The trucking industry is known for its versatility. The sundry functions a truck performs keeps things interesting when it comes to vehicle maintenance, but sometimes power take-offs (PTO), the very devices that enable truck versatility, require diligent maintenance or repair.
PTOs are devices that connect a vehicle’s drivetrain to a piece of auxiliary equipment, such as a winch or a crane. They make possible the use of a variety of equipment from cranes to emergency vehicle apparatus. Without them, trucks would be confined to providing the most basic transport functions.
Before addressing a PTO maintenance problem, make sure the correct unit is being used for the job. Improperly-speced PTOs are trouble waiting to happen; they can cause major damage and pose immediate safety threats.
Talk to your customers to learn the primary use of their PTOs, then check to see that they’re using correctly-speced PTOs.
This will save them from longer downtime and all the expenses associated with that occurrence. According to John Hylton, application engineering, Allison Transmission, “The PTO gears are in constant mesh with the transmission gears after installation. If the gears do not mesh at installation, the wrong PTO has been installed.”
Don’t assume that a PTO intended for a specific purpose will suffice for another. With PTOs, obtaining the correct gear meshing from the start is crucial.
If a driver reports PTO noise, it’s likely the PTO’s gears are not merging properly. A complaint of a rattling noise likely means that the gears are too loose, which creates a wear pattern at the tip of the teeth. Whining indicates an overly-tight gear fitting, confirmed by a wear pattern found at the root of the teeth.
Note, however, that a rattle also could result from torsional vibration from the engine. When a certain resonance is reached, the vibration can cause wear on the PTO’s spline shaft and teeth. If the rattle doesn’t change when the engine’s rpm is increased, you can bet that the rattling problem is indeed backlash related.
“When PTO gears do not mesh with those of the transmission, you have the wrong PTO for the application,” said Arnie Slutsky, product manager, S&S Truck Parts Inc. “The gears must mesh properly or the PTO will fail and the transmission also will not operate properly,” he explained.
Dave Douglass, director of training, Muncie Power Products, agreed about the need for absolute accuracy when matching PTO gears to those of the transmission. “If they don’t match, damage can result to the PTO housing and PTO and transmission gears if proper backlash is not established when the PTO is installed,” he explained.
Backlash, the amount of play that results from a loose connection between gears, determines how well the PTO gears will mesh with the transmission gears. The proper setting should be between .006″ and .012″. If this interval isn’t maintained, the gear teeth will cease to fit together correctly and premature wear will occur.
“The correct backlash is achieved by installing a combination of gaskets/shims that are provided with the PTO,” said Douglass.
“We recommend the use of a dial indicator to ensure proper backlash. Tight mounting also may result in physical damage to the PTO input gear and, in extreme cases, cause the PTO housing to crack,” he warned.
Once the correct PTO is speced and installed, make sure it’s attached with the proper amount of support. According to Douglass, “Direct coupled hydraulic pumps weighing more than 40 lbs.-the combined weight of the pump, hoses and fittings-must be installed with a support bracket attaching the rear of the pump to the transmission housing.
“The support bracket should be a four-point mount; two to the pump and two to the transmission. Failure to properly support a pump properly can result in a fracture of the PTO housing.” Housing damage also can occur when a truck runs over a large obstacle off road.
Slutsky concurred. “Flange bolts break when support brackets are not used on direct-mounted pumps that are excessively heavy.”
With the increasing use of automatic transmissions, it’s prudent to note the differences between automatic transmissions and manual ones when it comes time for PTO maintenance.
“The primary difference is in the type of PTO that’s commonly installed and it relates to how the PTO is engaged,” said Douglass. “Manual transmissions generally are equipped with mechanically-shifted PTOs that require the vehicle to come to a complete stop before the PTO is engaged.
“Clutch shift-type PTOs, commonly found on newer automatic transmissions, can be activated safely with the vehicle in motion as long as the engine speed is kept below 1000 rpm. This is common on residential refuse collection vehicles,” said Douglass.
The type of transmission also plays a role in PTO lubrication issues. If a driver complains of squealing emanating from the PTO, it could be caused by insufficient lubrication, which makes bearings squeeze.
“If proper lubrication is not maintained, the gears, bearings and bushings will seize,” said Slutsky. This will result in significant downtime for your customer.
On manual transmissions, PTOs often are attached in a manner that encourages easy bearing lubrication.
Automatic transmissions are not as easily lubricated, however. Sometimes the angle at which the PTO is attached to an automatic transmission makes bearing lubrication difficult if a pressure lube idler shaft is not employed. “Automatic transmissions provide lubrication to the PTO through a pressure lubrication hose that is supplied with the PTO and attached to a specific port on the transmission,” said Douglass.
“With manual transmissions, the PTO aperture is below the transmission fluid level and the PTO case is flooded. PTO installers should use about a quart of additional lubricant when refilling the transmission after installing a PTO,” he said. The experts also recommended checking for oil leakage at the end covers.
In addition to backlash and lubrication issues, another common PTO maintenance issue ties into the shifting mechanisms used. Manual transmission PTOs commonly make use of either an air-shift system or a cable-shift system. When your customer comes in with a shift problem, make sure he is using proper shifting practices.
“For a manual transmission with a mechanically-shifted PTO, bring the vehicle to a complete stop and set the brakes, then place the transmission in neutral,” explained Douglass.
“Next, fully depress the clutch and wait a few seconds for the transmission gears to stop. Engage the PTO and then slowly release the clutch for PTO operation. To disengage the PTO, depress the clutch, disengage the PTO and release the brakes.
“For automatic transmissions with clutch shift-type PTOs, first bring the vehicle to a complete stop and set the brakes. Place the transmission in neutral and activate the PTO rocker switch for engagement. Reverse the procedure to disengage,” Douglass explained.
When servicing a PTO, “Check for cable control wear or air cylinder leaks,” said Slutsky. “Check the air lines and air controls for the PTO or pump.”
If a customer complains that an air-shift system PTO is becoming difficult to put into gear, it could mean that not enough air pressure is available. If you don’t find any impediments along the air line, examine the truck’s air system. If it’s hard to shift an air system PTO out of gear, there could be a problem with the spring used to disengage.
Cable-shift systems should be inspected to see that a shift cable doesn’t have inappropriate bends, which also will impede system functioning.
There has been some concern in the marketplace about PTO placement and size in the new 2007 trucks with larger engines. “Concerns about exhaust routing do exist, mostly for right hand side PTOs, but most OEMs appear to be trying to keep enough clearance to allow for PTO installations,” said Hylton.
“Like today, there always will be some PTO installations that will not fit, just as there were in pre-2007 engine vehicles. Left-side PTO installations typically are open, but some interference can occur as well. Any given engine and chassis combination has to be evaluated by the PTO installer,” he said.
There is a truck for every task, but an array of their uses depend upon proper PTO function. Stress the importance of correct PTO usage to your customers and don’t cut corners with maintenance and repair.
For More Information
For more information on power take-offs (PTOs), you may contact the following companies directly. Other companies offering information on PTOs can be found in the Truck Parts & Service Aftermarket Buyers’ Guide & Directory as well as in the Buyers’ Guide section on our web site at www.truckpartsandservice.com.
Allison Transmission
Indianapolis, IN
800-252-5283
www.allisontransmission.com
American Powertrain
Ocala, FL
888-409-GEAR
www.americanpowertrain.net
Metaris Hydraulics
Toronto, ON
888-477-2737
www.metaris.com
Muncie Power Products, Inc.
Muncie, IN
800-367-7867
www.munciepower.com
Proheat/Telflex
Richmond, BC
604-270-6899
www.proheat.com
S&S Truck Parts Inc.
Newstar Parts Component Group
Schaumburg, IL
800-621-1558
www.sandstruck.com
ZF Sales & Service
North America
Vernon Hills, IL
800-660-9631
www.zflife2.com
