Maintenance Helps Drivetrain Components Perform Under Pressure

Robert Deierlein

Fleet managers agree that today's wide selection of drivetrain models are more durable and reliable and require less maintenance than ever before.

However, regular preventive maintenance - consisting of thorough inspections and lubrication - is crucial to efficient operations and lower repair costs.

The most expensive component of a drivetrain is the clutch (see chart). If the fork has moved, the clutch probably needs to be adjusted. The smallest movement of the pedal should move the release fork. If it doesn't, it's likely that "false free- play" or "false travel" has resulted from rust, dimensional distortion or wear, which can mask the need for a clutch adjustment.

Adjust the linkage and the clutch together; otherwise, the linkage may absorb the load intended for the clutch disc, which reduces clamp load at the plates and may lead to slippage, heat damage and premature clutch failure.

Incorrect release-bearing clearance can result in clutch dragging or loss of brake squeeze. Since clearance measurements differ from one clutch manufacturer to another and the transmission may not be synchronized, technicians should refer to a maintenance manual or the manufacturer's guide for proper release-bearing clearance.

As technology increases, fleet managers are inclined to upgrade equipment. Specifications for replacement, for example, are on the upswing. Diaphragmspring clutches are gaining popularity among many fleet managers and drivers who reportedly find them easier to use and more dependable. The 15 1/2-inch clutch has replaced the 14-inch as the most common size.

Clutches can be composed of ceramic or organic facing materials. Although engine torque output dictates lining type, ceramic facings are most popular because of the wear life. A ceramic facing may last as much as 75 percent longer. However, ceramic clutches may not operate as smoothly as most organic clutches and tend to become rough as the material heats up.

All modern clutches are designed for relatively easy inspections and adjustments to compensate for the effects of disc wear; these must be made in sequence and involve more than just the linkage. Some general rules of thumb include:

* Never adjust the linkage until the internal parts of the clutch have been inspected and serviced. Adjusting the linkage first may actually pull the in-cab clutch-pedal travel dimensions back in place despite internal wear, so that when the inspection plate is removed and the clearances are checked, everything may look okay - but it's not. For instance, without internal adjustment, the releasebearing housing may continue to move until the retainer contacts the rear friction disc, damaging both components.

* Ask the driver to help pinpoint the problem. The service technician should ask the driver to describe the symptoms. Signs of clutch trouble can include too short or too long pedal free-travel, clutch slippage or hang-up, hard shifting, chatter, vibration and early activation of the clutch brake.

* Check the clutch pedal stroke, which can indicate internal problems such as friction material wear. Every clutch has three critical pedal stroke characteristics: pedal height, free-play or free-travel and downstroke.

The clutch pedal height refers to the position of the pedal relative to the cab floor. Drivers often customize this setting to suit their personal preferences without realizing that pedal height is the fundamental starting point for most truck linkages.

* When setting or adjusting the clutch linkage, set the pedal height according to the truck operator equipment manufacturer's (OEM) specifications.

The length of pedal free-play when the clutch pedal is first depressed ensures sufficient clearance at the other end of the linkage between the clutch-release fork and the release bearing that houses wear pads. The clearance, or clutch free-travel, re-leases the fork fully when the clutch is engaged. If there's not enough clearance and the fork does not release completely, the clutch might remain partially disengaged, which causes slippage, excessive wear and heat build-up.

Some clutches specify 1/8-inch of clutch free-travel, which translates inside the cab to a typical clutch pedal free-play of 1 1/2 inches. De-pending on the truck make and model, this value can range from 1 1/8 inches to 2 inches for the same 1/8-inch fork clearance. Less than 1/8-inch of free-travel indicates disc lining wear.

The total clutch-pedal downstroke is about 6 to 8 inches. The clutch brake-squeeze area is a critical segment of about 1 inch near the bottom of the full pedal travel, where the stroke becomes considerably stiffer. The driver pushes the pedal to this position to apply the clutch brake. The release-bearing housing moves against the brake, squeezing it against the transmission-bearing retainer to slow and stop the transmission input shaft.

* All drivers should understand the clutch-brake function. A brake-squeeze area that rises higher than 1 inch could accidentally cause premature clutch-brake wear and failure during upshifts and downshifts. The drag can alter the shift timing and make it more difficult to switch gears.

If the brake-squeeze area is too high, it shortens normal clutch-pedal travel and could prevent the linkage from pulling the release-bearing housing back far enough to completely disengage the clutch.

* Look inside the transmission bell housing to check the clutch pedal. When the inspection plate is re-moved, it should be easy to check the clutch-release travel clearance between the release-bearing housing and the clutch brake on non-synchronized transmissions or the clearance between the release-bearing housing and the clutch cover on synchronized transmissions. Also check the clutch free-travel clearance between the release fork and the release-bearing pads, which gives the release bearing room to move back far enough to completely disengage the friction discs.

The clutch pressure plate is under constant, heavy clamping force (approximately 2,800 to 3,600 pounds) when the clutch is engaged. With this much force, the pressure plate gradually moves forward along the transmission input shaft, taking up the gap caused by disclining wear. Due to the internal lever ratio, the release-bearing housing moves at 6.5 times the pressure plate travel.

All these components are designed to move with reasonable freedom along the splines of the transmission input shaft. This international design feature allows a certain amount of movement to compensate for lining and part wear.

Fleet managers have found that without proper inspections and internal adjustments, the clutch will be back in the shop again, soon.

Other Major Components The transmission is reportedly the most improved driveline component. Following the manufacturer's lube-change requirements is crucial.

Always drain the lube while it's still warm, then clean the housing around the filter plug and remove the plug from the housing side. The transmission model and inclination will determine the amount of lube needed. Don't overfill the openings, which will force the lube out of the case through the front bearing cover. Clean the plug before reinstalling.

When assembling the auxiliary cover, hold the oil seal on the outer diameter to install it in the output bearing retainer. Touching the inner diameter of the seal will cause contamination and a leak between the shaft and the seal.

Follow the lubrication guidelines for drive axles as well. Lubricants help carry dirt away from moving parts and cool the axle carrier. Be sure to use the lube specified by the OEM and axle manufacturer; use the proper amount of lubrication; follow the lubrication interval for the axle; and replace the oil filter when the lubricant in the axle is drained and replaced.

For all the components of a drivetrain system, taking the time to perform preventive maintenance procedures will go a long way toward pre serving your investment.

The clutch is often blamed for many drivetrain performance problems, but sometimes other factors can disguise themselves as "clutch trouble." For example:

Claim: The transmission is hard to shift and/or the clutch won't release.

Probable cause: Transmission internals are worn or need adjustment, or the linkage is out of adjustment and/or severely worn.

Claim: The clutch vibrates or is out of balance.

Probable cause: Driveshaft U-joints are severely worn or caps and needles are missing; axle pinion end play is excessive; or transmission internals are worn or need adjustment.

Claim: The clutch is slipping.

Probable cause: The linkage or clutch is out of adjustment or the engine mounts are worn, broken or missing.

Claim: The clutch loses free play while driving.

Probable cause: The mechanic forgot to install the pilot bearing when servicing the transmission and clutch, or the pilot bearing has failed.