Technical Papers

Causes of and Countermeasures for
Allophone Trouble in a Piston Seal
System for Cylinders
1. Introduction
In recent years, to accommodate requests regarding

2. Example of Packing Structurefor Hydraulic Cylinders

hydraulic machinery, hydraulic systems have been

Generally, several packing and parts are used in high-streamlined by increasing the operating pressure andpressure cylinders including cylinders for hydraulictemperature and reducing the size and weight ofshovels. Figure1 illustrated the structure of a high-hydraulic systems. Accordingly, the performancepressure cylinder.

requirements of hydraulic cylinders including packings

Rodfor hydraulic cylinders have changed, and so theparts
Pisto

performance of various packings has been improved.

n parts

Spring 2017

We have developed a seal system for high-pressure
cylinders as a maintenance kit for cylinders for hydraulicshovels in the construction machinery industry. Whenevaluating actual equipment during the development
stage, we found that allophone（stick-slip phenomenon）
occurs in cylinders under certain conditions. During

Figure1

Structure of a hydraulic cylinder

allophone of the cylinder, a stick-slip phenomenon

２-1）Structure of Seal System in the Rod Segment

occurs at the contact zone of a reciprocating packing,The packing used in the rod segment plays ancausing abnormal noise and oscillation while the cylinder

important role in preventing external leakage, and isis moving. Such allophone is a problem of hydraulican important function of cylinders. Figure 2 illustratedcylinders.

the structure of the seal system in the rod segment.

This report describes an allophone trouble and its

Table1 showed the component parts.

solution. In this particular case, we considered that theallophone trouble was caused by the defective formationof an oil film at the contact zone between the packing andits opposing face, so we designed the packing to improve
its ability to absorb the oil film.

⑵ U-shaped packing
Backup ring for
u-shaped packing⑴ Buffer ring⑶ Dust seal

Figure2

Structure of seal system in the rod segment

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No.32

Causes of and Countermeasures for Allophone Trouble in a Piston Seal System for Cylinders

Table1 List of component parts of the seal system in the rod segment

Table2 List of component parts of the seal system in the piston segment（１）Buffer ring（４）Piston seal
The main component that maintains the oilpressure inside the cylinder for operation. Its
components are as follows:①：Sliding ring to enhance the tribologicalproperties②：Rubber back ring to ensure tight sealing③：Backup ring to support the pressure
resistance

Absorbs the initial high pressure to reduce
the strain on the u-shaped packing. The
pressure resistance is enhanced by the use of
the backup ring in reinforced plastic.
（２）U-shaped packing
The main sealing component to prevent fluid
leakage. The pressure resistance is enhancedby the use of the backup ring in reinforcedplastic.

（５）Wear ring
Functions as a bearing when the piston is inmotion. Typically made of cloth-reinforced
phenolic plastics.

（３）Dust seal
（６）Sliding ring

Spring 2017

This packing prevents the infiltration of external
contaminants such as sand and gravel. It also
helps to ensure the prevention of minor leakage
from the u-shaped packing.

Prevents the contaminants in the oil frominfiltrating the piston seal. Typically made of
Polytetrafluoroethylene (PTFE).

２-2）Structure of Seal System in the Piston Segment

resistance）acts on the contact surface. The stick-slip

The packing used in the piston segment retains

phenomenon is when both stick and slip conditions

hydraulic pressure and provides appropriate thrustoccur instantaneously and cyclically on the contact

and load-retention ability to move the cylinder.2）Figure

surface.２,３）

3 illustrated the structure of the seal system in theThis phenomenon occurs in hydraulic cylinders mainly

piston segment. Table 2 showed the component parts.

at low speed, and sometimes causes troubles such as
noise and oscillation. This noise is called allophone.２,４）⑷ Piston seal
⑹ Sliding ring⑸ Wear ring

4. Evaluation of Allophone

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We have developed a seal system for high-pressure
cylinders. We mounted actual equipment in a cylinderof a hydraulic shovel to conduct an operation test ofthecylinder.

Asaresult,allophone （stick-slipphenomenon） occurred in the cylinder when theshovel was operated at low speed and when the oil
Figure3

Structure of seal system in the piston segment

temperature was high. However, we did not confirm
allophone at the lab evaluation during development.

3. Allophone（Stick-slip Phenomenon）

This meant that the operating conditions of the actual
equipment had not been recreated, and revealed thedifficulty of evaluating allophone.

Static and kinetic frictional forces act on the contact
surface between the packing and the opposing face;

4-1）Lab Evaluation

the two forces are closely interrelated. Sticking is theWe conducted lab evaluation（stick-slip test）of a sealcondition in which static frictional force （startingsystemresistance）acts on the contact surface, and slipping isdevelopment stage under the conditions shown inthe condition in which kinetic frictional force（sliding

Table 3. The test results showed no allophone or stick-39forhigh-pressure

cylinders

duringtheTechnical Papers

slip phenomena.
It is difficult to conduct a quantitative assessment of
the stick-slip phenomenon; we need to precisely

5. Relationship between Stick
Slip and Oil Film on Packing

replicate the usage conditions and environment inPoor lubrication and frictional resistance at the contact

order to fully understand the characteristics of thesurface affect the stick-slip phenomenon on packing.

packing, and then to plan and conduct a test. However,That is, periodical and recurring cycles of sticking andin the lab evaluation, the operating conditions including

slipping at a contact surface cause the phenomenon. In

the cylinder’s transverse load and high oil temperature

the sticking condition of the cycle, the packing deforms

had not been replicated.

itself to remain in the original position, and tries to４）

Table3Oiltemperature

return to its original shape when it is forcefully slipped

Conditions for stick-slip test

Parameterby an external force, etc. The stick-slip phenomenon ofTesting conditions
Ambient temperature (temperature without
artificial control: approximately 40°C)

the packing is affected by various factors including

Velocity

0.01 m/sec

temperature, speed, pressure, surface roughness,

Weight25 kg

viscosity of hydraulic oil, and oiliness. Although theseLubricantVerification
methodHydraulic lubricant (kinetic viscosity at 40°C:46.0 mm2/sec)
Verification is based on the wave form depictedin the accelerometer measuring the up/down
movements of the cylinder.

factors have not been quantitatively investigated,qualitatively, the low viscosity of hydraulic oil oftencauses rupture of the lubricating film when equipment
is operated at low speed under high pressure andAccelerometer
WeightTo alleviate the stick-slip phenomenon of a packing,the overall frictional resistance is sometimes lowered.

Laser displacement sensor

One way to do this is to improve the retention of oil
film between the opposing face and sliding surface.２）

Spring 2017

when insufficient lubricant is applied.５）

Atmosphere
Up/down motion5-1）Formation of Oil Film on Packing
Regarding retention of an appropriate oil film on thesliding surface of a common reciprocating packing, thepressure gradient of the pressure pattern on the sideof the fluid inlet and the same pressure gradient on theCylinderside of atmosphere are closely interrelated.６）Absorption
Figure 4

or scraping of the oil film may occur depending on theEquipment for stick-slip test

angle of the pressure gradient; this absorption and加速度(G)Acceleration
rate (G)0.04

scraping affects the thickness of the oil film and sealing
properties.

0.02

In the process of developing a high-pressure rod seal（U-packing）, we focused on the shape of the packing0on the heel side and the pressure gradient of pressure-0.02

patterns in the heel segment. We verified that a seal
with excellent absorbability could be developed by-0.04050
Sliding摺動距離(mm)distance (mm)Figure5

Results of stick-slip test100

setting both the shape and the pressure gradient tooptimal values.１）

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No.32

Causes of and Countermeasures for Allophone Trouble in a Piston Seal System for Cylinders5-2）Application of Forming Method of Oil Filmto a Piston Seal System（FEA）. As a result, greater peak contact pressure
and a sharper pressure gradient were confirmed in theIn the cylinder in which allophone occurred, the rod

C-chamfer at both ends.

seal system of the cylinder used a U-packing developed

It was found that the developed U-packing had functions

by Valqua, and an appropriate oil film was retainedof both scraping and absorbing the oil film. As abetween the opposing face and sliding surface. Taking

countermeasure against problems associated with thethese conditions into consideration, we assumed that

C-chamfer, the U-packing was applied to the system toallophone occurred in the piston seal system.

make use of the two functions. Table4 shows the results

Lipof the analysis and investigation of the countermeasure.

Heel

Innerdiameter
sideChamfer surface[Fluid side]
Surfacepressure
distribution

The absorbability of the U-packing was used in shaping
both ends of the wear ring into an R-shape to form anappropriate oil film on the sliding surface. No allophone

Peakwas confirmed in an allophone evaluation of actual[Atmosphericside]

Based on this result, we consider that an appropriate

equipment in which this wear ring was provided.
oil film was formed.

[Fluid side]
Lip

InnerdiametersideSurfacepressure
distribution

Heel

Spring 2017

Table4[Atmosphericside]

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 esults of analysis of wear ring and investigation
Ron countermeasure

Parameter

Chamfering (edge)
Chamfer surface

and when transverse load was applied to the piston
seal system. When transverse load was applied to a

Round

Chamfersurface
Localized surfacepressure peaks appear in
the chamfer surface ofthe edges.

The results of allophone evaluation of actual equipment
viscosity of the oil was low due to high oil temperature

Alternative productevaluation results

Edgeform

Figure6 T echnique to absorb oil film used in thedeveloped productsuggested that allophone occurred both when theAnalysis outcomes

Surfacepressure
distributionof sliding
surface

Reduces the pressure
gradient by applying
rounded chamfering on
the edges.
Moderatepressure gradient

Fluid pressure
peak[Fluid side][Fluid side]

shaft bearing, wear ring of the shaft bearing wasstrongly pushed to the opposing face, and then thecontact pressure reached its maximum value. As a

[Piston seal side]

result, the pressure gradient of pressure patterns ispresumed to have become sharper, too. Therefore, weconsider that allophone was caused by higher frictional
resistance due to scraping of the oil film by the wearring.
To verify this hypothesis, the pressure pattern of thewear ring was confirmed by finite element analysis
41

Material
properties[Piston seal side]

Glass-fiber reinforced nylon. Based on the cost factor,
It is stronger and more cost the raw material will noteffective than conventional be replaced.
materials such as PTFE andcloth-reinforced
phenolic
plastics. It is inferior to PTFE
in its frictional characteristics.
Thus, some products havethe sliding surface treatedwith knurling.２）

Technical Papers

6. Allophone Evaluation of
Cylinders of Actual Equipment

cylinders.
To retain an appropriate oil film, the functions ofscraping and absorbing the oil film are essential. By
understanding these functions, this technique can be

hydraulictheapplied to other products to reduce allophone andcountermeasure described above. Figure7 showed thefrictional resistance. However, we still do not have afrequency of the allophone sound. No allophone wave

quantitative

shape was confirmed in the evaluation after taking theappropriate thickness of oil film for each product, andcountermeasure.

other factors, and many challenges remain. Therefore,shovel

beforeandaftertaking

The waveform of the squeak is identified in the diagram below.

understandingofusage

conditions,further product development is needed for verification.

8. References

Frequency

１）Yamashita, J., Valqua Technology News No. 30, 2016,
Time
The waveform of the squeak is not identified in the diagram below.

pp. 3–8２）Ando, T., Hydraulics & Pneumatics, May 2003, pp.
28–33３）Nakano, K., Journal of the Society of Rubber Science

Frequency

and Technology, Vol. 80, 4th issue, 2007, pp. 134–139
4）Takamure, T., Tsuji, K., Muramatsu, A., and Yoshida,Time

Figure7

Allophone evaluation of cylinders

7. Conclusion

T., Hydraulics & Pneumatics, May 2003, pp. 34–37
5）Akai, H., Hydraulics & Pneumatics, May 2003, pp.
13–20

Spring 2017

After countermeasure application Before countermeasure application

An allophone evaluation was conducted with an actual6）Kaneda, S., Journal of the Society of Rubber Scienceand Technology, Vol. 72, 4th issue, 1999, pp. 194–201

This study verified that retention of an appropriate oilfilm alleviated allophone in packings for hydraulickenichi Takahashi
Corporate Research andDevelopment Group
Development Division

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