Interesting push-pull cable failure

[molson309]

This happened to me on my F1 Rocket but the same type of control cable is in use in many RV's - the ACS 920 type push-pull cable. Copied from my post on the Rocket group:

Today I had an ACS 920 cable fail - the one used for the mixture control - in a way I had not heard of before. I had been cruising at 8500 feet and had been leaned back - about 1/2 travel of the mixture lever. As I descended I began enriching the mixture and without warning the lever suddenly froze and would not move in any direction. From the way it felt, I came to the conclusion the engine end of the cable was jammed in some manner. Thankfully it was set in a position where I could make a normal approach and landing but I would not have wanted to do a full power go around.

Once on the ground I removed the cable and cut it apart. It became clear that it was frozen somewhere in the very last part of the engine end of the cable. My guess was that the cable was frayed and the loose ends had jammed it; but when I cut the end apart this was not the case. Something had jammed solidly in the telescoping metal pieces. I put it in a vise and pulled, and after using considerable force the end pulled free with a piece of cable attached that was in very good condition. I then put it back together and now there was no impediment to travel. I could see no scoring or other signs of blockage, so the cause remains unclear. My best guess is that some piece of foreign matter got into the end of the cable and jammed it solid, and when I forcibly pulled it apart this fell out.

This makes me more than a little concerned - if it had failed with the mixture in a position where the engine would have been too lean to run at lower altitude, for example, or if it happened to the throttle cable the outcome could have been different.

I've heard of these failing when the cable inside gets frayed and breaks, but never one locking up in a mysterious manner with no signs of what caused the jam. There was only about 100 hours of use on this cable, so in my mind it was in no way close to being worn out.

Should I look for a more robust replacement for this cable? Does anyone have a substitute they could recommend? I'd sure like to prevent this from happening again.

 

[Ironflight]

First time I have heard of the failure you describe Mark - but there have been LOTS of failures of these cables if they get to hot - the nylon liner seems to swell and grip the moving inner core tighter and tighter, until things just get too stiff to move. Lots of posts on this in the past. Your case sounds different - did you have the rubber boots on the ends? They sometimes get lost during installation.

Paul

 

[molson309]

The boots were still in place, though I have found that over time these tend to dry out and crack. Once I cut the engine end off, the cable slid very smoothly in its liner. In fact, once I pulled the engine end apart it slid smoothly too. It seems like some lube in that area could help a lot - on mine it was dry. Don't think much lube would get to the actual inner cable itself but would do wonders for the metal ends that slide against each other.

Maybe metal-to-metal galling? But I certainly didn't see anything obvious that would lead me to this conclusion.

 

[chaskuss]

If it has moving parts, it requires maintenance.

Mark,
You might want to consider adding cable lubrication to your routine maintenance regime. You can purchase a nice tool to make this job fairly easy. You will need to temporarily slide the black end boots off the outer cable to accomplish this task. The link below will show you what the tool looks like. It clamps over both the inner and outer parts of the cable. An aerosol can of lubricant is then inserted into the tool [at the cockpit end so that any debris will be driven out by the lubricant] Spray lubricant till it comes out clean from the engine compartment end. Remove the tool and snap the black rubber ends back onto both ends of the cable. See

http://motorcycle-accessories.cruis...echampion&utm_medium=sli&utm_source=altavista

http://www.ehow.com/how_6979622_lube-motorcycle-cables.html

I've been using these on all sorts of cables for decades [1970s] They work great. Just an idea for your consideration.
Charlie

 

[rocketbob]

These cables have a teflon lining and do not need lubrication. I suspect one of the telescoping pieces got bent because if they are not perfectly straight they will bind like this. I ruined a mixture control cable once trying to squeeze it into a bracket betwen it and a firewall obstacle. A slight amount of bend in one of the tubes caused it to freeze up.

 

[lrfrey]

Does anyone know where to buy new rubber ends. As previously stated they get brittle and crack.

Larry

 

[Walt]

Check THIS PAGE from spruce.

 

[flynwest]

Stuck cable

I had one fail like that. When I cut it open the teflon liner had pulled loose at the end and raveled or bunched and that froze the cable.

 

[molson309]

As an experiment I took the ends I had cut from the engine end of the cable and inspected them. As before there was no sign of any burrs, bends, etc., that might have caused the jam, but I did notice that the swages on both ends of the cable had been ground down - presumably by the factory, after swaging, to make sure there wasn't a sharp edge that could catch on something during movement of the cable. However, this does present a way by which foreign matter could wedge itself between the swage and the outer sleeve. So, I tried inserting a .020 piece of safety wire into this area and pushed on the threaded end - presto, instant jam. Immovable in both directions, until I pulled hard enough to dislodge the wire.

So, I am thinking this is what happened to me. Something wedged itself at this point and, once I pulled hard enough to free it, fell out and was lost. How it got there is a mystery - perhaps it was a piece of liner such as happened to a previous poster. Not sure how to prevent this in the future, but if it was this, pushing on the mixture lever with more force might have freed it. If it ever happens to me again this is what I will try.

 

[TX7A]

About those grommets...

Those grommets at the cable ends need to be made of something else. Mine have cracked and I haven't evan started my engine. (I know. I'm slow. )
The large one seems to be the worst.
When I replace them, I think I'm going to take some of this stuff and carefully wrap the grommet with it. It should conform to the steps of the grommet without streching it too tightly.
Probably want to make sure it doesn't interfere with the push-pull action.
May be worth a try.

 

[molson309]

Offhand, I would have one issue with using this tape as a replacement grommet. The inboard, stepped, one is seemingly mostly a dust seal and only faces the motion necessary to accommodate the 8 degree tilt the cable end is capable of. The outboard grommet, though, needs to handle the telescoping motion of the nested tubes, and maybe this tape isn't the best option here as it could perhaps get drawn into the gap and cause at the very least excessive friction. Or were you only talking about the inboard grommet? On my cable both of these grommets, or seals, cracked.

 

[TX7A]

Offhand, I would have one issue with using this tape as a replacement grommet. The inboard, stepped, one is seemingly mostly a dust seal and only faces the motion necessary to accommodate the 8 degree tilt the cable end is capable of. The outboard grommet, though, needs to handle the telescoping motion of the nested tubes, and maybe this tape isn't the best option here as it could perhaps get drawn into the gap and cause at the very least excessive friction. Or were you only talking about the inboard grommet? On my cable both of these grommets, or seals, cracked.

Mark
Yes, I was thinking mainly about the inboard grommet. And you're right. Seems like the least little bit of foreign stuff gets in there and problems begin to happen. The manufacturer just needs to use a better-suited material; even if it costs a little more.