Pump Failure after thumb install

[racerboy832]

I had a pump failure on my mini excavator after about 30 hours of a thumb install.. It may have nothing to do with it but I want to pick a few brains. I have a spare circuit on my spool block that can my plumbed for a hammer or whatever. I only planned on using it for a thumb. I plumbed it from one port to a relief and to the closing side of the cylinder. The relief is tee d back to the tank. The retract side is just plumbed to the other side of the spool.

The reason I put the relief in line was if I grab something and curl the bucket too much is will open the relief before it will bend a thumb cylinder. Also is you squeeze the object it will relief.

Here is my question. If I grab a log and I squeeze it with the bucket and it rolls back the thumb cylinder. It will open the relief. Now it pushes the fluid out of the cylinder to the tank. Now the cylinder acts bouncy till I cycle it a few times. Does this hurt anything? Introduce air into the system or a void of fluid I had the relief set at 1500lbs and excavator runs at 3000lbs.

It may be a total fluke but figured I would ask.

 

[oldnslo]

Here is my question. If I grab a log and I squeeze it with the bucket and it rolls back the thumb cylinder. It will open the relief. Now it pushes the fluid out of the cylinder to the tank. Now the cylinder acts bouncy till I cycle it a few times. Does this hurt anything? Introduce air into the system or a void of fluid I had the relief set at 1500lbs and excavator runs at 3000lbs.

It may be a total fluke but figured I would ask.

Racer,
Since the relief is venting oil back to tank and there is no oil going into the other end of the cylinder you are drawing a vacuum and filling that end with air. And option would be to install an anti-cavitation check valve to the opposite end of you cylinder. This check would connect tank to the cylinder work port and allow the cylinder to drawn oil in while it is being forced open.

I would suspect the pump failure was just a fluke.

 

[Dirthog]

You may also be able to adjust one side of the valve for the thumb and not need another relief valve we do that with all the excavators we install thumbs on.

 

[GLyford]

You could also switch to a crossover relief valve, which connects both ends of the cylinder to each other and vents between them when the pressure is reached, rather than back to tank. That would help supply "make up" oil to the other side of the cylinder as the relief hits, and you could still have it set lower than your main system relief pressure so the bucket cylinder could push harder than the thumb.

Sometimes called a cushion valve:
1/2 NPT 3 GPM 5-15 PSI Hydraulic Cushion Valve | Prince Mfg | Brands | www.surpluscenter.com

 

[racerboy832]

I know my machine uses tank pressure to keep the flow of oil to the suction side of the pump. When the thumb reliefs back to the tank does that cause an imbalance in the tank pressure?

 

[oldnslo]

I know my machine uses tank pressure to keep the flow of oil to the suction side of the pump. When the thumb reliefs back to the tank does that cause an imbalance in the tank pressure?

No, this would not cause any significant imbalance.

 

[GLyford]

The pressure imbalance is in the supply side of the cylinder. Since it is moving without the oil pushing under pressure when the other side lets oil go, you are now creating a vacuum. Since the seals are made to keep oil in, not air out, the space fills with air. After a few cycles the air will work out again. So if you want to stop it from getting that "bouncy" feeling from the air in the system, the easiest place to get oil from is the other side of the cylinder, rather than sending all that oil to the tank. Similar idea to a regen circuit. Also, it means you no longer need a separate line to the tank from the dedicated relief valve on that circuit.

 

[racerboy832]

Does anyone have a picture of the cross over valve plumbed? I have a hard time understanding the drawings but not an actual picture.

 

[GLyford]

Go to this page, and scroll all the way to the bottom:
Ken's Bolt on Grab Hooks <<Hydraulics>>

 

[oldnslo]

You could also switch to a crossover relief valve, which connects both ends of the cylinder to each other and vents between them when the pressure is reached, rather than back to tank. That would help supply "make up" oil to the other side of the cylinder as the relief hits, and you could still have it set lower than your main system relief pressure so the bucket cylinder could push harder than the thumb.

Sometimes called a cushion valve:
1/2 NPT 3 GPM 5-15 PSI Hydraulic Cushion Valve | Prince Mfg | Brands | www.surpluscenter.com

This will not work on single rod cylinders due to the volume difference. I will try and sketch up a circuit tomorrow showing why you need the relief connected to tank