I have a snowblower designed for the three point of a tractor. 6', 2-stage, specs call for 30 to 50 hp.
I have a simple 3pt quick attach plate for our Bobcat S250 skidsteer. S250 aux hydraulic specs state 20.7 gpm @ 3,300 psi.
I want to find an appropriate hydraulic motor to run at 540 rpm in order to run the snowblower with the skid steer.
The closest that I have found is the following...
Do you think this would work? Do you have any other suggestions?
First, be aware that you probably will not get 20.7gpm @ 3300psi. Off-road equipment manufacturers often bend the truth by stating a maximum flow and pressure, but not explicitly stating Xgpm @ Ypsi. The stated max flow is often only theoretical or only achievable at <90% of rated pressure. Three things happen when you get close to relief pressure.
1. The relief valve may start to crack open a little when operating within 100-300psi of rated pressure. The actual range is dependent on oil viscosity and the relief valve/cavity design.
2. The internal leakage of the pump and valve is increasing with pressure.
3. At high flow rates the pressure drop through the fittings and couplers can be enough to drop the max pressure available at the attachment by 200-500psi. That is partially why the factories offer high flow kits with larger fittings. (BTW – Usually the quick couplers are the biggest culprit, so going to the next largest size will help a lot.)
So, with a 20.7gpm @ 3300psi supply theoretically you have 39.9 hp available. Assume 10% losses due to pump efficiency and pressure drop and you are at 35.9hp.
To get the general size of the motor needed:
20.7gpm supply / 540rpm desired speed
= 4781 in3/min / 540rpm
= 8.86 in3/rev max
To be conservative: 8.86 – 10% losses = 7.97 in3/rev max
Option 1: The 130cc version comes closest at 7.86 in3/rev, any bigger and you won’t get the shaft speed. With a supply of 3250psi @ 19gpm you are looking at 487rpm and 272 lbf/ft of torque. This comes out to 25.2hp of mechanical power at 70% efficiency. (And remember that most of the 30% lost to efficiency is turned into heat, so watch the oil temp!)
Option 2: Looking at the 110cc version we get 548rpm and 230 lbf/ft of torque. This comes out to 24.0hp of mechanical power at 67% efficiency. You don’t want to use this motor though because if the pressure requirement is below 2700psi you will overspeed the attachment, up to ~650rpm.
Option 3: Let’s try to 100cc version and gear it down. We get 624rpm and 209 lbf/ft of torque. At lower pressure requirements the motor will spin up to ~720rpm. If we gear it down 4:5 output is 499rpm and 261 lbf/ft of torque, coming out to 24.8hp of mechanical power at <69% efficiency. (need to account for gear/chain efficiency)
Options 1 & 3 will only give you ~25hp at best. You almost certainly would be better off going with a higher speed motor as the efficiencies will be better, then gear it down appropriately. I won't suggest a specific motor as there are hundreds to choose from, but you will probably want to start looking for something around 4cc/rev and 1100rpm.
Wow, that's a lot of good information. So it looks like it will produce less HP than I'd hoped. But it would still be better than what I'm doing now.
Right now, I'm running this snow blower behind my small utility tractor. A Kubota L245DT with a PTO rating of 20hp. It is sorely under powered especially for the snow that we got two days ago.
The skid steer I can drive forward instead of backing up the tractor.
The bobcat will have better speed control than the tractor. Instead of plowing into a drift, the bobcat could 'ease' into a drift.
The cost of the ws355-130 motor is $350 and it is available locally.
In addition I'd have to get a drive shaft made and some hydraulic hoses.
But it would still be a lot less cost than buying a new snowblower for the bobcat.
Is there a way to test the pressure and flow of the bobcat hydraulics to see how close they are to spec?
Pressure is fairly easy to check. Just plug a gage into one of the quick couplers. This should also be adjustable at the valve.
Flow is easy, but you need a flow meter which is $$. You might be able to get a dealer to do it for you as most of them have a flow meter for diagnostic purposes. But in the long run you can't do anything about the flow except replace the pump, so save the $$.
BTW - I hope that skid steer is enclosed or you are going to get a face full of snow!
It won't be half the face full I get backing up on that little tractor. And with even a little bit more power, I hope to throw the snow just a little bit further away from where I'm sitting.
too bad your S250 doesn't have the hi-flow option. If I get a chance, I'll throw my flow meter on our S250 at work and let you know what flow and pressure one can actually get.
I've done it before, but don't have anything recorded.
only have a blade for ours and a blower would be nice.
I'll be curious to find out what you find out. I wonder how much the hi-flow option costs to add. Probably more than I want to spend.
I like that.
I tell our students to try and "learn from other people's mistakes because you won't live long enough to make them all yourself."
23.5 GPM at 275 psi (load valve wide open)
20.5 GPM at 2500 psi
18.9 GPM at 2900 psi
after 2900, the flow drops fast, the relief is starting to bail around 2800 psi.
With the load valve closed, I was at 3320 psi.
I didn't have a tach, but by ear, the engine was having no issue with the load.
If the blower really needs 3300 psi (hard to imagine), you might have an issue with the relief.
There is a dual pressure main relief used in some Bobcat skid steers that could be plumbed for higher pressure with a solenoid controlled supply of charge pressure to bump it up (momentary "clogged chute" switch perhaps).
And just FYI, there is generally no port reliefs from the factory in the attachment lines of an S250, but one can be installed on the male coupler port in the DCV. The female port has no provision in the casting. I should make sure no one has installed one in ours. I'm not even sure which way I was going on the flow... my meter is bidirectional, so I wasn't concerned... but I should have gone female out, male back to make sure I wasn't leaking at a port relief
"The male wears the protection" is the saying I made up for remembering that.This message has been edited. Last edited by: whywhyzed,
Now, about that 275psi back pressure at no load:
to save space, and probably $$, and eliminate the need for a charge pump, many Bobcat skidsteers return oil from the Lift/Tilt/Aux valve into the hydrostatic charge system.
I can't recall if ours does that, but would explain the 275, and I think it does as it has the Sauer hydros, but if you can give me the serial number of your machine I can tell you if you will encounter that....
Later S250's have a proper charge pump in the hydrostatic pumpset.
The real delta P is much less to any attachment as a result and can certainly affect the performance adversely. Bobcat did some weird stuff.
Do they take that oil from a power beyond port and feed it to charge checks, or right from the tank port of the directional control valve, back preesuring the spool seals in the process?
If they take it off a PB port, and you run the boom across relief while traveling, do you cavitate the drive pumps?
Again, I'm new at working with hydraulic numbers but given those real world numbers and given this formula to calc HP...
Formula to calc HP:
(gpm x psi)/1714 = hydraulic hp
The skidsteer will still be able to produce 30hp for the attachment.
This should be a better than 50% increase in HP than the little tractor that we're using now. And much better maneuverability of the snow blower by using a skidsteer instead of a tractor.
If the hydraulic motor that I'm looking at has the following specs.
Max. Speed of 675 RPM
Max. Flow of 20 GPM
Max. Torque 2,715 lb-in
Max. Pressure of 3,000 psi
I'd think that would work for my application.
If I need to slow down the rpm of the attachment, I can slow down the rpm of the engine. Or select variable speed for the attachment and control it that way.
Thanks for taking time to check the S250 on your equipment. I can get the s/n of our machine.
WhyWhyZed - What was the oil temp when you took the measurements? With regular oil you should have it at 90-120°F so that the oil is at normal operating viscosity. Colder oil with increase pump efficiency but at the expense of increased pressure drops.
275psi at no load while pushing almost 24gpm through a 1/2" coupler sounds OK to me. The coupler alone is approximately 30psi backpressure. Plus there should be a return filter for another 10-25psi. And add in all the fittings and the pressure drop through the valve.
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