Nabtesco RV series high-precision gearbox

RV series precision cycloidal reducer technology full solution: high stiffness, small backlash, high torque precision transmission core

Introduction: The cornerstone of transmission in precision motion control

In modern industrial robots, CNC machine tools, assembly and conveying equipment, and various precision positioning systems, reducers play a core role in reducing speed, amplifying torque, and improving control accuracy. Although traditional gear reducers have a simple structure, they often suffer from problems such as large backlash, insufficient stiffness, and weak impact resistance, making it difficult to meet the dual requirements of high-precision trajectory control and heavy-duty working conditions. The cycloidal pinwheel reduction technology, especially the two-stage composite transmission scheme of "planetary spur gear+cycloidal pinwheel" adopted by the RV series, achieves compactness, lightweight, high stiffness, low backlash, and excellent overload bearing capacity through the unique structure of multi tooth simultaneous meshing, becoming a benchmark solution in the field of precision transmission.

This article is based on complete technical data of the RV series products, providing a comprehensive and systematic technical analysis of the three sub series RV-E, RV-C, and RV from multiple dimensions such as structural principles, performance parameters, selection process, installation specifications, lubrication and maintenance. It provides a professional, detailed, and directly applicable technical guide for mechanical design engineers, system integrators, and equipment maintenance personnel in engineering practice.

Series Overview and Core Advantages

The RV series is a general term for the products of the cycloidal pinwheel reducer family. Its core design concept is to accommodate as many teeth as possible in a limited volume, thereby achieving high stiffness, small backlash, low vibration, and long service life. Compared with traditional planetary or harmonic reducers, the RV series has significant advantages in the following aspects:

High stiffness: Multiple teeth support the load simultaneously, with extremely high torsional stiffness, and can maintain precise angular displacement even under heavy loads.

Lost motion: Through precision machining and pre tightening design, the clearance angle is extremely small, suitable for position servo control.

Overload resistance: The instantaneous maximum allowable torque can reach 500% of the rated torque and can withstand impact loads.

Compact and lightweight: The two-stage deceleration structure is integrated into one housing, and the output end is equipped with angular contact ball bearings, which can directly withstand external bending moments and axial forces.

Multiple configurations: Supports multiple working modes such as shell fixed shaft output and shaft fixed shell output, with a wide range of speed ratios.

According to the application scenarios and structural characteristics, the RV series is divided into three sub series:

RV-E series: Basic type, input gear directly connected to motor shaft, suitable for most general industrial robot joints, machine tool turntables, and positioners.

RV-C series: equipped with a central gear for more flexible input configuration, suitable for situations where wiring (cables, hoses) needs to be threaded from the center of the gearbox, such as robot wrists and rotary axes.

RV series (universal type): Similar to the E series, but including some older models (such as RV-15, RV-30, etc.), attention should be paid to specification differences when selecting and replacing.

The following will provide detailed explanations for each series, but first start with their common working principles.

Working principle: Implementation of two-stage precision deceleration

2.1 First stage - planetary spur gear reduction

Taking the RV-E series as an example, its input end is the input gear. When the servo motor drives the input gear to rotate, the input gear meshes with the spur gear installed on the crankshaft. According to the different gear ratios, the first stage can provide multiple speed ratio options. The input gear is usually machined and installed on the motor shaft by the user themselves, or standard parts can be ordered from Nabtesco.

2.2 Second Stage - Cycloid Needle Wheel Reduction

In addition to spur gears, the crankshaft also has two eccentric parts that are offset by 180 ° phase angle from each other. Install a cycloidal gear (RV gear) on each eccentric section. When the crankshaft rotates, the cycloidal gear undergoes eccentric motion, and the cycloidal teeth on its outer contour mesh with the cylindrical pins fixed on the inner ring of the housing.

Due to the special geometric shape of the cycloid gear profile, the cycloid gear moves one gear position relative to the needle teeth for each rotation of the crankshaft, and the direction of motion is opposite to that of the crankshaft. Simultaneous meshing of multiple teeth - usually with nearly half of the teeth in contact at the same time - results in even load distribution, greatly improving load-bearing capacity and torsional stiffness.

2.3 Output Modes

The RV gearbox provides two basic output modes:

Fixed shell and rotating output shaft: the most common working mode, suitable for robot joints, turntables, etc.

Fixed output shaft, rotating housing: suitable for situations requiring large hollow holes or special installation methods.

Speed ratio calculation formula (taking shaft output as an example):

R=1+Z2Z1⋅Z4

R=1+ Z 1Z 2⋅Z 4

Among them, Z1Z1 is the number of input gear teeth, Z2Z2 is the number of spur gear teeth, and Z4Z4 is the number of needle teeth. Due to the large size of Z4Z 4, the overall speed ratio can be achieved very high (from tens to nearly 200).

RV-E series: the benchmark for universal precision reducers

3.1 Model and Specifications

The RV-E series covers a variety of specifications from small to large, including models such as RV-6E, RV-20E, RV-40E, RV-80E, RV-110E, RV-160E, RV-320E, and RV-450E. The rated torque, speed ratio range, and external dimensions of each model gradually increase.

Definition of rated torque: For RV-6E, the rated output speed is 30 r/min; For other models, the rated output speed is 15 r/min. The output torque at this speed is used as the benchmark for calculating the lifespan. At different speeds, the allowable output torque varies according to the principle of "N · T=constant".

3.2 Detailed Explanation of Performance Parameters

3.2.1 Torque and Capacity

Taking RV-160E as an example, under the condition of speed ratio 129 (shaft output):

At an output speed of 15 r/min, the rated output torque is 13887 in lb (approximately 1568 N · m).

As the speed increases to 30 r/min, the allowable output torque decreases to approximately 11894 in lb, and the input capacity increases accordingly.

The peak torque allowed during acceleration and deceleration is 250% of the rated torque (200% for RV-6E).

The instantaneous maximum allowable torque under emergency stop or impact conditions is 500% of the rated torque.

3.2.2 Main bearing capacity

The RV-E series has built-in angular contact ball bearings that can directly withstand external loads. The main parameters include:

Moment stiffness: The value of the bending moment (in lb/arc. min) required to deflect the output shaft by one degree. For example, the bending moment stiffness of RV-160E is 26038 in lb/arc.min, which means its ability to resist bending deformation is extremely strong.

Allowable moment: The maximum allowable moment applied to the output shaft during normal operation. The allowable bending moment of RV-160E is 34718 in lb (approximately 3920 N · m).

Instantaneous maximum allowable bending moment: usually twice the allowable bending moment, used to withstand unexpected impacts.

3.2.3 Stiffness and backlash

Torsional stiffness: Apply torque to the output shaft while fixing the input shaft, and measure the torsional angle. The higher the stiffness, the smaller the elastic deformation under the same torque.

Lost motion: The torsional angle corresponding to the midpoint of the hysteresis curve measured during reciprocating loading within the range of ± 3% of the rated torque. The typical hysteresis value of the RV-E series is 1 arc minute (1 arc. min), which means that the backlash during forward and reverse switching is extremely small, making it very suitable for high-precision positioning.

Backlash: The twisting angle at which the torque is zero. The RV series controls the backlash within 1 arcminute through precision assembly.

3.2.4 No load starting torque and low temperature characteristics

Idle starting torque: The torque required to rotate the gearbox from the input side under no-load conditions. This value is affected by the viscosity of the lubricating grease and assembly accuracy.

Backdriving torque: When a certain torque is applied to the output shaft, the input shaft is driven in the opposite direction to rotate. This feature requires special attention in certain applications such as force feedback and gravity balance.

Low temperature characteristics: In low temperature environments (such as below -10 ℃), the viscosity of lubricating grease increases, and the no-load starting torque significantly increases. The product information provides measured curves under different low temperature conditions.

3.2.5 Angular Transmission Accuracy

Angle transfer error is defined as the difference between the theoretical output angle and the actual output angle. The RV series controls the angular transmission error within a very small range (usually within ± 1 arcminute) through high-precision cycloidal profile and strict manufacturing tolerances, thereby ensuring the smoothness and repeatability of the robot trajectory.

3.3 Installation and Assembly

The RV-E series is not pre lubricated at the factory, and users need to inject the specified amount of lubricant (Nabtesco Molybdenum RE00) during installation. The required amount of oil varies depending on the installation direction (horizontal or vertical).

Installation accuracy is crucial:

The coaxiality tolerance requirement between the motor flange and the input side of the reducer is 0.03-0.05mm (depending on the model).

The flatness and verticality of the installation surface must be controlled within the recommended range, otherwise it may cause vibration, noise, and abnormal wear.

There are two ways to fix the output shaft:

Bolt clamping type: Directly press the output shaft and load side flange with bolts.

Bolt+taper pin clamping type: Adding a taper positioning pin on the basis of bolt connection to transmit greater torque and withstand alternating loads. Suitable for high torque or high impact working conditions.

3.4 Selection Process

When selecting, the following steps should be followed:

Determine load conditions: including starting torque, uniform torque, braking torque, corresponding speed, and duration of action.

Calculate the average output torque and average output speed.

Calculate the expected lifespan based on the rated lifespan formula:

Lh=K × NoNm × (ToTm) 103L h=K × N m N o × (T m T o) 310, where K is the rated life base (usually 6000-10000 hours), NoN o and ToT o are the rated speed and torque, and NmN m and TmT m are the actual average speed and torque.

Verify whether the peak torque (acceleration/deceleration, emergency stop) exceeds the allowable value.

Verify whether the external bending moment and axial force are within the allowable range of the main bearing.

Select the input gear form and speed ratio based on the motor shaft diameter and speed ratio requirements.

RV-C Series: Center Thread Precision Reducer

4.1 Structural Differences and Application Scenarios

Unlike the E series, the RV-C series integrates a center gear at the input end. The central gear is a hollow structure that allows cables, air pipes, and liquid pipes to pass through the center of the reducer, thereby achieving "hollow wiring" of the robot joints. This design is crucial for situations where cables need to be avoided from being exposed or worn, such as robot wrists and base rotation axes.

The main models of the RV-C series include RV-10C, RV-27C, RV-50C, RV-100C, RV-200C, RV-320C, and RV-500C. Some models (such as RV-500C) cover extremely heavy-duty applications.

4.2 Performance parameter characteristics

The basic performance indicators of the RV-C series are similar to those of the E series, but there are the following differences:

The bending moment stiffness and allowable bending moment are generally higher than the E series of the same size, because the design of the C series focuses more on bearing larger external loads.

Accuracy requirements for central gears: Central gears are divided into large central gears and small central gears, and the tooth surface accuracy of both must reach JIS level 4 and 5. The center distance tolerance during installation is ± 0.03 mm.

Speed ratio calculation: The total speed ratio of RV-C is equal to the speed ratio of the built-in reducer multiplied by (the number of teeth on the large central gear/the number of teeth on the input gear), making the speed ratio configuration more flexible.

4.3 Installation precautions

The fixing method of the output shaft is also divided into bolt clamping type and through-hole bolt clamping type.

The clearance between the central gear and input gear should be controlled within the recommended range (such as 0.060-0.140 mm for RV-100C) to ensure smooth transmission and longevity.

Due to the presence of a central threading hole, the sealing requirements for lubricating grease are higher. O-ring or sealant (such as Loctite 5699) should be used on the mating end face to prevent grease leakage into the hollow area.

RV Universal Series: Inheritance and Compatibility

The RV universal series includes models such as RV-15, RV-30, RV-60, RV-160, RV-320, RV-450, RV-550, etc. Its structure is highly similar to the E series, with the main difference being:

Some old models (such as RV-15, RV-30) use 2 crankshafts instead of 3, so the allowed peak torque is 200% of the rated torque, while other new models are 250%.

In terms of speed ratio options, the universal series includes some intermediate speed ratios that are not available in the E series.

The external dimensions and installation interfaces are slightly different from the E series, so careful verification is required when selecting.

For new designs, it is recommended to prioritize the RV-E or RV-C series for better performance support and spare parts supply.

Lubrication and maintenance

6.1 Lubricating grease selection

Nabtesco recommends using Molybdenum RE00 grease. This is a special lubricating grease containing molybdenum disulfide, which has excellent extreme pressure resistance and low-temperature flowability. It is strictly prohibited to mix with other lubricating greases.

6.2 Oil injection volume

The oil injection amount depends on the installation direction:

Horizontal installation: Less oil, such as RV-160E requiring 448 cc.

Vertical installation: More oil is needed to prevent insufficient lubrication of the upper bearing, such as RV-160E which requires 514 cc.

When injecting oil, it is necessary to reserve about 10% of the internal space to avoid damaging the oil seal due to internal pressure increase caused by thermal expansion or agitation.

6.3 Replacement cycle

Under clean conditions with an ambient temperature below 40 ℃, the standard replacement cycle is 20000 hours. If the ambient temperature is higher or there is dust or moisture pollution, it is necessary to shorten the replacement cycle or regularly check the condition of the lubricating grease.

6.4 Exhaust and Sealing

During installation, it is necessary to exhaust the internal air through the exhaust hole (or oil injection hole) on the housing, otherwise the oil seal may leak due to air resistance. O-ring or sealant (such as Loctite 5699 RTV silicone) should be used to match the end face, and copper containing materials are prohibited (copper will react with lubricant additives).

Application Examples and Selection Worksheet

7.1 Typical Applications

Industrial robots: base rotation axis, shoulder, elbow, wrist joints. The RV-C series is used for wrist joints that require hollow wiring.

CNC machine tools: fourth/fifth axis turntable, tool magazine ATC cam box.

Automated assembly line: precision positioning table, indexing plate, pressing unit.

Exoskeletons and medical devices: high stiffness, small backlash, smooth motion.

Aerospace and military turntables: impact resistant, high reliability.

7.2 Summary of Selection Worksheet

When inquiring or selecting from Nabtesco, the following information should be provided:

Application device name and purpose.

Gearbox model (or target speed ratio and torque).

Load conditions: including starting torque, constant speed torque, braking torque, corresponding speed and time.

External loads: radial force, axial force, bending moment, and their point of action distance.

Installation method: shell fixed/shaft fixed; Horizontal/vertical installation.

Enter gear specifications: self-made by the user or ordered as standard parts; Motor shaft size.

Motor model and parameters.

Special environmental requirements: cleanroom, food contact, low temperature, vacuum, etc.

Quality Assurance and Service

Nabtesco provides material and process quality assurance for RV series reducers for 1 year from the date of delivery or 2000 hours of actual operation (whichever comes first). The warranty does not include dismantling and installation labor costs, transportation costs, and indirect losses caused by downtime. For products that need to be exported (especially for military purposes or weapon manufacturing), they must comply with the export control regulations of the relevant countries.