UR1 hydraulics

Hi everybody!

I have some questions on hydraulics.
In several topics the poor quality of eastern made rubber hoses has been mentioned. Can this also be the case for the o-rings originally mounted in Zetor tractors. My problem is that the front loader (Trima 912HD) on my Zetor 5245 is relatively week and can hardly lift a full bucket of snow. The hours on the tractor are only 1500 and it seems to me that it is to early for hydraulic pump failure although the tractor has been equipped with the front loader since it was new. After what I can understand there are several o-rings in the hydraulic circuit and I suspect that a leakage in one or more of these o-rings would impair hydraulic pressure. Could anyone tell me the most probable point for this leakage, so I know were to start looking.

There has also been suggested that I should test the pump capacity with 1000 rpm PTO but from what I can see this option does not exist on my tractor, there is simply no leaver at the position where it should be according to the manual???. Is it possible that there was a version of Zetor 5245 from 1985 with only 540 rpm PTO, the tractor was originally sold in Sweden where it still remains, by the way.

I bought both the operators and the workshop manual (www.tractor-manuals.com) last week so I have tried to identify the possible problems following the manuals.
I must say that I was mighty impressed by the delivery time of the manuals. I ordered them on Wednesday from London and got them on Friday to Sundsvall, Sweden.

According to the workshop manual the hydraulic pump pressure relief valve should open at 14 14.5 MPa but the pressure at the quick coupling for the external hydraulics should be 16-18 MPa with a properly working hydraulic system. Could someone explain this pressure increase?

Thanks
Bjore

Hi,
Zetor 'o' rings are as good as any other makes. You really need to test the system with a pressure gauge.
The problem is more likely to be in the pump / relief valve area, so if you can't get access to a gauge drop the pump plate down and give everything a good look over. (Drain the oil first)
Don't forget the problem could be with the loader itself before you get too carried away.

Regards

Mike

Yes, I have been thinking the same thing about the spool valve of the loader. It is acting a bit strange but it is perfectly tight when it comes to hold the pressure, the load I manage to lift stays there for at least 12 hours with no noticeable drop. The strange thing is that I cant make the loader go in the floating mode without holding the leaver forward in that position, it doesnt latch. And when I pull the leaver backwards to its maximum lifting position the loader stops rising. I suppose this is a simple matter of adjusting the cable length.
I will try to get hold of a pressure gauge and measure the pressure at the quick coupling that feeds the loader spool valve.

Does anybody know if this tractor model, 5245, came with the option only 540 rpm PTO in 1985.

And how should I interpret the pressure specifications, 14 MPa at pump relief valve and 16-18 MPa at external hydraulics.

Hope you dont find the last question to nagging. Being a radiation physicist, and only a weekend hobby farmer, I am really interested in understanding things from a theoretical point of view as well before I leave my desk and get my hands dirty.

Thanks
/Bjore

And how should I interpret the pressure specifications, 14 MPa at pump relief valve and 16-18 MPa at external hydraulics.

Click to expand...

This has to be a typo in your pressure specifications.

For a given hydraulic pipe size, if the pressure leaving the hydraulic pump is 14 MPa, this is the max hydraulic pressure that you can have in the system. The pressure at your external hydraulics has to be less than 14 MPa due to pumping losses which appear as heat. Otherwise you would have a perpetual motion machine!

Hi!
Great, thanks! Then we agree and my thermodynamics are not totally crap.
You could of course use the pump to drive a second pump stage to increase the pressure but with a resulting decrease in flow rate from that stage but I suppose that is, as they say, beyond the scope of this topic.[/]

I simply measure the pressure at the outlet and se what I get. And in a static system with no flow the pressure at the outlet should be about the same as at the pump after what I can understand.

Which of the two stated pressures is the correct one at the outlet 14 or 16 MPa in your experience? 14 sounds a bit low, the loader specification is at 17 MPa.

The workshop manual describes the pump relief valve and that it can be adjusted with 0.4 mm thick washers placed above the spring in its housing to increase the pressure of the spring on the valve. Has anybody out there tried this?

/Bjore

A hydraulic pump should deliver 2500 psi which is about 170 bar which is 17MPa

BUT

like ourselves, pump's output's disimprove with age. If you are are getting 14 MPA = 140 bar = 2100 psi this is still usable.

The pump relief valve is an overload device designed to trip when the pressure is reached. If you start messing about with this you may do more damage elsewhere in the system.

I did a front loader hydraulic cirucit drawing that you might find useful. I can't post it on the forum but if you want it, send me an e-mail at tompreaney@gmail.com

Hi Bjore,
The reason you may not have 1000 pto could be if your tractor is equipped with a forward/reverse shuttle lever.
Regards
Mike

Hi! Back at work after a weekend at the farm and thanks for the many useful comments!

The method of adjusting the pressure valve with washers was described in the workshop manual and should be performed together with measurements of the trip level of the valve (14-14.5 MPa). I am not going to try this at home, so to speak. I will send the valve to a friend, who is working with hydraulic systems, for a check and if necessary adjustment if it seems to be down to the pump assembly in the end. I cant do this until later this spring though, since the tractor is needed for snow duty, witch it barely can manage.

Up in the Scandinavian mountains at my farm, 300 km west of Sundsvall were I work, we have at the moment over a 100 cm of snow and more to come, temp -20C. Not so fun being outside dismantling hydraulic valves and hoses.

When the hydraulic/gearbox oil (universal 10W30) is cold and therefore thicker the loader seems to work better (a bit quicker and certainly stronger), what might this indicate?

I suppose a test of the lifting capacity at the three-point hitch will reveal if the pump assembly and the valves for the internal hydraulic circuit are OK. If the lifting arms can lift the test weight (1750 kg) in the stipulated time according to the WS manual I suppose the pump assembly is OK?

Mike, I dont quite understand what the forward/reverse shuttle lever means. Is it a separate forward/reverse gear leaver like in more modern tractor designs? Other than the lack of 540/1000 rpm PTO leaver I cant se any difference between the operators manual and the tractor when it comes to the leavers.

Again thanks for taking your time answering and discussing my questions
/Bjore

Hi,
Yes you've got it right regarding the forward/reverse shuttle. It could have a separate lever.

Regards

Mike

I didn't have 1000 rpm on my hydraulic-pump on my 1977 6748. I took away the lever right in front of the seat, and then could switch it to the right side too, and got it into 1000 rpm. There was "missing" a "trace" in the levers axel. I grinded a similar trace, and had 1000 rpm.