Drawing

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E5
Analyzing the Stress on the Trebuchet Axle continued
Will a Trebuchet Axle Fail?
The bending or breaking of the axle is likely to be the most frequent source of failure in a
trebuchet system. In order to prevent axle failure, it is necessary to determine if the axle can
support the loads imposed by the standing beam system as well as the more significant loads
imposed by throwing.
It is possible to determine if the axle can support the loads in several different ways:
1. Trial and error. Build a trebuchet and use it. If the axle breaks, build another one. This is
the costly, time consuming and inefficient method of axle design employed by ancient
trebuchet builders.
2. Use the simplified flexural formula and the force information supplied by the GEARS-
TrebStar simulator to develop a general approximation of the answer to the question:
“Will it break?”
3. Use SolidWorks COSMOSXpress or other CAD modeling software to perform virtual
tests of the axle’s performance. This is a great way to obtain a “visual” understanding of
how the axle will react to the applied forces.
4. Use all three methods. This is a great way to expand your engineering knowledge and
skill, and arrive at a working solution to the question: “Will it break?”
Mathematical Model of the Axle
A trebuchet axle and the forces acting on it are illustrated in figure C9. This is a simplified
example where the load on the axle is assumed to be acting on one point, at the center of the
axle span. This is a “Worse case” scenario. An axle that can support the design load acting at
the center of the span will likely support the loads generated by a working trebuchet. The actual
loads of a working trebuchet will be distributed over a portion of the axle length and not applied
directly to the center of the axle span.
The axle is assumed to be a rod with a span L, a diameter D, and supported on the ends, with a
load P in the center of the beam. The load comes primarily from the weight of the
counterweight plus the centrifugal force generated by the motions of the counterweight, beam
and projectile during the throw.
P
½ P ½ P
Deflection
D
Length L
Fig. C9 An axle supported on two points equidistant from load P
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