Are you seeing broken gear teeth in your shop? Cracked shafts? Burnt bearings? It’s probably not the hardware. More than likely, your customers’ driving habits need adjusting or transmission maintenance practices could use a transformation. A close look at failed internal components can help you tell.
Driver impact
Impact fractures to transmission gears are alarmingly common, and almost always a direct result of improper driving techniques or downright abuse. Such damage occurs when a vehicle is suddenly stopped under load – such as during an accident – or when a wheel spinning on a slick surface suddenly grabs terra firma and transmits a shock load through the drivetrain. It also can happen when a driver “pops” the clutch at elevated rpm on a loaded vehicle. In any case, one gear usually is stopped, and another wants to move it in a hurry. With nowhere for the energy to go, something’s gotta give. Usually, that means “a tooth or teeth are ripped out below the ‘root,'” says Mike Kidd, account and technical training manager, Transmission Technologies Corp. (TTC). Usually, it’s two or more teeth, since the gears are meshed during impact.
The gear shown in Photo 1, sans two teeth, was the victim of impact damage, and the missing-tooth bases display a brittle, granular texture typical of sudden breakage. In a twin-countershaft transmission, “The odds are good that you’ll find similar damage in two places, 180 degrees apart,” says Charles Allen, national service director, ArvinMeritor Commercial Vehicle Systems. “This one was probably the result of an accident.”
Opening a transmission at Southeastern Freight Lines revealed similar damage, as shown in Photo 2. “We’re pretty sure the driver backed under a trailer too fast,” says shop foreman Robert Dominick. “When the king pin seated, he stopped real fast.”
This type of failure often will cause vibration, rattling or knocking, or even complete inoperation of the transmission. In any case, the sooner the symptoms are noted and the transmission opened up, the better. Dominick and others recommend that, when teeth are missing, the entire transmission should be cleaned and inspected, since even a single piece of stray metal can take a fluid-borne joyride around the rest of the components, leaving widespread destruction in its wake.
In the event of a shock-related transmission failure, it also should be standard practice to check for evidence of other drivetrain component damage, such as twisted driveshafts, fractured U-joints and differential trauma.
Shifty operation
Another way transmissions can be damaged by drivers is through incorrect or abusive shifting. Forcing or “grinding” gears usually leaves partially damaged teeth that appear polished and shiny like those on the sliding collar in Photo 3. “The operator is very much part of the transmission,” says ArvinMeritor’s Allen. “And it’s important that he shift properly. The electronics in some newer engines can make it difficult to ‘float shift’ without the clutch, and the driver shouldn’t try to force it.”
