Transmission choice and power

[Jon Bos]

I posted this question on the US board, and was dissappinted with the results - 0 replies. Highly dissappointing. Oh well, lets see what the truely intellegent people have to say to this one


Does the choice of transmission on a tractor affect the power it puts out? If so, how?

On 4wd tractors, you usually see a 12x3 or 4 transmission with no hi-lo. And on 75 hp tractors, you usually see a shuttle shift or transmission with 4 gears without shifting. And these machines do two totally different jobs. So why do they have different trannies.

Anyone care to explain the whole transmission to job application theory? Which trannys work best for what jobs? I know that a shuttle shift is good for loader work, but when do full powershifts shine? and stuff?

Lets see what you guys think?

[Friedhelm]

Hello Jon.

This discussion is as old as tractors are. In general trannys have to transfrom Hp and torque from the engine to different km/h at the tyres. If the tractor is used for ploughing and other hard pulling jobs it is very usefull to have as much gears as possible in the space between 4 up to 12 km/h . If you want to plough with around 8 km/h and there is only one Gear the engine has got to do the job if it goes harder, maybe you shift down to the next lower gear and will plough with 6,5 km/h . This downshift is easy with Hi/lo , but if you have change manually the gear you think twice before you shift up again. This is this reason for 4x Powershift trannys or the high class 18/6 Powershift. The approx. workingspeed is higher because it is easier to use the full power of the engine. Stepless is at the moment the highes level of transmission you can get. But like everything this things don´t come only with a nice site. More and more gears, espacilly with wet clutches inside reduce the effciency of the tranny. There are 4x Powershift trannys which eat up to 20 hp from the engine to the wheels in General they are around 15 HP. Full Powershift 20 - 25 hp. And stepless eats also around 20 HP. This is the reason why low power tractors don´t have this Trannys because the tractor would have work with himself. Take for example a 75hp tractor with PowrQuad or Dynashift or a Fendt 406 Vario, they´re all the smallest tractor you can get with the big trannys. Those ones are very slow and they don´t pull as much as the same tractor with a smaller tranny.

[Jon Bos]

Good reply - exactly the info that I was looking for - thank you. Anyone else care to add?

Maybe those 250hp Fendt tractors, aren't generating as much power to the ground as they should, because of all the dinking around with clutches and stuff. I'd almost rather buy a full powershift, or a 4wd tractor - more efficient. Is my statement true?

[HPP]

No, your statement isn´true!
You see, that´s the thing about the Fendt Vario - there aren´t any clutches! It´s only a planetary gear and a hydrostatic unit!
Now, when we look into "the Followers", that´s a different story. There you´ll find your clutches. And, that would be the expanation to why there´s a difference in efficiency between Vario and the others.

[Friedhelm]

It is not that easy. What do you want to do with the tractor? I´ve you want to plough and do the hard work on your ground 4-wd and a Full Powershift is something I would choose for in the HP Class above 150. Is this tractor doing more allround work a 4x Powershift is more usefull. Do you only want to mow grass make hay or something like that a very simple gearbox does the job. But that just are hard facts. What do you like most? I know several farmers here which own a tractor with stepless transmission who really don´t need this, but they like it. It is the same like driving 135 HP and you only need 100 HP. Stepless transmissions try to be comfortbale und efficient in one. But IMO we will have to wait untill the final versions are ready and what this will cost finally.

[Friedhelm]

HPP.

Vario also looses up to 20 % form the Engine to the wheels. The "followers" loses are almost the same. Will have a look at the testing reports. You´re right about the clutches, but Hydrostatic loses which you have in heat and leaks in the hydraulics are the main reason for the loses at stepless transmission. Because of that the Followers invented the 4 steps for their Trannys instead of the 2 at the vario. The reason is to have more mechanical transmission.

[HPP]

And still they don´t reach the efficiency of that in the Vario! (More than just around 8-9 km/h). I´ve read the test reports.
Hey, by the way, weren´t you selling AGCO-products? If so you´ll be out there soon demoing "Varios"!

[Fred]

I agree with the idea of the gearbox to suit the application and tractor size.
When there was only one Full powershift available in the UK many anti g***n drivers would comment on losses through the gearbox, fair enough - but compare the productivity by having clutchless shifts or now infinetely variable speeds to a clutched syncromesh or constantmesh gearbox and those extra gearbox losses are less significant. Where I worked the boss traded a TW25 (16 speed crash gearbox 153hp) for a 8630 (18 speed powershift 132hp). The 8630 could bale more Hesston 4800 bales a day and plough more acres and mow more acres due to its ability to change on the move. With the TW you chose a gear and stayed there. The land is not flat but still same thing.

The question with the vario is how many extra acres due you need to cover the extra purchase price.

[HPP]

And, by the way, as I stated on another post awhile ago: The 20% losses you were talking about in the Vario could be divided into : 8.0% in the Vario itself and 10.9% in the rear end (differential and planetary geras in rear axle). Total 18.9%.
There are some full power shift tractors reaching this level, but not all of them. And, why not go for stepless if you reach the same level of efficiency that you do in the "step-boxes"?
Besides, the efficiency in the conventionals vary with what step you´re in, the efficiency varies very little through the whole speedrange.

[Friedhelm]

HPP.

I am not demoning stepless. I just mentioned that stepless is not that wonderthing marketing tries to tell people. It is a comfortable way to drive , but the efficieny is only a little bit better than other Full powershift trannys. For example there are several 4x Powershift who have got a overall efficieny about 10 - 15%. I drove on CVX, Vario and on Autopowr. Like I said a comfortable way of driving and with some work it is also usefull or fits better than other trannys do. But the price for stepless is higher . It is a the customers who decide which they want.
BTW: Soon we will get Vario also. So it would be stupid to crack this down.

[eppie]

In general, you could say: The higher the working speed, the bigger the needed power varies.

For deep tillage, 3 tines at 70 cm depth on medium heavy sand soil, our 5245 can do the job in one gear, with good equal engine load, engine torque raise is sufficient to compensate differences in implement's pulling power requirement.

When deep tilling at 3.5 km/h, the roll resistance and mass powers have less impact than with sowbed cultivating at 12 km/h, because :

When speed gets 2 times higher, mass forces get 4 times higher.

When you drive through loose land or mudspots, the tractor is continously 'climbing up onto' the soil, to 'compensate' the sinking away in the tracks it makes.

So, if you double the working speed, the implement pulling power doubles too, and roll resistance, mainly due to mass forces, get four times as high.

Therefor, tillage can be done with gearboxes with a small spacing of powershifts/engine torque.


...I know this is not a good example, but the principle is true..