Standard Transmissions

1
Chapter 15

• Standard Transmissions

2
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.

3
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.

4
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.

5
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

6
Gear Design (1 of 2)
7
Gear Design (2 of 2)
8
Gear Ratios
9
Speed Versus Torque
10
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.

11
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 1512 here

12
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.

13
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.

14
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.

15
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.

16
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).

17
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.

18
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.

19
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.

20
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.

21
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.

22
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.

23
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.

24
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.

25
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

26
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

27
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

28
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.

29
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.

30
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.

31
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.

32
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.

33
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.

34
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.

35
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).

36
Shift Bar Housing (2 of 2)
37
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.

38
Operation (2 of 2)
39
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.

40
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.

41
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.

42
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.

43
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.

44
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.

45
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 twicethe
  first time with the auxiliary section engaged in
  low gear or low range the second time engaged in
  high gear or high range.

46
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.

47
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

48
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.

49
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

50
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.

51
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.

52
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.

53
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.

54
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.

55
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.

56
Slave Valve In Neutral Position
57
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.

58
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.

59
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.

60
Eighteen-speed Transmissions (1 of 2)
61
Eighteen-speed Transmissions (2 of 2)

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

62
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.

63
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.

64
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.

65
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.

66
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.

67
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 ratiosfifth, sixth,
  seventh, and eighth.

68
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.

69
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.

70
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.

71
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.

72
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.

73
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.

74
Transfer Cases (3 of 6)

• Transfer cases can transfer drive torque directly
  using a 11 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.

75
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.

76
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.

77
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.

78
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.

79
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.

80
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 crankshaftdriven PTO, a flywheel PTO
  permits continuous operation.

81
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.

82
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.

83
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 drivers 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.

84
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

85
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.

86
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.

87
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.

88
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.

89
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.

90
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.

91
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.