Why this matters before you order
Two 20 t excavators can carry nominally similar tandem pumps and behave nothing alike at the levers. The difference is not the pump body - it is the control philosophy bolted to it, and it decides three things a buyer notices immediately: what happens at neutral, what happens when the machine runs out of flow, and how much heat the system makes while it waits.
When you order a replacement, the control variant on the nameplate is the part that must match. This page sets out the four arrangements we meet most often.
The physics behind all four
Every one of these systems meters oil through a spool opening, and every spool opening follows the same orifice law:
- Q = K · A · Δp^m - flow rises with opening area A and with the pressure difference Δp across the edge
- thin-wall or sharp-edged opening: m = 0.5, flow varies with the square root of Δp
- long narrow passage: m = 1, flow varies directly with Δp
- short tube / practical metering edge: 0.5 < m < 1
Because flow depends on Δp, a plain spool meters load as well as lever stroke. Everything that follows is an attempt to remove that dependency - which is exactly what the four control philosophies do, in four different ways.
1. Negative flow control (NFC)
Centre-open valve with a bleed to tank at neutral. The pressure in that bleed line is the pump's instruction: high bleed pressure means the operator is asking for little flow, so the pump destrokes; close the spools and the bleed pressure falls, so the pump upstrokes.
- Neutral: oil circulates through the open centre at low pressure - cool, but the pump keeps some displacement
- Saturation: the signal saturates and the functions share flow according to their openings and loads; the lightest load usually wins the argument
- Feels like: smooth, slightly soft, forgiving on a rough operator; standby flow costs a little fuel and heat
- Typical: widespread on 20-30 t Japanese and Korean machines of the 1990s and 2000s
2. Positive flow control (PFC)
The mirror image: the pump reads the pilot demand rather than a bleed pressure. Push the lever further and pilot pressure rises; the pump upstrokes in proportion. At neutral there is no bleed, so standby flow and heat are low.
- Neutral: pump nearly destroked - quiet and cool
- Saturation: same load-driven sharing problem as NFC, unless a compensator is added
- Feels like: crisper than NFC, with flow tracking lever stroke more closely
- Watch out: the pilot signal is a pressure corridor - a contaminated or leaky pilot circuit changes the pump's behaviour without the pump being at fault
3. Load sensing (LS)
Centre-closed valve with a load-signal gallery. The pump regulates to a fixed margin (typically 20-30 bar) above the highest load pressure, so flow is set by the spool opening alone - up to the point where the pump reaches its maximum displacement.
- Neutral: no flow, pumps sits at standby pressure; the coolest of the four
- Saturation: once the pump is at maximum, the highest-load function takes what it needs and the lighter functions starve - the classic "everything slows down when I crowd two functions" complaint
- Feels like: precise, repeatable, proportional; less forgiving if the margin is mis-set
- Watch out: the margin spring and the load-signal shuttle are what most often go wrong on a rebuilt unit
4. LUDV / flow sharing
Load-independent flow distribution adds a post-compensator downstream of each spool. Each function gets the same pressure drop regardless of its load, so flow is distributed in proportion to spool opening even when the pump is at its limit.
- Neutral: like LS - closed centre, low standby
- Saturation: when the pump runs out, every function slows proportionally instead of the heavy one stalling; the machine stays coordinated
- Feels like: the most predictable of the four under combined operation, which is why it dominates on machines sold on cycle time
- Watch out: the compensators are the part of the system that a "rebuilt pump" cannot fix - if the valve block is worn, a new pump will not restore flow sharing
Side by side
| Behaviour | NFC | PFC | LS | LUDV |
|---|---|---|---|---|
| Valve centre | open | closed | closed | closed |
| Signal the pump reads | bleed pressure | pilot pressure | highest load pressure | highest load pressure |
| Standby flow and heat | some | low | lowest | lowest |
| At pump saturation | load decides | load decides | heaviest load starves the rest | all functions slow together |
| What wears first | bleed orifice, spool clearance | pilot circuit | margin spring, LS shuttle | post-compensators |
What to send when you order a replacement
- The pump model with its full suffix - the control variant is in the suffix, not in the name
- A nameplate photo; the regulator housing and the number of control lines are visible there
- The machine model and, if the machine has been re-pumped before, what is fitted now
- If the complaint is "slow when I use two functions", say so - that is a valve-block symptom, and a new pump will not cure it
Send those four items and we confirm the variant against the catalogue line before dispatch - see how to verify a part number for the format each brand uses.