.

– This low-low gear is engaged by activating a split shifter or dash-mounted deep-reduction valve.

– It can be operated in the low range only.

Deep-reduction Transmissions (2 of 3)

• Constant air is supplied to the reduction cylinder center port.

• With the deep-reduction lever in the “Out” position, the valve is opened and air is used to disengage the deep-reduction gearing.

• When the lever is moved to the “In” position, the valve is closed and no air is supplied to the center port.

• Constant air from the air filter/regulator assembly then moves the piston to engage the reduction gearing.

Deep-reduction Transmissions (3 of 3)

• Powerflow is routed through both countershafts and countershaft deep-reduction gears, to the output shaft deep-reduction gear, which is locked to the output shaft by the sliding clutch.

• In shifting from low-low to low, the driver double clutches, releasing the split shifter and moving to low range low.

• Low through fourth gears are low-range gear ratios.

• The driver then range-shifts into high range for gears five through eight.

Transfer Cases (1 of 6)

• A transfer case is an additional and separate gearbox located between the main transmission and the vehicle drive axles.

• It functions to distribute torque from the transmission to the front and rear drive axles.

• Although not commonly used in trucks intended primarily for highway use, transfer cases are required when axle(s) in front of the transmission are driven.

Transfer Cases (2 of 6)

• The term all-wheel drive (AWD) in heavy-duty trucks usually refers to a chassis with a front drive axle in addition to rear tandem drive axles.

• Vocational trucks use these three-axle drive configurations that are essential in some on/off and off-highway applications.

Transfer Cases (3 of 6)

• Transfer cases can transfer drive torque directly using a 1:1 gear ratio or can be used to provide low-gear reduction ratios additional to those in the transmission.

• The drop box design of a transfer case housing permits its front drive shaft output to clear the underside of the main transmission.

• Most transfer cases are available with power takeoff (PTO) capability and front axle declutch.

Transfer Cases (4 of 6)

• The front axle declutch is used to option-drive to the front axle when negotiating steep grades or slippery or rough terrain.

• Both the PTO and front axle drive declutch are driver-engaged by dedicated shift levers.

• In addition, a transfer case might be equipped with an optional parking brake and a speedometer drive gear that can be installed on the idler assembly.

Transfer Cases (5 of 6)

• Most transfer cases use a countershaft with helical-cut gears.

• The countershafts are usually mounted in ball or taper roller bearings.

• Some transfer cases use an auxiliary oil pump externally mounted to the transfer case.

• Transfer cases may also be equipped with a driver-controlled, air-actuated differential lockout to improve traction under extreme conditions.

Transfer Cases (6 of 6)

• Another type of transfer case is the cloverleaf four shaft design.

• This two-speed, four-shaft design can also be adapted to incorporate a PTO and a mechanical-type auxiliary brake.

Power Take-offs (1 of 5)

• A variety of accessories on heavy-duty trucks require an auxiliary drive.

• Auxiliary drive can be sourced directly from the engine or by means of the transmission or transfer case.

• Auxiliary drive systems on trucks are usually known as PTOs.– The PTO is simply a means of

using the chassis engine to power accessories, eliminating the need for an additional auxiliary engine.

Power Take-offs (2 of 5)

• There are six basic types of PTOs classified by their installation location or drive source: – Side-mount PTO is bolted

to the side of the main transmission and is the most common type found on trucks.

– A split-shaft PTO transmits torque from the chassis drive shaft and is located behind the transmission; split-shaft PTOs require special mounting to the chassis frame.

Power Take-offs (3 of 5)

• Clutch-type crankshaft-driven PTOs are used so that engagement/ disengagement can take place while the engine is running.

• A flywheel PTO is sandwiched between the bell housing and the transmission.

• Rear crankshaft or flywheel-driven: Like the forward crankshaft–driven PTO, a flywheel PTO permits continuous operation.

Power Take-offs (4 of 5)

• The objective of a PTO is to provide driving torque to auxiliary equipment such as pumps and compressors.

• The driven equipment can be mounted either directly to the PTO or indirectly using a small drive shaft.

• The PTO input gear is placed in constant mesh with a gear in the truck transmission.

Power Take-offs (5 of 5)

• Establishing the correct mesh between the PTO drive gear and its partner in the transmission is critical.

• Too much or too little backlash can produce problems.

• Gear ratio is also critical in PTO operation. – Gear ratio must be set to

the torque capacity and operating speed required of the driven equipment.

Summary (1 of 9)

• Engine torque is transferred through the clutch to the input shaft of the transmission, which drives the gears in the transmission. – The transmission manages the drivetrain.– It is the driver’s means of managing drivetrain torque

and speed ratios to suit chassis load and road conditions.

• A transmission enables the engine to function over a broad range of operating requirements that vary from a fully loaded standing start to cruising at highway speeds.

Summary (2 of 9)

• Light-duty truck transmissions have a limited number of gear ratios and a single set of gears called main gearing, contained in a single housing.

• Most heavy-duty truck transmissions consist of two distinct sets of gearing: the main or front gearing, and the auxiliary gearing located directly on the rear of the main gearing.

• Auxiliary gearing compounds the available ratios in a transmission. – Most heavy-duty trucks use at least one compound;

some use two compounds

Summary (3 of 9)

• Gear pitch refers to the number of teeth per unit of pitch diameter.

• The three stages of contact through which the teeth of two gears pass while in operation are coming-into-mesh, full-mesh, and coming-out-of mesh.

• The relationship of input to output speeds is expressed as gear ratio.

Summary (4 of 9)

• Torque increase from a driving gear to a driven gear is directly proportional to speed decrease. – So, to increase output torque, there is a

resultant decrease in output speed, and vice versa.

• The major types of gear tooth design used in modern transmissions and differentials are spur gears and helical gears.

Summary (5 of 9)

• A heavy-duty standard transmission consists of a mainshaft and one, two, or three countershafts.

• Standard transmissions can be generally classified by how they are shifted. – Sliding gear, collar shift, and synchronized

shift mechanisms are used to effect shifts in standard transmissions.

Summary (6 of 9)

• Synchronizers have two primary functions.– First, they bring two components rotating at

different speeds to a single, synchronized speed.

– Second, they lock these components together.• Block or cone and pin synchronizers are the

most common in heavy-duty transmissions.

Summary (7 of 9)

• Most standard truck transmissions use mechanically shifted main sections. Most current compounded transmissions use air controls to effect shifts in the auxiliary section, though some older trucks used gear levers for both main and auxiliary section shifts.

• An air-actuated gearshift system consists of an air filter and regulator, slave valve, master control valve, range cylinder, and connecting air lines.

Summary (8 of 9)

• Auxiliary section gearing can be optioned to include a third gear in addition to the high- and low-range gears. – This third gear is engaged or disengaged by a splitter

shift system air activated by a button on the shift lever.• A transfer case is an additional gear box located

between the main transmission and the rear axle. – Its function is to divide torque from the transmission to

front and rear drive axles and, in addition, to option driving force to the front axle.

Summary (9 of 9)

• The accessory drive requirements on trucks are met by using power takeoff (PTO) units.– Hydraulic pumps for pumping loads off trailers

and compressors for blowing loads off sealed bulk hoppers would be two examples of PTO-driven equipment.

• The six types of PTOs, classified by their installation, are side mount, split shaft, top mount, countershaft, crankshaft driven, and flywheel.