Backhoe B26 hydralics bypassing

[Egon]

Have you got flow/pressure curves for the pump? They may be quite useful:thumbsup:!

 

[wdchyd]

Lower? I am not saying it is not possible to have a work port RV set lower
than the system pressure, but all the hoe work port RVs I have seen are
set considerably higher than the system RVs. I often see 3000psi settings
in these "shock-relief" valves.

A normal WPR Valve is set at a lower pressure rating on a typical control valve than the main relief (at the inlet section ) to protect that circuit from overpressure from the hyd pump ......

When you mention a "Shock-relief" valve set at a higher pressure then you are talking about a "Cushion Valve" protecting the circuit from an induced load.....(pressure exerted from an outside load).....ie: snowplow hitting a snowbank/curb or manhole cover........then it bypasses to protect from overpressure exerted from the external source, not from internal pressure from the hyd pump.....

Snowplow controls do this all the time.......And I've seen it on BH swing circuits because of the overrunning load when swinging at high speed and the control lever is quickly stopped or reversed......

Yes, In that case the cushion relief valve will be set higher than main PRV.....

 

[dfkrug]

A normal WPR Valve is set at a lower pressure rating on a typical control valve than the main relief (at the inlet section ) to protect that circuit from overpressure from the hyd pump ......

Protecting the circuit from overpressure from the hyd pump is what the
RV on the IN port of any valve does. Work port RVs are there to protect
the work circuit when the valve is closed, so you would want it set
higher than the other RVs, not lower.

I suppose you could design a system where you had work port RVs that
were LOWER than the system pressure, but it would be a more efficient
approach to use smaller cylinders, or a different bore-to-rod ratio on the
cyls in the work circuit.

 

[woody]

thanks for all the input, going to pick it up in the morning after working a 12 hour midnight shift. I hope all is well with my hoe.

 

[wdchyd]

Protecting the circuit from overpressure from the hyd pump is what the
RV on the IN port of any valve does. Work port RVs are there to protect
the work circuit when the valve is closed, so you would want it set
higher than the other RVs, not lower.

I suppose you could design a system where you had work port RVs that
were LOWER than the system pressure, but it would be a more efficient
approach to use smaller cylinders, or a different bore-to-rod ratio on the
cyls in the work circuit.

I suppose in tractor hydraulics this makes sense but in construction and logging equipment I see it all the time (WPR set lower).....WR ported valve sections can be different settings on the same spool or plugged to go with the main RV.....full size JD TLB's and skidders do it all the time to limit power to some BH functions......

 

[woody]

Picked my B26 up and tried it out no better than before but he did say they did not do anything but try it out. The rep. said the pressure is 2360psi and that it will pick up the backhoe. I agree if it picks up the BH then I cannot ask for more. I carried it home and tried it out if I put the bucket down a few feet from the tractor it will pick it up, but if you reach out to dig the prv will open and you have to baby it to dig. I loaded it up and carried it to a buddy's shop checked the pressure at the prv and true enough we read 2340psi close enough to the rep reading. I think I'll just have to get use to using an inferior machine than what I was using, they said it is doing all it will do. I admit if this had been my first BH I probably would not have though anything about it but compared to my other BH it sucks.

 

[J_J]

Have you checked for restrictions in the work ports of the BH? .

I am thinking that a pressure gage and flow meter on the work ports will tell you the true story,

 

[dfkrug]

I suppose in tractor hydraulics this makes sense but in construction and logging equipment I see it all the time (WPR set lower).....WR ported valve sections can be different settings on the same spool or plugged to go with the main RV.....full size JD TLB's and skidders do it all the time to limit power to some BH functions......

I imagine that a manufacturer of a hydraulic attachment will have a
certain max hyd pressure he will expect to encounter on the various
tractors that are in his target market. He will have an IN port RV to
protect from those with higher pressures. (That is why I recommend
that you match your attachment to your tractor's hyd pressure
capabilities. If the attachment has a lower RV vs the tractor RV, you are
weakening your other hyd implements, like the FEL.)

OTOH, if the manufacturer is protecting one hyd circuit of a multi-circuit
attachment with a work port RV set below the system pressure, then
he can not use shock RVs (higher pressure RVs) in that circuit. Hoes
often use shock RVs in at least the swing and boom circuits. It would
also be more efficient design to design for the same system pressure
in all circuits. This is exactly the issue you have in designing a hyd
thumb for a hoe.

The OP's BT801 hoe probably has 4 or more shock RVs.

Can you give a specific example of a hyd implement's work port RV that
is lower than the system pressure? Construction or ag implement. It
does not make sense to me.