Why Do Industrial Robots and Heavy-Duty Equipment Prefer Dual-Disc RV Reducers?

Many engineers have the same question when they first encounter an RV reducer: if a single cycloidal disc can already achieve speed reduction, why are RV reducers used in industrial robots almost always designed with two cycloidal discs?

Adding a second disc means more components, more complicated assembly, and higher manufacturing costs. So what is the reason?

The answer can be summarized in one sentence: the second disc is not primarily there to provide reduction; it is there to improve accuracy, stability, load distribution, and service life.

In this article, we use our RV reducer assembly model and motion simulation to explain why the dual-disc configuration has become the mainstream architecture for industrial robots, machine tools, rotary tables, and other heavy-duty precision transmission systems.

Note: The numerical data and images in this article are based on simulation calculations and are provided for reference only.

How Does an RV Reducer Work?

An RV reducer, or cycloidal pinwheel reducer, has a relatively short transmission chain.

The input shaft drives an eccentric mechanism. The eccentric mechanism carries the cycloidal discs, which engage with the pins inside the pin housing. The resulting motion is then transmitted to the output through the output pin mechanism.

The following is an exploded view of our NX assembly model, including the pin housing with 40 pins, crankshaft, eccentric bearings, two cycloidal discs, and output disc. Each component in the model is generated directly from the engineering parameters of the transmission system.

The reduction principle can be summarized simply.

The pin housing has 40 pins, while the cycloidal disc has 39 teeth. During one revolution of the crankshaft, the cycloidal disc moves backward by one tooth position. Therefore, after 39 revolutions of the crankshaft, the output completes one revolution:

i = 39

The motion simulation provides a more intuitive way to understand this transmission process.

What Are the Problems With a Single-Disc RV Reducer?

Problem 1: Radial Force Imbalance

The cycloidal disc operates eccentrically. Its eccentric mass rotates around the center, while the meshing load is also concentrated within a particular sector.

In our numerical lever-method calculation, among the 40 pins, 19 pins meet the basic load-bearing conditions, while approximately 15 pins are actually loaded under the analyzed conditions.

The calculation shows that approximately 90% of the load is concentrated on 11 pins within a 90° sector, giving a load distribution coefficient of:

Kγ = 1.71

In a single-disc configuration, this concentrated load sector rotates together with the eccentric mass.

The result is increased vibration, noise, and bearing loading.

For heavy-duty equipment and industrial robot joints, these effects become increasingly important because the reducer must operate repeatedly under high torque and frequent acceleration and deceleration.

Advantages of a Dual-Disc RV Reducer

A dual-disc RV reducer places two identical cycloidal discs axially within the transmission.

Disc A operates on the +e eccentricity, while Disc B operates on the −e eccentricity. The two eccentric sections on the crankshaft are therefore positioned 180° apart.

However, simply installing two identical discs 180° apart is not enough.

Disc B must also be rotated by a specific phase angle:

Δφ = −180° / zc = −180° / 39 = −4.615°

This detail is particularly important during CAD modeling and assembly.

If an engineer simply duplicates the first cycloidal disc and installs the two discs with zero phase difference, the pin holes will not align correctly and the intended backlash-reduction effect cannot be achieved.

The left side of the comparison below shows the incorrect 0° phase arrangement, where the pin holes are visibly misaligned. The right side shows the correct −4.615° phase arrangement.

Benefit 1: Inertial Force Cancellation

Disc A operates at +e, while Disc B operates at −e.

The eccentric masses therefore move in opposite directions, allowing their first-order inertial forces to largely cancel each other.

This reduces vibration and helps the RV reducer operate more smoothly.

For industrial robot joints, this is one of the most direct benefits of the dual-disc architecture.

Lower vibration can also reduce dynamic loading on bearings and other transmission components.

Benefit 2: Backlash Can Be Reduced Through Preloading

To understand why the dual-disc structure can reduce backlash, we first need to understand where backlash comes from.

In our model, five major error sources were converted to the output angle using a lever-based calculation.

Under the example tolerance conditions, the output pin-hole clearance accounted for approximately 48% of the total calculated backlash, which was roughly equivalent to the combined contribution of the other four sources.

The cycloidal tooth profile itself accounted for only about one quarter of the total.

This means that a major part of the backlash comes from the clearance chain, rather than simply from the cycloidal tooth profile.

The phase difference between the two cycloidal discs provides a way to compensate for part of this clearance.

By adjusting the phase, one disc can be positioned against one side of the clearance while the other disc contacts the opposite side.

The principle is similar to the anti-backlash mechanism of a preloaded double-nut ball screw.

However, there is an important engineering window that is often overlooked.

Our calculation gives an assembly phase window of approximately:

ε* = 79″

When the phase error remains within this window, the backlash-reduction effect remains effective.

Once the phase error exceeds the allowable window, the two discs can begin to oppose each other and partially lock up. Backlash may continue to decrease, but friction and temperature rise at the same time.

Therefore, dual-disc RV reducer acceptance should evaluate both backlash and no-load torque.

Looking at only one of these parameters can give a misleading impression of the actual assembly quality.

Benefit 3: Load Sharing

The two cycloidal discs provide parallel load paths.

In an ideal theoretical condition, each disc would carry approximately 50% of the transmission load.

In an actual reducer, manufacturing tolerances and phase errors mean that the load is not perfectly divided.

Nevertheless, the dual-disc configuration significantly reduces load concentration.

More teeth can participate in load transmission, which can reduce contact stress and improve wear distribution.

This load-sharing characteristic is particularly valuable in high-torque and long-service applications.

Benefit 4: Higher Torsional Stiffness

The dual-disc configuration creates an additional parallel transmission path.

This can increase the effective torsional stiffness of the transmission system and reduce output-end angular movement under load.

For applications such as welding robots, material-handling robots, industrial rotary tables, and machine tools, torsional stiffness is particularly important because the output must maintain its position under changing external loads.

Single-Disc vs. Dual-Disc RV Reducer

The additional cycloidal disc increases component count and assembly complexity, but it also provides improvements in force balance, load sharing, backlash control, and torsional stiffness.

Where Are Dual-Disc RV Reducers Used?

Dual-disc RV reducers are widely associated with applications that require high torque, high stiffness, accurate positioning, and long-term reliability.

HONPINE RV reducers are currently used in applications including heavy-duty robotic arms, industrial positioners, industrial rotary tables, and machine-tool systems.

RV Reducers for Industrial Robots

Industrial robot joints do not primarily prioritize saving the cost of one cycloidal disc.

They require a combination of:

• High positioning accuracy

• High torsional stiffness

• Low backlash

• High load capacity

• Long service life

• Stable operation

• High reliability

The dual-disc RV architecture adds manufacturing and assembly complexity, but provides benefits in backlash reduction, vibration control, load sharing, and stiffness.

For this reason, dual-cycloidal-disc RV reducers with phase preloading are widely used in industrial robot joints.

Single-disc RV configurations are more commonly found in teaching systems, experimental equipment, or cost-sensitive applications where the requirements for stiffness, vibration, and long-term heavy-duty operation are lower.

RV Reducers for Industrial Positioners

Industrial positioners share an important characteristic with robot joints: they often handle large loads while the output shaft is also exposed to eccentric loads and overturning moments.

A welding positioner, for example, needs to rotate a workpiece while maintaining stable positioning.

If the reducer has excessive backlash or insufficient torsional stiffness, the output can experience small angular movements during acceleration, deceleration, direction reversal, or external loading from the welding process.

Industrial positioners therefore pay close attention to:

• Output torque

• Torsional stiffness

• Backlash

• Allowable overturning moment

• High-frequency start-stop operation

• Long-term operating life

These are also areas where the dual-disc RV architecture can provide important mechanical advantages.

RV Reducers for Industrial Rotary Tables

Heavy-duty industrial rotary tables, robotic rotary tables, welding tables, and precision positioning tables place similar demands on the transmission system.

The requirement is not simply to make the table rotate.

The system must accurately rotate a large load to the target position and maintain stable positioning after the motion stops.

For this reason, important parameters for an RV reducer used in a rotary table include:

• Backlash

• Torsional stiffness

• Axial and radial load capacity

• Allowable overturning moment

• Output torque

• Positioning accuracy

• Long-term operating reliability

A dual-disc RV reducer can therefore be particularly suitable for rotary applications where high load and stable positioning are required.

Why Is the Dual-Disc Configuration the Mainstream RV Architecture?

The reduction function of an RV reducer comes from the cycloidal pin-tooth transmission.

The additional cycloidal disc does not fundamentally create the reduction ratio.

Its primary value lies in improving the mechanical behavior of the transmission system.

The single-disc structure addresses the basic question:

Can the mechanism achieve speed reduction?

The dual-disc structure addresses more demanding questions:

Can it reduce vibration, distribute load, control backlash, increase stiffness, and operate reliably over a long service life?

This distinction explains why dual-disc RV reducers are widely used in industrial robots, machine tools, heavy-duty rotary tables, and other precision transmission applications.

The small −4.615° phase difference in this example is therefore much more than a CAD assembly detail. It is one of the key factors that turns two cycloidal discs into a coordinated precision transmission system.