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.