rkcarguy wrote:I think I am making some sense of the hydraulic pump and motors sizing.
I've found the CMM50 series motors offered by Prince...
Speaking from experience, I do not recommend Prince hydraulic products, especially their motors. Caveat emptor!
...they are reversible instantly by reversing fluid flow...
That's true of virtually all hydraulic motors.
...and this model has an 800 RPM rated max continuous.
The 800 RPM for a Prince motor is optimistic, in my experience. Most of them don't work well at all when they are operated near their rated maximum speed.
Flow chart info varies from 113 rpm at 2GPM to 578 rpm at 8 gpm. With the additional 2.3:1 chain reduction to the wheels, this gives me an RPM range of 49-251 at the wheels. Torque is 40 ft/lbs at the motors, which will be multiplied via the gear reduction to around 90ft/lbs, x 2 motors, total 180ft/lbs of torque if my math is correct.
This is all at 1400 PSI pressures.
I've reviewed the flow charts for pumps, and one offered is 16 gpm. Flow chart is 4 gallons per minute at around 1000 rpm, and 16gpm at 3000 rpm, but this flow chart is at 2500 PSI and the HP required exceeds my 16HP motor by quite a bit at this pressure. However, it appears that at 1500 PSI it's almost perfect at about 1HP=1GPM ratio.
No aspersions, but you are going about it all wrong.
Firstly, the most efficient design will use 1:1 "gearing" between motor and axle. There is no reason to set up the 2.3:1 drive ratio you speak of when you can achieve all the reduction you need in hydrostatics.
A fundamental design characteristic of hydraulic pumps and motors is that they possess "positive displacement." What this means is, neglecting losses, they move a fixed amount of fluid per revolution. The amount of fluid moved is the motor's or pump's "displacement," which is measured in cubic inches or cubic centimeters per revolution.
In a closed hydrostatic circuit, the ratio between pump displacement and motor displacement dictates the drive ratio, and hence the motor speed and output torque for any given pump speed and input torque. As a simple example and
assuming zero losses in the system, consider the following:
- Pump displaces one cubic inch per revolution (CI/rev),
- Motor displaces 10 CI/rev,
- Pump speed is 3600 RPM,
- Pump input torque is 10 pounds/feet (Lb/ft).
The motor's behavior would be as follows:
- Speed: 360 RPM,
- Torque: 100 Lb/ft
In other words, the effect of 10:1 gear reduction is achieved. Incidentally, the input power to the pump in the above example would be approximately 6.8 horsepower. Disregarding losses, the motor will output 6.8 horsepower as well.
The three other ratings that get into the picture are flow, maximum pressure and maximum RPM.
- Flow is measured in gallons per minute (GPM) or liters per minute (LPM). In the above example, the pump's theoretical flow at 3600 RPM would be approximately 14.4 GPM, computed by the formula:
where RPM is the pump speed and d is the pump displacement.
In a closed system, the same flow will occur through the motor, whose RPM for any given flow may be computed by the formula:
where d is the motor displacement.
Maximum flow for a motor is related to the maximum safe RPM at which it can operate. If you know that RPM number you can use it in the first formula to figure out the approximate maximum allowable flow.
- Maximum pressure is a limiting factor that is determined by the mechanical strength of the components. Good design practice avoids operation at maximum pressure, as high pressure accelerates wear on components and decreases the life of the seals and hoses.
The amount of pressure developed in a hydrostatic propulsion system depends on input power to the pump, the resistance of the load and losses in the piping and valves. The torque generated by the motor is a function of pressure. As it is desirable to avoid very high pressures, I have long used the design philosophy of larger displacement components, which can transmit a given amount of power at lower pressure. This is because larger displacement motors have more "mechanical advantage" and thus do not have to be pressurized as much to develop a given amount of torque.
The flip side is that larger displacement components are physically larger and owing to the higher flow rates, may result in lower efficiency due to pumping losses in the circuit.
- Maximum RPM is a limiting factor that, like maximum pressure, should be avoided. Most pumps will tolerate some overspeeding, but usually will suffer significant efficiency loss. Overspeeding a motor can cause damage or seizure.
My general rule of thumb for selecting pump displacement is 1 CI/rev per 10 horsepower, assuming good piping practice is followed (e.g., minimal use of elbows and hoses). From that, select the motor size that will produce the desired overall "gear ratio."
For example, my F7 has a 16 horsepower engine. I used a Barnes G25 pump with a displacement of 1.4 CI/rev. I used Dynamic motors with a displacement of 5.9 CI/rev. As the motors are piped in series, they look like a single 5.9 CI/rev motor to the rest of the system. Hence the effective drive ratio is 4.21:1.
As the maximum governed engine speed is 2400 RPM and the driving wheel diameter is approximately 5.188 inches, the maximum ground speed would be 8.8 MPH, which is 66 MPH in scale. It's a freight engine, so that speed is realistic.
The formula for computing speed is:
- MPH = (RPM × D ) / (G × 336)
where
D is wheel diameter and
G is overall drive ratio.
Hope this information is useful to you.