CAT 301.6C jerky boom cylinder movement

[RedQueen]

Hi.

New to the forum so a quick introduction: My name is Andrew and I am a proud owner of 1.6 tons CAT mini excavator. I am amateur excavator operator with a couple of hundred hours in my log book. I am also an amateur mechanic - meaning that I will take on just about anything that needs to be done although that might mean that later I will have to call on the professionals to finish what I have started :)

My CAT is 301.6C mini hydraulic excavator. It has around 3000 hours and I got it a couple weeks ago used. Since I have been using it for a couple of days and I noticed two things:
When operating boom cylinder especially with the arm fully extended and not loaded I frequently (but not always) experienced jerky cylinder movement. Sometimes so much that I had to stop and start movement of boom again. It seems (but I am not sure) that it happens more often when the machine is hot.
The other thing is track movement. If I want to move slow it starts to move very slow but smooth. That's OK. But if I push the levers a little bit further to say half-speed it starts to move jerky and I have to stop and start over. Unlike boom cylinder issues this happens always (no matter hot or cold). If I push the levers all the way (forward or reverse) it moves smooth but with top speed.

Otherwise I noticed no leaks and have just changed hydraulic oil and filter. The engine runs fine. The power is always OK also under heavy load.
Can anyone point me in the right direction what would be the steps to try to solve this problem?

Thanks!

[Muz]

Welcome to the forum Some hydraulic setups use shock valves in the circuits to remove or soften sudden movements .. could be a fault related to that ? although it sounds as though theres not enough oil in the tank but you say you've checked all that ? was it the correct oil you used?

[RedQueen]

Hi Muz. Thanks for welcome and quick reply.

Oil level is OK. I am checking it frequently in order to spot any potential leaks. But as I said - no leaks and oil level OK. Used original CAT HYDO Advanced 10 oil so I believe also the oil type is OK. True - I don't know what type of oil was before in the machine since I got it used with no info on type of liquids used.

I will certainly check the valves as I suspect them also. I'm just not sure so I would like to get as much advice and input as possible before I start tearing the machine apart. :)

[v8druid]

Hi Ian,
Pick up/outlet pipe gauze in the tank perhaps, not allowing enough flow at higher demand, if it has one. Maybe sludge-ed up, causing cavitation in the pump
Were it just the cylinder I'd say bent/bowed rod
Gra

[RedQueen]

Hi v8druid,

Yes the machine has an outlet pipe in the oil tank fitted with the screen. So far I did not check that so it could be all you mention. I'll check it.
The cylinder does not look bent or bowed also no leaking at the seal although there is evidence of some servicing being done on it in the past.

Cheers.

[v8druid]

Hi v8druid,

Yes the machine has an outlet pipe in the oil tank fitted with the screen. So far I did not check that so it could be all you mention. I'll check it.
The cylinder does not look bent or bowed also no leaking at the seal although there is evidence of some servicing being done on it in the past.

Cheers.

It's odd it only does it with the main boom and tracking - dipper/crowd/slew/blade all ok? sure does sound like air in there though?
Have you tried cracking a pipe to see if you get any air/froth out or just pure oil flow??

I'd certainly expect the boom to self clear though if it did.

[RedQueen]

My gut feeling is telling me that it has nothing to do with oil or the cylinder itself. Rather it has something to do with some valve somewhere acting up on this particular hydraulic circuits (hydro-motor and boom cylinder). It feels like the pressure builds up but as soon as the movement starts it stops and then engages again... And while this always happens on mid speed with hydro-motor on tracks in often (but not always) happens on boom cylinder so it has some randomness into it to make it more interesting

But only my gut feeling, no evidence... So I need to do some more research. I really don't like chasing things blind...

[Muz]

Ive posted before about hydraulic systems groupings .. Is the same track and boom lift controlled with the same relief valve on the block ? Perhaps you would need a schematic to see because pipework isn't always obvious although often they are segmented in groups on the block too, just to make oil transition ports cheaper to produce .. for example .. if you pull the boom lift service at the same time as that track, does one or the other change speed ?

[Mick-the-fitter]

My gut feeling is telling me that it has nothing to do with oil or the cylinder itself. Rather it has something to do with some valve somewhere acting up on this particular hydraulic circuits (hydro-motor and boom cylinder). It feels like the pressure builds up but as soon as the movement starts it stops and then engages again... And while this always happens on mid speed with hydro-motor on tracks in often (but not always) happens on boom cylinder so it has some randomness into it to make it more interesting

But only my gut feeling, no evidence... So I need to do some more research. I really don't like chasing things blind...

A few questions for you! Which filter(s) did you change? Did you experience either problem prior to the filter and oil change, does this model have swash pump and pilot or triple gear pump?

Mick

[RedQueen]

Hi Mick,

According to maintenance manual there is only one filter to be changed for the hydraulic oil. It is situated in the hydraulic fluid tank and if I understand correctly filters the return oil flow from radiator. I used original CAT filter as per specifications.
Before I did an oil change I used the machine as it was shipped to me only for a couple of hours. I can for sure say that the problem with jerky movement of tracks when in medium speed was present already then. For the problems with boom cylinder I am not so sure as I was also getting used to the machine so it is hart to tell whether it was me or the machine .
As for the pumps there are two piston, one gear-type pumps so yes it has a swash pump.

Cheers.

[RedQueen]

Ive posted before about hydraulic systems groupings .. Is the same track and boom lift controlled with the same relief valve on the block ? Perhaps you would need a schematic to see because pipework isn't always obvious although often they are segmented in groups on the block too, just to make oil transition ports cheaper to produce .. for example .. if you pull the boom lift service at the same time as that track, does one or the other change speed ?

Hi Muz,
I will obtain the schematic in the next week so I will be able to check that and get back to you. Also I will try the second thing you suggested next week and certainly let you know.

Thanks!

[Mick-the-fitter]

Hi Muz,
I will obtain the schematic in the next week so I will be able to check that and get back to you. Also I will try the second thing you suggested next week and certainly let you know.

Thanks!

Hi,

I think it sounds like you could have pilot pump problem! As it runs a swash pump, pilot pressure is important to advance or retard swash plate angle, i.e give pressure or flow! If the plate doesn't move smoothly then jerky hydraulics can occur, check pilot pressure and flow.

Secondly you may want to check that the seal around the tank cover is not leaking air!

Mick

[RedQueen]

Hi.

Guys I am really grateful for all inputs you gave me. In this or next week (I really don't know exactly it depends on many factors outside my control ) and test all mentioned and report back with the results.

Cheers!

[v8druid]

Hi,

I think it sounds like you could have pilot pump problem! As it runs a swash pump, pilot pressure is important to advance or retard swash plate angle, i.e give pressure or flow! If the plate doesn't move smoothly then jerky hydraulics can occur, check pilot pressure and flow.

Secondly you may want to check that the seal around the tank cover is not leaking air!

Mick

Why Mick?
Surely the tank has a breather to atmosphere anyway?

[Mick-the-fitter]

Why Mick?
Surely the tank has a breather to atmosphere anyway?

The he breather is metered, the leak is not! If it is a vacuum or pressure system, with a leaking gasket or seal, it will be both.

Mick