Fully Hydraulic Tractor

[MH49]

Joel, I am confused with your question because a Hydrostatic transmission is a pump tied to a motor into a differential. If you are refering to one pump driving all machine functions, the main problem is that you loose dynamic braking with a system like that. In other words, you have no hold-back when you go down a hill. As someone mentioned, for heavy pulling like plowing, discing, they really stink. But for PTO jobs like mowers etc, they work real good.

A side note, most modern ag tractors with a CVT type transmission use a hydrostatic transmission in parallel with a mechanical drivetrain to get it's variable speed. Also large dozers like the D8 and up Cat uses a hydrostat to control the differential steering. I don't know all the details on that one but think what happens when you jack a car up and you turn one wheel clockwise and the other with go counter-clockwise.

 

[JoelD]

Hi MH,

My thought is a pump, running a hydraulic motor (gear pump type), which is mated to a gear box with low med and high. Very similar in concept to the current hydros, however, the hydraulic motor I was thinking of is essentially a gear style pump.

They are so easy to maintain and yet with a valve body in the middle would be infinately variable (in theory) in speed.

The pump / motor combo is so easy to maintain.

 

[MH49]

Ok, I see where you are going with this. You are correct that gear pumps/motors are real easy to maintain. They also will swallow a more dirt than a piston type pump/motor with less damage. However, the efficiency is a lot less than a piston type pump. The other problem that may be a bigger one is the dynamic braking issue that I mentioned. When you are going down a hill, the motor turns into a pump. In a hydro, this oil is forced against the pump which can't swallow all of it. This drives the pressure up in that side of the loop braking the machine. With a pump/valve/motor situation, that oil is 'pumped' directly back to the tank with little restriction and the machine runs away. You can get around this but it takes some extra valving which adds costs. Older Case/Ingersoll garden tractors a system similar to the one you described and they could be a little hairy on hills.

So IMHO, the lack of efficiency of the gear product and the risk of run away is why you don't see tractors like this. I think to cover all the things that the hydro does, the system would actually end up being more complicated.

 

[dfkrug]

The points about dynamic braking and axial piston pump/motor efficiency
are critical ones. I don't see any advantages to using gerotor pumps or
motors; there is no more ease of maintenance that I can see. Just keep
the fluid up and clean and that's all you need to do for either type.

 

[Wayne County Hose]

A fully hydraulic tractor would be more efficient than a conventional. Many large pieces of construction equipment and some newer forklifts are fully hydraulic. You see it more in industrial equipment than homeowner stuff. For a hydraulic piece of equipment to run it's best, it needs to run at the engine's torque peak. Most homeowner types wouldn't want to run their engines at this speed all the time.
The hydraulics "running away" is a non issue and is solved with some very simple valving. With a hydraulic drive it doesn't matter uphill or downhill, the travel speed will only be how far the valve is open. Close the valve, come to a stop. Hydraulic drives have brakes that last forever because they are rarely if ever used, the pump, motor, and control valve act as the brake. There is way more hydraulic drive stuff out there than you think. Tractor manufacturers are stuck in the old school mode and are very conservative when it comes to their engineering. I don't think I've seen any technology on a tractor that is from this decade yet, it's all 90's stuff. There could be newer technology, I just haven't seen it.

 

[JoelD]

Thanks for the note Wayne, it just seems to make sense, I also get the point about running at torque max as all controls would be through valve adjustment rather than throttle.

Interesting topic to me, just seems relatively bulletproof and sounding like a broken record, super easy to fix.

Thanks again,
Joel

 

[dfkrug]

A fully hydraulic tractor would be more efficient than a conventional.

I think you have to be careful with a statement like that, Wayne. HSTs may
allow more efficient changes in direction and speed within a given range, than
changing gears. If you just look at the most efficient conversion of
combustion energy into work, clutches and gears will be more efficient. That
is why you will see HSTs on more and more construction or industrial
equipment, and still mostly gears and clutches on AG equipment. The latter
often uses constant speed and direction.

As for a hydraulic machine operating its engine at maximum power, that
can be quite wasteful if max power is not constantly demanded by the
application.

 

[Wayne County Hose]

Didn't really have HST's in mind, more the pump-valve-hyd motor system. I do believe there are less power losses thru gears, but for fuel efficiency, you can't beat hyd drive. The automotive industry has been experimenting with it for a while. That's another reason they keep adding more gears to the automatics, to keep the engine closer to the torque peak. Also the theory behind the CVT that Nissan and a couple others have out.
There are also computer managed systems out that have a servo motor on the throttle controlled by the computer. The computer reads the amount of power called for the application and opens the throttle and hyd valving accordingly. There is a forklift made in Germany and imported here to the states called a Linde that does this very well. It takes some getting used to to drive, but is almost idiot proof. The machine has no gears, it's all engine and pumps.

 

[dfkrug]

Continuously Varaible Transmissions (CVTs) have been around for many
decades, often utilizing belts and pulleys with variable diameter. That
setup could not handle much power so it was limited to small engines
mostly (lawn tractors, tiny cars, etc). Nissan has a CVT today that
handles over 250hp (Murano), primarily by eliminating the belts.

Automotive transmissions utilize a hydraulic fluid coupling (torque converter)
and numerous hydraulicly activated clutches (Toyota has 8 speeds!) plus
gears to always be ready to transfer max power if needed. Because of the
inefficiency of the torque converter, smart ATs lock it up when the vehicle is
moving so there is a more direct transfer of power to the wheels. ATs are
truly amazing, but very complex mechanisms.

One of neat things about HSTs in our CUTs is they use variable displacement
pumps with no valves. When the swash plate is centered, there is no flow.
With a fixed displacement pump and a spool valve (as I use with my hydraulic
PHD), the flow rate is only effectively controlled with engine speed.
Feathering the spool valve is not very effective. I don't know what kind
of hydraulic transmissions they use in dozers and skid steers, but I hope to
acquire a SS this year.

 

[JoelD]

CVT's have been on ski mobiles for ever, biggest problem was belts disintegrating. My bunton mower uses a CVT.

I guess when you think about it, we may all end up with electric motors sooner or later, torque seems to be their big claim to fame.

Thanks to all for their thoughts.

Joel