{"id":754,"date":"2026-06-03T02:05:14","date_gmt":"2026-06-03T02:05:14","guid":{"rendered":"https:\/\/planetary-gearboxes.com\/?p=754"},"modified":"2026-06-03T02:05:14","modified_gmt":"2026-06-03T02:05:14","slug":"planetary-gearbox-selection-industrial-robot-joint-j1-j6","status":"publish","type":"post","link":"https:\/\/planetary-gearboxes.com\/ja\/planetary-gearbox-selection-industrial-robot-joint-j1-j6\/","title":{"rendered":"\u7523\u696d\u7528\u30ed\u30dc\u30c3\u30c8\u95a2\u7bc0J1\uff5eJ6\u7528\u904a\u661f\u6b6f\u8eca\u6a5f\u69cb\u306e\u9078\u5b9a"},"content":{"rendered":"
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Korea Ever-Power<\/span>
\nRobotics Application Guide<\/span><\/div>\n

Planetary Gearbox Selection for Industrial Robot Joints J1 to J6 \u2014 Why Every Axis Needs a Different Specification<\/h1>\n

With 542,076 industrial robots installed worldwide in 2024 \u2014 the second-highest annual figure in history \u2014 Korean OEM manufacturers are under intense pressure to specify servo gearboxes correctly the first time. A single incorrect joint specification on a 6-axis robot means either early bearing failure on an underspecified unit, or unnecessary cost and inertia penalty from an overspecified one. This guide provides the axis-by-axis framework.<\/p>\n

Get Joint-by-Joint Selection Support \u2192<\/a><\/p>\n<\/div>\n<\/div>\n<\/section>\n

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Why One Planetary Gearbox Series Cannot Serve All Six Robot Joints<\/h2>\n

The six axes of a standard industrial robot differ not just in torque requirement \u2014 they differ fundamentally in what physical property of the gearbox matters most. J1 and J2 are dominated by inertia and torsional stiffness requirements that standard precision planetary gearboxes cannot adequately address at their torque class. J3 is a torque-and-efficiency balance problem. J4 and J5 are primarily a packaging problem where axial depth determines whether the robot wrist stays within its target envelope. J6 is a speed-and-mass minimisation problem.<\/p>\n

Applying the same gearbox series across all six joints \u2014 a common shortcut in early-stage robot design \u2014 results in some joints being overspecified (heavy, expensive, high inertia) and others being underspecified (insufficient stiffness or axial load capacity). The correct approach is to treat each joint as an independent selection problem, resolved in sequence from J1 outward.<\/p>\n

\n\n\n\n\n\n\n\n\n\n\n
Joint<\/th>\nPrimary Design Driver<\/th>\nTypical Torque Range<\/th>\nTypical Ratio<\/th>\nIP Requirement<\/th>\nRecommended EP Series<\/th>\n<\/tr>\n<\/thead>\n
J1 \u2014 Waist<\/td>\n\u306d\u3058\u308a\u525b\u6027
\nInertia always >5:1<\/span><\/td>\n		800\u20133,000+ N\u00b7m<\/td>\n20:1 \u2013 40:1<\/td>\nIP65 preferred<\/td>\nEP-ZDS-142\/190<\/a><\/td>\n<\/tr>\n
J2 \u2014 Large Arm<\/td>\nTorque + Stiffness
\nPeak gravity torque<\/span><\/td>\n		600\u20132,000+ N\u00b7m<\/td>\n16:1 \u2013 25:1<\/td>\nIP65 preferred<\/td>\nEP-ZDS-115\/142<\/a><\/td>\n<\/tr>\n
J3 \u2014 Small Arm<\/td>\nTorque + efficiency<\/td>\n250\u2013800 N\u00b7m<\/td>\n10:1 \u2013 20:1<\/td>\nIP54<\/td>\nEP-ZDS-115<\/strong> or EP-ZDE-160<\/a><\/td>\n<\/tr>\n
J4 \u2014 Wrist Roll<\/td>\nAxial depth (compact)<\/td>\n20\u201380 N\u00b7m<\/td>\n8:1 \u2013 16:1<\/td>\nIP54<\/td>\nEP-ZDWE-80<\/a> or EP-ZDE-80<\/td>\n<\/tr>\n
J5 \u2014 Wrist Bend<\/td>\nAxial depth (compact)<\/td>\n15\u201360 N\u00b7m<\/td>\n8:1 \u2013 16:1<\/td>\nIP54<\/td>\nEP-ZDWE-60\/80<\/a><\/td>\n<\/tr>\n
J6 \u2014 Tool Rotation<\/td>\nMass minimisation<\/td>\n5\u201320 N\u00b7m<\/td>\n3:1 \u2013 8:1<\/td>\nIP54<\/td>\nEP-ZDE-60<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/section>\n

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\"Precision<\/p>\n
Industrial robot arm joints require different planetary gear reducer specifications at each axis \u2014 from high-stiffness IP65 units at J1\/J2 to compact right-angle input units at J4\/J5. View EP series planetary gearbox \u2192<\/a><\/div>\n<\/div>\n

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J1 and J2 \u2014 Why Torsional Stiffness Matters More Than Backlash<\/h2>\n

J1 (waist rotation) and J2 (large arm) are the most demanding joints in any 6-axis robot. At J1, the entire robot body plus maximum payload rotates about the base. At J2, the combined weight of the forearm, wrist, and payload acts at maximum moment arm when the arm is fully extended horizontally. Both joints have one defining characteristic: their load inertia structurally exceeds the servo motor rotor inertia by 10\u201335\u00d7 even at gear ratios of 20:1.<\/p>\n

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Why J1\/J2 Always Exceed the 3:1 Inertia Ratio \u2014 and What That Means<\/div>\n

For a 100 kg payload robot, the effective load inertia at J1 is approximately 540 kg\u00b7m\u00b2 \u2014 the entire robot body and payload rotating about the base. A large servo motor for this class has rotor inertia J_motor \u2248 0.15 kg\u00b7m\u00b2. At 20:1 gear ratio: J_reflected = 540\/20\u00b2 = 1.35 kg\u00b7m\u00b2<\/strong>, giving an inertia ratio of 1.35\/0.15 = 9:1<\/strong> \u2014 well above the “safe” 3:1 target. At J2 with 20:1 ratio, the ratio improves to approximately 2:1, making 20:1 the preferred ratio for J2.<\/p>\n

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J1 inertia ratio at 20:1: 1.35 \/ 0.15 = 9.0:1 \u2190 always high for waist axis<\/div>\n
J2 inertia ratio at 16:1: 0.38 \/ 0.12 = 3.2:1 \u26a0\ufe0f marginal \u2014 use 20:1<\/div>\n
J2 inertia ratio at 20:1: 0.24 \/ 0.12 = 2.0:1 \u2705 ideal<\/div>\n
J3 inertia ratio at 16:1: 0.09 \/ 0.05 = 1.7:1 \u2705 ideal<\/div>\n<\/div>\n<\/div>\n

The Engineering Solution: Torsional Stiffness Raises the Resonant Frequency<\/h3>\n

When inertia ratio exceeds 3:1, the standard approach \u2014 increasing servo Kv gain \u2014 excites the drivetrain’s mechanical resonant frequency. For J1 and J2, this resonant frequency must be pushed above the servo control bandwidth (typically 50\u2013100 Hz for robot joint controllers) to prevent oscillation. The resonant frequency of the load-gearbox system is:<\/p>\n

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f_resonant = (1\/2\u03c0) \u00d7 \u221a(Ct_output \/ J_load_output)<\/div>\n
where Ct_output = torsional stiffness at output shaft [N\u00b7m\/rad]; J_load_output = load inertia [kg\u00b7m\u00b2]<\/div>\n
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EP-ZDE-160 (Ct=38 N\u00b7m\/arcmin \u2192 130,000 N\u00b7m\/rad): f_resonant \u2248 2.5 Hz<\/strong> at J2 \u2014 below servo BW \u2192 oscillation risk<\/div>\n
EP-ZDS-115 (Ct=20 N\u00b7m\/arcmin \u2192 68,755 N\u00b7m\/rad): f_resonant \u2248 4.2 Hz<\/strong> at J2<\/div>\n
EP-ZDS-142 (Ct=44 N\u00b7m\/arcmin \u2192 151,260 N\u00b7m\/rad): f_resonant \u2248 6.3 Hz<\/strong> at J2 \u2014 manageable range<\/div>\n<\/div>\n
1 arcmin = \u03c0\/(60\u00d7180) rad \u2248 0.000291 rad. Ct[N\u00b7m\/rad] = Ct[N\u00b7m\/arcmin] \/ 0.000291.<\/div>\n<\/div>\n

This calculation explains why robot OEMs historically used strain wave gearboxes (zero-backlash, extremely high stiffness) for J1 and J2, and why the EP-ZDS high-stiffness series \u2014 with torsional stiffness up to 130 N\u00b7m\/arcmin and 28,000 N axial capacity \u2014 is the relevant EP series for these joints rather than the standard EP-ZDE. The backlash specification (<8 arcmin for EP-ZDS) is secondary to the Ct value at this axis.<\/p>\n

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J1 specification checklist<\/div>\n