Broken mechanical tach

[Joel]

I just got this neat little mechanical tach for cheap, but it does not work in the middle speed range. It is made by the Clarence J. Marx Co. The high and low ranges operate through gearing and work great.





The middle range (2,000) appears to work 1:1 off the 2 larger center gears. The ends of each of these gears have 2 little V shaped nubs that look to be undamaged, but I am failing to see how they are supposed to engage each other.

[Wirecutter]

That's funny - just last night I was wandering aimlessly around the shop, and I came across a similar unit made by a company called Jones Motrola. (No, not "Motorola") It's probably very similar inside, but it's got a bit more corrosion.

Mine has a cover that comes off of the back, and the "speed change" knob is on the front. Looking at the gears you have there, you see the two parallel bars running vertically from the top to the bottom of the gear cage? On my unit, the speed change knob has a pin that rides between those vertical bars. It turns the entire gear cage about +/- 20 degrees or so. The middle position on mine gives the 1:1 ratio, deflecting the cage to one side engages the "gear up" function for low speed, and deflecting the other way gears it down for the high speed range. When the gearbox is in the "middle" position, the large gears move closer to each other and the "V" parts engage. When the gearbox is deflected left or right, the big gears move away from each other and one of the gear pairs on the sides will engage to increase or decrease the speed ratio.

The meter movement on mine comes off separately, and I was able to clean it up so it works. I've got to dig up a really small allen wrench so I can take more of it apart for cleaning.

Oh, and in case you might have wondered. The buttons that protrude from the sides control a slider that allows the reading to be held on the meter when the tach is disengaged from the rotating part.

Hmmm. Maybe I'll try to post some pics. I'll need to take some other pics down to do it.

-M

[Wirecutter]

In your last photo, you have the high range engaged - it's reducing the speed coming from the input shaft to the main movement.

In the second photo, I think you should have the mid range engaged - that is, the "V" bits should catch on each other, giving you 1:1.

Ok, I'll try to get some photos up.

[Joel]

Thanks Wirecutter.
You have the operation correct, except in my case, the cage wouldn’t move up and down. I couldn’t figure out how it would allow the gears to touch ends as there was no play in any of the parts and they all moved smoothly. You inspired me to take it all the way apart and I saw that the cage was symmetrical (gears and all) except for some tabs on the edge of the big hole on one end. Who ever had it before apparently took it apart and put the cage in backwards. When I flipped it end for end, it allowed the movement required for the gears to engage. Now it works great!

[Wirecutter]

Joel -
That's great - I got a new camera last week, and I was having a bit of fun with it. You saved me having to clean up my web site to post photos. Your tach looks a lot cleaner than mine, anyway.

-Mark