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Glued and pinned cylindrical joints

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Trevorc:
Hello
Having had a disaster attempting to machine a single throw crankshaft from solid i am now following the
fabrication route with round shaft and journal fitted into reamed holes in the webs. I plan to use Loctite 638 with a 3/32 taper pin.  Being  curious  i decided to do some simple tests to measure the torque strength of the joints. I tested a joint using glue only, another using tapered pin only and finally a pinned and glued joint. The joints are 5/16 inch dia into reamed holes.
I found that the glued only was slightly stringer than that for the pinned only joint.
However what was a bit surprising was that the strength of the combined glued and pinned joint  was very similar to the glued joint alone. My question is why didn't the strength of the pinned and glued joint become anywhere near the sum of the strengths for pin only and glue only.
Can anybody please offer an explanation?
Am i wasting my time pinning the joint?
Thanks
Trevorc

Roger B:
I think the answer may be based on the different failure modes  :thinking: The measured strengths of pinned and glued separately are similar. If you have a combined joint as long as the glue is holding there will be no load on the pin. Once the glued joint breaks there is already enough load to break the pin. The glued joint is not very elastic. The pin relatively will deflect by an appreciable angle before it breaks.

steve-de24:
Hi Trevorc,

I think the answer is :-

With both the glue and pin you have two parallel load paths for transmitting the load between shaft and web. In that situation the load distribution is decided by which is the stiffest load path. Assuming you are testing the joint in shear the resisting shear is building up in the glued joint before the pin can take a lot of load. When the glued joint fails (probably without too much strain) the load is transferred fully to the pin. It probably takes a lot more strain to fail the pin.

And now my glamorous assistant will demonstrate the correct way to machine a crankshaft..........see pic.

Regards,
Steve
(Added later)  Roger B posted his reply just before I posted mine - I'm glad we agree on the answer.

crueby:
Also, if you look at the strength of the various Loctite versions in thier charts, time is a factor - longer cure times (days and days) for full strength.

I've done several that used both glue and taper pin, but never tested for true strength. This is very interesting!

derekwarner:
The answers from both Roger and Steve are absolutely on the mark......progressive rotational force will shear the Loctite bond, 'then' shear the pin

One consideration here is the setup......it would be far earsier to drill, ream & glue the webs & pins and set them up & clamp them down in a true 90 degree relationship without a series of pins protruding through the components

After the glued components have the recommended cure time, then set up & complete the drilling & taper reaming for the pins

No amount of vibration or and cutting fluids will damage the bonds and the latter process will be far easier to complete

So effectively by gently tapping the taper pins will add a mechanical stress within the joint between web & pin joint

A little reading will confirm the load/gentle hitting force required for the insertion of each pin, however importantly, this now can be achieved over a holed anvil 

I have spoken in this Forum about ...after trial fitting each pin, mark them, place them in a refrigerator freezer for a few hours, then taking out one at a time & insert + tap in..  :hammerbash:

Derek

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