Zenith Aircraft Builders and Flyers
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CH 750 Landing Gear Pads Deterioration
This a.m. I was washing my STOL 750 in preparation for tomorrow's flight to Mexico, MO, for the Zenith Homecoming. While washing the landing gear legs, I noticed the urethane rubber pads in the landing gear saddles were cracked! I have 3 pads on each side - 1 above and 2 below. I added the 3rd pad on each side ?2-3? years ago - Buzzy recommended it to give a larger range of adjustment and further cushion the ride while taxiing. So, the "new" pads were excellent condition but the 2 original pads were hard as rocks and some were severely cracked/crumbling while a couple were still intact but the corners were breaking off:
I re-installed the good pads on top of the gear on each side and for under the gear cut some new pads from a very high density closed cell foam - it's what I had on-hand and it was either use that or cut some old tires up since I'm flying in the a.m. I plan to pick up some new urethane rubber pads while at the Zenith Homecoming.
The deteriorated pads were my original pads - they were probably manufactured about 11-12 years ago. I wanted to give a heads-up to inspect them closely if they are very old.
John
N750A
Replies to This Discussion
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Permalink Reply by Bob Simmons on
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Just as long as it doesn't keep you from getting to Mexico.
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Permalink Reply by Clint L Gosch on
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Good catch John! See you tomorrow!
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Permalink Reply by Jim and Amelia Isaacs on
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Looks like I need to add these to the five year rubber and bungee replacement task!
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Permalink Reply by Jimmy Young on
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John,
If I had known, we could have discussed this at the Homecoming! I had the same problem in 2017, when mine crumbled away. I replaced with a rubber product from McMaster-Carr. I ordered a 1/2” thick and a 3/8” thick piece which come in 12” x 6” sizes, the 6” being perfect for the fit on our 750’s. I used the Durometer rating of 60. It has happened to a few 750 builders, one in particular in less than 1 year of first flight and within 2.5 years of receiving the kit. This McMaster-Carr product so far seems indestructible from normal use and abuse.
Go to McMaster-Carr.com, look up part #86335K35
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Permalink Reply by John Austin on
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Thanks for the tip! I purchased new pads while at the Homecoming. I'll keep that part # from McMaster on-file and try them out the next time a replacement is necessary.
Where did you position the 1/2" piece and the 3/8" piece?
John
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As previously mentioned, I picked up some new rubber pads while at the 30th Homecoming at Zenith Aircraft. If you didn't attend the Homecoming, definitely put it on your bucket list for next year! I've mentioned in other posts that I think punching holes in the pads (rather than drilling, sanding, or cutting with a saw or knife) results in precisely located holes that have absolutely smooth and clean edges and are likely less prone to cracks around the periphery of the holes. On the original build, I used a vise to drive a leather hole punch through the pads and it worked great. However, this time I tried my hydraulic press and liked it much better:
Since you're driving the punch with the press vertically rather than horizontally as with the vise, it's easier to hold the punch in position while pressing it down into the rubber:
This picture shows how nicely the punch cuts a neat hole:
In the original installation, I used a carbon steel leather punch. This time, I wanted a slightly larger hole so the rubber wouldn't bind on the studs and welds alongside the studs. I didn't have a larger leather punch, so I made a punch by sharpening the end of a short piece of metal tubing. However, the tubing wasn't carbon steel, so it quickly starts to dull and deform. So, I've ordered an 18mm id carbon steel leather punch which should hold up nicely. I'll report back how that works next week after I receive the new punch.
John
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I received the 18mm carbon steel punch yesterday and completed the replacement of the pads today. As I expected, the carbon steel punch stayed sharp and didn't have any problems with the cutting edge deforming with the pressure. I also tried lubricating it with a bit of silicone grease and that seemed to help it make an even cleaner cut. I checked with Roger at Zenith and on the top pads, I made the outboard edge of the notch about 10mm from the outer edge of the pad. This positions the pads just a bit deeper into the landing gear mount for more support. The outer edge of the top pad still slightly impinged on the front strut attaching bolt, so I notched that area, too, so the pad was not binding or deformed by the nut on the bolt. The lower pads were centrally notched as above so as to evenly straddle the L angle support underneath.
John
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A follow-up to my post above ... What with at least one previous pad change and changing-out the pads with a temporary fix for the trip to the Homecoming at Mexico, MO and then changing-out those temporary foam pads for the proper urethane pads after the trip, I had run the MS21045-8 lock-nuts off and on the mounting studs several times and obviously, they no longer had much, if any, lock-nut tension! So, ordered some new lock-nuts from Spruce and decided to experiment a little in the process (typical for me ... I never leave well-enough alone! Ha!)
Reflecting on Jimmy's comments above, I decided to get some of the 1/2" thick 60 durometer rubber from McMaster-Carr and cut some new pads to replace the ones between the top of the landing gear and gear mounting structure on the fuselage. It had always bothered me that even if you notch the upper pads very close to the outboard edge of the pads, the gear mount just barely captures the pad along the inboard edge - in fact, although I notched the urethane pads within 10mm of the outboard edge, you could see the rub marks where the landing gear mount runs at an angle along the top of the inboard edge and since it runs at a slight angle, it actually runs off the pad before it gets all the way across it. Obviously, with this configuration, you're not getting full support from the pad and I suspect since the area of the landing gear mount that is bearing pressure on the pad runs off it at an angle, there's a tendency to try to push the pad out laterally from the pressure.
So, considering this, I decided to make these new pads 3" wide rather than the 2" as-supplied from Zenith. I then notched them similarly, so this puts almost a full additional inch of pad inboard past the area where the landing gear mount rests on the pad. I left the urethane rubber pads from Zenith on the underside of the gear as they fit perfectly and were new and saw no reason to waste them! BTW, the Zenith pads are definitely firmer than the McMaster-Carr material, so I'm guessing they must be ?75? durometer hardness?
I reassembled everything with the new lock-nuts and adjusted the tension so the gear wasn't loose in the pads. There is no torque value specified since you really need to adjust the torque to get the gear firmly held but with slight "give" if you grab a strut and try to roll the tire with your foot (Roger has a video on this), but in my case, with the softer, 60 durometer upper pads and firmer, ?75? durometer lower pads (I use two lower pads on each side - this was a tip from Buzzy and he said it allows a greater range of adjustment of tension) it turned out that with the new lock-nuts, it took about 65 inch-lbs of torque on each nut to get the desired firmness/security of the gear in the pads.
There's no earth-shaking (pun intended!) difference with the new pad configuration, but it seems to work well. Durability/longevity of the softer, thicker upper pads remains to be seen, but Jimmy told me they have held up well in his experience. The best part, however, is that the rubber is only $6.73/ft. and one foot of the 6" wide rubber will make several pads!
John
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