Flexible bearing (harmonic reducer wave generator bearing)

01

Size the bearing for deformation, speed and torque

LKPB describes the harmonic drive as a wave generator, flexspline, flexible bearing and rigid spline: the bearing's inner bore fits the oval cam, its outer ring deforms elastically to fit inside the open end of the flexspline, and the flexspline's external teeth engage the rigid spline's teeth, which LKP says outnumber the flexspline's, on the long axis and separate on the short axis. [3]

ZYS lists flexible bearings for harmonic reducers in precision class P4 with, for each designation, bore d, outside diameter D, width B, corner radius, maximum radial deformation, input speed and output torque: for example ZRE904/P4 at 18.8 × 25 × 4 mm, 0.2 mm, 3,000 r/min and 2 N·m; ZR3E812/P4 at 60 × 80 × 13 mm, 0.5 mm, 3,000 r/min and 120 N·m; ZR3E830/P4 at 150 × 200 × 30 mm, 1.25 mm, 1,500 r/min and 2,000 N·m. [1]

Editorial calculation: from ZR3E812 to ZR3E830 the bore grows 2.5 times (60 to 150 mm) while listed output torque grows about 16.7 times (120 to 2,000 N·m) and listed input speed halves; pick the bearing from the reducer size and torque, not from bore alone. [1]

Give the supplier the reducer type and size (cup, hat or pancake), gear ratio, cam profile and wave height, rated and peak output torque, input speed, duty cycle and life required, operating temperature, lubrication, precision class, fits to cam and flexspline, cage preference, and inspection data required.

02

Control fits to the cam and flexspline and the radial clearance

ZYS states that the outer ring of its flexible bearings fits the flexspline bore at H7/h7 and the inner ring fits the cam at H7/js6, that rings and balls are GCr15 high-carbon chromium bearing steel, and that “the cage of light bearing is mostly made of nylon cage with integral type”, with separable brass cages in some high-power harmonic drives. [1]

W. Ostapski (Warsaw University of Technology, Bulletin of the Polish Academy of Sciences, 2010) reports a reliability test on an HP80-83 harmonic drive loaded at 120–200 N·m with the generator shaft at 1,200–1,500 rpm that ended after 72 hours (1.16 × 10⁷ cycles at an average 160 N·m) when the outer ring of the generator bearing cracked. Analysing the cam and bearing fits, the paper found interference that eliminated the bearing's radial clearance, and recommends a fit of the inner ring on the cam with a diametral gap of 0 to 0.025 mm (optimal 0.01–0.02 mm), which it shows can be achieved with a corrected cam radius and bearings sorted into two size groups. [2]

Editorial note: ask the supplier for the bearing's radial clearance before and after mounting on your cam, and match the cam's tolerances to the bearing's bore tolerance before ordering in volume.

03

Estimate gear life from the wave generator bearing's rated life

Harmonic Drive LLC's CSG/CSF component set catalogue states that the life of a strain wave gear is determined by the life of its wave generator bearing, and lists the wave generator's parts as a ball separator, the wave generator bearing, plug, insert, rub washer, snap ring and hub, with an Oldham coupling. Its life table gives an L10 life of 7,000 hours for the CSF series (with CSD, SHF, SHD and CSF-mini) and 10,000 hours for the CSG series, and L50 (average) lives of 35,000 and 50,000 hours, all at the rated torque and rated input speed from the rating tables. Rated life converts to the application by Lh = Ln × (Tr ÷ Tav)³ × (Nr ÷ Nav), where Tr and Nr are the rated torque and input speed and Tav and Nav the average output torque and input speed of the duty cycle. The catalogue says the CSG series gives 30% more torque and 43% longer life than CSF, and that the wave generator's L10 life also depends on grease temperature. [4]

Editorial calculation: for a CSF gear whose average torque is 70% of rated at rated speed, Lh = 7,000 × (1 ÷ 0.7)³ ≈ 20,400 hours; if the average input speed is also 1.5 times rated, life falls to about 13,600 hours. Ask the flexible bearing supplier for the basis of its own rating and the reducer maker for the duty-cycle life calculation, since the bearing sets the gear's life. [4] [1]

04

Compare what suppliers offer

Suppliers linked to this page list flexible bearings for harmonic reducers; each card links to the page reviewed.

The suppliers reviewed list flexible bearings with cross-roller, thin-section and other robot bearings; ask each for the designation table (dimensions, deformation, speed, torque), precision class, fits, radial clearance, steel and cage, and life test data.

05

Trace the wave generator parts

This is a functional reference built from the sources cited; it is not one maker's design.

06

Fill the specification fields with values for your reducer

A flexible bearing quotation needs values for your reducer. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the reducer, load and fit data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Reducer: [type; size; ratio]
Cam: [profile; wave height; tolerance]
Load: [rated and peak torque; input speed]
Life: [hours; duty cycle]
Bearing: [d × D × B; class; clearance]
Fits: [cam; flexspline]
Materials: [steel; cage]
Inspection: [dimensions; clearance; noise]

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