What PSI and GPM actually mean for attachment buyers. Why you can't just match one number and ignore the other. How relief valves protect (and sometimes damage) your system. And what to check before buying any hydraulic attachment.
Most attachment spec sheets list two hydraulic numbers: a pressure requirement (PSI or bar) and a flow requirement (GPM or L/min). A lot of buyers look at one, ignore the other, or assume that if the numbers are "in the ballpark" the attachment will work fine. That's where expensive mistakes come from.
Pressure and flow are not independent variables. They work together to define the hydraulic power available to an attachment. Understanding how they interact — and what relief valves do — helps you avoid mismatches that either damage attachments or leave you with an attachment that underperforms on your machine.
Think of hydraulic pressure as the force the fluid exerts. Think of flow as the volume of fluid moving per unit of time. The analogy to electricity isn't perfect, but it's close: pressure is like voltage (the push) and flow is like current (the volume moving). Power — the actual work the attachment can do — depends on both.
Or in metric: Power (kW) = Pressure (bar) × Flow (L/min) ÷ 600
A high-pressure, low-flow system produces a different kind of power than a low-pressure, high-flow system. They can generate the same total power (same wattage), but they do different things:
This is why a hydraulic breaker and a drum mulcher have very different requirements even if they're both "high-demand" attachments. The breaker needs sustained high pressure. The mulcher needs high flow to spin its rotor at productive speed.
Most skid steers offer an auxiliary hydraulic circuit in one of two configurations: standard flow or high flow. Standard flow machines typically deliver 17–25 GPM (65–95 L/min) at auxiliary pressure. High-flow machines deliver 30–45+ GPM (114–170 L/min). The pressure spec is similar across both — typically 3,000–3,500 PSI (207–241 bar) system pressure.
An attachment that requires 28 GPM to operate at its rated performance will run sluggishly and produce less than rated output on a standard-flow machine. That's not a catastrophic failure — the attachment works, just slowly. But for a drum mulcher that needs 25–35 GPM to spin its rotor at productive speed, running on a 17 GPM machine means the rotor isn't spinning fast enough to mulch effectively. You're working, but you're working at 50–60% of rated productivity.
The standard flow vs high flow guide covers the full attachment compatibility breakdown. The short version: attachments with hydraulic motors (mulchers, brooms, snow blowers, stump grinders, cold planers) generally benefit from high flow or require it. Attachments with hydraulic cylinders only (buckets, grapples, blades, forks) are largely indifferent to flow above a minimum threshold — they don't spin, they just extend and retract.
Every skid steer hydraulic system has a main system relief valve — a pressure-limiting valve that dumps excess pressure when system pressure exceeds a set threshold. This protects the pump, the hoses, and the cylinders from overpressure damage. The main system relief is typically set by the manufacturer to 10–15% above maximum working pressure.
Many quality attachments also have their own internal relief valves — particularly in the hydraulic motor circuit or cylinder circuit. These protect the attachment's internal components from pressure spikes that the machine's main relief might not catch fast enough.
If your machine's operating pressure (the working pressure, not just the relief setting) regularly exceeds what the attachment is rated for, the attachment is being stressed beyond its design. This is most common in one scenario: high-performance skid steers from manufacturers like Bobcat, Case, and Caterpillar that run 3,300–3,600 PSI systems, paired with lower-cost aftermarket attachments rated for 3,000 PSI systems.
The mismatch might not cause immediate failure. But over time, seal life is shortened, cylinder rods experience more stress, and motor components wear faster. A cylinder rated for 3,000 PSI continuous working pressure running on a 3,400 PSI machine is seeing 13% over-spec pressure on every cycle.
Less common, but it happens with heavy-duty attachments on smaller machines. A large hydraulic breaker may require 2,500–2,800 PSI to operate at rated impact energy. If the machine's auxiliary circuit only produces 2,200 PSI under load (due to pump condition, system leakage, or a lower-pressure relief setting), the breaker delivers less than rated impact energy. It works, but it's not doing what the spec sheet says it should.
Heavy hydraulic breakers in particular are sensitive to minimum pressure — below a threshold, they don't cycle at all. If a breaker isn't cycling on your machine, insufficient pressure is one of the first things to check, along with flow rate and accumulator condition.
The most practical way to assess whether an attachment and machine are matched is to calculate actual hydraulic power available versus what the attachment needs. Using the formula above:
An attachment that requires 45 HP of hydraulic input to run at rated performance physically cannot run at rated performance on the standard-flow machine — there isn't enough hydraulic power available. This is the correct way to think about hydraulic compatibility, not just checking one number against a threshold.
When you're comparing attachments, the spec sheet will typically show:
The range is significant. An attachment requiring "25–40 GPM" will work on a 25 GPM machine — at minimum performance. It will work at peak performance on a 40 GPM high-flow machine. If your machine delivers 32 GPM, you're in the middle of the range — the attachment performs at roughly 70–75% of its maximum rated productivity. For many applications, that's fine. For a mulcher or cold planer where you're paying by the acre, it's worth knowing.
Many attachments — augers, auger/trencher combinations, some brooms, hydraulic tilting forks — require bi-directional auxiliary flow. This means the attachment needs the hydraulic circuit to flow in both directions (reversing the flow direction reverses the motor rotation or cylinder direction). Most modern skid steers with a joystick control system support bi-directional flow as standard. Older machines or some entry-level models may only support one-direction auxiliary flow.
This matters specifically for augers (you need reverse to back out of a stuck hole), two-direction brooms, and some couplers. Check your machine's auxiliary hydraulic configuration before purchasing any attachment with a bi-directional requirement. The how to read a spec sheet guide covers this and other spec interpretation questions.
This takes about five minutes with a spec sheet. It's the difference between an attachment that performs as expected and one that either underperforms or has a shortened service life because the hydraulics weren't matched properly.
Every attachment in the catalog includes hydraulic flow and pressure requirements — compare against your machine specs before buying.