Auxiliary Power on Demand

Power take-off (PTO) capability is important to many contractors. But there are a variety of systems that fit into this broad category.

Mike Eaves, product manager, General Motors, says engine pulley driven or electric PTOs are sometimes used for light-duty applications to reduce the cost of the overall upfit. "In a lighter-duty application, they may not tie into the transmission," he says.

On some of the larger diesel engines, equipment may also be driven off the front crankshaft of the truck. But for the majority of vocational applications, you are going to want to connect to the PTO port on the transmission.

If you plan on using a PTO, don't treat it as an afterthought in the spec'ing process. "When spec'ing other chassis equipment options, it's important to ensure PTO access points remain clear for PTO installation," says Matt Gervais, product manager, Sterling Truck Corp. "Also, a clear understanding of the intended application of the truck helps ensure that truck specs (wheelbase, weight ratings, horsepower and torque requirements, transmission selection, etc.) will be suitable for optimal performance."

The driving factor
The driven device will dictate the correct PTO. "A lot of the decision is going to be based around the amount of work being done by the driven object," says Jeff King, marketing manager, Chelsea Products Division, Parker. "Make sure your PTO is able to deliver the proper horsepower and rpm that the driven object needs. Some transmissions have slower speeds; it is hard to achieve the rpm you want out of the PTO shaft."

According to Nathan Stansell, upfitter integration, General Motors, "What it boils down to is the output horsepower is directly a function of the size of the drive gear in the transmission. As a rule of thumb, most of our transmissions have the max output around 40 hp as limited by the size of the gear. The vast majority of PTO applications would not even approach the 40-hp need. But if you have a high power application, you can use a split shaft where you come right off the driveshaft of the transmission and go into a secondary transmission. Then you can get the full horsepower of the powertrain applied."

It's important to understand the relationship between the internal transmission gear and the PTO gear. "We have a wide variety of gear ratios to give you some very high and very low speeds," says King. "We cover a pretty wide gamut in PTO speeds. Once in a while, a unique transmission comes into the marketplace, where we have to work to get the gear geometry so we can match to the gear in the transmission."

Using the PTO to drive a hydraulic pump is a common application. You will need to identify the flow rate for the hydraulic pump and the output shaft speed required. "That backs into what is the gear ratio of the PTO gear itself?" says Stansell. "All of that work is done directly with the PTO supplier. Where we come into play is when you need to adjust the engine speeds or program for special needs. There are a lot of options about how the idle up can be controlled."

The capability of the PTO is dependent on a couple of factors. "Both engine and transmission specs play an equal role in PTO capability," says Gervais. "Diesel engines typically provide better PTO characteristics, with more available torque earlier in the rpm range."

The type of duty cycle also plays a critical role in PTO selection. When used in continuous operations, you will need to de-rate the PTO output. "Make sure with the de-rating that you have sufficient horsepower and torque to meet the requirements of your operation," says King.

Output speed is critical
It is critical to calculate the speed of the PTO and the corresponding engine speed. "The desired PTO speed - expressed as a percentage of PTO shaft speed to engine speed - is determined by comparing the required input speed of the driven component to the engine speed during normal operation," says David Douglass, director of training, Muncie Power Products.

The output of a given PTO is often determined by the gearing in the transmission. "Transmissions with high Pitch Line Velocity (PLV) provide faster PTO speeds, as well as increased torque capability," says Douglass. "PLV is measured in feet per minute (fpm) and is a function of the transmission's internal ratios and the diameter of the PTO drive gear."

The PLV directly impacts performance. "A transmission with a low (less than 1,000 fpm) PLV may be acceptable for a single- or tandem-axle dump truck, but would not be well suited for a high-torque application like a dry bulk blower or a vacuum pump," says Douglass. "For these applications, a transmission with a PLV of 1,400 fpm or more should be specified."

He explains, "Most modern engines provide a means of electronically setting a higher engine idle speed for PTO operation. (A range of 1,000 to 1,400 rpm is common.) In many cases, the vehicle computer senses PTO engagement and automatically increases engine rpm to a predetermined speed."

This is a major improvement over the previous-generation mechanical controls. "Electronic controls are more precise; not subject to sticking or coming out of adjustment; and are tamper resistant," says Douglass. "Additionally, other operating parameters can be set, such as brakes set [with] transmission in neutral or park; maximum PTO operating speed, etc."

This electronic integration is what really differentiates the performance of different makes of trucks in PTO operation. "Our PTO system has specific software that runs in the engine controller," says Stansell. "We provide switches in the cab that you can use to turn the PTO system on. This causes the controller to run the PTO program. We use it in conjunction with our cruise control inside the cab."

The cruise control switches give you two more speeds on the basic programming - one for the set button and one for the resume button. You also get an initial speed when you turn the system on.

"Then there is additional programming you can do where the set and resume buttons actually tap up or tap down, like cruise control when you are on the highway," says Stansell. All of this functionality is also available outside the truck via two interface connectors.

The system is programmed so the PTO can't be engaged if conditions aren't right, such as if you're not stationary. There are also safety features, such as overspeed control. "If you get the engine above a specified level, it will drop out," says Stansell.

"There is a lot of integration between the engine and the transmission controller," he continues. "It is quite a sophisticated offering of parameters that are available."

Choose the best PTO type
PTOs are available in many different configurations, including single, double or triple gear and clutch shift type.

Single gear PTOs are pretty rare in the U.S. market due to their low-tech design. "While least expensive, single gear PTOs do not offer direct pump mount features or multiple speed choices," says Douglass.

"Multiple gear PTOs provide additional torque output," adds Gervais.

Multiple gear PTOs are the type most commonly found on manual transmissions. They offer a variety of speed ratios, output shaft types and shift options, making them the most versatile, as well.

"Muncie's TG Series multiple gear models offer a wide range of speed, output shaft and shifting choices for manual transmission-equipped trucks," says Douglass. "This model is easy to service, and interchangeable parts make it easy to change gears, shifters and shaft assemblies. "

Double gear PTOs are perhaps the most popular. "For most PTO manufacturers, that is going to be the bread and butter of their lineup - including ours," says King. "They are popular on manual transmissions, but they are mechanical engagement. So if you don't shift them properly, you are going to have gear clashing. The operator has to make sure he pushes the clutch in, then he has to make sure the gears come to a standstill, then he can engage the PTO."

Automatic transmissions are equipped with clutch-pack style or "hot shift" transmissions. "There is no gear clashing; it is very easy to engage," says King.

The biggest advantage of clutch-type PTOs is their ease of use and ability to be engaged without bringing the vehicle to a stop.

Chelsea has also developed three gear units with hot shift to address tight clearance applications. "The three gear design is there to get clearance away from the transmission, and also to give clearance with the driveline coming from the transfer case for the four-wheel-drive options," says King. "They have really evolved because of a necessity for clearance in some very tight spaces we are fitting underneath the truck."

Shifting options
On medium-duty and larger trucks with manual transmissions, you often have PTO shifting options that include a cable, air or electronic/hydraulic actuation.

"The cable controls are your basic dumb, dirty standard option that comes with the PTO," says King. "It is a no-cost option. It is like getting your car with manual crank down windows instead of electric windows.

"The air shift option is really a lot more dependable because it doesn't have any exposed parts to the weather like a manual cable," he continues. "It is a lot easier to install and it is a lot more reliable."

Douglass adds, "Air shifting is more positive, easier and does not require periodic re-adjustment as do lever or cable systems."

Gervais agrees, noting, "Sterling offers air or electronic/hydraulically actuated PTOs due to their reliable and durable characteristics."

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