There are a few general types of gearboxes, including planetary, spur/helical gear and worm gear. Each has their advantages and applications that they are best suited for. The most common type for mobile and off-highway machinery is the planetary drive. These consist of a center “sun” gear with three or more circumferential “planet” gears, which in turn are surrounded by a ring gear. Planetary gearboxes are very robust, and are also capable of very high radial loads, especially when the load is correctly centered. This is why they’re often found in wheel drive applications.
Spur- or helical-gear style gearboxes use two or more gears meshed side by side, and are often used in industrial applications, such as a rolling mill. Their size and possible gear ratios vary widely, with the upper end of torque capacity only limited by the size of gears able to be manufactured. These types of gearboxes can have input and output shafts on either the same or opposing sides.
Also quite popular are worm gear style gearboxes. They employ a helical axial input shaft, which directs force across a perpendicular gear ring attached to the output shaft. Operation is always with input and output 90° from each other. Worm gear units are generally light to medium duty.
Gearboxes are applied to either increase torque or increase speed, and it should be clear that increasing one decreases the other. When used to increase torque, they can achieve this at ratios anywhere from 2:1 relative to input torque, to 10,000:1 or more…which of course would slow the output speed drastically. When used to increase speed, sometimes planetary or helical/spur gearboxes can be used in reverse. A common use for overdriven gearboxes is for pump drives. A 1000-rpm PTO speed doesn’t take advantage of a hydraulic pump capable of 3000 rpm, so a speed-increasing gearbox/pump mount would allow for the use of a smaller pump. Please note, worm gear reducers do not allow input energy at their output shaft.
Selecting a PTO
When selecting a PTO, you will need to provide your supplier with various parameters, as these units are not universal. You will need to know your transmission model number, your output speed, your torque and horsepower required, and the shifter style desired. The shifter engages and disengages the PTO, and is available with air shift, electric shift and mechanical (cable) shift. Some newer PTOs are operated via a clutch instead of sliding gears, but the application information changes little otherwise.
You will also need to know what pad mount is necessary for the pump you’re running off the PTO, such as SAE 2- or 4-bolt flange, and the series, such as SAE B or SAE C 4-bolt flange. Special consideration should be paid when mounting a pump to the PTO, as port location could interfere with chassis components or the transmission itself. These issues are often resolved by installing an intermediate shaft, allowing the pump to be mounted in a clear location a couple feet behind the transmission.
Even with the high torque able to be transmitted by tractor PTOs, there are occasions when even more torque is demanded than a tractor can transmit (or slower speed than a standard 540 or 1000 rpm PTO). Although implements often employ pulleys and chain drives, which can be used for torque multiplication, the heaviest of demands require a gearbox for reliable and efficient operation. Of course, the use of gearboxes are not limited to farm applications, and are most often seen in wheel drives and other massive-torque low-speed applications.