Homemade 6 wheel drive cart- problems, please help!

[justinlovestobuildstuff]

First I like to thank anyone who takes the time to help me with this. Im 35 yr old who has always loved to build random things- I built this 6 wheel drive skid steer cart to move dirt and materials around my yard (1/2 acre) which is mostly steep hillside, therefore the cart was designed with low speed low torque in mind with lots of wheel surface area for grip. I'm new to hydraulics and did my best to research and learn as much as i could. I did my best to use common formulas to determine pump and motors and hp requirements but its still all quite confusing.

The engine- 6.5 HP
The pump- 0.73 cu in- 5.88 GPM rated-10.58 GPM max-2000 RPM rated-3600 RPM max.
The motors- Prince Manufacturing CMM50-4RP 3.0 cu. in Max RPM 1000- Max PSI 2200- Max Torque in lbs 787- Max Flow GPM 12-(I find conflicting specs everywhere!)
The hoses- 1/2"
The valve- NorTrac Monoblock Hydraulic Control Valve - 12 GPM, 4 Spool - 3 way- open center
The filter- 25 Micron plumbed right before pump inlet
The tank- mounted above pump inlet approx 2 gallons
The drive system- direct drive from hydraulic motor to wheel hub (18" wheels) with sprockets and chain connecting 3 wheels together on each side

The problem- with wheels off ground unloaded they spin just fine when valve handle are actuated- far too fast actually. If this cart went 10 mph id be happy. When wheels on the ground under weight of cart (approx 400 lbs) the cart will lurch forward for just a second but then stop. Even with revving the engine it goes nowhere. As I increase the pressure using the regulator built into control valve block the motor struggles to stay alive. I have a pressure gauge installed, hard to read with the needle going crazy but ive seen close to 1000 psi.

What did i do wrong?
So keeping in mind that this cart only needs to go 5-10 mph what is my easiest/cheapest option to get this thing finished?

More HP?
Is the some weird cavitation issue maybe with the pump?
The fitting on the inlet of the pump that adapts 12 SAE to 3/4 NPT has an id of 1/2" Too restrictive?
Is the filter on the pump inlet too restrictive? its 25 micron with bypass.
Are the motors a poor choice?
I have another set of motors- 3.2 cu. in. / rev.-1750 PSI cont.-730 in-lbs. cont- 400 RPM cont. - 6 GPM cont. 10 GPM int.

Again thank you, i've spent alot of time and money on this thing and im starting to worry- Justin

 

[oldnslo]

Justin,
If the wheels are 18" diameter actual rolling diameter then 187 RPM would = 10 MPH.

187 hydraulic motor RPM at 3 CIR = 2.43 GPM

At present if I read your specs correctly you have 10.58 GPM so yes these will run real fast.

.73 CIR pump @ 3,600 RPM will produce a theoretical 11.37 GPM

11.37 GPM

6.5 HP will provide around 850 PSI @ 11.37 GPM.

Any idea on what the cart will weigh loaded?

I suspect you will have to either gear down the motors you have or get larger motors and a smaller pump to get the tractive effort required to make this machine perform.

 

[Leejohn]

I would get the psi up to at least 2100.

 

[justinlovestobuildstuff]

Ok it's starting to make a little more sense thank you. I can purchase a .24 cu in pump for $100. Id like to keep the motors i have installed ($310) for those. Any idea if simply changing out the pump would get me going? Even 5 mph would be fine. Fully loaded the cart wouldn't ever exceed 500 lbs. I just so happen to have some 60:1 gear reduction units i could squeeze in the system. Those were left over from the electric motor attempt. I hate failing...

 

[oldnslo]

Justin,
Here are some vehicle drive formulas for hydraulic drives. They might help you figure out what your system will do.

 

[ProjectKing]

Your hydro motors do not have enough torque. You will either have to replace them or gear them down.

 

[Rustyiron]

I hate failing...

I'd call it learning :thumbsup:

 

[otlski]

Where is your hydraulic reservoir tank? Perhaps I missed it. Also, read this link. Hydrostatics I – Charge! >> CD Industrial Group Inc.

 

[justinlovestobuildstuff]

Its at the right hand side welded to the vertical tube, i just made it with some 3x4" square tubing. I still dont know what i should do. Kinda discouraged. I guess i need to keep reading.

 

[oldnslo]

Justin
My suggestion is to look through some of the vehicle drive formulae to figure out the torque required to provide the tractive effort / drawbar pull required. Then figure what you can do with the HP you have. Use this information to start seeing what you can do with the components you have.

NOTE: Skid steering can require as much or more torque than driving straight

 

[1stDeuce]

This is a cool little project, though it appears you may need quite a bit of ballast on the back of it if you plan to haul a loader full of dirt...

The above recommendations are on point. You have too much pump for the HP of the motor, and too much pump in general if you were targeting 10mph, which is probably too fast for your hilly 1/2 acre property. I would think 5mph plenty fast for running through trees and maneuvering around things...

Actions you need to take: 1st, you need to figure out the pressure needed to the motors to provide the tractive force that you desire for moving the thing around. If it weighs 500lbs, and is going to haul a loader full of dirt, that's ~1000lbs. Driving that up a 30% (20ｰ - pretty steep!) slope would require 340lbs of tractive force. Even though you have two motors sharing the load, so 170lbs each, I would do calculations based on one, because as oldnslo also points out, skid steering will take significantly more torque than going straight... Hmm, actually, you're not only providing tractive force, but also skidding tires as you go. Perhaps use 500lbs as the required tractive force just to be sure it'll be able to turn if you're going up hill.

Knowing the torque required of the motor to provide the tractive force, you can get the pressure needed to generate that torque. If it is greater than the max operating pressure of the motors, you will have to gear down until it is below the operating pressure, or buy larger displacement motors. If it is not above the motor max pressure, you now have a required pressure needed to maneuver the rig.

Using that pressure, calculate the max pump displacement that your motor will run while providing that pressure. That is your maximum pump size.

Now, knowing the pump displacement, you can calculate the speed your unit will go with the engine running 3600rpm's. You can further reduce the pump displacement to slow the thing down, and give it more tractive power, but you can't speed it up without sacrificing power.

I'll take a swag and say that your 6.5hp motor might be able to hit 2mph going straight once you have the pump sized appropriately, and the gearing or motor size corrected if needed. Crunch the numbers and let us know how it works out.

 

[1stDeuce]

BTW, for continuous use, I think you'll overheat the fluid fairly quickly... It likely needs a bigger tank, or a cooler of some sort. Easy enough to keep an eye on temp and make changes if necessary.

 

[SilentJ]

Motors too small, pump too big

Engine maybe on the small side.