Questions about hydraulic motors for a front mounted shredder build

[tmc_31]

Hi all,

I am considering building a brush cutter that I can use on my skid steer and on my NH TN70D tractor. The tractor has a quick attach plate and front mounted hydraulic remotes.

My question is regarding motor and gearbox selection. The cut width for the mower is to be 72". I intend to use two blades on a stump jumper. The blades will be 27"X4"X1/2" shredder blades.

I am trying to achieve a blade tip speed of about 16,000 fpm (I am using the Rhino SM72 as a pattern. My tractor will flow about 16 gpm at 2770 psi, the skid will flow 21 gpm at 3000 psi.

I have tentatively selected a char-lynn 104-1024 motor (8.0 cubic in). It is my intention to drive a 1:1.47 gear box through an over running clutch. I think the over running clutch will ease the load on the tractor hydraulic system during shut down.

One problem is that this motor requires a case drain. Neither the tractor or the skid is plumbed for this. I am wondering if there is a motor in this size and capacity that does not require a case drain.

I intend to cross post this in the "Build it Yourself" forum as plans develop.

I am looking for a reality check. Any help or advise you can offer will be most welcome.

Thanks,

Tim

 

[J_J]

I am assuming that you have done the math for the blade tip speed.

Just run a case drain line from the motor direct to the reservoir. No problem.

You need to add a relief valve across the motor also set at about 2700.

Using 16 GPM and 2750 psi, requires about 30 HP, Motor HP developed will be less.

Using 21 GPM and 2750 psi , requires about 40 HP. Motor HP developed will be less.

Blade tip speed will be faster using the skid steer if using max GPM's.

16 GPM through the 8 cu in hyd motor will produce an rpm of 462.

21 GPM through the 8 cu in motor = 606 rpm

 

[Farmerford]

Tim:

Are you sure about the case drain requirement? I think that is a Series 2000 motor and I use a couple of them. I believe they have cross check valves from the case to both motor ports, so that the case will be connected to whichever port is low pressure (exhaust). I don't recall the exact numbers, but it seems that as long as the exhaust port pressure is less than 300 psi (maybe 600 psi) no separate hookup to the case drain is necessary, because the shaft seals will tolerate that much case pressure. If your exhaust travels straight to the reservoir (perhaps through the return filter) you should not have nearly 300psi in the exhaust circuit.

Instead of the overrunning coupling you can just insert a check valve across the motor ports to let fluid recirculate through the motor if the flow stops suddenly. A motor type control valve that opens the work ports to the reservoir in neutral will also work most of the time; but a motor valve does not allow overrun if the engine stalls.

 

[J_J]

The specs don't say anything about case drains. I took your word for that.

Only see two ports.

http://www.google.com/url?sa=t&rct=...rUUCEBmgchqZl_rIQ&sig2=woVsjkp3VGqDJmarEeHWeA

http://www.buycharlynn.com/reference/s-series.pdf

In the 25 digit code for the motor, item #13 there is an option for a case drain, #3, but the standard is, Internal check valves

 

[tmc_31]

Hey Guys, J-J I think I saw a option for an internal relief for this motor. If not the relief circuit shouldn't be hard to pipe up. Since I posted this, I called the technical help desk at Surplus Center and he confirmed that this motor should be appropriate for this application. He did say that with the relief circuit, there would be no need for the over running clutch.

farmerford, after talking with the tech rep, he indicated that a case drain would not be needed. He said that typically a case drain was required when more than one motor was run in series. How did you come up with the return line pressure? I looked on the motor specs and it appears that as long as the return line pressure is less that 500psi, no case drain is required. This is a good thing as I was not looking forward to plumbing in a case drain line on the skid or the tractor.

Thanks guys,

Tim

 

[J_J]

The case drains are not a problem hooking up.

My machine uses 4 wheel motors and they have case drains, and they all collect at a manifold and then go to tank. So not a big deal at all. They are 1/4 in hose and QD connected.

You might check what the extra cost of the internal relief is.

Here is a relief you could put across the hyd motor.

Surplus Center - 1/2" NPT 30 GPM 1500-3000 PSI RELIEF VALVE RV-H4

Surplus Center - 1/2 NPT 30 GPM 1500-3000 PSI HYD CUSHION VALVE

Reading through the 25 digit code for the motor, I did not see anything about an internal relief valve.

 

[tmc_31]

J_J, The tech rep from Surplus Center suggested the cushion valve too. These can be adjusted in the field while the optional (spaces 19&20 in the model code) have to be ordered with the relief setting that you want. Looks to me like it should be pretty simple to hook up and the price is right so that is most likely the way I will go.

Tim

 

[Farmerford]

Tim:

Char-Lynn motors with a 1 1/4" shaft have one oddity: they use a 5/16" key rather than 1/4". The SAE standards allow either size key, but in my experience the most commonly available pulleys, sprockets, couplings, etc. with a 1 1/4" bore have a 1/4" keyway. The motors are usually available with a 1" shaft; SAE requires a 1/4" key in all 1" bore devices, so they will have the correct keyway. I think the 1 1/4" shaft will tolerate a bit higher radial load than the 1"shaft, but that should not be a problem for you.

I am confused about the cushion valve. As you say it will provide a relief in both circuits, but I suspect the system relief on your tractor and skid steer will protect the motor while it is running since the motor will be connected to the system through the directional control valve.

If the purpose of the cushion valve is to vent pressure buildup in the exhaust circuit in case the directional control valve is closed suddenly while the motor is rotating at high speed, that assumes that the directional control valve does not have motor spools, which would provide a path for the motor exhaust fluid to reach the reservoir when they are closed. If the valve has motor spools, then the cushion valve is not needed.

If you don't have motor spools in the directional control valves, then the cushion valve will provide relief when pressure builds to to relief setting, say 3,000 psi. But as soon as enough fluid flows from the closed exhaust circuit to reduce the pressure to drop below 3,000psi the relief valve will close and you will still have a pretty abrupt stop to motor rotation; it will usually chatter a bit, depending on how much flexibility there is in the closed exhaust circuit (amount of hose, rigidity of hose, etc.).

If you plumb in a check valve across the motor ports you will have the hydraulic equivalent of an overrunning clutch; the check valve should allow flow from the exhaust port to the input port. The check valve has the same disadvantage as the overrunning clutch: you can't transmit power in reverse, so if the blades get hung up they can't be turned backwards (at least by power) to free them.

We look forward to some pictures.

 

[J_J]

The motor has a rating of 3000 psi. but it also says the continuous pressure is about 2000 psi

Pressure bar [PSI] 135 [2000] Cont.***.

So you might want to consider setting the relief to 2000 psi.

Your tractor is probably about 2500 psi, and the skid steer about 3000 psi.

 

[tmc_31]

J_J,

The 5/16" key on the 1-1/4" shaft should be no problem as I planned to make the coupler myself anyway. If I could find a coupler with a 1/4 keyway, it is not too difficult to widen and deepen the keyway to accommodate a 5/16" key.

As you have probably guessed, I am a complete hydraulics neophyte so I appreciate your assistance. I don't know if I have motor spools in the aux circuit or not. I will have to look in the service manuals for the tractor and the skid. The specs that I have for this motor show 3000psi cont. and 4500psi max. I did run a Rhino SM60 shreddder on the skid a few weeks ago. It worked well but I did notice a little shudder upon shutdown of the blades.

I have cross posted this post in the "build it yourself" forum. I added a preliminary drawing of the deck to this post. It may give you a little better idea of what I am trying to do.

Thanks

Tim