Many maintenance teams grade the health of their hydraulic system by the color of the oil in the sight glass. The problem is that oxidation is a chemical reaction that advances silently: by the time the color gives away the damage, varnish has been forming for months.
These are the five early warnings we check in our audits:
1. The acid number (TAN) starts to climb
TAN (Total Acid Number) measures the concentration of acidic compounds in the oil. New hydraulic oil typically reports values near 0.15 or 0.20 mg KOH/g. The exact figure matters less than the trend: when TAN breaks away from its historical baseline and accelerates, degradation is already underway. The acids attack the bronze and steel in pumps and actuators, and they are the raw material of varnish.
2. Persistent foam and entrained air
Oxidized oil loses the effectiveness of its antifoam additives. A thin layer of foam that never clears from the sight glass, or oil that takes minutes to release bubbles after shutdown, indicates micro-aeration. That compressed air implodes inside the pump at points of extreme temperature that locally burn the oil and accelerate oxidation even further: a vicious cycle.
3. Acrid smell when opening the breather
An acidic or sharp smell in the reservoir is a sign of advanced thermal degradation. It is a crude but immediate test: healthy oil smells like oil.
4. Slow actuators or loss of repeatability
Cycles that take fractions of a second longer, presses that lose precision, arms that hesitate: before blaming the electronics, suspect varnish on the servo valve spools. The resinous film increases internal friction and forces the solenoids to work harder than designed.
5. Dark color and sludge in the filters
A dark tea color and soft deposits on the filter elements are the final signal: polymerization is advanced and the oil no longer protects as it should. At this stage, changing the oil without cleaning the system only buys time: the varnish bonded to the surfaces will contaminate the fresh charge within weeks.
What to do: attack the cause without capital investment
The traditional answer, replacing thousands of liters of oil or buying filtration equipment, turns a chemical problem into a capital expense. FLOWTECH's FMaaS model solves it as a service:
The oil gives you warning. The difference between a routine adjustment and a catastrophic failure lies in listening to those signals in time.