JJ and others - Post Hole Digger

[ponytug]

Dear Carl,

I'm not sure I completely understand the question, but here goes.

Attachments for tractors are typically designed for operation at a fixed speed, but ground speed is adjusted to compensate. Many tractor speedometers are marked with PTO points in various gears.

There are two standard PTO speeds, 540, the old US standard, and 1000rpm, the old European standard. Tractor manufacturers try to have the gearing on the PTO output such that the engine rpm at the desired PTO speed (540/1000) is in the middle of the engine torque/HP curve, where it will be most fuel efficient. Many (most?) tractor will operate at either speed.

You can operate attachments at speeds other than the PTO benchmark speed, but there are drawbacks, e.g. fragmenting attachments, or carbonized engines, etc.

So, in your case, the blower wants 540rpm input, so you would want to design your hydraulic pump to put out 540 when you enough have enough HP to move around while operating the blower, which will consume some number of HP to run at 540. i.e. unless you plan to use the leaf blower with a wide open throttle, you might want to size the motor such that it hits 540 at say 2/3rds throttle, but checking to ensure that the motor can handle the full flow of the PT PTO pump at WOT.

Does that help?

All the best,

Peter

So here is a real dumb question JJ. I assume 540 RPM PTO speed is what you get when the engine is cranking - ie maximum top speed. And that most people are probably running there standard PTO at 400 RPM or so unless they are flogging the machine?

I ask because I am looking at this chipper that wants 35 HP and 540 RPMs Just making sure when I don't hit the RPM I am still OK...

 

[woodlandfarms]

Indeed it does.

What it comes down to is the amazing amount of choices on these motors. And for me, not understanding why one motor is different form another motor when the cubic inch specs are the same. Just learning I guess...

 

[J_J]

Dear Carl,

I'm not sure I completely understand the question, but here goes.

Attachments for tractors are typically designed for operation at a fixed speed, but ground speed is adjusted to compensate. Many tractor speedometers are marked with PTO points in various gears.

There are two standard PTO speeds, 540, the old US standard, and 1000rpm, the old European standard. Tractor manufacturers try to have the gearing on the PTO output such that the engine rpm at the desired PTO speed (540/1000) is in the middle of the engine torque/HP curve, where it will be most fuel efficient. Many (most?) tractor will operate at either speed.

You can operate attachments at speeds other than the PTO benchmark speed, but there are drawbacks, e.g. fragmenting attachments, or carbonized engines, etc.

So, in your case, the blower wants 540rpm input, so you would want to design your hydraulic pump to put out 540 when you enough have enough HP to move around while operating the blower, which will consume some number of HP to run at 540. i.e. unless you plan to use the leaf blower with a wide open throttle, you might want to size the motor such that it hits 540 at say 2/3rds throttle, but checking to ensure that the motor can handle the full flow of the PT PTO pump at WOT.

Does that help?

All the best,

Peter

Peter, I agree mostly with what you said, but let me add something. The PT has three pumps, and those pumps are sized to use a certain HP. I don't know which came first, the engine HP and then the pumps to match. or the work required, meaning the pumps and then the engine to supply the necessary HP to get the job done. They probably had to capitulate somewhere. Because of the size and rpm they were matched up with, PT didn't allow very much HP left over when all pumps are ruining at max. So, to the point, although you can run the engine at a lower rpm, and not use the power from a certain pump. You can not use the left over HP to get more work, because the pump was designed to work at a certain level and is already working at maximum in some situations, no more, no less unless you put the PT under a heavy load, such as when cutting up hill with bush hog.

I have figured out the HP required for my pumps, and it uses just about all the HP that I have available.

An analogy might be something like the P3C Lockheed aircraft that I flew as a crew member in the Navy. It had 4 large turboprop engines, with 4500 HP each. We used all 4 to takeoff and land, but once on station, 2 engines were shut down, and in an emergency, it could fly on 1 engine. What I am trying to say, is that extra HP is good.

Now if you could have purchased a PT-1445 with 60 HP, with the same pumps, then you would have some extra, and would never run out of HP. I believe your 1445 can lift more weight than mine, although we have the same HP. You may have larger lift cylinders, or larger pumps, Yours is diesel, and mine is gas.

 

[ponytug]

JJ: I agree. Not knowing the calculated HP of the pumps makes it hard to know what the load share is/could be.

One can certainly imagine an engineered solution with the HP split 40:40:20 or 100:100:20. A back of the envelope suggests that the PT is 60:?:8 PTO:wheels:aux
( I get 108rpm, at an unknown gpm for the wheel motors. Anyone who knows the model number could finish the calculation.)

Bottom line: my 1445 has a hydraulic HP rating of just of 26HP.

All the best,

Peter

Peter, I agree mostly with what you said, but let me add something. The PT has three pumps, and those pumps are sized to use a certain HP. I don't know which came first, the engine HP and then the pumps to match. or the work required, meaning the pumps and then the engine to supply the necessary HP to get the job done. They probably had to capitulate somewhere. Because of the size and rpm they were matched up with, PT didn't allow very much HP left over when all pumps are ruining at max. So, to the point, although you can run the engine at a lower rpm, and not use the power from a certain pump. You can not use the left over HP to get more work, because the pump was designed to work at a certain level and is already working at maximum in some situations, no more, no less unless you put the PT under a heavy load, such as when cutting up hill with bush hog.

I have figured out the HP required for my pumps, and it uses just about all the HP that I have available.

An analogy might be something like the P3C Lockheed aircraft that I flew as a crew member in the Navy. It had 4 large turboprop engines, with 4500 HP each. We used all 4 to takeoff and land, but once on station, 2 engines were shut down, and in an emergency, it could fly on 1 engine. What I am trying to say, is that extra HP is good.

Now if you could have purchased a PT-1445 with 60 HP, with the same pumps, then you would have some extra, and would never run out of HP. I believe your 1445 can lift more weight than mine, although we have the same HP. You may have larger lift cylinders, or larger pumps, Yours is diesel, and mine is gas.

 

[woodlandfarms]

My 1850 would be only 30 HP at the PTO or did I miscalculate? 18GPM, 3000 PSI, 60 HP engine driving the whole kit and kaboodle.

 

[J_J]

My 1850 would be only 30 HP at the PTO or did I miscalculate? 18GPM, 3000 PSI, 60 HP engine driving the whole kit and kaboodle.

Carl, I came up with 37 hp to run your PTO pump , 18 gpm at 3000 psi.

Your steering/lift will use abut 5 gpm, and that takes up 10 hp

The rest of the available hp goes to the VSP and drive motors.

 

[ponytug]

Remember folks, we are talking about how much power might be available for the PTO function, which is a maximum possible power. Just to clarify a bit. 18gpm at 3000psi uses a MAXIMUM of 37HP. You could use less, much less, on the order of 2HP. I should also point out that while 18gpm at 3000psi requires 37HP to drive it, only about 31HP is available at the motor due to pumping losses and motor efficiency.

Typically, it takes about 5% of a pump capacity to idle it, or in your case, a little less than 2HP load.

How much of the remaining 58HP is available for the VSP and auxiliary? Good question. Potentially, you could have 56HP left over to run your VSP, if PT put in a drive pump that large.

While I dislike suggesting what others were thinking, I would guess that the design engineers undoubtedly assumed that you would want to be moving when using the hydraulic PTO and that you might need (want?) some power available to the wheels while spinning the mower blades....

Using the chipper is a stationary use, probably not anticipated. It does bring up a major safety issue, if you aren't in the seat while chipping.

Have you looked at the brush clearing chippers that attach to the front of skid steers? It might be just what the doctor ordered for your blackberries.

All the best,

Peter

Carl, I came up with 37 hp to run your PTO pump , 18 gpm at 3000 psi.

Your steering/lift will use abut 5 gpm, and that takes up 10 hp

The rest of the available hp goes to the VSP and drive motors.

 

[J_J]

Remember folks, we are talking about how much power might be available for the PTO function, which is a maximum possible power. Just to clarify a bit. 18gpm at 3000psi uses a MAXIMUM of 37HP. You could use less, much less, on the order of 2HP. I should also point out that while 18gpm at 3000psi requires 37HP to drive it, only about 31HP is available at the motor due to pumping losses and motor efficiency.

Typically, it takes about 5% of a pump capacity to idle it, or in your case, a little less than 2HP load.

How much of the remaining 58HP is available for the VSP and auxiliary? Good question. Potentially, you could have 56HP left over to run your VSP, if PT put in a drive pump that large.

While I dislike suggesting what others were thinking, I would guess that the design engineers undoubtedly assumed that you would want to be moving when using the hydraulic PTO and that you might need (want?) some power available to the wheels while spinning the mower blades....

Using the chipper is a stationary use, probably not anticipated. It does bring up a major safety issue, if you aren't in the seat while chipping.

Have you looked at the brush clearing chippers that attach to the front of skid steers? It might be just what the doctor ordered for your blackberries.

All the best,

Peter

Peter,

I disagree about the left over HP. The pumps running at max rpm will use what it is designed for. If you run the engine at less rpm, you decrease the power available, and can not do the stated work. The efficiency of the pumps to motor is about 85 %. There are three loads on the PT. Only on certain occasion, are all three pumps used. When not in use, You could say it is wasted potential HP. If you are sitting still ,just running the PTO at maximum, you are using 37 hp to run the pump. So with an efficiency of 85%, the hydraulic motor is developing 31.45 HP, and the remaining HP that is being put out just doesn't do anything. You might say, why run the engine that fast if you are running one pump. The answer is so the PTO pump is working at full power. As far as the VSP pump, we can creep along only using the HP required to power the hyd motors at a certain speed.

Have you tried to pick up a heavy load at half engine speed, or bush hog up a hill at half power? That's my story, and I am sticking to it.

 

[ponytug]

JJ- I think we're strongly agreeing here. I think that perhaps a point to clarify is that if the load isn't developing the full backpressure at full gpm, i.e. 3000psi at 18gpm, you don't use all of the horsepower. If you do, you are. Lower backpressure at full rpm is reduced load, just like shunting the PTO back to the tank.

It is all about backpressure and therefor load.

I agree with all of your math below. As you point out, if you are using the PTO, and you need all 31HP, then you have to have the engine spun all the way up, since it is the only way you can get to 18gpm at 3000psi. The amount of wasted HP in the other pumps will depend on the backpressure in each system and the pump efficiency. If both are in 'neutral', then about 5% is a typical power wastage.

All the best,

Peter

Peter,

I disagree about the left over HP. The pumps running at max rpm will use what it is designed for. If you run the engine at less rpm, you decrease the power available, and can not do the stated work. The efficiency of the pumps to motor is about 85 %. There are three loads on the PT. Only on certain occasion, are all three pumps used. When not in use, You could say it is wasted potential HP. If you are sitting still ,just running the PTO at maximum, you are using 37 hp to run the pump. So with an efficiency of 85%, the hydraulic motor is developing 31.45 HP, and the remaining HP that is being put out just doesn't do anything. You might say, why run the engine that fast if you are running one pump. The answer is so the PTO pump is working at full power. As far as the VSP pump, we can creep along only using the HP required to power the hyd motors at a certain speed.

Have you tried to pick up a heavy load at half engine speed, or bush hog up a hill at half power? That's my story, and I am sticking to it.

 

[jgrreed]

This is a straight forward solution for you reversing needs. It is only a 12 gpm valve.

Burden Sales Surplus Center Item Detail

This is a 12 v DC valve for reversing motors. Once wired up correctly, a flip of the switch will put the valve in detent for continuous operation. You need a single pole double throw switch. When you need to run it in forward, flip the switch to up or fwd, To shut the motor off, toggle it to neutral. To go in reverse, flip the switch to dn, or rev. Whatever the switch is labeled. The PTO lines go the new valve, and the output lines go to the motor. You can put QD on the lines going to the new valve, so anything you are using now will still work.

Whoever you purchase the post hole digger from, ask if it will reverse. Most of them should.

The easy way would be to just reverse the lines from the motors. You probably will not need reverse very often. except to get out of trouble such as a jam.

Hey JJ,

Quick Q. On the product description of this valve it says that there is no neutral position, which confuses me and makes me think if I use this valve it's either forward or reverse, with no off setting.

I'm thinking of using a PTO driven system, and I'd like to leave the PTO on all the time and just use a 3 position toggle for forward/off/reverse as you describe.

Thanks,

-Jer.