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Gearbox loadingplays a big partin systemefficiency

calculations.

Fritz FaulhaberPresident of EngineeringMicroMoClearwater, Fla.

gearboxes use oil coolers and pumps to compen-sate for gearbox inefficiency.

Thus, gearbox efficiency depends on friction.This in turn depends on the quality of the gearing,the number of tooth engagements (how manytimes one wheel drives another) and the loadtorque (how much moment the gearbox has to

deliver).Most manufacturers will specify an intended

gearbox operating point. Gearbox efficiencies ina spur gearbox at a 16-mmdiameter vary fromabout 87%at a gear ratio of 6.3:1 to about 40%ata ratio of 10,683:1. A basic rule that designersuse for spur gears is a 10%loss per engagement.One gear wheel in contact with another is definedas an engagement and the loss in that engagementis approximately 10%.

A general rule is the lighter the load and thehigher the ratio, the less likely it is that the gear-box will actually reach the manufacturers speci-fied efficiency. Light loading and high ratios tend

to produce poor gearbox efficiencies. But withheavy loading and high ratios, the gearbox willapproach its theoretical efficiency.

Overall systemefficiencies depend on the effi-ciency of the motor and gearboxtogether. If the efficiency of themotor and gearbox is each 50%,the two efficiencies are multi-plied together to yield the systemefficiency (0.5 x 0.5 or 0.25, or asystemefficiency of 25%).

At low ratios, motors are moreheavily loaded than the gear-boxes. A low reduction ratio lets

the motor see more of the loadthan at a high ratio. For example,the maximumefficiency of a 22:1gearbox is about 76% and themaximumefficiency of the motoris about 80%. However, the twodont occur at the same time.When the motor reaches its peakefficiency, gearbox efficiency iscloser to 63% instead of 74%.Therefore, when the motor is at

MACHINEDESIGN www.machinedesign.com82 JUNE20, 2002

Basics of design engineeringMotioncontrol

A second look at

Gearbox efficiencies

Gearboxefficiency isusuallyspecified at a

specific torque.The graphshowsefficiencyincreasing withincreasingtorque.

Manufacturers often specify motorefficiency. Ditto for gearbox effi-ciency. However, total systemef-ficiency (motor plus gearbox) isneither clearly understood nor

easily calculated. This makes gearbox efficiencyspecifications found in product catalogs unreli-

able. Catalogs typically provide only a single effi-ciency rating which is not entirely accurate. Effi-ciency depends on a number of factors, espe-cially gearbox loading. Most manufacturers donot list efficiency tolerances, or the difference inefficiencies between a heavily-loaded gearboxand one running under normal loads.

The electrical input power to a gearbox (theproduct of voltage and current to the motor),multiplied by the motor efficiency is the inputpower to the gearbox. Output power is the gear-box speed and load torque. The ratio of outputpower to input power equals efficiency.

Power loss in the gearbox is mostly due to fric-

tion, which generates heat. In miniature gear-boxes, heat is not much of a problembecause thepower losses and the absolute amounts of powerinvolved are relatively small. However, large

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peak efficiency the gearbox is not in lowratios.

This is a critical issue. Assuming thegearbox has a constant efficiency leads toincorrect calculations. In this case, a 10%efficiency difference could mean a lot in theoverall systemefficiency. At higher gearboxratios, motor and gearbox efficiencies fol-low similar curves because at that point the

gearbox sees more of the load than does

the motor. This leads to peak efficiencies inboth gearbox and motor.

Variable motor speeds present anotherset of variables in the overall equation.However, at about 150:1 to 200:1, the gear-box and motor efficiencies peak at thesame time. To use the least amount ofpower, its critical to match the motor,gearbox, and load closely to get the best

systemefficiencies.s

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Basics of design engineeringMotioncontrol

Equations used incalculating efficiency

Motor efficiency (%):effmot = Pout/PinPout =Output powerPin=Input power

Motor output power:Pout = T nT= torquen= speed

Motor torque:T = (km I) (km I0)km= torque constant (fromcatalog);I0= measured no-load currentI = Loaded current

Assembly efficiency:effassy= Pout assy/ Pin motPout assy= Assembly output powerPin mot= Motor input power

Gearhead efficiency:effGH= effassy/effmot

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The graphshows how systemefficiency follows combined gearhead and motorefficiencies.

Simplifying servodrive

Rockwell Automation, Milwaukee, has addedthe Kinetix 6000 Servo Drive to its expandingline of integrated motion products. TheKinetix 6000 features a fiber-optic Sercos interfacelinking the drive and controller. The single, digital,fiber-optic link eliminates up to 18 wires/axis to pre-vent phasing problems and electrical noise often as-

sociated with discrete wiring.Built-in smart motor technology provides auto-

matic identification of motor configurations for easyintegration with the drives. The drives feature a PowerRail mounting and wiring system. It replaces tradi-tional dc bus bars, DIN rails or fasteners, auxiliarypower wires, and inter-module cabling.

The drive offers a velocity bandwidth greater than 400 Hz and acurrent-loop bandwidth greater than 1,300 Hz. The 230-V unit isjust 185 mmdeep and the 460-V unit is 260 mm. The shallowdepth lets drives fit standard 200-mm(8 in.) or 300-mm(12 in.)

deep enclosures.Target applications include packaging, material handling, con-

verting, and assembly.s

The Kinetix 6000 Servodrive is the latest additionto the Kinetix line ofintegrated motionproducts.