Directional Valve Flow and Circuit Response

Directional Valve Flowand Circuit Response

Select the flow path that produces the required movement—not simply the valve with the largest port or shortest switching-time claim.

1. Engineering answer: valve capacity and circuit response are different questions

A directional valve must connect the correct ports in every required state and pass enough air through the relevant supply and exhaust paths for the actuator to achieve its specified movement. Its suitability depends on the pressures during that movement, downstream volume, tubing, load, pilot conditions, control timing and the required end condition. A catalogue flow number or electrical switching time cannot establish that complete result by itself.

The useful selection question is therefore: can this exact valve, in this installed circuit, produce the required pressure and motion response over the intended operating conditions? Answer it by keeping function, capacity, timing and verification separate. An acceptable valve symbol does not prove sufficient flow. Sufficient steady flow does not prove a fast command response. Fast movement does not prove acceptable stopping, product force or restart behaviour.

Begin with the actuator's duty and the existing circuit requirements. Identify the start event, required movement, load, permitted time, minimum dynamic supply and exhaust arrangement. Then compare manufacturer flow data for each active path, account for the connected volume and restrictions, and verify the assembled system using synchronized command, pressure and motion evidence.

The output should be a traceable valve-response comparison: what is known, what has been calculated, what still needs supplier information and what must be demonstrated on the actual machine. It should never disguise a missing exhaust characteristic or an unmeasured delay behind a single green selection badge.

2. What owns—and what remains with adjacent pages

Its required working asset is the valve-response comparison worksheet.

Use this page when selecting a valve for a new machine, investigating a slow or inconsistent actuator, comparing substitutions, relocating a valve island or changing tubing. It is also relevant when a machine performs correctly alone but fails during simultaneous actions. The same investigation may identify an unsuitable regulator, shared exhaust or pilot arrangement rather than a defective main spool.

The foundation is explicit: preserve dynamic point-of-use pressure; distinguish flow descriptors; include exhaust and tubing; separate command, movement and confirmation; and keep normal control distinct from protective functions. The supplied bookmark documents are resource locators, not valve ratings.

No actual candidate manufacturer, customer circuit or operating limit is selected here. A project requires the exact model documentation, installation information and responsible engineering review. The worksheet can expose that evidence requirement without pretending to supply missing catalogue values.

3. Write a response requirement before choosing a valve size

Define the physical outcome at the mechanism: movement from which position to which position, under what load, with what permitted force, arrival speed and settling behaviour? Then define the timing boundary. “A 300 millisecond response” is incomplete until its start and end events are identified. A command-to-first-motion limit differs from a full-travel limit or a command-to-accepted-position limit.

Keep extension and retraction separate. Different chamber areas, external loads and exhaust paths can make the two directions behave differently. Record whether the requirement applies after a long idle, during repetitive production, at minimum supply, after a permitted pressure recovery or with neighboring consumers active. These are separate operating cases, not reasons to apply one unexplained percentage allowance.

Separate a functional requirement from a diagnostic timeout. The timeout may report missing confirmation; it does not establish that the actuator has stopped or that the next movement is safe. Likewise, a sensor can confirm a position without proving the required clamp force. Agree these meanings before comparing candidate response times.

4. Map the complete supply, working and exhaust paths

Create a path register for each relevant valve state. Follow air from the local supply through preparation, regulation, isolation, manifold passages, the selected valve, working-port fittings, tubing and actuator connection. Follow the displaced air back through its return tubing, flow control, valve exhaust passage and final discharge arrangement. A drawing that stops at the actuator ports omits half the response problem.

Use the exact manufacturer's port designations and symbol. Familiar labels such as P, A, B, R and S are useful on a teaching record, but the final register must identify the real connections. Do not infer a numbered port arrangement from appearance alone. Identify separate pilot supplies and pilot exhausts, especially when they remain connected after the main supply changes.

A high-capacity supply path does not compensate for an inadequate exhaust path. Nor does an open exhaust symbol prove that the chamber is immediately at atmospheric pressure. Tubing, silencers and simultaneously exhausting users can maintain an opposing pressure during movement. Preserve those conditions in the comparison rather than assuming that all exhaust ports discharge into the same ideal sink.

5. Read the valve function before its flow coefficient

Confirm whether the circuit needs a 3/2, 5/2, 5/3 or another documented function, and how it is actuated and returned. A monostable valve, a bistable valve and a three-position valve have different uncommanded behaviour. Selection by port size and voltage alone can silently change the machine's functional sequence.

For a three-position valve, read the actual centre connections. Closed-centre operation retains volumes but does not create a rigid, leak-free mechanical lock. Exhaust-centre operation may remove supporting force and allow an external load to move. Pressure-centre operation can produce a force imbalance in an unequal-area cylinder. The word “centre” says nothing about whether the resulting physical condition satisfies the machine requirement.

Then distinguish the fully established position from the transition between positions. A data-sheet flow coefficient normally describes an identified open path under its stated test conditions. During movement of the internal element, the effective opening and connections can change. Without manufacturer transition information, the fully open coefficient cannot be treated as available from the first instant of an electrical command.

For replacement review, preserve the symbol, return mechanism, pilot arrangement, sealing configuration, manual-override behaviour and permitted mounting. Keep functional compatibility ahead of arithmetic ranking. A candidate that provides greater nominal flow but the wrong uncommanded connections is not an improved replacement.

This page's comparisons concern normal pneumatic performance. Assigning a safety function requires the appropriate machine-level architecture and evidence; neither an ordinary centre-position choice nor a fast switching specification supplies that assessment.

6. Interpret Cv, Kv, nominal flow, effective area and C/b correctly

A capacity descriptor needs its definition, units, test conditions and path. The supplied circuit requirements specifically warn against treating Cv, sonic conductance, nominal flow and port size as interchangeable. The manufacturer's flow-characteristic documentation provides the applicable interpretation. [S1; R1]

A pure coefficient-unit conversion cannot repair missing gas-state information or create a second flow parameter. If the supplier provides Cv only, use the supplier's applicable gas-flow method or obtain additional data; do not invent b to make a two-parameter model run. Similarly, retain the stated reference basis of litres per minute rather than assuming the flow is actual compressed volume inside the tube.

Record where the rating was obtained. A bare valve, a valve on a particular sub-base and a complete manifold assembly can represent different boundaries. Check whether the cited rating already includes a connector or accessory before adding a separate allowance. For an unresolved boundary, the correct comparison status is incomplete evidence, not a hidden correction factor.

7. Understand why flow changes as the chamber pressure changes

Compressed air does not pass through a restriction at one constant catalogue rate regardless of downstream pressure. The relationship depends on upstream state, downstream state and the restriction's characteristics. Under a suitable choked condition, reducing downstream pressure further does not increase mass flow for unchanged upstream conditions and geometry. Outside that region, downstream pressure affects the result. [R1]

For a forward-flow comparison define r = p_down,abs / p_up,abs. Use absolute pressures for this ratio. Gauge differences can be useful for a pressure-loss record, but dividing one gauge pressure by another does not give the required gas-state ratio. The dictionary and formula register remain the shared references for pressure and temperature meaning. [S1; N2]

During filling, downstream pressure can rise while upstream pressure at the valve also changes because of supply restrictions. During exhaust, the actuator chamber is the upstream side of that exhaust path; its pressure decreases toward the exhaust-system pressure. Both directions therefore require time-dependent interpretation. A flow figure measured while exhausting to atmosphere is not automatically available while charging a nearly pressurised chamber.

Do not turn a choked-flow observation into a universal pressure-setting recommendation. The machine still needs adequate differential force, acceptable impact and operation within component limits. Nor should an ideal nozzle pressure ratio be substituted for a valve's measured critical ratio. A real component's adopted curve and parameters must govern its model.

For a useful first comparison, examine several pressure pairs spanning the intended event. This reveals where candidates differ before investing in a detailed circuit model. The comparison remains a set of flow snapshots, not proof of the resulting stroke time.

8. A declared two-parameter flow model for teaching and screening

The following expression is a transparent, classical C/b model using an elliptical subsonic branch. It is included to explain pressure-dependent flow, not as a complete implementation of every edition or variant of a flow-test standard. Apply it only when the supplier's C/b definition supports this form. [R1; N2]

For q_ref in litres per second, C in dm³/(s·bar), upstream absolute pressure in bar, and absolute temperatures in kelvin:

• q_ref = C × p_up,abs × √(T_ref/T_up) × φ(r)
• φ(r) = 1 for 0 < r ≤ b
• φ(r) = √{1 − [(r − b)/(1 − b)]²} for b < r ≤ 1

The teaching implementation requires C > 0, 0 < b < 1, positive absolute pressure and temperature, and p_down ≤ p_up. Equal pressures give zero forward flow. Reverse flow requires its own supported direction and parameters; the routine does not quietly take an absolute pressure difference. The comparison record must retain the reference-state identity associated with C.

Dimensional checking is straightforward: dm³/(s·bar) multiplied by bar gives dm³/s, and both the temperature factor and φ are dimensionless. Multiplying the result by 60 gives litres per minute on the same reporting basis. There is no extra factor of ten when pressure has already been entered in bar rather than MPa.

The source presents practical-unit formulas using rounded temperature constants and a stated atmospheric approximation. This register instead accepts absolute pressures directly and carries the explicit reference temperature. Its synthetic examples use 293.15 K. That choice is not permission to alter the declared reference state of a real manufacturer's coefficient.

Do not extend this simple function to changing gas composition, unprovided coefficients, full network conductance or transient spool opening by assumption. Those require additional evidence and a suitable model.

9. Compare every directional path, not only the best catalogue row

For a double-acting device, evaluate the supply and exhaust paths active during each movement. Keep the path identity attached to every coefficient or table value. A supplier may provide different values for supply-to-working-port and working-port-to-exhaust paths. Even where values match, the connected tubing and external pressure conditions can differ.

At minimum, the candidate record should contain the required valve function, exact configuration, relevant path data, local supply envelope, exhaust pressure basis, temperature/reference basis, response definition and pilot requirements. A record with all fields filled by guesses is less useful than a partially completed record that clearly identifies the missing supplier information. [N1 applying S1]

Compare candidates at the same pressure pairs and reporting basis. A flow value at one drop and another at a different drop are not a fair ranking. Where a manufacturer curve is used, preserve the original curve, its revision and how the selected point was obtained. Label interpolation as interpolation; do not extrapolate through an undocumented change in valve operation.

Do not add component flow ratings in series. The same mass flow passes through the series elements under a steady no-leak condition, but the pressures between them must be resolved. The smallest headline rating may flag an investigation, yet it is not a rigorous assembled-circuit rating. Parallel branches also need pressure and load-sharing context before their capacities can be combined.

A candidate can remain acceptable for further review despite having a lower maximum flow if it better supports the relevant pressure range, timing or installation. Conversely, a high-flow option remains unsuitable when it lacks the required function, pilot margin or exhaust performance.

10. Treat tube length, internal diameter and location as design variables

Record actual tube internal diameter, length, material, fittings and routing between valve and actuator. A hose described only by its external diameter or nominal thread does not define the flow path. Include adapters, swivel connections, moving loops and any restricted entry through a fitting. Preserve these dimensions in the as-built record so a later substitution can be assessed.

Tube size has two different effects. A larger internal diameter can reduce resistance at a given flow, but it also increases the gas volume whose pressure changes when the line is charged or exhausted. A shorter line can reduce both resistance and inventory, although valve relocation introduces other constraints such as environment, access, moving mass or communications. This is a coupled choice, not a universal instruction to use the smallest or largest tube. [N2]

For a uniform circular line, V_line = πd_i²L/4. Use consistent units and the internal diameter. This gives geometric volume only; it does not determine flow resistance through the complete line or fittings. If a line contains multiple sizes, calculate each segment and sum the volumes while retaining their individual identities for flow analysis.

Distinguish a line that changes pressure each cycle from a continuously supplied header. The same geometric volume does not create the same recurring consumption in those two arrangements. The location of the directional valve determines which volumes are repeatedly switched.

Where manufacturer sizing charts are used, retain their pressure, cylinder, load and installation assumptions. A chart for one tube length or duty does not automatically cover an arbitrary remote valve cabinet. The physical layout belongs in the selection, not in an installation note added after the valve has been ordered.

11. Use gas inventory to explain filling—without pretending it is a stroke simulation

For a fixed volume changing from one known pressure state to another, the newly supplied gas is the inventory difference, not the entire final inventory. Under an isothermal ideal-gas assumption, with the reference temperature equal to the chamber temperature:

ΔV_ref = V_fixed × (p_final,abs − p_initial,abs) / p_ref,abs.

This principle is already developed in the supplied cylinder material. A line initially containing gas at atmospheric pressure is not empty. Charging it to a higher pressure does not require replacing that initial gas as well as adding the pressure increment. Retain the initial condition, including any residual pressure.

For unequal known temperatures or compressibility factors, use the complete inventory relationship from the formula register. Do not apply a simplified pressure difference without its thermal assumption. A moving cylinder chamber also changes volume; the fixed-volume calculation must not replace the swept-volume model indiscriminately.

A useful bound is t_fill ≥ ΔV_ref/q_ref,max when q_ref,max is a defensible upper bound on the net inflow throughout the defined filling interval. It is not sufficient that q_ref,max happens to be a catalogue nominal flow. The bound excludes any additional command delay and does not guarantee an attainable filling time. Leaks or flow falling as pressure rises can increase the required duration. [N2]

For an ideal-gas chamber, use absolute pressure p, volume V, gas mass m, absolute temperature T and the specific gas constant R in consistent SI units. Differentiating m = pV/(RT) at constant temperature gives dp/dt = (RT/V)(ṁ_in − ṁ_out) − (p/V)dV/dt. This exposes why inflow, outflow and piston motion interact. A proper transient model also needs mechanical load, friction, valve opening, heat transfer and initial conditions where relevant.

12. Account for exhaust backpressure in both speed and force

An exhaust restriction does more than slow the removal of air. It can maintain pressure in the opposing chamber, reducing the net force available for the intended movement. A supply-pressure reading alone can therefore misdiagnose a weak or slow actuator. Record both chamber pressures when their difference is central to the conclusion.

For extension of a single-rod cylinder in a common atmospheric environment:

F_available = p_cap,g A_piston − p_rod,g A_annulus − F_internal.

Here F_internal is a separately defined internal resisting force. Compare the remaining available force with the external process load and required acceleration, without subtracting the same resistance twice. Gauge pressures are appropriate in this stated force balance because both use the same atmospheric reference; gas-state conversions still require absolute pressures. [S1; N2]

Backpressure can arise in the return tube, flow control, valve passage, silencer or shared exhaust gallery. A recorded pressure does not identify which part created it. Segment the path and preserve operating state before choosing a correction. A silencer that appears acceptable at low duty may behave differently when several users exhaust together or when contamination changes its condition.

Do not simply remove exhaust hardware to obtain speed. Noise, discharge location, contamination control and the approved machine behaviour remain part of the design. A suitable correction may involve an appropriately selected exhaust arrangement, maintenance or changed architecture, followed by verification of force, travel and stopping.

Improving exhaust performance can increase speed and impact as well as useful force. Retest the full movement rather than declaring success as soon as a pressure difference becomes smaller.

13. Specify speed control, cushioning and intermediate behaviour separately

Meter-in and meter-out arrangements influence different sides of the actuator's pressure evolution. The appropriate choice depends on load direction, whether the load resists or assists motion, supply conditions, friction, desired speed and failure behaviour. The supplied circuit material deliberately avoids a universal instruction based only on “extension” or “retraction.”

Keep the directional valve's flow capability separate from the setting of a speed-control device. A restrictive setting can dominate movement even when the main valve is generous. Conversely, opening a throttle further cannot create capacity that the upstream regulator, valve or tubing lacks. Record the device type, orientation and approved adjustment position for each movement.

A quick-exhaust arrangement changes the physical release path near the actuator. Its suitability depends on the supported configuration and the consequences for release rate, noise, backpressure and stopping. Any changed path needs a revised state and residual-pressure review. [S1; N1]

Cushioning near the end of travel can intentionally restrict exhaust to absorb motion energy. A slower final portion of a stroke is not automatically a valve defect. Identify where the slowdown starts, the load at that position and whether arrival behaviour meets the requirement. Removing intended cushioning to meet an arbitrary travel timer can overload the mechanical assembly.

Intermediate positioning is another distinct requirement. Blocking ports does not remove air compressibility or leakage. For controlled positioning or broad load-dependent speed accuracy, assess the supported feedback and control architecture rather than treating an ordinary on/off valve as a proportional motion controller.

14. Check pilot air and the electrical interface under the real switching condition

A pilot-operated valve may require a particular pressure condition before its main element can switch as intended. Verify that condition at the relevant pilot connection, including startup and simultaneous-demand cases. The main supply indication, electrical command and pilot availability are separate pieces of evidence. External pilot supply also creates a separate connection and isolation responsibility.

Record the exact coil and connector option, command voltage at the operating condition, permitted duty and the manufacturer's switching-time definition. An indicator lamp or controller output bit is not proof that the main pneumatic path reached its intended state. Direct spool-position evidence, where available and applicable, should remain distinct from ordinary output-command indication.

Electrical protection details can affect timing. SMC's identified SYJ installation manual states that response time depends on the selected surge-suppression method. This supports a narrow practical requirement: retain the documented suppression and driver arrangement when comparing or replacing a valve. It is not a wiring instruction or a universal numerical delay to apply to all coils. [R2]

Separate switch-on and switch-off behavior. A candidate may improve one while changing the other. Include the documented test voltage, pressure, temperature and load conditions; do not transfer a response figure between different options merely because the body family is the same.

At initial pressurisation, establish whether pilot air, main air and command power arrive in a controlled order and whether retained commands or manual overrides can change the result.

15. Evaluate the manifold as a shared pneumatic system

A valve island's individually rated stations can share supply passages, exhaust galleries, pilots and electrical resources. Record which stations operate together and which passages they share. A station that passes a single-actuator test may experience different inlet or exhaust conditions during a synchronized multi-axis event.

Use the actual machine sequence to establish simultaneous supply and exhaust demands. Do not sum only average consumption or assume that a program revision leaves the pneumatic demand unchanged. Shortened dwell, overlapping motions or simultaneous release can create a new restriction problem without adding any equipment.

Where shared exhaust is implicated, distinguish a pressure disturbance from a confirmed adverse motion. Record both, together with the affected valve states. SMC's manifold precautions explicitly identify backpressure as a possible cause of actuator malfunction; any proposed individual-exhaust or isolation arrangement must still be selected for the actual product and application. [R3]

A shared pilot source deserves its own review. Pressure recovery in the main manifold does not necessarily describe the pilot connection during switching. Similarly, adding supply entries without understanding the manufacturer's arrangement may not produce the intended pressure distribution. Use the exact manifold documentation rather than assuming unrestricted internal cross-connections.

For acceptance, include the governing simultaneous sequence and record the configuration that passed. Preserve station count, blanking and separation components, supply/exhaust connections and tube lengths. A later expansion, changed silencer or rearranged valve position can invalidate the earlier comparison even when each individual valve part number remains unchanged.

16. Build a timing record from observable events

A practical response record combines the command timeline with pressure and movement. Select a common time reference or document clock offsets. Define each event objectively: accepted controller command, electrical signal at the valve, main-path change where actually measured, first movement, arrival and accepted feedback. Do not label a pneumatic pressure edge as direct spool-position measurement unless that relationship is supported. [S1, S2; N1]

Use measurement resolution appropriate to the question. A one-second historian cannot resolve a twenty-millisecond switching claim. Sampling interval is only one part of timing capability; sensor response, logger filtering, timestamp origin and synchronization also matter. State whether values are instantaneous observations, averaged samples or controller-cycle records.

Keep raw records and the corrected analysis view. A clock adjustment should have an explicit sign, magnitude, basis and revision. Do not drag traces into alignment until they look plausible. If the timing uncertainty prevents a conclusion, retain an unresolved comparison rather than reporting excessive digits.

Do not add overlapping delays twice. Pressure buildup can begin before a valve reaches its fully established position, and movement can begin before the chamber reaches a later steady pressure. A set of disjoint intervals between observed events can be summed; a collection of independently quoted maxima may not represent a valid chronological decomposition.

For repeated cycles, report the observation count, operating conditions, individual exceptions and the chosen summary. A mean can conceal an occasional missed requirement. A percentile from a small trial set should not be described as a guaranteed future limit. Missing or invalid cycles remain visible in the evidence record rather than being removed simply because they are inconvenient.

17. Use a staged comparison instead of a single unqualified pass

Evaluate function first, then admissible operating conditions, then relevant flow behavior, then response evidence. A missing mandatory item produces a hold on that item. A known incompatibility remains a failure even when another attribute is unverified. The overall status should expose both conditions, not average them into a score. [N1]

Choose the level of model according to the consequence and available evidence. A transparent screening calculation is appropriate for identifying missing information or ruling out an impossible time bound. A supplier sizing tool or a suitable dynamic model may support detailed selection. Commissioning then checks the installed result, because actual fittings, settings and loads can differ from the calculation basis.

A larger valve is not automatically the preferred candidate. It may leave the governing tube or exhaust restriction unchanged, add cost or alter controllability. Compare the smallest complete arrangement that has the required supported operating range—not the smallest nominal port, and not a default maximum-size valve.

Record why a candidate remains under consideration. This preserves a useful engineering decision even before the final supplier and trial evidence are complete.

18. Worked example 1: a larger conductance does not win at every pressure ratio

All coefficients and conditions in this example are fictional. Compare two fully open forward paths using the declared model in section 8. Both coefficient sets are assumed to use the same reference state, with T_ref = T_up = 293.15 K. The upstream pressure is held at 7 bar absolute. Candidate A has C = 1.50 dm³/(s·bar), b = 0.20; Candidate B has C = 1.40 dm³/(s·bar), b = 0.50.

At a downstream pressure of 1 bar absolute, both paths are in their choked region and A produces the larger flow. At 6 bar absolute, A's subsonic factor is √{1 − [(6/7 − 0.20)/0.80]²} = 0.570311408. Therefore q_A = 1.50 × 7 × 0.570311408 = 5.988269784 L/s, or 359.296187 L/min.

For B, the factor is √{1 − [(6/7 − 0.50)/0.50]²} = 0.699854212. Its flow is 6.858571280 L/s, or 411.514277 L/min. B therefore provides more calculated flow at this particular near-loaded pressure pair despite its lower C value. The governing comparison is the matched characteristic, not conductance alone.

These values are not actuator speeds. Filling a chamber would cause the downstream pressure to change, and the local upstream pressure may also change. The example does not include transition opening, tubing, exhaust, load or heating. It demonstrates why the path data and operating pressure range must be retained before a candidate is ranked.

The zero-flow equality case is also important for implementation. When both absolute pressures are equal, this forward-flow model returns zero—not a nominal flow, an error caused by division or a small invented leakage. Leakage, where relevant, requires separate evidence and a different representation. [N2]

19. Worked example 2: line inventory creates a bound, not a guaranteed filling time

Consider a fictional fixed 1.000 L test volume connected by a uniform 4.0 m line of 6.0 mm internal diameter. For this arithmetic illustration, the chamber and line start at 1.0 bar absolute and finish at 6.0 bar absolute. Temperature is constant and equal to the declared reference temperature; reference pressure is 1.0 bar absolute. The volume is mechanically fixed throughout this example.

The line volume is π × 0.006² × 4/4 = 0.000113097336 m³, or 0.113097336 L. Combined geometric volume is therefore 1.113097336 L. The required inventory increase is 1.113097336 × (6 − 1)/1 = 5.565486678 reference litres. The original gas at 1 bar remains part of the initial inventory; it is not counted again as newly supplied gas.

Assume a separately justified fictional upper bound of 20.0 reference L/s on net inflow throughout the interval. Then the fill time cannot be less than 5.565486678/20 = 0.278274334 s. This is a lower bound under the stated model, not a prediction that the chamber will reach the target in that time.

Now change only the line to 8.0 mm internal diameter. Its volume becomes 0.201061930 L, and the combined inventory increase becomes 6.005309649 reference litres. At the same assumed inflow upper bound, the lower time bound becomes 0.300265482 s. The line volume increased by 77.7778%, but the actual flow resistance would also change; holding the flow bound unchanged deliberately isolates inventory arithmetic.

It would be wrong to conclude that the larger line necessarily makes the real circuit slower. Lower restriction may increase actual inflow enough to offset the added inventory. Likewise, the smaller line's lower inventory does not prove it can achieve the required flow. A proper comparison needs both effects and the actual pressure-dependent behavior.

Do not transfer these fixed-volume bounds directly to a moving cylinder. A moving chamber changes geometry, load and pressure together. The example is useful precisely because it states a limited physical problem instead of disguising a volume division as a complete stroke calculation. [N2 applying S2]

20. Worked example 3: exhaust pressure changes the available working force

A fictional horizontal cylinder has a 50 mm bore and a 20 mm rod. Its cap-end gauge pressure during the assessed condition is 5.0 bar. A separately assumed internal resisting force is 80 N. The external working requirement is 800 N; acceleration is excluded from this static comparison and would need an additional check for actual movement.

Piston area is π × 0.050²/4 = 0.001963495408 m². Annular area is π × (0.050² − 0.020²)/4 = 0.001649336143 m². Both chamber gauge pressures use the same atmospheric reference.

The first case gives F = 500,000 × 0.001963495408 − 120,000 × 0.001649336143 − 80 = 703.827367 N. Supply pressure appears substantial, but the stated working-force requirement is not met.

The second case reduces opposing pressure without changing cap pressure or cylinder geometry. The force becomes 868.760981 N. The increase of 164.933614 N comes from the changed opposing pressure, not a larger compressor or cylinder. It identifies why the exhaust path belongs in the diagnosis.

Rearranging the same static balance gives a maximum opposing pressure of approximately 0.616901 bar gauge for exactly 800 N under these assumptions. This is a calculated threshold for this fictional load case—not a recommended exhaust setting, an allowable component pressure or an approved design margin.

The arithmetic does not establish which exhaust component caused the pressure, how fast the cylinder will move or whether stopping remains acceptable. A correction requires the physical path evidence and a controlled trial. If improved exhaust raises speed, the cushion and mechanical impact assessment must be revisited. For a different load orientation or an assisting load, the entire force and control arrangement must be reconsidered. [N2]

21. Worked example 4: a faster valve can make only a small change to total response

The following fictional timestamps share one synchronized clock. They represent observed events, not catalogue maxima assembled from different tests. In this teaching record, direct instrumentation identifies the main valve's established open position; a normal installation may not have that measurement.

Command-to-first-motion time is 95 ms. Physical travel time is 385 − 95 = 290 ms. Command-to-accepted-completion time is 405 ms. Summing the five disjoint intervals gives the same 405 ms, while summing 95 ms start delay and 385 ms command-to-arrival would count the start interval twice.

Suppose a proposed replacement reduces the 20 ms valve interval to 8 ms, and assume every other interval remains unchanged. The conditional total is 393 ms, a 12 ms or 2.96296% reduction. Describing the main-valve interval as 60% faster would not justify describing the entire machine response as 60% faster.

For an illustrative upper response limit of 400 ms, the original nominal result is outside the requirement while 393 ms is nominally inside. But suppose the revised response estimate has an agreed interval half-width of 8 ms. Under a selected upper-bound acceptance convention, 393 + 8 = 401 ms does not establish acceptance. The comparison remains indeterminate under that convention; the rule is not asserted as universal.

The proposed replacement might also change pressure buildup or motion, so the unchanged-other-intervals assumption needs verification. An actual repeat trial should measure the new complete response rather than subtracting a catalogue delay from the old trace. This example demonstrates timing attribution, uncertainty and the value of measuring the dominant part of a response before buying a nominally faster component. [N1, N2]

22. Commission, diagnose and verify the installed response

Record the as-tested valve, manifold, pilot arrangement, tubing, fittings, controls, load and exhaust configuration. Verify that the observed motion corresponds to the required machine state and that measurement connections are suitable. Trials must follow the approved machine procedure, with competent control of pressure, electrical, motion and stored-energy hazards. The worksheet is not an instruction to expose personnel to a moving actuator or loosen a pressurised connection.

Change one well-defined factor where the approved investigation permits it, or record the combined intervention when factors cannot be separated. Preserve unsuccessful trials and adverse observations. Acceptance needs the relevant demanding operating conditions, not only the quietest cycle.

After a correction, verify travel, arrival, force and confirmation as well as the original complaint. Record the accepted settings and conditions that trigger reassessment. A new valve, tube length, silencer, pilot source or production sequence can require a revised baseline. Faster motion is not automatically better when the process required controlled motion within a bounded window.

23. Editable record A: duty, topology and measurement definition

Use one record for each materially different movement or response requirement. These are manually editable Word fields; the companion JSON provides the same structure for implementation. An unresolved field should contain an explicit status and owner rather than a guessed value. [N1]

Attach the current drawing or a clearly identified layout. The record should allow a second engineer to locate the same measurement points and reconstruct the same supply and exhaust paths. A photo can support identification but should not replace part numbers, connection identities or dimensions.

Keep the physical requirement separate from a proposed solution. “Complete this movement under the stated conditions” remains the requirement; “use this valve” is a candidate decision that follows evidence. This separation makes the record useful when a supplier proposes a different valve or when the original component becomes unavailable.

24. Editable record B: directional capacity and candidate response comparison

Create a candidate record for the exact valve and installation option. Preserve the supplier's original values and a separate normalised view. Do not replace a qualified or incomplete curve with a single calculated rating. Identify which data are published, supplier-confirmed, measured, interpolated or assumed. [N1]

For curve-derived data, preserve the point-selection method and the original axis meanings. A line drawn through two points is not an independently supplied full curve. When the required operating condition lies outside the available data, request the missing information rather than extending the curve silently.

Keep known failures alongside unresolved questions. An exhaust-flow shortfall must not disappear because a later row is marked “supplier clarification pending.” The comparison should support a defensible next action: reject the candidate, obtain evidence, revise the circuit or proceed to a controlled installed trial—not automatically order the largest option.

25. Editable record C: trial, diagnosis and change closeout

A selection becomes reusable engineering knowledge when the installed evidence is retained. Use this record to connect the approved requirement, configuration and trial results. Separate as-found observations, hypotheses, authorised modifications and as-left verification. A component replacement is an action, not proof that the original cause was identified. [N1]

Preserve individual cycles where response variability matters. Keep the recorded start/end definitions with the data; otherwise a later analyst may compare incompatible durations under the same “response” column. Record whether a pressure or position channel was filtered and whether the reported timestamp precedes or follows that filtering.

For handover, include the accepted configuration and a concise fault-investigation route. Operators should know what outcome requires investigation and which settings are controlled. Maintenance personnel need the replacement attributes that preserve function and flow. Neither group benefits from a single unexplained “valve passed” statement without the conditions under which it passed.

26. Frequently asked engineering questions

Does a larger valve always make a cylinder faster?

No. The governing restriction may be a tube, fitting, regulator, exhaust or speed control rather than the main valve. A larger valve can also change motion and stopping behaviour. Compare the complete path under the required load and pressure, then verify the installed result. The correct objective is the specified movement, not maximum possible speed.

Is valve switching time the same as cylinder response time?

No. The valve figure concerns a defined component event under stated conditions. The machine response can include command processing, electrical and pilot behavior, pressure buildup, physical travel, settling and accepted feedback. Record the start and end of each quantity. A faster component may change only a small fraction of the full response, as example 4 shows. [S1; N2]

Can I select from port size alone?

No. Port size identifies an interface, not the complete flow characteristic. Internal geometry, the selected state, manifold connections and accessories matter. Match the required function first and obtain the path-specific data. A replacement with the same thread and voltage may still have different capacity, pilot requirements or uncommanded behavior.

Why must supply and exhaust be checked separately?

They perform different parts of the movement and can have different restrictions. Exhaust pressure opposes the supplied chamber's force as well as affecting release rate. A generous supply-to-working-port rating therefore cannot prove a satisfactory return path. Shared galleries and silencers must remain inside the installed boundary where they affect the result. [S1; N2]

Can Cv be converted directly into a complete C/b curve?

Not from the number alone. The definitions and applicable model must be known, and a missing second parameter cannot be recovered merely by changing units. Use the manufacturer's gas-sizing method or obtain the required characteristic. Do not borrow a critical pressure ratio from a different valve to make a calculation appear complete. [S1; N1]

What does choked flow mean for a filling chamber?

It identifies a flow condition for a particular restriction and upstream state. It does not mean the same maximum flow persists until the chamber reaches supply pressure. The pressure ratio changes during filling, and the adopted characteristic must follow that change. Example 1 illustrates flow decreasing near equal pressures without claiming a complete filling simulation. [R1; N2]

Should a long tube always be replaced with a larger one?

Not without checking both resistance and volume. Greater bore can reduce restriction but increases gas inventory in a repeatedly switched line. Shorter routing or a different valve location may also matter. The selection should consider the actual actuator duty, fittings and pressure response rather than a universal tube-size rule. [S1, S2; N2]

Why does the circuit work alone but fail with other actuators?

The simultaneous event may change shared supply, pilot or exhaust conditions. Compare synchronized pressures and command timing with the successful single-user case. A revised sequence can alter the pneumatic peak without adding equipment. The observation suggests a shared-system investigation; it does not by itself identify the valve as faulty.

Can a closed-centre valve hold a load indefinitely?

Do not assume that it can. Blocking ports does not eliminate gas compressibility, component leakage or external forces. Holding, stopping and safe isolation are distinct requirements. A load-retention function needs its own defined performance, duration and competent design evidence; a normal directional-valve state is not automatic approval for that role.

What should I send for a valve-response review?

Provide the exact valve and actuator identities, required motion and timing, current circuit/layout, dynamic supply and exhaust observations, tube internal diameters and lengths, pilot details, relevant load and synchronized command/motion records. Include recent changes and the actual failed outcome. Clearly mark unavailable information so the investigation can target the missing evidence rather than assume it. [N1]