A hydraulic power unit is the self-contained package that makes the pressure everything else in your system runs on. An electric motor or engine turns a pump, the pump pulls oil out of a reservoir, and a valve block sends that pressurised oil out to your cylinders and motors. You size one by three numbers: the flow your actuators need, the pressure they need it at, and how hard the machine runs across a shift. Get those right, give the oil somewhere to cool, and the unit will usually outlive the equipment it feeds.
We build, repair and stock power units at our Concord and Barrie counters, so what follows is the conversation we have across the parts desk, written down. The parts, the maths, and the decisions that actually change the outcome.
Motor on top, pump coupled underneath, reservoir carrying the weight. Almost every unit you meet is a variation on this stack.
What does a hydraulic power unit actually do?
It converts electrical or engine power into fluid power, then stores nothing and delivers everything on demand. The motor spins the pump, the pump moves a fixed volume of oil per revolution, and pressure only builds when that oil meets resistance at the far end. A cylinder trying to lift six tonnes is resistance. An open return line is not, which is why an unloaded pump can run all day at almost no pressure and still be doing its job.
That distinction matters when you troubleshoot. Flow makes things move, pressure makes them push. A machine that moves slowly has a flow problem, and one that moves at normal speed but stalls under load has a pressure problem. Two different faults, at opposite ends of the unit.
Hydraulics earn their keep because oil barely compresses, so a small package puts out force that would need a far larger electric or pneumatic drive. The tradeoff is that everything depends on the fluid staying clean, cool and free of air. The safety rules live in ISO 4413, the international standard for hydraulic fluid power systems, and any hydraulic power unit built for a plant should follow it.
What are the main parts of a hydraulic power unit?
Five parts do the work and three more keep it alive. Here is the whole stack, what each piece is for, and the failure we see most often on each one.
| Part | What it does | What usually goes wrong |
|---|
| Motor or engine | Turns the pump. AC for fixed plant, DC for mobile, gas or diesel where there is no power. | Undersized for the real duty, so it trips on thermal overload every afternoon. |
| Pump | Moves a set volume of oil per turn. Gear pumps for simple duty, piston pumps for high pressure and variable flow. | Cavitation from a starved or blocked suction line, which sounds like gravel and eats the pump. |
| Reservoir | Holds the oil, sheds heat, lets air escape and dirt settle out. | Too small, so the oil never gets a chance to cool or de-aerate. |
| Valves and manifold | Set the pressure ceiling and route flow to each function. | A relief valve cracked open and dumping energy straight to heat. |
| Filtration | Keeps particles out of close-tolerance parts. Suction strainer, pressure filter, return filter. | Elements never changed, so the bypass opens and the filter becomes decoration. |
| Cooling | Air or water heat exchanger for units that run continuously. | Left off the build to save money, then retrofitted after the seals cook. |
| Instruments | Sight gauge, thermometer, pressure gauge, filter indicator. | No gauges at all, so nobody notices anything until it stops. |
| Plumbing | Suction, pressure and return lines, hose or hard pipe. | A suction line one size too small, which starves the pump from day one. |
If you remember one row, make it the suction line. More new units come back to us for suction trouble than anything else, and it is the cheapest thing in the build to get right. We go a size up on suction as a matter of course, and we would rather bend hard tube than run a long soft hose that can collapse under vacuum. Our hose and fitting counter builds the assemblies to length while the unit is still on the bench.
Is a hydraulic power unit the same thing as a hydraulic power pack?
Yes. Hydraulic power unit, HPU, power pack and power supply all describe the same machine, and no standard separates them. In practice the words drift by size and by trade: “power pack” tends to get used for smaller portable and mobile units, “hydraulic power unit” for the fixed industrial ones bolted to a plant floor, and the mining and construction trades use both interchangeably in the same sentence.
So do not let anyone tell you a power pack is a lesser thing. Ask for the flow, the pressure, the reservoir volume and the duty rating, and you know what you are buying whatever the quote calls it. If yours runs rough, our notes on hydraulic power pack service cover the winter start problems behind most December call-outs.
How do you size a hydraulic power unit?
Size a hydraulic power unit backwards from the actuator, in this order: pressure, then flow, then motor power, then reservoir. Each number falls out of the one before it, and jumping straight to “how many horsepower” is how people end up with a unit that is either gutless or wildly oversized.
Step one, pressure. Take the force the cylinder has to produce and divide it by the piston area. A 3 inch bore cylinder has about 7.07 square inches of area, so 14,000 pounds of push needs roughly 1,980 psi at the cap end. Add a working margin, usually 15 to 25 percent, and set the relief valve there rather than at the pump’s maximum.
Step two, flow. Take the swept volume of the cylinder and divide it by the cycle time you want. That same 3 inch bore over a 24 inch stroke displaces about 170 cubic inches, which is 0.73 US gallons. If it has to extend in five seconds, you need roughly 8.8 gallons per minute. Two cylinders that move at once need the sum, not the larger of the two.
Step three, motor power. Hydraulic horsepower is flow times pressure divided by 1,714. Our example needs 8.8 gallons per minute at 2,000 psi, so about 10.3 hydraulic horsepower. Divide by a realistic overall efficiency of 0.85 and you are at 12.1, which puts you on a 15 horsepower motor. Never round down here. A motor that is one frame size short will run hot, cycle its overload and fail early.
Step four, reservoir. The working rule we use on shop builds is two to three times pump flow per minute, so an 8.8 gallon per minute pump wants roughly 20 to 26 gallons of oil. Continuous duty, hot environments and tight enclosures all push you to the top of that range or into a cooler.
Do not buy the cheapest motor on the sheet
The motor is the part that quietly spends your money. Natural Resources Canada reports that motor-driven systems use 39 percent of all electrical energy consumed in Canada, and on the same page NRCan makes the point that matters at quoting time: the purchase price of a motor is less than 3 percent of what it costs to run that motor over its life. Everything else on the invoice is electricity.
Read that as two numbers on a bar: about 3 percent purchase, about 97 percent electricity, over the life of the system.
Lifetime cost of a motor-driven system (Natural Resources Canada)
Purchase price of the motor, under 3%
Electricity to run it, about 97%
That is the argument for a premium efficiency motor and for sizing honestly. Oversize the motor and you pay for the extra kilowatts every hour it runs. Undersize it and you pay in downtime instead.
AC, DC or engine drive: which power source fits the job?
Pick the power source before anything else, because it constrains the pump, the reservoir shape and where the unit can physically live.
| Drive | Best for | The catch |
|---|
| AC electric | Presses, balers, conveyors, test benches, anything bolted down near three phase power. | You are tied to the panel. Confirm available amperage before you spec 20 horsepower. |
| 12V or 24V DC | Liftgates, dump bodies, vehicle mounted equipment, compact waste gear, tight installs. | Intermittent duty only. Long runs flatten the battery and cook the motor. |
| Gas or diesel | Remote sites, road work, portable rigs, anywhere with no service power. | Fuel, exhaust and noise, plus an engine to maintain on top of the hydraulics. |
The 12 volt and 24 volt units are the ones people push hardest past their limits. They are built for short bursts, so a liftgate cycling a few dozen times a day is fine and a machine wanting two minutes of continuous flow every five minutes is not. If that is your duty cycle, specify an AC or engine driven hydraulic power unit instead.
Why the reservoir is the part everyone undersizes
The tank is not a bucket. It is a heat exchanger, an air separator and a settling chamber, and it is doing all three jobs at once while the machine runs.
Oil returning from the system arrives hot and carrying entrained air, and it needs dwell time in the tank to shed both before the pump picks it up again. That is the logic behind the two to three times flow rule: at 8.8 gallons per minute into a 25 gallon tank, any given litre gets roughly three minutes to sit still, cool against the walls and let bubbles rise. Halve the tank and you halve that recovery time.
Heat kills hydraulic oil and the seals around it. Once a unit is running above roughly 60 degrees Celsius we start pushing customers toward a cooler, because the damage shows up soon after as darkened fluid and weeping rod seals. Baffles between the return and suction zones, a return line discharging below the oil level and a suction pickup well above the tank floor all cost almost nothing while the hydraulic power unit is being built, and none of them can be added later without cutting the tank open.
What goes wrong with hydraulic power units?
Most hydraulic power unit failures trace back to four things: heat, dirt, air and the wrong fluid for the season. None of them arrive without warning, which is the useful part.
- Running hot. Usually an undersized reservoir, a relief valve set too low, or a cooler that has been blocked solid with dust. Check the oil temperature at the end of a shift, not at start-up.
- Contamination. Dirt is abrasive and it goes straight for pump clearances and valve spools. Change filter elements on a schedule rather than when the indicator pops, and keep the fill port capped.
- Cavitation and aeration. A rattling pump means it is trying to draw oil it cannot get, or it is pulling air past a suction fitting. Both destroy a pump quickly. Our guide to hydraulic system troubleshooting walks through telling the two apart.
- Wrong viscosity in winter. An Ontario yard swings from summer heat to hard frost, and oil that flows nicely in July can be treacle on a January morning. The pump is starved before anyone touches a lever.
When is a custom hydraulic power unit worth building?
Go custom when a catalogue unit forces you to compromise on something you cannot change: the footprint, the duty cycle, the voltage, the number of functions, or where the ports have to face. Buy off the shelf when your requirement is ordinary and the catalogue already covers it, because a standard unit ships faster and costs less.
In practice the four reasons we end up building a custom power unit instead of pulling one from stock are these:
- The space is fixed and awkward. A tank has to be L-shaped, or lie flat under a deck, or clear a beam that is not moving.
- The duty is harder than the catalogue assumes. Continuous running, high ambient temperature, or a cycle rate that needs a cooler and a bigger tank than any stock unit carries.
- Several functions run from one unit. Multiple circuits at different pressures means a purpose-built manifold rather than a stack of adapters.
- It has to drop into an existing machine. Port positions, mounting centres and pipe runs all have to match what is already there.
That last one is where the fabrication side earns its place. We cut and thread black pipe in Schedule 40 and 80 and bend tube for tight routing, so the hydraulic power unit lands on the existing bolt pattern instead of needing a week of on-site adaptation. We also build the 12 volt and 24 volt mobility units for liftgates, vehicle mounted equipment and compact waste handling gear, where weight and duty cycle drive the design rather than tank volume.
We build most often for mining and aggregates, construction and heavy equipment, manufacturing and automation, and waste management. Different machines, same four questions: how much flow, at what pressure, how often, and where does the heat go. The wider range is on our hydraulic systems page.
How long should a hydraulic power unit last?
Longer than the machine it feeds, if the oil stays clean and cool. The frame, tank and motor are simple, robust things. What wears is the pump, the seals and the valve spools, and each wears at a rate set by fluid condition and temperature rather than by the calendar.
So service life is a maintenance outcome, not a specification. A hydraulic power unit running clean oil at a sensible temperature will go through several pumps and never need replacing as a unit. One running hot on dirty oil chews through pumps often enough that the pump repair bills start to look like the price of a new build. Fluid analysis once or twice a year is cheap, and it tells you which of the two you own.
Hydraulic power unit questions we get at the counter
What is a hydraulic power unit in simple terms?
It is a motor, a pump and an oil tank packaged together to create hydraulic pressure on demand. The motor spins the pump, the pump draws oil from the tank and pushes it out under pressure, and valves decide where that pressure goes. Everything else on the unit exists to keep the oil clean, cool and free of air.
What size reservoir does a hydraulic power unit need?
The working rule is two to three times the pump’s flow per minute, so a 10 gallon per minute pump wants a 20 to 30 gallon tank. Go to the top of that range for continuous duty, hot rooms or enclosed installations. If you cannot fit the volume, add a cooler instead, because the tank is doing thermal work you have to replace somehow.
How much horsepower does my unit need?
Multiply flow in gallons per minute by pressure in psi, divide by 1,714, then divide again by about 0.85 for real world efficiency. Ten gallons per minute at 2,000 psi works out near 14 horsepower, so you would specify a 15 horsepower motor. Round up to the next standard frame size, never down.
Can I run a 12V power unit continuously?
No. DC units are designed for intermittent duty, typically short cycles with cooling time in between, and a continuous demand will overheat the motor and flatten the battery. If your application needs sustained flow, move to an AC or engine driven unit and size the reservoir accordingly.
Do you repair power units or only sell them?
Both. We service and rebuild existing units, supply replacement pumps, motors, valves and filtration, and build custom units from scratch when a catalogue model will not fit. Bring the unit or the nameplate details to either counter and we can work from that.
If you are specifying a hydraulic power unit and want a second opinion on the numbers before you order, bring us the cylinder size, the cycle time and the pressure you need, and we will size it with you. Call the Concord counter on (905) 760-1711 or the Barrie shop on (249) 315-4673, or send us the details and we will come back with a build sheet.
