Thread: Help!! Roll-over pool table HSC student

Hi,
You certainly have a bit to do. My points have to be brief as it is late, sorry! My detailed reply was lost when I went to submit it

Gears , pulleys and belts, levers all do similar jobs and could be applied to your project.

The key point is that if you can find the tables centre of gravity and set your pivots so that as the table is rotated the centre of gravity always acts downwards through the line of the shaft axis.

If you can do this the table will always be nicely balanced regardles of its rotation angle. If you get this wrong it will take effort to turn it and effort to stop it running away from you once you reach a certain point of rotation. Bit like if you were to rotate a wheel using a axis that was off centre.

Now imagine tabel as a solid rectangular block. And on each side you placed a dot at the centre of the centre of that face. If you joined opposite dots with an imaginary line, at the intersection of the three lines would lie your centre of gravity.

Now picture looking end on to your table in the direction of your shaft, you would only see two of these lines, the intersection of which marks the centre point of your shaft.

Pool tables are generally symmetric in two planes. Therefore (still looking end on) the location of teh vertcial line will most likely be half distance along the horizontal edge.
Your BIG problem is that it is very unlikely that the horizontal line will be located half distance through the tables vertical thickness ( height). Finding this is difficult and hard to do accuratley.

You are trying to find the pivot point of a see saw in two planes. One way is to stand the table on edge and use a broom handle as the "see saw" pivot line, keeping the handle parrallel with an edge of the table. At some point the table will be evenly balanced, it is at this location that the tables centre of gravity is working and located but only for one plane of reference. try it with a book or somtheing and you will see what I mean. Now this method is Ok with light object but quite dangerous otherwise as a pool table on edge on a rolling pivot is very likely to fall. I would not be doing this with a pool table, I'm allergic to being crushed. Accuracy is also questionable.

I would build a pivot that can be adjusted in height relative to the tables height. locate it on the vertical line mentioned earlier and set its height somewhere below the tables halfway thickness (height), possible starting point could be 1/3 up. This way you can adjust the devices height to find the balanced position.

Check out 'back inverters' on the web the principle is the same.

Check out SKF or NSK or other bearing webistes it will give you ideas and generally contain useful info.

Shaft is determined by your design and the tables weight, and your bearing choice, if for a HSC project, I would think if you ask an engineerring business nicley they would likely help you out if not too busy.

Find a basic structural mechanics text or websearch shaft design/engineering statics/engineering dynamics or similar.

If you can provide a couple of pictures or even a hand skecth of what you are doing it would be useful.


Good luck

Hi,

I am so concerned as to how the slate will remain supported! Will it want to fall or slump when upside down?

I see that this multi purpose model Here is designed with major framing and a steel bed.

I hope it works out for him.

Dear Woodworm1,

I'm feeling really guilty that I never thanked you for your well thought out advice. Lots of your comments were really helpful! He ended up getting a shaft to go through the centre supported by bearings. Unfortunately they ran into problems and it did not get finished in time for marking so it was marked as an incomplete project. He plans to get back to it once his HSC exams are over.

I suggested he might send you some pics for advice, but he was not willing at that stage. Thanks for the offer. His teacher has an engineering background, but this was not overly helpful. Early research at an engineering firm may have helped.

Disappointing that it did not get done... probably too ambitious right from the start. As parents you want to support your kids in what they do, especially when they are so passionate about it (as he was). Now I wish we'd been able to talk him into something less ambitious. Still... its all part of the learning process and I'm sure he did learn a lot from this. I look forward to the day it is finished and then we ask the question "Where do we put it?"

Thanks again for you interest.

glad i could help pdg. Shame that time ran out, hope the table still got finished though!

Unfortunately not yet finished. He was waiting until exams were over and has had tonsillitis since. He plans to get back to it as soon as he is better, but is still feeling a little unsure about some of the structural problems. I'll be very glad when it is finished!
Thanks again for your interest.

Originally Posted by woodworm1

The key point is that if you can find the tables centre of gravity and set your pivots so that as the table is rotated the centre of gravity always acts downwards through the line of the shaft axis.

If you suspend the table from a horizontal rail, its centre of gravity will be in the same vertical plane as the rail.

If you suspend the table from the horizontal rail attached in another position, you will have a second plane, and the centre of gravity is at their junction.

This assumes the rail has negligible mass; in practice I might use triangulation to locate the centre..

The "rail" might be the axle you will use.