Tach cable and tach failure

[Bruce Fenstermacher]

Went flying today. It was the first fairly cold day of the season at about 35 or so.

All was well till I glanced at my tack to see that it was bouncing from about 2400 to 3200 RPM. The throttle was set at 2500 before this started.

Knew that it was the probably the cable and congealed lube. No problem have seen this before. But then the tack stopped bouncing and just started slowly climbing holding various setting over 3000 rpm and at on point pegged at 3500. I started hearing a noise either from the tach itself or the cable. Decided it was time to land.

After landing I disconnected the tach drive at the engine and tried to pull the cable out. It wouldn't come out. I've done this before at least with my Cub and I'm sure with the 170. The cable usually comes right out. This time it wouldn't. The cable does turn free.

I removed the cable drive the plane and looked in the tach side. It looks like there is a brass bead of some sort that is stuck to the end of the inner cable not allowing the cable to slide out of the outer sheathing.

I don't remember this. Is it normal or has something attached itself to the cable drive shaft?

Second I've seen plenty of speedometers and tachs bounce and understand how that happens with the tach cable binding and winding like a spring then releasing. What puzzles me is how does the tach get driven at steady pace, faster than the tach cable is being spun. Or I suppose something is binding in the tach itself causing the needle to register at a higher rate. Does anyone have any idea.

[mod cessna]

The tach works like an automatic transmission. It is not directly connected to the cable but rather through a drag cup system. When this gets oily it slips less then it should and thus you get higher reading. It will eventually fail reading full scale high.

[jrenwick]

Bruce,

I've seen different end fittngs on tach cables. The stock one should not have any sort of a fitting on the end that would prevent it from being pulled out of the casing from the tachometer end, but the cable on my 170 was not made that way. Last year when I replaced it, I ended up destroying the end fitting to get it out of the casing. I replaced it with a stock part from Wentworth, but it would have been cheaper to order a new one from Spruce.

Best Regards,

John

[GAHorn]

Bruce, when tach drive cables are factory-made they are formed with the square drive at each end and can be removed by pulling them out either end.
Aftermarket (and virutally ALL universal automobile speedo cables) are cut to length in the field and a brass square fitting is swaged onto the end. That end fitting has a collar which prevents it being pulled out the opposite end. This sounds like your situation....you probably have a replacement cable that has been mfr'd in the field with a replacement core placed into the existing housing. It's likely your replacement was acquired at the local Napa and fitted with the brass fitting.

Mechanical tachs have a couple of different methods of operation...the most common being the centrifugal and the magnetic.

The centrifugal tach depends upon the action of fly-weights which move outward from a rotating shaft owing to the centrifual force generated as the shaft rotates. When the weights move outward they compress a spring-loaded scissor which, thru a lever, moves the needle to indicate RPM. If the oil is congealed or lost, or the scissor becomes worn due to lack of lubrication, the mechanism may seize, the spring may fracture, and provide inaccurate readings, including excessive RPM.

The magnetic tach relies upon a magnet being turned inside a cup which is in turn pulled by the magnet against a hairspring. The balance of the forces between the pull of the magnet and that of the hairspring makes the drum take up a position that is a measure of RPM. Again, congealed oil (or lack thereof) will cause an indication error, but in the magnetic tach an additonal error can occur.... that of metal filings accumulating due to wear... which can jam between the magnet and the drum thereby increasing the effective RPM of the jammed drive-and-driven-drum... resulting in an overspeed condition (and resultant excessive RPM.)

Your tach may return to normal if warmed.... but it likely needs cleaning, calibration, or (probably cheaper) replacement. You might try disassembling it and using electronic cleaner to blast it clean and relube it with light machine oil like LPS 1 or WD 40. Don't over do it, tho'. A Q-tip is a good applicator.

[blueldr]

George,
You really know thar WD-40 is a rotten long time lubricant, don't you.
Just look at the tip of an old can of wd-40 and see the gummy residue around it.

The stuff was designed as a "Water Displacer #40", and it works really good for that purpose.

[GAHorn]

Yeah..I really shouldn't have suggest that stuff for that purpose. But it was Bruce's tach!

The ideal lubricant would be instrument oil, preferably a synthetic. It may seem weird but a visit to the local aircraft shop and a drop or two of ordinary synthetic turbine oil (such as used in medivac helicopter turbine engines) is probably a good substitute.

[Bruce Fenstermacher]

Thanks George. I was thinking I'd just submerge the tach in WD-40 and fix the lube problem, stop any corrosion and I won't have to polish the tach as much.

At the very least I'd be sure there was no water in the tach which is what the 40th formula for Water Displacement was designed to do.

I'n not a big fan of WD-40 as a lubricant for much other than my house door hinges. A light oil such as turbine oil might be the ticket. I'll be looking at the tach next week. FYI talking to the folks at Keystone Instruments a few years ago the owners son told me the FAA likes to see Vaseline as the lubricant in lots of places like the tach cable and in the case of the Cub tach on the oil pad at the tip of the needle.

George I'm pretty familiar with tach and speedometer cables and the squared of ends. The end going into the engine will have a shaft with a key swaged on to it. The shaft with the key being slipped into the slot in the oil pump gear shaft inside the tach cable attachment housing. The other end should be just square and slip into the square socket in the tach. My cable doesn't have anything swaged on to it to make it square but the cable itself is square as you would normally find and George described. But down inside about an inch from the tip of this end seems to be a bead of some type that is stuff to the cable preventing removal.

A new cable is probably in order. Didn't know NAPA could be a source, thanks for the tip George. I'll check out Spruce which might be less trouble though.

[bsdunek]

If instrument oil cannot be obtained, I suggest Remoil. It is marketed by Remington for guns, and is a synthetic oil that will not gum. You can get it a gun shops, or department stores with a hunting section.
I would use a toothpick to apply oil in this case.

[N2540V]

There are automotive shops that overhaul tachs and build new cables.
A bit of their business is aviation tachs.
For the Seattle area flyers, there is one down near Tacoma on Hy 512. I cannot think of its name right now.

[GAHorn]

When my tach cable gave me trouble in 2002 I bought a replacement from Spruce and it's been good so far. Drill a 1/8" hole at the 6 o'clock position in the "bell housing" at the knurled nut where it attaches to the engine drive. (It'll allow a leaky garlock seal to drop oil into the cowl instead of pumping it up into your tach and cockpit.)

[33alfa]

You guys are killin me. Do not mess with that Tach. Most of the old ones are crimped together. Not worth messing with, besides everyone knows the only thing WD 40 is good for is fishing Halibut. On a serious note as an instrument guy who will not work on them. You shouldnt! After all were talking about ruining a $15,000 plus engine for trying to save money on a$200.00 Tach. WD 40 Ug!!!! 33alfa

[GAHorn]

If you have the standard prop you'd have a hard time ruining the engine due to rpm/overspeed whether you have a tach or not. And WD40 is good for a lot of things besides Halibut. It's a great cleaner/bug/tar-remover/penetrating oil. I wash my airplane with it instead of soap.
(It's illegal to work on instruments without the proper certification, of course. It's the rebel in me, (and the incredibly cheap construction of the typical guage and my good luck) that keeps me tinkerin' with them anyway. I've repaired oil temp gauges several times when they read incorrectly, a lot cheaper'n buyin' another one which arrives ready-to-fail.)

Tach cables and tachometers are a different matter, I agree. Better to just buy another one.

[Bruce Fenstermacher]

There is a lot to be said about buying new. There just under $200 but without markings. That cost $30 more. You do get a 2 year warranty.

Keystone Instruments will repair, calibrate and yellow tag the tach for $159. They will also restore the face plate to original specs or your specs if you want.

If your doing an absolutely original rebuild or have a Cub with unique instruments this is the way to go. The instrument will essentially be new when you get it back.

While I may end up lowering my originality standards just a bit or at least enough to put a new tack in my somewhat original plane, at this point I haven't done that yet.

In the mean time I have learned a lot about the workings of a tach. Not my airplane tach of course, that wouldn't be illegal. But others of strikingly similar design.

For example for bonus points when your tach is reading 2400 RPM what RPM is the tach cable spinning?

[hilltop170]

2400 rpm* X gear ratio of the tach drive system = tach cable rpm

How many bonus points do I get? Do they count toward my score on the next judging? Is there a catagory for "Rat" 170s?

*assuming tach is reading correctly

[Bruce Fenstermacher]

Well you haven't won yet Richard.

If you know anything about our engines you will be able to figure this out to the exact RPM of the tach cable. it just takes some thought.

For this contest you can assume the tach is accurate when it's reading 2400 RPM.

Richard a "Rat" 170 category is a GREAT idea. But you'd be hard pressed to beat my plane. And I do like yours just the way I saw it at Galveston.