Case J70 tiller

rblincoe61 said:

My research has told me i need a 8.5 to 9.5 gpm hydraulic pump to run tiller, (correct me if I'm incorrect), just not sure what HP engine is needed to run pump for tiller along with oil resevoir.

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This is actually a pretty straightforward problem, although it requires referencing a few tables/documents. You can calculate the horsepower needed for hydraulic work using the following formula:

Horsepower = Pressure (psi) * Flow (gpm) / 1714

If you were happy with the performance of the tiller with your 444 running it we can use that as a baseline.

The 444's C26278 Parker / Cessna pump substitutes to a more modern Parker D27X05. At 0.641in^3 displacement, that pump moves ~9.99gpm at 3600rpm. It has a pressure relief on the travel circuit (the circuit the PTO uses) of 2000psi. Let's assume that's the pressure relief the PTO was set to as well because we don't have your tractor here to check. Thankfully Parker still has information on how flow degrades with pressure on these pumps, so we can take a look at the Parker D series pump specs to see how pressure effects a D27.


At 2000psi the pump is only at ~94% volumetric efficiency, pumping just 7.80gpm at 3000rpm instead of the 8.32gpm you'd expect from the 0.641in^3 displacement. They don't show a 3600rpm data point in their chart, but If we take the 2000psi data we can extrapolate to get a 3600rpm flow rate at 2000psi, which in this case would be 9.45gpm.



If we go back to our horsepower equation that gives us:

Horsepower = 2000 * 9.45 / 1714 -> Horsepower = 11.02

So, your 444 was pumping 11hp worth of hydraulic work through the tiller when you enjoyed using it. Some of that is used to drive the wheels, but you're moving slow and in low range, so let's call it 8hp or so to the tiller? If you want to size a pony engine driving a hydraulic pump you need to remember that there's losses from converting mechanical (engine spinning) energy into hydraulic (the fluid being pumped) energy, so you'd probably want a ~10hp pony motor to drive a hydraulic pump so you can get ~8hp worth of work into that hydraulic tiller to replicate the performance you had with your 444.

That being said, if you are ok with less performance than your 444 had you can use a pump with less flow rate or a system with a lower pressure relief. Then use the equations here to figure out how much smaller of a motor you could get away with. There's certainly tillers with their own engines running much smaller motors than a 10hp, but we did our math based on replicating your 444's performance.

At the end of the day though between buying a hydraulic pump, buying another engine, buying a valve body, and doing the fab work, you're gonna be in this pretty deep in time and money. If it were me I'd probably either sell the tiller and buy one that's shaft drive like your Cub is, or I'd buy a 224 / 226 / 444 / 446 etc with a sleeve hitch and a PTO already attached if you really want to keep that specific tiller. They're set up well for doing exactly what you're trying to do and can be had pretty affordably. 🤙

Gordy said:

So no the flow is NOT split between the wheels and tiller.

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DDudeinar said:

So how does that work then?.

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That's a great point @Gordy ! I think it's a common misconception just because mentally it's easy to think of the flow being split between the two hydraulic motors. In actuality, it's the pressure that gets split between the two motors.

When you talk about pressure, it's always a difference in pressure. When you air your tires up to 35psi, it's actually ~50psi absolute pressure, but because you're comparing it to an atmospheric pressure of 14.5psi, your tires are only at an effective 35psi or "delta pressure" (referred to as 35psi gauge pressure, because it's what would show up on a gauge). If you were somewhere with a much higher atmospheric pressure, your tires would look underinflated at 50psi absolute pressure because the deltaP is lower.

In the theoretical example I drew up above where the pump is making 11hp of work, the drive motor is using 3hp, and the tiller is using 8hp, even though both motors have 9.45gpm flowing through them, the tiller motor is only seeing ~1450psi and the drive motor is using ~550psi. Because this is all just napkin math, I assumed that the pressure after the axle drive motor (from the axle to the hydraulic reservoir) is 0psi gage pressure, or atmospheric pressure. In reality that's probably incorrect, it's likely ~20psi or so just due to the restrictions from moving such a thick fluid through the return line and cooler so quickly. I've never actually measured the return line on a 444, there's a chance it is much higher or lower than that.

Here's the hydraulic diagram for a 200/400 series with a PTO while it's both driving the tiller motor and driving forward. I marked it up to show the pressure at each individual step in the drive circuit from the example I wrote out:



Then we look at the change in pressure across each motor, because that shows us the actual work (in horsepower) done by the motor:



Tiller motor, 8hp:



Drive motor, 3hp:



So obviously, I've never even seen rblincoe61's 444, let alone measured system flow and individual pressure at multiple points in the system to calculate exactly how much horsepower each motor is using. I came up with those pressure values based on my guess of 3hp being used to drive the tractor forward while tilling and that's why the math all comes out exactly like my earlier post would suggest. But as you can see, with the motors in series, all 9.45gpm has to flow through the PTO to get to the drive motor. If you were to do something that made it harder to turn the drive motor (like driving up hill, or driving in high range) you would increase the pressure before the drive motor, taking effective (delta) pressure away from the tiller. It should be noted that this scenario assumes the drive lever is in full forward position! If you had it at less than full forward the PTO would see all 9.45gpm, but once the 9.45gpm got to the TCV part of it would go straight to the return instead of continuing through the drive motor.

@DDudeinar if you want to take a look at more hydraulic circuit situations the hydraulic service manuals (where I took the graphic from) are pretty great, and they include text descriptions talking about each circuit in each situation.


I'm more than happy to mark up more diagrams too if you think they help! 🤙