Sizing a hydraulic motor

[Weldon K]

I need some input from some of you guys who have knowledge in mechanical engineering . Needed are recommendations for selecting the size of a hydraulic motor that will power a device that I plan to build and install on skid steer loader. The device will be used to gather loose rocks from surface of tilled soil by driving slowly across land and having device sweep/kick rocks into bucket. Rocks would be from cantaloupe size down . I intend to build a framework that will be bolted to the bucket of the skid steer loader. Frame will be mounted in front of bucket and will carry a rotor mounted on pillow block bearings. Rotor will be built of square steel tubing (either 4 inch or 6 inch) with 4 inch long flat bar stock teeth welded to flat surfaces of the tubing. Spacing of teeth will be probably 3 inch and alternating from one surface of tubing to the next in effect giving a 1-1/2 inch spacing. Rotor will be chain driven by a hydraulic motor powered from skid steer axillary valves. I would like the rotor to turn at about 100 rpm or so but yet be adjustable to more or less depending upon conditions and how it turns out that the thing works. Depending on the rated rpm speed of the motor used, a portion of the speed reduction from motor to rotor could be by ratio of sprockets. Additional speed control will be through hydraulic flow control valve plumbed into input line to hydraulic motor.

Specs for hydraulic system of skid steer are as follows:
13.0 gal. min. at 1150 psi
System relief 2600lbs at quick couplers.

I want a motor that will have adequate power and speed capacity to do the job but also do not want overkill. With the above information , can anyone make a recommendation for a size motor that will do what I need?

Knowing that hydraulic motors are reversible and realizing that there may be times when the rotor may need to be reversed , will it be necessary to have two flow control valves, one on input and one on return to prevent excess rpm of rotor when switched to reverse? Info on flow control valves says "designed to provide flow control and shut off in one flow direction while providing free flow in the opposite direction". I would not want to have the rotor set at 100 rpm in use direction and then switch it to reverse and have it too fast, so wouldn't flow control in both directions be required? The flow from the electric controlled axillary valves is not adjustable by the operator as the system's valves are either on with full flow or off with none (no feathering action as could be obtained with conventional manually operated spool valves).

I am thinking of using # 60 roller chain and sprockets. Will this be sufficient? I'm thinking 1-1/8'' shaft through the square tubing rotor will be sufficient ( rotor will be 60" ). Is use of chain drive and speed reduction by sprocket ratio really necessary? How about a motor of sufficient size coupled directly to rotor shaft?
Most motors I am looking at in catalog have 1 inch shaft. Would 1 inch shaft through the rotor be sufficient? It would eliminate some of problem of connecting motor shaft to rotor shaft.

Any suggestions or other info that anyone can provide will be greatly appreciated.
Thanks, Weldon
 

[David G]

You have already set the limit on the HP for the motor at 5.7 according to the Eaton calculations. You must now make your mechanical system live within those limits.
 

[Dave in Tx]

I have a bunch of slow turning wheel motors off commercial golf equipment very reasonable if you are interested. 325 642 7647 .

Dave
 

[fixerupper]

Sounds like a neat idea. Would work like a charm on my farm. One idea is to put an adjustable relief valve somewhere in the pressure line so you can dial it in. Maybe a steel screen in front of the driver in case the reel catches a half buried rock, builds up some pressure and then flings the rock up. I've used a commercial pull behind picker with a hydraulic driven reel and it would do that if it got into a tough rock or if I accidentally got it in the dirt. Let us know how it turns out when you're done. Jim
 

[JMS./MN]

I described my similar idea here the other day. It would have only one row of teeth, to sweep in the laggards. There is a commercial product like you describe- local dealer rents it out. No info on his website tho. I would go with an 80 chain, or at least a 60H. (slightly heavier than a 60).
 

[Mark-Mi]

Forget the chain drive. Direct drive motor off end of shaft.
 

[moresmoke]

HP = GPM x PSI / 1714

GPM x 231 / RPM = cubic inch displacement of motor.

Open or closed center system on skid steer? Need to know this for flow control. 1 1/2 inch shaft on your rotor.
 

[DiyDave]

Go to surpluscenter.com website, they have a page with all the formulae, to determine the speed and displacement you need to know. I built my own snowblower, out of a 3 point hitch model, to mount on the front of my ASV RC-30. Most helpful, and they have good prices on Hyd motors, and good tech help.
 

[jm.]

I have an rear attachment that is about what you describe except it just rakes them in a windrow. Unit is made by a company called ATI . Your best bet is surplus center. Get the largest unit that you can turn with your aux flow and go with it. By using chain drive you can play with the gear ratio until you get it right. Making it on a rock bucket or one with the open bottom might be a better option. In any advent please copy my e mail and keep me informed of progress. Something I have always wanted.
 

[buickanddeere]

It would cost less and work better to just attend an auction sale and purchase a stone picker.
 

[JOHNNYfromMI]

Go to a rental shop that rents skidsteers and ask to see a rockhound. Sounds like that is what your trying to invent/build.
 

[Weldon K]

I have a Schulte rock picker. It gets the bigger ones but leaves lots of fist sized ones. They are the ones that I want to get with this thing I will build. Plus there are areas on this farm that rocks are so numerous that they lay touching each other. Sometimes I wonder if there is soil in there!
 

[Weldon K]

Jim, I may wind up building a bucket with expanded metal bottom and definitely will put a screen at top of bucket to deflect any errant flying rocks.
 

[Weldon K]

Did you arrive at hp figure by calculating the amount of flow at pressures described? Do you mean that components of device should be constructed to handle 5.7 hp? When I build anything I always overbuild it anyway.
 

[greygoat]

Pressure equals Torque
Gpm and displacement (Cubic inch per revolution)
converts to RPM,(speed)