Hendricks door latch

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Installing the Hendricks Door Handle, by Don Johnston.

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Any errors, typo's or suggestions, let me know. Please check here for the original article. For pictures and video, you can view the blog entry here.

Disclaimer: If you don't know what you're doing. don't do it. I take no responsibility for anything. If you use this to modify your door handles and you get stuck inside your airplane, too bad. If your door pops open in flight, tough luck. If it then departs the fuselage and takes out your prop, not my problem. Get the picture?

Lyle Hendricks sells these through Aircraft Spruce but you can buy them direct for about $100 less. Go to http://www.hendricksmfg.com/latchprod.htm to order.

I also ordered about 2 feet of 7/16" x .049" steel tubing. (AS PN 03-01800-2) and some 5/16" T3 aluminum rod just in case any of the shafts couldn't be straightened (AS PN 03-41600-2). I got 4 pieces 2 feet long..

In the box were two handles (each with a set of keys), a dimension sheet and a template for creating the opening for the handle.

The first thing I did was to swap the inside handles. In other words, the left inside handle became the right inside handle and vice-versa.

I found it easier to do the mounting with the handle disassembled. The handle comes apart with three hex-head screws. Remove the two outer screws first to remove the inside handle. I had to be VERY careful when removing the center screw to disassemble the handle further. Underneath the driver plate hub (the large part with the two lobes that the inside latch attaches to) is a spring loaded ball bearing that detents the handle in the open and closed position. As the hub slides comes off, the ball bearing will no longer be captive and once it sees light, it WILL make a break for freedom. I did the disassembly on a flat surface with a moving blanket on top. Ball bearings can't run very fast on a moving blanket so I had a better chance of intercepting it's escape. Once the handle is apart, I stored the parts until later.

Removing the old latch and handle.

1) Pull the cotter pins holding the existing three shafts and one pin (with intermediate link) to the cam. 2) Remove the shafts, pin and link and them aside. 3) Set the clevis pins aside for later use. 4) Remove the E-clip from the over center linkage. 5) Remove the screw holding the handle shaft to the cam. 6) Remove the exterior handle and cam/inside handle. 7) Remove overcenter linkage. 8) Drill out rivets holding the latch bracket to door skin. 9) Heat the bracket and using a putty knife, remove from door skin.

Even doing a real careful job, there were be holes and gouges. What worked best for me was to dig out the rivet guts and then fill the holes and gashes with epoxy/micro. When it cured, then it's time to prep for the handle.

Locate the center of the hole for the existing handle.
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Draw a level line .10" (about 7/64"... it doesn't have to be perfect, just get it close) below the center of the hole.
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I used the bottom of the window as my "level" line. I did mine from the outside and drilled two small holes on that line to transfer it to the inside. Then cut out a template for the door handle. Put the hole for the lock over the existing hole in the door and align the template handle opening with the level line. (Don't forget that the lock hole is .1" above the centerline of the handle!!!) Draw a line around the perimeter creating a rectangle 7" x 1.5". Now cut through the inner door skin and foam but do NOT cut the outer skin. Then remove the foam and scrape the inner skin until it's free of foam. Check that he handle housing will fit in the opening. Trim as needed. Once it fits, check that it's level with your line.
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Now take your template and place it in the opening and draw a line for the handle and lock cylinder. Confirm the lock hole is higher than the handle centerline. Then cut out the holes and trim and sand the openings until the raised part of the handle fits in the hole.
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From the outside, verify the handle is level. You'll get a much better result if the raised portion of the housing is proud of the outside of the skin just a little.

Sand the area of the handle housing that will mate with the inner skin. Mix up some gray-goo/cabo and glue it in place. You can put some weight on the housing to insure it's fully seated. I also used a combination of duct tape and Vaseline to keep any adhesive from sticking where it wasn't supposed to.

Cut six pieces of steel tubing 1-1/4" long and cleanup/deburr the ends.

Then using a 1/8" piece of aluminum stock, make two cams and eight intermediate links. Two of the links won't need to be as long as the rest, but rough cut eight pieces and clamped them together for finishing/drilling.
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Test fit the cam by mounting it to the driver hub using the two screws that are used to mount the inside handle. Unless you've got a milling machine, you'll spend some quality time with your file. All the holes are 3/16".

Once the adhesive has cured, you can relieve the inner skin and foam for the screw ends on the lobe and the lock arm. I used trial and error. Basically, put the lock cylinder in and remove foam until it moves. Then put the drive hub in, put one of the screws in the lobe and remove foam until the hub and rotate 90 degrees. Then repeat for the other lobe. When you're done, it'll look something like this:
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The steel sleeves in the door edge are not aligned with the new door handle. Most of mine came out by turning them with a pair of vice grips but a couple required some heat provided by a soldering iron. Take a sleeve, put it on a nice straight rod and put it in one of the holes OTHER THAN THE UPPER/FORWARD HOLE. The goal is for the end of the rod to point directly at the center of the driver hub and touch the edge of the cam that you just made. Grind on the hole until you can do that on those three holes. For the upper/forward pin, a new hole will have to be drilled. Draw a line 1-1/4" above and parallel to the level line that you used to locate the handle housing. When all the holes allow the shaft/sleeve to be in the correct alignment, file the part of the sleeve that protrudes out the outside of the frame so that is conforms with the edge of the door. Then using gray-foo/cabo, glue them in place. I suggest leaving the aluminum shafts in place to guarantee the alignment. I put some Vaseline on the shafts to keep them from becoming permanent. The upper/forward sleeve will need some internal reinforcement. The previous sleeve was located in a wood hardpoint. What I did was flip the door upside down, drill a small hole in the channel below (but now it's above) the sleeve, mix up a batch of running micro/cabo and inject it into the channel. It runs down and surrounds the sleeve. It can go much further because it runs into the wood hardpoint.
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At this point, you COULD skip the next few steps. Simply adjust the length of you shafts and attach them directly to the cam. But you're not going to be happy. When the cam rotates, the shafts have to flex to follow it around. This will cause them to bind in the sleeves and the door edge and makes it real hard to open and close the latch.

Here's how to make the handle move like... butter. A big stick of butter.

Mount the cam to the driver hub with an intermediate link in the rear hole and use clevis pins to mount two intermediate links to the bottom holes on the cam. It will look something like this:
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Then take one of the 1-1/4" long pieces of steel tubing you cut earlier and slide it over the aluminum shaft. from the inside. Point the shaft at the end of it's corresponding link. You want to position the sleeve so that it's 1-1/2" from the end of the intermediate link. You'll need to use some scrap foam to make a standoff so that the shaft maintains the correct elevation relative to the cam. Just swing the link out of the way and slide the shaft all the way to the cam to make sure the elevation is right. When you're done, it should look something like this:
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When you've done the other two stand-offs, cover the foam with some epoxy/micro and then cover the whole thing with a layer of BID.

The upper/forward link is custom length. To determine the dimension, attach a link to the short ping using a clevis pin. Then extend the pin through the sleeve as far as it will go. Now measure the distance from the hole of the pin to the hole in the forward lobe of the driver hub. Remove the, drill a new hole to that dimension (it will be somewhere between 3-1/2" and 4-1/16"), cut it and round off the new end.
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Homestretch

The other three shafts are too long. What I did was insert the shaft through the outer sleeve and the stand-off sleeve all way until it hits the end of the intermediate link. Then measure the distance from the hole at the end of the link to the outer edge of the outer sleeve.
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Once you have that dimension, add 1/2" to it and cut the shaft at that point. Say the distance from the tip of the pin to the hole on the link was 7-3/8" (7.375"). Add 1/2" and that gives you 7-7/8" (7.875). That's how long the shaft will now be.
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Once you've cut the shaft to length, you'll need to make a flat spot. My kingdom for an end-mill!!! But a file will have to do. I like taking 1/8" off one side and 1/16" off the opposite. When you've got the flats done, drill a 3/16" hole 1/4" from the end. Round it off and you're almost done.
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The result will be that the tip of the pin will stick out of the outer sleeve by about 1/4". That shouldn't be enough to hit the door frame and it will allow more contact into the door frame sleeves when the door is latched.

Final step.

The plane of the cam is not the same as the shafts. So it may be necessary to bend some of the links to match the angle of the shafts. Make sure that you bend the link at least 2" from the cam end to prevent binding.

Optional spring loaded over-center. Get a compression spring with an inside diameter of just over 5/16" and a length of about 4". Partially remove the lower/forward shaft and put a 5/16" washer, the spring and another 5/16" washer between the outer sleeve and the stand-off. With the handle in the open position, slightly compress the spring against the outer sleeve and mark the shaft. You can also put a small c-clamp on the shaft and test the compression on the spring by operating the handle. Once you're satisfied with the load, drill a small hole in the shaft for a cotter pin.
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Because the placement of the holes in the cam are overcenter, the spring will "load" the mechanism in the closed or open position.

That's it for the door. Now the sleeves on the door frame will have to be removed and reinstalled for the new position of the pins. A soldering iron and a pair of vice grips was all it took to get all of mine out. The upper/forward sleeve will need some internal reinforcement but I haven't got to that point yet. I'll probably do some variation of the trick I used on the door edge sleeve.
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