Slow down, it moves too fast!

Contributing editor Rex Narraway examines the options open to designers who want to slow things down by using speed reducers

Conventional electric motors still remain the prime movers in most automatic systems. However, without recourse to mechanical speed reducers, they are inherently too fast to maintain the accurate positional control required for many of the tasks that contemporary systems are called upon to undertake.

Speed reduction gearboxes using traditional involute toothed gear wheels are very cumbersome to install and the high weight limits their use to static applications. Consequently, a number of manufacturers have developed much lighter units that can be used dynamically at the point of application.

Speed reducers from two of the major manufacturers, Sumitomo and Teijin Seiki, are based on a cycloidal disc which is given an eccentric rotation between a set of rollers. The edge of this disc is machined so that the circumference consists of a series of lobes, thereby generating a wavy profile on the surface instead of the conventional serrated shape generated by the involute teeth of a spur gear. In addition, the pitch of the lobes is considerably greater that that normally associated with gear wheel teeth.

The rollers are arranged in a circular pattern on an annular housing and mounted so that they run within the lobes: the number of rollers being one more than the number of lobes on the cycloidal disc. When the input shaft is rotated, the cycloidal disc is given an oscillatory motion between the input shaft and the rollers. This causes a reaction force between one of the lobes and its associated roller which drives the disc in the reverse direction to the input shaft while allowing the next roller to occupy its own position in the next `wave-trough’.

The actual distance that the disc moves is directly related to the difference between the numbers of the rollers and lobes. Consequently, its speed of rotation is reduced by a similar amount. The motion is continuous and the geometry is such that the rollers remain in close contact with surface of the lobes on the disc. Thus, backlash is reduced and the load is shared simultaneously by about one third of the total number of rollers, giving the unit a smooth drive. In addition, the arrangement can sustain shock loads of up to five times the rated torque.

In the design of the standard FC-A range of Sumitomo Cyclo reducers, one or more cycloidal discs are located on a central bearing mounted on an eccentric section of the input shaft. A ring of holes is machined through each cycloidal disc and mates with a set of pins mounted on a plate which connects to the output shaft. The size and position of the holes allows for the eccentric movement of the cycloidal disc and, thus, the pins are able to transfer a smooth drive to the output shaft.The original design was developed by Cyclo. This was acquired by Sumitomo and has been totally included as an affiliate company since 1994. The Cyclo is ideal for highly dynamic applications by virtue of its very low inertia, and has been used in heavy-duty production machines, large microscope tables and conveyors. The basic unit is now being integrated with a fixed helical output stage to provide a parallel offset gearbox assembly. In addition, by fitting either a fixed-ratio bevel or a worm output stage, the manufacturer has produced a right angle gearbox.

A variation of the principle, using a form of coarse involute gearing on epitrochoid discs in place of the standard wavy-profile disc, has been incorporated into the Cyclo F2C-T. Here, two discs are made to move eccentically to the input shaft by three bearings mounted on a crankshaft. The crankshaft is rotated by simple involute gear set driven by the input shaft.

A similar system is used to impart the oscillatory motion to the cycloidal disc used in the Teijin Seiki design. Here, the arrangement relies on three crankshafts driven from the input shaft via three reduction spur gears. The design normally uses two cycloidal discs- known as RV gears-which act as epicyclic gears and are offset to one another by 180i to provide a balanced load.

The assembly is supported on a double tapered-roller bearing housed in a flanged case. Output is delivered via a rigid structure supporting three pillars which extend through the cycloidal discs, and drive can be taken either from the case or from a shaft: if the case is fixed, the shaft provides the output; conversely, if the shaft is fixed, the output is taken from the case.

There are three types in the Teijin Seiki range of speed reducers. These include the standard design RV-AII with a tapered roller bearing, the more rigid RV-E with an angular ball bearing, and the hollow-shaft type RV-C. A 2-stage reduction principle based on the RV-AII uses a spur gear system as a pre-stage and an epicyclic reduction gear as a second stage. The design of the RV series simplifies the installation and reduces the number of components used in it. It has been incorporated in the Vigoservo RV reduction gear which has been used on industrial robots.

Teijin Seiki also supplies a speed reducer known as Flexdrive which is manufactured in the USA by its affiliate company, Harmonic Drive Techniques. A similar unit is also made in Germany by Harmonic Drive Antriebstechnik, who are represented in UK by Harmonic Drive Systems. While there are subtle differences between the designs offered by the Japanese and German companies, their operation principle is basically the same.

Each unit depends on three major components: a flexible splined ring or Flexspline; a rigid Circular Spline; and a bearing-mounted elliptical element known as a Wave Generator. The Flexpline is a thin-walled flexible steel ring with an external spline machined on the outer face and effectively replaces the cycloidal disc used in the Sumitomo and Teijin Seiki devices as the method of translating the oscillatory motion of an eccentric input component into a slow rotary motion imparted on the output shaft. Being flexible, the ring is distorted from its natural circular form and takes up the shape of the elliptical profile of the Wave Generator.

In an assembled unit, the three components are mounted coaxially, one inside the other, with the bearing-mounted Wave Generator attached to the input drive. The Flexspline is fitted over the eccentric profile of the Wave Generator so that the teeth on two small diametrically opposite arcs of the circumference are brought to bear on teeth of similar profile machined into the internal bore of the Circular Spline; the spline of latter component having two more teeth than that of the Flexspline.

When the Wave Generator is rotated, the region of tooth engagement travels with the major elliptical axis. However, after it has turned through 180i, say in a clockwise direction, the Flexspline will have regressed by one tooth relative to the Circular Spline. As a result, each turn of the Wave Generator moves the Flexspline back by two-teeth on the Circular Spline giving a reduced speed in the anti-clockwise direction.

The original Harmonic Drive speed reducer used a Flexspline in the form of a cup-shaped cylinder with a spline composed of teeth with involute profiles. Recently, the tooth profile was changed to an approximately sinusoidal shape which increases the region of tooth engagement by comparison with the traditionally-used profile.

The IH profile raises the total number of teeth in contact at any one time from 15% to 30%, resulting in an increase in torsional stiffness and zero backlash. The new design also features an enlarged root radius, which results in a higher allowable stress, while its use gives a more even loading of the Wave Generator bearing. Together, this has led to an increase in torque capacity and life expectancy.

Applications for the Flexspline have included drives for robots and aircraft servo-systems. Specials have also been made for flight simulators. A version with a shortened Flexspline offers a significant reduction in the axial length and is now being sold with the designation HFUC. For companies wishing to incorporate a Flexspline into their own designs, Harmonic Drive Techniques offers a `pancake’ version which is supplied as components only.

New to the field is the Reduto from Infranor, designed to complement a range of brushless motors. Operating on the cycloidal principle, and using an arrangement akin to the Harmonic Drive but without a flexible ring, the Reduto has four basic parts: an eccentric shaft; a combined input/secondary rotor; an output rotor; and the outer casing.

At the heart of the unit is the input/secondary rotor machined as a single component forming two concentric rings joined together by a common web: the inner ring representing the input rotor while the outer ring is effectively a secondary rotor which engages with the case. A serration of specially-shaped toothed elements is machined on the outer face of both rings and the inner ring is bored to take a needle-bearing. The output rotor is connected to the output shaft. It consists of a bored cylindrical disc with a serration, which is complementary to that used on the input rotor, machined into the inner surface of the hole. The outer casing is designed to contain the complete assembly and, again, consists of a ring with similar complementary tooth elements machined on the inside face.

The motor shaft is connected to the input shaft of the Reduto, a portion of which has been machined to form a large eccentric feature. The eccentric section is fitted directly through the needle bearing so that, as the input shaft is rotated, the input/secondary rotor is given a reverse oscillatory motion between the rings formed by the outer casing and the output rotor. The specially profiled serration inches the output rotor around in the same direction as the input shaft.

The serrated faces on both the casing and the output rotor use elements with a profile that follows an alternating pattern of triangular and trapezoidal teeth. To mate with this, the serration on the output rotor consists of a pattern of two triangular teeth, followed by a gap, which allows it to mesh with each of the elements on the casing, as it oscillates. The profiles can be held to a close tolerance giving the arrangement a very low backlash and obviating the need to pre-load the gears to eliminate or minimise free-play.

Component parts of the Reduto can be assembled as units which give either reduced or increased speed, or as a differential gear. To date, applications have included prototypes for plasma cutting machines, wire winding equipment and paint sprayers. However, other potential markets include the control of registration on printing machines, web tensioning and converting machines.

{{Harmonic Drive SystemsTel: Chichester (01243) 545400Enter 470

SumitomoTel: Kingston-upon-Hull (01482) 790340Enter 471

Teijin SeikiTel: Germany +49 02 11 17 37 90Enter 472

InfranorTel: West Sussex (01403) 223500Enter 473}}

{{Table 1: Performance figures of some of the available speed reducerson the market. Data was abstracted from manufacturers catalogues andis for comparison purposes only

No. in OD Size Nominal Reduction Radios Range (mm) Torque (Nm) available

SUMITOMO CYCLO FC-A 6 115 to 310 149 to 3900 29:1 to 179:1SUMITOMO CYCLO F2C-T 6 156 to 330 345 to 3920 81:1 to 171:1TEIJIN SEIKI RV-AII 8 120 to 370 58 to 3726 31:1 to 192.4:1HARMONIC DRIVE TECHNIQUES 10 38 to 330 3.4 to 3160 60:1 to 200:1HARMONIC DRIVE SYSTEMS HFUC 13 70 to 330 5.4 to 3550 50:1 to 160:1INFRANOR REDUTO 3 110 25 20:1}}

{{ MaxI/P Speed Backlash Efficiency Pos’n’l (rev/min) (arc sec) (%) (arc sec)

SUMITOMO CYCLO FC-A 6150 to 2000 0 85 to 90 <28SUMITOMO CYCLO F2C-T 50 o/p to 25 o/p 0 80 to 95 <28TEIJIN SEIKI RV-AII 10300 to 2000 60 60 to 80 26HARMONIC DRIVE TECHNIQUES 15000 to 2250 <50 70 to 85 150 to 15HARMONIC DRIVE SYSTEMS HFUC 14000 to 2500 <36 70 to 85 120 to <60INFRANOR REDUTO 10000 <90}}