Tractor transmissions - The benefits of the various systems

Tractors are a part of daily life for many groundsmen and greenkeepers. They are the beasts of burden: loading trailers, towing trailers, tipping loads, towing mowers and powering attachments like aerators and sprayers.

Since the invention of the modern tractor (by Sir Harry Ferguson in the 1930s), there have, of course, been many changes to the size, power and comfort of a tractor. If you compare the old steel pan seat on a leaf spring, to the modern full suspension seat, possibly in an air conditioned cab, of today's machines, it is easy to see just how far tractors have evolved.

One of the major changes has been to the transmission systems available on a modern tractor, and the choice that the potential buyer faces when purchasing a new machine. In this article, we look at the current offerings and try to establish the benefits of each transmission type.

Manual Transmissions

The original drive system relied on operating a clutch to change gears, range and to engage/disengage the PTO drive by use of a 2 stage clutch, where full depression stopped all drives, and partial depression disengaged only the gearbox.
Today's PTO drives are all independent, removing the need for a two-stage clutch.

Originally, the gear levers were situated between the driver's legs, on top of the gearbox (as they still are in a car). There were two levers, one for the gears and a second lever which changed the ranges, allowing a wide spread of forward speeds and full engine power. There was a separate reverse gear, just like a car.

Gear levers were moved to either the side console or the steering column, as they made getting on or off the tractor much more difficult. Moving the levers, and developments in cab design to make a flat floor, allowed the easy access that we take for granted today.

Shuttle shift: Today, there isn't a separate reverse gear. Instead, we have a forward and reverse lever, allowing a change of direction without having to change the gear the tractor is in. So, if we have twelve gears, we have twelve forward and twelve reverse gears!

Note: Whilst you are in the same gear, your tractor will run slower in reverse (for safety reasons).

The basic manual transmissions of today still require clutch operation to change direction on the shuttle lever, and to change gear. The more advanced versions of this have a hydraulically operated forward and reverse shuttle, which allows a direction change without using the clutch. The addition of a progression engagement control now allows the driver to set the speed at which the tractor reacts, allowing them to drive without using the clutch at all.

Intelligent Transmissions: The latest manual transmissions now allow the driver to change gears, and to change direction, without using the clutch. An electronic gear change is achieved by pressing an up or down button on the gear stick, and forward-reverse is selected by using a shuttle lever (usually beside the steering wheel). The shuttle gives an additional neutral, making it easier for the driver to operate safely. The only time the clutch has to be used is to select the speed range the tractor is going to be working in.

So, from an operational point of view, the modern manual transmission has come a long way. Near clutchless operation and semi-automatic gear changes allow the driver to select the correct speed and power output to match the task. The benefits of positive gears are a fixed forward speed, and this is critical for accurate spraying, fertiliser applications and, of course, the essential turf maintenance task of aerating.

HST (Hydrostatic Transmission)

Hydrostatic transmissions have been around for a long time, and they have also evolved over the years to be much more efficient and economical. The core benefit of an HST transmission has always been the gear-free driving experience; no clutches or gears to shift, just press the pedal and go! It is the automatic transmission for tractors and, when less experienced staff are driving or multiple staff are using it, an HST transmission can be a real blessing.

HST tractors have always been great for loading in the yard, or pulling a set of gangs around; this is when the instant forward and reverse feature really comes into its own. However, the Achilles heel has always been the requirement for a fixed forward speed when using an aerator or sprayer. Until recently, an HST tractor would not give the constancy required for even hole spacing. This has now changed with the introduction of electronic cruise control on tractors. Cruise gives a consistent forward speed, whilst a resume function allows the tractors to return to the pre-set speed after a turn, meaning that hole spacing is consistent.

Auto throttle: Another previous drawback of the HST transmission was having the engine running at a high speed, all the time, in order to provide power when it was needed or, if that was too noisy, then constantly changing the hand throttle setting to try to achieve a compromise. The advent of the auto throttle system, where the engine speed increases as the foot pedal is pressed, brings a near fully automatic transmission to a tractor. It's particularly useful when towing a trailer and for road driving, as the engine returns to tick over when the transmission returns to neutral. This reduces noise and vibration, reduces fuel consumption and, consequently, engine emissions as well.

So, from an operational point of view, the ease of driving has always been the main attraction of an HST system. Now, development of the auto throttle and the addition of cruise control has removed many of the negatives associated with HST transmission tractors.

CVT (Continuously Variable Transmission)

The latest trend in tractor transmissions has been the move to CVT transmissions. CVTs have been around for some years, and many mopeds, scooters, golf cars and utility vehicles use a simple belt drive CVT transmission. Tractor transmissions use a hydraulic motor to provide an instant forward and reverse facility, and then a mechanical drivetrain to give the efficiency and power delivery required.

On larger horsepower tractors, the transmissions have become hugely complicated (think about a 24-speed gearbox!) and, subsequently, very expensive. A CVT transmission is less complex and it allows the transmission to easily match engine power as required.

Without pre-set gears, the engine revs can vary to meet the power demand, whilst the forward speed can remain constant. This makes it easier to drive and can make it more fuel efficient.

CVT transmission also provides an "auto-stop" feature. When the drive is returned to neutral, the transmission is effectively locked. There won't be any rolling, or motion, until the control lever is moved, and the system returns to positive forward or reverse drive. In this way, CVT can show some good safety features.

Whilst well established in the agricultural market, CVT transmission has yet to really catch on in the groundcare sector. There are now some 40 and 50 horsepower models available, which will be tempting for some customers. Going forward, we are likely to see more CVT models available.

Conclusions

So, which system is right for you? The simple manual transmission will give years of reliable service, and it's always going to be the lowest priced option. Going up to an intelligent transmission will give near clutchless operation, seamless gear changes and smoother driving.

HSTs will still be popular for grass cutting - with a mid-mount deck they are very good - and the auto-throttle versions make general driving duties a pleasure. Add a cruise control and there is not much to choose between them. If you have occasional or less experienced tractor drivers, they are an obvious choice.

The newer CVT systems are yet to become fully established, but they potentially offer a real mix of the best features of the other systems, the variable speed and instant forward and reverse of the HST, and the mechanical efficiency of the manual transmission.

It all depends on your application and situation; get a demonstration of each type of transmission system so you can see for yourself how they can meet your needs.