Hydraulic questions

unclehan · Sep 23, 2004

It seems hydraulic systems are generally less efficient than mechanical or electrical.

What's the main reason for this? Is it because of fluid viscosity?

Out of all the three types of efficiencies in a hydraulic pump:

1. Hydraulic efficiency
2. Mechanical efficiency
3. Volumetric efficiency

Which one do you think is the most significant factor in a piston pump?

I heard the piston have no seal, pistons are lapped into the cylinder bores. Cylinder bypass accumulates in pump body which lubricates swash plate assembly. Your output shaft and plate adjuster will be the only seals.

Did anyone ever need to service their hydro pumps? Is so, how often?

Are cylinder bypass significant? How many hours (or min) of operation at 2000 rpm do you think will leak 5 ml? Are they filtered and recycled back into the reservoir?

slowrev · Sep 23, 2004

I can't really comment on the piston pumps, All I have worked on were either vane or gear types.

boustany · Sep 24, 2004

I honestly don't understand the question(s). I can't imaging lifting a 1000 pound rock with an electrical linear drive. It seems to me it depends what you are doing.

A hydraulic pump offers fluid power which can impart rotational motion, lifting motion, you name it, and also provide 'variable leverage' in a way most other systems can't: take 2000 psi and apply it to 4 square increase you get 8000 pounds force. Apply it to 8 square inches you get 16000 pounds force. Try that with an electric system.

In terms of efficiency, I would imagine viscosity is a big issue.

I don't see in most cases where hydraulics are used, it is the best way to go, or the only practical way.

Henro · Sep 24, 2004

I can't imaging lifting a 1000 pound rock with an electrical linear drive. It seems to me it depends what you are doing 

I think it really does depend...

In one steel mill where I worked, we had over head travelling electric cranes that were rated at 400 tons lift capacity. Yes, that is 800,000 lbs...electric motor driving everything...so electric can do the work too, in the right application. In this case ladels filled with 350 tons of molten steel were regularly lifted and carried around. That is much heavier than most rocks we deal with to say the least... /forums/images/graemlins/shocked.gif

Also not sure if overall hydraulic systems are really extreemly more inefficient that other type systems or not. At least when all aspects of what is being done is considered...

Probably the best comparison is between a gear drive and hydraulic drive, driven by the same prime mover...e.g.... a engine of some type. The mechanical system would win the number battle, but the hudraulic system would win the war in the long run I expect. Why? Because it is self limiting to a degree, and would suffer less mechanical failures, under hard use, in my limited experience... /forums/images/graemlins/tongue.gif

Pulling electric systems into the question ignors the losses that are suffered elsewhere like at the power generation plant...and these should be considered, since it is part of the overall picture. If you factor power plant and transportation (power transmission system losses) into the picture the apparent higher efficiency of an electrical system degrades somewhat...

boustany · Sep 25, 2004

Oh, electricity has its uses, but my point was that hydraulics has its uses too. I haven't heard of an crane or any other thing like an FEL that uses electricity. Year ago, they used steam engines and the like with cable systems and drums to move and lift, but I don't think we would want to have that type of system on our backhoe.

The hydraulic ram is an amazing thing.

Henro · Sep 25, 2004

Oh, electricity has its uses, but my point was that hydraulics has its uses too. 

I did not mean to sound like I wan't agreeing with that! /forums/images/graemlins/blush.gif

Myself I see great functionality in hydraulic systems. /forums/images/graemlins/smile.gif I wanted to say great beauty, but feared that I might start getting a reputation as a romantic... /forums/images/graemlins/grin.gif

markls8 · Sep 26, 2004

The efficiency of any system is defined as the percentage difference between the energy input minus the energy put out to do the work. So the most efficient system will have the least energy lost in doing the work. Speaking in generalities here because hydraulic systems themselves vary considerably in efficiency by their design and construction of pump, pump energy source, actuator, cylinders, valves, seals, lines diameter and material etc, as do mechanical systems - hydraulic systems are less efficient than mechanical systems mainly because of internal frictional losses, manifested as and most readily observable as heat.

If you consider, say, a given electric motor (or a diesel motor, or anything else) driving a lever (a mechanical system) to raise a certain weight versus driving a cylinder of a hydraulic system raising the same weight, the motor will use more energy (ie. electricity, or fuel) in the hydraulic system. The hydraulic system will give off more heat while doing the same work because of friction in the fluid against the internals of the system. The hydraulic system can be more complex and applying pressure to the fluid will cause heat to be given off throughout the entire pressurized system even if no other work is done. No such loss to this degree occurs in a mechanical system.

Cylinder bypass (I'm assuming you mean leakage past the seals) depends on the condition of the seals and the seating surface in the cylinder and is also a function of the original design. It is possible to design a cylinder so that the fluid bypassing the seals will be almost nil. Note that with o-rings, and proper design, the more pressure is applied, the tighter the seal becomes, and at low pressure leakage can ocurr because the ring is not seated. In practise, the cylinder bypass is just one of many areas for loss, others being valves, pumps etc. These also contribute to the efficency loss of the system. Yes, some cylinders do not have seals but just rely on the viscosity of the fluid to seal them, most commonly in pumps. The fluid that gets by just enters another part of the same system usually.
Hope this answers your question- it's the short answer. /forums/images/graemlins/smile.gifRegards - Brian.

MossRoad · Sep 26, 2004

I haven't heard of an crane or any other thing like an FEL that uses electricity 

Go to google and do a search on "walking draglines" and then do a search on "big muskie". All of these mamoth dragline cranes and shovels that excavate coal strip mines are electric.

Also, look at trains. 99.999% are diesel electric. The diesel generates power for the electric motors that turn the drive wheels. Look at the torque a train has to generate to get a mile of coal cars moving. Ever read the side of a coal car? It is not uncommon to see it rated for 200,000 plus pounds. Multiply that by 100 or more cars. Thats 20,000,000 pounds! I've seen coal trains excellerate from 0 to 30 in about a mile and a half. That is an amazing display of smooth power. /forums/images/graemlins/smile.gif

Now how efficient is it? I have no idea. But I doubt we will ever see a gear driven or hydraulic driven train put into production any time soon. You would need too many gears to cover the speed range from 0 to 60+ MPH and I cannot imagine the amount of PSI and GPM of hydraulic fluid it would take to get a train moving, let alone how to dissapate the heat it would generate.

For really big loads... really, really big loads, electric is the only way to go.

boustany · Sep 26, 2004

I meant in terms of linear actuators (i.e. the electrical equivalent of a hydraulic cylinder). I''m well aware of the use of large electric motors in things like trains and the like. My understanding is that electric motors work well near their rated outputs and are very innefficient at lower outputs (back EMF & stuff like that).

There is a Canadian company making diesle/electric/hybrid (like a Prius) locomotives that are supposed to be very feul efficient for shuttling railcars around.

Still. for the things hydraulics as good at, they are very good at.

rambler · Sep 27, 2004

(

Also not sure if overall hydraulic systems are really extreemly more inefficient that other type systems or not. At least when all aspects of what is being done is considered...

Probably the best comparison is between a gear drive and hydraulic drive, driven by the same prime mover...e.g.... a engine of some type. The mechanical system would win the number battle, but the hudraulic system would win the war in the long run I expect. Why? Because it is self limiting to a degree, and would suffer less mechanical failures, under hard use, in my limited experience... /forums/images/graemlins/tongue.gif

)

I understand the nature of your message, but I totally & 100% disagree with your conclusion.

Farm equipment is full of mechanical & hydraulic systems that often parrallel each other in funtion, and the mechanical system is always cheaper & more reliable & cheaper to mantain/ repair.

There are many good reasons to choose hydraulic systems - I very often do & often prefer them for many chores.

Reliability or cost savings would _very_ rarely be a possible reason. I believe you are 100% wrong on that point.

I do not wish to relive any long threads here, & I do not want to overly influence you. /forums/images/graemlins/smile.gif Just stating my position, and I will leave it at that. I enjoy your posts here. You have a good one. /forums/images/graemlins/smile.gif

--->Paul