40C vs 420C hyd pump

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groo

Re: 40C vs 420C hyd pump

Post by groo » Mon Sep 10, 2018 5:27 pm

As I explained with math previously, if you retract a cylinder, the rod goes into the cylinder. fluid gets displaced.

Where does the oil go? It can't just go from the base side to the rod side since the is less volume on the rod side. Thanks there is on the base side.

The piston seal is needed to move the piston, not to hold it. When moving oil is not trapped. It is flowing in and out of the cylinder.

I dealt with one cylinder that lost its piston nut, and the piston fell off. This cylinder had a stroke limiter so it didn't just fall apart completely before the extention was stopped by the geometry of the mechanism. This piston would flow freely when the valve was actuated. When the valve was closed it was impossible to manually retract it.

Look at the fluid level on a machine when all pistons are extended vs all pistons retracted. The fluid level in the resivior is higher when you retract the piston. The fluid in the cylinder flowed out of the cylinder and into the resivior. If you were to extend the cylinders, top off the resivior, then seal it, it would explode when you retract a cylinder.

Ps. There are two types of single acting single stage cylingers.
1 have piston seals, but rod side is vented to the atmosphere. I've seen these on backhoe swing in pairs.
2 have rod seals, but the piston only has wear rings.
These generally have larger rods and are similar to multi stage single acting cylinders in that the rod side is ported to the base side. Often these have hollow rods.these can also have more flexibility when placing the port. I've seen these commonly on snow plows.

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Lavoy
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Re: 40C vs 420C hyd pump

Post by Lavoy » Mon Sep 10, 2018 9:15 pm

And as I have tried to explain repeatedly, in spite of what your math may or may not explain, when one of the cups splits in these cylinders, the blade will settle, it is just that simple, period. I have been diagnosing, repairing, and selling parts to people that I have diagnosed their cylinder problems over the phone for about 30 years now, as well as repairing my own, and customers in my shop. It is seldom a valve failure, because the two cylinder crawlers do not really have anything in the valve that can instantaneously fail like a split or blown cup. I can say with utter confidence that I have never rebuilt a two cylinder control valve due to blade settling. Can it occur, I imagine, but I have never seen it, or encountered it in owning a couple hundred of these crawlers, working on a substantial number more, and helping an ever larger number with theirs. Hence why I stock several sets of leather cups, and no hyd valve parts.
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B Town
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Re: 40C vs 420C hyd pump

Post by B Town » Wed Sep 12, 2018 7:10 am

Groo- your explanation works in theory minus one real world factor. Load. We want our blades and buckets to hold dead still(at work and at the shows). If the leathers or the Orings are worn or cracked and let oil bypass, the blade/bucket/ cylinder will drift.

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Stan Disbrow
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Re: 40C vs 420C hyd pump

Post by Stan Disbrow » Wed Sep 12, 2018 7:43 am

Hi,

I am an engineer, and this happens often. One has a lovely theory, backed by math, and then things just don't quite work that way.

I refer to it as: A beautiful theory torpedoed by an ugly fact. In this case, as mentioned, loading forces oil transfer.

So many blades and buckets that don't stay put, and so much time spent eyeballing valves, only to find seal and/or scoring issues in the cylinders to be the issue. So much so, one learns to look there first.

Stan
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Have: '58 JD 420c 5-roller w/62 inside manual blade
Have: '78 JD350C w/6310 outside manual blade
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Ray III
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Re: 40C vs 420C hyd pump

Post by Ray III » Fri Nov 23, 2018 6:50 am

groo wrote:
Mon Sep 10, 2018 5:27 pm
As I explained with math previously, if you retract a cylinder, the rod goes into the cylinder. fluid gets displaced.

Where does the oil go? It can't just go from the base side to the rod side since the is less volume on the rod side. Thanks there is on the base side.
With a bulldozer blade, the rod is extended as the blade drops. So rather than there being a volume of oil to displace, there is a vacuum. And what you don't see is that air gets drawn through the rod seal, which is probably worn if the piston seals are. After a blade has settled due to seal wear, you can put a floor jack under it and jack it up quite a ways because the cylinders now have air in them.

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