Clutches, Rear Axles, and Steering

Clutches

Eaton Valeo Clutch Break-In

With a new or newly installed Eaton Valeo clutch, the clutch may slip for a short time while the friction surfaces break-in. However, allowing the clutch to slip for more than two seconds can severely damage the clutch disc, pressure plate, and the flywheel.

During initial operation of a new vehicle or a vehicle with a new clutch, check for clutch slippage during acceleration. If the clutch slips, decelerate until the clutch does not slip. Allow the clutch to cool 15 to 30 seconds, and then gradually accelerate again. If the clutch continues to slip, repeat the procedure. If necessary, repeat the procedure up to five times. If the clutch slips after five attempts, stop the vehicle. Allow the clutch to cool for at least one hour. Notify your Freightliner dealer of the problem.

Do not allow sustained slippage of the clutch; this could severely damage the clutch disc, pressure plate, or flywheel. Damage caused by clutch slippage due to improper break-in is not warrantable.

General Information

The major reason clutches wear out too soon is excessive heat. Clutches are designed to absorb and dissipate more heat than encountered in typical operation. The temperatures developed in typical operation will not break down the clutch friction surfaces. However, if a clutch is slipped excessively, or required to do the job of a fluid coupling, high temperatures develop quickly and destroy the clutch. Temperatures generated between the flywheel, driven discs, and pressure plates can be high enough to cause the metal to flow and the friction facing material to char and burn.

Slipping and excessive heat are practically nonexistent when a clutch is fully engaged. But during the moment of engagement, when the clutch is picking up the load, it generates considerable heat. An incorrectly adjusted or slipping clutch will rapidly generate sufficient heat to destroy itself.

The most important items that a driver should be aware of to ensure long service life of the clutch include: starting in the right gear, recognizing clutch malfunctions, and knowing when to adjust a clutch.

Clutch Operation

Starting the Vehicle in the Correct Gear

To drive forward, always start in first gear. After the clutch is fully engaged, the engine should be accelerated to the correct rpm for the upshift into the next higher gear.

Gear Shifting Techniques

Many drivers upshift into the next gear, or even skip-shift into a higher gear, before the vehicle has reached the correct speed. This type of shifting is almost as bad as starting off in a gear that is too high, since the engine rpm and vehicle speeds are too far apart, requiring the clutch to absorb the speed difference by generating heat. For transmission operating instructions, see Chapter 5 .

The clutch brake (if equipped) is applied by fully depressing the clutch pedal. Its purpose is to stop the transmission gears from rotating in order to engage the transmission gears quickly in making an initial start.

Never apply the clutch brake when making downshifts or upshifts. The clutch pedal should never be fully depressed before the transmission is put in neutral. If the clutch brake is applied with the transmission still in gear, a reverse load will be put on the gear. At the same time, it will have the effect of trying to stop or decelerate the vehicle with the clutch brake. Rapid wear of the friction discs will take place necessitating frequent replacement. Considerable heat will be generated, which will be detrimental to the release bearings and transmission front bearings.

Vehicle Overload, or Overloading the Clutch

Clutches are designed for specific vehicle applications and loads. These limitations should not be exceeded.

Overloading will not only result in damage to the clutch, but also to the entire powertrain.

Riding the Clutch Pedal

Riding the clutch pedal is destructive to the clutch, since partial clutch engagement permits slippage, generating excessive heat. Riding the clutch pedal will also put a constant thrust load on the release bearing, which can thin out the lubricant. Release bearing failures can be attributed to this type of misuse.

Holding the Vehicle on an Incline With a Slipping Clutch

A slipping clutch accumulates heat faster than it can be dissipated, resulting in early clutch failures. Never use the clutch as a hill holder.

Coasting With the Clutch Released (Pedal Depressed) and the Transmission in Gear

Coasting with the clutch released and the transmission in gear can cause high driven disc rpm through multiplication of ratios from the final drive and transmission.

Do not coast with the clutch released (pedal depressed) and the transmission in gear. High driven-disc rpm could cause the clutch facing to be thrown off the disc. Flying debris could cause injury to persons in the cab.

Engaging the Clutch While Coasting

Engaging the clutch while coasting can result in tremendous shock loads and possible damage to the clutch, as well as to the entire drivetrain.

Reporting Erratic Clutch Operation Promptly

Reporting erratic clutch operation as soon as possible will give maintenance personnel a chance to inspect the clutch components.

On mechanical clutch linkages, free-pedal travel should be included and commented on daily in the driver's report, since clutch free-pedal travel is the maintenance department's guide to the condition of the clutch and the release mechanism.

Operating the vehicle with incorrect clutch pedal free-travel could result in clutch damage. See Group 25 of the Business Class® Trucks Service Manual for free-pedal adjustment procedures and specifications.

Clutch Adjustments

Some clutches have an internal adjustment. See the applicable section in Group 25 of the Business Class® Trucks Service Manual for clutch adjustment procedures and specifications.

Operating the vehicle with the clutch incorrectly adjusted could result in clutch or clutch brake failure.

Lubrication

The release bearing should be lubricated at frequent intervals. See Group 25 of the Business Class® Trucks Maintenance Manual for intervals and procedures.

Failure to lubricate the release bearing as recommended could result in release bearing damage and damage to the clutch.

Hydraulic Clutch Fluid Reservoir Checking

Make sure that the clutch fluid reservoir ( Fig. 6.1 ) is full. The reservoir is full when the fluid level is up to the "max" mark. The fluid level must always be above the "min" mark. Use only heavy-duty brake fluid, DOT 3, in the hydraulic-clutch system.

1. Clutch Fluid Reservoir

Fig. 6.1, Check Clutch Fluid Level

Rear Axles

Meritor Single-Drive Axles with Traction Equalizer

Some Meritor single-drive axles are equipped with a traction equalizer that is a load-sensing, self-actuating feature. A traction equalizer provides normal differential action where traction is good. When one wheel begins to spin faster than the other, clutch plates in the differential housing automatically engage, delivering power to both wheels. There is no operator control with this feature.

A traction equalizer occasionally tends to slip in a jerking motion, producing irregular intervals of sharp noises. This generally occurs when the vehicle is operating at low speeds on fairly sharp turns. This condition, called slip-stick, is corrected by adding a friction modifier to the axle lubricant. This additive tends to reduce the static coefficient of friction to a value equal to, or lower than, the sliding coefficient.

See Group 35 of the Business Class® Trucks Maintenance Manual for additional information on friction modifiers and when to add them to axle lubricants.

Tire sizes on both rear wheels should be the same on axles equipped with a traction equalizer. If not, excessive wear may occur in the traction equalizer.

Meritor Two-Speed Rear Axle

The two-speed rear axle can be used to operate the vehicle in low range when operating off-highway or when starting out with a heavy load on-highway. See Fig. 6.2 . After the vehicle is moving on the highway, the axle can be shifted to high range.

A. Push down for low speed range.
B. Pull up for high speed range.

Fig. 6.2, Range Selection Lever Operation

To shift the axle to the high range: keep the throttle pedal down, then move the range selection lever to high; release the throttle pedal until the axle shifts, then accelerate.

To shift the axle to low range: keep the throttle pedal down, then move the range selection lever to low; release and then depress the throttle pedal quickly to increase engine rpm. The axle will shift to low range.

NOTE: When parking the vehicle, put the axle in the low range with the engine running. Engage the clutch and transmission to be sure the axle has completed the shift into the low range. Some vehicle motion is needed to ensure engagement of the axle.

Eaton and Meritor Drive Axles with Main Differential

The Eaton and Meritor main differential lock is a driver-controlled traction device operated from the vehicle cab. A rocker switch allows the driver to lock or unlock the differential. An indicator light in the switch comes on when the differential lock is engaged.

The main differential lock provides maximum traction under slippery conditions. When the differential lock is engaged, the clutch collar completely locks the differential case, gearing, and axle shafts together, maximizing traction of both wheels and protecting against spinout. Under normal traction conditions, do not engage the differentiallock.

Operate the axle with differential action between both wheels.

Be especially careful when driving under slippery conditions with the differential locked. Though forward traction is improved, the vehicle can still slip sideways, causing possible loss of vehicle control, personal injury, and property damage.

Eaton and Meritor Main Differential Lock Operation

To lock the main differential and obtain maximum traction under slippery conditions, depress the control switch.

Lock the main differential only when the vehicle is standing still or moving less than 25 mph (40 km/h). Never lock the main differential when the vehicle is traveling down steep grades or when the wheels are slipping. This could damage the differential or lead to loss of vehicle control, causing personal injury and property damage.

If the vehicle is moving, maintain a constant vehicle speed while engaging the differential lock. Briefly let up on the accelerator to relieve torque on the gearing, allowing the differential to fully lock. The indicator light comes on when the differential is fully locked. When the differential is locked, the turning radius will increase because the vehicle understeers. See Fig. 6.3 . Drive cautiously and do not exceed 25 mph (40 km/h).

A. Turning Radius When Differential is Locked (engaged)—Understeer Condition
B. Turning Radius When Differential is Unlocked (disengaged)

Fig. 6.3, Turning Radius

To unlock the main differential, depress the bottom of the control switch. Briefly let up on the accelerator to relieve torque on the gearing, allowing the differential to fully unlock.

When the differential lock disengages, the indicator light will go off.

Eaton Single Reduction Axles with Controlled Traction Differential

The controlled traction differential system is a differential assembly incorporating a friction plate assembly designed to transfer torque from the slipping wheel to the one with traction. The unit is basically a multiple disc clutch designed to slip above predetermined torque values. This controlled slipping characteristic at higher torque values enables the vehicle to negotiate turns in a normal manner. Resistance to slippage at lower torque values enables the vehicle to maintain an appreciable amount of tractive effort when one wheel encounters relatively poor traction.

A control valve in the cab is actuated by the driver to engage and disengage the controlled traction feature.

Disengaged, the axle has differential action all the time. One wheel will spin independently of the other, if slippery conditions are encountered.

Engaged, wheel slippage and spinout are minimized.

NOTE: The controlled traction differential can be engaged at any speed, except during spinout.

Eaton Two-Speed Tandem Axles

Each axle of the two-speed tandem contains a high range single reduction gear set and a low range double reduction gear set. The operator selects the desired axle range by moving the range preselection lever located on the shifter knob (see Fig. 6.2 ).

Dual Range Two-Speed Tandem Axle Operation with Multispeed Transmissions

NOTE: See Chapter 5 , "Transmissions," for detailed information on how to use the range preselection lever. On multispeed transmission applications, the two-speed axle should be used as a dual range. Use the low range when operating off-highway or when starting out with a heavy load on-highway. After the vehicle is moving on the highway, the axle can be shifted to high range.

To shift the axle to the high range, make sure the interaxle differential lockout is disengaged, keep the throttle pedal down, move the range preselection lever to high, release the throttle pedal until the axle shifts, then accelerate.

To shift the axle to the low range, keep the throttle pedal down, move the range preselection lever to low, release and depress the throttle pedal quickly to increase engine rpm. The axle will shift to low range.

NOTE: When parking the vehicle, put the axle in the low range with the engine running. Engage the clutch and transmission to be sure the axle has completed the shift into the low range. Some vehicle motion is required to ensure engagement of the axle.

IMPORTANT: See "Eaton Interaxle Differential Lockout Operation" for precautions that must be taken when shifting axles in relation to the interaxle differential lockout.

Eaton Interaxle Differential Lockout Operation

Interaxle differential lockout systems include a lockout control valve located in the cab, and an air-operated shift unit mounted on the forward rear axle.

When the interaxle differential lockout control valve is in the LOCK position, the interaxle differential is locked out and the driveshaft becomes a solid connection between the two axles. Power entering the forward axle is also transmitted straight through to the rear axle, so both axles turn together at the same speed. The LOCK position should be used when additional traction is needed.

Engage the lockout only when stopped or at slow speeds and never when the wheels are spinning. Do not operate the axles on dry pavement with the lockout engaged for prolonged periods. Use only when additional traction is needed under adverse road conditions. Disengage the interaxle differential lockout before shifting the axle to a higher range.

When the interaxle differential lockout control valve is in the UNLOCK position, the interaxle differential allows differential action between the axles thereby compensating for different wheel speeds and variations in tire size. Keep the interaxle differential lockout in the UNLOCK position for normal driving on roads where traction is good. On two-speed axles, the interaxle differential must be in the UNLOCK position before attempting to shift the axles out of low or high range.

Steering System

General Information

When there is no load on the vehicle, and the front tires are pointed straight ahead, the steering wheel spokes should be at the 4 and 8 o'clock positions, or within 10 degrees of either side. See Fig. 6.4 . See Group 46 of the Business Class® Trucks Service Manual for steering adjustment procedures.

1. 8 o'Clock
2. 4 o'Clock

Fig. 6.4, Steering Wheel Centered

Power Steering System

The power steering system consists of an integral steering gear (which includes a manual steering mechanism, a hydraulic control valve, and a hydraulic power cylinder), hydraulic hoses, power steering pump, reservoir, and other components. The power steering pump, driven by the engine, provides the power assist for the steering system. If the engine is not running, there is no power assist.

If the power assist feature does not work due to hydraulic fluid loss, steering pump damage, or some other cause, bring the vehicle to a safe stop. Do not drive the vehicle until the cause of the problem has been corrected.

Driving the vehicle without the power-assist feature of the steering system requires much greater effort, especially in sharp turns or at low speeds, which could result in an accident and possible injury.

Drivers should use the power available with a power steering system carefully. If the front tires become lodged in a deep chuckhole or rut, drive the vehicle out, instead of using the steering system to lift the tires from the hole. Also, avoid turning the tires when they are against a curb, as this places a heavy load on steering components and could damage them.