論文 / 著書情報
Article / Book Information
Title

Measurement of Creep Deformation of Resin Structural Parts for a
Lightweight Industrial Robot

Authors

Yuta Tsukamoto, Kenji Sekiguchi, Hiroyuki Nabae, Gen Endo

Citation

Proceedings of the 2024 IEEE/SICE International Symposium on
System Integration, , , pp. 592-597

Pub. date

2024, 1

Copyright(c) 2024 IEEE. Personal use of this material is permitted. Permission
from IEEE must be obtained for all other uses, in any current or future
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DOI

http://dx.doi.org/10.1109/SII58957.2024.10417463

Note

This file is author (final) version.

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Measurement of Creep Deformation of Resin Structural Parts
for a Lightweight Industrial Robot*
Yuta Tsukamoto1 , Kenji Sekiguchi1 , Hiroyuki Nabae1 , and Gen Endo1
Abstract— Lightweight industrial robots are increasingly required to reduce power consumption. In this research, we focus
on lightweight resin materials such as 3D printed parts, and ourultimate goal is to develop practical industrial robots using thesematerials as structural parts. Unlike conventional light metalmaterials such as high-strength aluminum, resin materials havecreep deformation properties even at room temperature, which
deteriorates the end-point position accuracy in practical use
over a long period. In this paper, we fabricated an experimentapparatus that can measure creep deformation by applying
40 kgf of load at the end of a beam specimen. A laser
displacement meter accurately measured the creep deformation
for approximately two months (1600 hours). For comparison,
we measured the creep deformation of six specimens of ﬁve different lightweight resin materials and conventional aluminumalloys (A5052). The results showed that 3D printed materials
had large creep deformation, whereas A5052 and FELCARBO
showed almost no creep deformation.

I. INTRODUCTION
Industrie 4.0, has led to introduction of industrial robots to
production lines to improve factory productivity. Reducing
the energy consumption of industrial robots is a crucial
goal for achieving sustainable development and reducing
production costs. One of the most straightforward methods to reduce energy consumption is to reduce the weightof industrial robots. In particular, weight reduction in the
structural parts of industrial robots can reduce the actuator
power consumption in operation. Furthermore, the weight
reduction in the structural parts is an effective method
because it permits the robot to use a smaller actuator thateventually reduces the weight of the actuator and the total
weight of the industrial robot. The authors’ research groupis conducting fundamental research to reduce the weightof conventional industrial robots by 25% by applying newlightweight materials for structural parts and reduction gears.
One of the essential design items for the structural parts ofindustrial robots includes durability against generated stressand vibration that occur during use. The generated stressand deformation should be predicted based on stress analysis,
and the appropriate material and shape should be determinedbased on the design speciﬁcations, such as the workspaceand payload capacity. Then, structural modiﬁcations basedon vibration analysis are applied to avoid deterioration ofcontrollability and damage due to resonance. There are many
*This research is subsidized by New Energy and Industrial Technology
Development Organization (NEDO) under a project JPNP20016.
1 All authors are with the Department of Mechanical Engineering,Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo
152-8550, Japan, {tsukamoto.y.ak, sekiguchi.k.ai,

nabae.h.aa, endo.g.aa}@m.titech.ac.jp

examples of research on structural optimization throughdesign based on the above analysis [1][2].
However, this design method is applicable in the caseof metal materials because the metal materials can be approximated by a linear material, and the stress and vibration
analysis are sufﬁciently accurate. On the other hand, a resinmaterial is a non-linear material. Moreover, its mechanical property also depends on the processing methods. For
example, the mechanical properties of a resin specimen
manufactured by machining process from bulk material
would be different from a resin specimen manufactured by
a 3D printing process, which is represented by the Fused
Filament Fabrication (FFF) method, due to lamination modeling. Therefore, it is necessary to experimentally accumulate
the fundamental mechanical properties of a resin material,considering a realistic use case.
In our previous works, we fabricated standard specimens
with different materials that simpliﬁed the structural part ofthe link of an industrial robot and one degree-of-freedom
experimental apparatus with a harmonic drive actuator unit.
We measured vibration damping property with the specimens[4] and compared the stiffness of the specimens as a manipulator structure [5]. We also have proposed a new fastening
method to connect the specimens to a rigid metal part [6].
In this paper, we particularly focus on the creep deformation of the specimens. It is generally known that resinmaterials gradually deform when a static load is applied for along time, even at room temperature. This creep deformation
changes the relative position and orientation of the joints ofthe manipulator and degrades the accuracy of the end-point
position control. Therefore, measuring creep deformation isessential to evaluate the applicability of resin materials for
an industrial robot.
Although many studies have been conducted on creep
testing of standard specimens of materials such as ABS and
PLA, commonly used for 3D printing [7][8]. However, thestandard specimen is very small, and there is no guaranteethat the result is scalable and directly applicable to the largesized industrial robot. Therefore we assume a more realistic
link length and load condition.
This paper aims to fabricate a large-scale experimentapparatus with laser displacement sensors and measure creep
deformation when 40 kgf of the load is applied to the endof the specimen. The six specimens with different materials
are measured simultaneously, and obtain creep deformation
data for two months (67days, 1600 hours). We quantitatively
compare and discuss the ﬁndings.

(a) A5052(b) FELCARBO(c) MC901(e) Onyx(f) Onyx+CF

292 mm
86 mm

P.C.D. 77(d) POTICON

Fig. 1: Shape of the specimen. (a) A5052 is the aluminum alloy. (b) FELCARBO and (c) MC901 are resin parts
machined from bulk material. (d) Onyx, (e) Onyx + CF, and (f) POTICON are 3D printed parts.
TABLE I: Weight of links (the metal sleeves for connection
shown in Fig. 1 are not included).
Link
1
2
3
4
5
6

Material
A5052
FELCARBO
MC901
POTICON
Onyx
Onyx+CF

Mass [g]3531.81756.11502.5
658.4
609.3
646.0

Processing method
Machining
Machining
Machining
3D printing
3D printing
3D printing

292 mmCost [yen]19,64057,66014,30051,35530,00031,900Specimen

Jigs

WeightII. S PECIMENS
Fig. 2: Load condition of the experiment.
III. E XPERIMENT A PPARATUS
The specimens are shown in Fig. 1 and their masses,
processing methods and costs are indicated in Table I. A
specimen is fabricated for each of the following six materials:
an aluminum alloy (A5052), which is a conventional material
used for industrial robots for comparison; two resin materials, FELCARBO (Futaba) and MC901; three 3D-printed
materials, Onyx (Markforged), Onyx with continuous carbon
ﬁber (CF) (Markforged), and POTICON (Otsuka Chemical).
MC901 is a nylon resin material widely used in engineering and is from bulk material via a cutting process.
FELCARBO is a resin material produced by laminating
layers of non-woven carbon ﬁbers, high-pressure molding,and cutting. Unlike conventional carbon ﬁber reinforced
plastics (CFRP), FELCARBO is a new lightweight material
with high machinability and mechanical properties similar toisotropic materials.
POTICON (NTL34M) is a nylon resin reinforced withpotassium titanate ﬁbers. The specimen is fabricated using
the Raise 3D Pro2 (Raise 3D) 3D printer. Onyx is a nylon
resin reinforced with short carbon ﬁbers, and Onyx + CF
is a nylon resin reinforced with continuous carbon ﬁbers.
Two parts are fabricated with these materials using X7(Markforged). The 3D printed parts were fabricated with an
inﬁll rate of 37%, a triangular ﬁll structure, eight solid layers,
and two walls. The nominal mechanical properties of theabove ﬁve materials and A5052 with comparative materials
are shown in Table. II.

A. Measurement Condition
The creep deformation of resin materials is greatly affectedby temperature and load. The temperature in the experimentchamber is set to 25◦ C, which is the room temperature used
for industrial robots. For the load condition, a weight witha mass of 42.08 kg is added at a distance of 292 mm fromthe ﬁxed part to the tip of the cantilever beam, as shownin Fig. 2. We derive this load condition by assuming that anindustrial robot whose maximum reach is 1000 mm picks up
a weight of 12.3 kg in a horizontal posture. Under these test
conditions, measurements were conducted simultaneously for
1600 hours on specimens of six materials.
B. Mechanical Condition
The ﬁxed end of the specimen is fastened to a ﬁxture jig of
SUS304 (shown in Fig. 2) using M4 screws with a fastening
torque of 3.6 Nm for the 16 equally-distributed through holes
in the specimen. For MC901 and the three 3D printed parts,
we apply our previously proposed fastening method [6]. The
diameter of the through-holes is 6.2 mm, and thin-walled
pipes made of SUS304 with an inner diameter of 4.0 mm
and wall thickness of 1.0 mm is inserted into each. A screw
is inserted inside the thin-walled pipe to fasten the parts.
This fastening method prevents the screws from sinking into
the resin part due to the high compressive force applied to
the weak resin material. For FELCARBO and A5052, thediameter of the through-holes is set to 4.2 mm and screws

TABLE II: Material properties. [9][10][11][12][13][14][15](A large industrial robot link is manufactured by iron casting. FC300 is a typical cast iron. PEEK is a typical highperformance engineering resin and has material property values comparable to nylon’s. It has superior performancewith signiﬁcantly less moisture absorption. However, its high heat resistance makes it difﬁcult to use as a 3D printerﬁlament. Both FC300 and PEEK are not used in the experiment and are shown only for comparison.)Material

FC300

A5052

FELCARBO

MC901

POTICON

Onyx

Onyx+CFPEEK

Density [g/cm3]

6.8-7.8

2.7

1.3

1.2

1.3

1.2

1.4

1.3

Tensile strength[MPa]

190-300

260

270

96

100

37

800

100

Tensile modulus
[GPa]

92.4

68

22

3.4

5.6

2.4

60

3.8

-

-

300

110

167

71

540

163

-

-

18

3.5

6.3

3.0

51

3.5

Bending
strength[MPa]
Bending
modulus[GPa]

HeaterFan

Warming chamber

Specimen

Warm airTest machine

Current temperature

Fig. 3: Experiment apparatus without laser displacement
sensors and the temperature control device.

are inserted through them to fasten the parts directly this wasdone because no deformation was observed when the parts
were fastened with a screw in the preliminary experiments.
Then, four weights are each adjusted to 10.52 kg using
stick-on weights. A collar is inserted into the weights’ center
hole to eliminate mechanical play between the weights’ innerdiameter and the specimen’s inner diameter. The 42.08 kg
load is assembled by connecting the collar to a shaft madeof SUS304. In addition, FINE U-NUT (Fuji Seimitsu) areinstalled on the ends of the shaft to restrain the weights inthe axial direction.
Figure 3 shows the six specimens installed on the experiment apparatus, with the load at the end of each cantileverbeam. In order to minimize the difference in test durationand specimen deformation during preparations, jacks are
installed to support the weights. These jacks are loweredat the beginning of the experiment and removed from theexperiment apparatus.

Fig. 4: Temperature control. Warm air is circulated byblowing on a heater whose surface rises to 200◦ C.
C. Temperature Management
The temperature setting must be controlled because it
affects the creep deformation and can cause measurement
errors due to the temperature dependency of the laser displacement transducer used in this study. For this reason, a
simpliﬁed thermal insulation chamber, as shown in Fig. 3,
is installed around the experiment apparatus, and the air
conditioning system controls the temperature in the room
where the experiment apparatus is installed. The temperature
control equipment (shown in Fig. 4) is also installed within
the thermal insulation chamber, and temperature control isconducted by circulating warm air inside the chamber whilefeeding back the temperature obtained by thermocouples
installed in the center of the experiment apparatus.
D. Measurement System
We record time series data of the specimens’ displacements, temperature in the testing chamber, and temperatureof the surface of the laser displacement transducers. The
displacement of each specimen is measured using an LKH150 (KEYENCE) laser displacement meter and logged by
an LK-G5000 (KEYENCE). Displacement is measured at

TABLE III: Measured displacement drift for calibration.
Inner temperature measurement point
Height adjuster

Link 6

Link 5

Link 4
Link 3

Link 2

Link 1

Aluminum frame to be measured

Fig. 5: The conﬁgurations of the preliminary experiment. In
this experiment, the top of the aluminum frame is alignedwith the height of the measurement points in the main
experiment.

a chamfered point on the end face of the shaft used toconnect the weight to each specimen. The temperature inside
the chamber and the temperature of the surface of the laser
displacement transducer are measured using a thermocouple
installed in the center of the experiment apparatus and a thermocouple attached to the surface of the laser displacementtransducer, respectively.
IV. C ALIBRATION OF THE LASER DISPLACEMENT METER
As a preliminary experiment, an aluminum frame wasﬁxed across the apparatus instead of the specimens (shownin Fig. 5) so that the displacement would match the initialdisplacement during the main experiment. The displacementof the top surface of the aluminum frame, the temperatureinside the chamber, and the temperature of the surface ofthe laser displacement meter were acquired as time-series
data over the course of four days. Fig. 6 shows the timeseries changes in the temperature in the chamber and thetemperature of the surface of each laser displacement meter.
We conﬁrmed that the temperature ﬂuctuations in the chamber and on the surface of each laser displacement meter was
within 0.4◦ C and 1.7◦ C respectively, which was a sufﬁciently
negligible deviation. Fig. 7 shows the time-series variation of
the displacement obtained by each laser displacement meter.
We conﬁrmed that each displacement measurement drifted at
a constant rate as time passed. Therefore, the amount of drift
per unit of time (shown in Table III) was used to calibrate
each laser displacement meter.
V. M EASUREMENT P ROCEDURE
The measurement procedure is described below.
1) The test specimen is attached the test specimen to
the ﬁxture using M4 screws with fastening torque of
3.6 Nm. The ﬁxture is then attached to the main frameof the experiment apparatus.
2) The weights are attached to the tip of the specimen
while supporting it with a jack.

Link No.

Drift [μm/50hour]

1

6.0

2

4.5

3

3.8

4

4.7

5

2.1

6

6.5

3) The temperature control system is turned on andallowed to stabilize until the temperature inside thechamber at 25◦ C.
4) The laser displacement meter is turned on and its
reading allowed to stabilize over the course of 4 hours.
5) The measurement of thermometer and laser displacement meter are started simultaneously, the jacks supporting the weights are removed, and the measurementfor 1600 hours is started.
VI. R ESULTS
The righthand ﬁgure of Fig. 8 shows the measurement
results of the temperature inside the chamber and the temperature on the surface of the laser displacement meter. The
temperature deviations were approximately less than 1.5 ◦ C.
In the preliminary experiment, the temperature ﬂuctuation
suppressed less than 0.7 ◦ C; thus, the temperature changes
were much more signiﬁcant than we expected. This is dueto the difference in the outside temperature. The preliminary
experiment was conducted in May, and the outside temperature is close to the target temperature. On the other hand,
the main experiment was conducted from July to September,where the outside temperature is often 10 ◦ C higher than thetarget temperature. Although the ambient air temperature in
the experiment room was kept at approximately 17 ◦ C by the
air conditioner in our building, the temperature control device
developed in the experiment apparatus was affected by theoutside air conditioning performance because the experimentapparatus did not have a cooling function. There is a strong
correlation between the inner and surface temperature of thedisplacement meter. Thus, we must consider the displacement measurement errors due to temperature ﬂuctuations.
We observe irregular temperature changes marked by orange
areas numbered (1) and (3). These are due to the unexpected
power down of the air conditioning system in our building.
We also lost the measurement data in the light blue area (2)and (4) because of the instantaneous power failure of thelogging system. We observed small spikes in the light blue
area (5) because of an earthquake. Although the experimentencountered the troubles mentioned above, we successfully
obtained the primary trend of the creep deformation in thelong time range.
The left ﬁgure of Fig. 8 shows the displacement measurement results for 1600 hours. The baseline of each displace-

25

Link 5
Link 4

32.5

Link 3
Link 2
Link 632

Link 1

31.5
0

20

40

60

80

100

Temperature [°C]

Temperature [°C]

33

24.8
24.6
24.4
24.2

Inner temperature

24
23.8
0

20

40

Time [hour]

60

80

100

Time [hour]

0.17
0.16
20

40

60

80

100

-0.88
-0.89
0

20

Link 4

-0.74
-0.75
-0.76
0

20

40

60

80

Time [hour]

60

80

100

-1.18

Link 3

-1.19
-1.2
-1.21
0

20

Time [hour]

100

Displacement [mm]Displacement [mm]Time [hour]

40

Link 5

-1.34
-1.35
-1.36
0

20

40

60

40

60

80

100

Time [hour]

80

100

Displacement [mm]0Link 2-0.87

Displacement [mm]Link 1

0.18

Displacement [mm]Displacement [mm]Fig. 6: The temperature measurement result of the preliminary experiments.

-1.17

Link 6

-1.18
-1.19
-1.2
0

20

Time [hour]

40

60

80

100

Time [hour]Fig. 7: The displacement result of the preliminary experiments.

ment value was obtained from the measurement value at 5
minutes after removing the jack of the ﬁrst specimen because
removing all jacks took a few minutes.
Negligible small deformation was observed in the timeseries for A5052, which is consistent with the fact thatcreep deformation does not occur at room temperature. 1
Furthermore, the time-series deformation of FELCARBO isalso small. This indicates that FELCARBO does not havecreep deformation under the load and temperature in these
conditions, suggesting an excellent candidate to replace thealuminum material if we can afford to use it. (One of the
drawbacks of FELCARBO for industrial applications is thematerial cost and machining process shown in Table I.)
The deformation of MC901 was about 0.2 mm, but thedisplacement did not signiﬁcantly change over time after
500 hours, indicating that the creep deformation could be
suppressed to 0.2 mm. The deformation of the 3D printed
parts, Onyx+CF, POTICON, and Onyx, at 1600 hours was
0.7 mm, 1.7 mm and 3.8 mm, respectively. These values
were signiﬁcantly large compared to MC901, whereas the
1 The measured deformation is slightly positive at 1600 hours, which

is not physically correct. This is because of the calibration of the laser
displacement meter described in Section IV. The calibration was based on
the short period of preliminary measurement. Thus, the measurement results
shown here include approximately ±0.05 mm errors.

ﬁlaments of these 3D printed materials are made from thesame nylon resin material. This is due to the low in-ﬁll rateof 37 % of the 3D-printed specimens. In other words, the 40
kgf load was too large for the 3D printed parts with a 37 %in-ﬁll rate.
As for the comparison with or without the continuous ﬁberin Onyx, the continuous ﬁber suppresses the creep deformation. However, we still observe the creep deformation with
Onyx + CF that does not agree with the mechanical propertyof the CF in Table II. This is because of the improper
allocation of the CF. The CF is automatically allocated by
the slicing (modeling) software “Eiger” supported by Markforged, and the CF is placed on the front/back surface of thespecimen shown in Fig.1(c). These surfaces are perpendicular
to the load direction and did not contribute much to suppress
the creep deformation. The CF allocation in the top/bottom
surface of the specimen would signiﬁcantly decrease thecreep deformation.
VII. CONCLUSIONSIn this paper, an experiment apparatus that measures the
creep deformation under practical shape and load conditions
was fabricated for structural parts of industrial robots. The
apparatus measured the creep deformation of two machined
bulk resin materials, three 3D printed parts, and conventional

Fig. 8: The result of the creep deformation of the links. (left) the result of displacement, (right) the result of temperature.
aluminum alloy A5052. Almost no creep deformation wasobserved in FELCARBO under the test conditions of thetwo bulk materials. The creep deformation was observedin MC901, and the deformation settled after approximately
500 hours. The creep deformation continued in all 3Dprinted parts, which was signiﬁcant compared to A5052,
FELCARBO, and MC901.
As for future work, structural improvements are necessary
for 3D printed parts, such as modiﬁcation of the in-ﬁll rate,ﬁlling structure, optimum allocation of the reinforcementﬁbers, and the shape of the structural parts to suppress creep
deformation. This paper measured simple shape structure andsingle load condition. It is necessary to clarify the speciﬁcations of applicable industrial robots for each material in
the future by conducting measurements under various loading conditions with the more practical shape of structures.
Additionally, the problem of heat dissipation of the actuatoris also crucial for using thermoplastic materials in industrial
robots. Materials with high heat transfer coefﬁcients should
be considered in the future.
ACKNOWLEDGMENT
This research is subsidized by New Energy and Industrial
Technology Development Organization (NEDO) under aproject JPNP20016. This paper is one of the achievementsof joint research with and is jointly owned copyrighted material of ROBOT Industrial Basic Technology Collaborative
Innovation Partnership.
We thank Prof. Naoyuki Takesue (Tokyo Metropolitan
University), Prof. Yusuke Ohta (Chiba Institute of Technology), and Prof. Takeshi Takaki (Hiroshima University) for
their valuable comments and discussion.
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