Backhoe thumb retrofit

[bobsphotos]

I switched my Woods BH-80X manual thumb to a hydraulic thumb which runs off my 3rd SCV remotes. It works awesome but I do have one question for the guru's. When I swing the thumb out about 1/2 way and then curl the bucket to meet it, I can move the bucket backwards if I swing the thumb towards it. If I then release the thumb pressure the bucket bounces forward. My concern is that I am not sure what is happening from a hydraulic standpoint. I think that the thumb has more mechanical advantage. Any thoughts?

 

[AKKAMAAN]

I switched my Woods BH-80X manual thumb to a hydraulic thumb which runs off my 3rd SCV remotes. It works awesome but I do have one question for the guru's. When I swing the thumb out about 1/2 way and then curl the bucket to meet it, I can move the bucket backwards if I swing the thumb towards it. If I then release the thumb pressure the bucket bounces forward. My concern is that I am not sure what is happening from a hydraulic standpoint. I think that the thumb has more mechanical advantage. Any thoughts?

If the force from thumb can move the bucket so the bucket cylinder get pushed in (retract), there is a hydraulic problem.....If not the movement in bucket comes from excessive play in the linkage between the cylinder and the bucket

You need to tell how much you can move the bucket with the thumb....

There is a natural play in any linkage. After some hundreds of hours of use, this play increases (wear prevention by using the grease gun every 8 hrs of digging) an will show some considerable movement on the edge of bucket...there are 1 connection pin on the capped side of cylinder and 3 connection pins on the rod side of cylinder....manually check the play in these connection pins...together they will add up to a considerable free play

 

[bobsphotos]

Thanks for the info. The backhoe only has about 50 hrs on it so it shouldn't have much linkage play but I will take a much closer look at it tonight. I also thought that I might put a piece of masking tape around the rod 9 about 2 inches out and try to get a sense of actual rod movement back into the cylinder.

 

[J_J]

You could add a relief valve to the thumb cyl and set the pressure to whatever to suit your purpose. If the cyl are the same, as in tons of force, then the one with the best mechanical advantage will overpower the other, and perhaps break something.

There might be air in that cylinder or the valve is leaking. Does the cyl extend back out when you let off the thumb?

 

[bobsphotos]

When I back of the thumb, the bucket snaps forward.

 

[bobsphotos]

Could I add an inline relief valve?

 

[J_J]

https://www.surpluscenter.com/item.asp?item=9-6135-50-H&catname=hydraulic

 

[bobsphotos]

JJ,
If I decide to go the route of an in-line relief valve, I have to assume tht the relief port gets sent back to the tank, but is there an easy way to do that. I am using a set of rear remotes.

 

[J_J]

Sure, just tee the relief outlet to any return hose.

Does your SCV have work port relief?

 

[bobsphotos]

JJ,
If I only put a relief valve on the line that extends the cylinder, can I tee the relief port back to the line that retracts the cylinder or would that try to pressurize that line?

 

[bobsphotos]

Sure, just tee the relief outlet to any return hose.

Does your SCV have work port relief?

I have no idea.........

 

[bobsphotos]

I have done some more checking and it turns out that the issue is not a linkage play issue but I can indeed force the bucket ram to retract back inside it's cylinder by aplying pressure against the bucket with the thumb. If I do some rough measuring and figuring, it looks like the tip of the thumb has at least a ton more mechanical advantage over the edge of the bucket. That being said, is the relief valve for the bucket being opened by the thumb? My plan will be to add a relief valve to the thumb circiut and adjust it to balance the bucket but I am confused about what is happening. Does anyone want to advise me as to the easiest way to plumb in an inline relief valve? I am adding a pic to show how the thumb is plummed to my remotes.

 

[AKKAMAAN]

JJ,
If I decide to go the route of an in-line relief valve, I have to assume tht the relief port gets sent back to the tank, but is there an easy way to do that. I am using a set of rear remotes.

You need to fill the void on opposite side of piston with oil, when relief valve drains....so you need an anti cavitation valve together with the relief valve...
I actually think it should be a relief valve on rod side of bucket cylinder too....
so a crossover relief would be the way to go if there isn't any RV's there now...

btw..

 

[kennyd]

You can/should plumb in a cross-over relief valve aka "cushion" valve on the thumb, that way it can't overpower the bucket. Like this one: https://www.surpluscenter.com/item.asp?item=9-4019-50-H&catname=hydraulic

 

[bobsphotos]

Kennyd,
Could you draw up a simple plumbing schematic for the cushion valve.
Bob

 

[bobsphotos]

Kennyd,
I just looked at the drawing on the Surplus Center Link. I assume you just break both lines at the cushion valve and adjust accordingly. It must dump the overload back to the other side of the cylinder.
Bob

 

[J_J]

Another option is to move the thumb cyl mounting pin holes closer to the pivot point, decreasing the push force. Just bore some new holes about half the distance of where the pins are now. My thumb has three sets of mounting holes.

 

[J_J]

I believe you have the cylinder pins in the wrong holes. Use the holes closer to the pivot point. The pin mounting does not look correct. Wrong size hole for the pins.

If the holes are to big for the pins, add some bushings.

If you are going to use the relief valve, and the SCV is at the back, then just tee the relief output into a return line to tank.

 

[AKKAMAAN]

Kennyd,
Could you draw up a simple plumbing schematic for the cushion valve.
Bob

Here is a typical drawing for across over used on a double action single rod cylinder...The low pressure side of the RV needs to be vented to tank since the cylinder piston are displacing different volumes on each side .....(the rod is part of the displacement), this drainage is necessary to prevent cavitation in cylinder (these two check valves on the picture, are aka anti cavitation valves).
That means the cross over valve need 5 ports, the 5th for the connection to tank or return line...so this cross over valve is best plumbed close to the control valve, to eliminate the extra hose over the boom, but for the best function its better to have it close to the source of pressure, the cylinder....

Read comment on picture....

Edit:
Cross over relief valve with out drain are used on applications like motors, cylinders with double sided rods, or cross connected twin cylinders....(equal volume or piston area on each side of piston(s)...)

This is the reason way one always need to pay the extar bucks for a control valve in which there are options for work port relief valves.....no extra plumbing when problems with poor design start popping up... see picture...

 

[kennyd]

Kennyd,
I just looked at the drawing on the Surplus Center Link. I assume you just break both lines at the cushion valve and adjust accordingly. It must dump the overload back to the other side of the cylinder.
Bob

That is 100% correct, it only has 4 ports. The two lines from the cylinder go through the valve. This is the same valve that's installed on snowplows to allow the blade to trip left or right when a solid object is encountered. Below is a picture of the one installed on my snowplow. You would adjust the relief valve so that the thumb could not move the bucket.