Basic science of Hydraulics?

[MKIHALL]

Hello all...in my search for a small TLB machine, I'm trying to compare multiple brands/sizes/features. I have very little experience with hydraulics, anyway these are my questions (I figure if you're reading this forum you'll probably know the answers)
1.) All these GPM pump numbers are confusing, for example the CC - Yanmar 2450 has more total GPM than others in scut range, but its split btwn two separate 3.5 and 3.6gpm pumps, does this increase or decrease the backhoe pistons speed and load capabilities?
2.) Can an attachment max out on GPM, in so far as, no matter what, a small backhoe runs the same on 3 or 5 GPM?
3.) Does the GPM affect a BH or Loaders capacity, or is that stricly a matter of piston and cylinder diameters and length? I think it does affect speed, right?
4.) Is horsepower more or less important than torque for running these pumps... or is it just a simple impeller type thing where its speed is governed by pressure and viscosity, not the load being applied to the attachment?
5.) What specs should i be looking at to truly gauge each machines loader or backhoe capabilities?
:confused2:
thx

 

[Zebrafive]

For #3, GPM determines how fast you can cycle any function. More GPM the quicker the cycle times. Also gives the ability to do more than one function at the same time. System pressure and cylinder size (diameter of piston and rod). It's a function of the pumps PSI x piston area. More pressure (PSI) more capacity, larger piston area, more capacity. Length does not affect capacity. Rod size reduces area of the piston when pressure is to the rod side of the cylinder. I would not worry to much about the rod size as far as capacity goes.

 

[KWentling]

#1, When there is more than one pump, they have a dedicated purpose, such as power steering, hydrostatic transmission, etc., so the combined flow is not used for any one function, such as the backhoe.

#2, Possible. Depends on the maximum flow capability of the components. Probably not going to be seen in a 3 vrs. 5 gpm application though.

#3, Just as Zebrafive indicates.

#4, Small tractors like these are almost always open center fixed displacement pumps directly driven so gpm is directly linked to engine rpm. The hp needed to drive the pump is directly linked to the pressure required to do the work.

#5, Compare the lift and breakout capacity of the loaders and the breakout force of the backhoes along with the digging depth, lift heights, cycle times, etc., for each. As far as the hydraulic capacity of the tractor, more gpm and higher pressure is always a good thing. If the attachments are designed for the specific tractor, (as in TLB packages) performance should be acceptable. Some will be more capable than others.

Kim

 

[MKIHALL]

Thank you very much Z5 and KW, this is fascinating stuff.
So the magic number for strength/ability of these implements seems to be PSI, as long as I'm not concerned with cycle times?
Also, I'm curious, would it be true to say that the larger the piston area (or volume it displaces) the higher the breakout #s? if so wouldn't a smaller and longer rod be desirable?
What does "open center" mean?
Finally, it would seem that regardless of the GPM numbers the more linear or unobstructed the hydraulic lines to/from the pump to/from implements the stronger the system should run? Does friction/turbulence matter?
Really appreciate the feedback

 

[KWentling]

Yes, the higher the pressure, the more force that can be produced. The larger the piston area the oil pushes on, the more force applied. When a cylinder is retracted, (oil flows into the rod side of the cylinder) the piston area is reduced by the area occupied by the rod, so a cylinder can push more that it can pull. The bigger the rod diameter, the less the area of the piston has for the oil to push against. The rod length has no bearing. We are talking area, not volume. Smaller rods are desirable only as long as they have enough strength to do their job. Since we are talking about fixed displacement pumps in these small tractors, they are always pumping oil. That oil has to have someplace to go, so the valves are "open center" meaning that when the valve lever is in neutral, the oil is flowing through the valve body and back to the sump. When the valve is moved to enact movement of a cylinder, the oil is diverted to flow to one side of the cylinder, the movement of the cylinder's piston forces the oil from the other side back through the valve back to the sump. If the valve is held until the cylinder is fully stroked (or is stalled because a lack of sufficient pressure to move) and is continued to be held, a pressure relief valve should open and the oil goes back to the sump. The alternative to an open center system is a closed center system. A closed center system uses a variable volume pump that can be de-stroked to zero flow or stroked to full flow as is required. Since there is no constant flow, the valve center is closed. When the valve is operated, it sends a signal to the pump to stroke, creating flow. The advantage being that you aren't using up power pumping oil that isn't doing anything. There are many variations of both systems, but that is the basic idea. And yes, one of the inefficiencies of hydraulics is the friction loss of the oil going through the lines and around corners, etc., so the shorter and straighter the lines, the more efficient they are.

Kim

 

[kennyd]

:thumbsup: Nice job Kim

 

[slowzuki]

1.) Total only matters in old single pump systems. Implement pump is the number to look at. The other pump is power steering only. The flow only affects speed, not power.

2.) Hydraulic cylinder speed is proportional to gpm, if you try for too high of gpm like say 4x normal, friction in the hoses will cause all kinds of problems with fluid heating.

3.)GPM only affects speed.

4.)None of the pumps are sized to use all the engines hp. At full flow, with the relief valve just shy of relieving, my Kubota's pump uses 5 hp. They are fixed displacement pumps so the hp they need is directly related to the load on the cylinder. Light load, low hp.

5.) Implement pump flow, and a matching size loader. A really strong rated loader has big cylinders, so you need a big pump to make it operate at a reasonable speed.

Hello all...in my search for a small TLB machine, I'm trying to compare multiple brands/sizes/features. I have very little experience with hydraulics, anyway these are my questions (I figure if you're reading this forum you'll probably know the answers)
1.) All these GPM pump numbers are confusing, for example the CC - Yanmar 2450 has more total GPM than others in scut range, but its split btwn two separate 3.5 and 3.6gpm pumps, does this increase or decrease the backhoe pistons speed and load capabilities?
2.) Can an attachment max out on GPM, in so far as, no matter what, a small backhoe runs the same on 3 or 5 GPM?
3.) Does the GPM affect a BH or Loaders capacity, or is that stricly a matter of piston and cylinder diameters and length? I think it does affect speed, right?
4.) Is horsepower more or less important than torque for running these pumps... or is it just a simple impeller type thing where its speed is governed by pressure and viscosity, not the load being applied to the attachment?
5.) What specs should i be looking at to truly gauge each machines loader or backhoe capabilities?
:confused2:
thx

 

[Transit]

Think of a rod as a column, a long thin column in compression is easily buckled, that is why FEL's should never be used as a bull dozer.

Volume has nothing to do with the force generated, it is all area and pressure.

Pascal's law:


A1 x P1 = a2 x p2

http://www.google.com/images?client...PJ4SQ0QH9o63eAw&ved=0CC4QsAQ&biw=1024&bih=615

 

[J_J]

slowzuki,

Your tractor implement hyd pump at 9.8 GPM's at 2500 psi will take about 17 HP.

If your power steering psi is set for 1500, then the 4.5 GPM will take about 5 HP.

 

[Ductape]

I'll just add two comments.

Its not always as black and white as the numbers that manufacturers use.

1) Numbers such as lift capacity can be very decieving. One manufacturere will use the lift capacity at the center of the bucket, and a different manufacturer will use the lift capacity at the bucket pivot pins. In my opinion, the manufacturers do this to make it difficult to compare apples to apples. They will often use numbers that make their product seem better, stronger, etc.

2) As far as breakout force.... its not necessarliy as simple as the larger hydraulic cylinder automatically having more breakout force. A cylinder placed so that it gives a greater mechanical advantage (leverage) will have more brakeout force than an identical cylinder with less leverage.