Medium Duty: Transmission and PTO Upkeep

Maintaining transmissions and PTOs is serious business.


It is a tragedy that could still happen in any shop.

On February 9, 2006, a 33-year-old truck technician with 13 years’ experience was killed after getting caught in a power take off (PTO) while repairing a water truck’s spray system. Working underneath the truck while it was still running, a protruding set screw on the PTO drive shaft caught the collar of his coveralls and quickly entangled him.

According to the National Institute for Occupational Safety and Health, two truck drivers were killed this year in PTO-related accidents. Bob Johnson, director of fleet relations with the National Truck Equipment Association (NTEA), says these deaths are preventable with some common sense and consistent maintenance.

“If the PTO shaft is turning, stay out from under the truck,” he says. “A lot of (PTOs) have square-headed set screws for tightening down the yokes on the shaft, and that’s just a guaranteed thing to catch (clothing). The most common reason people are up underneath the truck is because it didn’t shift properly—either it didn’t go in or it didn’t release, and they’ll get under there and try to knock it out, and they’ll get their sleeve caught in the PTO and wrapped up and that’s the end of it.

“That’s the reason it’s so critical to make sure the shift linkage is properly working—just double-check that everything is in gear properly.”

PTO BASIC TRAINING

Aside from the obvious dangers, maintaining PTOs is fairly standard. Dave Douglass, director of training at the Muncie, IN-based Muncie Power Products, says the biggest change in recent years is the devices’ increasing integration with electrical systems, which can cause engagement and operations problems for installers.

“Before automatic transmissions got very popular, putting a PTO on a manual transmission was pretty much a mechanical connection—making sure that the gear of the PTO meshed properly with the gear of the transmission, and hooking up some kind of a mechanical linkage, and it was a very mechanical, relatively simple installation,” he says. “As automatic transmissions became more popular, where they started having to interface, not just mechanically with the transmission, but also interface electrically with the vehicle; that’s where a lot of installation problems started to occur, and our experience has been that a lot of that has been simply due to electrical errors rather than mechanical errors. There is a lot of stuff to keep track of, and the vehicle OEMs sometimes aren’t as good as they should be about communicating changes to the people that have to do the installations, so there tends to be a lot of confusion at the level of the installer.”

Muncie offers PTOs for both automatic and manual transmissions for a variety of applications, though there is more for a technician to be aware of with automatics, Douglass says.

“PTOs for automatic transmissions are usually what are referred to as a clutch-type PTOs, where you have friction disks inside that perform the engagement, and in manual transmissions, the PTOs tend to be just a mechanical shift, where you have one gear moving in and out of mesh with another gear,” he says. “So the PTOs for automatics have more parts, but troubleshooting-wise, most PTO problems have to do with—if the initial installation was performed properly—an operator not following the proper shifting procedures. And if that’s a manual transmission, what that usually means is they’re not disengaging the clutch before they try to engage the power take off, and if it’s an automatic transmission, one of the most common things is engaging the PTO at too high an engine speed.”

Douglass says PTOs—while still potentially dangerous—are much easier to repair these days because equipment installers are using direct coupled hydraulic pumps instead of remote-mounted pumps, in order to eliminate the troublesome driveshaft.

“Not that we even recommend anyone get under a truck while it’s running,” he says.

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