Beck TD, Part 56: Understanding my M41 Overdrive (Laycock D-Type)

At the end of Part 2 of this overdrive saga, when I had just gotten the transmission working (minus the overdrive unit), I promised I would try to actually understand how the overdrive works. I've made some good strides toward that goal, so I'm going to try to tell you what I've learned. Nothing like trying to teach something to tell you what you don't understand...

Here's the problem: most of the multitudes of books and web pages I reviewed assume you know certain things. For instance, what all the parts inside of the overdrive are called, and maybe how they go together. So, if it refers to the "annulus" you have to have some sense of what that is. In addition, there are cryptic, color-coded cut-away diagrams that are difficult to read if you've never held the parts in your hands. Not ideal for beginners.

I'm going to try it a different way, leading you through discovering certain points based on pictures of the individual components. Let's start with this one:

That is, of course, an M41 transmission minus the overdrive unit, and it is identical internally to the M40 four-speed, except for that long shaft sticking out the back. The overdrive unit engages that at the far end on those splines.

When I started trying to learn about overdrives, I first started researching "planetary gearsets" because that's what's inside this type of overdrive. Big mistake! Those gearsets have about eight different modes of operation depending on which element is locked from rotating, and this type of overdrive only uses one of those modes. First, let's pretend the gearset isn't even there.

This picture shows the very back end of the overdrive, just before the output shaft exits the unit.

The tail housing is just the case everything is mounted in, and it bolts to the rest of the overdrive. The annulus is that internal gear, and for the moment you can ignore those teeth. The part that took some understanding is the bit in the middle. Most of the manuals call it the uni-directional clutch, but I found the name "over-running" more descriptive. It has the characteristic that if it is being rotated clockwise, it locks and pulls the annulus along with it. In the other direction, it free-wheels and allows the clutch to rotate backward.

See the splines in the middle? They engage with the main shaft. Thus, the center of the over-running clutch always spins at the same speed as the main shaft. So, you need to take a second to visualize this: if the annulus is stationary, the over-running clutch can freely turn backward. But if the over-running clutch is stationary, the annulus can freely turn forward. And here's the point: if the annulus is somehow made to spin faster than the main shaft, the over-running clutch's action allows that to happen. The center of the clutch continue to spin at the same speed as the main shaft (since they are splined together), but the annulus is free to spin even faster.

So, how do we spin it faster? That's how the rest of gearset comes into play, and we start by wondering why that internal gear is called an "annulus." It's related to the work "annual" which is related to the orbit of the earth. The annulus forms the orbit within which the planetary gearset revolves. 

The three gears are the planets, housed in a carrier. The teeth of the gears engage the inside teeth of the annulus. Also, see the spline in the carrier at the bottom of the left photo? Those engage the main shaft and thus the planet carrier always spins at the same speed as the main shaft also. Here's a photo of the planets and carrier nestled in the annulus:

But if we're making a solar system here, we need the sun, and here it is:

The teeth to the left side engage the planets on the inside of the carrier:

But here's an interesting thing: the sun is not attached to the main shaft at all - it's designed to let the shaft pass right through:

This took me a long time to figure out. In all the cut-away drawings, it looks like the sun should spin the planets, which then spin the annulus. The sun never spins anything! As we'll see, its job is to stop something from spinning...

There are other parts to consider, but first let's look at a very brief video I prepared using my transmission test fixture. You'll first see the input and output turning at the same speed because the overdrive is not engaged. Then, I manually engage it by holding the sun stationary, and the output shaft suddenly is going faster than the input. Watch the video a couple of times, and then I'll explain what's going on. Email subscribers, use this link: https://youtu.be/oKAxQhT22Fg

When the overdrive is not engaged, the main shaft is spinning the over-running clutch at the same speed. It locks, and spins the annulus at the same speed also, as if the overdrive isn't even there. Of course, the planet carrier is also spinning at that speed, but that's irrelevant. The planets and sun gear are just along for the ride, and don't turn at all relative to the annulus.

But when the sun gear is locked from rotating, so are the planets because they are mated with the sun gear. But the carrier still wants to rotate with the main shaft, so it carries the locked planet gears along with it. These are also mated to the gear teeth in the annulus, and that motion has to go somewhere! The ratios are chosen so that the annulus is forced to rotate faster than the main shaft, and the over-running clutch allows that to happen. The center of the clutch continues to rotate with the main shaft, but the annulus and therefore the output shaft is moving even faster.

Easy, right? Only one more thing to ponder: how do we manage to lock the sun gear? There's one more part, called the cone clutch:

You can see that there's friction material inside and out, and both faces are tapered. The inner face mates with the tapered outside of the annulus, and at first I assumed it was providing the driving action just like a regular transmission clutch. Now we know that's not so - it's the over-running clutch that does that. The clutch surface that mates with the annulus is there to smooth things out during transitions, and provide engine braking when you let off the gas. Otherwise, the free-wheeling action of the over-running clutch would just let the car coast. The cone clutch is held against the annulus by four very strong springs.

The other friction material mates with a steel brake cone, and its purpose comes clear if you scroll up and look at the picture of the sun gear again. See the splines on the end opposite the gear? They mate with the splines in the center of the cone clutch. When the cone clutch is pulled back from the annulus against the brake cone, it stops cold, and stops the sun gear with it. The main shaft is still free to spin inside the sun gear, but now the overdrive is engaged.

Overcoming those four strong springs takes a lot of force, and that's why there's a built-in hydraulic pump that operates in excess of 500 pounds per square inch. That's what pulls the cone clutch back, and engages the overdrive by locking the sun gear.

Wow, that's a lot of detail. It probably is as incomprehensible as all the other descriptions, but it is mine and I'm standing by it. I'll close with one more link: this was the best of the online descriptions, with clear figures and pretty good text. I could understand it all after I figured it out my way.

https://www.uniquecarsandparts.com.au/how_it_works_laycock_overdrive

I welcome comments below or via email - help me make this description more clear!

Continue on to Part 57....