Yankee Siege Destroys Main Axle Bearings - October 5, 2009

Just finished a weekend of throwing. Saturday was raining and no throws. Sunday was a beautiful fall day in the 60's and perfectly calm. Several shots exceeded the 2000 foot range, but some shots released very late and were very low. We don't know what exactly is causing the late release (we did throw some rather light pumpkins in the 8 pound range and that may account for the late release but even some of the ten pound pumpkins would release somewhat late).

Today (Monday) we entertained a high school physics class. This was a small class with very attentive students. It's nice to see a well behaved class with students that appear to be interested in physics and math. I'm afraid I may have talked too long. I have a tendency to be long winded and perhaps too technical when it comes to trebuchets. I get very excited. Sometimes it's hard to talk to a class that has just been introduced to projectile motion and not be too technical. I could talk for days. They had to shut me up and get on with the demo or we would have run out of time.


Bearing Failure

Just before the class, Chuck and I went to inspect the main axle bearings for the throwing arm. We have been noticing that the bearings on one side had appeared to be loosening and causing one of the towers to move inward, causing a binding of the throwing arm against the frame. Upon closer inspection we found that two of the four bearings had cracked housings. These bearings are forty year old Dodge pillow block bearings obtained from a junk yard. We suspect that the new throwing arm is the culprit. The new throwing arm is so light and has such a low moment of inertia, that the counterweight literally "falls" for 12 feet and slams to a near halt at the end of its travel. This action of the counterweight causes a significant increase in the reaction force on the axle leading to failure of the bearing. (We don't know exactly when this cracking occurred, it may have started last year and went unnoticed until this year with a new throwing arm). There is about a 3/16 th's inch wide crack on the lower part of the bearing housing.

Now we have to figure out how to solve the stress on the bearings. We have decided to put six bearings on the axle (we now have 4 bearings). We will also move the two inner bearings on each side as close to the throwing arm as possible (the inner bearings take most of the load).

This is just another example of unanticipated stresses with a change in the configuration of the machine. A lighter throwing arm can lead to vast increases in the reaction force on the axle. Yankee Siege has a four inch solid steel axle through the throwing arm and turned down to 3 and 7/16 th's inches in the bearings

We have ordered four new bearings that will be here Thursday. Changing the bearings will not be an easy task. It will require removing the new throwing arm, removing the counterweight and stub throwing arm, beating off the old bearings and then reassembling. Hopefully we will be ready for Saturday's demos. Actually, the hardest part of the whole project is telling my wife how much I just spent on the new bearings! Wish me luck!

Steve Seigars, YS