The Heliocentric Principle

The Universal heliocentric reduction unit is much more than just another speed reducer. Actually it is the first development in half a century adding anything vital to conventional speed reducer construction. Many leading engineers and machine designers agree that the heliocentric is a definite improvement over anything that preceded it.

Until the heliocentric principle is understood there may appear to be an unusual number of moving parts. It requires only two major moving parts, 1) an input shaft carrying a centrally located eccentric, and 2) an output shaft attached to a plunger holder which travels within a stationary rack at the rate of one tooth for each revolution of the input shaft and eccentric. The plunger holder is slotted radially to carry a battery of reciprocating plungers. As the eccentric on the input shaft forces the plungers outward and into the stationary rack, the contour of the rack teeth impose a uniform motion. Since the plunger holder is rigidly connected to the output shaft, a smooth uniform rotation results as the plungers come into contact progressively. As each reaches the limit of its stroke, it is retracted on the no load cycle by the retaining ring working in unison with the eccentric. The heliocentric depends on this conversion of the reciprocal motion of the battery of plungers into slow speed rotary motion through the sliding or rolling wedge action of the plunger tips into the teeth of the stationary rack or internal ring gear(but with a much different tooth form).

As there is almost no energy lost due to friction in this concentric drive, high ratios of reduction can be obtained in a remarkably small compact unit.

The motion of the plungers is essentially the same as the sliding contact of standard gears through their "arc of action" with one important difference. The relative speed of the contact is substantially less in the heliocentric reducers than in any other type of geared reducers. The plunger motion is one complete cycle for each revolution of input - hence quite fast, but the length of travel is extremely small, being two times the eccentricity which sets the reciprocal motion. This radial eccentricity is a function of reduction - varying inversely with the ratio - and the diameter of the output member, hence varying directly with the torque capacity.

In the accompanying chart, the action of the plunger is shown throught a full cycle of engagement with the rack. The plunger point follows a double sine curve while the contact starts on the radius or the shoulder of the plunger and gradually advances to take the load near its point. The action avoids plain sliding friction by producing a combined rolling and sliding contact which progressively rolls the lubricant in at the point of greatest pressure. The action has been proven by actual tests, and accounts for the high efficiencies obtained on high ratios./

It will also be noted that the plunger extends part way into the tooth space before making contact. This, however, is a variable factor and the variations work favorably so far as capacity is concerned. In the diagram, the contacts presented are normal for the unit running idle (no load). As a load is imposed and as the torque increases, slight deflections occur on the contacting faces of the plunger holder so that the rack teeth start the plungers into engagement earlier. This means that as the load increases a greater number of plungers are contacted at the same instant and under test we have found as many as 9 plungers in contact in an element carrying 24 plungers. Fully thirty percent of the plungers are always in contact, thus giving load distributiion and making possible a very compact unit without undue strain on the individual elements. Contrast this with the ordinary tooth to tooth gear where usually one tooth (never more than two teeth) carries the load at any instant.

By following the actions of one plunger, it will be noted that for one complete revolution of the eccentric, the plunger advanced a distance equivalent to one tooth of the rack and so on for each complete revolution of the eccentric. Thus it will be seen that the speed reduction ratio is equal to the number of the teeth in the rack plus one when the input and output shafts rotate in the same direction. However, the output shaft may just as readily be able to rotate in the opposite direction from the input with no additional parts or change in the dimension or general design of the reducer. This is a design feature that cannot be found in any other gearing.

For maximum smoothness, quietness, and balance of working parts, every Heliocentric unit is now made with two eccentrics, two plunger assemblies and two racks. Each set, times directly opposite or 180 degrees apart, has a load-carrying arc of approximately 120°.

One set of plungers is engaging its rack diametrically opposite the other battery of plungers. The opposing thrust of each set of plungers - on a short couple - provides perfect balance, relieves bearing loads of both high and low speed shafts, and at the same time doubles the driving contacts. The total effect is a balanced distribution of driving power over two-thirds of the complete circle, an amazing accomplishment in comparison with ordinary gear drives in which, not to exceed two teeth per gear, carry the load at any given instant.

The plungers are numerous and intentionally so since the plungers and their operating racks correspond to the stationary teeth in ordinary gearing. There is this important difference. The plungers are independently movable and because of the eccentric action it is possible to make many more plunger contacts or engagements than tooth to tooth engagements in ordinary gearing. Approximately one-third of the plungers carries the load.

In heliocentric design it is desirable to utilize as many plungers as the space and ratio will permit. More plungers will mean more contact, greater strength and driving power for a given case. Furthermore, this design permits an increasing number of plungers to be utilized as we go to higher ratios. This benefit operates inversely to conditions in ordinary gear to gear reductions where two teeth may be in engagement at low ratios and only one tooth at high ratios.

Our reducers are made with a standard number of plungers. For heavy duty and high torque the number of plungers can be increased. Always bear in mind, ratio is determined by the number of teeth in the rack. The number of plungers affects the strength of the unit and the direction of rotation. For these reasons it is obvious why this principle is not adaptable to extremely low ratios. The lowest ratio manufactured by Universal in the Heliocentric design is 20-1 and this is only for light-duty units where slight losses in efficiency are unimportant. The low ratio limit for heavy-duty single stage is 30-1. As the ratios increase (more teeth per rack) the pitch of each tooth becomes sharper so that less motion is imparted per tooth to the slow speed shaft. This particular method of transmitting power shows but slight loss in efficiency as ratios increase.

UNIVERSAL UNITS SERVE MANY INDUSTRIES

Whatever may be your requirement for gear drive equipment, consult us. The adapting of the Heliocentric principle of reduction will assure you a high efficiency, quiet, dependable, trouble-free operation to fit your particular need.