THE SIMPSON GEARSET IN OPERATION
Presenting a detailed history and description of each production transmission to use a Simpson gearset is well beyond the scope of this article. However, we will explain the basic principles of its operation, which will help you understand the production applications and their relatively minor variations on the theme.
A simple planetary gearset has three elements: a sun gear, a set of planet pinions journaled on a planet carrier, and an annulus (ring gear). All the gears are in constant mesh: The planet gears mesh with the sun gear and the annulus meshes with the planets. Obviously, the planet carrier is not a gear; rather, it’s the armature holding the pins on which the planet gears rotate, which may itself rotate at a different speed than the planets, the sun gear, or the annulus.
A Simpson gearset is a compound planetary gear train with five elements: the annulus of gearset A (which we’ll abbreviate AA); a set of planet pinions on the planet carrier of gearset A (which we’ll abbreviate CA); the sun gear (S); the annulus of gearset B (AB); and another set of planet pinions on the planet carrier of gearset B (CB). Annulus B is permanently affixed to the output shaft.
(To be clear, what we’re describing as “gearset B” is the gearset whose annulus is connected to the output shaft, regardless of actual physical position. Depending on the specific layout, gearset A may be located behind gearset B or vice versa; this description and all the calculations below are based on each gearset’s function, not its position.)
Any Simpson gear train requires at least two brakes and two clutches:
- A forward clutch that connects the input shaft to annulus A in the forward gears. (In some variants, annulus A is permanently connected to the input shaft and the forward clutch instead connects planet carrier A to the output shaft.)
- A direct/reverse clutch that connects the input shaft to the sun gear in third and reverse. (Simpson also designed some split torque variants where this clutch instead connects the sun gear directly to the engine, usually through the torus cover of the fluid clutch.)
- A low/reverse brake that can hold planet carrier B stationary in certain gears.
- An intermediate brake that can hold the sun gear stationary in certain gears.
We say “at least” because some transmissions use multiple brakes of different types, a point we’ll discuss in more detail below.
Simpson gearsets obtain three forward and one reverse speed by combining the above elements in the following combinations:
- First (low): The forward clutch engages, allowing the transmission input shaft to drive annulus A (or connecting planet carrier A to the output shaft). A low/reverse brake is applied to planet carrier B. As annulus A rotates, reaction torque created by the inertia of carrier A causes the sun gear to turn backward. The sun gear’s reverse rotation attempts to turn planet carrier B backward as well, but the low/reverse brake holds the second carrier stationary instead. This causes annulus B to rotate around the stationary carrier in the opposite direction — which in this case is forward — at reduced speed. Since carrier A is also affixed to the output shaft, it must also rotate forward at the same speed.
- Second (intermediate): The forward clutch remains engaged, so the input shaft continues to drive annulus A forward. The low/reverse brake disengages, allowing planet carrier B to rotate freely. An intermediate brake is applied to hold the sun gear. As annulus A rotates, reaction torque attempts to turn the sun gear backward, but the brake holds the sun gear stationary instead. This forces planet carrier A — and the output shaft — to rotate forward around the now-fixed sun gear at reduced speed. The rotation of the output shaft drives annulus B forward at the same speed, causing planet carrier B to rotate idly forward around the stationary sun gear.
- Third (high): The forward clutch is still engaged, allowing the input shaft to drive annulus A. Both the low/reverse and intermediate brakes are released, allowing planet carrier B and the sun gear to rotate freely. The direct/reverse clutch engages, locking the sun gear to either (depending on the version) planet carrier A or annulus A. This forces all elements of both gearsets to turn at the same speed, putting the transmission in direct drive. (In split torque variants, the sun gear turns at engine speed, annulus A turns at input shaft speed, and carrier A resolves the difference.)
- Reverse: The direct/reverse clutch engages, allowing the input shaft (or, in split torque versions, the torus cover) to drive the sun gear forward. A low/reverse brake is applied to hold planet carrier B. The sun gear’s rotation attempts to turn carrier B forward, but the brake prevents the carrier from moving, forcing annulus B to rotate backward at reduced speed. The forward clutch is released, disconnecting annulus A from the input shaft (or, if annulus A is permanently connected to the input shaft, disconnecting carrier A from the output shaft) so that gearset A can’t transmit any torque. (If annulus A is released, it spins idly backward; if the planet carrier A is disconnected, it turns idly forward at engine speed or close to engine speed.)
- Neutral: All clutches and brakes are released. No element is connected to the input shaft (or the torus cover) and no element is held.
BRAKES AND CLUTCHES
In principle, a Simpson gearset can use any type of brake or clutch; Simpson’s earliest versions of this layout used cone and dog clutches along with band brakes. Most production applications use a mix of multi-disc clutches, band brakes, and one-way clutches.
Unlike other types of brake, which keep the elements to which they’re attached from turning in either direction, a one-way clutch (or overrunning clutch) allows a gear element to rotate in one direction, but not the other. Many Simpson gear trains (and other automatic transmission layouts) use sprag-type or cam-and-roller one-way clutches to simplify the mechanics of shifting.
For example, you’ll notice in the above summary that in first gear, the sun gear of a Simpson gearset rotates backward (i.e., opposite the direction of engine rotation and attempts to rotate planet carrier B backward as well. In second gear, carrier B rotates idly forward. If carrier B is connected to a one-way clutch that allows it to turn forward but not backward, that clutch will hold carrier B stationary in first and automatically releases upon the shift to second, or vice versa. The one-way clutch requires no external mechanism or synchronization.
There are two drawbacks to using a one-way clutch in this way. First, the one-way clutch isn’t effective in reverse, when the forward rotation of the sun gear attempts to rotate carrier B forward. Second, if the output shaft overruns the engine (for example, when descending a steep hill in first gear), the rotation of annulus B will also rotate carrier B forward, causing the one-way clutch to immediately unlock and leaving the transmission effectively in neutral. Consequently, the one-way brake must be supplemented by a band or clutch-type brake, which is used in reverse or when the driver manually selects “Low.”
Some transmissions with Simpson gearsets, such as Chrysler’s TorqueFlite, use both a one-way clutch and a separate brake for first gear, but only a single band- or clutch-type intermediate band for second. To use a one-way clutch to hold the sun gear in second, there must be some means of neutralizing that clutch in first gear, when the sun gear has to turn backward. Simpson proposed doing that by attaching the outer race of the one-way clutch to a brake drum and band brake. Reverse rotation of the sun gear would always lock the inner race against the outer race, but with the brake released, the whole drum would simply rotate backward. Some production transmissions, including GM’s Turbo Hydra-Matic, accomplished the same effect by using a multi-disc clutch to lock the one-way clutch’s outer race to the transmission case.
As with a one-way clutch on carrier B, a separate overrun or coast brake is still needed to keep the sun gear locked when coasting. Turbo Hydra-Matic included an overrun band for this purpose, functional only when manually selecting a lower speed range and disengaged in Drive. This gave the driver greater ability to keep the transmission in second gear for engine braking in hilly terrain.
While this arrangement is obviously more complex than a simple two-brake/two-clutch Simpson gearset, it allows each shift in Drive to be accomplished by engaging or disengaging one element. In a Turbo Hydra-Matic, for example, engaging the forward clutch from a neutral condition would give first gear; also engaging the intermediate clutch would give second; and engaging the forward, intermediate and direct/reverse clutch gave third. Downshifts were just as straightforward; from third, releasing the direct/reverse clutch would produce an immediate downshift to second and then releasing the intermediate clutch put the transmission back in first. The additional mechanical complexity was repaid with simpler (and usually smoother) shift action between all forward speeds.
On the following page, we’ll explain how a Simpson gearset’s ratios are calculated.