Other Articles - January/February - 2018

There is More than One Way! Selectable One-Way Clutch Technology

The first one-way clutch was designed in 1869 for use in a bicycle. In the years since, we have seen this technology advance and now one-way clutches are used in everything from hay balers to helicopters. Several different types of one-way clutches are in use, these include:

  • Roller designs
  • Sprag designs
  • Spiral designs
  • Diode designs

While one-way clutches share the same function, their designs as well as advantages/disadvantages vary. One design that is making a major transition into the transmissions of today is the mechanical diode (MD).

Mechanical diodes were first introduced into mass produced automatic transmissions with the Ford 4R70 family of transmissions. Since that time, we have seen MD devices in the 5R110, CD4E, 6L50, 6T** GM, 6F** Ford, 8F** Ford, 9T**GM as well as some Asian manufactures such as Mazda.

The mechanical diode is a fairly simple concept using a series of “struts” to lock the inner and outer races together, while allowing the struts to lay down when the MD is overrunning. With continued pressure to improve fuel economy and to lower a vehicles carbon foot print, a new design product has been introduced, the “CMD” or “Controllable Mechanical Diode” selectable one-way clutch. CMD’s were developed by the engineers at “Means Industries” as a progression of their successful MD one-way clutch technology. While Means’ selectable one-way clutch is called a CMD other terms for this type of device include:

  • 2-way one-way clutches
  • 2 mode one-way clutches
  • 4-way one-way clutches
  • 4 mode one-way clutches
  • Selectable One-Way Clutches “SOWC”

The advantages of a CMD include increased fuel economy (1-2%), lower emissions, lower production costs, mass reduction, improved package and compact design improvements allowing the transmission to fit into tighter spaces. With a CMD, the manufacture can improve fuel economy and lower production costs by removing clutch components from the transmission (figure 1).

As an example, the CMD may be used to replace a Low/Reverse clutch, low one-way clutch combination in a typical planetary transmission. Large fuel economy losses occur in a transmission because it takes power to rotate a clutch that is in the released position. Known as “spin loss”, (drag torque) force is required to rotate the friction discs, intertwined with the steel discs as happens when the clutch is in the released position. Utilizing a CMD selectable one-way clutch, eliminates the cost of the clutch components as well as the spin loss issues because of the minimal amount of torque required to rotate a CMD in the overrun direction.

CMD’s are capable of operating in up to 4 modes:

  • Locking/torque transmission in the clockwise direction
  • Locking/torque transmission in the counter clockwise direction
  • Locking in both directions
  • Freewheeling in both directions

A CMD can be controlled via a direct acting solenoid as in some applications or via a hydraulic actuator as used in some GM and Ford 8/9 speed transaxles.


The CMD consists of (figure 2):

  • A reverse pocket plate
  • A forward notch plate
  • A forward/reverse plate
  • A selector plate
  • Forward/Reverse struts
  • Actuator Arm

The actuator arm determines the operational mode of the CMD. The actuator arm connects to the selector plate and determines the position of the selector plate (figure 3). As the actuator arm moves, the selector plate is moved enabling or disabling the forward, reverse or forward and reverse struts. Depending on the desired operation, the struts allow the CMD to transfer torque or to free wheel.

The actuator arm may be moved in several ways, via a direct acting solenoid/electronic linear actuator, or with a hydraulic actuator.


As the name implies, the direct acting solenoid design connects the pintle of the solenoid to the actuator arm (figure 4). As the solenoid is turned on or off the solenoid pintle moves the actuator arm into the desired position.


Hydraulic actuation is accomplished via a servo piston within the CMD. In GM and Ford applications the CMD is connected via ports to the valve body (figure 5). In the case of the 9T** GM applications, a valve body solenoid controls the position of a clutch select valve. The clutch select valve controls which port on the CMD receives pressure. The servo piston then positions the CMD actuator strut into t he desired position. In 1st gear the forward port receives pressure enabling the operation of the forward struts allowing the CMD to function as a standard one-way clutch. In reverse, both ports are pressurized, locking the CMD from rotating in either direction. This makes the CMD act as a brake holding the planetary carrier.


Service of a CMD is limited to inspection and cleaning of the internal CMD components. If a component is damaged, the CMD will require replacement as the internal parts are not serviced individually.

In our industry we tend to think of technology improvements as “electronics changes or updates”. As you can see, technological change is also happening to the mechanical devices within the transmission.

As always, ATRA is dedicated to working with the manufacturers and suppliers to make sure you are kept up to date as changes occur. A great “thank you” to the engineers from Means Industries and GM Customer Care and After Sales for their help with the material for this article.