O-RING

Strut seal leaks and flat struts can be caused by roll or twist of the main O-ring seal. It is very frustrating to carefully check all the parts and surfaces and put a new seal kit in the strut only to find it flat again after a relatively short time. We have seen this happen when a film of MIL-5606 hydraulic fluid dries out on the exposed chrome strut shaft. MIL-5606 by nature has a tendency to get sticky as it is exposed to air and dirt and then dry.

Very slight dried oil films are sometimes hard to detect and they can get past the plastic wiper seal. When this happens the sticky strut has a tendency to grab the O-ring and roll or twist it when the strut slides in or out causing the seal to distort. A very small O-ring twist or distortion will cause a leak. This phenomena is aggravated by the low pressure that the Cessna strut is designed for. Wipe the chrome strut down with Stoddard solvent (mineral spirits) periodically to soften and remove any dried oil film, dirt, dust and bugs.

The Viton gasket and screw seals do not require any sealer and must not have any lubricants. Do not over tighten the attachment screws as over tightening can extrude the gasket out from under the transmitter and tear the gasket in the screw holes. This will cause a leak. Tighten the screws evenly until you see the gasket just start to extrude a little. Any lubrication will aggravate the tearing of the gasket.

There are many components all connected to keep the nose gear in line. If one component is worn or out of tolerance it can cause shimmy problem. Reference Dave McFarlane’s article: Can You Stop Nose Gear Shimmy instructions and suggestions.

Overlooked items also include the rod ends, shimmy dampener mounting and attachment, shimmy dampener, and steering collar. The steering collar is where the steering rod tubes connect and the upper torque link is attached to, along with the shimmy dampener on most aircraft. If the steering collar has play vertically and laterally and is allowed to tilt; that will cause excessive wear and force on the torque links, steering, and strut components. There are three different thicknesses of shims to help get the collar in place.

Ensure that the shimmy dampener is working properly with no dead spots in dampening action. The cylinder could be worn on the inside or on a piston that warrants replacement of components. All of the nose strut components are tied to each other and any movement is transmitted through to the tire and back through the dampening system, which if remedied will continue to be more pronounced.

When hydraulic oil changes temperature, the volume of the oil also changes. This volume change from a temperature reduction will create a vacuum in the oil chamber of the original Cessna uncompensated shimmy dampener. This vacuum will cause the oil to vaporize giving the oil a foamy expanded mixture that is compressible. The shimmy  dampener action is then drastically degraded. An increase in temperature will increase the oil volume causing a drastic pressurization of the dampener oil chamber. This pressure will force small quantities of oil past the dampener shaft seals. The decrease in oil will then aggravate any temperature reduction with increased chamber vacuum and related oil vaporization. This process explains why continuous servicing of the original shimmy dampener is required.

The temperature compensation system works by having a small chamber of oil that is spring pressurized through a very small passage into the main dampening restrictive orifice of the shimmy dampener. The spring loaded oil chamber can adjust for oil volume changes as temperature changes. A similar system is built into your car shock absorbers. The temperature compensated hydraulic system requires very little service over extended periods of time and assures stable shimmy dampening action.

A rubber based dampener is continuously fatiguing the rubber components as it changes direction of motion. The rubber system depends on stable friction of the rubber riding in a metal tube. This is very difficult to achieve over extended usage. There are inherent differences in static friction of rubber and dynamic friction of rubber that affect dampening performance. Long term use changes the performance of the dampener caused by all of these un-repairable factors. The hydraulic system works in a film oil with stable performance for long periods of time and it is totally repairable.