Robot Cycloidal Reducer Pin Carriers: How to Specify Pin-Hole Position, Pitch Circle and Bearing Interfaces

A cycloidal reducer uses an eccentric input and a cycloidal disc to reduce speed and transmit torque. In a common design, output pins mounted to a carrier pass through holes in the disc, transferring the reduced rotation to the output assembly. The outer ring pins and output pins perform different jobs. 

For a CNC-machined pin carrier, the key requirements are pin-hole position, pitch circle diameter (PCD), and bearing alignment.

The pin carrier supports the output pins and connects them to the output structure. The pins pass through the cycloidal disc's clearance holes, allowing the disc's eccentric motion while transmitting the reduced output rotation.

A typical design may include:

1:A central bearing interface

2:Equally spaced pin mounting holes

3:Output pins and their retention features

4:A mounting face for the output flange or connected assembly

The pitch circle diameter is the diameter of an imaginary circle passing through the centers of the pin holes.

A manufacturing drawing should define:

1:PCD and number of holes

2:Whether the holes are equally spaced

3:Hole diameter and required fit

4:Position tolerance relative to the functional datum axis

5:The reference mounting face and any required perpendicularity

Hole size and hole position are separate requirements. A pin hole can have the correct diameter but still sit in the wrong position.

One published cycloidal-drive design illustrates why the dimensions must be considered together: it uses 9 mm output pins passing through 10 mm disc holes, with 0.5 mm eccentricity. These are example-specific dimensions, not universal specifications. 

If the carrier supports or locates a bearing, define the interfaces that control its fit and position:

1:Bearing-seat diameter and required fit

2:Axial shoulder or seating face

3:Relationship between the bearing axis and pin-hole pattern

4:Face flatness or perpendicularity where function requires it

5:Any required runout or axial location control

Bearing selection and fit depend on load direction, stiffness, mounting arrangement, lubrication, and whether preload is required. These requirements should be agreed with the reducer designer or bearing supplier rather than assumed from the part geometry alone.

Imagine Mike, a robotics engineer, sends a pin carrier drawing that specifies all the hole diameters but leaves the PCD and datum axis unclear.

A supplier could machine each hole to size, yet the pattern might not align with the cycloidal disc or output assembly.

Before production, check that the drawing defines the pin pattern, central reference, bearing interface, and required fits as one coordinated system.

Our recent CNC Machining Datum Selection Guide explains how functional datums establish consistent feature relationships. You can also read our CNC Machining Tolerances Guide for guidance on specifying critical dimensions.

For related robotic components, see CNC Machining for Robotics: Custom Brackets, Motor Housings, Mounting Plates, and End-Effector Parts.

Cycloidal reducer pin carriers need a clearly defined pin-hole pattern, PCD, datum axis, and bearing interface. Hole diameter alone is not enough to ensure that the carrier fits the disc and output assembly.

If you are also interested in custom CNC products, send your 2D drawing, 3D CAD file, material, quantity, critical tolerances, surface finish, and any cooling, sealing, or inspection requirements.