Function of Range and RPM on an HST

[Vigo327]

I don't understand. You say that hydros (hydrostatic transmissions) are variable displacement pumps hooked to fixed displacement motors...and then you go on to explain in terms of motors that vary in displacement form max to zero. How is it that a fixed displacement motor varies from max to zero displacement?

He was trying to say that if you had a fixed displacement pump and a variable displacement motor, it wouldn't work because instead of your speed going from say 0-5mph, the range would go from 'already too fast' to 'even faster' to 'its trying to break itself'. So conceptually having that arrangement just doesn't work.

What is it that the typical L/M/H three speed range shifting is actually shifting ?

As i said in one of the pictures I posted, your 'speed ranges' are literally just an old school manual transmission. If you took an old 3spd manual gear tractor and put an HST unit between the clutch and the 3spd, you would now call it HST with 3 speed ranges (low, medium, high). You can essentially think of it as two transmissions in a row. One of them is a CVT of sorts, and the other one is an old-school manual with 2, 3, or 4 gears. Most old gear tractors also basically had two transmissions in a row. For example my Kubota B6100 is a 3spd followed by a 2spd (high/low range). Most 4wd trucks also have this arrangement since the transfer case usually has high and low range.

What does HST plus do in addition to the standard range shifting ?

Makes it easier to drive since you don't have to manage a clutch and don't have to shift gears to change directions. Arguably it also has advantages over other forms of 'automatic' like a torque converter because you can have 'full engagement' and put full torque across it at any point, whereas for example if you put a torque converter in that spot it won't transfer any torque below a certain engine rpm, and then have a very non-linear 'take-up' as the engine speeds up to the converter 'stall speed'.

 

[Vigo327]

Can you explain a bit how the dual displacement works? Not familiar with how shifting between the two ranges takes place.

I know very little about how it's mechanically accomplished inside the motor but you can compare it to the function of a 'rapid pump' floor jack, or a log splitter. In a floor jack you often have two pumps, one a high displacement which moves the jack quickly with little strength, and one a small displacement which moves the jack slowly but with full power. At first they both work, but once the resistance on the jack causes the hydraulic pressure in the system to go past a certain point, a combination of a pressure relief valve and a check valve (together called an unloader valve, i think) allows the large pump to 'recirculate' while the small pump continues working alone.

Log splitters often function the same way.. a large displacement pump and small displacement pump work together to cause the cylinder to move quickly until it hits the log, at which point the larger pump goes into 'bypass' and the smaller pump continues to push at a much slower speed, but with great strength.

Apply the same idea to a motor (but opposite since it would be a small displacement motor to go fast and a large displacement motor to go slow) and you have a similar result.. high speed and low torque at low pressure, but once pressure reaches a certain point a valve or valves will open and exposes more of the motor to fluid pressure, which causes it to slow down but generate greater torque.

I read a little into the first PDF i found from googling it, and it looks like on a piston pump some of the cylinders' inlets are just blocked off until pressure hits a certain point, at which point fluid flows into those cylinders too, increasing the displacement of the motor, lowering rpm and increasing torque.
Dual Displacement Motor

 

[MHarryE]

I don't understand. You say that hydros (hydrostatic transmissions) are variable displacement pumps hooked to fixed displacement motors...and then you go on to explain in terms of motors that vary in displacement form max to zero. How is it that a fixed displacement motor varies from max to zero displacement?

What does Kubota use? Does anyone really know if Kubota uses a variable displacement pump or variable displacement motor? That would be a nice place to start.

What is it that the typical L/M/H three speed range shifting is actually shifting ?

What does HST plus do in addition to the standard range shifting ?
rScotty

You misunderstood what I said. The motors on high end hydros are dual displacement. For example, a pump on one of my machines is variable from 0 to 56 cc/rev. The motor on the final drive has 2 displacements, 56 cc/rev gives low range and 28 cc/rev gives high range. A motor can never be zero - that is like pumping oil against a plug.

However, there are variable displacement motor systems that are variable between the 2 displacements. I used the example of 56 cc max displacement motor coupled with a 28 x 56 dual displacement motor. I have one product that was controlled by the pump displacement going from 0 to 56, but once at 56 cc, the motor displacement then varied from 56 to 28. It was balanced to maintain the most efficient operating pressure without stalling. But zero motor displacement, never.

 

[rScotty]

Thank you for the explanation.

BTW, in your explanation above I believe you should change
"I used the example of 56 cc max displacement motor coupled with...."

to instead read:
"I used the example of 56 cc max displacement pump coupled with....."

rScotty

 

[MHarryE]

You’re right, I meant 56 max displacement pump coupled with 28 min, 56 max dual displacement motor. Torque is pressure times displacement. 56 cc is 3.356 cu. In. 3.356 x 6000 psi = 20,136 in lb or 1678 ft lb. Cut the displacement in half and peak torque is 839 ft lb.

 

[MHarryE]

Thank you for the explanation.

BTW, in your explanation above I believe you should change
"I used the example of 56 cc max displacement motor coupled with...."

to instead read:
"I used the example of 56 cc max displacement pump coupled with....."

rScotty

Here

Here is the cross section of a 2 position variable motor. It might be hard to see how it works if you aren’t familiar with them. It’s shown in max displacement. Note the pistons have along travel between min and max. Next see the minimum displacement limiter screw. The servo piston on the end controls how the bearing plate moves. This design is very common on construction equipment, not so much on CUT’s because it’s pretty spendy.

 

[CalG]

I'm a bit late to the party, but...

One aspect that appears to be overlooked in this discussion is the reality that the external hydraulic functions such as a loader, or three point lift are an ENTIRELY SEPARATE system from the HST.
Add to that, the two are at odds.

The HST wants High pump /Engine RPMS for Maximum hydraulic fluid flow and MINIMUM mechanical stress on the tractor drive components.

While the auxiliary hydraulic pump that serves the power steering, 3pth and loader is only compromised in SPEED if the engine revs are reduced. Positive displacement systems being what they are.

So, To the OP, The HST and the Engine RPM as related to the other hydraulic functions are TWO DIFFERENT ISSUES.

There are two pumps, just because they share a common fluid reservoir has no significance. HECK! The B7200 HST has separate fluid FILTERS for each system!

 

[MHarryE]

It’s typical to have finer filtration for the high pressure hydro vs the medium pressure, commonly gear pump, auxiliary hydraulic system. I just traded a L6060. It has a suction filter out of the transmission case with 3 flow paths. One path is oil going to a pump supplying steering oil and PTO clutch oil. Path 2 is to the auxiliary system powering 3-point, loader, and 3rd function. Those are gear pumps. Third path is to a gerotor type charge pump. Oil from this pump is now under higher pressure so those having a Kubota Grand L will notice the hydro filter has a much heavier can and is tightened to a higher torque to resist the pressure. Having a pressure filter allows the oil to be filtered to a finer level protecting the piston pump and motor. Still the oil being supplied to the hydro charge pump goes through a suction filter. Having this suction filter is a reason for Super UDT2 - it has lower low temperature viscosity so charge oil can get to the charge pump without cavitation. Kubota says regular UDT is okay in places like FL where a cold start is 70.

 

[npalen]

Lots of good hydrostat on Youtube and this one explains in layman's terms for the most part.

 

[rScotty]

Thanks, npalen for the link. Very nice explanation. Worth spending the time to watch the moderatordevelop the hydraulic circuitry. The moderator doesn't think much of simple suction filtration and makes a good argument for low viscosity fluid - or other alternate low temperature flow.
rScotty