Reverse Cam Modification

[Al Schober]

Thinking of modifying a reverse cam (OXKA-26) so the gear will stay in reverse without holding the shift lever. Wondering what some wiser heads on the board think of the idea.
Photo shows a spare reverse cam, cam height varies from .30" in neutral to a max thickness of .48". The proposed modification would grind down the nose of the cam to about .40" or so, letting the roller ride onto the parallel section. The actual height of the cam isn't really critical as the hex nut will adjust the tightness of the reverse band. It seems the critical part is to have the parallel part get to the roller just as my shift linkage gets to the full reverse position.
Of course, I'll have to make measurements of linkage travel both connected and disconnected. No sense having lots of motion in the gear if my linkage can't provide it.

[Dave Neptune]

Al, I see no reason it would not work. Built and modified many cams over the years so I have no fears here. If it were me I'd do some measuring and put a high spot on the cam and then a slight taper (smaller) forward. This would engage and the "over-center" lump would be like the detent in forward as it would hold it in place.

The problem would be whether the motion would stress the linkage of the band. IE the side motions is what I would really look at as the pressure of the clamping actions of the cam are pretty straight forward.

Dave Neptune

[ndutton]

I'm no reversing gear guy evidenced by my disappearance when the topic appears but I believe the continued run-out of the original brake band reversing bar applies increasing clamping pressure on the band the farther the shifting arm is moved. According to the drawing the modification under consideration loses the final 40% (approx.) of pressure application if needed.

The question becomes at what point on the ramp do you design the over-center? Too little ramp and she'll slip, too much and you'll never reach the quasi-detent. I can see where it might introduce a much narrower adjustment window.

It will be interesting where this goes.

[lat 64]

Perhaps an exernal detente fasioned against the shift lever in the cockpit. Visible, adjustable, and eventually removable after the accident.

[Dave Neptune]

Now Russ, that is a real confidence booster.

Dave Neptune

[thatch]

This is definitely some out of the box thinking, I love it! I will certainly open up the trans on my spare A4 and see where the best place for the detent dip should be. As Neil pointed out, I have a feeling that the reverse adjustment may be a little more critical with this mod.
Tom

[Al Schober]

Went out and played with the garage engine a bit. Fwd is obviously where the 3 fingers are up and in the groove on the operating sleeve. Neutral would seem to be where the fingers are against the smallest part of the operating sleeve. And maximum reverse travel (reverse adjusting nut loose) is where the operating sleeve hits against the screw collar for adjusting forward. During this full motion, the reverse cam moves about 1.25" - .7" going to neutral and another .55" to get to reverse. Interestingly, at the full reverse position, the roller is about 1/2" from the fattest part of the reverse cam.
On the boat, I wanted to measure things with the linkage disconnected, but had trouble getting the pin out of the shift arm. So, just backed off the reverse adjusting nut and cycled things with my shift lever - got the same full stroke. My linkage will move the operating sleeve fully against the screw collar (as well as into full forward). This is good.
With things in full reverse, I tightened up the adjusting nut until it was snug. Then I shifted into neutral and tightened the adjusting nut 3 flats. When I shifted back into reverse, I lost 1/8" of travel on the reverse cam. Back into neutral, tighten another 3 flats, lost another 1/8" of travel.
I'm thinking of grinding down the cam to the first 1/8" point - I can always go further, but putting metal back on is a pain.
A side benefit I see is that the flat area on the reverse cam will prevent overloading the roller if you push on the shift lever too hard. The loads will be limited by the setting of the adjusting nut - an extra push on the shift lever will do nothing - just move the cam further. Things won't get any tighter, and you won't shear off the roller! I'm liking this more and more...

[romantic comedy]

Is there any comparison to other trannies to be made here?

I have never fooled with marine trannies at all..Mine just keeps going.

[joe_db]

Maybe a Model T?
I cannot think of any automotive transmission that works even slightly like an A4 other than that and even that is not a very close match.
Speaking of which, if you have ever worked on a car changing a clutch you will realize how utterly ridiculous it is to think the A4 engine ever powered a Jeep, tractor, or any kind land vehicle.

[Al Schober]

OK, here's a photo of the modified reverse cam. Did the rough work with a grinding wheel, final work with a belt sander (120 grit), and finished up with a carbide oilstone. High spot measures .421" at .66" from the end and tapers down to .419. Should get it installed this weekend. Will adjust the nut so things don't slip at full reverse, and see if it stays in reverse without holding the lever. If it doesn't, I'll have to make the flat a little longer.

[edwardc]

If this works out, I propose we name it the "Schober" modification!

[thatch]

"Name it"

I'm leaning toward "Al dente"

[Al Schober]

Got the modified cam installed today. Easy job, took about 15 minutes. Only one glitch - I was using a 10d nail to turn the adjusting nut, and I dropped it into the gear case. It's still there.
Still not sure how much to tighten the adjusting nut - first try I had no reverse at all. Tightened a couple of flats, and now the shaft turns in reverse, but I can't tell if it's slipping. But so far, it seems to stay in reverse without holding the lever.
Next step is to use a couple of tachometers to watch engine RPM and shaft RPM. I've got an old Heathkit tune-up meter for the engine, and I've also got an optical tachometer that uses a hunk of reflective tape on the coupling. I think I'm also going to find out how much oil leaks out of the gear box cover when it doesn't have any bolts installed..
On the cam I removed, the worn area extends to about .59" from the end, so the modified cam (with the flat going to .66") should work fine.

[Marty Levenson]

oil shower

" I think I'm also going to find out how much oil leaks out of the gear box cover when it doesn't have any bolts installed.."

Been there, done that: do not advise it. It doesn't leak: it sprays!

[Administrator]

The reversing gear is a Paragon product, right? I think they're still in business. I wonder if they'd be really interested in what you're trying to accomplish.

Bill