View Full Version : Hydraulics

04-08-2012, 03:59 PM
Hi. Today I wonder about hydraulics. Mainly interested in variable displacement pump running off a gasoline engine, driving two wheels on independent suspension. About 6hp max at 7000rpm motor speed tops.. More likely 3600rpm average. I desire about 300RPM max at the wheels that will be 26" diameter for a 46mph top speed. (Obtainable with the motor I desire to use mounted on a bicycle, unsure if my cart will get that speed, but that is the plan)

My main concerns are as follows:

Most hydraulic gear motors I see mention 400rpm min 3600rpm max.
Does this mean they are ineffective below 400rpm flow? Or are they just highly inefficient?

What kind of efficiency can I get from a lawnmower variable displacement pump and cheap gear motors?

What would the efficiency be for a more typical transmission involving 1 gear mesh, 2 chains rear differential and U joints? (Independent rear suspension)

Should I use gearing/chains/etc to reduce the 3600rpm of the gear motor to the 300rpm desired speed?

How well will such a system maintain low output RPM (in the range of 30rpm at the wheel, so 360rpm at the motor if gearing is used) under varying torque loads? will a high torque load make the motor stop or significantly slow down compared to free speed if flow is maintained? (ie: Will low flows just leak pass the motor?

04-08-2012, 05:23 PM
Just off the top of my head, I would think that a gear motor would be able to operate from 0 rpm- if it was a vane motor I can see that a minimum rpm would be required to make sure the vanes always contact the inside of the housing. Of course, if the gear motor rpm was at or near 0, there would be little or no power output even if the pressure was high. There would be maximum torque though, and power usage would rise as the rpm came up. If you applied pressure but didn't let the shaft turn, then the leakage rate at the applied pressure would be the sole user of the power. It's probably more an efficiency thing than anything else.

My own thought is that for maximum efficiency in a hydraulic system, you would try to take the most advantage of the torque possible. In other words, if you had the motor running at 2000 rpm, but your torque usage was only a fraction of the motors capability, then you are supporting more pumping and friction losses than you need to. What I'm saying is I'd rather keep the pressure up and have the motor turning slower than the other way around. You would try to size the displacement of the motor such that at maximum continuous pressure rating, you'd get about as much torque as you needed or maybe 20% more- this is for hill climbing or burning rubber, whatever. Then your maximum speed would be related to the maximum flow rate your pump can produce. If that keeps the gear motor well under its maximum rpm, that would be fine. If the pump can produce a high enough flow rate to turn the gear motor past its rated rpm, then you probably need a larger displacement gear motor, which in turn would give you more torque from a standing start, but at the cost of a higher fluid flow rate. The larger displacement motor might be less efficient in that case.

From my own calculations, done years ago and forgotten, I came up with a torque requirement figure needed to boost my three wheel vehicle weight plus my own weight up a certain maximum slope which is an overpass here in my town. I figured I would need to meet that as a minimum, otherwise I couldn't go up that slope. If I met that with some extra, then I could go anywhere in the region. Then on level ground, or from a standing start, the degree of acceleration I could get would be based on that torque capability and I would accept whatever that was. If my system had more torque capability, then it would also be less efficient over most of the operating conditions. I did have this worked out, but I'd have to calculate it all over again. My idea at the time was to have my drive motor integrated with the drive wheel axle, and turning at the same speed- in other words direct drive.

04-08-2012, 07:19 PM
As a side note a friend at the airport took a gas weedeater mounted a prop & throttle cable to a bicycle has to pedal to get going than cruises over 30mph without pedeling & has almost nothing in it, all yard sale stuff.:D

04-09-2012, 12:16 AM
Hydraulic motors are for the most part very inefficient. I've never seen one on any type of device designed for speed. There is a lot of resistance to overcome pushing oil through lines and the faster you try to push it the warmer it will get. The main use is slow speeds and power.


04-09-2012, 05:50 AM
Yea, I know hydraulics is not perfect for this application.. but it seemed to have its own advantages. Sealed drive train, simplicity compared to making my own gearbox and U joint arrangement, CVT transmission via variable displacement pump, Option for limited slip differential with a flow divider and cross over valve. Ease of adding 4 wheel drive if desired.

I guess cost will be rather high however.. and losses... And weight really.
It had nice appeal that it would be easy to add an electric or pedal power input to the circuit at a later time... And maybe an accumulator used to start the engine.

04-09-2012, 01:44 PM
There are a huge number of "it depends" involved in the design of such a system. Losses can easily exceed 50% at some speed ranges and even 100% at very low power and speed. Best efficiency is at full power assuming the system is operating at a power level consistent with the actual capability of the pump and motor and that they are matched. Under those conditions it is around 10% to 15%.

Different types of pumps vary a lot with lots of tradeoffs for oil viscosity, temperature and volumetric efficiency at various rpms. Hydraulic pumps are not a good match to small displacement high rpm engines. They don't like more than around 2000 rpm because of cavitation issues.

Have a look here: