Not all the work you do involves simply hauling the load down the road. Sometimes the commodity needs to be picked up, spun around, or pumped out. There’s plenty of special equipment that can do the job. How to power it is another matter.

On a truck or tractor, there are two ways to drive hoists, pumps, mixers, etc.: with an auxiliary engine, or by using a power take-off to redirect output from the vehicle’s main engine.

Auxiliary engines work best when you need lots of power and torque (like oilfield drilling) or a dedicated source (welding rigs or power generation). And on occasion, you’ll see an auxiliary engine on a bulk tanker trailer to pump off some product or other.

But when you’re pumping air (as with the pneumatic unloading of dry bulk goods) or hydraulic fluid (to power cylinders or motors used on dumps, mixers, snow plows, or refuse haulers), a PTO will suit you best. Each application requires different equipment, and the capacity, installation ease, manufacturer, and maintainability you need will vary depending on the use (and the experience of the buyer). Thus we won’t try to begin to list all the driven equipment available. Instead, let’s look at the various types of power take-offs that can be installed on a truck or tractor to power the equipment.

Basic Designs

A PTO takes power and torque from the engine either directly (straight off the engine in some way) or via the drivetrain (by tapping into the transmission or driveline). The engine and transmission must have special openings or fittings to accommodate the PTO; sometimes the truck frame must be lengthened or cleared for mounting, and the driveline shortened or altered in some way.

Depending on where it’s mounted, a PTO can be fitted directly with one or more pumps or can send power through one or more shafts to the pump or other driven equipment. So while the PTO itself may be relatively inexpensive, adding on the other equipment and engineering the chassis to accommodate it adds cost.

A PTO mounted on a transmission or in the driveline is said to be “clutch dependent”; one mounted on the engine will be “clutch independent.” These describe whether the truck’s clutch must be engaged for the PTO to get engine power, or whether power is constantly coursing through it.

Most PTOs have gears and shafts enclosed within a case and, depending on their intended use, resemble 4×4 transfer cases or even transmissions. The more complex types can have two or more speeds and/or a reversing gear, and be engaged or disengaged with clutches or operated with powershift mechanisms.

The gears may be the “spur” type with straight-cut teeth (that is, the teeth edges run perpendicular to the gear’s face and parallel to the axis on which the gear spins), or the “helical” (teeth are cut diagonally across the face). Helical gear teeth are angled in “left hand” or “right hand” fashion, and one of each is required to mesh two helical gear sets.

Levers, knobs, and/or switches inside the cab let the driver engage or disengage the PTO and work the equipment it drives. Controls will vary with the type of application and, of course, the PTO and equipment package installed to handle the job.

If the PTO is used while the truck is under way (as with trash trucks or cement mixers), normal engine revs are probably sufficient. With the truck stationary, the engine usually must be speeded up to a certain point after the PTO is engaged. The driver does this by pulling out the manual throttle or using cruise control switches. Electronic engine controls can be programmed to respond to electronic PTO controls, automatically raising engine speed to the correct level when the PTO is engaged. The latest electronically controlled automatic transmissions also work with electronic controls on engines and PTOs.

Among the advantages of some (but not all) electronic PTO controls on the popular six-bolt side-mount type is that they shut off the PTO when the driver switches off the engine. This prevents accidental engagement with dangerous consequences later (such as a rising dump body hitting an overhead bridge). Also, if the LED warning light is burned out, the PTO won’t run until the LED is replaced.

PTO Types

PTOs are described by mounting type, location, how they work, or how they themselves are driven.

A side-mount PTO attaches to the side (and sometimes the bottom) of the main or auxiliary transmission. It takes power by meshing a PTO gear with a gear inside the transmission. In a heavy-truck transmission, that gear will be on one of the two countershafts. Many transmissions are built with side apertures covered by plates; a plate is removed to accommodate the PTO unit, which bolts up by means of a standard six- or eight-bolt pattern. The six-bolt PTO usually goes on the right side of a heavy-truck transmission, and the eight-bolt on the left side or the bottom. Midrange transmissions usually have six-bolt apertures on both sides.

The six-bolt pattern is also used on both sides of Allison’s new 1000/2000/2400 series automatic transmissions for use with Chelsea PTOs. Allison’s MD and HD World transmissions use a special 10-bolt pattern designed by Allison and Chelsea.

Because of its versatility, ease of installation, and low cost, the side-mount PTO comprises more than 90% of installations today. Six-bolt types are typically used on dump trucks and side- and rear-loading refuse packers, and stronger eight-bolt types are common on bulk-hauling tractors.

A top-mount PTO sits on the top of an auxiliary transmission. This is more common in extra-heavy-duty applications, such as in oilfield trucks, where the top mount PTO drives a “power tower” to operate big pumps and other equipment.

A split-shaft PTO mounts to the frame within the driveline, behind the transmission, “splitting” the vehicle’s driveshaft. The driveshaft still propels the vehicle, while the PTO’s output shaft powers the driven equipment, though seldom at the same time.

A front-mount/crankshaft-driven PTO takes power off the front of the engine, which is especially useful where power is required while the vehicle is in motion. Sometimes a clutch (within a “clutch pack”) will engage or disengage to start or stop the PTO. The truck’s frame rails must extend beyond the grille to mount the PTO and the hydraulic pump it drives, and, of course, the bumper extends ahead of this assembly.

You’ll see this on dump trucks outfitted to plow snow. The PTO drives the hydraulics, which raise, lower, and swing the plow blade (or blades, if a wing plow and/or underbelly scraper are also used). Because the PTO can be entirely disengaged, it’s dormant in warmer months when the truck performs hauling and utility duties.

The front-mount PTO once was common on cement mixer chassis and front-loading refuse packer trucks. But some users of these vehicles have been buying hydraulic pumps mounted right to the crankshaft, a workable arrangement because hydraulic power is needed almost all the time in such service. The front-mount PTO is also relatively inexpensive.

Either way, the PTO sits on a forward frame extension, and needs a special radiator to allow the pass-through drive shaft.

A flywheel PTO is sandwiched between the engine and clutch or transmission and driven by the flywheel. Since it supplies constant power, it can be an alternative to the front-mount PTO, especially if the vehicle’s overall length is an issue. But the flywheel PTO is about 6 inches in length, and the cost of moving the drivetrain back to accommodate it can be considerable.

A gear-driven PTO is driven off the front gear train of Volvo diesels. A hydraulic pump is bolted directly to it, and the resulting package claims power and torque capacity and a cost approaching that of the six-bolt, side-mount PTO. A disadvantage is possible clearance problems with other components within the engine bay.

Gearing & Capacity

The gearing and capacity you need depends on what’s required by the driven equipment. A pump, for example, is made to run in a given speed range and takes a certain amount of power.

Hydraulic pumps ideally run at 80% to 120% of engine rpm, while an air compressor or “blower” will run at 150% to 300% of engine rpm. “Product pumps” run at 50% of engine speed.
Matching desired speeds of pump and engine mean the PTO needs the correct input and output ratios.

The PTO must also have the capacity to handle the power and torque running through its gear set. In order of strongest to weakest in capacity are the flywheel PTO (typically 800 to 1000 pounds-feet); top-mount (up to 650 pounds-feet); split-shaft and eight-bolt, side- or bottom-mount (up to 500 pounds-feet); six-bolt, side-mount (300 pounds-feet); front mount/crankshaft driven (250 pounds-feet); and gear-driven (150 pounds-feet).

Among the design considerations is lubrication of the PTO gears. Sometimes transmission lube will be shared with the PTO; in heavier duty or “continuous operation” applications, it may require a pressurized lube system. Continuous is defined as five or more minutes of operating time in a 20-minute period.

Spec’ing Considerations

Aside from the characteristics and advantages/disadvantages of each type of PTO are considerations regarding the vehicle itself. So you have to look at the engine, chassis, PTO, and driven equipment as a package.

Among things to watch out for are interference from various chassis components. These include the front suspension, especially with a setback front axle, where the rear spring hangers and shackles can be close to the transmission. PTO manufacturers make angled geared adapters just to get around these problems.

The exhaust system can sometimes get in the way of your desired PTO setup, and one may have to be modified to accommodate the other.

Remember, too, that the more crowded the truck’s frame rails, the more difficult the PTO installation.

Suppliers of PTOs and driven equipment, up to and including body makers, will work together to make sure you get the performance you need. But it doesn’t hurt to look over their shoulders.