I have seen this term used a lot for farm tractors. No idea what it means except it is the best thing ever. No tractor without it is worth looking at etc etc..
It usually means the tractor is propelled hydraulically and you can reverse from forwards to backwards by just tipping a pedal, or moving a lever. Now the tractor might still have gear ranges, for different speeds, but to go from forwards to reverse, you don't have to clutch, hit nuetral and shift into reverse to do it. Used to run a Case 480 that was that way, the little International I had wasn't like that, you had to shift it manually.
What Brian said. The shuttle is useful for say a front end loader where you need to change directions often and this saves using the clutch. For plowing and discing where brute strength is important some prefer a gear drive tranny. Some people prefer a "hydro tranny" This unit has two petals and you push on one with your foot to go foreward or the other to go backward and that is all there is too it. The farther you push the petal the faster you go. Hydro's are very smooth and easy to use so they are great for mowing and loader work.
A lot of fork lifts use shuttle systems such as this . Just push throttle to desired speed and push on top of pedal, kick with heel and you go into reverse, seen lots of first time operators jirk their heads off or hit a wall . Cat had them as far back as 1970
My tractor has 12 manual speeds. After picking a gear, the shuttle is used to go forward or reverse, or neutral, while leaving the transmission in whatever gear you picked. You can also use the foot clutch to pause with the shuttle in forward or reverse. The shuttle is a pretty handy feature.
The device is a Hydrosatic transmission. Try a Google search.
Terminology varies in different regions but most responses here are not correct. The "hydrostatic" transmission is *not* a "shuttle shift". It is an infinitely variable hydraulic transmission. One pedal does it all--stops/starts/reverses + varies speed.
IMHO--after a year on a manual gearbox tractor you'll be so tired of grinding the teeth in the (typically) non-synchro gearbox, that you'll wish you'd bought a "hydro".
The shuttle shift is an improvement on the manual transmission (especially for loader work where frequent changes of direction are required) --see ColoradoBoy's description.
It is *not* "the best thing ever". Such statements are highly dependent on what it is you need to do with the tractor, and to some degree what one compares this feature to. Yes, it's a big improvement over a straight manual, but again, depends on what you need to do with the machine.
Shuttle shift basically adds forward/reverse gearing *after* the main transmission--giving you instant access (via a second shift lever) to either direction while in *any* speed.
A typical gear-drive tractor will have a four speed tranny, coupled with a hi/lo or hi/med/lo range transmission giving 8 or 12 speeds. The addition of shuttle shift provides forward and reverse for all available speeds.
AS has been pointed out, for heavy *ground engaging* work, most prefer gear drive over a hydro. Also, typically hydros are only available on "compact" tractors.
Some modern gear drives have synchromesh boxes and others go even further with designs that allow rapid "clutchless" shifting. These are a huge improvement over non-synchro manuals.
But if your needs can be met with a tractor of 50 hp or less, and you can pay the entry fee--Hydro is by *far* the sweetest system you will ever use.
I have heard, but have no direct experience, that early shuttle shifts had reliability problems and were expensive to repair.
The "shuttle" section is just a reversing gearbox--nothing special. It is purely mechanical just like the rest of a manual transmission.
This reversing gear is handled like any other shifting operation: clutch disengaged, stop tractor, shift to/from reverse and re-engage clutch.
Failing to come to a complete standstill will result in the gnarling and gnashing of teeth. Most shuttle shift transmissions are plenty rugged but, it stands to reason that this particular mechanism is going to be employed heavily when doing loader work and the like.
Many operators, if careless and/or in a hurry, are going to inevitably fail to stop completely before attempting to change direction and therefore will be trying to force a shift with moving gear teeth. In other words, the shuttle will have to withstand a lot of abuse and is likely going to be the first thing to break if not of a bullet-proof design.
My 1958 Oliver OC-4D dozer has a mechanical
"Travel Reverser" located between the engine and the main 4 F + 1R main transmission and requires the clutch on the engine flywheel to be disengaged when shuttling between Forward and Reverse. A great time saver even though you have to clutch every time !
My son's 1973 John Deere 350-B dozer has a F - R shuttle transmission located between the engine and main 4 F transmision. This shuttle transmission uses hydraulically actuated clutch packs to shift between Forward and Reverse and does not require the clutch on the engine flywheel to be disengaged to "shuttle shift". An even greater time saver for dozer ( and loader )work !!