Hydraulic Steering Control Unit Displacement: Calculate Cylinder Volume, Flow, And Turns Lock To Lock

A familiar casting can be misleading. Displacement tells you how much oil is metered during one revolution of the steering wheel, so it changes lock-to-lock turns, response, flow demand, and manual effort after a pump failure. It does not tell you whether the valve section suits the machine. That comes from the center type, reaction behavior, ports, shaft, and built-in valve options carried elsewhere in the model code.

Begin with the cylinder. Work out how much oil it takes to steer left and right, then divide each figure by the intended number of wheel turns. The quotient gives you a displacement to investigate, not a part number to order. Before approving it, compare the candidate with the machine’s priority flow, neutral circuit, feedback behavior, pressure protection, port details, steering-column connection, and safety requirements.

BLINCE lists a 100 series hydraulic steering control unit with valve block for tractor and heavy-machinery applications. The public listing identifies the product family and construction but does not publish a complete displacement-by-model matrix. Buyers should therefore treat the calculations below as sizing examples and request a model-specific drawing, code, pressure/flow data, and valve-block function before ordering.

Quick Selection Table

If the supplier cannot complete this table, the quotation is not ready for technical approval.

What Displacement Means in a Hydraulic Steering Unit

Steering-unit displacement is the theoretical volume metered per steering-wheel revolution, normally stated in cubic centimeters per revolution (cc/rev or cm³/rev). A 160 cc/rev unit theoretically sends about 160 cm³ toward the selected work port for one full wheel turn. Actual behavior also reflects internal leakage, oil temperature, pressure, air, valve timing, and steering-cylinder geometry.

Displacement does not state pressure capacity. It does not prove that the unit is open center, load sensing, reaction, non-reaction, or fitted with the required valves. It does not identify port threads or the input spline. Treat it as one high-impact selection field inside a complete code.

For unit conversion:

1 in⊃3;/rev = 16.387 cc/rev

Therefore, a 9.76 in⊃3;/rev steering unit is approximately:

9.76 × 16.387 = 159.9 cc/rev

Round only for comparison. Order from the manufacturer code and drawing, not from a rounded conversion scratched onto a work order.

Step 1: Find the Cylinder Oil Volume for Each Steering Direction

The steering unit meters oil into a cylinder chamber. The first calculation is therefore chamber volume, not pump pressure. Use dimensions from the cylinder drawing when possible. A tape measurement of the tube outside diameter does not reveal the bore.

For a cylinder cap-end piston area:

A_cap (mm²) = π × bore⊃2; / 4

For the rod-side annular area:

A_rod_side (mm²) = π × (bore⊃2; − rod⊃2;) / 4

For chamber volume:

V (cm³) = area (mm²) × stroke (mm) / 1,000

Worked cylinder example

Assume one single-rod cylinder has:

• 63 mm bore;

• 36 mm rod;

• 200 mm stroke.

Cap-end area:

A_cap = π × 63⊃2; / 4 = 3,117 mm²

Rod-side area:

A_rod_side = π × (63⊃2; − 36⊃2;) / 4 = 2,099 mm²

Cap-end volume:

V_cap = 3,117 × 200 / 1,000 = 623 cm³

Rod-side volume:

V_rod_side = 2,099 × 200 / 1,000 = 420 cm³

This cylinder has unequal chamber volumes. If the steering circuit fills one chamber directly in one direction and the other chamber in the opposite direction, equal steering-unit displacement will not create equal wheel turns. Linkage geometry can add another difference. That is why a buyer should calculate left and right from the actual schematic rather than enter one vague “cylinder volume” number.

Balanced double-rod cylinders have equal effective areas on both sides when the rods are equal. Two-cylinder articulated or axle-steering layouts require a circuit-level volume calculation: identify which chambers fill and which chambers exhaust during each direction, then total the receiving volume. Do not simply double the bore-side volume because two cylinders are visible.

Step 2: Convert Cylinder Volume Into Target Turns Lock to Lock

The basic estimate is:

Turns lock to lock, N = steering cylinder volume, V / steering-unit displacement, q

Or, when choosing displacement:

Steering-unit displacement, q = cylinder volume, V / target turns, N

Suppose the correctly calculated receiving volume is 640 cm³ in each direction. The theoretical comparison is:

The table does not rank one size as universally better. Target turns depend on vehicle speed, wheelbase, tire scrub, axle load, articulation geometry, operator expectations, emergency-steering requirements, and the original equipment design. A fast response that feels useful on a slow forklift may be too sensitive for another vehicle.

Take a system that receives 640 cm³ while steering left and 560 cm³ while steering right. With a 160 cc/rev unit, the arithmetic comes to 4.0 turns in one direction and 3.5 in the other. Unequal results are not automatically a fault; they may follow from the cylinder and linkage. They are, however, a reason to recheck which chambers were included. Keep both figures on the selection sheet instead of averaging them away.

Step 3: Calculate the Steering Flow Required

A larger displacement meters more oil per wheel revolution. Pump and priority flow must support the desired steering speed.

Use Q = q × n / 1,000, where q is displacement in cc/rev and n is steering-wheel speed in rpm. For a 160 cc/rev unit, the flow request changes quickly with the driver’s hands:

Now compare a 200 cc/rev candidate. At 100 rpm it calls for 20 L/min. If the priority branch can deliver only 14 L/min, turning the wheel faster will not make the cylinder consume 20 L/min. Movement remains limited by the oil available, and part of the steering effort shifts back to the driver.

The nominal calculation is not a pump-selection guarantee. Add the supplier’s leakage/efficiency guidance and verify flow across engine speed, hot oil, other hydraulic functions, and the exact priority circuit. A pump can deliver adequate total flow while the steering branch receives too little.

Step 4: Check Steering Time Against Vehicle Behavior

Travel time provides a useful cross-check. Divide the receiving volume by the flow that actually reaches the steering circuit:

Time (s) = volume (L) × 60 / available flow (L/min)

A 0.64 L chamber takes a theoretical 2.4 seconds to fill at 16 L/min. Reduce the measured flow to 10 L/min and the same stroke takes 3.84 seconds. This is the practical distinction between what the driver requests and what the circuit can deliver. When the stopwatch and the calculation disagree, investigate relief flow, priority loss, leakage, trapped air, restrictions, and the cylinder data before reaching for a different displacement.

Step 5: Calculate Cylinder Force and Pressure—Separately

Displacement determines metered volume per wheel revolution. Cylinder force comes from pressure acting on effective area.

Force (N) = pressure (bar) × area (mm²) × 0.1

Using the 63/36 mm cylinder example at 120 bar differential pressure:

Cap-end theoretical force:

F_cap = 120 × 3,117 × 0.1 = 37,404 N = 37.4 kN

Rod-side theoretical force:

F_rod_side = 120 × 2,099 × 0.1 = 25,188 N = 25.2 kN

Real force is lower after friction and pressure losses. More importantly, steering force must be checked against tire contact, axle load, scrub radius, linkage leverage, mechanical stops, and permitted pressure. Selecting a larger-displacement steering unit does not make the cylinder stronger.

Do not raise a relief setting to correct too many wheel turns. Relief pressure and displacement solve different problems. An unjustified pressure increase can overload cylinder mounts, hoses, axle parts, and steering linkage.

Open Center, Closed Center, and Load Sensing Must Match the Pump Circuit

An open-center steering unit provides a neutral flow path from pump to tank and is typically paired with a fixed-displacement pump arrangement. A closed-center unit belongs to a different supply logic. A load-sensing unit communicates steering demand through an LS signal so a priority valve or controlled pump can provide the required pressure and flow.

These are circuit behaviors, not marketing options. The existing article on open-center, closed-center, load-sensing, reaction, and non-reaction steering units is a useful companion, but the approved replacement should still be checked against the machine schematic and original model code.

Installing a closed-center unit in a circuit expecting neutral through-flow can load the pump or force oil across relief. Installing an open-center unit where closed-center or load-sensing behavior is required can remove control authority or waste flow. A unit may bolt on and steer during a yard test while creating heat, priority problems, or unsafe behavior in real work.

Circuit details to send with an inquiry

1. Pump type and displacement, engine or motor speed range, and measured warm flow.

2. Whether a priority valve is used and how excess flow is routed.

3. P, T, L, R, LS, EF, or other port identification from the schematic.

4. Original steering-unit complete code, not only a cast number.

5. Neutral behavior and whether auxiliary functions share the pump.

6. Relief, shock-valve, suction/anti-cavitation, and check-valve requirements.

If no schematic exists, trace and label the hoses before removal. Do not infer port function from physical position.

Reaction vs Non-Reaction Steering

With reaction steering, external forces at the steered wheels can produce corresponding movement at the steering wheel when the driver is not steering. That feedback may support a designed return tendency where cylinder volumes and steering geometry are suitable.

With non-reaction steering, the unit isolates the steering wheel more strongly from wheel forces in neutral. It is often used where the steering wheel should remain in position and road feedback is not desired. “Non-reaction” does not mean the axle cannot kick, wander, or drift when the cylinder, checks, linkage, or circuit has a fault.

Do not switch between reaction and non-reaction simply to change steering feel. Confirm cylinder arrangement, original specification, vehicle behavior, applicable requirements, and manufacturer approval. A reaction unit in an unsuitable unequal-volume system can behave very differently from the unit it replaces.

Smaller vs Larger Displacement: Two Honest Pros-and-Cons Sets

Smaller-displacement unit

Advantages

• More steering-wheel turns give the operator finer metering.

• For a given hand-wheel torque, a smaller metering displacement can support higher manual steering pressure than a larger unit, subject to the exact design and losses.

• Required flow is lower at the same steering-wheel rpm.

Disadvantages

• Lock-to-lock movement takes more wheel turns.

• Rapid maneuvering may feel slow.

• Operators may spin the wheel faster, so actual requested flow can still become significant.

Larger-displacement unit

Advantages

• Fewer turns lock to lock for the same cylinder volume.

• Faster commanded cylinder movement when enough priority flow is available.

• Can suit larger chamber volumes without excessive wheel rotation.

Disadvantages

• Needs more flow at the same wheel rpm.

• Emergency manual steering generally demands more input torque for the same pressure.

• Too large a unit can make a vehicle overly responsive and leave inadequate control resolution.

The buying decision is not “largest available” or “same cc as another tractor.” It is the displacement that meets the target turns and safety behavior within the available flow and circuit architecture.

Pump Flow, Priority Flow, and the Difference Buyers Miss

Total pump delivery is not automatically steering flow. A shared mobile circuit often uses a priority valve to reserve flow for steering and send the remainder to implements. The priority setting, LS signal, spool condition, pump speed, oil viscosity, and downstream demand decide what reaches the steering unit.

Suppose a pump produces 36 L/min at working engine speed. The steering calculation requires 16 L/min at the target wheel speed, and the priority valve is designed to supply up to 18 L/min. The remaining nominal 18 L/min may serve auxiliary functions. At low idle, however, the pump might supply only 17 L/min total. Steering can consume most of it, leaving the attachment slow. That is expected system behavior unless the machine was designed otherwise.

If steering becomes heavy only while another function operates, measure pressure and flow around the priority section instead of ordering a different displacement immediately. BLINCE’s hydraulic steering troubleshooting guide organizes checks for hard steering, kickback, hot-oil drift, and shared-flow faults.

Pressure Drop, Return Back Pressure, and Heat

The useful pressure at the cylinder is lower than pump outlet pressure when oil loses pressure through the priority valve, steering unit, hoses, fittings, and return path. Tank-port pressure also matters. Excess return back pressure reduces available differential pressure and can increase effort, leakage, temperature, and seal stress.

Hydraulic power converted to heat across a restriction can be estimated by:

Power loss (kW) = pressure drop (bar) × flow (L/min) / 600

At 16 L/min and an avoidable 8 bar combined loss:

Power loss = 8 × 16 / 600 = 0.213 kW

That is about 213 W of continuous heat while the path is active. The number may look small beside engine power, but a compact reservoir and repeated maneuvering can expose it. Measure pressure at meaningful points under the same warm, loaded condition. A gauge at the pump alone cannot locate the loss.

Valve Blocks: Name Every Function

A steering control unit may be offered with an integrated valve block. The block can include relief, shock, suction/anti-cavitation, inlet check, or other functions. The words “with valve block” do not define which functions exist or their settings.

For each proposed model, request a hydraulic symbol or section drawing and list:

Settings should come from the vehicle design, not from the highest number a seller can supply. Ask whether pressure values are nominal, tolerance bands, or verified test settings.

Mechanical and Port Compatibility Checklist

Two steering units with the same displacement can still be mechanically incompatible. Approve a drawing that covers:

• mounting bolt pattern and pilot diameter;

• overall length and available clearance;

• steering-column connection, spline, thread, and shaft engagement;

• maximum permitted column load and alignment;

• port location, function, thread standard, sealing method, and adapter stack;

• valve-block envelope and service access;

• hose bend radius through the full articulation range.

P, T, L, and R labels must be mapped correctly. LS and excess-flow ports must not be capped or connected by guesswork. A thread that starts is not proof of compatibility: BSPP, metric, SAE O-ring boss, and tapered pipe threads seal differently.

The broader hydraulic fitting identification guide can help with visual screening. Final assembly still needs an approved port specification and compatible hose/fitting system.

A Worked Procurement Case: 160 cc or 200 cc?

A slow industrial vehicle uses two steering cylinders. From the schematic and drawings, the total receiving volume is calculated as 720 cm³ left and 700 cm³ right. The team wants about four turns lock to lock. The original unit label is unreadable, and the pump/priority branch can provide 17 L/min at normal working speed but only 12 L/min at idle.

Starting displacement from the larger volume is:

q = 720 / 4 = 180 cc/rev

There may be no exact 180 cc/rev candidate in the relevant family. Compare nearby choices rather than rounding blindly.

With 160 cc/rev:

• left turns = 720 / 160 = 4.50;

• right turns = 700 / 160 = 4.38;

• flow at 100 wheel rpm = 16 L/min.

With 200 cc/rev:

• left turns = 720 / 200 = 3.60;

• right turns = 700 / 200 = 3.50;

• flow at 100 wheel rpm = 20 L/min.

The 200 cc/rev option gives fewer turns but requests more than the available 17 L/min at 100 wheel rpm. At idle, even the 160 cc/rev option cannot sustain 100 wheel rpm because only 12 L/min is available; achievable speed would be approximately 12,000 / 160 = 75 rpm, before leakage allowances.

The decision now needs vehicle behavior, circuit type, manual-steering requirement, supplier performance data, and test validation. The calculation has not selected the part by itself. It has exposed the tradeoff and prevented a casual “bigger is faster” purchase.

Common Selection Mistakes

Mistake 1: Copying only the displacement

The new unit is 160 cc/rev, just like the old one, but it has the wrong center type or reaction behavior. The machine may steer and still overheat, lose priority, or feel unsafe.

Mistake 2: Calculating with tube outside diameter

Cylinder volume requires bore, rod, stroke, and the actual receiving chambers. Tube OD is not bore. A modest dimensional error can shift the selected displacement enough to change wheel turns noticeably.

Mistake 3: Ignoring unequal left/right volume

A single-rod cylinder has different cap and annular areas. Two-cylinder circuits can be balanced or unbalanced depending on plumbing and geometry. Calculate both directions.

Mistake 4: Treating total pump flow as priority flow

The steering branch receives what the priority circuit can provide under the operating condition. Test low engine speed and simultaneous implement demand.

Mistake 5: Using relief pressure to change response

Pressure controls available force; displacement and flow control volume per wheel turn and speed. Raising pressure will not reduce theoretical wheel turns.

Mistake 6: Assuming “with valve block” means protected

A block may omit a needed shock, suction, or check function, or use settings that do not match the vehicle. Approve the hydraulic symbol and values.

Mistake 7: Ordering from photos

Castings are reused across variants. Photos cannot reliably prove displacement, spool/sleeve logic, valve content, port threads, shaft, or calibrated settings.

Mistake 8: Ignoring emergency manual steering

Manual behavior changes with displacement, pressure requirement, vehicle load, and internal design. A larger unit can reduce lock-to-lock turns under power while increasing manual effort when the pump is unavailable.

Mistake 9: Validating only with the axle unloaded

A shop test with the steered wheels raised removes tire scrub and much of the real load. Commission at controlled pressure first, then validate under permitted loaded conditions and across temperature.

Who This Selection Method Is For

This method is useful for:

• OEM engineers defining a hydrostatic steering circuit for slow mobile equipment;

• repair shops replacing an unreadable or obsolete steering unit;

• tractor, forklift, loader, agricultural-machine, and industrial-vehicle buyers;

• distributors preparing a technically complete cross-reference inquiry;

• maintenance teams trying to separate a sizing problem from a supply or leakage problem.

It is especially valuable when cylinder drawings and a hydraulic schematic are available. It turns “we need the same orbitrol” into a requirement that a supplier can check.

Who Should Not Select or Install a Steering Unit From This Article Alone

Do not use a web calculation as sole approval for road-going vehicles, high-speed vehicles, autonomous steering, safety-rated functions, machines governed by type approval, or any application where steering loss can create severe risk. Follow the vehicle manufacturer, applicable laws and standards, and a competent steering-system engineer.

Do not buy from displacement alone if the old code, circuit type, cylinder geometry, port mapping, pressure settings, and emergency behavior are unknown. Do not substitute an open-center unit into a load-sensing circuit, change reaction type, or raise relief settings without system-level validation.

Do not use a new steering unit to mask worn kingpins, a binding column, a leaking cylinder, contaminated oil, a starved pump, or a faulty priority valve. Replacement will not repair those causes.

Procurement Worksheet

For a BLINCE comparison, the hydraulic steering control unit category provides the product-family starting point. Send the completed worksheet rather than only the machine model.

FAQ

What does 100 cc/rev mean on an orbitrol steering unit?

It means the metering section theoretically moves about 100 cm³ of oil per steering-wheel revolution. It does not state circuit type, pressure rating, valve content, port threads, or reaction behavior.

What is the practical way to choose steering-unit displacement?

Start with the oil volume entering the cylinder during one steering direction. Divide it by the intended wheel turns (q = V / N), run the calculation again for the opposite direction, and then see whether the machine can supply the resulting flow.

Is there a standard lock-to-lock figure for tractors or forklifts?

No single figure is safe for every machine. Vehicle speed, axle load, linkage geometry, tire behavior, feedback type, and local requirements all matter. For a replacement job, the original manufacturer’s target is the sensible baseline unless the steering system is being formally redesigned.

Does a larger displacement reduce steering effort?

Do not assume that it does. The wheel makes fewer turns, but every turn asks for more oil. With the pump unavailable, producing the same hydraulic pressure through a larger metering section can also demand more torque from the driver.

Is 200 cc/rev a direct replacement for 160 cc/rev?

No—not on displacement evidence alone. The change is 25%, which alters both wheel turns and flow demand. It also needs the same circuit type, feedback behavior, valve settings, ports, mount, input shaft, and approved vehicle response.

Why does the machine steer a different number of turns left and right?

Possible reasons include unequal cylinder chamber volumes, linkage geometry, mechanical stops, air, leakage, or an incorrect volume calculation. Calculate and measure both directions before condemning the steering unit.

How much flow should I allow for a 160 cc/rev unit?

First decide how quickly the steering wheel must be turned. The calculation gives 9.6 L/min at 60 rpm, 16 L/min at 100 rpm, and 19.2 L/min at 120 rpm. Those are theoretical requests; the final allowance should follow the exact unit data and its leakage at working temperature.

Is an open-center steering unit interchangeable with a load-sensing unit?

No. Neutral flow and signal logic differ. A physical fit does not make the hydraulic circuit compatible.

What is the difference between reaction and non-reaction steering?

Reaction units can transmit external wheel forces back toward the steering wheel in neutral; non-reaction units isolate that feedback more strongly. Selection must match the cylinder arrangement and vehicle design.

Do I need a steering unit with a valve block?

That depends on whether the circuit requires integrated relief, shock, suction/anti-cavitation, inlet-check, or other functions. Request the block symbol and settings; the phrase alone is insufficient.

Why is steering still slow after installing the calculated displacement?

Available flow may be low, the priority valve may not reserve enough flow, the pump may be worn or starved, oil may contain air, a return path may be restricted, or the cylinder/unit may leak internally. Measure before changing size again.

What information should I send for a steering-unit quotation?

Send the full old model code, machine type, cylinder bore/rod/stroke and arrangement, target turns, pump and priority flow, pressure settings, circuit type, reaction requirement, port and mounting drawings, oil, temperature, quantity, and safety constraints.

Final Buying Rule

Start with the cylinder, not the catalog. Cylinder volume divided by desired wheel turns gives a defensible starting displacement. Displacement multiplied by intended wheel rpm exposes the required steering flow. Those two calculations eliminate many bad matches, but they do not replace circuit and safety validation.

For a technical inquiry, send BLINCE the worksheet, old-unit label, hydraulic schematic, cylinder drawing, measured hot flow, pressure settings, port and shaft photos, and required quantity. The goal is not merely to find a steering control unit that bolts on. It is to specify one whose displacement, circuit logic, valve protection, interfaces, and vehicle behavior agree.

Tel: +86 132 4232 1601

✉️ Email: sales16@blince.com

Website: https://blince.com/

Disclaimer

This article is a general engineering guide. Final component selection should be based on machine drawings, measured hydraulic data, working conditions, safety requirements, and confirmation from a qualified hydraulic engineer or supplier.

Blince Hydraulic Team

Blince Hydraulic is an industry-leading company dedicated to precision-engineered fluid power manufacturing and custom hydraulic solutions. Backed by decades of deep field expertise in industrial machinery and thousands of successful global deployments, our engineering team focuses entirely on high-performance hydraulic component manufacturing, including specialized orbital motors, high-pressure travel drives motor, and robust directional control valves. Our production infrastructure utilizes state-of-the-art multi-axis CNC machining systems and is fully ISO 9001 certified to guarantee repeatable volumetric accuracy across every single manufacturing run.

We deliver fast, highly dependable, and cost-efficient hydraulic solutions to heavy industry distributors, machinery OEMs, and maintenance crews across more than 150 countries. Whether your active project calls for a small-volume batch of customized shaft profiles or a large-scale production run of severe-duty cast iron gear pump, we configure our flexible production schedules to meet your target lead times with total pricing predictability. Partnering with Blince means securing maximum system efficiency, elite material quality, and uncompromised fluid power professionalism.

To learn more about our complete product lineup, visit our official website: www.blince.com.