Boom stress in fairground ride 1

[jbourke]

Hi all,

Newbie here and not a mechanical engineer; please be patient

I have been fascinated for a long time with a flat pack ride called Enterprise (see

http://pointbuzz.com/content/ask-rideman-seat-belts-witches-wheel

for some interesting info on this 70's product still going strong).

The main boom is lifted by a single-ended hydraulic cylinder. When the boom is horizontal the cylinder is only at a shallow angle to the boom and so most of the force exerted by the cylinder must be trying to rip the boom from its pivoting point instead of raising it. I do get that because of the various pivoting points the resulting action is actually raising the boom but some nagging questions remain:

- is it just a matter of properly sizing the mounting points to cope with this "ripping" stress? (interestingly enough cracks near the boom mounting point have been found on some of these rides)

- as the boom rises and the angle increases the cylinder must become more "effective" (probably not the right term). Am I correct in assuming oil pressure needs to be reduced to prevent the boom raising at an accelerated rate?

Your expert replies are much appreciated.

Cheers,

John

 

[MikeHalloran]

Forces are vectors; they have a magnitude and a direction.
They add at pivot points in a sort of geometric fashion.
Sophomore engineering majors should learn how to add forces, and to resolve them into components aligned with the multiple members at a joint.

Using a tool known as a free body diagram and some trigonometry, the forces acting on a given part or assembly can be resolved into component forces for each member of that part or assembly.
You have correctly intuited that the forces within an assembly can be quite a bit larger than the external forces acting on the assembly.

Hydraulics are ideal for this sort of situation, where they may work with a disadvantageous mechanical advantage for part of their stroke.
You have also correctly intuited that a cylinder supplied with a constant flow may generate large velocities when its mechanical advantage is disadvantageous as above, so some sort of flow control may be provided, as in a variable bypass valve, or a pump with variable capacity.

( In the specific circumstance you mention, the pressure must be raised to compensate for the lack of mechanical advantage, while the flow must be reduced to limit the velocity, which is otherwise magnified by the same mechanical disadvantage. )

In the above, I hope that I have not further confused you, and have provided you with some search phrases that may help your eventual understanding.

Yes, it's as simple as making the mechanical parts big enough to withstand the predicted forces, and before that, of correctly predicting the forces, and of allowing enough margin to compensate for unknowns and for wear associated with regular disassembly and reconstruction, and of making it difficult to reconstruct the assembly in the wrong way, and of making it all strong enough to do its job and also light enough so that the pieces can be handled by a small crew, and small enough so that the collapsed ride will fit on a finite number of trailers with finite width, and go under bridges with standard clearance, and to ensure that the pieces will fail in a graceful way, e.g. by cracking in a place that will be visible during a casual inspection, and, and,
... if it was easy, anybody could do it.



Mike Halloran
Pembroke Pines, FL, USA

 

[jbourke]

Thanks Mike; no confusion, got the answers I was looking for and great pointers for further topics of interest to boot. BTW being part of conception and design of flat pack rides must be a great career, should have listened to that little guy inside all those years ago