KYB TECHNICAL REVIEW No. 67 OCT. 2023

ProductIntroduction

Development of KCH-8 High Pressure Cylinder for
Construction Machinery
KOBAYASHI Toshio

1Introduction

KYB manufactures and sells cylinders for construction
machinery including hydraulic excavators as its main
products. Among these, hydraulic excavators of an
operating mass of the 20 to 40 ton class are high indemand and can be often seen in various places
throughout the world from urban construction sites tomines. KYB has continued manufacturing cylinders forhydraulic excavators of the class stated above (hereinafter"KCH") for about 50 years. However, the company is
now exposed to a very competitive environment due tothe globalization of hydraulic excavator manufacturers,
and the emergence of competitive cylinder products.
Particularly in terms of price competition, it is difficult
for us to find a competitive edge.
To improve this disadvantageous situation, we havedeveloped a new model (KCH-8: Kayaba Cylinder Highpressure model-8) that is not bound by the existing
structure. This paper introduces the existing product wefocused on this time, its technical challenges, and thenewly developed product.

2

Piston rod

Seal

2. 1 What is a Hydraulic Cylinder?
A hydraulic cylinder uses hydraulic fluid supplied by apump as a means to apply a pressure to its pressure area,
which will thereby generate a thrust. A hydraulicexcavator uses the thrust and expansion/contraction of its
hydraulic cylinders linked to a bucket to accomplish
various tasks including digging, transport, and lifting ofheavy soil (Fig. 1).
Arm cylinder
Bucket cylinder
Boom cylinder

Example of use of hydraulic cylinders in construction machinery

Cylinder headCylinder tubeCushion

PistonBearing

Fig. 2
Table 1

Component
Cylinder tube
Piston rod
PistonSeals
Bearings
Cushion

Cylinder modelComponents and typical functions of a hydrauliccylinder

Cylinder head

Hydraulic Cylinders

Fig. 1

2. 2 Major Cylinder Components
A hydraulic cylinder consists of less than 50 kinds ofparts, which is fewer than other hydraulic components(such as pumps and valves). The mechanical structure ofa hydraulic cylinder is rather simple. Although a detailed
introduction of cylinder parts is omitted here, Fig. 2
shows an overview of a hydraulic cylinder and Table 1
shows its typical functions.

Typical function
Structure, sealing
Structure, sliding
Structure, sealing, bearing
retention
Structure, bearing retention
Sealing, pressure resistance
Structure, sliding
Buffering

2. 3 Characteristics of Hydraulic Cylinders
Hydraulic cylinders used in a hydraulic excavator play
the twin roles of performing the required exterior workand accommodating the counterforces and external forces
applied to the excavator. They can deliver various
functions related to workability/operability of thehydraulic excavator including sealing of hydraulic fluidfor ensuring thrust generation, slidability for smooth
expansion/contraction, and buffering with cushions at thestroke ends. At the same time, many of the cylinder
components serve as structural members to bear thepressure of hydraulic fluid, tensile/compressive loads,bending, buckling, and vibration as well as holding thestructure themselves.

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Development of KCH-8 High Pressure Cylinder for Construction Machinery

The major dimensions of cylinders are decided by thethrust and stroke required by the hydraulic excavator.
Since many cylinder products are long and narrow, theircomponents are required to be long in the direction of thecylinder axis.

3

required strength (requirements). The results are shownin Table 2.

Development Challenges andImprovement Points

3. 1 Clarification of Challenges
KCH is exposed to severe price competition as statedin the Introduction. Cost reduction is the top priority. To
clarify the challenges, we conducted a cost analysis on
the cylinder. The analysis revealed that the material costof cylinder tubes accounted for the largest proportion of
the cylinder cost (piston rod material is the secondlargest). These long parts are dominant cost contributors(Fig. 3). Therefore, it can be expected that thinning thewalls of these long parts will substantially reduce thecylinder cost.

Fig. 3

Cylinder cost contributors

3. 2 P roperties and Technical Hurdles of
Cylinder Tubes
Examples of hydraulic cylinders used in a hydraulicexcavator (Fig. 1) include boom, arm, and bucket
cylinders. Since the way in which cylinders are used
depends on where they are installed, the load applied tothe cylinders naturally depends on their location.
Therefore, the required strength varies by cylinder, socylinders are specifically designed to attain their own
relevant strength. Fig. 4 shows the cylinder tube
assembly, which is one of the components of theh y d ra u l i c c y l i n d e r, a n d i t s c o n f i g u r a t i o n . O u rconventional cylinder tubes are designed to have anappropriate wall thickness selected from up to four
thickness levels, in order to attain an optimal design for
the required strength.
The wall thickness of the cylinder tube can be reduced
as long as it has adequate strength. We analyzed the rateof change in various properties of a cylinder tube with awall thickness 20% less than the conventional thickness
and determined whether the cylinder tube met theFig. 4
Table 2

Existing cylinder tube assembly model

Major mechanical properties of cylinder tube withthinner wall

Property
Endurance strength
Strength against
external force
Compressive strength
Pressure deformation
Rigidity

Rate of change-57.0%

Requirements
Not acceptable-24.7%

OK-18.5%-16.0%-4.3%

OKOKOK

Among the various properties, the endurance strength
decreased at the highest rate. The analysis implies that
simply thinning the wall of the cylinder tube will notmeet the requirements. Particularly, the cylinder bottomweld (Fig. 4) is the weakest point, posing a strength
bottleneck. You might question why thinning the cylinder
tube wall should affect the endurance strength of thecylinder bottom weld. However, endurance strength isinversely proportional to stress concentration. The
endurance strength of the cylinder bottom weld is
proportional to the wall thickness of the cylinder tube. Astrength analysis (using the Finite Element Method;
hereinafter the "FEM") showed, through a trial calculation,that thinning the cylinder tube wall would increase thestress in the weld and lower the endurance strength.
It should be noted here that the cylinder bottom weld
is a welded joint between the cylinder tube and thecylinder bottom and has a dramatic change in geometry
in contrast to the straight cylinder tube. Due to thisgeometry change, stresses are concentrated in the weld.
Moreover, the weld, which is directly connected to thecylinder tube, has a higher mean stress in proportion to
the lower wall thickness of the cylinder tube. Thus, thealready stress-concentrated weld has an even higher stress.
In the light of these characteristics of the cylinderbottom weld, we thought that, if the stress concentrationin the cylinder bottom weld was suppressed, a cylinder
tube with a thinner wall could maintain a conventional
endurance strength, in spite of the increased mean stress ofthe cylinder bottom weld caused by thinning of the tube wall.
Then, we set a challenge of "thinning the cylinder tube
wall by improving the endurance strength of the cylinderbottom weld" and decided to work on the improvementof the weld.
3.3 Conventional Cylinder Bottom WeldStructure and its Characteristics
Fig. 5 shows various cylinder bottom variants (four― 28 ―

KYB TECHNICAL REVIEW No. 67 OCT. 2023

geometries) that have been used so far. These geometries
have been developed to ensure the necessary functions of
the cylinder bottom weld, including pressure resistance,endurance, and coaxiality of the cylinder tube and thecylinder bottom.
The cylinder bottom weld geometry before thedevelopment of KCH-8 was Type 4.
Type 1: The initial geometry of the hydraulic cylinder
tube assembly. Type 1 features the simplest
design, lower number of parts, and a rather good
production yield. But the endurance strength is
the worst among the four types.
Type 2: Compared to Type 1, the weld is farther awayfrom the cylinder bottom to reduce the stressconcentration, delivering better endurance. Type2 consists of the same number of parts as Type 1,
but shows a lower production yield.
Type 3: Designed to reduce the stress concentration inthe cylinder bottom weld by considering
deformation due to hydraulic pressure applied tothe cylinder bottom section. Type 3 shows better
endurance than Type 2. Uses the same number ofparts, but involves complex machining to cut acircular groove in the bore of the cylinder bottom.
Type 4: To further reduce the stress concentration due tohydraulic pressure applied to the cylinderbottom, a three-piece structure has beenintroduced. Type 4 is constructed to weld thecylinder bottom to the cylinder tube using a collar(a part equivalent to a back board). Type 4 delivers
the best endurance among the conventional types.
The production yield of the cylinder bottom
section has been improved from Type 3.
Cylindertube

CylinderbottomCylindertube

CylinderbottomCylindertube

CylinderbottomCylindertube

Cylinderbottomconfigurations were analyzed, including different wall
thicknesses of the cylinder tube, the presence/absence ofa collar, and different machining geometries of thecylinder bottom section. From the results of the analysis,
we predicted that forming a groove-like cavity inside thebore of the cylinder bottom weld section would enable
the stress concentration to be suppressed without
modifying the conventional configuration. We thoughtthat the presence of the groove would allow the axial
load to be evenly distributed to the structure, and notintensely applied to the stress concentrated part, thereby
enabling improvement of the endurance strength (Fig. 7)
4. 2 Confirmation of EffectivenessThe geometry variants of the cylinder bottom weld
subjected to the stress analysis were narrowed down to
six good candidates, which were further subjected toendurance tests. We checked whether the candidates
exceeded the endurance strength requirements andwhether they showed the endurance strength as analyzed.
As a result, assuming that the endurance test result of theconventional cylinder tube was 1.0, the finally selected
geometry variant showed about 1.1 even with a reduced
wall thickness. Thus, we confirmed a higher thanconventional endurance strength for the selected variant.
Also, an endurance test on a cylinder tube assembly withconventional tube wall thickness with only the cylinderbottom geometry modified showed an endurance strength
twice that of the conventional counterpart, proving thatthe geometry change adequately improved the strength(Fig. 8).
In addition to endurance, machinability and workability
were added to the test evaluation items to build ageometry variant that also considered productivity.

CollarFig. 5

4

Bottom weld geometries that have been used so farFig. 6

Stress distribution around the weld

How to Improve Weld Endurance

4. 1 Structural Study
We studied how to improve the endurance of thecylinder bottom weld by modifying the geometry of theweld section. The farther the weld is from the cylinderbottom, the lower the stress. This is a typical stress
reduction measure, but unavoidably extends the total
length of the structure, posing a configuration
disadvantage. Then, we conducted an FEM analysis tolook for an optimal geometry (Fig. 6). Various― 29 ―A groove is formed in the bore to alleviate
stress concentration in the part indicated by thecircle.

Fig. 7

Geometry of bottom weld of newly developed
KCH-8

Development of KCH-8 High Pressure Cylinder for Construction Machinery

geometry and successfully improved the endurancestrength of the cylinder tube assembly even with thethinner tube wall. I am pleased with this achievement.
Based on the outcome of this development project, Iwill continue my efforts as part of the team developing
new hydraulic cylinders.
Finally, I would like to take this opportunity tocordially thank all those concerned from the construction
machinery manufacturers, related corporate partners, andrelated KYB departments who extended cooperation tothe development of this product.
Fig. 8Comparison of endurance strengthReferences51) TAKAI: "Changes in Cylinder for Hydraulic Excavators", KYB
Technical Review No.50 (April 2015)

Concluding Remarks

In this development project, we were told by ourpredecessors that this type of product is difficult toimprove. However, we have kept the conventional2) FUNATO: "Inverse Type Hydraulic Cylinder for Medium Class
Excavators", KYB Technical Review No.57 (October 2018)3) H ARA: "Memory, Foundation of KCH", KYB Technical
Review No.56 (April 2018)

Author 

KOBAYASHI Toshio
Joined the company in 2007.
Experiment Sect. No. 2,
Developmental Experiment Dept.,
Hydraulic Components Operations
Used to be engaged in thisdevelopment in the former Design
Sect., Gifu South HydraulicEngineering Dept., HC Operations― 30 ―