What does it mean when you have a "synchronized" transmission?

[RoMad]

I could shift my 06 4cyl Tacoma without the clutch if I gave it a little throttle as I shifted. I only did it a couple of times gently just to see if I could.

 

[dave1949]

As Ralph said, tractor gears are not synchronized like in a car. You can change gears while rolling with the clutch depressed, but the gears won't mesh easily unless they are turning at a compatible speed. My tractor, rolling along in fourth cannot be downshifted to second for example, until the tractor is going slow enough for the gear rotation speed needed by second gear.

Truly synchronized transmissions will allow you to go into a gear that makes no sense, for example your car can get into third gear when you really wanted fifth, and your speed is way too high for third causing the engine to over-rev.

Tractor-trailer manual transmissions have a fairly standard 400 engine rpm difference between successive gears. They are actually very easy to shift without the clutch in most situations, and lots of truck drivers do not use the clutch unless they know from experience that they are in a situation where they need to.

 

[dave1949]

RalphVA "synchronize the engine shaft rpm with the gearbox shaft rpm"

This is the key to getting non-synchronized gears to mesh.

I don't know if it helps understand down shifting tractor gears, but when driving a tractor-trailer and approaching a downhill grade, you need to get into the gear that gives the engine braking required to not run away before starting down that grade.

Once you pick up too much road speed to match the engine rpm with the gearbox rpm, you literally cannot downshift. Double clutching won't help because you cannot run the engine fast enough while out of gear to catch up with the gearbox.

If you start down a steep grade and realize, oh crap, I'm in too high of a gear for this one, you are supposed to use your brakes while you still have some to get the truck down to the slow crawl needed to get into a low gear, or better yet, stop completely.

 

[RoyJackson]

Actually, when it comes to tractors...who really need to shift through the forward gears that often?
Most of my work is finish and field mowing (I'd wager that applies to a lot of TBNers). I might use a higher gear to transport to the area to be mowed, but once I'm there and the mower/cutter is down....it's pretty much the same gear (except for occasional shifts into reverse).
I have never shifted up through the gears...just start off in the gear I think is most appropriate.

 

[k0ua]

If synchronized transmissions on tractors don't exist, then why does my Kioti manual say it is synchronized in both the main gears and the shuttle.? I believe many modern tractor transmissions are synchronized over all forward gears in a range, and some older tractors may be synchronized in some gears in a range. I can't see any reason why a tractor transmission cant have synchronizers. Like I said, I am no expert, but tractors advertise as being synchronized. I bet they are.

James K0UA

 

[RoyJackson]

If synchronized transmissions on tractors don't exist, then why does my Kioti manual say it is synchronized in both the main gears and the shuttle.? I believe many modern tractor transmissions are synchronized over all forward gears in a range, and some older tractors may be synchronized in some gears in a range. I can't see any reason why a tractor transmission cant have synchronizers. Like I said, I am no expert, but tractors advertise as being synchronized. I bet they are.

James K0UA

Yep...
Unless it's a collar shift, it's probably has synchronizers.

 

[dave1949]

Actually, when it comes to tractors...who really need to shift through the forward gears that often?
Most of my work is finish and field mowing (I'd wager that applies to a lot of TBNers). I might use a higher gear to transport to the area to be mowed, but once I'm there and the mower/cutter is down....it's pretty much the same gear (except for occasional shifts into reverse).
I have never shifted up through the gears...just start off in the gear I think is most appropriate.

I do. I might start in second gear on rough ground, come to a smooth area and up-shift directly to fourth--all at a constant throttle setting and rolling. In hi range (which I don't use often) I start in first gear and often shift up to second just because it's less wear on the clutch. Up-shifting is almost never a problem, but my tractor sure doesn't downshift like it has synchronizers.

I'm not saying all tractors are the same, but mine is said to have synchronized gears. I take that to mean you don't have to stop, but you do need to be traveling at some compatible speed.

 

[David_Kb7uns]

Grand L series Kubota tractors can be had with HST, FST (Fully Synchronized Transmission) and GST (Glide Shift Transmission) the GST is the most unique where the ECU (Electronic Control Unit) takes input from the shifter (lever with a potentiometer attached) the engine tach and travel speed sensor, and shifts the shifting forks and modulates the hydraulic clutch. So not only there are tractors that have synchronized transmissions but they can clutch, shift and declutch totally by themselves.

So yes hate to say the world is not flat either.

David Kb7uns

 

[TerryR]

A synchronized transmission means it has a mechanism to cause the input (engine-side) part of the transmission reach the appropriate rotational speed relative to the output (wheel-side) part so the gears can engage without "grinding" when you shift from one gear to the next. By contrast, an non-synchronized transmission (also called a "square box") has no such mechanism and depends on the skill of the operator to prevent grinding of the gears.

The intent is that one would still use the clutch to shift gears, but the synchronizing mechanism means the two parts of the transmission are automatically synchronized to the proper rotational speed while the clutch is released.

Double-clutching is a technique to manually synchronize the two parts of the transmission when there is no mechanism to do that automatically - that is in a non-synchronized transmission. When shifting to a higher gear this isn't generally needed, since pausing an appropriate amount of time will allow the input side to slow down by friction to engage properly. But when down-shifting you need the input side to speed up, which can be accomplished by a skilled driver by double-clutching. That is, shifting into neutral, engaging the clutch while accelerating the engine to the proper speed, then releasing the clutch while you engage the lower gear.

While a really skilled operator can accomplish all this without using the clutch, and do so more easily with a synchronized transmission, that's not what a synchronized transmission is designed to do, and doing so no doubt causes increased wear on the synchronizing mechanism.

Of course all this only matters when you shift from one gear to the next while the vehicle is moving, so is isn't that important for tractors, which typically are started from a standstill in the intended gear.

Terry

 

[milkman636]

Going between two properties that are almost 3 miles apart, I've up-shifted on a roll with my JD often when pulling onto the road and getting back up to speed from stops. Starting from a dead stop in the top gears would definitely put a strain on things, and sometimes isn't even possible. As a matter of practicality I think shifting on the fly is actually a common need.

From what I recall from reading about the synchronized shifting offered in my particular vintage of tractors is that it applied to shifting between gears within the same range, but I've also been around some newer powershift models can run up through all the ranges from a stop. For me I would want to hear from someone who owns an operating manual for my tractor before I tried anything between ranges though.