Hydraulic motor sizing - weird project

[cmaas]

I plan to build a amphibious barge to transport an electric car (Fiat 500). The car's 110hp electric motor will provide the motive power to the water propeller (sterndrive) and to the land drive wheels. I want to run the drive wheels with two hydraulic wheel mounted motors and am having trouble figuring out how to size them. Online calculators are just confusing me at this point so I'm hoping someone here can give me some advice.

I would like this thing to travel at about 4mph up a 23% slope. The complete package will weigh about 5,500 lbs. My engineering student friend tells me it will take 1,420 ft. lbs. of torque just to hold my place on the hill so I will need some amount more to actually move.

I plan to use 22" diameter tires which I think means they will rotate at 25rpm to travel 4mph.

What I don't understand is how to get that high torque and low rpm from a wheel motor? I mean if I have a motor with a rated torque of 1,400 in. lbs. at 500rpm does the torque go up as load increases and rpm decreases?

Thanks for any help. I imagine this falls under the stupid question heading but I am just not grasping it.

 

[LouNY]

The torque in a hydraulic motor is constant with a constant pressure,
if the pressure varies the torque will vary.

A 22" diameter tire is a fairly small tire it will rotate 885 times per mile so 4mph should be about 59 rpm. (if I calculated right)

Even a 45 cu in wheel motor only develops 9200 in pounds of torque, or about 766 foot pounds.
45.45 cu in Dynamic BMER-2-75-WS-T4 Wheel Motor | Wheel Mount Hydraulic Motors | Hydraulic Motors | Hydraulics | www.surpluscenter.com

It seems like you may need to use a planetary gearset to get your desired torque.
21:1 Bonfiglioli 2T75C2K22A14 Hydraulic Wheel Motor And Planetary Gear Drive | New Arrivals | www.surpluscenter.com

It seems to me that your required torque seems high.

 

[oldnslo]

How many driven wheels will you have?

Will this be a hydrostatic drive?

Like Lou stated pressure differential across the motor equals torque so higher pressure equals smaller components but will typically cost more.

If amphibious what type of fluid are you planning on using?

All of these factors plus many more will help determine component selection

 

[cmaas]

Thanks. That gives me a better understanding. It seems like I'm going to need to gear down somehow. Either with a motor like the planetary gear drive you found or with a chain drive.

And you are right about the tire rpm. I miscalculated.

The torque in a hydraulic motor is constant with a constant pressure,
if the pressure varies the torque will vary.

A 22" diameter tire is a fairly small tire it will rotate 885 times per mile so 4mph should be about 59 rpm. (if I calculated right)


It seems to me that your required torque seems high.

 

[cmaas]

How many driven wheels will you have?

Will this be a hydrostatic drive?

Like Lou stated pressure differential across the motor equals torque so higher pressure equals smaller components but will typically cost more.

If amphibious what type of fluid are you planning on using?

All of these factors plus many more will help determine component selection

The two front wheels are driven. Back two wheels caster.

I hadn't planned on hydrostatic drive. The distance this travels on land is short enough that 3 - 4 mph is acceptable.

I don't know what type of fluid to use. The vehicle is amphibious but the hydraulic system is not used when it's in the water. Other than to drive into the water.

If the hydraulic pump uses an appreciable amount of energy when it's idling I'll try to find an easy way to disengage it for the water portion of the trip.

Thanks for your thoughts.

 

[Industrial Toys]

You could try a wheel drive motor that often has gearing already built into the unit.

 

[oldnslo]

cmaas,
If I calculated correctly two 700 - 750 cc/rev (43 - 45 cubic inch/rev) motors operating at 3,000 PSI differential across the motor would provide the required torque with 22 inch diameter tires. At 2,000 PSI you need to be in the 1,000 cc/rev (61 cubic inch/rev) range.

If not hydrostatic you will also have to figure out how to control speed, direction, etc. Lots more to think about besides just the motors.

 

[cmaas]

Thank you for those calculations oldnslo. That is very helpful. Boy do I know there's a lot to think about with this project. Fortunately most of it I am qualified to figure out. Hydraulics is not one of those things.

So would I need a 40gpm pump to power those motors if that was its only task? Would that require a 50 gallon reservoir? That is more weight than I had hoped to carry. . Is there a way to cool the fluid other than that mass of fluid?

I am thinking the steering would be similar to a zero turn mower. And that the wheel motor speed and rotation direction would be controlled by flow control motor valves.

Do I need brakes? Or will reducing flow to the motors work?

 

[cmaas]

Thanks. I did not know there was such a thing. LouNY posted a link above to one.

You could try a wheel drive motor that often has gearing already built into the unit.

 

[4570Man]

I’d look for a scraped mini trackhoe, manlift, or tracked skid for donor motors. They’ll already have a planetary and be plenty strong enough. It might be easier to get a truck rear and and drive the yoke with either a single electric or hydraulic motor.