Hydraulic Pump Cavitation: The Real Causes (And Why Your Reservoir Tank Is Usually to Blame)
Why do hydraulic pumps cavitate even when the pump is new?
Most hydraulic pump cavitation problems start at the reservoir, not inside the pump.
When the tank outlet is undersized, the suction path is restrictive,
or oil is pulled from a turbulent zone, the pump draws air instead of solid oil.
We see this most often after reservoir swaps or pump upgrades
where the system flow increased but the tank design stayed the same.
A lot of people think the pump is the problem.
The pump starts making noise. Pressure goes down. The case drain gets hot. Someone replaces the pump, and it works for a bit, but then the same issues come back.
When I hear that kind of story, I don’t just look at the pump first. I check the tank.
I work on building hydraulic tanks for a range of machines, such as log splitters and lifts. From what I’ve seen, a lot of cavitation problems actually come from how the tank is set up, not the pump itself. On paper, the design might look okay, but in real life, oil moves around, the frame can bend, and things heat up. Suddenly, the pump is pulling in foam and vapor instead of just oil.
That’s what cavitation really looks like when you’re actually working with the equipment.
It’s not just something you read about in a textbook. This is what actually happens out in the field.
What Hydraulic Pump Cavitation Looks Like in the Field
Cavitation isn’t always easy to spot at first. Most of the time, it starts with small signs like these:
- Pump whine that wasn’t there before
- Rattling or gravel sound on startup
- Oil that looks milky or full of tiny bubbles
- Pressure that hunts under load
- Case drain temp climbing for no clear reason
We’ve had customers tell us, “It only does it when it’s hot,” or “Only when the machine is on a slope,” or “Only after running an hour.”
Those clues usually mean there’s something going on with how the oil gets to the pump, or how the tank is working when the machine is actually running.
How Reservoir Design Causes Cavitation
This is where most of the trouble begins. It’s not that people are careless, but tanks are often designed on a computer, not based on how oil really acts when the machine is working.
Suction Port Height and Pickup Tube Placement
One mistake I see a lot is when the suction port is welded too high up on the tank.
On paper, oil level looks fine. In the field:
- Machine on a hill
- Tank tilted during operation
- Oil level drops under flow
- Pickup tube starts pulling air
I’ve seen tanks where the pickup tube is just under the normal oil level. It might work fine in the shop, but once you use it for real, it starts to cavitate.
When I build a tank, I make sure to figure out where the oil level will be when the machine is running, not just when it’s sitting still. That helps me decide where to put the suction port and how long the pickup tube should be.
Suction Port Size vs Real Flow
Another problem is when people size the suction ports based on charts, instead of thinking about how the oil can get dirty, cold, and thick in real life.
Cold oil + undersized suction = vacuum at the pump inlet. That vacuum drops pressure below vapor pressure. Oil flashes. You get bubbles. They collapse in the pump. Metal gets eaten.
I’ve seen pumps get ruined even when everything is technically the right size. Sometimes the suction line is too long, has sharp bends, or the tank port is just a bit too small.
When I build custom tanks, I usually make the suction ports bigger than the minimum. It’s a lot cheaper to do that than to have to replace a pump later.
Baffles That Interfere With Flow
Baffles can help, but if they’re not designed right, they just make things worse.
We’ve opened tanks where a baffle was placed right in front of the suction area. Oil has to turn sharp inside the tank before it can reach the pickup. That creates turbulence and pressure drop inside the reserFrom the outside, the tank might look well-built, but inside, it’s actually making things harder for the pump.king the pump.
When we design baffles, we think about oil path, not just separation. Oil has to settle, but it also has to flow to the suction without restriction.
Foamy Oil and Aeration — A Hidden Cavitation Trigger
Foam is cavitation’s best friend.
Most of the time, foaming happens because the return oil comes back too close to where the pump picks it up.
Common issues we see:
- Return line splashing above oil level
- Return dumping directly toward pickup
- No diffuser tube
- High-velocity return jetting into still oil
Even if the oil is clean, it can still foam if too much air gets mixed in.
We’ve had customers swear their pump was bad. The real issue was the return port location. Oil was coming back hot and fast, whipping air into it. The pump was pulling aerated oil all day.
On ouWhen I build tanks, I usually add return diffusers inside or move the return ports so the oil has a chance to slow down and lose its air before it gets close to the suction.t’s not something you see in a catalog photo. It’s something you learn after cutting open tanks and seeing where the bubbles actually go.
Overheating Makes Cavitation Worse
Hot oil cavitates easier. Period.
As temperature goes up, oil’s vapor pressure goes up. That means it takes less vacuum at the inlet to start forming vapor bubbles.
Small tanks make this problem even worse.
I’ve seen a lot of power units with tanks that only hold enough oil to run, but not enough to keep things cool.
Thin wall tanks, tight packaging, no airflow. Oil stays hot. Cavitation shows up even if the suction layout is decent.
In fabrication, wall thickness and surface area matter. A heavier tank with more steel can actually help shed heat. Sometimes upsizing the tank does more than adding a cooler that clogs and leaks later.
Why Cavitation Leads to Hydraulic Pump Failure
This is usually the part everyone notices.
Cavitation bubbles collapse inside the pump. When they collapse, they create tiny shock loads on metal surfaces.
Over time, that causes:
- Pitting on gear or piston surfaces
- Erosion on valve plates
- Scoring
- Increased internal leakage
- Heat buildup
- Loss of efficiency
- hydraulic pump cavitation sound
By the time the pump is making enough noise to worry you, the damage has already happened.
That’s why just swapping out the pump without fixing the tank usually means you’ll have the same problem again.
Mounting and Vibration — Air Leaks You Can’t See
Another problem that’s easy to miss is when the tank cracks or gets tiny air leaks on the suction side.
We’ve repaired tanks where:
- Mounting tabs were too light
- Frame twist flexed the tank
- Weld Those leaks might not drip oil, but they let air in.
Those leaks don’t always drip oil. They suck air.
That air goes straight to the pump. Same result as cavitation, even though oil level looks fine.
When we build mounting tabs, we look at how the frame moves. Mobile equipment flexes. If you weld a rigid tank to a twisting frame, something gives. Usually a weld.
How We Design Reservoirs to Reduce Cavitation
This isn’t just something I say to sell tanks. Here’s what I actually change when cavitation keeps ruining pumps:
- Lower suction port placement
- Larger suction bungs
- Straighter internal pickup paths
- Baffle layouts that don’t choke flow
- Return diffusers to slow oil
- Thicker wall steel in high-vibration areas
- Reinforced mounting tabs
- Service access so you can inspect and clean
Doing these things might take a bit more time when building the tank, but it saves a lot of trouble and pump replacements later.
Signs Your Tank Is the Real Problem (Not the Pump)
If you see these, don’t just order another pump:
- Pump whines mostly when hot
- Foamy or milky oil
- Cavitation noise on slopes
- Repeated pump failures
- Filters clogging fast
- Oil temps creeping up over time
Those are signs that the real problem is with the tank or how the system is set up.
When a Custom Reservoir Saves More Than It Costs
We’ve had customers who were on their third pump in a year.
After redesigning the tank — moving ports, changing baffling, reinforcing mounts — the same pump runs for years.
It wasn’t the pump that changed, but the way the oil was handled.
From a buyer standpoint, it’s usually cheaper to fix oil delivery once than to keep replacing pumps and eating downtime.
Bottom Line From the Shop Floor
If your system is having cavitation problems, I wouldn’t start by looking at the pump specs.
Start with:
- Where oil enters and leaves the tank
- How oil moves inside the tank
- How hot the oil runs
- How the tank is mounted
- How the suction is actually fed
That’s usually where cavitation problems really start.