Gearing

Gearmotors
Gearmotors are an all-in-one combination of an electric motor and one or more pairs of gears (which is also known as a gearbox). A gearmotor simplifies combining a motor with a gear reducer system.

Planetary gears
Planetary gears consist of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier that may rotate relative to the sun gear. These systems often use an outer ring gear or annulus, which meshes with the planet gears.

The gear ratio in this type of system is not obvious, particularly because there are several ways in which an input rotation can be converted into an output rotation.

Typically, one of these three gear wheels is held stationary; one is an input providing power to the system, while another is an output, receiving power from the system. The ratio of input rotation to output rotation is dependent upon the number of teeth in each gear, and upon which component is held stationary.

Planetary gears offer several advantages over parallel axis gears. These include high power density, the ability to achieve a large reduction in a small volume, multiple kinematic combinations, pure torsional reactions, and coaxial shafting. The disadvantages include high bearing loads, inaccessibility, and design complexity.

In a planetary gearbox arrangement, one advantage is power transmission efficiency, which is typically 3% per stage. Thus, a high proportion of the energy transmitted through the gearbox is used rather than wasted on mechanical losses inside the gearbox.

Planetary gearbox arrangements distribute load efficiently too. The transmitted load is shared between multiple planets, which greatly increases torque density. The more planets in the system, the greater load ability and the higher the torque density. This arrangement is also very stable due to the even distribution of mass and increased rotational stiffness.

Strain wave gearing
Strain wave gearing is an approach to speed reduction using metal elasticity (deflection) of the gear to reduce speed. (Strain wave gearing is also known as Harmonic drives–a registered trademark term of Harmonic Drive Systems Inc.) The benefits of using this approach include zero backlash, high torque, compact size, and positional accuracy.

A strain wave gear is comprised of three components: Wave Generator, Flexspline, and Circular Spline.

The Wave Generator is an assembly of a bearing and a steel disk called a Wave Generator plug. The outer surface of the Wave Generator plug has an elliptical shape machined to a precise specification. A specially designed ball bearing is pressed around this bearing plug causing the bearing to conform to the same elliptical shape of the Wave Generator plug. The Wave Generator is typically used as the input member, usually attached to a servomotor.

Strain wave gearing uses the metal elasticity of gears to reduce speed. The teeth of the Flexspine and Circular Spline are engaged near the major axis of the ellipse and disengage at the minor axis of the ellipse. The elastic radial deformation acts like a very stiff spring to compensate for space between the teeth that would otherwise increase backlash.

The Flexspline is a thin-walled steel cup. Its geometry allows the walls of the cup to be radically compliant, yet remain torsionally stiff since the cup has a large diameter. Gear teeth are machined into the outer surface near the open end of the cup (near the “brim”). The Flexspline is usually the output member of the mechanism.

The cup has a rigid boss at one end to provide a rugged mounting surface. The Wave Generator is inserted inside the Flexspline so that the bearing is at the same axial location as the Flexspline teeth. The Flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This causes the teeth on the outer surface of the Flexspline to conform to this elliptical shape. Effectively, the Flexspline now has an elliptical gear pitch diameter on its outer surface.

The Circular Spline is a rigid circular steel ring with teeth on the inside diameter. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the Flexspline. The tooth pattern of the Flexspline engages the tooth profile of the Circular Spline (circular) along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse will contact a circle at two points. However, the gear teeth have a finite height. So there are actually two regions (instead of two points) of tooth engagement. Roughly 30% of the teeth are engaged at all times.

The pressure angle of the gear teeth transforms the output torque’s tangential force into a radial force acting on the Wave Generator bearing. The teeth of the Flexspline and Circular Spline are engaged near the major axis of the ellipse, and disengaged at the minor axis of the ellipse.

The Flexspline has two less teeth than the Circular Spline. Thus, every time the Wave Generator rotates one revolution, the Flexspline and Circular Spline shift by two teeth. The gear ratio is calculated by:

Number of Flexspline Teeth / (Number of Flexspline Teeth – Number of Circular Spline Teeth)

The tooth engagement motion (kinematics) of the strain wave gear is different than that of planetary or spur gearing. The teeth engage in a manner that allows up to 30% of the teeth (60 teeth for a 100:1 gear ratio) to be engaged at all times. This contrasts with maybe 6 teeth for a planetary gear, and 1 or 2 teeth for a spur gear.

In addition, the kinematics enable the gear teeth to engage on both sides of the tooth flank. Since backlash is defined as the difference between the tooth space and tooth width, this difference is zero in strain wave gearing.

As part of the design, the gearteeth of the Flexspline are preloaded against those of the Circular Spline at the major axis of the ellipse. They are preloaded such that the stresses are well below the material’s endurance limit.

In the gear as the gear teeth wear, this elastic radial deformation acts like a very stiff spring to compensate for space between the teeth that would otherwise cause an increase in backlash. This allows the performance to remain constant over the life of the gear.

Strain wave gearing offers high torque/weight and torque/volume ratios. The lightweight construction and single stage gear ratios of up to 160:1 allows the gears to be used in applications requiring minimum weight or volume. Small motors can exploit the large mechanical advantage of a 160:1 gear ratio to create a compact, lightweight, and low cost package.

A new tooth profile for strain wave gearing has become available in the last few years. This “S” tooth design allows more gear teeth to engage. The effect is to double torsional stiffness, double peak torque ratings, and lengthen operational life.

The “S” tooth form does not use the involute curve of a tooth. Instead, it uses a series of pure convex and concave circular arcs that match the loci of engagement points dictated by theoretical and CAD analysis.

The increased root filet radius makes the “S” tooth much stronger than an involute curve gear tooth. It will resist higher bending (tension) loads while maintaining a safe stress margin.

GAM introduces gearbox and motor mount kit

USA-GAM Gear, LLC (www.gamweb.com), a global provider of servo gearbox and coupling solutions used in the automation of machinery, is proud to announce the release of the EPL-H and the LSK – the company’s first-ever planetary gear head and motor mounting kit that can be adapted to Festo actuators.

“Many of our customers have had challenges developing and assembling brackets that connect motors and gearboxes to Festo® actuators,” said Craig Van den Avont, GAM Gear, LLC’s president. “We’re pleased to introduce a motor mount kit and planetary gearbox that comes ready to bolt directly to a Festo actuator.”

The new LSK from GAM is a customized mounting solution designed to connect any motor with a linear actuator. The LSK is also offered in multiple sizes and includes the mounting and coupling hardware.

The new EPL-H is a hollow output planetary gearbox that can directly mounts to any actuator and motor, and is offered in three frame sizes.

“We hope this universal solution will help make mounting to Festo actuators easier,” added Van den Avont. “We’re committed to meeting our customers’ needs as we strive to manufacture custom and universal solutions that make application design a more seamless process.”

2010 / 2011 Wittenstein (alpha) Gearbox Catalog Now Available

The new version of the Wittenstein (alpha) catalog is now available, containing all standard servo gearbox technical information as well as information for rack and pinion systems, servo couplings and sensor technology. Changes in this new release include:

• Information on the energy efficient SP+ gearbox – the SP+/L-version (low-friction)
• Technical data on the large SUMO hypoid gearboxes

All updated technical information can be found on product-specific pages at the Wittenstein website (www.wittenstein-us.com). Downloading the electronic version of the new catalog can be done from the Manuals page online.

Rino RW8 Miniature NEMA 23 Flanged Gearbox

The made in USA miniature Rino RW8 gearbox with NEMA 23 input flange has ratios from 1:1 to 120:1 and torques to 40 inch-lbs. Input speeds to 5000 rpm. The RW8-NEMA23 is easily customized to meet customer’s special needs. Available from stock.

The RW8-NEMA23 is made from a CNC machined aluminum block housing, dual ball bearings for shaft support, stainless hardened single piece worm/input shaft and stainless output shaft. It is designed for 24 hour per day operation.

Applications include: food machinery, semiconductor processing, packaging machinery, medical diagnostic equipment, and all sorts of usual and unusual applications.

Rino Mechanical
www.rinomechanical.com/gearbox_miniature.htm

Gearboxes Suit Washdown Applications

Corrosion resistant and washdown gear reducers are available nickel plated, lacquer coated or in stainless steel.

IP65 protection, food-grade grease, and stainless steel components are some of the features for the range. The washdown option is available as standard for the following alpha products: alpha LP+, alpha SP+, alpha TP+, alpha HG+, alpha SK+, alpha TK+ and alpha V-Drive.

Wittenstein
www.wittenstein-us.com

Automated Gearbox Testing Builds in Consistency

by William L. Winterbauer, Ph.D., Principal Engineer, QED Services, Ann Arbor, Mich.

The U.S. military has demanding requirements for the hardware it needs. Take, for instance, a set of gearboxes built by Excel Gear Inc., Roscoe, Ill, (excelgear.com) for missile launchers on the U.S. Navy’s new DDG1000 series of ships. The gearboxes are drive elements for the servo systems that rotate and elevate the missile launcher. For good servo performance the boxes must meet the Navy’s requirements for stiffness, efficiency, and low backlash.

A prototype was tested using a time consuming manual method. Although satisfactory, the method required careful checking to prevent data entry errors. Requirements for the test system called for high accuracy, elimination of measurement errors, and elimination of data entry errors. The company’s experience automating the test procedures provides a useful design lesson.

After successfully completing the prototypes, Excel Gear president N.K. Chinnusamy, decided the production run needed improved assembly procedures and to automat the test methods. Preload on the bearings was identified as an important factor – too little preload allowed excessive backlash while too much decreases efficiency and generates heat. A measurement accurate enough to size an optimum preload spacer is difficult because before the spacer is in place, the bearing can tip from side to side.

The company manufactured a set of fixtures to prevent bearing tipping and improve the repeatability of the preloads. The fixtures also improved the efficiency of the first production boxes and reduced their backlash from what had been attained in the prototype boxes.

The fixtures were developed to minimized bearing tipping. The fixtures resulted in improved efficiency and lower backlash.

Types of tests

The units called for several tests. For example:

Temperature tests during run-in: The primary sources of heat in the gearbox are seal friction, bearing friction, and oil churning. The heat generated by oil churning distributes throughout the box and dissipates through the case. Seal and bearing friction are concentrated and, if excessive, will cause failure. Temperatures are checked near the bearings on the high-speed shaft, where measurements on the prototype boxes showed the highest temperatures. These areas were also near the seals. Although it isn’t possible to separate the heat generated by the bearings and seals, the seals seem to generate the most heat.

Temperatures were recorded for two hours with the box running at maximum speed. After cooling, the test was repeated with the box running in the opposite direction. In the test, temperatures rise rapidly at first and then at a decreasing rate. While the temperatures do not reach equilibrium, in operation the boxes will not run continuously for two hours, and they will reach top speed only intermittently.

Gear-train stiffness: This characteristic, measured with the output shaft locked, is the ratio of the input-shaft motion to the torque applied, in Nm/rad. Torque was applied using a hydraulic actuator with two opposed cylinders driving two racks against a pinion. The racks are held in the pinion by a bushing. This results in friction force opposite to the direction of motion. Because the friction in the hydraulic actuator would cause measurement inaccuracies, the torque is measured between the actuator and the input shaft using a Dataflex 42/1000 torque transducer. This sensor has a capacity of 1,000 Nm in either direction. Torque is determined by measuring the twist in the transducer shaft using rotary encoders in a differential circuit. An encoder rotor is mounted at each end of the shaft. Because the encoder read heads are mounted to the stationary part of the transducer, there are no slip rings. The A-quad-B output from the transducer is converted to a voltage by an encoder electronic box. The voltage output range is 0 to 10V with a no load value of 5V, and the calibration constant is 0.200 Nm/mV. Rotary motion was measured using a 2,000 line rotary encoder (resolution of 0.018° ). Because the input shaft extends through the box, the encoder is mounted on the opposite end of the shaft from the hydraulic actuator. When the box is in operation, a brake is mounted on this end of the shaft.

The A-quad-B output from the encoder is converted to a voltage by the programmable encoder-control box. The range and number of volts per degree can be set depending on the amount of rotation to be measured. The output has a range of 0 to10V. For this test, the output was 8.100° per V and the no-rotation voltage was 5V.

Gear train backlash: Some backlash is necessary to provide running clearance for the gears. Too little backlash results in overheating and premature failure, and too much degrades servo performance. Backlash is determined from the data collected for gear train stiffness.

Breakaway torque: For these boxes, it was low and measured manually using a snap-torque wrench. Although automated tests are usually preferred, a few are so simple that the programming required is not justified. This test, for instance, was the only one not automated.

Gearbox power losses over the full range of speeds: Input torque was measured with the gearbox running at a set of speeds both clockwise and counterclockwise. The torque is a nearly linear function of speed with a small component of stiction. This is preferred in a servo system because it contributes to servo loop damping. Because the torque is nearly a linear function of speed, the power-loss curve is nearly parabolic.

Test equipment

Accompanying images show the test equipment and The test hardware table lists a few of its details. The software used, DASYlab, is a graphical programming language. It is programmed by placing block diagrams representing data collection operations on a screen and connecting them with “wires” to control data flow. In the system used here, processed data is written to disk in a tab-separated format suitable for further analysis using Microsoft Excel.

More detail of the test equipment is listed in the accompanying table

Programming the data collection: The DASYlab block diagram provides an example programming screen. Each block represents an operation on the data such as collecting, scaling, saving to disk, and displaying. This programming method is faster than writing code. For example, the voltage output from the torque transducer, encoder, and thermocouples was connected to the electronic interface box. This box has a built in reference junction for the thermocouples. The device also has digital and analog outputs but these were not used. The interface box scans its inputs and converts from analog to digital values. These are passed to the computer through a USB connection at about 1Hz. This is relatively slow for data acquisition, but more than adequate for these quasistatic tests.

The sample screen shows how DASYlab software is programmed, by running virtual wires from sensors to instruments.

Red curves (not data from a gearbox test) represents data from a typical output while the blue line show a best curve fit. The vertical portion on the zero axis represents backlash. The slope of the blue curve represents gearbox stiffness.

In the DASYlab program, the first box is an input box that “talks” to the hardware and places the input values on its outputs in digital form. Typically this is a special module that works only with particular hardware. Most other boxes do not depend on the type of hardware in the system. The other boxes used are numerical displays, graphical displays, and output boxes to record the data on disk. These boxes have corresponding components on a display screen. This screen can be a virtual instrument, that is, it can look like instruments such as voltmeters, oscilloscopes, and chart recorders. For this test the program converts the inputs to engineering units and displays the values several ways. Digital displays show the current numerical values of inputs.

Another display, an XY-plot, shows rotation on the Y-axis and torque on the X-axis. This feedback gives a preview of results. It can save much time because if something is wrong, such as a broken wire or failed thermocouple, it quickly becomes apparent. The test can be stopped, the problem corrected, and the test resumed. A problem that goes undetected until the data is analyzed wastes the entire test period.

A disadvantage of DASYlab is that this program had to be written with the computer attached to the interface hardware. It would be a great advantage to write the program sitting in front of a desktop computer rather than working in the test area using a laptop.

Details of the analysis

Backlash and Compliance: One advantage of automated data collection is the larger amount of accurate information than can be manually collected in a reasonable period. The additional data gives a better picture of the equipment characteristics than would otherwise be possible. In Backlash and compliance, the red line simulates points collected when the torque was varied from zero to maximum, to minimum, and back to zero three times. (The data shown are not actual values but they are an accurate representation of the type of data collected.) The data showed good consistency and repeatability, which produces confidence in the results. For instance, the blue line shows the curve fitted to the backlash and compliance data. The length of the vertical line at zero-load is reported as backlash. The slope of the lines fitted to the observations is the stiffness. This is a conservative method for determining such values. The normal manual four-point test would have given both lower backlash and compliance numbers. The four-point test uses two torques that are just a little higher than breakaway torque in each direction and two torques that are a quarter to one half of the full load torque.

The encoder (dark disc with cable) works with the automatic system while the inclinometer (meter mounted on the horizontal bar) was used for manual readings. The encoder has better resolution, but a difference between the two devices would quickly show scaling or programming errors.

Converting from manual to automated testing: The first use of an automated system involves debugging because the test system, as well as the tested device, may have problems. An advantage to starting this system was that previous manually collected results were available for reference. Problems with the test system are seen quickly. For the first test, some manual measuring devices were used in parallel with the new test equipment. This either verified the results or showed problems. For example, an incorrect scaling factor was quickly detected and corrected. This illustrates one principle of successful testing: Check the calibration of the test equipment before running the tests.

About the author: William Winterbauer is principal engineer with QED Services and a consultant to Excel Gear Inc.

Excel Gear Inc.
www.excelgear.com