Every cycloidal gearbox manufacturer will consistently get one question from their customers, typically several times a day. Most customers want to understand the difference: planetary vs cycloidal gearboxes.
The answer to this question is almost a no-brainer for companies like Omme, which has been manufacturing these kinds of products since 1961. However, in order to answer it as thoroughly as possible, we have decided to write a more detailed answer so as to help our customers make the most informed choice possible to find the best solution for their needs.
The short answer is that every piece of machinery has its most suitable gearbox, and it should be chosen according to its application.
But the fuller answer is found in this table, which identifies all the benefits of the two mechanisms and compares them using a boxing match analogy.
Who would win: planetary vs cycloidal?
| Feature | Cycloidal Gearbox | Planetary Gearbox |
| Reduced dimensions | ✔️ | ❌ |
| Higher accuracy | ✔️ | ❌ |
| “High reduction ratio ( for the same volume)” | ✔️ | ❌ |
| Greater sturdiness | ✔️ | ❌ |
| Lower maintenance | ✔️ | ❌ |
| “Higher torque (for the same volume)” | ❌ | ✔️ |
| Increased durability | ✔️ | ❌ |
| Cable routing (robotics) | ✔️ | ❌ |
| Coating factor | ✔️ Bearings (pressure area) | ❌ Gears (2 teeth grip) |
| Lower noise level | ✔️ | ❌ |
A sports comparison: planetary vs cycloidal
Imagine we have two boxers competing for the world championship title:
- The first boxer is smaller and more agile, extremely precise in punches; he is more robust and moves swiftly at higher speeds, but most importantly, he has greater endurance.
- The second boxer is bulkier, less accurate, possibly less robust, and does not move swiftly, but he has a strong enough punch to take off the opponent with a single blow.
The winner will be decided not by the skills of each boxer but by the rules of the fight.
Likewise, for gears such as planetary vs cycloidal gearboxes, it all depends on the application: if more force (i.e., more torque) is needed, the planetary is a must, though it is less robust and tends to break more easily.
Conversely, for those that require more speed, the cycloidal gear is required in the Planetary vs cycloidal comparison.
Both perform the same function, that is, they transmit rotational motion, but they do so with different mechanisms:
- The cycloidal mechanism is based on “rolling,” with a larger gripping area.
- The planetary mechanism is based on ” swiping,” with two teeth on the grip at all times.
Now, we will have to get a little more technical and talk about rotation/how the wheel works for better evaluating the comparison planetary vs cycloidal gearboxes.
Wheelwork
Wheelwork is defined as a system of toothed wheels geared to each other so that the movement of one causes a motion for others.
Planetary systems are distinguished from ordinary wheelwork by the fact that at least one wheel has a moving axis; they may be single-stage or multi-stage. Planetary gear systems have multiple degrees of freedom, that is, they have multiple shafts that are independent in their motion.
If you restrict a few shafts, for example, impose zero speed, you can have a system with only one degree of freedom. This means that two shafts have a fixed relationship between their angle speeds.
In this case, you have a gearbox when the angular speed of the output shaft is less than that of the input shaft; you have a multiplier gearbox when the opposite is the case.
The advantages of using planetary wheelwork, especially as gearboxes, are a transmission ratio that can reach even very high values; large transmittable powers with reduced overall dimensions and masses, compared to a transmission solution with ordinary gears, which is even more evident if several satellites are engaged in parallel.
Let’s find out together how they differ and how they can be transformed into power and speed by gears.
Kinematics of planetary vs cycloidal gearboxes
A planetary gearbox also known as a planetary gear consists of three characteristic elements: a sun gear (Sun), multiple satellites or planet gears (Planet), and an inner ring gear (Ring Gear). The input shaft connects to the Sun gear, which transmits the rotational motion to the planet gears, which in turn rotates the inner ring gear.
The planetary gears rotate on solid pins embedded in a plate called the planet carrier; this rotation of the planet carrier is what rotates the output shaft. As in all mechanical speed reductions, the output shaft has a lower rotational speed and higher torque than the input shaft.
A Cycloidal gear consists of four components: the input shaft, an eccentric element on which bearings are fitted, one or more cycloidal cams, an outer gear that mounts fixed pins and rollers, and an output hub that mounts movable pins and rollers. Unlike in planetary gears where sliding stress and bending loads persist, in cycloidal gears compressive loads occur and the transmission of motion is ensured by the pure rolling of the above-mentioned elements.
Reduction ratio
In a planetary reduction stage, in the Planetary vs cycloidal comparison to a gear reduction with the same number of teeth between the gear and the pinion, higher ratios are achieved in favor of the planetary by 20 percent. Typical reduction ratios range from 3:1 (single-stage) to 100:1 and above (multi-stage).
Let’s dig deeper in the comparison planetary vs cycloidal gearboxes.
In cycloidal wheelwork, the input shaft connected to the eccentric element rotates the first cam and due to special rollers the motion is carried to a second cam (double reduction) or a third cam (third reduction) ensuring a total reduction ratio of the single-stage up to as high as 119:1.
The reduction ratios could even increase (up to 300:1) in case we are talking about coupled reduction stages. This is because we take advantage of the concept of the difference in the number of teeth between the fixed gear (S) and the number of teeth of the planetary element (P):
Furthermore, when the eccentric bearing rotates one rotation, the cycloidal disk moves forward in the opposite direction by 360°/pivot/roller steps. The direction of rotation between the input and output in a cycloidal gearbox is discordant.
Transmitted torques and internal stresses
In planetary gear reduction, the gears have multiple gears as opposed to a parallel gear reduction where the mesh is single (overlay factor about 1.15). Consequently, for the same amount of torque to be transmitted this results in lower forces on the teeth, and thus for the same unit forces, the planetary reduction needs smaller dimensions than an ordinary transmission.
Once again, cycloidal gears, however, turn out to be the higher-performing gears in terms of transmitted torque and overload capacity. After all, thanks to internal compressive (not shear) stresses and an overlap factor between rolling elements that can reach 70 percent of the main surfaces in contact at the same time, higher torques can be transmitted compared to planetary gears.
In addition, when the eccentric bearing rotates by one rotation, the cycloidal disk advances in the opposite direction by 360°/pivot/roller steps. The direction of rotation between the input and output in a cycloidal gearbox is discordant.
In the comparison planetary vs cycloidal gearboxes, as a matter of fact, the load is absorbed by more pins/rollers and cycloidal lobes unlike planetary or ordinary gearboxes where you only have 1 or 2 teeth absorbing the internal shear stress. In addition, the main torque transmission components within OMMEGEARS gearboxes are made of steel from 100Cr6 and/or 100CrMo7 bearings and thermally treated to 63 HRC toughness.
This aspect is most important as it immediately shows us the great advantage of cycloidal gears over planetary gears:
· Firmness/Compactness: 70% net volume saving and substantial weight reduction.
· Zero maintenance: Significant reduction in maintenance due to the smaller number of elements used for the transmission.
· Overload capacity: cycloidal gearboxes can withstand loads of up to 500% of rated torque.
· Lower input powers: A significant reduction in maintenance due to the smaller number of elements used in the transmission.
· Extreme precision: Cycloidal gearboxes can be used in markets where positioning accuracy is strategic. In fact, thanks to the manufacturing technology, accuracies of less than 1′ can be guaranteed.
The next step
Since 1961, we have been serving very demanding customers with a strong focus on service and a competitive advantage that enable us to build increasingly reliable machines.
If we were to encapsulate our strength in one sentence: YOU DON’T HAVE TO ADAPT YOUR MACHINE TO THE GEAR, BUT WE ADAPT THE GEAR TO YOUR MACHINE, at no extra charge for the prototype!
Schedule a free consultation now to find out how our gearboxes can make your machines even more competitive.
Else, you can discover more about gearbox maintenance, clicking here.