by Goran Freske

I'll bet you all waiting with bated breaths for the continuation of my over-drive tale, which began in February's issue of 1800 NEWS. So with no further ado, let's resume.

Again quoting my manual; "In direct drive, power is transmitted from the gear box mainshaft, through the unidirectional clutch, to the output shaft". Let's what that means.

If one looks at an overdrive that is used in our cars, whether it be the real thing or a picture, "D" model or "3", it is comprised of three major sections, bolted together through four flanges with eight bolts per pair. The front section is just an adaptor to mate the overdrive to the gear box, and serves no mechanical importance in the operation of the overdrive. The middle section contains the whole hydraulic system, which we can forget about right now, since we assumed that it works. The only component in the middle section we have to remember is the "brake ring", the back of which you can actually see from the outside, as a black metal band, sandwiched between the middle and aft sections bolted flanges. The aft section is a little more complicated. It contains the "output shaft" that connects to the drive shaft that goes to the differential on the rear axle. The "output shaft" is shaped like two cones, connected at their big ends. The aft cone contains the "unidirectional clutch", which acts like a ratchet. The forward cone is truncated about 314 in. from where it joins the aft cone. On the inside of this truncated forward cone are gear teeth all around the circumference, which we will revisit later. The output shaft coming out of the GEAR BOX, referred to as the "mainshaft" in the manual, runs all the way through the overdrive and is splined to the "unidirectional clutch". I'd like to stop here for a moment, because THIS IS ALL YOU NEED for making the car drive in all four forward gears, (not in overdrive). As long as the "unidirectional clutch" is working properly, you always have use of all four forward gears, with no slipping. If you have understood everything so far (I'd like to flatter myself and believe it is all perfectly clear), we can add another component to the internal workings of the overdrive; the clutch, or "sliding member".

The next sentences in the manual read; "At the same time, the clutch sliding member is pressed by four clutch return springs against the tapered part of the output shaft. The torque is then transmitted through the clutch sliding member when reversing or when the engine acts as a brake". Hmmmmm

The "sliding member" looks like a flat bottomed cup with its straight sides the - same angle as, you guessed it, the forward cone angle ("the tapered part" in the manual) of the "output shaft" mentioned earlier. The center of the bottom of the cup is splined by devious means, the details of which we don't really have to know about right now, to the "mainshaft". Thus the cup is always rotating in the plane of its flat bottom in whichever direction the "mainshaft" is turning. On the flat sides of the cup are clutch linings, both on the inside and outside. During normal, or non-overdrive, operation, the inner clutch lining of the "sliding member" is forced onto the outside surface of the forward cone of the "output shaft" by four stiff coil springs, as the manual indicates. This is always the state of affairs in normal drive. Thus, in all forward speeds, not only is the "unidirectional clutch" engaged by its spline in transmitting power, but so is the inside clutch lining on the "sliding member" by its spline as well. The two splines are locked onto the "mainshaft", so in direct forward drive, the "unidirectional clutch" takes the load off the inner clutch liner. But let us see what happens when you try to use the engine as a brake, or, indeed, try to reverse the car. You know that the engine is turning in the same direction when reversing the car as it is in forward drive, and, if you did not have an overdrive in your car, you also know that it is the gear box that changes the direction of rotation of the driveshaft. If you brake with the engine, although the drive shaft is still rotating in the same direction as when driving forward, the resultant of the relative torques between the engine and the driving wheels are in the same direction as backing up. The same thing is happening inside the overdrive. When the "mainshaft" changes direction, whether actual or relative, the "unidirectional clutch" is released and becomes free wheeling and inactive for transmitting power. This is where the inside clutch liner of the "sliding member" comes into play, and all the "reverse" power has to be transmitted through it. This is what is not understood fully by the "experts" that said that my problem was a faulty "unidirectional clutch"! And it was almost a unanimous first, and in most cases only, diagnosis, even after I spent much time and effort explaining what the symptoms were before and after I changed the clutch! But we have just seen, corroborated by the manual, that the "unidirectional clutch" is released in reverse gear! And just the fact that the "unidirectional clutch" was working in all forward gears should be a hint. It probably is true that if the "unidirectional clutch" was not functioning, you could drive in the forward gears for a time, relying only on the "sliding member" clutch, but I doubt very much that I could have driven for almost 2000 miles, partly through the mountains, without noticing any slipping.

Now that that is clearly understood, we can put our overdrive in overdrive. But first we need to add another two components; the planetary gears, and the sun wheel.

Let us first see what the manual says. "In the overdrive position, the clutch sliding member is pressed against the "brake ring" (remember?) with the help of the pistons in the hydraulic cylinders, thereby locking the sun wheel. Since the planetary gear carrier is splined to the mainshaft, the planet gears are forced to rotate around the sun wheel. This causes the output shaft to rotate at a higher speed than the mainshaft".

You remember the gearteeth on the inside of the forward cone of the "output shaft"? That is where the "planet gears" run. Three small gears are assembled in a carrier in such a fashion that the outside of the gears are meshing with the inside gear teeth of said cone, while the carrier is splined to the "mainshaft", as it says in the manual. (This spline is the devious arrangement I mentioned earlier). The manual fails to mention that the "sliding member" is locked on to the "sun wheel" by another spline, and that the "sun wheel" in turn meshes with all three "planet gears". Also, it is not made clear that for the "sliding member" to lock into the "brake ring", the "sliding member" moves forward, releasing the the inner clutch lining from the "output shaft" and engaging the outer lining with the "brake ring". Now we have the power from the "mainshaft" going through the spline in the "planet gear" carrier, to the "sun wheel", through the "planet gears", onto the gear teeth on the inside of the cone of the "output shaft". But, again, the only reason this arrangement works is because of the "unidirectional clutch". The last sentence from the manual above holds the key in the relative speeds of the two shafts. Essentially, in overdrive, we are always braking with the engine in terms of relative torques!

If you and I are able to understand the preceeding, why is it that "experts" that brag about the high number of overdrives they have rebuilt, not to mention factory manuals and even courses that they have taken, can not diagnose my simple problem? I certainly hope that this little treatise gives the reader more insight into the mechanical operation of the overdrive, so he or she can avoid costly repairs by "experts" who misinterpret the symptoms.

What happened to my overdrive? After some more iterations in trying to come up with the proper linings, I got my money back from Southland. Then I ended up just using the clutch out of the old overdrive, which seems to be working fine. I could have saved all the work and aggravation all along! But then, I would not have learned how the overdrive works, would I, and isn't learning an important part of the old car hobby? On the advise of several sources, the best, and maybe the only, option for getting the cone clutch relined is to send it back to England, for reportedly less expense than I almost had to pay. In fact, if anyone reading this has a clutch he or she would like to have relined, let me know, and we can send a whole collection over there, and maybe get a break on the price, or at least save on the shipping. I have contacted the owner of the the old Laycock tooling, and, hopefully, we'll get yet another parts source for keeping out favorite cars rolling well into the next century!