help with hydraulic pump/motor system design

[chopperjoe]

Hello all, I've been a fan of the forum for a while but never had much to say until now. this is my first post so forgive me if its painful.

I hope you all can advise me on a project I have in the works. The area I need help in is hydraulic design. I haven't much experience in the finer points of hydraulic motor and pump sizing.

Background
My friend is in need of a compact piece of equipment for skidding logs on his wooded property. The terrain is varied and would challenge most any compact wheeled vehicle (four wheeler, tractor, etc). He also doesn't want/can't clear/grade extensive trails/roads. After researching compact skidders, we discovered some available equipment. The links below detail two we found.

Iron horse: basically a mechanical replacement to the horse. Neat design and capable for the scale we have in mind. They are 5 and 9.9 Hp.

Tilton Equipment :: IH 2055PW Iron Horse

[ame=http://www.youtube.com/watch?v=vVI9PzRZpH4]Ironhorse movie - YouTube[/ame]

Oxtrac: great little skidder, again a neat design. It is more than we need for the scale of skidding we'll do. They are 35 Hp.

Tremzac - Technical specifications

[ame=http://www.youtube.com/watch?v=IOh3PLvjKso]OXTRAC - YouTube[/ame]

Project
So, after surveying our options the most fun thing to do is to build one ourselves! After copying the most desirable features from the available designs I've come up with this initial model. It's intended to be a gasoline powered hydraulically operated skid steer tracked mosheen. Its dimensions are 44" wide and 84" long. see attached file for a CAD model.

Before I get too far in the mechanical design I want to iron out the hydraulic design. The following performance specs are what we're shooting for:

Skid/load capacity: 1000 lbs
Speed: 2-3mph while loaded (faster when not loaded)

I'm hoping this can be accomplished with the 18hp B&S I have available. This engine is a design criterion because we already have it. I also have a 13Hp B&S but I think that'll be too small.

Design

So now begins the hydraulic system design, I'm thinking hydraulic motors on drive sprocket for each track. The motors run in parallel. Obviously a pump (gear) is on the motor, a 2 spool directional control valve for movement and skid steering, plus the other requirements for the system e.g. reservoir, filter, hoses, PRV, etc.

My initial calculations with the equations of hydraulics working from the 18Hp yields 12.5 GPM at 1000psi (not loaded) and 5GPM at 2500psi (loaded) this takes into account halving the horsepower capability because of the gasoline engine. The system will in reality produce less than these calculated flows so I'm guessing like 10 and 3-4GPM.

Researched parts

Hydro motor:
I found this at surplus center- Surplus Center - 7.4 CU IN WHITE CE WHEEL MOUNT HYD MOTOR

The spec sheet is here- http://www.whitedriveproducts.com/pdf/catalog/2008/CEcatalog.pdf

Pump:
?????Not sure how to size the pump. Would a 12 GPM pump rated for 3000psi satisfy the lower 3-4 GPM at higher pressure?

Surplus Center - 1.22 cu in DYNAMIC GPF2020PC HYD PUMP

Control valves:
Not sure if its better to have one double spool or two single spools for the movement/steering.

Something like this
Surplus Center - 2 SPOOL PRINCE VALVE SAE PORTS

Operational analysis

Load/torque capacity for one motor in the not loaded state
I'm assuming (guessing) the pressure to be lower 1000-1500psi. With 6GPM (12.5GPM/2 motors) flow from the motor spec sheet puts the performance around 934 Inlbs and 182 rpm. With an 8 (.66ft) drive sprocket (circumference=2.01ft) I get 233.5 lbs pull force and 4.1Mph. see below for calculations.

Circumference=2*π*radius, C=2*3.14*.33ft=2.01ft
Torque=Force*radius, F=T/r, F=934inlbs/4in=233.5lbs
Speed=(C/rev)*(rev/minute)*(60minute/hour)*(mile/5280ft), (2ft/rev)*(182rev/min)*(60min/hr)*(mile/5280ft)=4.1Mph

Total pull force is 467 lbs at 4.1 Mph

Load/torque capacity for one motor in the loaded state
I'm assuming (guessing) the pressure to be 2500psi. At the lower flow of 2 GPM (4GPM/2 motors) Again for the motor spec sheet 2382 Inlbs and 33 rpm. 8 sprocket calculations yield 595 lbs pull force and .75 Mph

Total pull force 1191 lbs at .75 Mph. Seems slow!

Final thoughts
1. Does a 1000 lbs pull force seem accurate for skidding a 12-16 log about 16 long? Maybe 2 logs? My hunch is the force is lower but this has a factor of safety.
2. Is about 2Mph too much to ask from an 18Hp with a load?
3. Does the calculated GPM for the 18 Hp motor seem realistic?
4. Does the estimated 1000psi at 12 GPM and 2500psi at 4GPM flow for the 18 Hp motor seem realistic?
5. Would a 1.22 cu in pump work well?
6. Does the flow really get divided in half because of 2 motors?
7. Will the 2 spool valve work for movement/steering?
8. Do any of the above calculations make sense?
9. Will heat dissipation be an issue with the "double GPM for reservoir size" rule of thumb?
10. Is there a snowballs chance in hades that these specs will work?

let me know if I've blatantly missed something. I'm eager to hear your input. Please share your experience and knowledge.

I plan on posting the project as it moves through the phases. hopefully you'll get a kick out of it.

Thanks for reading.

 

[oldnslo]

Have you considered a small skid loader. They would have all the hydraulics and design work done for you. There are models that will fit through 3 foot wide doors but I am not sure how much they will pull.

On the hydraulics. Typical drive system would be closed loop hydrostatic where there is one pump and one motor for each track. This provides the ability of driving both tracks while turning.

Bobcat, Case, Gehl, are some companies that make these small skid steers.

Roy

 

[chopperjoe]

i did consider a small skid loader. the thing is i've never seen/imaged one of them going through the woods with varied terrain. my feeling is with the weight of the machine and wheel/track/suspension they couldn't do what we need. they are also a few thousand bucks plus the inevitable repair. i'd rather spend a few K on a new machine that fits our needs. and have the fun of building it.

i have since decided based on more research to do the pump to each drive motor combination. currently looking at hydrostatic drive pumps at surplus center. not sure about belt drives for multiple pumps or direct drive with this one. trying to determine if this one will meet the performance specs.

Surplus Center - 0.43 cu in HYDROSTATIC KANZAKI DUP-7A TANDEM PUMP

 

[oldnslo]

Chopper,
You did a lot of math on your first post and are on the right track. To design a hydraulic system and pick components you have to know force required and speed of operation required. You have already done the torque calculations and the speed with HST drive will be variable.

Now the fun comes in selecting components. You have to try and determine an operating pressure. With a pressure selected you can now start sizing the wheel motors.

Once you get the wheel motor size figured out then you back figure pump size based on desired motor RPM.

Then you calculate required prime mover ( engine) HP and discover it is only half as big as you need.

At this point you start making more decisions on what can I change. Force required or travel speed.

The nice feature with HST drive as that the operator controls the HP draw and travel speed.

Possibly option: Have you considered variable speed belt drive. Lot's lighter and simpler with far less power loss than hydraulic. Hydraulic drive figure 20 - 30% loss or more depending on component selection. Reading the specs on the tilton equipment it sounds it also has a gear box for different speeds.

Roy

 

[chopperjoe]

i was just going down teh trail of what you described in the post with some more accurate pull force i.e. "draft force" numbers. my recent investigation is exactly how much weight/force we are dealing with for these logs. the load should really indicate whether the hydraulics off this mosheen can actually do something. below are some numbers from a log calculator (Timber and Lumber Calculators at WOODWEB).


Species: Oak, White Small End Diameter: 10.00 Large End Diameter: 12.00 Length: 12.00' Quantity: 1.00 Estimated Weight: 527
Species: Pine, Red Small End Diameter: 10.00 Large End Diameter: 12.00 Length: 12.00' Quantity: 1.00 Estimated Weight: 360

im trying to determine what pressure this is created in the hyd system. this with flow will give a better idea of performance i.e. speed. working backwards from a log weight of 500lbs i'm using the motor (7.4in^3/rev) spec tables (http://www.whitedriveproducts.com/pdf/catalog/2008/CEcatalog.pdf) to get a pressure from the torque generated by the load.

here is my thought process: the load generates a torque in the motor. the torque in the motor generates pressure. the motor specs relate pressure and flow characteristics to RPM. the pump needs to cover the pressure range and the flow requirements. the RPM worked out thru the drive sprocket dimensions should give speed. yeah victory!

so, in the "worst case" draft scenario i have.......a

500lb log, divided by 2 (number of motors)250lbs; reduced to a torque (4" drive sprocket radius) of 1000inlbs. 1000inlbs in the motor table creates a pressure between 1000-1500psi and between 177-182 rpm around 6 GPM. the RPM is between 51-56 at 2 GPM. the DUP 7 HST is spec'd to be able to supply around 5-6 GPM at 18HP. Sprocket circumference is 2' so speed should

(2ft/rev)*(180rev/min)*(60min/hr)*(mile/5280ft)=4.1Mph

seem realistic?

 

[oldnslo]

Chopper your math seems logical with the exception of you are not including the weight of the machine itself.

Another question, how are planning on controlling direction and speed? Just curious since if you ride on the machine you just added another ??? lbs to your load.

Roy

 

[pat32rf]

Just to throw in another variable-the weight of a log doesn't always (seldom) relates to the force required to drag it...
Those small Forecat skidders are awesome in the bush....but pricey unless you are going to hire out for other work.

 

[chopperjoe]

regarding the weight of the vehicle, i'm using a fabulous principle i learned in school. if you don't want to account for some aspect of a math equation because it will complicate things, you simply assume it to be negligible. LOL. my thoughts about the weight/loading are that the drag/draft force would be orders of magnitude larger than what contributions the vehicle and occupants would be. the rolling friction vs. static and dynamic friction issue. FWIW i would like this vehicle to come in around 500lbs curb weight.

i found a custom vehicle that is a close match to what i'm building. its on another forum i read. the link is below. turns out it uses the little HST i intend on using. similar sized engine as well. i think the hydro motors have a larger displacement from what i'm going to use. i think that will allow me better speed with less torque.

Hydro Track Vehicle - Hydraulic Innovations

turns out he pulled a 5000 lb snowcat with this thing.

 

[oldnslo]

Yes smaller displacement motors will give greater travel speed and provide less torque with the same input parameters.

Roy

 

[temsan]

Beautiful and useful