CVT vs HST

[jagyzf]

In *theory* a CVT is the BEST transmission you can have because it is an Continously (infinately) Variable Transmission - meaning that you can operate the engine at it's most efficient speed (in terms of fuel, power or a combination as needs dictate) yet have complete control over ground speed. This allows (in theory) lower fuel use for the same job, CVTs are essentially mechanical HSTs, exactly.

In practice, they need TREMENDOUS amounts of pressure to keep the mechanical elements from slipping and destroying the trans in short order. With pressure like that, the actual transmission efficiency is MUCH lower than theory would suggest and therefore most OEMs have stepped away from CVTs and back towards GSTs with more ratios.

Comparing a 4 or 5 spd GST to a model with HST makes a pretty dramatic arguement for the HST because it is very easy to match the load to the ground speed, unless you get into a stall condition and that's another discussion.

Moving from a 12F/12R to a HST model is, well, pointless. Yes, there are 12 distinct "speeds" with the GST but in practice there is slmost no real difference with that many gears AND you get a greater percentage of FWHP to the PTO and drawbar with GST than HST or CVT.

HSTs and CVTs make a LOT of heat that the tractor needs to get rid of. How? A radiator of some sort and to move enough air over that radiator in all conditions you need a bigger fan than with a GST. Bigger fans take more HP to turn and the HP is a function of engine RPM, double the RPM and it takes 4 times the HP to turn the fan.

Wikipedia.org on CVTs: Continuously variable transmission - Wikipedia, the free encyclopedia

I think as someone pointed out, you are thinking of car trans, the auto industry is going away from there style. What OEM in AG is moving away from them? Deere, CNH, AGCO are all moving forward with CVT's, and coming out with more machines that use them. From what I have been told a CVT like Deere is using in the larger tractors makes less heat and is more efficent at putting power to the ground than HST. Deere has powershift with 15 speeds, and CNH 18, so if IVT/CVT were not an advantage, why would they be moving towards this? I can't imagine it would be just to offer it because Fendt did, Fendts are expensive and have no dealership support around here.

Here Deere is just adding CVT to the track tractor line. The article is from a few weeks ago.
http://www.fwi.co.uk/Articles/2010/05/05/121096/VIDEO-Tracks-v-tyres-John-Deere-8R-tractors.htm

 

[ModMech]

I think as someone pointed out, you are thinking of car trans, the auto industry is going away from there style. What OEM in AG is moving away from them? Deere, CNH, AGCO are all moving forward with CVT's, and coming out with more machines that use them.

The auto industry DID the same thing stating about 10 years ago. Everyone (in the press) was talking it up, sound familiar?

From what I have been told a CVT like Deere is using in the larger tractors makes less heat and is more efficent at putting power to the ground than HST.

Less than HST but still 3-4 times more than a GST, that adds up to wasted energy that you need to deal with.

Deere has powershift with 15 speeds, and CNH 18, so if IVT/CVT were not an advantage, why would they be moving towards this? I can't imagine it would be just to offer it because Fendt did, Fendts are expensive and have no dealership support around here.

You can't get get it in the similar US models, which would indicate competition is the driving factor, not building a more useable or reliable product.

 

[whistlepig]

Goodness. I didn't know they made tractors with CVT. I have a Polaris Sportsman with CVT. This works very well on the Polaris. It's a very good ATV transmission. But it is a "mushy feeling" type of drive system. I think about the CVT on the Polaris when driving my HST tractor. I wouldn't trade the hydro for the CVT. The hydro is much more precise and responsive. The CVT seems to sit and stall until the RPM's build. And then it surges forward.

 

[Duffster]

Less than HST but still 3-4 times more than a GST.

May I ask where you get this figure? I have to wonder if it is a apples to apples comparison.

You can't get get it in the similar US models, which would indicate competition is the driving factor, not building a more useable or reliable product.

Can't get what?

 

[Duffster]

Goodness. I didn't know they made tractors with CVT. I have a Polaris Sportsman with CVT. This works very well on the Polaris. It's a very good ATV transmission. But it is a "mushy feeling" type of drive system. I think about the CVT on the Polaris when driving my HST tractor. I wouldn't trade the hydro for the CVT. The hydro is much more precise and responsive. The CVT seems to sit and stall until the RPM's build. And then it surges forward.

Not the same thing.

 

[jagyzf]

The auto industry DID the same thing stating about 10 years ago. Everyone (in the press) was talking it up, sound familiar?



Less than HST but still 3-4 times more than a GST, that adds up to wasted energy that you need to deal with.



You can't get get it in the similar US models, which would indicate competition is the driving factor, not building a more useable or reliable product.

As stated, the Auto trans IS NOT THE SAME tran. They are totally different.

Where are you getting information on a powershift/GST vs IVT/CVT?

What trans can't you get in a similar US model? Deere actually has 15 speed, and 18 speed powershift, and up to 24 gear manual shift. These are just a few, all along with the IVT/CVT can be purchased here and overseas.

Here is a story on Valtra, they just came out with a new tractor that can get either powershift or CVT. The CVT came out ahead in testing of when measuring fuel usage for work being done.

VIDEO: Valtra - powershift versus CVT - 17/03/2010 - Farmers Weekly

 

[TripleR]

As stated, the Auto trans IS NOT THE SAME tran. They are totally different.

Where are you getting information on a powershift/GST vs IVT/CVT?

What trans can't you get in a similar US model? Deere actually has 15 speed, and 18 speed powershift, and up to 24 gear manual shift. These are just a few, all along with the IVT/CVT can be purchased here and overseas.

Here is a story on Valtra, they just came out with a new tractor that can get either powershift or CVT. The CVT came out ahead in testing of when measuring fuel usage for work being done.

VIDEO: Valtra - powershift versus CVT - 17/03/2010 - Farmers Weekly

Very interesting thread, I had never heard of this in tractors.

 

[ModMech]

May I ask where you get this figure? I have to wonder if it is a apples to apples comparison.

Can't get what?

Typical PUBLISHED efficiency for HSTs range from as little as 40% to as much as 91% (under ideal, steady-state operation).

Normal GSTs operate in the 89% to 94% efficiency range, we can use 91% as a respective average (it's not the same in every gear after all).

Low end HSt systems used in consumer goods (this would include tractors of most descriptions) rarely achieve 85% overall efficiency and most are closer to the 75% number with a few bad examples nearer to 60% efficiency. The pump and motor seperately are not too bad, but efficiencies of each are multiplied to make a bad overall efficiency.

A GST with assumed 91% efficiency "consumes" 9% of the FW power as it moves thru the transmission.

A HST with assumed (but typical) 85% efficiencies in both the motor and pump will result in a HST SYSTEM efficiency of about 72%. This means that 28% of the power produced by the engine is "consumed" by the HST drive. That is, 3 times more wasted energy than with a GST and it's in the form of heat.

GST example: 24 FWHP x 1.00 (clutch) x 0.91 (trans) x 0.89 (diff) = 19.4376 HP to the wheels

HST example: 24 FWHP x 0.85 (pump) x 0.85 (motor) x 0.89 (diff) = 15.4326 HP to the wheels

To jag, Auto or Ag, the DESIGN is the same. There are only so many patented CVT designs, you've GOT to pick one regardless if in a car or a tracked tractor.

 

[MessickFarmEqu]

there is alot of information in this thread thats not correct...I don't have time to set things straight right now.

 

[whistlepig]

I do know that they use CVT on some combines. My neighbor bought a used Gleaner combine last fall and he let me crawl around inside it. I was surprised to see a giant pulley and CVT belt on a piece of equipment this large. He gave me a ride in this combine while he was harvesting last fall. It's a nice combine and he is very pleased with it. This is the first post about tractor CVT's I have seen on TBN. I would very much like to read more.