Why do I seem to have problems getting my landing gear joints to accept grease? What grease should I be using?
(Search strings: gear pins, Aeroshell, Microgel, greasing, lubrication, Lubriplate, dry joint, knee pins, flush fitting)
I thought I’d better elaborate on this topic; primarily because I just finished reaming out another jammed grease hole, in a compressor plate pin off of a Sierra nose gear. Here are the key points:
- Our landing gear have knee pins and compressor plate pins. The design was originally intended to use a Lithium-complex based grease made by Lubriplate. Most folks are familiar with the ‘white grease’ by that name. It will develop ‘bleed-out’, but the remaining Lithium carrier does not harden up in place; it remains soft.
- The gear pins on the first 554 Model 23s had to be disassembled and lubricated by hand. It rarely got done, which is why I find Clevis retaining pins corroded away, and pivot pins corroded in place.
- Beech switched to a pin design made to be lubricated via Alemite flush fittings, using a needle fitting on a standard grease gun. To get the grease to the bushing-pin junction, they modified the pins. A .250 hole was drilled into the end of each pin, just long enough to seat the fitting in it (the nose pin passage is longer). A side hole was drilled to a diameter of .049”. The side hole originates in a circumferential groove, and intersects with the center hole for the lube fitting. The drawings say that the hole can be on a single side, or can go all the way through the pin. I find the in-service pins to be configured about half and half. If they are drilled only on one side, I always position the pin (on reinstallation) so that the radial hole exits into the primary load area of the bushing. Beech seems to have installed them randomly (some point up, some down).
- Grease goes from the lube fitting, through the tiny radial hole, into the narrow, shallow groove. From there it spreads out into the pin-bushing contact area. If you lube the gear with the wheel hanging free (you can do one leg at a time), the grease exits primarily via the loaded surfaces. That flushes them clean and puts the new grease where you need it. If you lube with the weight of the plane on the wheels, the grease comes out via the path of least resistance; the unloaded area beneath the pins. Very seldom will any grease make its way to the contact area where it is needed. This results in pin and bushing wear. On the nose gear in particular, it contributes to shimmy. Shimmy can also occur on a fixed main gear, if there is sufficient looseness from worn knee pin joints and loose Jo-bolts. Proper greasing is especially needed for the big nose pivot bearing (wheel hanging free, clear of the ground).
- This lubrication system design was completely compatible with the original Lithium-based Lubriplate grease. At some point in time, someone decided to specify the more common airframe grease in the Shop Manual, for the gear pins. Perhaps at the time the most readily available Mil-Spec grease was also Lithium-based. But when the most common Aeroshell products hit the scene, they were (and are) based on what Shell Oil calls ‘Microgel’. Microgel is Bentonite, AKA ‘fine clay’. I used to use it to make drilling mud, for drilling water wells. In my experience, the clay does a poor job of holding the lube oil in suspension. Over time, it dries out as the oil drains from it; just like a lump of wet clay will dry out on your back porch. Unlike other grease carriers, when it dries out, it hardens up. When our planes get greased only once per year (at best), it is very easy for the .049” drilling, and the thin, shallow groove, to have nothing left inside but dried out clay. If they have been greased with a gun containing an old tube of grease, already partially drained out, the stage is set for a ‘dry joint’. I have known owners who felt that if the 100-hour Inspection called for the gear lubrication, but they had only flown 50 hours prior to the Annual Inspection, there was no need to lubricate the gear for another 50 hours; despite the passage of a year. Ignorance isn’t always bliss; and we are all ignorant about many things. All that differs are the subjects.
- If you try to apply enough gun pressure to free the clay plug, you will most likely blow the pressed-in Alemite fitting out of the joint. I did an experiment once, in which I threaded the .250” fitting hole for a 6MM grease fitting, installed a 6MM Zerk, and used a standard gun on it. With my considerable weight on the handle, I still could not force the dried clay from the passage. Once completely dried, the only solution is .041” safety wire (or a wire-size drill bit), on a removed pin clamped in a padded vise. Last Sunday, with a pin in my padded vise, I was doing my usual grease-flow testing. I was leaning against the gun, to keep the nozzle seated and to hold in the fitting. One of the holes was initially jammed up. Just before I let up on the pressure (to go get the wire), the clay plug came free. The wad of dried clay, and the new string of blue grease behind it, nearly hit the top of the hangar. Another pin hole had to be reamed and flushed clean.
- The prevention is to use a Lithium-based grease. If you must stick with an Aeroshell grease, the only one I’m aware of is their newer Aeroshell 33. It can’t be a coincidence that Boeing drove a new spec that led to the AS33 development. One of the published reasons, as casually mentioned on the Aeroshell website, was to move away from the clay-based grease. I’d bet my bottom dollar that they encountered lubrication and servicing failures related to bleed-out, and the associated dry joints, with the clay-based greases. One advantage is that the AS33 is touted as an all-purpose aviation grease; airframe, wheel bearings, etc. The conventional (cheaper) Aeroshell greases aren’t a problem where there are large clearances or sliding surfaces. Frequent lubrication with fresh grease also minimizes the exposure. Most shops will have fresh grease due to turnover. Most owners have aged grease in their guns, when they only use them once per year.
Aeroshell website text follows:
- For many years aircraft operators have been seeking to rationalize the greases used on aircraft and to reduce the number of different greases in their inventories. Recently Boeing began research on a new, general purpose, corrosion-inhibiting grease. The aim was for a non-clay based grease that would provide longer life for components and mechanisms and possess improved wear and corrosion resistance. This led to the introduction of the new Boeing Specification BMS 3-33.
Owing to the wide range of operating temperatures, loads and other environmental conditions required for various aircraft components, several different types of grease with different desirable properties are used during routine lubrication of aircraft components. Boeing, in developing their BMS 3-33 specification, took account of the properties of the different grease types used on aircraft and wrote a specification for a grease which would provide improved performance and which could be used in the widest possible range of grease applications.
Shell worked closely with Boeing during the development of BMS 3-33 and formulated AeroShell Grease 33 to meet the improved performance properties required by this specification. It was the first, and for several years the only, grease approved to BMS 3-33. AeroShell Grease 33 is also approved to the MIL-PRF-23827C specification.