Photo: Courtesy of Aircraft Spruce
Photo: Courtesy of Matronics Wiki a Wikipedia for Aviation
Photo: Courtesy of Aircraft Spruce |
Photo: Courtesy of Matronics Wiki a Wikipedia for Aviation |
| Shorly after joining the UltraCruiser Yahoo discussion group, one of the discussion topics was about using Taper Pins for connecting the wing fittings. Being a glider pilot and having owned a Schweitzer 1-35c, I have great respect for taper pins over bolts and close tolerance bolts. After much consideration, I decided to use taper pins in place of bolts. Since I didn't know how to proceed, I contacted friends with more knowledge and researched taper pins on the web. | Advantages:
The
taper pins are very easy to insert (important when trying to align a
heavy 22 foot glider wing) and easy to remove (a slight tap, and they
pop out). Disadvantages of bolts: If tight, very hard to insert. Inserting can cause wear. If slightly loose, the hole will eventually elongate |
| RV builders/owners are replacing their nose gear bolts with taper pins LINK due to elongation of the bolt hole. |
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| Before making the choice to use
tapered pins, you must determine: |
- Does the taper pin you select
have the required tensile strength? - After reaming the tapered hole, do you have sufficient edge distance? |
I
made a test run on scrap aluminum which exactly matched the
composition of all the connecting pieces in the main spar fittings as
shown on the plans, sheet 6 (even including a piece of .020 sheet).
Here is how I did it
... |
For the fittings on the
main spar: Clamp the fittings in place ... block up to ensure clear space below the hole. Drill a pilot hole, then .... For the #3 taper pin, drill a 5/16 inch hole (which happens to be the size on the plans) ... if you have already drilled your fittings, the previous steps are complete. Replace the drill bit with a #3 reamer. Carefully advance the reamer until you can feel it at the bottom of the hole. Then advance the reamer another 1/8 inch. Remove reamer, turn off power, and slide fitting to one side. ... insert taper pin and count the threads sticking out the bottom. Calculate how much further you must advance the reamer. Make advances in small steps ... a bit too far ... and your fittings will be ruined. As a final note, the
taper reaming of the hole is very sensitive. If you take off
just a smidge too much, then the pin will drop into the hole a long
way.... You might practice once or twice....on scraps. (Chris
Bobka)
When the desired fit of the taper pin is achieved ... stop. |
 |
This
image shows the finished hole with the drilled taper pin special washer and castle nut. Note the indented portion of the washer. |
 |
Threaded end of taper pin extending through joint with a small shoulder. The special taper pin washer will fit over the shoulder. |
 |
Taper pin with special washer and castle nut
(shown finger tight). A one-notch turn with wrench snugs down
the bolt and aligns the castle nut with the bolt hole. Be sure to install a cotter pin or "safety pin". Notes: Do not over tighten. If over tightened, the taper may cause undue stresses in the fitting material. Chris Bobka says:
Taper pins
are to be used only in "shear" situations.
All layers pinned together must fit
tightly together.Remember that any taper pinned joint has the potential to put an axial load on the hole that can be quite large if you tighten up the securing nut too much. The idea is to have a perfect fit with minimum nut torque. |
|
The UltraCruiser wing fitting
design is double shear with the single fitting on the center wing
section (Sheet 6 of the plans).
/  |
 |
| AN386-3-14 ... Nomenclature for the B&S taper pin |
3-x is
for a 1/4 nut. An -x-6
is for 3/4" length which would be used to join a total thickness (grip)
of 3/4 - 1/8 = 5/8". The length you select should be
1/8" longer than the total thickness (grip) you are joining.
In other words, L = grip + 1/8". ... The length and the grip
do not include the threaded part for the nut.....just like when you
size regular AN bolts.....(from Chris Bobka) Note: The #3 pin is .317 inches at the narrowest end (wider than a 5/16 hole) The #3 reamer is .3099 inches at the narrowest end (narrower than a 5/16 hole) Thus, thus a 5/16 inch hole ( .0312) would be the appropriate pilot hole |
| Tooling: B&S #3 Taper Pin Reamer... Cat. No. 1-017-030 B&S #1 or #2 Taper Pin Reamer ... Cat. No. 1-017-020 Source: JTS Machinery & Supply Co ... Reamers Reamer; Finishing; Brown & Sharp Taper; Straight Shank; HSS; 3 B & S Taper; $26.35 Parts: four AN386-3-14 Taper Pin ... note two sizes larger than my plan four AN975-4 Taper pin washers four AN310-4 Castle nut two AN386-2-13 Taper Pins two AN975-3 Taper Pin Washers Source: From Aircraft Spruce Taper pin Taper Pin Washers Castle Nuts |
My
Costs: $26.35 B&S #3 Taper Pin Reamer (one time only) $ 0 B&S #2 Reamer (I will borrow from Chris for the rear spar.) $35.20 Four Taper pins for main spar $ 9.90 Two taper pins for rear spar -------- $71.45 Total Cost plus shipping .... minus peace of mind! Note: Pin costs can vary without logic with longer pins costing less than shorter pins. Select taper pin length to obtain the lowest cost. Oversize taper pins are not a problem since there is plenty of room for the taper pin to stick out on the front side of the spar fittings. Also, the extra length gives you something to hang on to. B&S or Brown & Sharp Standard Taper ... (1/2 inch taper to the foot) (0.0416" taper per inch of length). |
 |
While drilling close
tolerance holes, I discovered that my drill press had significant runout or
wobble. A check on the web found several complaints about the
Ryobi Drill Press in this regard after drilling many holes (and the
UltraCruiser requires a lot of holes!). Testing with a dial
indicator showed that the arbor is dead-on, so a
new chuck was ordered. While I was at it, I bought a
1/2 inch keyless chuck. Dale Johnson loaned me his dial indicator. Video on using Dial Indicators another link for same video |
