Solenoid valve Fault Tracing by Norgren

Fault Tracing

For pneumatic and electro-pneumatic systems

Contents

Safety Fault effect Common faults Component fault tracing Solenoids Switches

Monitored conditions Fault tracing procedure Simulated system

pneumatic

Simulated faults

Click the section to advance directly to it

Introduction

Valves

Simulated system PLC

System fault tracing

Introduction

Modern component design and manufacture coupled with planned preventive maintenance provide a high level of performance and reliability

In the event of a machine or system break down, the heavy cost of lost production time makes it essential to restore normal running in the minimum time

If a machine or system shows a fall off in performance or stops working, there is a race to trace and correct the fault as fast as possible

A methodical approach and experience of likely problems is the pre-requisite of the fault tracing engineer

Safety in fault tracing

Safety in fault finding

In any fault tracing exercise personal safety and the safety of others is paramount

Work should be carried out using approved practices and observing relevant legislation

Ideally all electrical and pneumatic power will be isolated, pressure exhausted and moving parts mechanically locked


In practice it may be necessary to have the machine or device partly or fully powered up to locate a fault

To gain access to test a suspect device, it may also be necessary to remove and override the guards.

This clearly presents many dangers and great caution and awareness is required by the fault finding team

Fault finders must keep well clear of the path of all actuators, mechanisms and other hazardous moving parts

Electrical equipment should be checked using only the proper test instruments


DANGER! Jammed actuator

The actuator will be pre-exhausted

Clearing the jam will cause the actuator to impact to the end of its stroke

This will be faster than you can react to remove your hand

Ensure the actuator is de-pressurised and use a tool to clear the jam


DANGER! Live trip valves and limit switches

Accidentally touching or leaning on a limit valve or switch while testing a machine can generate a signal

This can cause an actuator or sequence of actuators to operate unexpectedly


DANGER! An exhausted system can leave some components with trapped pressure

An actuator controlled by a 3 position valve may be pressurised regardless of whether the main pressure has been exhausted or not

Removing a connection to one end or overriding the valve will cause sudden actuator movement

Click the illustration to start and stop animation


Some of the dangers to consider include:

Sudden exhausts to the face:

noise can injure the ears ejected particles can

injure the eyes Electric shock Moving mechanical parts

Fault effect

Symptoms of a fault

In a typical pneumatic or electro-pneumatic machine a fault will first be apparent due to one or more of the following:

Poor performance, slow Faulty product, inaccurate Machine stops

A fault tracing and correction procedure must be

put into action

Common faults and solutions


SymptomThe machine is working but is falling off in performance due to slower operation

Possible Causes Upstream flow

restriction or air starvation

Downstream flow restriction

Lack of lubrication

Possible Solutions If weak supply pressure

Fit larger pipe Install local receiver Install larger

compressor If strong supply pressure

Check for kinked tube downstream and renew

Lubricate mechanisms Fit air line lubricator


SymptomOne actuator is falling off in performance due to slower operation

Possible Causes Flow regulators set too

low Tube kinked Lack of lubrication Out of alignment Objects in actuator path Piston rod bent Barrel dented

Possible Solutions Readjust flow regulators Renew tube Fit air line lubricator Clean, realign and

lubricate mechanisms Replace or repair

actuator


SymptomFaulty product due to machine adjustments

Possible Causes Stops or mechanisms

out of adjustment Limit valve or sensor

moved out of position causing a short stroke

Failed ON sensor causing a skipped step in the sequence

Insufficient power to a stamping or pressing actuator

Possible Solutions Readjust mechanisms,

stops and limit sensors Check suppressor and

circuit then replace sensor

Modify the logic control to stop the machine in the event of any sensor failing ON

Increase pressure to the actuator or replace the actuator with a larger one


SymptomMachine stops

Possible Causes Failed pneumatic or

electrical power supply Limit valve or sensor

moved out of position Mechanical jam

preventing an actuator from operating a limit switch

Failed to OFF sensor breaking the sequence

Possible Solutions Re-establish power

supplies, reset machine and restart

Adjust and tighten sensor

Ensure pressure is released from both sides of the actuator before clearing a jam

Check suppressor and electrical circuit then replace sensor

Component fault tracing

Solenoids

Solenoid pilot valves

Fault: the pneumatic output is continuously ON when the valve coil is de-energised

Check the manual override (if fitted) it may have been left in the ON (1) position

0 1


Fault: The pneumatic output is OFF when the coil should be ON

Check the electrical supply at the plug

If this is OFF trace the supply back to the source

If ON the fault may be: low voltage mechanical fault

preventing the armature pulling in

burnt out coil

DC

OFF

200mV

2V

20V

1000V

200V

750V

200V

20V

2V

200mV

200uA2mA

200mA

20mA20A

200mA20mA20A

2mA 200uA

DC

AC

20M

2M

200K

20K

2K

200

AC

20A mA uA COM V

FUSED

UNFUSED

!!!

1000Vdc 750Vac MAX

250mA MAX10A cont20A 30sec MAX

AUTO POWER OFF

-TECHIDM91ISO


A suppressor fitted within the plug may fail

If so it is likely to fail open circuit

This will not stop the coil from working but will leave it unsuppressed

This could damage the circuit that is switching the coil

Short circuit failure will blow a fuse or damage the switching circuit

+24 V 0 V


A weak or broken spring will prevent the armature from sealing the supply, causing air to flow to the outlet and exhaust

Alternatively the armature may be held in the up position by the air pressure after the coil is de-energised

2

3

1


Cut or deeply embedded seats, can cause leakage but needs to be extensive before the valve is unusable

12

3


Coils must be firmly fixed to the solenoid stem

For a.c. solenoids the reaction of the alternating magnetic field causes axial reactive forces on the coil

If the coil is loose it will vibrate

There will be heat build up, less efficient holding of the armature and eventual failure



Solenoids valves have matched coils and stems according to coil power

A low power coil will not pull in the armature against the stronger spring of a high power stem assembly

A high power coil causes slamming of the armature on a low power stem resulting in premature seal failure

1.6

0 1

1.6

2W = 1.0mm orifice diameter 6W = 1.6mm orifice diameter8VA = 1.6mm orifice diameter

24V =

6W


Solenoid coils are continuously rated which means they can be left energised indefinitely

When energised for a long time it is normal for a coil to become too hot to hold comfortably

Overheating can result if coils are continuously energised in a confined space with no means of ventilation

100%ED

Switches

Reed switches

Fault: Switch contacts permanently made

1. Used directly with an unsuppressed solenoid coil where the arcing has fused the contacts together

2. Too high a current has passed through the contacts

a surge current caused by very long leads (capacitive coupling)

too high a load

L + V

0 VLoad

Extended leadsStandard leads

470µH

+24 V 0 V

Fit a suppressor

Fit an inductor

Reed switches

Indicating light (l.e.d.) permanently ON although the switch functions normally

1. On three wire types this is most likely due to the brown and black wire being reversed

2. If this is not the problem it is likely the contacts have failed closed

BlueCoil

Black0 V

Brown + V

0 V

Protection diode

BlueCoil

Black0 V

Brown + V

0 V

Protection diode

Reed switches

Fault: Switch not turning ON when actuators piston is at the end of stroke

The switch is positioned too far to one end on the actuator

It is beyond the range of the magnet


Reed switches

Fault: System not responding to a switch at the actuator’s mid-stroke position

It is likely that the piston speed is too fast

The bandwidth of the magnetic field is approximately 6mm.

If this distance is covered in less time than the response time of the equipment being signalled then it will not be successful


Limit switches

Fault: The switch mechanism appears to be moving but the switch is not changing over

1.This may be due to a cam operating the roller in the pre travel band only

re-adjust to include the operating travel

2. Broken contact due to vibration and mechanical fatigue

3. Burnt contacts due to unsuppressed loads

Operatingtravel

Pre-travel

Valves

Power Valves

Fault: Valve spool not changing position

Low signal pressure The minimum operating

pressure can be from 1bar to 3 bar depending on the valve type

Power Valves


Opposed signals on bi-stable types (fault with other part of the control)

The signal on the left has not been removed so the new signal on the right has no effect

Check the valve giving the left signal and it’s control

Power Valves


Sticking spool due to incorrect lubrication (swelling of seals)

Lubricating a valve with a non-compatible oil can cause the seals to swell and grip the spool

Some non-compatible oils when dried out leave a residue that can glue the spool in position

Power Valves


Jammed spool due to the use of a long threaded fitting which has damaged the valve bore

Use only fittings designed for connection to pneumatic valve ports

Power valve

Fault: Slow changeover of the spool

A blocked vent hole The space behind the

pistons needs to breath If the valve is installed

firmly against a flat surface the breathing hole can be blocked

The restriction can cause slow or incomplete operation of the spool

Power valve

Fault: Valve spool not resetting

For mono stable valves this may be due to a broken spring

Power valve

Fault: Air escaping continually from exhaust ports

Leaking due to damaged or worn seals

For 5/2 valves this may alternatively be caused by leaking actuator seals

Simulated Faults

Simulated circuit

This simple circuit simulates an application

Click on the circuit to see it in operation

The lever valve is operated and the cylinder operates automatically

The lever valve is reset to stop the cylinder in the fully instoked position

The next series of slides show the effect on this circuit of various faults


Fault simulation 1

Click the circuit for fault 1 The cylinder stops in the

outstroked position where it should be operating the trip valve

The trip valve (a1) has not operated and cannot signal the 5/2 valve

The fault is likely to be loose mountings of valve (a1) so that it is not being tripped properly


Fault simulation 2

Click the circuit for fault 2 The cylinder stops in the

outstroked position operating the trip valve

The trip valve (a1) has operated but is not signalling the 5/2 valve

The fault is likely to be a kinked or trapped tube


Fault simulation 3

Click the circuit for fault 3 The cylinder completes

one cycle but cannot start the next cycle

The trip valve (a1) has operated but not reset

The 5/2 valve has opposed signals so remains in the reset state

The fault is likely to be jamming of the valve operating mechanism against an obstruction


Fault simulation 4

Click the circuit for fault 4 The piston rod stops in a

part stroked position The piston rod has

jammed against an obstruction

The cylinder is pre-exhausted and will impact to the end of stroke when the obstruction is cleared

Freeing the jam while there is pressure applied is dangerous


Systems Fault Tracing

System fault tracing

With a failed system it is usually difficult to know where to start the search

The fault tracing process is one of back tracking from the event that should have happened but has not

It may be local such as a jammed actuator or the back tracking process may take you through several stages of logic before the fault is revealed

Resources

The speed with which faults can be located within a system depend on the following facts:

How well the engineer knows and understands the machine

The number of monitored conditions

The quality of the documentation and if it is up to date

Sunday, April 9, 2023

Monitored conditions

Pressure gauges show the presence and level of pressure

Pressure indicators show the presence of pressure

Lamps show the presence of electrical power

LEDs show that a solenoid is receiving power or that a reed switch or sensor is turned ON

2

4 6

8

10

4080

120

lbf/in2

bar

Monitored conditions

In the absence of monitored conditions the fault tracing procedure will take a little longer:

Pneumatic connections have to be tapped to check for the presence of pressure

Electrical connections have to be accessed to check for voltage

OFF

200mV

2V

20V

1000V

200V

750V

200V

20V

2V

200mV

200

uA

2mA

200mA

20mA

20A

200mA

20mA

20A2mA 200

uA

DC

AC

20M

2M

200K

20K

2K

200

AC

20A mA uA COM V

FUSED

UNFUSED

!!!

1000Vdc

750Vac

MAX

250mA MAX10A cont20A 30sec MAX

AUTO POWER OFF

-TECH IDM91ISO

Fault tracing procedure

1. Identify the actuator that has failed to complete or make it’s stroke

2. Establish the direction of movement at this step, outstroke + or instroke –

3. Inspect the state of the inputs and outputs of the valve controlling the actuator

4. Are these correct for the intended stroke ?

If YES: the fault is with the actuator or between valve and actuator

jammed actuator flow regulator sealed tube squashed

If NO: are the inputs correct ?

If YES the fault is with the valve

If NO follow the incorrect signal path to find the fault

Inputs and Outputs

It will help if you become familiar with the pattern of inputs and outputs at a power valve

When a system stops the pattern will be a guide to the fault

Few systems will have such convenient monitoring indicators as shown on the next slides but the pattern can be established

Where push in fitting are used, pushing the tube towards the valve but not releasing the collet will give a springy or slack reaction indicating whether pressure is present or not

Building blocks

All circuits are made from basic building blocks

The circuits illustrated are single actuator building blocks pneumatic and electo-pneumatic inputs

Click each circuit to animate and observe the changing pattern of monitored conditions, inputs (signals) and outputs (power)

Click again to stop or start at different steps


Inputs / outputs for 5/2 Valve

These are the four patterns that exist normally during the operation of an actuator

If an actuator fails to move, patterns 1 or 2 suggest a jammed actuator or low pressure, patterns 3 or 4 suggest a failed signal

If an actuator stops at part stroke, any of these patterns suggest a jammed actuator

+ –

10

+ –

10

+ –

10

+ –

10

1 2

3 4

All combinations are shown on the next slide with suggested reasons why the actuator has not operated

5/2 Valve i/o patterns

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

+ –

10

Jammed or lost signal 0 ?

Jammed or lost signal 1 ?

Jammed at part - stroke ?

Jammed at part + stroke ?

Signal 1 not released ?

Signal 0 not released ?

Valve supply lost ?

Valve supply lost ?

Valve supply lost ?

Supply lost plus false signal ?

Valve not changing position ?

Valve not changing position ?

Split actuator or valve seal ?



Split seals plus false signal ?

Back tracking to the fault

The back tracking procedure is identified with the next two example simulations

1. System with pneumatic logic 2. System with PLC logic

Fault simulation (pneumatic logic)

Click the circuit to animate the sequence A+B+B-C+C-A- It stops near the end of the third cycle Follow the fault tracing procedure

1. Actuator C should have moved minus

2&3. The inputs are not correct follow either of these

4. The cascade valve has not changed. The lower input is not correct

5. Follow the path of this signal. This comes from valve c1. It can be seen the roller is missing and the valve has not operated


Fault simulation (PLC Logic)

Click to animate A+A-B+B-C+C-A+B+C+ABC-ABC+A-B-C- It stops at the end of the last A- Follow the fault tracing procedure

1. Actuator B should have moved minus

2. The input a0 from the previous movement A minus is not ON

3. As a result the output to send B minus has not been given

4. Following the fault path to the a0 sensor reveals that it has slipped out of range of the piston in actuator A


Solenoid valve Fault Tracing by Norgren

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