Power Take-Offs (PTOs) are designed to pick up engine power through rotation and transfer the power to another piece of equipment. For this to work, a piece of equipment can be mounted to the PTO or it can be connected by a driveshaft. The process begins with the PTO input gears meshing with one of the gears in a vehicle’s transmission starting the rotation. This rotation created from the engine drives the transmission and results in turning the PTO gear and rotating the PTO output shaft. Input gears must mesh properly with the transmission’s PTO drive gear for the PTO to work. But there are a series of gears that must be considered to determine the final output ratio of the PTO.
When analyzing the gears, a measurement term to be familiar with is gear pitch. Gear pitch is the measure of the size of the teeth and is determined by the number of teeth in a given area. To calculate the gear pitch, you would divide the number of teeth by the pitch diameter of the gear. Knowing the gear pitch is important since the PTO gear must have the same pitch as the transmission gear to function properly.
Another measurement term to be familiar with is gear ratio. Modification of the operating speed of the engine to the PTO driven device can be created through the gear ratio. To understand the PTO gear ratio, it measures the revolutions of the small and large gears. Looking at a smaller gear with 12 teeth driving a 24 teeth gear, the small gear makes a revolution with the larger gear only making half a revolution during the 1 small gear revolution. This means that the speed of the larger gear is half of the smaller gear, but the torque and twisting force is twice of the smaller gear.
The gear ratio in this scenario equates to the number of teeth in the driven gear (24) divided by the number of teeth in the driving gear (12). This results in a gear ratio of 2 to 1. The change in torque in this scenario is 1 to 2 resulting from dividing the number of teeth in the driving gear (12) over the driven gear (24). With the assumption of knowing the engine horsepower and the revolutions per minute (RPM) of the smaller gear, torque can be determined.
T = Horsepower x 5252/Speed (RPM) = Lbs. Ft. Torque
(As you can see from the above photograph, the two gears would lock as so with the red marked teeth)
Product series can have multiple gear ratio options or have just one gear ratio option. To select what ratio makes the most sense, you must know the RPM you want your vehicle’s engine running at for the application and the required operating speed of the driven equipment being used in the application. The ratio of a series of gears creates the speed for the output shaft. Those gears include the input driver gear, the input ratio gear, and the output ratio gear. Their relationship with one another will determine how fast the output of the PTO is spinning in relationship to the engine. The required speed for the driven equipment must be known in order to select the proper PTO ratio. When utilizing pumps, flow rate and displacement are needed to be determined beforehand to make sure the pump input shaft will work with the given speed from the PTO.
When looking into specific product series offered by Parker Chelsea, we want to highlight two different scenarios for series model codes. Starting with our new 210 Series for 2020 Ford Super Duty 10R140 transmissions, this series only has one gear ratio being 46/36 (internal ratio). When all of the gears in series are considered, the final output ratio is 144% of the engine speed. With this gear ratio, and using a 90% efficiency rating, specific pump options are offered for the 210 Series which include the CGP-P11, PGP-315, and P16 pumps. It is important to remember pump productivity is determined by the pump size in relation to the pump speed. Therefore, certain pump options may be more suitable than others depending upon the requirements of the application.
The 524 Series Rear Mount PTO is a little different compared to the 210 Series in relation to the gear ratio(s). The 524 Series has gear ratios of 1:1.00, 1:1.33, and 1:1.80. The design of the PTO itself is a two gear mechanically shifted Rear Mount PTO that is attached to rear mount apertures of a transmission. Rear mount apertures are becoming more common in the U.S. with European based transmissions becoming more popularized in the U.S. market. With the three gear ratios, this leads to different torque ratings being available in the market, therefore, increasing the number of applications that can be used with the 524 series along with optimizing the driven equipment.
This article was contributed by Michael Mabrouk, marketing leadership associate, Chelsea Products Division, Parker Hannifin Corporation.