trim tab-jack screw-too much play??
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
trim tab-jack screw-too much play??
I was surprised this morning while doing the walk-around to find some play in the elevator trim tab. Getting under the tail the attachments for the push rod ends are ok but on remaval of the inspection cover under horizontal stab i noted about 1/16th play in the jack screw: ie when I move the trim tab up and down I get fore and aft movement at the gear end of the jack screw. The jack screw assembly is tight in the horizontal stab so I am thinking I have some slack in the screw assembly itself. Have any of you seen this and is this normal or something to worry about?
Gary, I don't have an answer for you exactly. I don't have any "specs" for allowable play in the screwjack actuator.
But I do have 3 actuators here in the hangar, and none of them have that much play. Two of them are from other models, and one is on my B-model. There is only a barely noticeable amount of end-play in my 170B's actuator, resulting in about 1/32" of play total at the trim tab itself.
The other two Cessna trim screwjacks have virtually no end-play at all in them.
I've had one apart several years ago. They're very simple affairs inside, and if the screwjack and screwnut themselves are not excessively worn, then shimming the unit shouldn't be too difficult. I recommend marine-type grease (waterproof) when lubricating these units. I've used boat-trailer wheel bearing grease to personal satisfaction on low altitude airplanes like these.
But I do have 3 actuators here in the hangar, and none of them have that much play. Two of them are from other models, and one is on my B-model. There is only a barely noticeable amount of end-play in my 170B's actuator, resulting in about 1/32" of play total at the trim tab itself.
The other two Cessna trim screwjacks have virtually no end-play at all in them.
I've had one apart several years ago. They're very simple affairs inside, and if the screwjack and screwnut themselves are not excessively worn, then shimming the unit shouldn't be too difficult. I recommend marine-type grease (waterproof) when lubricating these units. I've used boat-trailer wheel bearing grease to personal satisfaction on low altitude airplanes like these.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
George, Thanks for your speedy reply. Been back to the airport and wiggled every cessna trim tab mostly 172s and 150s and many seem to have the same play. Anyway for my own mental health I am going to remove it and inspect it and go from there. I got 40 hours on the bird and this is the first wrinkle I hope no big deal tho that darn trim is pretty important. I will let you know what i find. Thanks again G
OK, Gary. Let me offer a couple of ideas for your project.
1. Have several short lengths of safety wire to hold the chain in place while the unit is out of the airplane. Keep tension upon the chain cables while you're doing your work. (You don't want the trim cable to fall out of place where it goes around all those small pulleys throughout the fuselage.)
2. Don't move the trim wheel while the unit is removed, AND... start the project with the index at zero, and the trim tab itself confirmed to be in the nuetral position.
3. Before you remove the push-pull rod from the tab and the actuator...look it over very carefully for orientation. You'd be surprised how many of these get re-installed upside/down/backwards, and no one notices for years until the chafeing is noticed where it rubbed the paint of the tube.
4. Confirm that your trim tab has the correct amount of nose-up/nose-dn angle, both before AND after the repair. This is adjusted by the cable-stops (clamps) inside the fuselage.
5. This is a good time to inspect your trim cable for frayed strands, general condition. You can use the old cable to pull a length of nylon string or safety wire thru, then use the string/wire to pull the cable back into/thru all the pulley's/fairleads (but you can only do it if/and after the cable stops are removed.)
1. Have several short lengths of safety wire to hold the chain in place while the unit is out of the airplane. Keep tension upon the chain cables while you're doing your work. (You don't want the trim cable to fall out of place where it goes around all those small pulleys throughout the fuselage.)
2. Don't move the trim wheel while the unit is removed, AND... start the project with the index at zero, and the trim tab itself confirmed to be in the nuetral position.
3. Before you remove the push-pull rod from the tab and the actuator...look it over very carefully for orientation. You'd be surprised how many of these get re-installed upside/down/backwards, and no one notices for years until the chafeing is noticed where it rubbed the paint of the tube.
4. Confirm that your trim tab has the correct amount of nose-up/nose-dn angle, both before AND after the repair. This is adjusted by the cable-stops (clamps) inside the fuselage.
5. This is a good time to inspect your trim cable for frayed strands, general condition. You can use the old cable to pull a length of nylon string or safety wire thru, then use the string/wire to pull the cable back into/thru all the pulley's/fairleads (but you can only do it if/and after the cable stops are removed.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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Joe Moilanen
- Posts: 600
- Joined: Sun Dec 28, 2003 5:45 am
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LakeHood170B
- Posts: 29
- Joined: Tue Mar 21, 2006 10:36 pm
There are two 5/16" ID by about .024" thick external "C" clips inside the elevator trim jackscrew mechanism, one up against each end bushing. If one of those were to have broken, that might produce the slop you're describing.
The Cessna pins that hold the end bushing in are a pain to remove and reinstall -- you'll need a punch no larger than 1/16" diameter, preferably smaller, to drive them out. The 170B parts book shows two per bushing, but found only one.
Since the slots in the bushings seem to be hand-made (hence probably off-center and crooked as both of mine were), take note of their orientation coming apart so you reassemble it the same way, else you'll try to create an interference situation or have excessive slop. In fact, you may have nothing more than an improper reassembly by someone before you, with the symmetrical bushing (the one on the sprocket end) turned around 180 degrees (in pitch or yaw, not in roll) from way it was when the hole for the retaining pin was drilled. The bushing on the other end is asymetrical -- it only goes back in one way to accomodate the rotating internal part fitting inside it.
The Cessna pins that hold the end bushing in are a pain to remove and reinstall -- you'll need a punch no larger than 1/16" diameter, preferably smaller, to drive them out. The 170B parts book shows two per bushing, but found only one.
Since the slots in the bushings seem to be hand-made (hence probably off-center and crooked as both of mine were), take note of their orientation coming apart so you reassemble it the same way, else you'll try to create an interference situation or have excessive slop. In fact, you may have nothing more than an improper reassembly by someone before you, with the symmetrical bushing (the one on the sprocket end) turned around 180 degrees (in pitch or yaw, not in roll) from way it was when the hole for the retaining pin was drilled. The bushing on the other end is asymetrical -- it only goes back in one way to accomodate the rotating internal part fitting inside it.
1952 170B, UNCERTAIN (1983 337 says Doyn) 180hp conversion, EDO 2000s
Got ther trim tab actuator back from my friendly AME. I wasn't a party to the rebuild but he did shim it and all the play is gone. Re installed and careful not to disturb the trim cable and chain positions. Happily the clunk at the trim tab is gone. Again thanks for your guidance on this one. Cheers G