Power takeoffs: The what, where and some of the why

What is a PTO?Put simply, PTO means power takeoff. PTOs are mechanical equipment used to take power off or from a primary drive system — generally to do some type of work.The primary drive system can be mechanical, as in the driveline of a commercial truck or other vehicle, usually at the appropriately equipped transmission, the engine, or the propeller shaft itself. Alternatively, PTOs can be electrically driven with or without a dedicated motor, sometimes in a combustion engine vehicle, but often in a zero-emission electric vehicle (EV).What should be considered for a PTO application?First, it’s critical to understand how a PTO will be used. Consider the type of equipment it will drive, its speed, power/torque requirements and duty cycle.For any given application, only an appropriately selected and applied PTO can provide a durable and safe operating solution. In addition, the spatial requirements of the PTO — and its driven equipment — must be established and accommodated to permit mounting. This includes required piping (for lubrication and/or engagement) and electrical control equipment.This means the PTO application must be fully defined prior to the selection of a chassis. Often, a chassis has been received without the knowledge or specific details of the operating conditions for the driven equipment. An under-spec’d PTO might lack the necessary operating conditions or potentially damage the equipment which drives it. This may result in costly repairs or require sourcing of an appropriate replacement chassis with necessary capabilities to meet the operating duty.Duty cycle is another important factor in a PTO application. When selecting a PTO model and type, consider the loads and heat produced during PTO operation to ensure these parameters fit within the operating capabilities of the PTO and the equipment driving it.PTO vs. PTO provisionDistinguishing the PTO as added auxiliary equipment from the PTO’s provisions is key. Provisions are the features and equipment included with the chassis that make it capable of receiving and driving the PTO. The most frequent example of this is at the transmission, where mounting the PTO requires an aperture, or opening, in the transmission case. (We’ll discuss this in more detail). Within the transmission, there must be a gear to drive the PTO, and the chassis usually requires electrical connections and an electronic module or controls to activate the PTO. It can be an expensive mistake to assume that the transmission can receive and drive a PTO, only to later discover its capability is lacking these provisions. For example, a necessary transmission was not ordered for a given chassis because that particular chassis model does not normally have a PTO-type transmission in a base configuration.PTOs can serve many purposes. For example, a pump might be directly mounted to the PTO to drive hydraulic equipment. In other applications, a PTO may be used for pumping gases, liquids and bulk solids, or for driving blowers, cooling equipment and electric generators. In contrast, a driveshaft may connect the PTO to the driven equipment, rather than being directly mounted. In this case, shaft length and diameter, as well as operating speeds and joint angles, must be considered just as they are for the propeller shafts in a vehicle driveline.PTO driven equipment types include: hydraulic pumps, gas/liquid pumps, blowers, compressors and electric generators. PTOs can be mounted by chassis manufacturers (OEMs) or their distributors/dealers, body and equipment installers, or end-users.Traditional PTOsTransmission make and modelThis information is critical in order to select a PTO manufacturer and model that will work with the transmission. Application guides published by PTO manufacturers are arranged by transmission make and model.Transmission PTO aperture — Where will it connect?Left (driver) side/right (passenger) side: six-, eight-, 10-bolt openings Some transmissions also have PTO mounting provisions on the front (e.g., with two- and four-bolt openings), bottom, top, and rear. Depending on driveline capability and the application requirements, more than one aperture may be utilized.The mounting bolt arrangements for North American sourced and sold transmissions are usually based on SAE standard patterns. For some North American sourced transmissions, and for those globally sourced, the bolt pattern may use the manufacturer’s proprietary pattern or follow another industrial standard. An adapter spacer can change the mounting pattern of the transmission aperture to accept an alternate PTO flange pattern. Sometimes a larger PTO input gear diameter may be accommodated with a flat spacer between the transmission and PTO. Limited frame space or component interference, such as exhaust pipes, may be addressed with an angled spacer.Speed and PTO percentageThe operating speed of the PTO is expressed as a percentage of the engine speed. For example, the output of a 125% PTO turns at 1.25 times the speed of the engine. Rotational directionThe driven equipment almost always has a required direction of rotation, so it is critical to note the driven direction of the PTO. Rotation is described as the same as the engine (ENG) or opposite it (OPP). If necessary, and if space permits, the direction can be reversed with the addition of a geared adapter. If the driven equipment requires rotation in both directions, a reversible PTO is a possible drive solution.The driven equipment has a required power value, expressed in horsepower or kilowatts and speed. Since PTOs are rated by maximum torque capability, the power value is used with PTO operating speed to determine the applicability of a given PTO.Torque = Power x 5252/Speed … Torque in lb-ft, Power in hp, Speed in rpmTorque = Power x 9549/Speed … Torque in Nm, Power in kW, Speed in rpmIt’s important not to exceed the maximum torque rating of the PTO or the transmission, whichever is lower. PTO mounting and type of engagementA critical step in mounting a PTO to the transmission is the proper setting of the backlash (spacing between the drive gear in the transmission and the driven gear in the PTO). Backlash is controlled by the mounting space between the PTO case and the transmission case by using gaskets of various thicknesses or a compression gasket. This can be checked during mounting with a dial indicator. A PTO with excessive backlash will produce a chattering noise. One with too little backlash will create a whining sound. Due to excessive pressure on the PTO idler shaft and bearings, a tight backlash can also cause premature wear. Checking for noise before installing a PTO establishes a baseline that can be used to diagnose any change in sound after PTO installation.While the transmission gear and driven PTO gear are constantly meshed, rotation of the PTO output is controlled by engaging the PTO idler gear. This can be controlled manually with a cable or “wire” shifted lever, electrically with a solenoid or motor, by vacuum, by air (pneumatic), or hydraulically with a pressure line from the transmission.In some instances where space is available for a larger PTO case, the output rotation can be clutch-shifted using a clutch or clutch pack within the case. This permits “hot shifting” of the PTO without having to bring the PTO drive gear to a stop.Intermittent vs. continuous duty cyclesAs mentioned, duty cycle is an important aspect of the application. In general, intermittent duty is considered five minutes of operation out of 15. Continuous duty operation is use greater than five minutes in 15. For continuous duty operation, maximum power and torque ratings are typically reduced by the manufacturer. Note that applications with PTO output shaft speeds above 2000 rpm are considered continuous duty independent of the operating duration.Overspeed protectionA speed sensor can be used to automatically disengage a remotely-controlled clutch-engaged PTO to prevent excessive speeds in the driven equipment.Other considerationsDepending on the application, additional space on the chassis frame or within the body may be required for air or hydraulic fluid reservoirs, and/or heat exchangers. Together these components are described as a wet line kit. PTO vs. clutch pump A clutch pump is a rotary device, similar to a PTO, that is used on a vehicle to do auxiliary work. Like a PTO, a clutch pump uses rotational energy from the vehicle. It is a pulley-equipped hydraulic pump that’s driven with one or more belts, usually located at the engine front end accessory drive (FEAD), but sometimes at the transmission or even the propeller shaft. In this way, a clutch pump is similar to a PTO that drives a hydraulic pump. The clutch pack in the pump is used to independently control output while the engine turns. It can usually be engaged using a control switch operated by the vehicle driver or automatically, based on inputs from hydraulic system requirements.PTO output configurationsPTOs offer two output types for transferring power to driven equipment: shaft and direct-mount.Shaft outputThis permits the application of remotely mounted driven equipment which may be required due to size in situations with limited available space, or due to body design or functional configuration.Direct pump mountThis eliminates drive shaft and complications involving shaft-induced noise and vibration and speed limitations.What’s a FEPTO or a REPTO?Front engine power takeoff (FEPTO) is driven directly by the engine crankshaft and is mounted at the front of the engine. When available space is limited, the crankshaft may be connected through the radiator to the driven equipment on or near the front bumper. FEPTO applications include hydraulic pumps for refuse applications, heavy-duty snow control equipment, and remotely operated equipment on emergency vehicles.A rear engine power takeoff (REPTO) is also driven by the engine crankshaft but mounted at the rear of the engine instead of at the transmission.Note, in either case, the input shaft to the PTO turns when the engine turns, so drive control is required at the PTO output or the driven equipment to vary operational speeds from maximum to full stop.Engine flywheel PTOThis PTO is mounted between the engine and the transmission and offers up to 100% of the engine’s available torque. Flywheel PTO applications include pumps, generators, blowers and vacuum pumps.Split-shaft PTOThis PTO type is located aft of the transmission and is driven by the vehicle propeller shaft. For operation, the PTO disconnects the driveline from the rear axle and routes power to an auxiliary jack shaft. This configuration offers maximum horsepower and torque from the engine. Because the PTO is driven through the transmission, the vehicle must be stationary during split-shaft PTO operation. A familiar example of a split-shaft PTO is in the operation of a fire apparatus pump.Electric power takeoffs (ePTOs)Electric PTOs are used in vehicles with high voltage power, such as a dedicated battery electric vehicle or a hybrid combustion-electric type. Instead of being driven mechanically through a gear, shaft or belt, an ePTO is powered by electric current connected to a high voltage system. This method can offer improved flexibility for equipment placement on the chassis or body; further, it allows PTO function to be independent of the vehicle’s driveline operation. Electric PTO applications are often similar to mechanical PTOs, and are used to drive pumps, hydraulics, refrigeration equipment, or motors and actuators. Since it is powered electrically, an ePTO can have non-rotary functions. For example, it might supply converted power to a generator or be used off-vehicle for jobsite power or electric power tools. These significant variations make it important to properly interpret objectives when an ePTO solution is requested. For higher power applications, an ePTO can use internal oil cooling or even external water cooling.A proposed connector: the Recommended Practice SAE J3253, Electric Power Take-Off Interface Requirements for Commercial Vehicles describes guidelines for high voltage (from 60 to 1200 volts) DC power transfer between the OEM chassis electrical system and electrically powered body accessory equipment. It describes a recommended interface connector design and communication modes for operation, diagnostics, and isolation management.This article provides a basic understanding of power takeoffs and their use and applications in the commercial vehicle industry. In many cases, a vehicle uses one or more PTOs to perform auxiliary work secondary to the vehicle’s use in hauling cargo. In other cases, such as the fire apparatus mentioned earlier, PTO operation is the fundamental purpose of the vehicle. No matter the scenario, understanding the needs and operating conditions of the driven equipment and the vehicle while engaged is key. PTO manufacturers can provide additional product and application-specific assistance through online selection programs, guides, and their technical sales representatives.Learn moreMembers may contact NTEA’s technical services team for additional information. NTEA offers access to publications and reference materials on current regulations, safety standards, and other technical issues at ntea.com. To learn more about the tools, resources and solutions available to members, visit ntea.com/memberbenefits.

Power takeoffs: The what, where and some of the why

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