Cycloidal Wheel
Parts & RV Sets

Cycloidal wheel parts are the precision components that form the heart of a cycloidal speed reducer — a type of planetary gearbox where the planet (cycloidal disc) has a hypocycloid profile and rolls inside a ring of pins rather than gear teeth. The main components are: cycloidal disc(s) — the eccentric planet, one or two per stage; pin housing (outer ring) — cylindrical housing with precision bores holding the outer pins; output pin flange — picks off the output motion from holes in the cycloidal disc; eccentric bearing — creates the eccentric orbit of the disc.

Cycloidal reducers have several advantages: High reduction ratio in one stage (10:1 to 200:1) compared to 3:1–10:1 for a single planetary stage. High torque density — all lobes are in contact simultaneously (50–70% of lobes engaged), vs. 2–3 teeth for a planetary. Zero backlash achievable — the cycloidal profile is a conjugate fit with the pins, so there is no tooth backlash in the traditional sense. High shock load tolerance — used in robot joints that experience impact loads. These advantages make cycloidal drives standard in collaborative robot wrists, RV reducers for industrial robots, and AGV drives.

Cycloidal disc profile accuracy is the most critical manufacturing tolerance: ±0.005 mm for standard industrial cycloidal reducers. ±0.003 mm for robot joint RV reducers and precision servo drives. ±0.002 mm or better for the highest-precision harmonic-competing drives. Profile accuracy is measured point-by-point with a CMM at 1° angular increments and reported as a radial deviation map. HXC provides this report with every cycloidal disc order.

GCr15 (52100 bearing steel): the standard for high-performance cycloidal discs — through-hardened to HRC 58–62, excellent dimensional stability, Ra 0.2 µm achievable after superfinishing. 20CrMnTi carburized to HRC 58–62: used in RV reducers where a tough core is needed under shock loads. Bearing steel SUJ2: Japanese-standard bearing steel equivalent to GCr15, used in robot RV reducers. 42CrMo4: for larger heavy-industrial cycloidal drives where GCr15 is impractical in large sections. Never use unhardened steel for production cycloidal discs — the lobe flanks will wear rapidly under the high contact stresses.

The eccentricity is the offset between the cycloidal disc centre and its rotation axis — which equals the eccentric bearing eccentricity. The eccentricity (e) is a fundamental design parameter: too large → higher contact stress on each pin; too small → difficulty manufacturing the cycloidal profile. Typical eccentricity for robot joint RV reducers: 1–3 mm. The eccentricity, lobe count, pin count (= lobe count + 1), and pin circle radius together define the complete cycloidal drive geometry. HXC engineers can calculate eccentricity from your reduction ratio and envelope size requirements.

Yes — HXC supplies matched cycloidal disc(s) + pin housing + output pin flange sets measured together to confirm the correct pin-to-lobe clearance and output pin-to-disc hole clearance. All three components are produced in the same facility in coordinated production runs, ensuring the assembly clearances are within the design specification. Sets are bagged together with a single inspection report. This is particularly important for robot joint applications where incorrect clearance causes backlash or binding.

Cycloidal profiles are generated by 5-axis CNC milling from the mathematical profile equations (hypocycloid modified by the pin radius equidistant curve), followed by profile grinding to remove the heat treatment distortion and achieve the final tolerance. The CNC program is generated directly from the cycloidal geometry equations — the profile is not drawn manually but calculated. After hardening, the profile is ground on a CNC profile grinder using a small-diameter grinding wheel (typically Ø5–10 mm) that can follow the tight convex / concave curves of the cycloidal lobe profile.

Single-disc cycloidal stages: one cycloidal disc per stage, simpler and lighter, but produces an inherent unbalanced input force and torque ripple. Used in low-cost reducers. Double-disc (180° offset) stages: two cycloidal discs phased 180° apart on the same eccentric shaft, cancelling the input unbalance force and halving torque ripple. Standard for robot joint RV reducers and precision servo drives. HXC machines both single and double disc configurations — for robot joint applications, always specify double disc.

The clearance between the cycloidal disc lobe and the pin housing pins determines the backlash of the reducer. For zero-backlash designs: the cycloidal disc is designed with a slightly smaller pitch than the pin circle (negative clearance fit) — the disc is preloaded against the pins. This requires very precise matching between disc profile and pin housing, which is why matched sets are important. Standard industrial reducers: 5–15 µm pin clearance. Robot joint RV reducers: 0–5 µm (near-zero or preloaded). HXC measures pin clearance in each matched set and reports it on the inspection certificate.

Single cycloidal disc prototype (5-axis milled, unhardened): 7–10 days. Profile-ground, hardened disc: 12–18 days. Matched set (disc + housing + flange): 14–20 days. Double-disc stage set: 16–22 days. Rush production available for single discs — contact us with your timeline.