One of our members wrote me with this question:
“On Cat diggers and some other makes they have two hoses off the pumps that are plugged. I call them hoses to nowhere. Some mechanics have told me they are there to reduce spike pressures. And others have said they are to make the pumps run quieter. Can you please tell me which one it is, or are they there for other reasons?”
I can’t confess to knowing the historical reasons for their presence on hydraulic excavators, but these ‘hoses to nowhere’ are in effect, an accumulator. Which means to some extent, they will perform both the above mentioned functions: smooth pressure ripple to make the pumps run quieter and take the top off pressure peaks by absorbing extraneous oil volume. And I suspect it’s the latter function that these hoses are most effective at, and therefore the primary reason for their presence.
When the excavator is working and a cylinder stalls out or reaches the end of its stroke, this hose-made accumulator will absorb some of the 40 or so milliseconds of pump flow that has nowhere else to go while the pump’s control is reacting and reducing its displacement to zero, and/or the direct-acting, spike relief valve in the system is opening. Absorption of this excess flow reduces pressure overshoot and shock.
In fact, an accumulator is a better solution for controlling pressure transients than a direct-acting relief valve. But the reality is, it’s rarely employed because installing a gas-loaded accumulator solely for pressure spike minimization is many times the cost of a relief valve.
The crude, accumulator-effect of a plugged length of hose is therefore a cheaper compromise to this ideal. And certainly better than subjecting the hydraulic system to pressure shocks, something which can be a very costly mistake. And to discover 6 other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.