Which side has more power?

Donman · Sep 19, 2005

Which side has more power?

While surfing an implement and miscellaneous auction Saturday, a friend and I noticed several hydraulic cylinder applications where the two way cylinders appear to be installed backwards. An example would be the main arm lift cylinders on a fel with the rod end on the bottom instead of the top. Common sense would say that the rod end of a two way cylinder has less square inches and therefore would produce less lifting power. Why would they be installed this way? Would they be faster acting because it takes less fluid to fill that end of a cylinder? Can anyone with hydraulic knowledge enlighten us?

chim · Sep 19, 2005

Re: Which side has more power?

One of us is "backwards" and it could well be me. I can do better with a pen and napkin when discussing some of this stuff.

If the ram is fastened to the bottom support, and it needs to extend to lift the bucket, the surface of the end of the rod is the entire ID of the cylinder.

For down pressure, the area is the ID of the cylinder, less the OD of the ram, making it less powerful?...............chim

Tim_in_IA · Sep 19, 2005

Re: Which side has more power?

regardless of which way it is installed it should have the same power, unless you change where it mounts. You are still pushing or pulling with the same force. I would think this had more to do with keeping stuff out of the cylinders such as rain etc so they are more commonly installed ram side out??

hazmat · Sep 19, 2005

Re: Which side has more power?

As others have said orientaion doesn't matter

Extension force = pressure*area of cylinder
Retraction force = pressure*(area of cylinder-area of ram)

CTyler · Sep 19, 2005

Re: Which side has more power?

I don't understand how you would have the same power when retracting. Isn't there less surface area to push against when retracting? You no longer have the full end area to push against. The higher the surface area the more push.

RobS · Sep 19, 2005

Re: Which side has more power?

True, there is less force when retracting as Hazmat has shown in his equation. The force in a given direction has nothing to do with mounting attitude though /forums/images/graemlins/smile.gif

CTyler · Sep 19, 2005

Re: Which side has more power?

Agreed, doesn't matter right side up/down.

I was questioning the statement about pushing or pulling.

After re-reading it though I understand he meant you have the same power for a given direction wether its mounted right side up or upside down.

TonyV · Sep 19, 2005

Re: Which side has more power?

If my old brain remembers the stuff I learned in a first semester college hydraulics course, then F=PA (force equals pressure times area) where force is in pounds. As long as the hydaulic pressure remains constant, then the more area that it works against the higher the force will be. Therefore (this is a word us college profs like to use /forums/images/graemlins/grin.gif) the force acting on the end of the cylinder with the rod must be less than the force acting on the end without the rod. I also recall my old hydraulics prof of making a particular point of this.
(I reserve the right to blame old age on the misinterpretation of these facts as prestented /forums/images/graemlins/grin.gif)

...Tony

Donman · Sep 19, 2005

Re: Which side has more power?

I just realized when reading your replies that the fel cylinder was a bad example because when the cylinder expands it is pushing on the full diameter of the piston whether or not it is upside down. We did see other applications where we thought that more power was needed on the retraction stroke and then the diameter of the rod takes away some of the needed area for maximun power.

DaveNay · Sep 19, 2005

Re: Which side has more power?

( I just realized when reading your replies that the fel cylinder was a bad example because when the cylinder expands it is pushing on the full diameter of the piston whether or not it is upside down. We did see other applications where we thought that more power was needed on the retraction stroke and then the diameter of the rod takes away some of the needed area for maximun power. )

In strictly theoretical terms, one of the mounting directions will have very slightly more lifting capacity due to the fact that either the rod and piston assembly or the cylinder (bore) assembly will weigh more than the other one. This would mean a lifting capacity difference of perhaps 5-10 lbs depending on the bore dimension and stroke length.

Dave

blackd · Sep 20, 2005

Re: Which side has more power?

A real world reason: when building my FEL the bucket mounts for the cylinder end was narrower than the rod mounting. The cylinders would mount with the ram reversed; therefore, I mounted them in that direction as opposed to reworking the FEL. Works great, only need to watch the hydraulic lines so that they don't get damaged as they are more exposed.

DaveNay · Sep 20, 2005

Re: Which side has more power?

( A real world reason: when building my FEL the bucket mounts for the cylinder end was narrower than the rod mounting. The cylinders would mount with the ram reversed; therefore, I mounted them in that direction as opposed to reworking the FEL. Works great, only need to watch the hydraulic lines so that they don't get damaged as they are more exposed. )

Another valid reason might be to put the rod end down, and therefore lessen the chance of moisture collecting on the seal, and rotting it away.

Dave

MikeD74T · Sep 22, 2005

Re: Which side has more power?

The most logical reason for mounting the cylinder in a stationary location is to keep the hoses shorter. For example on a woodsplitter the hoses don't move at all ( they move a little on a FEL, usually rotationally). If the rod were anchored , both hoses would have to be lengthened to move with the cylinder.
In vertical applications sometimes the rod is mounted down to allow less dirt accumulation on the packing. In such cases the hoses usually go around a rotation point but do not change length.
As for power, it's strickly a function of surface area. A 4" cylinder with a 2" rod (as found on many splitters) will push twice as much as it will pull.

AntrimMan · Sep 22, 2005

Re: Which side has more power?

Geee,
I just spoke with Cliff Clavin about this and he said that a 4" cylinder with a 2" rod will push about 1.3 times what it will pull.
Who is correct?

All the best,
Martin

TonyV · Sep 22, 2005

Re: Which side has more power?

Ok...hydraulics class is now dismissed (where we learned that force = pressure times area) and geometry class is now in session. /forums/images/graemlins/grin.gif

The area of a circle is equal to pi times r squared. A 4" diameter cylinder has a radius of 2" so its area is about 12.6 square inches. A 2" diameter rod has a radius of 1" so its area is about 3.14 square inches. Subtracting the area of the rod from the area of the cylinder gives us 9.46 square inches. This is the area that the hydraulic fluid engages on the rod end. The ratio of 12.6 to 9.46 is about 1.33. Therefore the force exerted on the open cylinder end will be 1.33 times greater than the force exerted on the rod end. Or, if you'd rather the force on the rod end is about 75% of that on the cylinder end. Geometry class dismissed.

Tomorrows topic will be "Hair Tonics...do they really work?"

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
Tony

Defective · Sep 22, 2005

Re: Which side has more power?

( Tomorrows topic will be "Hair Tonics...do they really work?")

Sorry Tony...

Looking at your avatar, the answer's already been determined.

/forums/images/graemlins/tongue.gif

TonyV · Sep 23, 2005

Re: Which side has more power?

 ( Tomorrows topic will be "Hair Tonics...do they really work?")
Sorry Tony...
Looking at your avatar, the answer's already been determined.
/forums/images/graemlins/tongue.gif )


This is a new low in my forum posting experience...dissed by a defective visitor. /forums/images/graemlins/crazy.gif /forums/images/graemlins/grin.gif
I'm glad to see that my brand of humor isn't lost on everyone here though. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

....Tony

willfick · Sep 24, 2005

Re: Which side has more power?

I think you're right about the determining factor being the hydraulic line layout rather than lifting force. On the FEL on my sister's JD 5310 the lift cylinders are set up what I would call upside down. The hyd. lines for all four cylinders on the loader use short hoses to jump from the tractor to the loader frame then have hard lines to close to their respective cylinders, then another short hose to connect from frame to cylinder. She doesn't have to deal with any of the two to four foot hoses that my old Ford has rubbing themselves to death on both sides.
Wm

bobodu · Sep 24, 2005

Re: Which side has more power?

"With equal pressure at
either end, a cylinder
can exert more force
when extending because
of the greater piston
area. In fact, if equal
pressure is applied to
both ports at the same
time, a cylinder will
extend because of a
higher resulting force
on a head end."

Taken from US ARMY Hydraulics Manual FM 5-499

For what it's worth... /forums/images/graemlins/grin.gif

MikeD74T · Sep 24, 2005

Re: Which side has more power?

Tony V, You got me on my own math. I should have subtracted the smaller diameter rather than compared their diameters. And obviously the correct ratio is 1.333 to 1. where the rod is 1/2 the cylinder diameter.
The real point being the closer the diameter of the rod is to the diameter of the cylinder, the greater the difference in force. i.e. telescoping truck body hoists have great pushing force with no pulling force or hydraulic cranes where large forces are needed to extend the boom & much smaller to retract. The difference in needed force is crucial in piston selection along with the need for rigidity of the extended rod. If equal force is needed in both directions it becomes difficult to have large diameter differences yet maintain rigidity. Thats why many backhoes have 2 single acting swing pistons mounted to oppose each other.