Hydraulic Flow Divider Guide: Types, Working Principle, and Selection Tips

A hydraulic flow divider decides whether two cylinders lift a platform evenly, whether a drill carriage advances straight, and whether the two drive motors on a mobile machine turn at the same speed. Its entire job is to take the flow delivered by one pump and split it into two or more streams at a fixed ratio. This article explains the working principle, compares the main types, gives you a concrete selection checklist, covers crossport relief valves, and walks through common problems so you can specify and maintain flow dividers with confidence. If you only remember one thing, remember this: for any application that needs synchronized or proportional movement, a correctly sized flow divider usually delivers the result at a fraction of the cost of adding a second pump.

What Is a Hydraulic Flow Divider?

A hydraulic flow divider is a flow-control device installed downstream of a pump. It accepts the full pump flow at its inlet and distributes that flow to two or more outlet circuits according to a fixed or adjustable ratio. The most common configuration delivers 50 percent to each of two outlets, but three-section and four-section units are standard, and units with unequal ratios can be built to order.

It is important to distinguish a flow divider from other components:

• A flow divider is not a throttle valve that creates a pressure drop to reduce flow. Throttle valves generate heat, and their dividing accuracy collapses as soon as the load on one branch changes.
• A flow divider is not a priority valve, which gives one circuit everything it needs and sends only excess flow to the secondary circuit.
• The opposite function, flow combining, merges two inlet flows into one outlet. Many gear-type and motor-type dividers can operate in both directions if the datasheet confirms it.

When a single pump feeds multiple actuators, unequal loads naturally cause the lightly loaded actuator to move first and fastest. A flow divider forces each branch to receive the same volume of oil over time, so the actuators move in step despite different loads. That is the reason flow dividers appear in scissor lifts, aerial work platforms, hydraulic presses, and slurry tankers. Understanding where the divider sits in the circuit is easier when you review the working principle of a hydraulic system first.

How a Flow Divider Works: The Core Principle

The gear-type flow divider is the best place to start because it is the most widely used in industrial and mobile hydraulics. Picture two fixed-displacement gear sets mounted back to back in one housing, with both gear shafts connected on a common drive shaft. Pressurized oil from the pump enters a common inlet passage and flows into both gear sections simultaneously. From there it exits through separate outlet ports. Because the gear sections are mechanically locked to the same shaft, they must rotate at exactly the same speed. Each output therefore delivers a fixed percentage of the incoming flow, set by the displacement of that gear section.

Consider a unit with two identical gear sections. The pump delivers 12 L/min to the inlet, and each outlet receives 6 L/min no matter how much the pressure at each outlet differs. If the cylinder on outlet port A encounters a heavier load, its pressure rises and it tends to resist the flow. The gear set in section A tries to slow down, and the gear set in section B tries to speed up. But the common shaft transfers torque from the less-loaded section to the more-loaded section, so both sections continue rotating at exactly the same speed. The flow division stays accurate within about 2 to 3 percent.

This is called pressure-compensated flow division, and it is the key advantage of gear-type and motor-type dividers over simple orifice splitters. The same principle allows the unit to act as a flow combiner: push oil into both outlet ports, and the internal shaft equalizes the two inlet flows into one combined outlet flow.

One important detail: if one outlet is completely blocked, the pressure in that section rises until the common shaft stalls or the system relief valve opens. That is why flow dividers are often supplied with integral crossport relief valves, covered in a dedicated section below.

Main Types of Hydraulic Flow Dividers

Not all flow dividers use gears, and the type you choose changes the accuracy, pressure capability, and cost. Here are the four main categories.

1. Gear-type (rotary) flow dividers: Positive-displacement gear sections on a common shaft. Compact, robust, accurate to about 2 to 3 percent, available with four or more sections, and tolerant of standard mineral oils. The best all-round choice for synchronized cylinder applications.
2. Motor-type flow dividers: Essentially two hydraulic motors coupled on a common shaft. They behave like a gear-type divider but are built with larger ports and heavier bearings for higher-flow, higher-pressure circuits. Accuracy is slightly lower at around 3 to 5 percent.
3. Spool-type (pressure-compensated) flow dividers: Precision lapped spools with internal compensators hold a constant division ratio. Accuracy can reach 1 to 2 percent, but spools are more sensitive to contamination and cost more.
4. Orifice or restrictor-type dividers: A fixed orifice in each branch. Simple and cheap, but division accuracy depends on the square-root relationship between pressure drop and flow. If a branch load changes by 50 bar, the flow split can shift by 10 percent or more.

Gear-Type Flow Dividers in Detail: Construction and Sizing

Since gear-type dividers dominate the market, it pays to understand their construction. Each section consists of a gear pair, a housing cavity, and a set of bushings that form the bearing and sealing surfaces. The sections are stacked and bolted together in one housing, and a common drive shaft passes through all of them. Many manufacturers also offer an optional crossport relief valve block that connects the outlet ports to each other.

Sizing considerations

• Nominal flow: Choose the divider so the inlet flow at maximum pump output is within the rated range. Operating below 30 percent of rated flow increases the influence of internal leakage and can hurt accuracy.
• Number of sections: If you need three synchronized actuators, use a three-section divider rather than splitting a two-section divider with a tee.
• Displacement ratio: Standard units are equal-section 50:50. Unequal ratios such as 60:40 or 70:30 are available for applications that need proportional rather than equal flow.
• Port size and type: BSPP threads dominate in European and Asian equipment, SAE O-ring boss in North American markets. Confirm the port size matches your hoses.
• Operating pressure: Check continuous and peak pressure ratings. A divider running at its pressure limit will wear faster.
• Fluid compatibility: Standard nitrile seals work with mineral oils; specify FKM seals for high-temperature or biodegradable fluids.
• Mounting style: Inline units can be bracket-mounted; manifold-mounted units bolt directly to a valve block, which reduces plumbing.

A typical 20 L/min gear-type divider with two equal sections weighs roughly 8 to 12 kg depending on the housing material and port size. Aluminum housings keep weight down, but cast-iron housings tolerate higher pressure spikes.

Key Specifications and a Selection Checklist

When you open a datasheet for a flow divider, you will see around six important numbers. Here is what to check first:

• Inlet flow capacity, both maximum and minimum
• Output flow per section at rated pressure
• Maximum operating pressure, continuous and intermittent
• Accuracy stated as a percentage of rated flow
• Internal leakage at stall
• Port sizes and the hydraulic connection standard

Step-by-step selection process

1. Define the maximum pump flow from your power unit.
2. Determine how many outputs you need and the required ratio.
3. Calculate the flow per outlet.
4. Note your system relief valve setting.
5. Choose a divider whose continuous pressure rating is at least 1.25 times the relief setting.
6. Select the port type used by your hoses or valve block.
7. Check whether you need crossport relief valves.
8. Verify the oil viscosity range matches your fluid and ambient temperature.

A common mistake is choosing a divider by inlet port size alone. The inlet port tells you nothing about the displacement ratio or the accuracy class. Always work from the hydraulic data, not from the thread size.

Common Applications in Industrial and Mobile Hydraulics

Flow dividers appear wherever a single pump must serve multiple paths reliably. Their most familiar role is keeping two or more actuators in step.

Synchronized lifting platforms and scissor lifts

A typical 4-post lifting platform uses two cylinders at opposite corners. Without a divider, the heavier side always rises first and the platform tilts. A gear-type divider with two equal sections ensures both cylinders receive the same flow, keeping the platform level within the cylinder leakage tolerance. If you are building this kind of application, the power unit choice is as important as the divider itself. A lifting platform powered by a dedicated AC power unit with a manifold-mounted flow divider is usually the simplest construction.

Mobile equipment

Tailboard lifts, dump trailer hoists, and aerial work baskets use dividers to share pump flow between steering and lifting circuits. DC-powered power units on truck tailboards frequently work with a divider because vehicle battery power limits pump size, but the tailboard must still rise evenly on both sides.

Machine tools

Clamping systems with multiple hydraulic clamps close each clamp at the same rate when fed through a flow divider, reducing vibration and work-piece distortion.

Lubrication systems

Gear-type dividers split a small pump flow to multiple lube points, and the compact construction handles high outlet resistance.

Conveyor and belt drives

In systems where two hydraulic motors drive one belt, a flow divider roughly synchronizes the motors until mechanical belt tension balances the speeds.

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Crossport Relief Valves and Pressure Considerations

A gear-type flow divider behaves as a positive-displacement machine. If one outlet is blocked or its actuator stalls, the gear section in that branch continues to receive flow until the pressure rises to the pump relief setting or the divider fails. Crossport relief valves connect the two outlet passages to one another. When the pressure in one branch exceeds the valve setting, the valve opens and allows oil to pass from that branch into the neighboring branch, so the stalled section cannot overpressurize.

The relief valves also protect against pressure intensification. If a suspended load pushes a cylinder faster than the divider feeds it, that cylinder port pressure drops and the opposite port pressure rises. A crossport relief limits the spike before it damages hoses or seals.

Setting the crossport relief

Typical settings range from 20 to 50 bar above the maximum working pressure of the corresponding branch. Never set it above the pressure rating of the actuator hose or cylinder seal.

For circuits that use a flow divider together with a power unit relief valve, keep the power unit relief setting above the crossport setting. Otherwise the crossport valve never opens and offers no protection. Some divider manufacturers build relief valves into the divider body; others offer a separate sandwich plate. For a power-unit engineer, a divider with integral crossport relief is usually the easiest to plumb. When you assemble the valve stack for a mobile power unit, a compatible power-unit relief valve on the main manifold protects the whole circuit at the pump output.

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Matching the Flow Divider to Your Hydraulic Power Unit

The flow divider does not create energy; it distributes the energy that the power unit supplies. So the first step in sizing is to look at the pump output of your power unit.

Example: a DC power unit with a gear pump delivers 8 L/min at 150 bar. A two-section equal flow divider sends 4 L/min to each branch. Each cylinder with a 40 mm bore and 800 mm stroke moves at roughly 0.04 m/s, ignoring cylinder friction losses. The pressure needed at each branch depends on the load, but the power unit relief valve determines the maximum pressure the divider will see.

Rules for matching a divider to a power unit

• The sum of the divider outlet flows must equal the pump flow. Many engineers forget that internal leakage in the divider means the outlets deliver slightly less than the inlet; the difference is the divider internal leakage.
• The maximum pressure of the power unit relief valve must not exceed the continuous pressure rating of the divider.
• The flow divider adds a pressure drop of roughly 5 to 15 bar depending on flow and oil viscosity, so verify that the pump can still produce the required flow at the reduced pressure margin.

For mobile applications, DC series power units are frequently paired with two-section dividers to run a pair of tailboard cylinders. A DC double-acting power unit with a built-in valve scheme can be combined with an external flow divider when two cylinders have to remain synchronized through both the extend and retract strokes. Note that a flow divider divides flow in one direction only unless it has a combiner function; check the datasheet for bidirectional capability.

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Common Problems, Troubleshooting, and Maintenance

Flow dividers are simple and durable, but they still fail when oil is contaminated, pressures are set too high, or the unit is oversized. The table below lists the issues you are most likely to see in a workshop or on a machine site.

Maintenance best practices

• Keep oil cleanliness to ISO 4406 Class 20/18/15 or better. Gear-type dividers are as sensitive as gear pumps to particle contamination.
• Change oil on schedule to prevent viscosity increase and sludge buildup.
• Check the torque of mounting bolts; vibration loosens them over time.
• Listen for unusual noise; it often signals cavitation or aeration.
• Monitor system pressure; a gradual pressure rise usually indicates wear bypass in the pump rather than in the divider.

Frequently Asked Questions About Hydraulic Flow Dividers

What is the difference between a flow divider and a flow combiner?

A flow divider takes one inlet flow and splits it into multiple outlet flows. A flow combiner takes multiple inlet flows and merges them into one outlet flow. Many gear-type and motor-type dividers operate both ways, but you must check the datasheet because some models are directional.

Can I use a flow divider to synchronize cylinders of different bore sizes?

Not with an equal-section divider. Different bore sizes consume different volumes per millimeter of stroke, so they need different flows. Use a divider with unequal section ratios or a closed-loop proportional valve in that case. For identical cylinders, an equal-section divider works well.

How accurate is a gear-type flow divider?

For a new unit at rated flow and pressure, division accuracy is typically within plus or minus 2 to 3 percent. This value degrades as the gear sections wear and internal leakage increases, especially when operating at low flow.

What does a crossport relief valve do?

It connects the two outlet ports through a set of relief valves. When pressure in one branch exceeds its set point, the valve opens and dumps oil to the other branch, preventing pressure spikes during load overrun or actuator stall.

How do I size a flow divider?

Start with the pump flow and the number of branches. The inlet flow must match the rated flow of the divider. The total of outlet flows equals the inlet flow minus internal leakage. Check the pressure rating against your relief valve setting, choose the port standard, and decide if you need crossport relief and unequal ratio sections.

A hydraulic flow divider is a simple component, but it solves a problem that would otherwise require a second pump, an electronic synchronization system, or a costly feedback control loop. For synchronized movement, proportional flow sharing, and reliable operation under unbalanced loads, a gear-type divider with integral crossport relief is the most practical starting point. Spec it from the pump flow, set the reliefs correctly, keep the oil clean, and it will work quietly for years.