Backhoe Valve Questions

   / Backhoe Valve Questions #1  

hbaird

Silver Member
Joined
May 16, 2005
Messages
194
Location
Deadwood Falls Louisiana
Tractor
Jinma JM200LE
A prince SV sectional control valve catalog page from surplus direct has both serial and parallel spool options. It says I need serial spools for movement of two or more rams at the same time. The serial spools will split the total pressure available between them. Is this the best way to control a Backhoe? The cost is higher for the serial spools but not enough to hurt. I want to be able to move the bucket, boom extension, and boom at the same time and get the max force for digging. All three cylinders will be 2in bore. Please help me with this.
Thanks,
Harold
 
   / Backhoe Valve Questions #2  
Series is the way to go for a BH circuit. Also get the float detent option on the Boom section.
 

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   / Backhoe Valve Questions
  • Thread Starter
#3  
To help me better understand, could you please explain the advantage of serial. Parallel gives flow to all the circuits when a valve is moved also. I think I know what float does but don't follow how to use it on a Boom. Any enlightenment would be appreciated.
Harold
 
   / Backhoe Valve Questions #4  
HBaird,

Advantages or no, it's the tractor's installed system that dictates whether additional hydraulic elements may be serial or in parrallel, as I understand it so far. While there are installed systems that use parrellel circuits or serio-parrellel types, those are generally installed in large machines with very involved control systems. Our relatively small tractors are predominantly using serial flow systems and unless new pieces are of that type they will not work successfully or smoothly. Some attempts to do such installations could result in damage to the preexisting parts as well. A case of 'and never the twain shall mix' it seems.
 
   / Backhoe Valve Questions #5  
hbaird said:
To help me better understand, could you please explain the advantage of serial. Parallel gives flow to all the circuits when a valve is moved also. I think I know what float does but don't follow how to use it on a Boom. Any enlightenment would be appreciated.
Harold

Harold,
Did you read the description at the link provided by Shaley? It explains very clearly the advantages of the series circuit for your application.
 
   / Backhoe Valve Questions #6  
hill said:
HBaird,

Advantages or no, it's the tractor's installed system that dictates whether additional hydraulic elements may be serial or in parrallel, as I understand it so far. While there are installed systems that use parrellel circuits or serio-parrellel types, those are generally installed in large machines with very involved control systems. Our relatively small tractors are predominantly using serial flow systems and unless new pieces are of that type they will not work successfully or smoothly. Some attempts to do such installations could result in damage to the preexisting parts as well. A case of 'and never the twain shall mix' it seems.

Hill,
I think you are confusing the series parallel question for "open center" and "closed center". That is not related to the original question.
 
   / Backhoe Valve Questions #7  
Harold:

This is long winded, and probably has errors because I am doing it hurriedly, but I have some thoughts you might consider before ordering your valve.

I am personally familiar with only two backhoe hydraulic control valves: a Case 580 TLB from the past and a John Deere 9310 backhoe on my 455D crawler. The control valves for them (at least on the bucket, dipper, boom, and swing circuits) are parallel (I never noticed for the stabilizers).

I never thought much about it until your question. Series and parallel valves each have advantages and disadvantages, as Shaley and Kennyd point out. But it seems to me that parallel valves are preferred, on the whole, for a small backhoe.

In series valves the return flow from the upstream valve is directed to the downstream valves and, if a downstream valve is activated, the return flow from upstream valve is directed out a work port on the downstream valve. Suppose that in the backhoe the crowd (dipper) valve is first (upstream) and the curl (bucket) valve is second (immediately downstream of) in the valve bank. If you move the crowd valve fully to extend the crowd cylinder (pull the bucket end of the dipper back toward the tractor), all the hydraulic flow from the pump will be directed to the base end of the crowd cylinder. The return flow from the rod end of the crowd cylinder will be directed by the crowd valve downstream to the curl cylinder. If you then activate the curl valve to close the bucket, the fluid that is returning from the crowd cylinder will be directed to the base end of the curl cylinder. That means the movements of the two cylinders are “locked” together: if the crowd cylinder stalls because you are digging too deep, no fluid will flow into the crowd cylinder and, therefore no fluid will flow out of the crowd cylinder and into the curl cylinder---so the curl cylinder stops too (note that this assumes that both valves are full activated). My backhoe does not operate that way and I would not want it to. In many instances, I will want to keep full pressure on the crowd while I am still curling the bucket so that the bucket will be pulled tightly against the tractor side of the trench.

Not only does the series arrangement tie the movement of the two cylinders together, it divides the hydraulic pressure between them. Assuming for argument the same size cylinders, that the return from the upstream cylinder is going into the corresponding end of the downstream cylinder (ie, base into base, rod into rod), and equal mechanical leverage, the force will be divided equally between the crowd and curl functions when one stalls.

The only advantage I see in this case is that the curl and crowd functions are faster (but not stronger) because each receives the full volume of the pump. That is, in my example above, if 20 gpm is going into the crowd cylinder, then 20 gpm is also going into the curl cylinder. Thus, if there is no load present, the speed of a function (crowd or curl) is not affected by whether or not the other function is activated. If the crowd takes 4 seconds to move from full out to full in, it will still take only 4 seconds if the curl is activated at the same time, because the same 20 gpm flows through the crowd in each case. When the curl is not activated the crowd 20 gpm return (ignore the rod effect) flows to the tank. When the curl is activated the crowd 20 gpm return flows to the curl cylinder, but the crowd cylinder receives 20 gpm in each case.

The cost of this “full speed” on both cylinders is that the potential force at each cylinder is less that the full force of the pump. In my example above each cylinder has only half the force it would have if it were acting separately.

A parallel arrangement, in contrast, shares the flow, but not the pressure. In a parallel valve, the activation of any spool (assume full activation for simplicity) directs the flow from the pump to all valves. Of course, only the valves that are activated will in turn direct the flow to the cylinder. Each valve has the potential to direct flow directly from the pump to its cylinder at the same time as the other valves. If a valve is activated, the return flow from its cylinder is directed to the tank, rather than to a downstream valve as in the case of a series arrangement. If two valves in a parallel arrangement are fully activated, then the pump flow can travel into both cylinders. Assuming the same size hoses, restrictions, etc. if neither cylinder is under load, the pump flow will be divided equally. In my example, each cylinder would receive 10 gpm. However, if one cylinder stalls (because it strikes an obstruction) while the other cylinder remains unloaded, all the flow will go to the unloaded cylinder (because there is no resistance to the flow) until it travels fully and then stalls against the stops; at that point no more fluid will flow to the second cylinder, pressure will begin to build up, and the increased pressure will increase the force on the first cylinder that stalled on the obstruction.

Consider again my first example of the curl and crowd cylinders activated at the same time. If the valves are parallel, when the crowd (dipper) stalls, the curl keeps closing the bucket because the full pump pressure is available to each cylinder, and the fact that one cylinder stalls (and therefore has no return flow to go to the other cylinder) does not stop the other cylinder as in the series arrangement. I may just be an inept operator, but a large percentage of my operating time on the backhoes is spent curling the bucket while crowding the bucket toward the tractor and, unless I am extremely lucky the dipper often stalls because I take too big a bite. But the bucket will usually still curl and load. Then when the bucket curls fully, I raise the boom to take the bucket out of the cut.

Just as in the case of the series arrangement, there is a trade off. Since the pump flow is divided among all valves activated in a parallel circuit, when two valves are activated each cylinder gets only half the flow (in my simple example of two unloaded cylinders). In the 20gpm example, each cylinder gets only 10 gpm and therefore moves only half as fast as it would in the series arrangement where each cylinder receives 20gpm. But it receives full pressure.

So, it seems to me that the best tradeoff on a backhoe is in favor of the parallel arrangement. The cylinders move slower because the pump flow is divided among the cylinders rather than flowing fully through each cylinder. However, each cylinder can apply full force to its function at stall rather than one-half force in the series arrangement. And if one cylinder stalls, the other cylinder(s) keep moving because they are not dependent on return from the upstream cylinder for power.

If you have a front end loader, try this. Dump the bucket fully and then drop the loader to the ground. Run the tractor at idle. Now pull the joystick back fully to raise the booms. After the booms raise a foot or two, keep the stick back and pull it full left to curl the bucket. If your loader valve is parallel like mine, the booms will continue to rise, but much slower, because the pumps flow is now divided between the boom and bucket circuits. Once either the bucket or boom reaches the end of its movement, the other function will speed up because it is now receiving full flow.

If the loader valves were in series, the rate of movement would not slow when both valves were activated (because full pump flow goes through each circuit), and both movements would stop when one of them reached the end of its movement (because then there would be no exhaust flow to operate the downstream cylinder or if the downstream cylinder reached stops first, because the return from the upstream cylinder would have no place to go.

Before you choose the valve, perhaps you could try someone else’s backhoe to determine what kind of valve it has. That would give you the best basis for deciding what kind to get.
 
   / Backhoe Valve Questions
  • Thread Starter
#8  
Farmerford, thank you for the lucid and comprehensive analysis of my questions. You gave me an understanding of why you supported your point of view. I agree that parallel is the best solution for me. I did look at several control systems and found series was only supported for two cases. the first when flow rate is very limited, and when two cylinders are working in opposition. The why is sometimes missing in the replies and I am always suspect of answers given to complex issues with no justification. I am a retired engineer and trained to doubt unsubstantiated technical statements.
Thanks to all repliers,
Harold
 
   / Backhoe Valve Questions #9  
Harold:

Glad we could help.

Shaley's recommendation for a float on the boom valve is a good one. When you finish a dig and just want to scrape the loose soil off the bottom rather than to dig deeper into the undisturbed soil, you can put the boom in float so that it puts no down pressure (other than weight) on the bucket. Then the bucket will usually just scrape along the hard surface as you crowd, picking up the loose soil, but not cutting into the hard soil underneath.

I am an ex-farm boy and engineer too, by training, but only practiced a few years before changing professions. I started as a EE, but LaPlace transforms and thermodynamics in the same quarter in 1964 convinced me that IE was the place for me.

Post some pictures of your projects when you can.
 
   / Backhoe Valve Questions #10  
kennyd said:
Hill,
I think you are confusing the series parallel question for "open center" and "closed center". That is not related to the original question.

Yep, seems so. It's all larnin'
 
 
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