Clutches
Six Steps To Longer Clutch Life
Deborah Lockridge
Senior Editor
Clutches don't really need a lot of preventive maintenance, and new trends in clutches and transmissions can mean even less. But that doesn't mean there aren't some things to be aware of that can help you get longer life out of your clutches.
In its Recommended Practice 639 on spec'ing heavy duty clutches, the Technology and Maintenance Council lists four primary factors affecting heavy-duty clutch service:
• Powertrain specifications (engine, clutch, transmission, axle and tires)
• Control of powertrain torsional activity
• Driver technique (starting gear selection, engagement technique, driving habits)
• Clutch maintenance (adjustment, lubrication, protection from rust and contamination)
"These factors are not independent of each other," TMC notes. "For example, powertrain specifications can have a major effect on the third factor listed - driver technique."
1 Spec it Right
The first step in getting maximum clutch life is spec'ing the right one in the first place.
The main factor in spec'ing clutches is the rated torque capacity, which must be equal or greater than the peak torque of the engine. If different-sized clutches are available for a given engine and torque rating, the larger clutch with more heat capacity will provide longer life.
The transmission selected also affects clutch life. "The number of gears and gear ratio steps are important for clutch life," TMC says. "It is essential that a comfortable start-up gear with acceptable startability be available for any normally encountered vehicle operating weight and application condition. Selecting a higher gear for start-up will increase clutch engagement slip time and may significantly reduce clutch life."
You can reduce maintenance costs by spec'ing self-adjusting clutches (but keep in mind that like any "maintenance-free" component, these still require regular inspections.) Self-adjusting clutches are also expected to last longer than ordinary types, because so many clutches in real life operate out of adjustment.
"There's really not a lot to tell about clutch maintenance if you're using self adjusters," says Darry Stuart, who works with fleets on equipment issues through his DWS Fleet Management Services. "Sometimes the self-adjusters do seize up and don't operate after quite a few miles, but even with some of the haphazards you may run across, you're still way ahead [spec'ing self-adjusting clutches], in my opinion."
Spec an automatic transmission, and obviously you have no clutch maintenance at all. But even the new semi-automated mechanical transmissions make a difference. The driver may use his clutch foot only to start the truck off and to stop it. By taking the driver largely out of the shifting process, these transmissions lead to longer clutch life.
2 Lubricate Properly
The clutch release system, which includes the release bearing and sleeve bushing, cross shaft bushings, and all of the release linkage, needs to be lubricated monthly or according to the manufacturer's recommendation, says the Technology and Maintenance Council in its Recommended Practice 638 on clutch maintenance.
"A common misconception is overgreasing the bearing, and you really can't," says Matt Blomeke, customer satisfaction representative at Eaton Clutch Division. "When you're greasing a release bearing on a clutch, you want to be sure to grease it till you see grease purging out of the housing."
Even self-adjusting clutches do need to have the bearing lubricated at an interval recommended by the manufacturer.
Use a high-temperature, lithium-based, multipurpose wheel bearing grease. "This has become more important as chassis component temperatures have risen with higher horsepower aerodynamic vehicles," TMC says.
Blomeke also says it's important to ensure you're getting some grease on the input shaft. "Inside of the sleeve that the release bearing rides on, there's a bronze bushing that rides against the input shaft, and the only way you can get that lubed is to have some lube on the input shaft," he says.
3 Know When To Adjust
Proper clutch adjustment helps reduce wear. Unlike many other preventive maintenance issues, however, generic time or mileage intervals are not a good guide. Instead, wear is the underlying reason to adjust a clutch. The Technology and Maintenance Council recommends using the history of your specific operation to guide you in developing a clutch adjustment frequency. Generally, the more severe the application, the more frequent the adjustment.
Free pedal travel, also called free play, free pedal or free travel, has been the classic clutch adjustment indicator. Proper free pedal results from the 1/8 inch clearance between the release fork (yoke) tips and the clutch release bearing wear pads. When the amount of free pedal travel drops below a certain point, it's time to adjust the clutch.
In fact, one of the most common clutch complaints from drivers, says Eaton's Blomeke, relates to the amount of free pedal. At the other end of the linkage, free play enables the clutch release fork to release fully when the clutch is engaged. If there's not enough clearance, the clutch might remain partially disengaged, causing slippage, excessive wear and heat buildup.
In looking at free pedal complaints, you need to keep in mind that the amount of free pedal in the cab will vary by truck make and model. "Some people have a preconceived notion that it should be an inch and a half no matter what," Blomeke says.
In fact, the free play value in the cab can range from 1 1/8 inches to as much as three inches. If the free play drops too low, it's a sign of disc lining wear.
TMC recommends adjusting the clutch when you've reached a 50% loss of yoke gap, from 1/8 inch to 1/16 inch. This will be signaled by a 50% loss in free pedal travel. To properly do this, you'll need to record what the free pedal travel is when the clutch is properly installed and adjusted so you have a point of reference.
"Drivers should not make the adjustment calls based on [free pedal travel] preference," says TMC's RP 638, "because of the difference between vehicle models and manufacturers."
4 Know How To Adjust
You not only have to know when to adjust the clutch, but how to do it properly.
"The biggest problem with clutches is people don't understand how to adjust them," Stuart says. "They think they do, but they don't. A lot of people go to the linkage before they go to the internal adjustment rings. Unless the truck has a lot of miles, you shouldn't have to adjust the linkage."
There are a number of steps to take before getting into the linkage, including in-cab inspection, and inspecting the clutch inside the transmission bell housing and adjusting the bearing. Linkage adjustments should never be made until the internal parts of the clutch have been inspected and serviced.
"Many internal problems can be overlooked if service technicians adjust the linkage before considering other potential problems," says Jerry Montgomery, vice president of engineering at Meritor in an "Issues & Trends" bulletin on troubleshooting clutches. "If a driver says the clutch seems to be 'acting up,' a thorough diagnosis should be performed."
Some technicians tend to go right from the in-cab clutch pedal inspection to linkage adjustment.
"You have trouble getting people really wanting to get under the truck and measure and see where things are at," Eaton's Blomeke says. "They want to start adjusting first."
Trying to adjust the linkage before, or without, inspecting and adjusting the internal clutch components, can cause problems. It can mask telltale differences in pedal travel and pedal feet. The changes in linkage may actually pull the in-cab clutch pedal travel dimensions back into place, even though there's internal wear.
"If that happens, everything may look proper when the inspection plate is removed and clearances are checked - but it won't be," says Meritor's Montgomery. "For instance, without internal adjustment, the release bearing housing may keep moving forward until the retainer contacts the rear friction disc, causing damage to both discs."
When troubleshooting, whenever possible, interview the driver in enough detail to pinpoint the problem. Symptoms could include things like pedal free travel being too short or too long, problems with clutch brake squeeze, clutch slippage, clutch hang-up, hard shifting, chatter, vibration or early activation of the clutch brake.
After learning the symptoms, technicians should climb into the cab and check the clutch pedal stroke, which can be a good indicator of internal problems. Every clutch has three critical pedal stroke characteristics, according to Meritor: clutch pedal height, clutch pedal free play or free travel, and clutch pedal downstroke.
Clutch pedal height is the position of the pedal relative to the cab floor. Drivers often "customize" this setting to suit personal preferences, not realizing that pedal height is the fundamental starting point for most OEM truck linkages. So when it's time to set or adjust the clutch linkage, first set the pedal height according to the truck OEM specifications.
Before adjusting the linkage, you need to inspect the clutch inside the transmission bell housing for several things:
1. On non-synchronized transmissions, measure the clutch release travel clearance between the release bearing housing and the clutch brake. (On synchronized transmissions, check the clearance between the release bearing housing and the clutch cover.)
Heavy-duty truck clutches require the release bearing to travel a minimum of 1/2 inch in order to completely disengage when the clutch pedal is pushed.
2. The clutch free travel clearance between the release fork (yoke) and the release bearing pads. This 1/8-inch gap is necessary to ensure the release bearing can move back far enough to completely disengage the friction discs. If the yoke gap is not correct after the release bearing gap has been set, the clutch will require a release linkage adjustment, according to TMC.
3. Inspect the clutch brake.
Another common complaint area, Blomeke says, is clutch brake squeeze. The total clutch pedal downstroke is about six to eight inches, depending on truck make and model. Near the bottom of the full pedal travel, there is a critical segment of about an inch, where the stroke becomes significantly stiffer. This is the clutch brake squeeze area. The driver pushes the pedal to this position only when he wants to apply the clutch brake. The clutch brake should be used only when shifting into first or reverse gears, with the vehicle stopped.
If the brake squeeze area rises higher than an inch, it could lead to accidental application during upshifts and downshifts when the truck is in motion. This could cause premature clutch brake wear and failure, among other problems.
If you get complaints about clutch brake squeeze, before getting under the truck, check in the cab to make sure the pedal is not obstructed. "Drivers sometimes want to put a piece of carpet on the floor under the pedal, and that could be limiting the pedal travel at the bottom," Blomeke says.
Incorrect clutch brake operation could indicate a worn or damaged brake unit, notes TMC. Examine the clutch brake for unusual friction material wear, broken drive teeth or damage to the main housing or body that could indicate the need to replace.
"We get calls where people can't get free pedal and brake squeeze. Maybe they get one or the other," Blomeke says. "That is a linkage travel issue. You need to ensure the linkage is in good condition - not worn - and that the cross shaft bushings are not worn, giving you excessive play in the system when you're pushing the pedal down."
If you have a self-adjusting clutch, it's still important to make sure the linage is in good condition to ensure you're getting clutch brake squeeze and that the clutch is going to adjust itself to compensate for wear.
When all adjustments and in-service maintenance procedures are completed, the vehicle should be driven to evaluate clutch function. If the clutch slips under high torque conditions, TMC's RP 638 says it must be considered to be damaged or worn out and should be replaced.
5 Use The Right Replacement Clutch
When choosing a replacement clutch, keep in mind the old adage of "you get what you pay for."
"The worst is a cheap rebuilt clutch," Stuart says. "If somebody uses a cheap rebuilt clutch, they'll get caught."
Just as when spec'ing a new clutch, it's important to get the proper replacement as far as torque capacity. "People tend to look at price more so than they do the proper application," Stuart says, "because they don't understand the torque ratings of clutches."
Putting in the wrong replacement clutch can not only lead to shorter clutch life, but also could affect the rest of the drivetrain. And keep in mind that a lot of the cost of replacing a clutch is labor to remove and replace the transmission. If your cheap clutch is failing early, those labor costs have suddenly negated a lot of those savings.
As far as the actual replacement process, Stuart recommends always refacing the flywheel. "The biggest mistake is that people think they can tell (if it needs replacing) by looking at it."
When you get into the replacement of the clutch, you need to make sure everything going back in is in good condition.
"Make sure the bushings and yoke aren't worn, and if so, replace them," Blomeke says. "Same with the cross shafts and even some of the pivot points on the linkage. You need to make sure all those points are in good condition and not worn, that could limit your linkage travel."
6 Train Your Drivers
How a truck is driven can have a big influence on the life of the clutch.
Remind drivers that they should not hold the truck on a hill using the clutch. This causes heat, increases wear, and is also a safety issue because the brake lights won't be on.
Coasting down hills with the clutch depressed can result in clutch disc speeds that can damage the clutch.
Drivers also need to use the proper gear to start the truck moving.
"If you're loaded at 80,000 pounds and starting in too high a gear, the driver's going to try to feather the clutch to get the truck moving smoothly," Blomeke says, causing more wear. "Where if you use the lower gear, you can engage the clutch normally and progressively shift from there. That's going to reduce your clutch wear and also, if you've got the manually adjusting product, it's going to help reduce your adjustment interval."
