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shimmy


thunderturk
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we have a problem at nose landing gear, during take off shimmy starts until 70 knot after that there is nothing shimmy after 70 knot. we have seen shimmy at the same time on steering control wheel. we have changed nlg wheel tires and we have checked steerıng control valf, and we wehave done nlg door rigging. we suspect steering control valf.

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check to make sure trunnion bolts and caps are tight. I see you have already changed the tires, so like another poster said to make sure that the treads are the same. Steering control valve could be the culprit, but normally it alwasy reverts back to the tires. Let us know what you find out.

Dave

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Have you checked the upper/lower torque arm bushings where they connect to the strut and where they connect together? They will wear out and cause a shimmy.

This is what I was trying to remember. In the desert we had some dirt get in there causing a shimmy.

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  • 9 months later...

Again shimmy , during take off, shimmy starts 40 knots continuing until 70 knot and at landing starts 70 knot stops at 40 knots.

upper and lower torgue arm are tight.tire pressure 60 psi and same level.tires condition good. nlg door rigged.steering control valf normal.fulcrum bearing replaced.steering cable tension checked.

there is no any fault at nlg .

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First step is replace NLG Tires, this INCLUDES the bearings.

Next step is to check the steering and rigging.

Also make sure you have the same type of tire on the nose, same tread pattern.

Doesn't have to be same tread pattern. That's an old wife's tale. Matched set means the outside diameter of the tire/wheel assembly is within the required measurements.

Have seen MANY, MANY fall to this and have two nose tires with SAME tread pattern yet the outside diameter difference between the two assemblies are out to lunch.

So long story short, just because you changed tires, if the "new" ones are not matched, the gripe is still there.

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Doesn't have to be same tread pattern. That's an old wife's tale. Matched set means the outside diameter of the tire/wheel assembly is within the required measurements.

Have seen MANY, MANY fall to this and have two nose tires with SAME tread pattern yet the outside diameter difference between the two assemblies are out to lunch.

So long story short, just because you changed tires, if the "new" ones are not matched, the gripe is still there.

According to the job guide, they do have to have matching tread pattern. At least that's the Air Force.

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I don't know that shimmy in the NLG is ever caused by just one problem area. Usually, there are two or more items that are "a little off" but together the result is a nose gear shimmy. If it happens on the roll, then first of all, the shimmy dampeners must not be able to compensate for it. The original reason will be something else, whether it is a scissors bushing problem, a nose tire problem, nose bearing problems, etc, etc. Conversely, we can frequently eliminate a shimmy for a short while just be fixing one of the problem areas, and I fear that is what we usually do.

If I have a shimmy on takeoff or landing roll, I always bleed the air out of the steering actuators to allow the shimmy dampeners to do their job. Like scissors bushings, that is a pain so many folks either don't want to do that or they won't do it thoroughly. In addition to that, I try to find out why the shimmy occured in the first place. Nose gear tires are allowed to be of different diameters by only 3/8th of an inch. It is not easy to measure on the line and should have been done before installation. Most folks want to disregard the scissors bushings as they are a pain to change out, so worn out bushings are fairly common.

If you have aggressive operators who like to taxi fast and make 90 degree turns, you will eventually get out-of-balance tires where they were skidded sideways. The only way I have been able to check them is to slow roll them when the nose is jacked up off the ground. I've had them either stop in the same spot most of the time, or I have had them stop and reverse direction of rotation all by themselves. However, even then, it usually takes another condition to permit the nose gear to shimmy, and I find that worn scissors bushings is the other condition. The shimmy dampeners can't work well if the nose gear is "loose."

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Check the measurements... Air Farce is out to lunch sometimes as well... Does it say "Shall" or "Should"?

Here is what the job guide says about the nose tire tread!!

NLG tires on the same axle require matching for size. The outside diameter of fully inflated tires shall be the same diameter within limits specified in Figure 2-1 of TO 4T-1-3. Nose tires should be of the same tread design when available.

‘‘Should’’ and ‘‘may’’ are used to indicate nonmandatory provisions!!

The diameter is the most important when dealing with a shimmy, usually it is something loose or worn causing the shimmy other than the tires! But of course tires and whel bearings are the easiest fix and more often than not the problem will reoccur.

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  • 7 years later...

Check the Nose Wheel Steering Actuator rod ends for play.  The bearing is press fit into the rod end, if you pull the bolts out, and the bearing falls out of the rod end, the actuator is bad. Both actuators usually go bad at the same time.  There are 2 modes for the steering control valve, shimmy dampening and steering.  In shimmy dampening mode a small amount of fluid is ported to both actuators for a shock absorbing effect. if the tires, bearings, Control Arm Bushings and cable tension is good... only then change the steering control valve.   

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21 hours ago, C130ron said:

Check the Nose Wheel Steering Actuator rod ends for play.  The bearing is press fit into the rod end, if you pull the bolts out, and the bearing falls out of the rod end, the actuator is bad. Both actuators usually go bad at the same time.  There are 2 modes for the steering control valve, shimmy dampening and steering.  In shimmy dampening mode a small amount of fluid is ported to both actuators for a shock absorbing effect. if the tires, bearings, Control Arm Bushings and cable tension is good... only then change the steering control valve.   

You got quite a bit wrong here.

1. The rod end bearing are slip fit.  If they fall out its just because the adhesive has failed.  Those bearings are literally glued in.  I have rebuilt/overhauled hundreds of them, the bearings slide right in and get glued in place, I have never seen one become loose enough that it would cause a shimmy, even if the glue fails.

2.  No fluid is ported during shimmy dampening.  The control valve goes to neutral and the sprung piston accumulator in the control vave takes the shock from the actuators to prevent abrubt movements.  Fluid is ported during steering mode only.

Edited by hehe
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