astronomy

Simpson planetary gearset

Simpson planetary gearset is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Simpson planetary gearset rather than just read about it. In short: The Simpson planetary gearset is a compound gear train consisting of two planetary gearsets with a common sun gear and a coupled ring gear and carrier. A Simpson gearset delivers three or four forward gears and one reverse, plus neutral, and is commonly employed in three- and four-speed automatic transmissions.

Key takeaways

Simpson planetary gearset belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Simpson planetary gearset to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Simpson planetary gearset from memory before moving on to harder problems.

Reference excerpt

The Simpson planetary gearset is a compound gear train consisting of two planetary gearsets with a common sun gear and a coupled ring gear and carrier. A Simpson gearset delivers three or four forward gears and one reverse, plus neutral, and is commonly employed in three- and four-speed automatic transmissions. It is one of the several designs invented by American engineer Howard Simpson.

Three-speed transmission

1950s: three-speed design

The two planetary gearsets are coupled to each other via two fixed connections, which typically—but not necessarily—use the same gearsets and gear ratios. The first connection links the sun gears together: In most cases, both gearsets share a common sun gear. The second connection links the carrier of the first gearset ("first" means closer to the input shaft) to the second gearset's ring gear. Owing to these linkages, only two brakes and two clutches are needed to command the gearsets.

Beside ZF all major manufacturer obtained a license for this design too.

First gear The first gearset actuates on the second gearset and output shaft; the second gearset reacts and makes the sun gear turn in reverse, causing the first gearset to increase the reduction ratio. The input is coupled with the first ring gear. The first carrier is coupled with the output and second ring gear. The second carrier is braked so its ring gear motion causes the common sun to spin retrograde. The retrograde spin reduces the first carriers reaction to its ring gear motion thereby doubling the reduction of the output of the first gearset.

Second gear The command is much simpler: the second carrier is released and the sun gear is braked. The input is coupled to the first ring gear, and the output is coupled to the first carrier.

Third gear It is always direct drive (1:1), by coupling the input to both the first sun gear and the first ring gear, each gearset is effectively blocked and rotates as a single unit, so the input directly drives the output.

Reverse gear The second sun gear is the input and the second carrier is braked, so the second ring gear is the output.

Gear changing One problem of changing from first to second gear is that two shift elements must actuate in synchrony: the planet carrier of the second gearset must release at the same time as the first sun gear is braked. Sprag clutches mitigate roughness when a shift requires a combination of elements to be released or actuated. GM's Turbo Hydra-Matic 350 and 400 transmissions utilized overrunning clutches in both the low and intermediate gears, allowing for fully progressive shifting with no "overlap". Gear changing patterns in some Simpson gearboxes, including modern four-speed units, avoid first gear as much as they can, going up to second as soon as possible, and avoiding reducing to first unless absolutely necessary (very slow speed plus floored throttle).

Four-speed transmissions

First designs with 3 planetary gearsets

Overview The first four-speed transmissions with Simpson gearsets need 3 planetary gearsets to get four speeds. The third planetary gearset adds an overdrive or slows down the Simpson gearset.

1964: slowed down Simpson gearset

Mercedes-Benz slowed down the Simpson gearset and obtained a licence for a variant—featuring cross-coupled ring gears and carriers instead of coupled sun gears and only one coupled ring gear and carrier—for its 2nd automatic transmission generation with four speeds, which was launched in 1964 for the 600 and was gradually introduced with slightly different ratios in all passenger cars from 1967 onwards. The 2nd automatic transmission generation variant with three speeds (1971–1980) was not based on a Simpson gearset design, but on the Detroit Gear.

1980: overdrive

The 1979 oil crisis, as the second one after the 1973 oil crisis, called for a swift response. In 1980 ZF came up with a four-speed version by simply adding a third planetary gearset to the 3HP from 1973, which is placed behind the original Simpson gearsets. This makes the transmission complex and costly to manufacture, long, and bulky, so it is only suitable for longitudinal installation. Beside ZF all major manufacturer offered transmissions of this design too.

Second design with 2 planetary gearsets

In 1995 ZF came up with a new design, which makes four-speed transmissions with overdrive possible in a Simpson planetary gearset layout without an additional planetary gearset: the overdrive is provided by an additional shift element, which is integrated in the original Simpson gearsets. This keeps the design cost efficient and compact, so it is suitable for different transmission models either for longitudinal or transverse installation. Beside ZF all major manufacturer offered transmissions of this design too. For niche applications some transmissions are kept in production because they are compact, cheap, reliable, well-known, and good enough for the target application.

Modern transmissions

In the 2000s, the Simpson gearset went out of fashion and was replaced by more complex transmissions. A Ravigneaux planetary gearset was combined with a simple planetary gearset to form a Lepelletier planetary gearset, that offers six useful forward gears.

Modern eight-, nine- or ten-speed transmissions have four gearsets. In some of the gears offered by these transmissions, two of them operate according to the Simpson principle.

References

Illustrations

Worked examples

Example 1 — a first encounter with Simpson planetary gearset

Start with the simplest possible case. Write down what Simpson planetary gearset claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Simpson planetary gearset before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Simpson planetary gearset ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Simpson planetary gearset

In research: Simpson planetary gearset appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Simpson planetary gearset in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Simpson planetary gearset is common in secondary-school and first-year university syllabi. It links to neighbouring topics Automotive transmission technologies, Epicyclical gearing, Gears, so understanding it makes those chapters shorter.
In everyday life: Look for Simpson planetary gearset outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Simpson planetary gearset in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Simpson planetary gearset means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Simpson planetary gearset out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Simpson planetary gearset in simple terms?

Why does Simpson planetary gearset matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Simpson planetary gearset?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Simpson planetary gearset.