Nose Gear Bushings

·         matthew dovey

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Hi,
I have a Jab SK and the bushings that the nose gear strut fits into have some play. I intend to replace the bushings but the manual has no procedure for replacing them. Can you tell me if the bushings are bonded in or pressed in?

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·         Doug Smith

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Hi Matthew,
They're bonded in.
The replacement technique, in brief, is:
1. Acquire some new bushes. If your aircraft is old enough (circa mid nineties) you will need to get bushes to suit an "un-machined" nose leg. If the part of your nose leg that goes into the bushes has been turned down then you can get the current bushes.
2. Remove the old ones. They'll generally disbond from their flock bed if you prise them out. Then you need to remove the remains of the old flock bed back to bare glass.
3. Ideally final reaming of the bushes happens after they're fitted to the housing but in your case your best bet is to ream them to be a nice fit on your leg before you assemble it.
4. Apply some release agent to your nose leg. Mould release wax or a small amount of grease will work.
5. Prep the outside of your bushes for installation. Give them a quick rub with some coarse sandpaper then clean them with acetone to make sure there's no oil etc on the bonding face. Do the same with the sockets in the nose leg housing.
6. Mix some flock (epoxy resin and cotton flock) and bond both bushes into the housing. Don't be shy with the flock: more = better (within reason). While the resin is still wet, insert the leg into the bushes. That will keep all the bushes in line. Stand back and check the leg is vertical in the housing.
7. Spend some time cleaning up the flock and if necessary clean excess resin off the leg so it will come out when you're done.
8. Let it cure.
9. Remove leg, clean up, grease and reassemble. You might need to re-ream The bushes a little if the leg binds in the housing. Make sure it turns smoothly without binding.

The whole job can be done with the housing still on the aircraft if necessary but if you do it on the bench you will have access to the inside of the housing which will make controlling where the flock ends up easier. It's not a job for anyone unfamiliar with resin etc so if you're not happy with your skills I'd leave it to the experts.

Regards,
Doug.

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·         matthew dovey

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Hi Doug,
Thanks for the reply. My SK has been flying since 2001! The leg is 1.5 inch dia. Not sure I have a reamer that large. Looks as if some machining has taken place.
Is the flock being used to add some rigidity to the resin? I can't see any evidence of flocking on the old bushings. Would chop strand mat cut finely be a substitute for the cotton flock?

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·         Doug Smith

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Sounds like you've got the later "machined" nose leg which is a good thing, they fit the bushes better so you get less free play in the assembly.
The flock makes the resin / attachment stronger and more resilient. Compared to fillers like q-cells or micro-balloons it's harder, stronger and heavier while compared to talc or colloidal silicon it's tougher and less brittle. In theory finely chopped glass would work but ideally you want a good volume of sub-millimeter fibres to get into smaller spaces and hold it together. Trying to achieve that manually would be tiresome so I'd still recommend trying to find some flock.
Regards,
Doug.

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·         matthew dovey

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Hi Doug, just checking my local Jabiru dealers web site. He has two types of bushing. One which say machined, with an ID of 37.6mm P/N 6194104. The other with a ID 38.2 mm P/N 6055134. If my leg is 1.5 inch (38.1 mm) the I want the unmachined bush. The machined bush will need reaming!
I did contact them to confirm part number but heard no more.
I'll buy some flock when I've got some bushes.

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·         Doug Smith

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The "machined" and "unmachined" in the name of those parts refers to the legs... so the "machined" bush is suitable for the machined leg and vice versa. Assuming your leg is the machined type, the upper portion where it fits into the leg housing will have had its outer diameter machined down to a little less than 1.5" at manufacture and so the "machined" bush should fit neatly. It should be pretty easy to see or, if you have a pair of verniers you could measure the upper tip of the leg - the little bit that stands above the steering link etc.

If your leg is the older, "unmachined" type then the chances are that the bushes will still need some reaming - that's because the tube isn't perfectly round. The "unmachined" assembly always needed reaming to fit (to allow for minor variations in the tube sizes for different batches of material) which was one of the reasons for the design change to the machined type. We have an adjustable reamer here but it can be done manually by wrapping some sandpaper (about 120 grit) around a tube a bit smaller than the bush (say 1.25") and using it as a big file to gradually shave it down. Obviously it's more time consuming that way and you won't get the end product as round as if you were using a ream but it can be done if your patience outweighs the cost of the big ream!

Regards,
Doug.