Teach me about hydraulic motors

[PHPaul]

Grandson has been pricing specialty mowers for mowing orchards and ditches and such. Lots of suitable technology out there, but the prices will take your breath away.

I'm thinking of building a hydraulic version of my diesel string trimmer. The idea is that the string head would be hinged so it could tilt up/down. It would involve a PTO mounted pump to run a hydraulic motor on the string head. What I need to know is whether or not my combination of components is suitable. I'm looking for 3-5 HP at the string head. RPM can be achieved by suitable pully ratios if needed.

Here are my first thoughts:

PTO Pump - Surplus Center 9-8902-3, 7.4 GPM @ 540 RPM
Motor - Surplus Center 9-16442-12, 5.2 GPM @ 1500 PSI
Tank - Surplus Center 9-8416-5, 5 gallon capacity

Insights/Suggestions appreciated.

 

[InTheWoods]

With 7.4 gpm from that pump, you could exceed the max flow of that motor. With hydraulics, you don't just add a simple 'valve' to reduce the flow rate. I'd start with a motor/pump combo that might work with no control other than 'on/off'.

According to TractorData, you have 5 gpm available from your B2650. Have you ruled using that out for some reason?

That aside, what RPM do you need? I'd suggest knowing that up front, as the selection of a motor can be optimized that way. Little gas weed whips are in the 5000-1000 RPM range. With a larger head, are you thinking something about half that?

 

[PHPaul]

My mechanical design works well at about 3000-3600 RPM. I based that on the walk behind 4-stroke trimmer that uses direct drive from motor to string head.

If this materializes, it'd be for my grandson's 3046R John Deere. Not sure where I'd tap into the system to use the tractor's hydraulics.

I'm assuming flow rate on the PTO pump is directly related to RPM. Yes or No?

 

[InTheWoods]

If this materializes, it'd be for my grandson's 3046R John Deere. Not sure where I'd tap into the system to use the tractor's hydraulics.

It has the flow. Are other hydraulics being used at the same time as the trimmer?

I'm assuming flow rate on the PTO pump is directly related to RPM. Yes or No?

Yes.

And similarly, the motor speed is directly related to the flow rate.

 

[PHPaul]

One remote will be used for the tilt cylinder on the cutting head.

 

[InTheWoods]

Assuming there's another remote, seems like it'd be worth considering.

 

[PHPaul]

Assuming there's another remote, seems like it'd be worth considering.

That would require tying the valve lever in the open position and restrict flow wouldn't it?

 

[TheOldHokie]

My mechanical design works well at about 3000-3600 RPM. I based that on the walk behind 4-stroke trimmer that uses direct drive from motor to string head.

If this materializes, it'd be for my grandson's 3046R John Deere. Not sure where I'd tap into the system to use the tractor's hydraulics.

I'm assuming flow rate on the PTO pump is directly related to RPM. Yes or No?

For fixed didplacement pumps like that PTO pump flow is proportional to RPM. I would suggest you add a control valve with a motor spool to your design. Only add $100 to the cost.

Dan

 

[InTheWoods]

That would require tying the valve lever in the open position and restrict flow wouldn't it?

I'd be inclined to select a motor with enough displacement that it hits the desired RPMs with the full flow your pump can provide. You'd end up with a motor having more displacement (and thus torque) than needed, but a pressure relief valve would effectively limit the torque, if needed.

That said, I've pondered similar projects, but never done it - so I'm not claiming to be an expert here...

 

[TheOldHokie]

I'd be inclined to select a motor with enough displacement that it hits the desired RPMs with the full flow your pump can provide. You'd end up with a motor having more displacement (and thus torque) than needed, but a pressure relief valve would effectively limit the torque, if needed.

That said, I've pondered similar projects, but never done it - so I'm not claiming to be an expert here...

1. Flow determines motor speed
2. Pressure determines motor torque.
3. In his application pressure will never go higher than needed to spin the trimmer head.

Dan

 

[InTheWoods]

I know - just suggesting a pressure relief valve as a safety measure. I'd expect the pressure to be quite low normally, unless something gets jammed...

 

[TheOldHokie]

Yes, he needs a relief for any size pump.

Dan

 

[InTheWoods]

So what do you think about using the tractor's hydraulics for this instead of a dedicated PTO-driven pump?

 

[PHPaul]

So what do you think about using the tractor's hydraulics for this instead of a dedicated PTO-driven pump?

It's certainly an option.

He already has a 3rd function for a grapple on the bucket and I'm not sure what plumbing would be required (or if ports are available) to tap into power-beyond.

Worth investigating if he/we decide to build it. Save about $700 what with the pump, tank, filters and fittings.

 

[PoTreeBoy]

So what do you think about using the tractor's hydraulics for this instead of a dedicated PTO-driven pump?

Should be doable, but there are considerations,
Many hydraulic motors have back pressure limits unless a case drain is provided,
Your other hydraulic functions might rob the motor when they're used simultaneously.

Have you considered mechanical drives? I'm not sure what flexibility you're after. But if it's just rotate the plane of the trimmer, could you pivot it on a jack shaft? Use belts similar to your picture, but run the shaft horizontally. Swing the trimmer support frame on the jack shaft, twist the outboard belts to get from vertical to horizontal. Then you'd need to drive from the PTO to the jack shaft. Maybe a shaft with u-joints from the PTO to a shaft on pillow blocks mounted parallel to the other shaft. V- belts interconnecting, with tensioner. With double speed increase by appropriate pulley sizes?

 

[TheOldHokie]

It's certainly an option.

He already has a 3rd function for a grapple on the bucket and I'm not sure what plumbing would be required (or if ports are available) to tap into power-beyond.

Worth investigating if he/we decide to build it. Save about $700 what with the pump, tank, filters and fittings.

Assuming you have sufficient flow that would be my choice.

Heat is the main enemy. A dedicated valve with motor spool and direct tank return is optimal. The problem with a direct tank return is it will disable all down stream functions.

Dan