{"id":256,"date":"2026-06-12T08:24:58","date_gmt":"2026-06-12T08:24:58","guid":{"rendered":"https:\/\/planetary-gearboxes.cn\/?p=256"},"modified":"2026-06-12T08:24:58","modified_gmt":"2026-06-12T08:24:58","slug":"how-to-select-planetary-gearbox-for-servo-motor","status":"publish","type":"post","link":"https:\/\/planetary-gearboxes.cn\/ko\/how-to-select-planetary-gearbox-for-servo-motor\/","title":{"rendered":"How to Select a Planetary Gearbox for Your Servo Motor \u2014 A Step-by-Step Engineering Guide"},"content":{"rendered":"
\n\"Precision<\/p>\n
\n
SELECTION GUIDE<\/div>\n

How to Select a Planetary Gearbox for Your Servo Motor<\/h1>\n

Motor power, target speed, torque requirement, backlash tolerance, and installation envelope \u2014 five parameters, six steps, one confirmed specification. This guide walks you from motor nameplate to gearbox model number with real calculation examples.<\/p>\n

\n
\n
6<\/div>\n
SELECTION STEPS<\/div>\n<\/div>\n
\n
3\u2013512<\/div>\n
RATIO RANGE<\/div>\n<\/div>\n
\n
\u22643\u2032<\/div>\n
BEST BACKLASH<\/div>\n<\/div>\n<\/div>\n

Skip to Engineering Consultation \u2192<\/a><\/p>\n<\/div>\n<\/div>\n

\n

<\/p>\n

Why Servo Motors Need a Planetary Gearbox<\/h2>\n
\n
\n

A servo motor excels at speed control and positioning accuracy, but it produces relatively low torque at high speed. Most servo-driven loads \u2014 conveyor rollers, robot joints, ball screws, rotary tables \u2014 require higher torque at lower speed than the motor delivers on its own. A planetary gearbox bridges this gap by trading speed for torque at efficiencies above 96%, while keeping the motor and load on the same axis in a compact package.<\/p>\n

Beyond torque multiplication, the planetary reducer also reduces the reflected load inertia seen by the servo amplifier. This improves the dynamic response of the entire motion system: the motor accelerates and decelerates faster, settling time shortens, and the servo loop remains stable under varying loads. The inertia reduction follows the square of the gear ratio \u2014 a 10:1 reducer cuts reflected inertia by a factor of 100, which is why selecting the right ratio is critical for achieving responsive servo control.<\/p>\n

Selecting the wrong planetary gearbox \u2014 whether undersized, oversized, or mismatched on backlash \u2014 degrades all of these benefits. An undersized unit fails prematurely under thermal and mechanical stress. An oversized unit wastes budget, adds unnecessary mass, and may create installation clearance problems. A backlash grade tighter than necessary inflates procurement cost by 3\u20135 times without improving system performance. The six steps below take you from your servo motor\u2019s nameplate data to a confirmed specification, with formulas, worked examples, and the most common pitfalls to avoid at each stage.<\/p>\n<\/div>\n

\"Planetary<\/div>\n<\/div>\n

<\/p>\n

Step 1 \u2014 Calculate the Required Gear Ratio<\/h2>\n

The gear ratio determines how much the planetary gearbox reduces the motor speed (and proportionally increases torque). It is the first parameter to calculate because it drives every subsequent selection decision.<\/p>\n

<\/p>\n

\n
i = Nmotor<\/sub> \u00f7 Noutput<\/sub><\/div>\n
Where i = gear ratio, Nmotor<\/sub> = motor rated speed (rpm), Noutput<\/sub> = required output speed (rpm)<\/div>\n<\/div>\n

<\/p>\n

Worked Example:<\/strong><\/p>\n

Motor rated speed: 3,000 rpm. Required output speed: 150 rpm.
\ni = 3,000 \u00f7 150 = 20:1<\/strong>
\nSince 20:1 is not available as a single-stage ratio (max ~10:1), a two-stage unit with a 20:1 ratio<\/strong> is required. Check your target manufacturer\u2019s catalogue for the exact available ratio \u2014 Korea Ever-Power offers 20:1 as a standard two-stage option across all frame sizes.<\/p>\n<\/div>\n

<\/p>\n

\u2139 Tip:<\/strong> If the calculated ratio falls between two available catalogue values (e.g., the calculated 17.5:1 falls between 16:1 and 20:1), always choose the higher available ratio<\/strong> (20:1 in this case). A slightly lower output speed is easily corrected by the servo drive\u2019s electronic speed parameter, but an overloaded gearbox running beyond its torque capacity cannot be corrected in software and will fail prematurely.<\/div>\n

<\/p>\n

Step 2 \u2014 Calculate Required Output Torque<\/h2>\n

The output torque determines the frame size of the planetary gearbox. You need to calculate it from the motor\u2019s rated torque and the gear ratio, then verify it falls within the gearbox\u2019s rated capacity.<\/p>\n

\n
Toutput<\/sub> = Tmotor<\/sub> \u00d7 i \u00d7 \u03b7<\/div>\n
Where Tmotor<\/sub> = motor rated torque (N.m), i = gear ratio, \u03b7 = gearbox efficiency (0.94\u20130.96 for two-stage)<\/div>\n<\/div>\n
Worked Example:<\/strong><\/p>\n

Motor: 750 W, 3,000 rpm \u2192 Tmotor<\/sub> = (0.75 \u00d7 9,550) \u00f7 3,000 = 2.39 N.m<\/strong>
\nGear ratio: 20:1. Efficiency (two-stage): 0.94
\nToutput<\/sub> = 2.39 \u00d7 20 \u00d7 0.94 = 44.9 N.m<\/strong>
\nThis output must be less than<\/strong> the rated torque of the selected gearbox frame. For an EP-PL80 at 20:1, the rated output torque is 110 N.m \u2014 well within the safe operating margin.<\/p>\n<\/div>\n

<\/p>\n

\u26a0 Common mistake:<\/strong> Using the motor\u2019s peak torque<\/strong> instead of rated (continuous) torque<\/strong> in this calculation. Peak torque applies only during brief acceleration bursts and may be 2\u20133 times the rated value. The planetary gearbox must be sized for the continuous duty torque that the motor delivers over sustained operation. If the motor\u2019s peak torque multiplied by the gear ratio exceeds the gearbox\u2019s emergency stop torque rating, the unit will suffer premature gear or bearing failure during high-dynamic acceleration events.<\/div>\n
\u2139 Safety margin guideline:<\/strong> As a general engineering practice, the calculated output torque should be no more than 80% of the gearbox rated torque<\/strong> for continuous duty applications. This 20% margin accounts for load fluctuations, start-up transients, and ensures the gearbox operates well below its thermal limit, extending service life from the rated 2,000 hours to 8,000\u201310,000 hours under derating conditions.<\/div>\n

\"Korea<\/p>\n

<\/p>\n

Step 3 \u2014 Select Frame Size by Motor Power<\/h2>\n

Planetary gearbox manufacturers designate frame sizes by the output flange diameter in millimetres (e.g., Frame 60, 80, 90, 120). Each frame accepts a specific range of motor shaft diameters and motor power levels. The table below shows the Korea Ever-Power standard mapping:<\/p>\n

\n\n\n\n\n\n\n\n\n
Frame Size<\/th>\nMotor Power Range<\/th>\nMotor Shaft (mm)<\/th>\nRated Torque (N.m)<\/th>\nTypical Motor Brands<\/th>\n<\/tr>\n<\/thead>\n
60 \/ 70<\/td>\n50\u2013400 W<\/td>\n6\u201319<\/td>\n13\u201360<\/td>\nPanasonic A6B, Delta ECMA-C<\/td>\n<\/tr>\n
80 \/ 90<\/td>\n200\u20131,000 W<\/td>\n8\u201324<\/td>\n27\u2013150<\/td>\nMitsubishi HG-KR, Yaskawa SGM7J<\/td>\n<\/tr>\n
120<\/td>\n400\u20132,000 W<\/td>\n14\u201335<\/td>\n63\u2013280<\/td>\nSiemens 1FK7, Mitsubishi HG-SR<\/td>\n<\/tr>\n
145 \/ 160<\/td>\n750\u20134,500 W<\/td>\n19\u201342<\/td>\n170\u2013720<\/td>\nYaskawa SGM7G, Siemens 1FL6<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

Frame sizes vary by series. EP-PL\/PF: 60, 80, 120, 160. EP-PBL\/PBF: 70, 90, 120. EP-HAB: 60, 90, 115, 145. Input coupling: OP1 (integral) for standard series, OP2 (split-type) for precision series.<\/p>\n

The frame size must satisfy both<\/strong> constraints: the motor shaft must fit within the accepted diameter range, and the calculated output torque from Step 2 must fall below the frame\u2019s rated torque. If the motor shaft fits but the torque exceeds the rating, move up one frame size. For our 750 W example, Frame 80 or Frame 90 is the correct match.<\/p>\n

\"Precision<\/p>\n

<\/p>\n

Step 4 \u2014 Choose the Right Backlash Grade<\/h2>\n

Backlash is the angular dead zone at the output shaft when the input reverses direction. It is measured in arcminutes (arcmin), where 1 arcmin = 1\/60 of a degree. Lower backlash means higher positional precision \u2014 but also higher cost. Specifying tighter backlash than your application actually needs wastes budget without improving performance.<\/p>\n

<\/p>\n

\n
BACKLASH GRADE \u2192 APPLICATION MAPPING<\/div>\n
\n
\n
\u22643\u2032<\/div>\n
Ultra-precision<\/div>\n
CNC rotary tables
\nRobot J1\/J6 joints
\nOptical positioning<\/div>\n<\/div>\n
\n
\u22645\u2032<\/div>\n
High precision<\/div>\n
Semiconductor
\nLaser cutting
\nAOI inspection<\/div>\n<\/div>\n
\n
\u22648\u2032<\/div>\n
Standard precision<\/div>\n
CNC feed axes
\nPackaging
\nGeneral automation<\/div>\n<\/div>\n
\n
\u226415\u2032<\/div>\n
Economy<\/div>\n
Conveyors
\nWinding machines
\nNon-positioning drives<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\u2139 Cost perspective:<\/strong> A \u22643 arcmin unit typically costs 3\u20135 times more than a \u22648 arcmin unit of the same frame size. If your application operates in one direction only (unidirectional positioning), backlash does not affect accuracy \u2014 you can safely specify a higher grade and reduce procurement cost significantly.<\/div>\n

Korea Ever-Power maps backlash grades to specific product series: the EP-PL\/PF standard planetary gearbox<\/a> delivers \u22648 arcmin at the most competitive price point. The EP-PBL\/PBF high precision series<\/a> reaches \u22645 arcmin with IP65 sealing. The flagship EP-HAB achieves \u22643 arcmin with the highest torsional stiffness in the catalogue.<\/p>\n

<\/p>\n

Step 5 \u2014 Decide Between Inline and Right-Angle Output<\/h2>\n

The output shaft direction determines whether you specify an inline planetary gearbox (motor and output on the same axis) or a right-angle unit (motor perpendicular to the output). This decision is driven almost entirely by the available installation space behind the output face.<\/p>\n

An inline planetary gearbox with a 750 W servo motor extends approximately 280\u2013320 mm behind the output face. A right-angle unit at the same power redirects the motor sideways, reducing the depth behind the output to approximately 140\u2013170 mm \u2014 a 40\u201350% reduction. However, the right-angle configuration adds a spiral-bevel input stage that increases backlash by approximately 3 arcmin and reduces efficiency by about 2% per stage compared to the equivalent inline unit.<\/p>\n

The decision rule is straightforward: choose inline whenever physical space permits, because it offers the best efficiency, tightest backlash, and lowest cost. Switch to right-angle only when the available depth behind the driven axis is physically insufficient for the inline motor-gearbox assembly length, or when the motor must be oriented perpendicular to the load axis for clearance, cabling, or maintenance access reasons.<\/p>\n

\n
\n
Inline (Coaxial)<\/div>\n

Motor and output shaft on the same axis. Highest efficiency (\u226596%), tightest backlash (\u22643 arcmin available), and lightest weight. Choose inline whenever the installation has sufficient axial depth behind the output face to accommodate the motor-gearbox assembly length. Series: EP-PL\/PF, EP-PBL\/PBF, EP-HAB.<\/p>\n<\/div>\n

\n
Right-Angle (90\u00b0)<\/div>\n

Motor perpendicular to the output shaft. Reduces machine depth by 40\u201350%. Specify when axial space is constrained (AGV wheel hubs, wall-mounted conveyors, panel actuators) or when the motor must clear an adjacent structure. Efficiency is ~2% lower than inline due to the bevel input stage. Series: EP-WPL\/WPF, EP-WPBL\/WPBF.<\/p>\n<\/div>\n<\/div>\n

<\/p>\n

Step 6 \u2014 Verify Inertia Match<\/h2>\n

Inertia matching determines how well the servo motor can control the load through the planetary gearbox. The reflected load inertia at the motor shaft should ideally be within 1:1 to 10:1<\/strong> of the motor rotor inertia (Jload<\/sub>\/Jmotor<\/sub>). A well-matched system responds crisply; a poorly matched system oscillates, overshoots, or fails to settle within the required time.<\/p>\n

\n
Jreflected<\/sub> = Jload<\/sub> \u00f7 i\u00b2<\/div>\n
The gear ratio squared reduces the reflected inertia. A 10:1 ratio reduces reflected inertia by 100\u00d7.<\/div>\n<\/div>\n
Worked Example:<\/strong><\/p>\n

Load inertia: 0.005 kg\u00b7m\u00b2. Motor rotor inertia: 0.0001 kg\u00b7m\u00b2. Gear ratio: 10:1.
\nJreflected<\/sub> = 0.005 \u00f7 10\u00b2 = 0.00005 kg\u00b7m\u00b2
\nRatio: Jreflected<\/sub> \/ Jmotor<\/sub> = 0.00005 \/ 0.0001 = 0.5:1<\/strong>
\nThis is within the ideal 1:1\u201310:1 range. The servo system will respond well.<\/p>\n<\/div>\n

If the inertia ratio exceeds 10:1 even after accounting for the gear ratio, consider increasing the gear ratio (which reduces reflected inertia by i\u00b2) or stepping up to a larger motor with higher rotor inertia. The planetary gearbox itself adds a small amount of inertia (the planet carrier and gears), but this is typically negligible compared to the load and motor inertia.<\/p>\n

\u26a0 Practical tuning note:<\/strong> Even with a theoretically ideal inertia ratio, the servo amplifier\u2019s PID gains must be tuned to account for the gearbox compliance (torsional stiffness). A low-stiffness planetary gearbox (\u22643 N.m\/arcmin) paired with high servo gains creates a spring-mass resonance that manifests as audible chatter at the output. For high-dynamic applications, specify a unit with torsional stiffness above 10 N.m\/arcmin and work with the servo drive manufacturer\u2019s autotuning algorithm before manually adjusting gain parameters.<\/div>\n

Selection Summary \u2014 One Table, All Six Steps<\/h2>\n

The table below consolidates all six selection steps into a single reference. For each step, confirm that your calculated value falls within the acceptable range before proceeding to the next step.<\/p>\n

\n\n\n\n\n\n\n\n\n\n\n
Step<\/th>\nParameter<\/th>\nFormula \/ Method<\/th>\nCheck<\/th>\n<\/tr>\n<\/thead>\n
1<\/td>\nGear ratio<\/td>\ni = Nmotor<\/sub> \u00f7 Noutput<\/sub><\/td>\nMatch to nearest catalogue ratio<\/td>\n<\/tr>\n
2<\/td>\nOutput torque<\/td>\nTout<\/sub> = Tmotor<\/sub> \u00d7 i \u00d7 \u03b7<\/td>\nMust be < gearbox rated torque<\/td>\n<\/tr>\n
3<\/td>\nFrame size<\/td>\nMotor power \u2192 frame table<\/td>\nShaft & torque both within range<\/td>\n<\/tr>\n
4<\/td>\nBacklash grade<\/td>\nApplication mapping table<\/td>\nDon\u2019t over-specify<\/td>\n<\/tr>\n
5<\/td>\nOutput direction<\/td>\nMeasure installation depth<\/td>\nInline preferred if space allows<\/td>\n<\/tr>\n
6<\/td>\nInertia match<\/td>\nJload<\/sub>\/i\u00b2 vs Jmotor<\/sub><\/td>\nRatio 1:1 to 10:1<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

\"Planetary<\/p>\n

<\/p>\n

5 Common Selection Mistakes to Avoid<\/h2>\n
\n
\u2717<\/span><\/p>\n
Sizing on peak torque instead of continuous torque<\/strong>
\nThe planetary gearbox must handle continuous duty torque, not brief acceleration peaks. Size on rated motor torque \u00d7 ratio \u00d7 efficiency. Check that motor peak torque \u00d7 ratio does not exceed the gearbox emergency stop torque.<\/span><\/div>\n<\/div>\n
\u2717<\/span><\/p>\n
Over-specifying backlash<\/strong>
\nA \u22643 arcmin unit costs 3\u20135\u00d7 more than \u22648 arcmin. If the application is unidirectional or tolerates 0.5 mm positional uncertainty at the working radius, \u22648 arcmin is sufficient. Over-specifying backlash is the most common source of unnecessary cost in servo gearbox procurement.<\/span><\/div>\n<\/div>\n
\u2717<\/span><\/p>\n
Ignoring radial and axial load limits<\/strong>
\nThe output shaft bearings have permissible radial and axial load ratings. If the load applies significant side force (belt tension, cantilever weight), verify the load falls within the gearbox specification. Exceeding the rating shortens bearing life dramatically.<\/span><\/div>\n<\/div>\n
\u2717<\/span><\/p>\n
Forgetting to check motor shaft diameter compatibility<\/strong>
\nEach frame size accepts a specific shaft diameter range. A 750 W motor from Brand A may have a 14 mm shaft while the same power motor from Brand B has a 19 mm shaft. Always confirm the exact motor shaft dimension against the gearbox input coupling specification.<\/span><\/div>\n<\/div>\n
\u2717<\/span><\/p>\n
Choosing 3 stages when 2 will do<\/strong>
\nEach additional stage adds length, weight, cost, and approximately 2% efficiency loss. A two-stage unit covers ratios up to 100:1. Only specify three stages if you genuinely need ratios above 100:1 (e.g., 160:1, 200:1, 512:1) and can accept the cumulative backlash increase.<\/span><\/div>\n<\/div>\n<\/div>\n

<\/p>\n

Frequently Asked Questions<\/h2>\n
\n
\n\u25b6 What three parameters do I need to start selecting a planetary gearbox?<\/summary>\n
Motor power (watts), required output speed (rpm), and backlash tolerance (arcmin). From motor power and output speed, you can calculate the gear ratio and output torque, which determine frame size. Backlash tolerance determines which product series to specify. With these three data points, any planetary gearbox manufacturer can recommend a confirmed model number.<\/div>\n<\/details>\n
\n\u25b6 How do I know how many gear stages I need?<\/summary>\n
Single-stage covers ratios from 3:1 to 10:1. Two-stage covers 9:1 to 100:1. Three-stage covers 64:1 to 512:1. Choose the minimum number of stages that includes your required ratio. For example, 25:1 is available as a two-stage unit \u2014 do not specify three stages unless you need ratios above 100:1.<\/div>\n<\/details>\n
\n\u25b6 Is my Mitsubishi \/ Panasonic \/ Yaskawa \/ Siemens servo motor compatible?<\/summary>\n
Yes. Korea Ever-Power planetary gearboxes are compatible with all major servo motor brands including Mitsubishi (HG-KR, HG-SR, HG-MR), Panasonic (MINAS A6, A5), Yaskawa (SGM7J, SGM7G), Siemens (1FK7, 1FL6), Delta (ECMA), and Beckhoff (AM8000). Six input coupling options (OP1\u2013OP6) cover different shaft and keyway configurations. Provide your motor model number and our engineering team will confirm the correct adaptor.<\/div>\n<\/details>\n
\n\u25b6 What is the ideal inertia ratio for a servo planetary gearbox system?<\/summary>\n
The reflected load inertia at the motor shaft (Jload<\/sub> \/ i\u00b2) should ideally be between 1:1 and 10:1 of the motor rotor inertia. Ratios below 1:1 mean the motor is oversized relative to the load \u2014 functional but wasteful. Ratios above 10:1 make servo tuning difficult and can cause oscillation or slow settling. The gear ratio squared is your main tool for reducing reflected inertia \u2014 increasing the ratio from 5:1 to 10:1 reduces reflected inertia by 4\u00d7.<\/div>\n<\/details>\n
\n\u25b6 How quickly can Korea Ever-Power confirm a selection and deliver?<\/summary>\n
Send us your motor model number, required ratio, and application description. Our engineering team returns a confirmed planetary gearbox specification within one business day. Standard configurations with common ratios ship within 5\u201314 business days from our Ansan-si facility. Custom shaft or non-standard ratio configurations require 15\u201325 business days.<\/div>\n<\/details>\n<\/div>\n

<\/p>\n

\n
Need Help Selecting the Right Planetary Gearbox?<\/div>\n

Tell us your servo motor model, required output speed, and torque. Our application engineers will return a confirmed model number, dimensional drawing, and quotation within one business day \u2014 no obligation.<\/p>\n

Get a Free Selection Recommendation \u2192<\/a><\/p>\n<\/div>\n<\/div>\n

\ud3b8\uc9d1\uc790: Cxm<\/p>","protected":false},"excerpt":{"rendered":"

SELECTION GUIDE How to Select a Planetary Gearbox for Your Servo Motor Motor power, target speed, torque requirement, backlash tolerance, and installation envelope \u2014 five parameters, six steps, one confirmed specification. This guide walks you from motor nameplate to gearbox model number with real calculation examples. 6 SELECTION STEPS 3\u2013512 RATIO RANGE \u22643\u2032 BEST BACKLASH […]<\/p>","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_et_pb_use_builder":"","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[19],"tags":[],"class_list":["post-256","post","type-post","status-publish","format-standard","hentry","category-application-and-technical-guid"],"_links":{"self":[{"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/posts\/256","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/comments?post=256"}],"version-history":[{"count":2,"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/posts\/256\/revisions"}],"predecessor-version":[{"id":259,"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/posts\/256\/revisions\/259"}],"wp:attachment":[{"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/media?parent=256"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/categories?post=256"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/planetary-gearboxes.cn\/ko\/wp-json\/wp\/v2\/tags?post=256"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}