BH77 Hydraulic Thumb options

[SlickShift]

Greetings all,

I now have a BH77 backhoe with the factory mechanical thumb. Like many on here, I'd like to convert it to a hydraulic thumb. I have read all of the posts I can find on this topic. I understand that many people use a solenoid & diverter valve to electrically re-route hydraulic pressure to actuate the thumb cylinder. I would prefer not to do that. Looking at the service manual (and the BH92 Hydraulic Thumb Kit details), it seems as though the easiest way to add hydraulic thumb control would be to add another Kubota valve to the control stack, just like the BH92 kit does. I looked, and unfortunately Kubota charges over $2000 for just ONE of the valves in the stack after you buy the pressure relief cartridges too. Or you can buy the whole stack for over $4 grand. But that's approaching what I paid for the entire backhoe.

My next thought is to tee into the P & T ports feeding the control valve body. (Should be the two lines shown in the left of the picture.) Then I can add a foot pedal (akin to the Walvoil SVM502-01B2) on the right floorboard of the backhoe. Then of course run the lines from the foot valve up the boom and to the thumb cylinder.

As others have mentioned, a pressure relief crossover could be easily added in this circuit to prevent blowing out the thumb cylinder.

The advantages are multiple:
1. With the pedal, the thumb should work at the same time as all the other controls, though the more you actuate, the slower they will be.
2. The only connections you'd have to make when removing and installing the backhoe are the same two hydraulic lines you always have to.
3. You don't have to tie in to your tractor's electrical system at all.

Has anyone tried anything like this? Can anyone think of any reason why this wouldn't work? Anyone who has installed a thumb cylinder, which one (brand & part number) did you use? Are you happy with it?

I'm planning to do this sometime this spring. If it goes well, I think the same approach is valid for adding the 3rd function on the front end loader (on the left side). Only in that case, I will have to add two more hydraulic couplings on that side in order to remove the loader.

 

[TheOldHokie]

Greetings all,

I now have a BH77 backhoe with the factory mechanical thumb. Like many on here, I'd like to convert it to a hydraulic thumb. I have read all of the posts I can find on this topic. I understand that many people use a solenoid & diverter valve to electrically re-route hydraulic pressure to actuate the thumb cylinder. I would prefer not to do that. Looking at the service manual (and the BH92 Hydraulic Thumb Kit details), it seems as though the easiest way to add hydraulic thumb control would be to add another Kubota valve to the control stack, just like the BH92 kit does. I looked, and unfortunately Kubota charges over $2000 for just ONE of the valves in the stack after you buy the pressure relief cartridges too. Or you can buy the whole stack for over $4 grand. But that's approaching what I paid for the entire backhoe.

My next thought is to tee into the P & T ports feeding the control valve body. (Should be the two lines shown in the left of the picture.) Then I can add a foot pedal (akin to the Walvoil SVM502-01B2) on the right floorboard of the backhoe. Then of course run the lines from the foot valve up the boom and to the thumb cylinder.

As others have mentioned, a pressure relief crossover could be easily added in this circuit to prevent blowing out the thumb cylinder.

The advantages are multiple:
1. With the pedal, the thumb should work at the same time as all the other controls, though the more you actuate, the slower they will be.
2. The only connections you'd have to make when removing and installing the backhoe are the same two hydraulic lines you always have to.
3. You don't have to tie in to your tractor's electrical system at all.

Has anyone tried anything like this? Can anyone think of any reason why this wouldn't work? Anyone who has installed a thumb cylinder, which one (brand & part number) did you use? Are you happy with it?

I'm planning to do this sometime this spring. If it goes well, I think the same approach is valid for adding the 3rd function on the front end loader (on the left side). Only in that case, I will have to add two more hydraulic couplings on that side in order to remove the loader.

Thats basically what I have suggested in several other threads.

To get the operation you want the valve for the thumb has to be a series type directional control valve and needs to br placed ahead of the backhoe valve.

Its almost exactly like a standard loader third function

Dan

 

[SlickShift]

Thanks Dan,
If you do it in series, you have to have a valve that will flow through back to tank even in the neutral position. If you have a valve that deadheads in the neutral position, then you'd need to plumb the valve in parallel (which looks like the easier of the two methods in that space). Correct?

 

[TheOldHokie]

Thanks Dan,
If you do it in series, you have to have a valve that will flow through back to tank even in the neutral position. If you have a valve that deadheads in the neutral position, then you'd need to plumb the valve in parallel (which looks like the easier of the two methods in that space). Correct?

You are a little confused about series and parallel.

A parallel valve puts all active cylinders in parallel. In that arrangement the cylinder with least load moves and stalls the cylinders with lesser loads. The only way to add a valve in paralllel to your backhoe is to add a parallel section to the the backhoe valve stack.

A series valve feeds the downstream cylinder(s) with the exhaust flow from the cylinder it is controlling. The cylinder controlled by a series spool cannot stall the downstream cylinders but the cylinder loads are additive.

Both of these type valves are open center. When a spool is shifted the center closes. The difference is the parallel spool sends exhaust oil to tank. A series spool sends it to the neutral port.

Most monoblock valves with power beyond are parallel and will block all downstream devices when they are active. The industrial solenoid type valves used for loader third function work like a series valve. They do not have a power beyond port and exhaust ool is sent to the tank outlet. They do not block down stream valves or actuators but they run in series with them and the actuator loads are additive.

Dan