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Tackling Tractor Transmissions

Here we explain transmissions to help the tractor buyer choose the best for his or her needs.

By Oscar H. Will III

Tractor manufacturers are continually upgrading and improving transmissions to help make their machines last longer, become more fuel efficient and help operators accomplish the tasks they require more safely and easily. And, as you might imagine, some of the more flexible and high-tech transmissions add cost along with incredible value, if you need it.

With new tractors, you’ll want to consider the categories of transmission systems available. Be aware that many transmission systems actually have two separate components, typically called the range transmission and the speed transmission. Most of the new bells and whistles pertain to the speed transmission, while the range transmission requires stopping and often clutching to change the selection. Range transmissions generally have two (high, low) or three selections (high, medium, low), while speed transmissions might have three or more (first, second, third, etc.) settings, or no settings at all, being infinitely variable over the range.

Geared Speed Transmission

The oldest and simplest configuration is with a geared speed transmission. You usually need to stop, clutch, shift gears and declutch to change your speed or direction with this tried-and-true design. In some cases, these transmissions will be “synchronized,” which means you can shift up or down while the tractor is rolling, but not underload; you’ll need a true powershift transmission to do that. Powershifts are still found in some larger tractors, but not in the smaller machines we’re discussing here.

Still others in this category offer a synchronized shuttle shift or reverser. The shuttle shift gives you a second hand lever to easily switch from forward to reverse without coming to a full stop. Other subtle variations of these combinations exist, but in general you operate the clutch to make the shift, and the clutch is often a dry design, meaning it is not bathed in oil and can wear more quickly.

Power Shuttle Transmission

Power Shuttle Transmission

Power Shuttle Transmission

An upgrade to the relatively simple gear transmission is the power shuttle transmission. This gear transmission features a forward-reverse shuttle lever that, when moved from one position, through neutral to the other, will automatically clutch the tractor, make the shift and reengage the clutch, based on tractor speed and rpm. To the novice, this may seem absurd, but power shuttle is wonderful if you do a lot of forward and reverse shifting, such as with a loader. The clutches in these machines are usually bathed in oil and hydraulically actuated for smooth control and long life.

Also, it’s good to know that some tractors offer a feature that allows a customer to fine tune the aggressiveness of the forward/reverse movement. This helps customers find the shift that’s just right for them.

Hydrostatic Transmission

The hydrostatic transmission, which, also, is usually coupled to a gear-type range transmission, relies on high-pressure hydraulic oil to spin the rear output shaft and, subsequently, the wheels. This design allows an operator to control the speed simply by adjusting the volume of oil the hydraulic motor receives.

Today, these transmissions are operated by pedal. Push the go pedal (or rock the pedal forward on single-pedal models) to move forward; the harder you push, the faster you’ll go. Press the reverse pedal (or rock the pedal backward on single-pedal models) to reverse. With most hydrostatic transmissions, the tractor stops when you take your foot off the pedal(s) or return to the neutral position yourself.

Know that there are also two types of hydrostats: non-synchronized often found in more basic models that do have a clutch or e-hydros that are often found in premium tractors. E-hydro can offer more features like cruise control, modulation control to fine tune the aggressiveness of that control, max speed limit and most importantly no clutch that can wear out.

Early hydrostatic transmissions had a reputation for overheating with heavy use, but that’s not the case with today’s highly engineered designs. Just make sure you keep squeaky clean hydro oil in the system, change the filters as recommended, and keep the transmission oil cooler fins clean.

Tops in Transmissions: The CVT

The pinnacle of tractor transmission design at the moment may well be the continuously variable transmission (CVT) system, which in AGCO’s tractors works via a patented pressure-lubed and variable pump and motor design. It’s the industry’s most efficient deign of transferring horsepower to the ground.

Having originated at AGCO, these transmissions are marvels of modern engineering, and manufacturers sometimes incorporate a CVT into a system that also uses a hydrostatic unit. An advantage to the CVT over the hydro is that it can be more efficient since the fluid connection of the hydro allows for considerably more energy lost to heat. There are only a few compact tractors with this type of drive system today, but large tractor innovations almost invariably trickle down. Stay tuned.