mini cat20 excavator hydraulic flow / pressure question.

[blakester]

I have a cat20, probably early 80's mini excavator, 7000lbs ish that has a foreign tag on it. i cant really find the specs but ill assume the main hydraulic pump is a gear pump.
Its my second machine, and this one feels like its quite slow comparatively. It also never bogs down when it encounters hard digging, it just slows down. If for example i am lifting the main boom and extending the arm, it slows down to well below half speed. I assume this is a flow rate issue rather than a pressure issue, but im not sure, maybe it's both. Is it possible to increase the flow rate on gear pumps as well as the pressure, or it that going to be completely related to rpm? Im also assuming since it affects all the hydraulics, that its not a bypass or regulator or something on each line, but something on the main pump. Thoughts?

 

[blakester]

oh yeah, and if the bucket gets into hard ground it slows down as well, but again does not bog down the machine really at all.

 

[dfkrug]

I assume this is a flow rate issue rather than a pressure issue, but im not sure,
maybe it's both.

You need to do a pressure test under load. The easiest way is to just put a gauge on the end of one of the
work port hoses and operate that circuit.

What can happen is your relief valve opens, perhaps at a too-low pressure, and half of your flow
goes thru that. What you see is your cylinder moving much slower.

You may also have worn out piston seals in your cyl(s). Half your flow may be going past your
seals under load. This will cause quite a hot-spot in your cyl, which you can detect with
a non-contact thermometer. And it will be pretty noisy there, too.

 

[blakester]

Hmm, so do I need to know what kind of pressure should be in those lines before i can tell if the relief valve is opening too early? And is the relief valve part of the pump or is it something that is part of the controls. And im guessing that relief valve is adjustable yeah?

 

[dfkrug]

Relief valves on industrial equipment will likely be on the outlet plumbing of the
main hyd pump, before the first valve. Modern industrial equipment generally
has more than one pump (and therefore more than one system RV). The RV may be
integrated into the first valve or on some sort of hydraulic block or manifold.
There will be RVs in your work-ports, too.

System RV pressure settings may be adjustable, but not always. Expect settings of anywhere
from 2000-4000psi for something like your Cat. You need a manual.

 

[Snobdds]

Yep, check your PSI on the hydraulics. Usually a person can tell when the relief valves opens. For example your curling your with a load and it's hung up on a root. The bucket will just seem to stop curling all of a sudden under load.

I have no idea about your Cat (never even heard of that naming nomenclature), but flow rate and pump capacity will dictate if two or more hydraulic functions can work simultaneously. However knowing how Cat builds stuff, you should have ample flow to do two circuits at once.

 

[blakester]

Yeah I need to get out there and take some pics. I'm sure that I'll be able to get a manual that's in english. I cant find anything on the net about a CAT20. The only thing I know about it is that it's a Mitsubishi engine, and it's in great condition, no leaks anywhere, extremely small amount of slop on the whole machine. Everything is tip top. Thanks for the tips, ill cehck it out this weekend.

 

[dfkrug]

I don't know much about Caterpillar myself, but the modern stuff will have a name like "303.5", and
designate an excavator that weighs about 3.5 tons. My avatar shows a Cat 315, about 30,000#.

In the old days (1930s), a Cat 20 was a 20hp gas dozer weighing in at about 7500#. It
had "TWENTY" cast into the radiator. I once rebuilt an Ensign carb from one of those units.