Beck TD, Part 57: Overdrive Success!

After my recent post about my attempt to understand how my M41 overdrive does its work, I was ready to reassemble. I used a lot of new parts, including all three bearings in the overdrive unit, and all the parts still available via Moss Motors - mainly o-rings, a spring or two, various seals and washers. I won't bore you with pictures of the assembly, because it is extremely well-documented in this pair of videos by John 'The Box' Roseby on The Sunbeam Alpine Channel on YouTube:

Disassembly: https://youtu.be/Ht6eb7w4gto

Assembly: https://youtu.be/WvAjwhIaSzw

Once it was done, I installed the overdrive unit on the transmission, and set it on my crude but effective test stand:

There were a few things to attend to before I could really test, beginning with the adapter I had welded up previously when I rebuilt the M40 transmission. It worked fine with the 150 RPM motor I was using to rotate the input shaft, but it had just enough runout to make the motor vibrate. I was not willing to risk that with speeds 10 or 20 times higher. I cut it apart, cleaned everything up and re-welded, and got a good result:

The other main bit of fabrication involved the pressure gauge. At first I just bought the adapters necessary to hook it up, but then realized that I needed something to hold the spring under the gauge in a very specific location. I made a pretty brass adapter with a shoulder in the right place to hold the spring. I don't know any way I could have accomplished that goal without making it - no chance of finding that exact adapter in the open market. Here's three views:

Once that was done, I could hook it all together and give it a try. Here's the whole test setup, still being spun by the 150 RPM motor:

At first the pump wouldn't prime, but after some fussing around, I got a solid 360 PSI of pressure at the gauge:

Two problems: First, that pressure should have been 575 PSI - 360 is not enough to actuate the overdrive. Second, I had pressure all the time. That's wrong. I should only see pressure on that gauge when the overdrive solenoid is "on."

Here's the parts diagram for the actuator mechanism under that gauge. When the overdrive is actuated by energizing the solenoid, the arm #4 lifts #41, lifting the ball bearing, #42, off a seat, allowing oil to flow to actuate the overdrive. I had replaced #42 and the copper washer, #45 - the only two parts still available. Kelly Williams was over to see what was going on, and after looking at it with me, asked the pertinent question: how can that small bearing plug that hole?

I had actually verified that the ball bearing was correct, according to the Moss website. But after Kelly's question, I rummaged around and found the bearing I had removed. Hmmm....

Moss had specified a 1/4" bearing, but I didn't notice that the one I removed was 5/16" instead! I put it back in, and the actuator immediately worked correctly, with zero pressure when the overdrive was not engaged, and pump pressure otherwise. However, the pump pressure was still 360 PSI - too low.

I wondered if the slow RPM was the problem, and had a couple of false starts trying to upgrade my motor. The first substitute was too weak to turn the trans, and the second rotated the wrong direction and was not reversible. I finally got the idea of using a heavy-duty 1/2" drill as the motor. That gave a new result - at the higher RPM, the pressure on the gauge wildly oscillated between about 200 PSI and 360 PSI. After pondering a bit, I realized that would be consistent with the relief valve rapidly oscillating.

I consulted the Volvo repair manual and found this interesting info (red box, click to enlarge if needed):

That triggered a memory. When I disassembled the relief valve, some beat-up washers fell out, including a partial thin one. I asked around and no one had a guess what that was about:

The relief valve consists of a sleeve with a (new) o-ring, a plunger, and a very heavy spring. The fault analysis info from the manual told how to choose the right washer to increase the pressure. It seemed obvious somebody was building the pressure with those washers, and things went south when the thin one broke. Even though they were bent up, I was able to to measure their thickness with a micrometer, and found 0.030 + 0.025 + 0.005 = 0.060.

Let's see... the manual says each 0.004 of thickness raises the pressure about 14 PSI. 60 thousandths divided by 4 thou = 15. 15 X 14 = 210. My pump was at 360 PSI. 360 + 210 = 570. The spec is 575! This can't be a coincidence.

I set out to make a solid spacer of 0.060 thickness, and found out how hard it is to us a parting tool to make such thin parts.  I made a few. This one landed at 0.065", and I decided that was good enough for a test.

It certainly helped, moving the pressure to the mid-400s. I tried another attempt at 0.077", and the pressure was about 500 - still not enough. A third washer at 0.090" gave me 575 PSI, and I heard that satisfying CLICK as the pistons moved the cone clutch and engaged the overdrive.

But I still didn't know if engagement was solid. I asked Phil Oles to bring his digital tachometer to measure the output shaft speed. This device uses a laser plus a small piece of reflective tape to precisely measure rotational speed. First up: measure the speed at the drill that was driving the system:

Then I measured the RPM at the output, first with the overdrive NOT engaged (thus, a gear ratio of 1:1) and then engaged. The picture tells the story!

More math. The overdrive ratio on a type D Laycock overdrive is 0.76. That means the output should spin 1 / 0.76 = 1.32 times faster. 1812 / 0.76 = 2384 RPM. I'm not surprised the indicated RPM is a little lower, because we heard the drill motor bog down a bit as the overdrive engaged.

This has been such an interesting learning project, and I now have an overdrive transmission ready to go. I'm actually not sure it's going into Beck TD. I've been pondering a second project car - perhaps a Volvo PV544. That would be very cool with an overdrive trans.

One more link: the guy that did such an excellent job of the videos referenced above has an absolutely magnificent test stand. Even if you don't watch the whole video above (and I wouldn't expect you to unless you have a transmission on your workbench), check out this link, which goes right to the point where he's testing. If I'm going to keep doing this transmission stuff, I need something like this!

https://youtu.be/WvAjwhIaSzw?t=1225

Continue on to Part 58...